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Automated monitoring and warning solution for concrete placement and vibration workmanship quality issues

Automated monitoring and warning solution for concrete placement and vibration workmanship... Placing and vibrating concrete are vital activities that affect its quality. The current monitoring method relies on visual and time-consuming feedbacks by project managers, which can be subjective. With this method, poor workmanship cannot be detected well on the spot; rather, the concrete is inspected and repaired after it becomes hardened. To address the problems of retroactive quality control measures and to achieve real-time quality assurance of concrete operations, this paper presents a monitoring and warning solution for concrete placement and vibration workman- ship quality. Specifically, the solution allows for collecting and compiling real-time sensor data related to the work - manship quality and can send alerts to project managers when related parameters are out of the required ranges. This study consists of four steps: (1) identifying key operational factors (KOFs) which determine acceptable workmanship of concrete work; (2) reviewing and selecting an appropriate positioning technology for collecting the data of KOFs; (3) designing and programming modules for a solution that can interpret the positioning data and send alerts to project managers when poor workmanship is suspected; and (4) testing the solution at a certain construction site for validation by comparing the positioning and warning data with a video record. The test results show that the monitoring performance of concrete placement is accurate and reliable. Follow-up studies will focus on developing a communication channel between the proposed solution and concrete workers, so that feedbacks can be directly delivered to them. Keywords: Real-time monitoring, Concrete quality, Placement and vibration, Workmanship 1 Introduction Mills et  al., 2009). For these reasons, project managers Concrete placement and vibration are important in have made considerable efforts to reduce the defects by achieving desired construction quality measures, because ensuring proper performance of concrete placement and inappropriate placement and vibration of concrete can vibration. significantly decrease concrete quality by causing con - However, many projects still face problems with con- crete defects, such as honeycombs and air voids. These crete quality, because faulty workmanship cannot be eas- defects can result in delays, cost overruns, and disputes ily detected on the spot and immediate corrective actions among project stakeholders, as well as frequent mainte- are rarely made during the concrete construction opera- nance, elevated safety concerns and shortened service tions (Lee & Skibniewski, 2019). The current method by life of facilities (Aljassmi & Han, 2012; Barber et al., 2000; human supervision and inspection is not a practical solu- tion for continuous monitoring of compliance, because (1) managers cannot perform all-time monitoring by *Correspondence: slee239@terpmail.umd.edu always hovering around the concrete workers; (2) their Department of Civil and Environmental Engineering, University of Maryland, sights of view on construction workers and equipment 1188 Glenn Martin Hall, College Park, MD 20742, USA are often disturbed and occluded by objects; and (3) Full list of author information is available at the end of the article © The Author(s) 2022. 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Lee and Skibniewski AI in Civil Engineering 2022, 1(1):4 Page 2 of 19 they have different levels of knowledge and field expe - knowledge, experience, and relevant rules and guide- riences, possibly leading to different interpretations of lines. If managers detect improper workmanship, they observed workmanship. Due to these limitations of the will typically ask for corrections from the foremen and/ visual monitoring method, concrete defects are typically or workers. inspected and repaired after the concrete is hardened. However, the vision-based monitoring of concrete Although many of such defects can be repaired, this reac- work entails several problems that affect its reliability. tive method is inefficient because inspection and repair First, the workmanship is often monitored temporarily would incur additional time and cost (e.g., related costs of and intermittently. In fact, visual monitoring is too time- material and equipment use, inspectors’ time for exam- consuming and labor-intensive to be performed continu- ining, evaluating, and reporting the defects). Moreover, ously, because it requires the project managers’ entire slipshod inspection and repair may lead to failures in focus on the concrete work. The managers need to stay at detecting defects or unsatisfactory quality of the repaired the placement sites all the time, while keeping their eyes concrete. These reasons explain why proactive and pre - on the workers and equipment. In fact, managers are also ventive quality control measures should be achieved. responsible for taking care of cost, schedules, and safety, This study introduces a monitoring and warning in addition to the quality of concurrent construction solution of concrete workmanship inspection to help activities performed by various crew and equipment. For managers ensure the quality of concrete placement this reason, nonconformity of rules and guides may not and vibration operations, thus to  prevent the concrete be detected when managers are tied up with other tasks. defects in a proactive manner. Since various studies have Moreover, visual monitoring and inspection is error- explored and developed many tracking technologies for prone and subjective (Kim et  al., 2015; Lundkvist et  al., concrete vibrators, this study will focus on achieving 2014; Park et al., 2013). Human visual feedback can be erro- monitoring and management support by utilizing the neous when their views are unclear or occluded by obsta- data collected from the tracking technologies. cles. Cast-in concrete is often poured into a formwork that is tightly sealed to prevent leakage, and it is hard to recog- 2 Literature review nize objects inside the formwork. Moreover, concrete is 2.1 C urrent monitoring and supervision of workmanship placed and vibrated by a crew consisting of workers who The quality of concrete is susceptible to the operators’ play different roles (e.g., adjusting the direction of the pump workmanship as well as the degree of skills in which the hose, handling concrete finishing tools, or operating con - construction activities are performed. Even concrete crete vibrators). In these dynamic circumstances, manag- made of good materials in a properly proportioned and ers’ view is frequently blocked, and it is challenging to track batched manner can be defective if performed with poor and monitor all workers simultaneously. Furthermore, workmanship (ACI Committee 311, 2007). When freshly since the information acquired by the visual monitor- mixed concrete is deposited, it will normally not be able ing methods may be inaccurate, supervisors often need to to completely fill the spaces in the formworks and around rely on their own subjective interpretation and evaluation, embedded items (e.g., steel reinforcements) with its own which may vary depending on the levels of their knowledge weight. Rather, it may become “honeycombed”; that is, and field experiences (Gong et al., 2015; Tattersall, 1991). it may contain entrapped air at a level of approaching Last, the process of learning and improving workman- 20% of its volume. Improper placement and vibration ship is an inefficient and long-time one. Inexperienced may account for non-uniform density and excessive air workers without the requisite expertise and know-how voids. Such defects would weaken the structural integrity often have to go through several trials and errors by per- and mechanical properties of concrete members, such forming the work and evaluating the resulting quality. as compressive strength and bond stress with reinforce- The next visit to a certain site by a concrete worker can ment (ACI Committee 309, 2017; Cement Concrete & be days and weeks later at which they may not remem- Aggregates Australia, 2006). Additionally, such defects ber the behaviors during the placement. This time gap may cause long-term durability problems by allowing reduces the learning rate of workers, meaning that more water seepage into the concrete members and expos- practice is needed to improve their workmanship. The ing the reinforcements to corrosion (Roy et  al., 1999; problem of low-learning rates also applies to project Siegfried, 1992). By monitoring and supervising how managers: the knowledge of concrete quality and work- concrete is placed and vibrated, project managers can manship only remains in their memories and is often assure the quality and minimize defects. Generally, they forgotten and distorted. Thus, not having tangible work - walk around the site on a regular basis to visually inspect manship data becomes an obstacle to extracting and and supervise the concrete operations. They assess sharing knowledge with other project managers and the observed behaviors of the workers based on their workers for effective improvement. L ee and Skibniewski AI in Civil Engineering 2022, 1(1):4 Page 3 of 19 Due to the problems of monitoring workmanship as in the experiment. Wang et  al. (2021) developed a com- described above, faulty site workmanship is common, puter-vision-based vibration quality monitoring method. and concrete defects in construction are frequently found The system, which used an infrared sensor to measure commonly in buildings and infrastructures (Ahzahar the distance between the sensor and the concrete sur- et  al., 2011; Lin & Fan, 2018; Love et  al., 2017; Neeley, face and a camera to capture concrete surface images. 1993). The resulting quality problems are managed by a The system was applied to a heavy vibrator machine diagnostic and corrective approach, which is to inspect which is equipped with a head of multiple vibrating bars. and repair the defects (Love, 2002; Lundkvist et al., 2014). Although the system could monitor undervibration and Although inspection and repair are important for meet- overvibration from the concrete surface, the vibrator’s ing the desired quality level of the final products, they are location was not recorded. Tian et  al. (2019) developed inherently reactive, as they merely correct non-conform- a real-time visual monitoring system for vibration effects ance in an "after-the-fact" manner. Repairing defects can by using the Global Navigation Satellite Systems (GNSS), be technically difficult and costly, requiring more efforts including GPS, Beidou, and GLONASS. This monitoring than taking preventative measures, because the concrete system could track the vibrator tip’s location based on the has already hardened and gained strength (ACI Com- location of the mobile antenna on the worker’s wrists. mittee 309, 2005; Kennedy, 2005; Rwelamila & Wiseman, They assumed that the locations of the tip were alone on 1995). Performing inspection and repair cannot guaran- the linear line between the two hands. The system had a tee good quality if the defects are not detected during warning function to instruct the construction workers data entry or when newly applied material is not bonded to act against observed undervibration. Liu et  al. (2015) with the existing concrete (ACI Committee 309, 2005; developed a quality monitoring system for storehouse Kim et al., 2015; Lundkvist et al., 2014; Park et al., 2013). surfaces of roller compacted concrete (RCC) dams. The For these reasons, concrete defects should be minimized system utilized GLONASS and GPS to acquire the posi- and prevented by following proper placing and consoli- tioning of roller compactors. It monitored rolling speed dating procedures (Neeley, 1993). and the number of passes of compaction and sent warn- ing messages to supervision and management personnel. 2.2 P revious studies on monitoring on‑site concrete Various technologies have been used to track vibration workmanship effort, and many of them adopt GNSS which requires a In this section, the existing research on advanced moni- clear view of the sky. The major focus of these studies was toring of concrete work is reviewed (Table  1). Studies on the technological development and visualization by that did not particularly focus on concrete work (e.g., testing and improving the applicability of the technolo- excavators and general worker location tracking) are not gies to concrete work. Some studies modified the design included. Lee and Skibniewski (2019) introduced a con- of internal vibrators to improving the tracking perfor- ceptual framework of a concrete placement and vibration mance, while others used different vibrating machines to work monitoring system. They proposed to utilize ultra - install and implement the tracking sensors. Regarding the sound (US) positioning and computer vision technolo- decision support features, Lee and Skibniewski (2019) gies, to track locations of objects and perform monitoring identified the important factors of concrete placement based on the key operational factors that can represent and vibration work, but failed to develop the proposed the critical workmanship related to concrete quality. concept of the monitoring system. Many of the existing Gong et al. (2015) attached a ultra-wideband positioning studies either focused only on undervibration of concrete sensor to a vibrator head, in order to acquire its position- or did not have a warning system that supports real-time ing data. They tested the positioning accuracy from sim - decision making. ulated work and visualized the locations of the vibrator Moreover, they monitored the output of the product- head. However, since the purpose of their research was to focused concrete work processes (e.g., concrete surface apply and test the tracking technology, there was no con- after vibration), rather than workers’ behaviors during sideration on how to support managerial decisions with concrete placement. For example, the existing monitor- the collected data. Tian and Bian (2014) tracked vibra- ing systems can display undervibrated areas of concrete tors’ locations by using the Global Positioning System and send an alert about these areas, but they did not (GPS) and the Global Navigation Satellite System (GLO- monitor the movement of the workers (e.g., short inser- NASS). In the study, they fixed a carbon fiber bar that tion duration, or insufficient insertion depth) which has a mobile antenna with the vibrator and estimated the triggered these warnings. Monitoring how the work is vibrator head’s location from the antenna’s location and performed would foster workers’ awareness of their rela- the length of the fiber bar. They also displayed the vibra - tive compliance to a set of guidelines and clarify certain tor status (insufficient, normal, and excessive vibration) behaviors that cause concrete quality issues and expedite Lee and Skibniewski AI in Civil Engineering 2022, 1(1):4 Page 4 of 19 the learning process by changing their workmanship this time gap, the violation of the KOF can be monitored immediately. and determined. Lastly, the studies of monitoring concrete placement In the development step, the major tasks of the solution are scarce. The existing studies were generally focused on were identified and classified into four groups, and the tracking concrete vibrators but lacked efforts in tracking modules dedicated to performing each task group were concrete-pouring activities. Proper placement of con- developed. These modules were used to extract design crete is vital, as excessive free fall of concrete and pour- information, collect and merge sensor data, update sta- ing concrete directly on vertical forms or reinforcing bars tus of placement and vibration, and perform real-time may cause segregation. Furthermore, the vibration status warning. The performance of the solution was tested and of concrete can be understood only when it is tracked evaluated at a construction site. The reliability of the col - with the poured concrete, because without knowing the lected data was assessed by comparing them with a video locations where the concrete is poured, it is impossible to recorded during the work. If data were not collected for a determine whether the concrete is vibrated or not. moment or the accuracy was too low, the possible causes The research question addressed in this study focuses and remedies were discussed. In addition, the sensor on (1) achieving real-time quality assurance for concrete data and the video record were used to analyze the warn- placement and vibration operations, and (2) overcoming ing instances and their impacts on the concrete quality. the limitations of the existing monitoring methods, such The resulting concrete quality was visually inspected to as the lack of monitoring and warning features for work- decide whether there was any quality problem not pre- ers’ behaviors and the issues with the placement work- vented by the warning module. manship quality. 3.1 Determining key operational factors (KOFs) 3 Methodology Fifteen KOFs are summarized in this study, and they are To help managers assure the quality of concrete work- what managers shall focus on to make timely feedback manship in real time, a monitoring and warning solution to the workers. The first six factors are about concrete is designed and developed in this study. Specifically, the placement, while the remaining nine are about vibration. solution will perform data collection on concrete place- Table  2 shows the KOFs and the consequences of viola- ment and vibration with ultrasound (US) and electro- tion. After determining the KOFs, their parameters and magnetic (EM) sensors attached to concrete equipment thresholds for measuring and determining the compli- (e.g., vibrators or pump hoses), continuously compare ance or violation were investigated (Table 3). the observed work-related parameters with the threshold values, and alerts managers when the workmanship does 3.2 Development of the solution and its modules not comply with the recommended practices. The US 3.2.1 Mo dule 1: Extract the design information positioning technology was selected based on a compari- Module 1 extracts the geometric information of concrete son by Lee and Skibniewski (2019), and the EM position- members and forms. From the corner points of a con- ing technology was chosen because its accuracy is not crete member that a user has entered, the module cre- affected by occluded line-of-sight. ates the concrete points and placement grids used for Fig.  1 shows the procedures for developing the con- tracking the status of concrete placing and vibrating. crete work monitoring solution. First, both appropriate Extracting design data is initiated by calculating a coor- and inappropriate practices of concrete construction are dinate conversion matrix, which will convert the coordi- determined. This study focuses on the common tech - nates of concrete members and forms in CAD drawings niques of concrete placement and vibration, which use into the US sensor coordinates. A coordinate conver- concrete pump hoses and internal vibrators (also as sion matrix (4 × 3) is calculated by using the following known as spud or poker vibrators). Manuals, reports Eq. (1) with a set of CAD points and their corresponding and guidebooks about concrete work and inspection are US points. By multiplying with the conversion matrix, reviewed to determine the KOFs. Then, these KOFs are the CAD points are converted and expressed in the US used to figure out their parameters for measuring and coordinate system. There should be at least three corre - determining the compliance or violation of the KOFs. For sponding non-collinear points in order to calculate the example, the parameter of the KOF “Compaction must conversion matrix. be done while concrete is still plastic” can be the time gap between placing and vibrating concrete. By measuring L ee and Skibniewski AI in Civil Engineering 2022, 1(1):4 Page 5 of 19     P P cad us a vibrator shaft. Figure  3 shows the procedures for pro- � �� � � �� � cessing and merging collected data from the sensors.     a a a �1 b b b 1x 1y 1z 1x 1y 1z         There are four groups of processes: (1) data collection a a a �1 b b b 2x 2y 2z 2x 2y 2z         and retrieval; (2) US data processing for a pump hose; a a a 1 b b b 3x 3y 3z � 3x 3y 3z   × C =  , (1)  . . . .   . . .  (3) US and EM data processing for a vibrator operator; . . . . . . .     . . . . . . .     and (4) integration of US and EM coordinates. The solu -     a a a 1 b b b nx ny nz nx ny nz tion first checks whether the data are available before initiating data processing. If it fails to retrieve any of the data required, it will move on to the next loop, trying to where P indicates a matrix of CAD points (n × 3), C cad retrieve the data again. indicates a coordinate conversion matrix (4 × 3), P indi- us After that, the data for the pump hose and the vibra- cates a matrix of US points (n × 3), and n indicates the tor are merged, respectively. When the data are merged, number of points used for the conversion. a linear interpolation method is applied to match their For example, if the four points of a CAD drawing are timestamps. The US and EM sensors have different coor - (2.4257, − 3.8481, 2.0000), (− 0.5743, − 3.8481, 2.0000), dinate systems. Therefore, the coordinates are aligned (− 0.5713, 1.2446, 2.0000) and (2.4257, 1.2446, 2.0000), before estimating the location of the vibrator head. and their corresponding points are (3.0000, 0.0000, Whereas the reference point of the US sensor coordi- 0.0000), (0.0000, 0.0000, 0.0000), (0.0030, 5.0920, 0.0000) nate system is fixed during data collection, the EM sen - and (3.0000, 5.0930, 0.0000) in the US coordinate system, sors’ reference point is based on the EM source. Thus, the then its conversion matrix is calculated in Eq. (2). cad us 2.4257 −3.8481 2.0000 1 3.0000 0.0000 0.0000 −0.5743 −3.8481 2.0000 1 0.0000 0.0000 0.0000 (2) × C = , −0.5713 1.2446 2.0000 1 0.0030 5.0920 0.0000 2.4257 1.2446 2.0000 1 3.0000 5.0930 0.0000   1.0000 0.0000 0.0000 points in the EM sensor system will change depending  0.0000 0.9999 0.0000  on the location and the orientation of the source. For this C = .   (3) 0.0000 0.0000 1.0000 reason, the degrees of tilt and orientation of the source 0.5743 3.8476 −2.0000 are measured by attaching the US sensors with inertial measurement units to the EM source, and the EM points After converting the coordinate systems of the points, are converted to the US coordinate system. the forms’ plane parameters are calculated, and the placement grids and internal points of concrete mem- 3.2.3 Module 3: Update the status of placement bers are created. The form’s plane parameters represent and vibration the shapes of the forms in three dimensions, and they The height of concrete in each grid is estimated based on are used to compute the distance between a point and the pumping rate and the duration that the pump hose the form as well as an intersection between a line and the stays toward the grid surface. This simplified approach form. The placement grids are created by dividing an area is significantly faster and less computationally expensive of a concrete member along with x and y axes, and simi- than scanning the actual shapes of deposited concrete. larly, the internal points are created by distributing points From the location information of the pump hose, the along with x, y, and z axes. The placement grids enable change of concrete heights over time can be tracked. The tracking of the height of fresh concrete deposited in the procedures for updating the heights of concrete based forms, while the internal points are used to monitor the on the location of the US sensors mounted on the pump vibration status at all critical points of the concrete mem- hose are presented in Fig. 4. bers. Figure 2 shows an example of a concrete member in First, the positioning data of the two US sensors the CAD coordinates and the US coordinates. mounted on the pump hose are retrieved. One sensor is attached to the upper part of a pump hose, so that it 3.2.2 Module 2: Collect and merge sensor data can collect the location data reliably after the hose tip is The solution collects positioning data from two US sen - immersed in the forms. Another sensor is attached to the sors for tracking a pump hose, two US sensors for track- bottom of the pump hose. If the pump hose is higher than ing a vibrator operator and three EM sensors for tracking Lee and Skibniewski AI in Civil Engineering 2022, 1(1):4 Page 6 of 19 Fig. 1 Procedure for developing the concrete work monitoring solution L ee and Skibniewski AI in Civil Engineering 2022, 1(1):4 Page 7 of 19 Table 1 Studies about monitoring on-site concrete workmanship Author Technology Purpose of collecting data Supported managerial decisions Sensor attachment location Lee and Skibniewski (2019) Ultrasound positioning sensors and Tracking a pump hose and a vibrator’s Authors determined key workmanship Ultrasound sensors at a pump hose, and computer vision location factors for monitoring cameras at the known locations of a site Gong et al., (2015) Ultra-wideband positioning sensors Visualization of a vibrator’s location No The vibrator’s head Tian and Bian (2014) Global Positioning System (GPS) and Tracking a vibrator’s location and visual- No At the tip of a carbon fiber bar attached to Global Navigation Satellite System izing the vibration effect the vibrator (GLONASS) Wang et al., (2021) Computer vision Monitoring concrete surface’s quality Undervibration and overvibration were The heavy vibrator machine detected based on the vibration depth and duration Tian et al., (2019) Global Navigation Satellite System Visualization of vibration effect, and Undervibrated area and its volume were The vibrator operator’s wrists (GNSS) delivery of warnings to construction detected based on the moving trajec- workers tory of a vibrator Liu et al., (2015) GLONASS and GPS Tracking the location of a roller compac- Warning messages about the rolling A receiver is installed on a roller compac- tor speed and the passes of a roller com- tor pactor are sent Lee and Skibniewski AI in Civil Engineering 2022, 1(1):4 Page 8 of 19 Table 2 Key operational factors and the consequences of violation Key operational factors Consequences of violation 1 Uniform placement layer Honeycombs, air voids, form offset, and sand streaking The crews should only deposit as much concrete as can be consolidated efficiently (ACI Committee 309, 2017; ACI Committee 311, 2007; Kosmatka et al., 2011) 2 Continuous placement Cold joints and pour lines Placement should continue before the initial set of the previous lift ( ACI Com- mittee 304, 2002; ACI Committee 309, 2017; ACI Committee 311, 2007; Cement Concrete & Aggregates Australia, 2006; Kosmatka et al., 2011) 3 Avoiding excessive free fall height Segregation Placing concrete should be from an appropriate height (ACI Committee 309, 2017; U.S. Department of Agriculture Natural Resources Conservation Service, 2008) 4 Vertical drop of concrete Segregation Concrete should drop vertically (ACI Committee 304, 2002; ACI Committee 311, 2007) 5 Avoiding direct placement on vertical forms Segregation Concrete should not be placed directly on vertical forms (ACI Committee 311, 2007) 6 Avoiding direct placement on reinforcing bars Honeycombs, sand streaking, and segregation. Reduced bonds between reinforce- Concrete should not be placed directly on reinforcing bars (ACI Committee 304, ment surfaces and mortar may be formed due to premature coating 2002; ACI Committee 311, 2007) 7 Appropriate vibration duration Undervibration: honeycombs, excessive entrapped air voids, blisters, subsidence Concrete should be vibrated for an appropriate duration (ACI Committee 309, cracking, pour lines, and cold joints 2017; Neeley, 1993; U.S. Department of Agriculture Natural Resources Conserva- tion Service, 2008) 8 Appropriate insertion depth Overvibration: segregation, loss of entrained air, form offset, and sand streaking Vibration sticks should be inserted into the sufficient depth (ACI Committee 309, 2017; ACI Committee 311, 2007; Kosmatka et al., 2011; Neeley, 1993; U.S. Depart- ment of Agriculture Natural Resources Conservation Service, 2008) 9 Appropriate distance between insertions Distance between insertions of the vibrator head should be appropriate (ACI Committee 309, 2017; ACI Committee 311, 2007; Cement Concrete & Aggregates Australia, 2018; Kosmatka et al., 2011; Neeley, 1993; U.S. Department of Agriculture Natural Resources Conservation Service, 2008 p. 6) 10 Inserting vibrators close to forms Vibrators should be kept close to the form face (ACI Committee 309, 2017; Neeley, 1993) 11 Continuity between placement and vibration Honeycombs and cold joints Compaction must be done while concrete is still plastic (ACI Committee 311, 2007; Cement Concrete & Aggregates Australia, 2018; Kosmatka et al., 2011) 12 Avoid touching forms with vibrators Form offset, form damage and deflection, and burn marks on the finished surface Vibrators should not be allowed to touch the forms (ACI Committee 309, 2017; Cement Concrete & Aggregates Australia, 2006, 2018; Neeley, 1993) 13 Appropriate penetration and withdrawal speeds of vibrators Air voids and blisters Vibrators should quickly penetrate the concrete layer and be removed slowly to remove the entrapped air (ACI Committee304, 2002; ACI Committee 309, 2017; ACI Committee 311, 2007; Cement Concrete & Aggregates Australia, 2006, 2018; U.S. Department of Agriculture Natural Resources Conservation Service, 2008) 14 Appropriate penetration and withdrawal angles of vibrators Pour lines and cold joints Vibrators should be inserted and withdrawn vertically (ACI Committee 304, 2002; Unsystematic insertions at haphazard angles may cause undervibration of the ACI Committee 311, 2007; Cement Concrete & Aggregates Australia, 2006; U.S. lower layer, while accumulating mortar at the top surface Department of Agriculture Natural Resources Conservation Service, 2008) 15 Avoiding dragging vibrators Sand streaking and segregation Dragging vibrators through the concrete should be prohibited ( ACI Committee 304, 2002; ACI Committee 311, 2007; Cement Concrete & Aggregates Australia, 2006; Kosmatka et al., 2011; U.S. Department of Agriculture Natural Resources Conservation Service, 2008, p. 6) a certain point at which both sensors secure the line-of- is located at a point vertically down from the top sensor. sight with the stationary sensors, the location data of both The horizontal distance between the top and the bottom sensors are used to estimate the deposited point. When sensors can be generally ignorable when a pump hose is the hose is down and may not secure the line of sight of lowered deeply to pour deep concrete members, such the bottom sensor, it is assumed that the bottom sensor as columns and girders, because reinforcing steel would L ee and Skibniewski AI in Civil Engineering 2022, 1(1):4 Page 9 of 19 restrict the pump hose’s horizontal movement. After cal- not within the set range. When a violation is detected, culating the deposit point, the height of concrete in each its details are delivered to the managers, with the warn- grid is updated based on the pumping rate and the dura- ing information displayed on the screen of a local com- tion that the pump hose stays in the grid. The internal puter and uploaded to a cloud server. points that are lower than the current concrete height are The data of the malpractice are presented in readable marked as “placed”, and the time of placement is saved. formats, such as tables and figures, facilitating the man - Figure  5 shows the procedures for updating the vibra- agers to understand the details at a glance. The warning tion status of the concrete internal points. The vibrator module compresses the sensor data together with the head’s location is first estimated based on the locations violation time points and saves them in an Excel data- of the EM sensors mounted on the vibrator shaft. This sheet. This sheet allows the managers to revisit what hap - solution fits the locations of the three EM sensors with pened around the time of violations. It also creates and a linear line in a top view and with a quadratic curve in a stores.jpg images to help managers interpret the coor- side view. Then, the actual distances between the vibra - dinates and pinpoint the locations. Figure  6 shows an tor head and the sensors are used to acquire the head’s example of KOFs #1 and #5. The images show the infor - location. With the location of the vibrator head and the mation about the violations, such as their time points and height of concrete, the solution updates the vibration sta- locations. The left figure below highlights the placement tus of the concrete members’ internal points. When the grids that exceed the acceptable height difference, while height of the vibrator’s head is lower than the height of the right figure shows the locations where concrete is concrete, the insertion duration begins to be counted. placed directly on the vertical forms. After a certain period of duration, the nearby points of the vibrator’s head are marked as “vibrated”, and the time 4 Site application of vibration is saved. Then, all the updated data of the A site application was conducted to confirm whether placement grid and internal points are passed onto the malpractices were caught as intended and to investigate warning module in order to determine whether there is whether there was any false alarm. The project is located any violation. at the University of Maryland, and its scope includes designing and constructing a 4-story building that has labs, conference rooms, and student lounges. The appli - 3.2.4 Module 4: Real‑time warning cation area was 7.3 m by 3.8 m with a thickness of 0.46 m. The warning module compares the acquired data with the The project used a concrete bucket instead of a pump predetermined threshold values, and it will send warn- hose for its placement method; and the concrete had ing messages to the managers if a violation is detected. a slump of 15.2  cm with the characteristic strength of The conditions for violation of KOFs are determined 24.13  MPa. Figure  7 shows the installed sensors on the as shown in Table  4. For example, KOF #12, “vibrators concrete bucket and the vibrator shaft. Given that work- should not be allowed to touch the forms”, can be con- ers would be more careful with their behaviors when they sidered violated when the head of the vibrator bar stays were aware of the experiment, the threshold values were within 0.l m of a form face for t consecutive seconds. This intentionally set tighter when acquiring the warning data. factor needs threshold values for its two variables (l and Prior to the concrete work, the researchers visited the t) to be determined. The threshold values can be refer - site to attach four stationary US sensors to the columns enced and calibrated from Table 3. where the sight is secured and four mobile US sensors The KOF #7 through #11 are monitored while mak - to two concrete buckets, while acquiring the CAD draw- ing sure that the vibrator head stays near the concrete ings of the application area and calculating the conver- points for a desired period of time. Then, for exam - sion matrix. 8 placement grids and 191 internal concrete ple, the solution may inform the managers of undervi- points were generated. The areas of placement grids brated internal points, which encourages an immediate ranged from 1.291 to 3.386  m after subtracting the areas remedial action such as re-vibrating the undervibrated of columns in the girds. The internal points had equal points. In this way, the monitoring is focused on “prod- distances of 0.389 m, 0.381 m, and 0.25 m along with x, y, ucts” to help achieve quality assurance for all inter- and z axes, respectively. The placing and vibrating opera - nal points of concrete. It is also important to monitor tion was performed for approximately thirty minutes. and manage the “behavior” of the vibrator operator. During the work, one researcher stayed near the vibra- The vibrator operator needs to know about the poor tor, taking a laptop to receive the sensor data remotely. workmanship, in order to learn about the faulty behav- At the same time, another researcher recorded a video ior and prevent its recurrence. In a word, the warning of the work to confirm the quality of the sensor data and module constantly checks the distance, duration, and the warning instances. The timestamps of opening and depth of insertions before alerting the users if they are Lee and Skibniewski AI in Civil Engineering 2022, 1(1):4 Page 10 of 19 Table 3 Parameters and recommended values of the key operational factors for general concrete work Parameters Values (in SI units) Sources 1 The layer depth of concrete - Not exceed 508 mm ACI Committee 309 (2005), ACI Committee 311 (2007) Kosmatka et al., (2011) 2 Time gap of placement between layers - 1.5 h (between mixing and compacting) Kosmatka et al., (2011) 3 Height of the discharging point at a pump - Not exceed 1.2 m Minnesota Department of Transportation (2003) hose 4 Angle of the discharging point at a pump hose - 90° (vertical) ACI Committee 304 (2002), ACI Committee 311 (2007) 5 Duration that concrete is placed on vertical - 0 s (not allowed) ACI Committee 311 (2007) forms 6 Duration that concrete is placed on reinforc- - 0 s (not allowed) ACI Committee 304 (2002), ACI Committee 311 ing bars (2007) 7 Duration that a vibrator head stays when - About 5 to 15 s ACI Committee 304 (2002), ACI Committee 311 inserted into concrete (2007), Cement Concrete & Aggregates Australia (2018), Kosmatka et al., (2011) 8 Depth of a vibrator’s insertion into concrete - At least 152 mm into the previously placed ACI Committee 309 (2005), Kosmatka et al., layer (2011) 9 Distance between insertions - 1.5 times the radius of action. (see Table 5.1.5 ACI Committee 309 (2005), Kosmatka et al., of 309R-96 Guide for Consolidation of Concrete (2011), U.S. Department of Agriculture Natural (ACI Committee 309, 2005)) Resources Conservation Service (2008) 10 Distance between forms (where concrete is - About 50 mm clear of the form face ACI Committee 309 (2005), Cement Concrete & placed) and their closest insertion - About 100 mm of the form Aggregates Australia (2006) 11 Time gap between placement and vibration - 1.5 h (between mixing and compacting) Kosmatka et al., (2011) 12 Duration that a vibrator touches a form - 0 s (Not allowed) ACI Committee 309 (2017), Cement Concrete & Aggregates Australia (2006, 2018), Neeley (1993) 13 Vibrator’s insertion/withdrawal speeds - At a rate of 30 to 61 mm/s (withdrawal speed) U.S. Department of Agriculture Natural Resources Conservation Service (2008) 14 Angle of the vibrator when inserted/with- - 90° (vertical) ACI Committee 309 (2005), ACI Committee 311 drawn (2007), U.S. Department of Agriculture Natural Resources Conservation Service (2008) 15 Distance of a vibrator’s horizontal movement - 0 m (not allowed) ACI Committee 311 (2007) while inserted closing the bucket gate were recorded to estimate the Table  5 summarizes the descriptions of the warning concrete pouring speed and location. data generated during the site application. The warning Figure 8 shows the compared locations of insertions of module detected the following: a large concrete pour the collected sensor data and video records. About 95% causing a height difference between nearby placement of the insertions (72 out of 76) were accurately located grids; two undervibrated concrete internal points; one in the construction environment, with 4 out of 76 show- withdrawal action of the vibrator that was too quick; and ing a reduced accuracy. The 4 red points are supposed to two occurrences of large tilt angles of the vibrator head. be observed at the blue points close to the tower crane. In addition, the vibrator head moved horizontally while A comparison with the video records revealed that the being inserted into concrete. The warnings were investi - wood panels enclosing the tower crane have blocked the gated and confirmed with the recorded video. However, line of sight between the stationary sensor and the vibra- one warning of KOF #15, which is moving horizontally tor operator, thus reducing the accuracy of the US sen- while being submerged into concrete, turned out to be a sors. The error of the red points ranged from 0.85 m  to false alarm. This case occurred when the operator turned 1.72  m. These results demonstrate that securing the line off the power of the vibrator machine and let the head of sight of the US sensors is critical to collect reliable become submerged while waiting for the next pour from insertion data, and this can be achieved by separating the the bucket. Due to the long insertion duration, the solu- path of the vibrator operator from that of bulky equip- tion cumulated the small errors of the vibrator’s location, ment and materials, installing stationary sensors at a high and it created a false warning. The remaining violations position to minimize occlusions, and zoning the tracking were confirmed by the video. The authors have finally areas to reduce blind spots. confirmed with the manager on site that these violations had negligible impact on the quality, considering the L ee and Skibniewski AI in Civil Engineering 2022, 1(1):4 Page 11 of 19 Fig. 2 A concrete member in (a) the CAD coordinates and (b) the US coordinates slump value, the amount of reinforcement, and the loca- an occurrence of a defect. Sharing the results of such tion, frequency, and severity of the violations. analyses will reinforce the learning and feedback loop, leading to a solid and proactive defect prevention plan. 5 Discussion Based on the workmanship analysis, project manag- From the site experiment, it was observed that the ers can give clear and unequivocal instructions on the locations were generally accurate, except for the case desired workmanship. For example, a specific range where the line of sight between the US sensors was not of distances and durations of insertions can be rec- secured. The monitoring solution was able to capture ommended, with particular site conditions taken into the violations of the key operational factors reliably. account. In addition, the warning data can be used to Six warning instances were generated, and one of them, coordinate training sessions for new workers. Inform- which detected excessive horizontal movement of the ing new workers of the circumstances and locations vibrator head occurred due to the abnormal insertion where the poor workmanship is more likely to occur duration. The remaining warning instances were gener - will help them avoid the malpractices, without learning ated properly as the operators’ behavior exceeded the through experiencing defects. set thresholds of KOFs. To maximize the benefits of the monitoring and warn - This system promises multiple benefits both for pro - ing solution, this study highlights the importance of pro- ject managers and concrete workers. First, the moni- ject managers considering the following points before toring and warning solution proposed in this study they adopt this solution. First, the managers should helps achieve proactive and data-driven management clarify and share the purpose of adopting this solution of concrete work and enables real-time sensor-based with the stakeholders. Since this solution can work as a feedbacks, so as to prevent concrete quality problems data collection and monitoring tool, its application may caused by poor workmanship. The modules of the solu - prompt conflicts among the stakeholders. If it is used tion translate the raw positioning data into the infor- to clarify the responsibilities of the stakeholders con- mation of workmanship and then deliver warning cerning certain concrete defects in case of a lawsuit, its messages to the managers for prompt managerial deci- deployment may create a distrustful atmosphere within sions. With these warnings, both managers and con- the projects, therefore inhibiting its full adoption by the crete workers can take immediate actions to prevent stakeholders. For these reasons, it is suggested that this defects, rather than waiting until the formworks are solution be used as a tool to improve the internal quality stripped for inspection and repair. assurance and enhance collaborative monitoring for con- In addition, the solution can expedite the learning crete work, rather than for assigning blames to any of the process of project managers and concrete workers by stakeholders. enabling collection, analyses, and sharing of data. Since Second, it is recommended to prepare a long-term the collected data of concrete placement remain in a application plan, so as to maximize the benefits from the database, managers can perform in-depth data analysis automated monitoring of concrete work. One of the limi- to understand how a certain malpractice contributes to tations of the proposed solution in the beginning stage is Lee and Skibniewski AI in Civil Engineering 2022, 1(1):4 Page 12 of 19 Fig. 3 Procedure for processing and merging sensor data L ee and Skibniewski AI in Civil Engineering 2022, 1(1):4 Page 13 of 19 Fig. 4 Procedure for updating the concrete grid’s height with the US data of a pump hose Lee and Skibniewski AI in Civil Engineering 2022, 1(1):4 Page 14 of 19 conditions, concrete equipment, and properties of con- crete mixes, the same thresholds may or may not be able to indicate certain poor workmanship. A long-term use of this solution could allow project managers to assess whether the set values are appropriate for delivering the desired quality. This experience will ultimately enable the data-driven calibration of the threshold values. Lastly, creating a communication channel between the solution and concrete workers will help them respond promptly to warning messages. The solution designed in this study can deliver warning messages to project man- agers and concrete foremen for real-time assessment, so that they can require workers to take corrective actions promptly. Project-level decisions on how to respond to the warning messages are provided. For example, if too much concrete is poured at one location, the manag- ers may instruct the pump hose workers to reduce the pumping rate and ask the vibrator workers to insert the vibrator’s head deep enough, while share the warning information with an inspector for a thorough quality investigation on all the locations where poor workman- ship is defected. If adopted by a concrete subcontractor, this solution could be more efficient to send the warning information directly to the operators, without passing through the project managers or concrete foreman. 6 Conclusion Problems with concrete quality are commonly detected at construction sites, and they can lead construction pro- jects to delays and cost overruns. Thus, project managers have long tried to prevent such quality problems by mon- itoring workmanship. However, since visual monitoring of concrete work is time-consuming and labor-intensive, managers can only perform the monitoring tasks tempo- rarily and intermittently. Current monitoring methods rely much on the visual feedbacks from managers, a pro- cess which is error-prone and subjective. Consequently, poor workmanship may not be detected on the spot, and immediate and corrective actions are often not made during the concrete operations. The current method, which depends heavily on inspect - ing and repairing defects, is costly, may fail to observe defects, and may be too late to initiate reparative actions when defects are detected. Moreover, due to the absence of instant feedback, inexperienced concrete workers Fig. 5 Procedure for updating vibration status of concrete internal may go through frequent trials and errors in learning points to improve their workmanship. The workers may forget the details of their movements when they are able to see the result of their work, which is after the fresh concrete the lack of knowledge on determining threshold values. has hardened and the formwork is removed. Obser- Project managers may not know the best performing vations and experiences of managers about concrete thresholds and may simply follow the values suggested work are often forgotten and distorted, and therefore, in Table  3. However, since projects have different site L ee and Skibniewski AI in Civil Engineering 2022, 1(1):4 Page 15 of 19 Table 4 Conditions for violation of KOFs Key operational factor (KOF) Conditions for violation 1 Uniform placement The height difference of concrete between two adjacent placement grids is greater than l m layer 2 Continuous place- A pump hose does not come back to the grid within t seconds after it leaves ment 3 Avoiding excessive The height of the discharging point of a pump hose from the surface of fresh concrete exceeds l m for consecutive t seconds free-fall height 4 Vertical drop of The angle of the sensor installed at a pump hose exceeds n degrees from the vertical axis for consecutive t seconds concrete 5 Avoiding direct The angle of the sensor installed at a pump hose exceeds n degrees from the vertical axis, and the line that connects two sensors intersects with a vertical form within l m from the main placement on vertical sensor for consecutive t seconds forms 6 Avoiding direct place- The line that connects two sensors intersects with a congested area within l m from the main sensor for consecutive t seconds ment on reinforcing bars 7 Appropriate vibration The vibrator fails to stay in l m from a cube point for consecutive t The vibrator head is submerged into concrete for less than t seconds or more than t seconds 1 1 2 duration seconds within t seconds after concrete is poured 8 Appropriate insertion The insertion depth (Z value of the vibrator head) is at least l m higher than the average depth of the recent depth insertions 9 Appropriate distance The horizontal distance between two consecutive insertions is at least l m lower or higher than the average between insertions horizontal distance of the recent insertions 10 Inserting vibrators N/A close to forms 11 Continuity between placement and vibration 12 Avoid touching forms The vibrator head stays within l m from a form face for consecutive t seconds with a vibrator 13 Appropriate penetra- For the insertions with t seconds or longer, the insertion speed is lower than v m/s, or the withdrawal speed is higher than v m/s 1 2 tion and withdrawal speeds of vibrators 14 Appropriate penetra- The angle of the vibrator head exceeds n degrees from the vertical axis for consecutive t seconds while it is submerged in concrete tion and withdrawal angles of vibrators 15 Avoiding dragging a For the insertions with t seconds or longer, the cumulative distance of a vibrator’s horizontal movement exceeds l m while the vibrator is inserted vibrator Lee and Skibniewski AI in Civil Engineering 2022, 1(1):4 Page 16 of 19 Fig. 6 Warning images for KOFs #1 and #5 Fig. 7 Installed ultrasound and electromagnetic sensors Fig. 8 (a) Observed and actual vibrator insertion locations and (b) occlusions at the site L ee and Skibniewski AI in Civil Engineering 2022, 1(1):4 Page 17 of 19 Table 5 Warning instances during the application, results of data validation, and their impacts on the quality KOF # Warning statistics Description Results of investigating the data and Impact on the quality video 1 - Number of warning instances: 1 There was one placement grid where the A large pour is suspected to be triggered Considering the concrete’s slump value - Number of placement grids: 2 amount of placed concrete was larger than by the placement method using a concrete (15.2 cm), the water cement ratio (0.4), and - Height difference: 0.33 m the nearby grids bucket the simple shape of the slab, the violation - Duration: 22.5 s is expected to have a minor impact on the concrete quality 7, 8, 9, 10 - Number of warning instances: 1 The warning module found that two con- Two nearby concrete points did not have a These points were in the middle of the slab, - Number of undervibrated internal points: 2 crete points among two hundred ones were vibrator submerged within 1.3 m for 10 min where the concrete can flow and fill from vari- - Location: center grids suspected to be undervibrated after the placement ous directions, so the warning was not critical to the quality of concrete 13 - Number of warning instances: 1 One quick withdrawal of the vibrator was The observed withdrawing speed was Since the quick withdrawal of the vibrator did - Withdrawal speed: 0.66 m/s detected 0.66 m/s, which is minimally faster than the not occur frequently, this warning is expected threshold of 0.5 m/s to be non-critical to the quality 14 -Number of warning instances: 2 There were two violations observed with The head angle was between 35°–40° from This case is expected to be non-critical for - Average degrees tilted: 39° a tilt head while it was inserted into the the vertical line, lasting for 2 s the quality, because the tilt of the vibrator concrete occur neither for a long time nor in a frequent manner 15 - Number of warning instances: 1 There was one warning of excessive horizon- From investigating the video, the researchers This was a false positive, and it does not - Duration: 9.6 s tal movement of the vibrator determined that this was a false alarm, which reduce the quality of the concrete - Cumulative horizontal movement: 1.0 m occurred due to the cumulated positioning errors during the prolonged insertion Lee and Skibniewski AI in Civil Engineering 2022, 1(1):4 Page 18 of 19 Received: 13 May 2021 Accepted: 18 March 2022 opportunities to share knowledge and analyze workman- Published: 18 August 2022 ship may be lost. 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Automation in Construction, 123, 103510. https:// doi. org/ 10. 1016/j. autcon. 2020. 103510 Publisher’s Note Springer Nature remains neutral with regard to jurisdictional claims in pub- lished maps and institutional affiliations. http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png AI in Civil Engineering Springer Journals

Automated monitoring and warning solution for concrete placement and vibration workmanship quality issues

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Abstract

Placing and vibrating concrete are vital activities that affect its quality. The current monitoring method relies on visual and time-consuming feedbacks by project managers, which can be subjective. With this method, poor workmanship cannot be detected well on the spot; rather, the concrete is inspected and repaired after it becomes hardened. To address the problems of retroactive quality control measures and to achieve real-time quality assurance of concrete operations, this paper presents a monitoring and warning solution for concrete placement and vibration workman- ship quality. Specifically, the solution allows for collecting and compiling real-time sensor data related to the work - manship quality and can send alerts to project managers when related parameters are out of the required ranges. This study consists of four steps: (1) identifying key operational factors (KOFs) which determine acceptable workmanship of concrete work; (2) reviewing and selecting an appropriate positioning technology for collecting the data of KOFs; (3) designing and programming modules for a solution that can interpret the positioning data and send alerts to project managers when poor workmanship is suspected; and (4) testing the solution at a certain construction site for validation by comparing the positioning and warning data with a video record. The test results show that the monitoring performance of concrete placement is accurate and reliable. Follow-up studies will focus on developing a communication channel between the proposed solution and concrete workers, so that feedbacks can be directly delivered to them. Keywords: Real-time monitoring, Concrete quality, Placement and vibration, Workmanship 1 Introduction Mills et  al., 2009). For these reasons, project managers Concrete placement and vibration are important in have made considerable efforts to reduce the defects by achieving desired construction quality measures, because ensuring proper performance of concrete placement and inappropriate placement and vibration of concrete can vibration. significantly decrease concrete quality by causing con - However, many projects still face problems with con- crete defects, such as honeycombs and air voids. These crete quality, because faulty workmanship cannot be eas- defects can result in delays, cost overruns, and disputes ily detected on the spot and immediate corrective actions among project stakeholders, as well as frequent mainte- are rarely made during the concrete construction opera- nance, elevated safety concerns and shortened service tions (Lee & Skibniewski, 2019). The current method by life of facilities (Aljassmi & Han, 2012; Barber et al., 2000; human supervision and inspection is not a practical solu- tion for continuous monitoring of compliance, because (1) managers cannot perform all-time monitoring by *Correspondence: slee239@terpmail.umd.edu always hovering around the concrete workers; (2) their Department of Civil and Environmental Engineering, University of Maryland, sights of view on construction workers and equipment 1188 Glenn Martin Hall, College Park, MD 20742, USA are often disturbed and occluded by objects; and (3) Full list of author information is available at the end of the article © The Author(s) 2022. Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http:// creat iveco mmons. org/ licen ses/ by/4. 0/. Lee and Skibniewski AI in Civil Engineering 2022, 1(1):4 Page 2 of 19 they have different levels of knowledge and field expe - knowledge, experience, and relevant rules and guide- riences, possibly leading to different interpretations of lines. If managers detect improper workmanship, they observed workmanship. Due to these limitations of the will typically ask for corrections from the foremen and/ visual monitoring method, concrete defects are typically or workers. inspected and repaired after the concrete is hardened. However, the vision-based monitoring of concrete Although many of such defects can be repaired, this reac- work entails several problems that affect its reliability. tive method is inefficient because inspection and repair First, the workmanship is often monitored temporarily would incur additional time and cost (e.g., related costs of and intermittently. In fact, visual monitoring is too time- material and equipment use, inspectors’ time for exam- consuming and labor-intensive to be performed continu- ining, evaluating, and reporting the defects). Moreover, ously, because it requires the project managers’ entire slipshod inspection and repair may lead to failures in focus on the concrete work. The managers need to stay at detecting defects or unsatisfactory quality of the repaired the placement sites all the time, while keeping their eyes concrete. These reasons explain why proactive and pre - on the workers and equipment. In fact, managers are also ventive quality control measures should be achieved. responsible for taking care of cost, schedules, and safety, This study introduces a monitoring and warning in addition to the quality of concurrent construction solution of concrete workmanship inspection to help activities performed by various crew and equipment. For managers ensure the quality of concrete placement this reason, nonconformity of rules and guides may not and vibration operations, thus to  prevent the concrete be detected when managers are tied up with other tasks. defects in a proactive manner. Since various studies have Moreover, visual monitoring and inspection is error- explored and developed many tracking technologies for prone and subjective (Kim et  al., 2015; Lundkvist et  al., concrete vibrators, this study will focus on achieving 2014; Park et al., 2013). Human visual feedback can be erro- monitoring and management support by utilizing the neous when their views are unclear or occluded by obsta- data collected from the tracking technologies. cles. Cast-in concrete is often poured into a formwork that is tightly sealed to prevent leakage, and it is hard to recog- 2 Literature review nize objects inside the formwork. Moreover, concrete is 2.1 C urrent monitoring and supervision of workmanship placed and vibrated by a crew consisting of workers who The quality of concrete is susceptible to the operators’ play different roles (e.g., adjusting the direction of the pump workmanship as well as the degree of skills in which the hose, handling concrete finishing tools, or operating con - construction activities are performed. Even concrete crete vibrators). In these dynamic circumstances, manag- made of good materials in a properly proportioned and ers’ view is frequently blocked, and it is challenging to track batched manner can be defective if performed with poor and monitor all workers simultaneously. Furthermore, workmanship (ACI Committee 311, 2007). When freshly since the information acquired by the visual monitor- mixed concrete is deposited, it will normally not be able ing methods may be inaccurate, supervisors often need to to completely fill the spaces in the formworks and around rely on their own subjective interpretation and evaluation, embedded items (e.g., steel reinforcements) with its own which may vary depending on the levels of their knowledge weight. Rather, it may become “honeycombed”; that is, and field experiences (Gong et al., 2015; Tattersall, 1991). it may contain entrapped air at a level of approaching Last, the process of learning and improving workman- 20% of its volume. Improper placement and vibration ship is an inefficient and long-time one. Inexperienced may account for non-uniform density and excessive air workers without the requisite expertise and know-how voids. Such defects would weaken the structural integrity often have to go through several trials and errors by per- and mechanical properties of concrete members, such forming the work and evaluating the resulting quality. as compressive strength and bond stress with reinforce- The next visit to a certain site by a concrete worker can ment (ACI Committee 309, 2017; Cement Concrete & be days and weeks later at which they may not remem- Aggregates Australia, 2006). Additionally, such defects ber the behaviors during the placement. This time gap may cause long-term durability problems by allowing reduces the learning rate of workers, meaning that more water seepage into the concrete members and expos- practice is needed to improve their workmanship. The ing the reinforcements to corrosion (Roy et  al., 1999; problem of low-learning rates also applies to project Siegfried, 1992). By monitoring and supervising how managers: the knowledge of concrete quality and work- concrete is placed and vibrated, project managers can manship only remains in their memories and is often assure the quality and minimize defects. Generally, they forgotten and distorted. Thus, not having tangible work - walk around the site on a regular basis to visually inspect manship data becomes an obstacle to extracting and and supervise the concrete operations. They assess sharing knowledge with other project managers and the observed behaviors of the workers based on their workers for effective improvement. L ee and Skibniewski AI in Civil Engineering 2022, 1(1):4 Page 3 of 19 Due to the problems of monitoring workmanship as in the experiment. Wang et  al. (2021) developed a com- described above, faulty site workmanship is common, puter-vision-based vibration quality monitoring method. and concrete defects in construction are frequently found The system, which used an infrared sensor to measure commonly in buildings and infrastructures (Ahzahar the distance between the sensor and the concrete sur- et  al., 2011; Lin & Fan, 2018; Love et  al., 2017; Neeley, face and a camera to capture concrete surface images. 1993). The resulting quality problems are managed by a The system was applied to a heavy vibrator machine diagnostic and corrective approach, which is to inspect which is equipped with a head of multiple vibrating bars. and repair the defects (Love, 2002; Lundkvist et al., 2014). Although the system could monitor undervibration and Although inspection and repair are important for meet- overvibration from the concrete surface, the vibrator’s ing the desired quality level of the final products, they are location was not recorded. Tian et  al. (2019) developed inherently reactive, as they merely correct non-conform- a real-time visual monitoring system for vibration effects ance in an "after-the-fact" manner. Repairing defects can by using the Global Navigation Satellite Systems (GNSS), be technically difficult and costly, requiring more efforts including GPS, Beidou, and GLONASS. This monitoring than taking preventative measures, because the concrete system could track the vibrator tip’s location based on the has already hardened and gained strength (ACI Com- location of the mobile antenna on the worker’s wrists. mittee 309, 2005; Kennedy, 2005; Rwelamila & Wiseman, They assumed that the locations of the tip were alone on 1995). Performing inspection and repair cannot guaran- the linear line between the two hands. The system had a tee good quality if the defects are not detected during warning function to instruct the construction workers data entry or when newly applied material is not bonded to act against observed undervibration. Liu et  al. (2015) with the existing concrete (ACI Committee 309, 2005; developed a quality monitoring system for storehouse Kim et al., 2015; Lundkvist et al., 2014; Park et al., 2013). surfaces of roller compacted concrete (RCC) dams. The For these reasons, concrete defects should be minimized system utilized GLONASS and GPS to acquire the posi- and prevented by following proper placing and consoli- tioning of roller compactors. It monitored rolling speed dating procedures (Neeley, 1993). and the number of passes of compaction and sent warn- ing messages to supervision and management personnel. 2.2 P revious studies on monitoring on‑site concrete Various technologies have been used to track vibration workmanship effort, and many of them adopt GNSS which requires a In this section, the existing research on advanced moni- clear view of the sky. The major focus of these studies was toring of concrete work is reviewed (Table  1). Studies on the technological development and visualization by that did not particularly focus on concrete work (e.g., testing and improving the applicability of the technolo- excavators and general worker location tracking) are not gies to concrete work. Some studies modified the design included. Lee and Skibniewski (2019) introduced a con- of internal vibrators to improving the tracking perfor- ceptual framework of a concrete placement and vibration mance, while others used different vibrating machines to work monitoring system. They proposed to utilize ultra - install and implement the tracking sensors. Regarding the sound (US) positioning and computer vision technolo- decision support features, Lee and Skibniewski (2019) gies, to track locations of objects and perform monitoring identified the important factors of concrete placement based on the key operational factors that can represent and vibration work, but failed to develop the proposed the critical workmanship related to concrete quality. concept of the monitoring system. Many of the existing Gong et al. (2015) attached a ultra-wideband positioning studies either focused only on undervibration of concrete sensor to a vibrator head, in order to acquire its position- or did not have a warning system that supports real-time ing data. They tested the positioning accuracy from sim - decision making. ulated work and visualized the locations of the vibrator Moreover, they monitored the output of the product- head. However, since the purpose of their research was to focused concrete work processes (e.g., concrete surface apply and test the tracking technology, there was no con- after vibration), rather than workers’ behaviors during sideration on how to support managerial decisions with concrete placement. For example, the existing monitor- the collected data. Tian and Bian (2014) tracked vibra- ing systems can display undervibrated areas of concrete tors’ locations by using the Global Positioning System and send an alert about these areas, but they did not (GPS) and the Global Navigation Satellite System (GLO- monitor the movement of the workers (e.g., short inser- NASS). In the study, they fixed a carbon fiber bar that tion duration, or insufficient insertion depth) which has a mobile antenna with the vibrator and estimated the triggered these warnings. Monitoring how the work is vibrator head’s location from the antenna’s location and performed would foster workers’ awareness of their rela- the length of the fiber bar. They also displayed the vibra - tive compliance to a set of guidelines and clarify certain tor status (insufficient, normal, and excessive vibration) behaviors that cause concrete quality issues and expedite Lee and Skibniewski AI in Civil Engineering 2022, 1(1):4 Page 4 of 19 the learning process by changing their workmanship this time gap, the violation of the KOF can be monitored immediately. and determined. Lastly, the studies of monitoring concrete placement In the development step, the major tasks of the solution are scarce. The existing studies were generally focused on were identified and classified into four groups, and the tracking concrete vibrators but lacked efforts in tracking modules dedicated to performing each task group were concrete-pouring activities. Proper placement of con- developed. These modules were used to extract design crete is vital, as excessive free fall of concrete and pour- information, collect and merge sensor data, update sta- ing concrete directly on vertical forms or reinforcing bars tus of placement and vibration, and perform real-time may cause segregation. Furthermore, the vibration status warning. The performance of the solution was tested and of concrete can be understood only when it is tracked evaluated at a construction site. The reliability of the col - with the poured concrete, because without knowing the lected data was assessed by comparing them with a video locations where the concrete is poured, it is impossible to recorded during the work. If data were not collected for a determine whether the concrete is vibrated or not. moment or the accuracy was too low, the possible causes The research question addressed in this study focuses and remedies were discussed. In addition, the sensor on (1) achieving real-time quality assurance for concrete data and the video record were used to analyze the warn- placement and vibration operations, and (2) overcoming ing instances and their impacts on the concrete quality. the limitations of the existing monitoring methods, such The resulting concrete quality was visually inspected to as the lack of monitoring and warning features for work- decide whether there was any quality problem not pre- ers’ behaviors and the issues with the placement work- vented by the warning module. manship quality. 3.1 Determining key operational factors (KOFs) 3 Methodology Fifteen KOFs are summarized in this study, and they are To help managers assure the quality of concrete work- what managers shall focus on to make timely feedback manship in real time, a monitoring and warning solution to the workers. The first six factors are about concrete is designed and developed in this study. Specifically, the placement, while the remaining nine are about vibration. solution will perform data collection on concrete place- Table  2 shows the KOFs and the consequences of viola- ment and vibration with ultrasound (US) and electro- tion. After determining the KOFs, their parameters and magnetic (EM) sensors attached to concrete equipment thresholds for measuring and determining the compli- (e.g., vibrators or pump hoses), continuously compare ance or violation were investigated (Table 3). the observed work-related parameters with the threshold values, and alerts managers when the workmanship does 3.2 Development of the solution and its modules not comply with the recommended practices. The US 3.2.1 Mo dule 1: Extract the design information positioning technology was selected based on a compari- Module 1 extracts the geometric information of concrete son by Lee and Skibniewski (2019), and the EM position- members and forms. From the corner points of a con- ing technology was chosen because its accuracy is not crete member that a user has entered, the module cre- affected by occluded line-of-sight. ates the concrete points and placement grids used for Fig.  1 shows the procedures for developing the con- tracking the status of concrete placing and vibrating. crete work monitoring solution. First, both appropriate Extracting design data is initiated by calculating a coor- and inappropriate practices of concrete construction are dinate conversion matrix, which will convert the coordi- determined. This study focuses on the common tech - nates of concrete members and forms in CAD drawings niques of concrete placement and vibration, which use into the US sensor coordinates. A coordinate conver- concrete pump hoses and internal vibrators (also as sion matrix (4 × 3) is calculated by using the following known as spud or poker vibrators). Manuals, reports Eq. (1) with a set of CAD points and their corresponding and guidebooks about concrete work and inspection are US points. By multiplying with the conversion matrix, reviewed to determine the KOFs. Then, these KOFs are the CAD points are converted and expressed in the US used to figure out their parameters for measuring and coordinate system. There should be at least three corre - determining the compliance or violation of the KOFs. For sponding non-collinear points in order to calculate the example, the parameter of the KOF “Compaction must conversion matrix. be done while concrete is still plastic” can be the time gap between placing and vibrating concrete. By measuring L ee and Skibniewski AI in Civil Engineering 2022, 1(1):4 Page 5 of 19     P P cad us a vibrator shaft. Figure  3 shows the procedures for pro- � �� � � �� � cessing and merging collected data from the sensors.     a a a �1 b b b 1x 1y 1z 1x 1y 1z         There are four groups of processes: (1) data collection a a a �1 b b b 2x 2y 2z 2x 2y 2z         and retrieval; (2) US data processing for a pump hose; a a a 1 b b b 3x 3y 3z � 3x 3y 3z   × C =  , (1)  . . . .   . . .  (3) US and EM data processing for a vibrator operator; . . . . . . .     . . . . . . .     and (4) integration of US and EM coordinates. The solu -     a a a 1 b b b nx ny nz nx ny nz tion first checks whether the data are available before initiating data processing. If it fails to retrieve any of the data required, it will move on to the next loop, trying to where P indicates a matrix of CAD points (n × 3), C cad retrieve the data again. indicates a coordinate conversion matrix (4 × 3), P indi- us After that, the data for the pump hose and the vibra- cates a matrix of US points (n × 3), and n indicates the tor are merged, respectively. When the data are merged, number of points used for the conversion. a linear interpolation method is applied to match their For example, if the four points of a CAD drawing are timestamps. The US and EM sensors have different coor - (2.4257, − 3.8481, 2.0000), (− 0.5743, − 3.8481, 2.0000), dinate systems. Therefore, the coordinates are aligned (− 0.5713, 1.2446, 2.0000) and (2.4257, 1.2446, 2.0000), before estimating the location of the vibrator head. and their corresponding points are (3.0000, 0.0000, Whereas the reference point of the US sensor coordi- 0.0000), (0.0000, 0.0000, 0.0000), (0.0030, 5.0920, 0.0000) nate system is fixed during data collection, the EM sen - and (3.0000, 5.0930, 0.0000) in the US coordinate system, sors’ reference point is based on the EM source. Thus, the then its conversion matrix is calculated in Eq. (2). cad us 2.4257 −3.8481 2.0000 1 3.0000 0.0000 0.0000 −0.5743 −3.8481 2.0000 1 0.0000 0.0000 0.0000 (2) × C = , −0.5713 1.2446 2.0000 1 0.0030 5.0920 0.0000 2.4257 1.2446 2.0000 1 3.0000 5.0930 0.0000   1.0000 0.0000 0.0000 points in the EM sensor system will change depending  0.0000 0.9999 0.0000  on the location and the orientation of the source. For this C = .   (3) 0.0000 0.0000 1.0000 reason, the degrees of tilt and orientation of the source 0.5743 3.8476 −2.0000 are measured by attaching the US sensors with inertial measurement units to the EM source, and the EM points After converting the coordinate systems of the points, are converted to the US coordinate system. the forms’ plane parameters are calculated, and the placement grids and internal points of concrete mem- 3.2.3 Module 3: Update the status of placement bers are created. The form’s plane parameters represent and vibration the shapes of the forms in three dimensions, and they The height of concrete in each grid is estimated based on are used to compute the distance between a point and the pumping rate and the duration that the pump hose the form as well as an intersection between a line and the stays toward the grid surface. This simplified approach form. The placement grids are created by dividing an area is significantly faster and less computationally expensive of a concrete member along with x and y axes, and simi- than scanning the actual shapes of deposited concrete. larly, the internal points are created by distributing points From the location information of the pump hose, the along with x, y, and z axes. The placement grids enable change of concrete heights over time can be tracked. The tracking of the height of fresh concrete deposited in the procedures for updating the heights of concrete based forms, while the internal points are used to monitor the on the location of the US sensors mounted on the pump vibration status at all critical points of the concrete mem- hose are presented in Fig. 4. bers. Figure 2 shows an example of a concrete member in First, the positioning data of the two US sensors the CAD coordinates and the US coordinates. mounted on the pump hose are retrieved. One sensor is attached to the upper part of a pump hose, so that it 3.2.2 Module 2: Collect and merge sensor data can collect the location data reliably after the hose tip is The solution collects positioning data from two US sen - immersed in the forms. Another sensor is attached to the sors for tracking a pump hose, two US sensors for track- bottom of the pump hose. If the pump hose is higher than ing a vibrator operator and three EM sensors for tracking Lee and Skibniewski AI in Civil Engineering 2022, 1(1):4 Page 6 of 19 Fig. 1 Procedure for developing the concrete work monitoring solution L ee and Skibniewski AI in Civil Engineering 2022, 1(1):4 Page 7 of 19 Table 1 Studies about monitoring on-site concrete workmanship Author Technology Purpose of collecting data Supported managerial decisions Sensor attachment location Lee and Skibniewski (2019) Ultrasound positioning sensors and Tracking a pump hose and a vibrator’s Authors determined key workmanship Ultrasound sensors at a pump hose, and computer vision location factors for monitoring cameras at the known locations of a site Gong et al., (2015) Ultra-wideband positioning sensors Visualization of a vibrator’s location No The vibrator’s head Tian and Bian (2014) Global Positioning System (GPS) and Tracking a vibrator’s location and visual- No At the tip of a carbon fiber bar attached to Global Navigation Satellite System izing the vibration effect the vibrator (GLONASS) Wang et al., (2021) Computer vision Monitoring concrete surface’s quality Undervibration and overvibration were The heavy vibrator machine detected based on the vibration depth and duration Tian et al., (2019) Global Navigation Satellite System Visualization of vibration effect, and Undervibrated area and its volume were The vibrator operator’s wrists (GNSS) delivery of warnings to construction detected based on the moving trajec- workers tory of a vibrator Liu et al., (2015) GLONASS and GPS Tracking the location of a roller compac- Warning messages about the rolling A receiver is installed on a roller compac- tor speed and the passes of a roller com- tor pactor are sent Lee and Skibniewski AI in Civil Engineering 2022, 1(1):4 Page 8 of 19 Table 2 Key operational factors and the consequences of violation Key operational factors Consequences of violation 1 Uniform placement layer Honeycombs, air voids, form offset, and sand streaking The crews should only deposit as much concrete as can be consolidated efficiently (ACI Committee 309, 2017; ACI Committee 311, 2007; Kosmatka et al., 2011) 2 Continuous placement Cold joints and pour lines Placement should continue before the initial set of the previous lift ( ACI Com- mittee 304, 2002; ACI Committee 309, 2017; ACI Committee 311, 2007; Cement Concrete & Aggregates Australia, 2006; Kosmatka et al., 2011) 3 Avoiding excessive free fall height Segregation Placing concrete should be from an appropriate height (ACI Committee 309, 2017; U.S. Department of Agriculture Natural Resources Conservation Service, 2008) 4 Vertical drop of concrete Segregation Concrete should drop vertically (ACI Committee 304, 2002; ACI Committee 311, 2007) 5 Avoiding direct placement on vertical forms Segregation Concrete should not be placed directly on vertical forms (ACI Committee 311, 2007) 6 Avoiding direct placement on reinforcing bars Honeycombs, sand streaking, and segregation. Reduced bonds between reinforce- Concrete should not be placed directly on reinforcing bars (ACI Committee 304, ment surfaces and mortar may be formed due to premature coating 2002; ACI Committee 311, 2007) 7 Appropriate vibration duration Undervibration: honeycombs, excessive entrapped air voids, blisters, subsidence Concrete should be vibrated for an appropriate duration (ACI Committee 309, cracking, pour lines, and cold joints 2017; Neeley, 1993; U.S. Department of Agriculture Natural Resources Conserva- tion Service, 2008) 8 Appropriate insertion depth Overvibration: segregation, loss of entrained air, form offset, and sand streaking Vibration sticks should be inserted into the sufficient depth (ACI Committee 309, 2017; ACI Committee 311, 2007; Kosmatka et al., 2011; Neeley, 1993; U.S. Depart- ment of Agriculture Natural Resources Conservation Service, 2008) 9 Appropriate distance between insertions Distance between insertions of the vibrator head should be appropriate (ACI Committee 309, 2017; ACI Committee 311, 2007; Cement Concrete & Aggregates Australia, 2018; Kosmatka et al., 2011; Neeley, 1993; U.S. Department of Agriculture Natural Resources Conservation Service, 2008 p. 6) 10 Inserting vibrators close to forms Vibrators should be kept close to the form face (ACI Committee 309, 2017; Neeley, 1993) 11 Continuity between placement and vibration Honeycombs and cold joints Compaction must be done while concrete is still plastic (ACI Committee 311, 2007; Cement Concrete & Aggregates Australia, 2018; Kosmatka et al., 2011) 12 Avoid touching forms with vibrators Form offset, form damage and deflection, and burn marks on the finished surface Vibrators should not be allowed to touch the forms (ACI Committee 309, 2017; Cement Concrete & Aggregates Australia, 2006, 2018; Neeley, 1993) 13 Appropriate penetration and withdrawal speeds of vibrators Air voids and blisters Vibrators should quickly penetrate the concrete layer and be removed slowly to remove the entrapped air (ACI Committee304, 2002; ACI Committee 309, 2017; ACI Committee 311, 2007; Cement Concrete & Aggregates Australia, 2006, 2018; U.S. Department of Agriculture Natural Resources Conservation Service, 2008) 14 Appropriate penetration and withdrawal angles of vibrators Pour lines and cold joints Vibrators should be inserted and withdrawn vertically (ACI Committee 304, 2002; Unsystematic insertions at haphazard angles may cause undervibration of the ACI Committee 311, 2007; Cement Concrete & Aggregates Australia, 2006; U.S. lower layer, while accumulating mortar at the top surface Department of Agriculture Natural Resources Conservation Service, 2008) 15 Avoiding dragging vibrators Sand streaking and segregation Dragging vibrators through the concrete should be prohibited ( ACI Committee 304, 2002; ACI Committee 311, 2007; Cement Concrete & Aggregates Australia, 2006; Kosmatka et al., 2011; U.S. Department of Agriculture Natural Resources Conservation Service, 2008, p. 6) a certain point at which both sensors secure the line-of- is located at a point vertically down from the top sensor. sight with the stationary sensors, the location data of both The horizontal distance between the top and the bottom sensors are used to estimate the deposited point. When sensors can be generally ignorable when a pump hose is the hose is down and may not secure the line of sight of lowered deeply to pour deep concrete members, such the bottom sensor, it is assumed that the bottom sensor as columns and girders, because reinforcing steel would L ee and Skibniewski AI in Civil Engineering 2022, 1(1):4 Page 9 of 19 restrict the pump hose’s horizontal movement. After cal- not within the set range. When a violation is detected, culating the deposit point, the height of concrete in each its details are delivered to the managers, with the warn- grid is updated based on the pumping rate and the dura- ing information displayed on the screen of a local com- tion that the pump hose stays in the grid. The internal puter and uploaded to a cloud server. points that are lower than the current concrete height are The data of the malpractice are presented in readable marked as “placed”, and the time of placement is saved. formats, such as tables and figures, facilitating the man - Figure  5 shows the procedures for updating the vibra- agers to understand the details at a glance. The warning tion status of the concrete internal points. The vibrator module compresses the sensor data together with the head’s location is first estimated based on the locations violation time points and saves them in an Excel data- of the EM sensors mounted on the vibrator shaft. This sheet. This sheet allows the managers to revisit what hap - solution fits the locations of the three EM sensors with pened around the time of violations. It also creates and a linear line in a top view and with a quadratic curve in a stores.jpg images to help managers interpret the coor- side view. Then, the actual distances between the vibra - dinates and pinpoint the locations. Figure  6 shows an tor head and the sensors are used to acquire the head’s example of KOFs #1 and #5. The images show the infor - location. With the location of the vibrator head and the mation about the violations, such as their time points and height of concrete, the solution updates the vibration sta- locations. The left figure below highlights the placement tus of the concrete members’ internal points. When the grids that exceed the acceptable height difference, while height of the vibrator’s head is lower than the height of the right figure shows the locations where concrete is concrete, the insertion duration begins to be counted. placed directly on the vertical forms. After a certain period of duration, the nearby points of the vibrator’s head are marked as “vibrated”, and the time 4 Site application of vibration is saved. Then, all the updated data of the A site application was conducted to confirm whether placement grid and internal points are passed onto the malpractices were caught as intended and to investigate warning module in order to determine whether there is whether there was any false alarm. The project is located any violation. at the University of Maryland, and its scope includes designing and constructing a 4-story building that has labs, conference rooms, and student lounges. The appli - 3.2.4 Module 4: Real‑time warning cation area was 7.3 m by 3.8 m with a thickness of 0.46 m. The warning module compares the acquired data with the The project used a concrete bucket instead of a pump predetermined threshold values, and it will send warn- hose for its placement method; and the concrete had ing messages to the managers if a violation is detected. a slump of 15.2  cm with the characteristic strength of The conditions for violation of KOFs are determined 24.13  MPa. Figure  7 shows the installed sensors on the as shown in Table  4. For example, KOF #12, “vibrators concrete bucket and the vibrator shaft. Given that work- should not be allowed to touch the forms”, can be con- ers would be more careful with their behaviors when they sidered violated when the head of the vibrator bar stays were aware of the experiment, the threshold values were within 0.l m of a form face for t consecutive seconds. This intentionally set tighter when acquiring the warning data. factor needs threshold values for its two variables (l and Prior to the concrete work, the researchers visited the t) to be determined. The threshold values can be refer - site to attach four stationary US sensors to the columns enced and calibrated from Table 3. where the sight is secured and four mobile US sensors The KOF #7 through #11 are monitored while mak - to two concrete buckets, while acquiring the CAD draw- ing sure that the vibrator head stays near the concrete ings of the application area and calculating the conver- points for a desired period of time. Then, for exam - sion matrix. 8 placement grids and 191 internal concrete ple, the solution may inform the managers of undervi- points were generated. The areas of placement grids brated internal points, which encourages an immediate ranged from 1.291 to 3.386  m after subtracting the areas remedial action such as re-vibrating the undervibrated of columns in the girds. The internal points had equal points. In this way, the monitoring is focused on “prod- distances of 0.389 m, 0.381 m, and 0.25 m along with x, y, ucts” to help achieve quality assurance for all inter- and z axes, respectively. The placing and vibrating opera - nal points of concrete. It is also important to monitor tion was performed for approximately thirty minutes. and manage the “behavior” of the vibrator operator. During the work, one researcher stayed near the vibra- The vibrator operator needs to know about the poor tor, taking a laptop to receive the sensor data remotely. workmanship, in order to learn about the faulty behav- At the same time, another researcher recorded a video ior and prevent its recurrence. In a word, the warning of the work to confirm the quality of the sensor data and module constantly checks the distance, duration, and the warning instances. The timestamps of opening and depth of insertions before alerting the users if they are Lee and Skibniewski AI in Civil Engineering 2022, 1(1):4 Page 10 of 19 Table 3 Parameters and recommended values of the key operational factors for general concrete work Parameters Values (in SI units) Sources 1 The layer depth of concrete - Not exceed 508 mm ACI Committee 309 (2005), ACI Committee 311 (2007) Kosmatka et al., (2011) 2 Time gap of placement between layers - 1.5 h (between mixing and compacting) Kosmatka et al., (2011) 3 Height of the discharging point at a pump - Not exceed 1.2 m Minnesota Department of Transportation (2003) hose 4 Angle of the discharging point at a pump hose - 90° (vertical) ACI Committee 304 (2002), ACI Committee 311 (2007) 5 Duration that concrete is placed on vertical - 0 s (not allowed) ACI Committee 311 (2007) forms 6 Duration that concrete is placed on reinforc- - 0 s (not allowed) ACI Committee 304 (2002), ACI Committee 311 ing bars (2007) 7 Duration that a vibrator head stays when - About 5 to 15 s ACI Committee 304 (2002), ACI Committee 311 inserted into concrete (2007), Cement Concrete & Aggregates Australia (2018), Kosmatka et al., (2011) 8 Depth of a vibrator’s insertion into concrete - At least 152 mm into the previously placed ACI Committee 309 (2005), Kosmatka et al., layer (2011) 9 Distance between insertions - 1.5 times the radius of action. (see Table 5.1.5 ACI Committee 309 (2005), Kosmatka et al., of 309R-96 Guide for Consolidation of Concrete (2011), U.S. Department of Agriculture Natural (ACI Committee 309, 2005)) Resources Conservation Service (2008) 10 Distance between forms (where concrete is - About 50 mm clear of the form face ACI Committee 309 (2005), Cement Concrete & placed) and their closest insertion - About 100 mm of the form Aggregates Australia (2006) 11 Time gap between placement and vibration - 1.5 h (between mixing and compacting) Kosmatka et al., (2011) 12 Duration that a vibrator touches a form - 0 s (Not allowed) ACI Committee 309 (2017), Cement Concrete & Aggregates Australia (2006, 2018), Neeley (1993) 13 Vibrator’s insertion/withdrawal speeds - At a rate of 30 to 61 mm/s (withdrawal speed) U.S. Department of Agriculture Natural Resources Conservation Service (2008) 14 Angle of the vibrator when inserted/with- - 90° (vertical) ACI Committee 309 (2005), ACI Committee 311 drawn (2007), U.S. Department of Agriculture Natural Resources Conservation Service (2008) 15 Distance of a vibrator’s horizontal movement - 0 m (not allowed) ACI Committee 311 (2007) while inserted closing the bucket gate were recorded to estimate the Table  5 summarizes the descriptions of the warning concrete pouring speed and location. data generated during the site application. The warning Figure 8 shows the compared locations of insertions of module detected the following: a large concrete pour the collected sensor data and video records. About 95% causing a height difference between nearby placement of the insertions (72 out of 76) were accurately located grids; two undervibrated concrete internal points; one in the construction environment, with 4 out of 76 show- withdrawal action of the vibrator that was too quick; and ing a reduced accuracy. The 4 red points are supposed to two occurrences of large tilt angles of the vibrator head. be observed at the blue points close to the tower crane. In addition, the vibrator head moved horizontally while A comparison with the video records revealed that the being inserted into concrete. The warnings were investi - wood panels enclosing the tower crane have blocked the gated and confirmed with the recorded video. However, line of sight between the stationary sensor and the vibra- one warning of KOF #15, which is moving horizontally tor operator, thus reducing the accuracy of the US sen- while being submerged into concrete, turned out to be a sors. The error of the red points ranged from 0.85 m  to false alarm. This case occurred when the operator turned 1.72  m. These results demonstrate that securing the line off the power of the vibrator machine and let the head of sight of the US sensors is critical to collect reliable become submerged while waiting for the next pour from insertion data, and this can be achieved by separating the the bucket. Due to the long insertion duration, the solu- path of the vibrator operator from that of bulky equip- tion cumulated the small errors of the vibrator’s location, ment and materials, installing stationary sensors at a high and it created a false warning. The remaining violations position to minimize occlusions, and zoning the tracking were confirmed by the video. The authors have finally areas to reduce blind spots. confirmed with the manager on site that these violations had negligible impact on the quality, considering the L ee and Skibniewski AI in Civil Engineering 2022, 1(1):4 Page 11 of 19 Fig. 2 A concrete member in (a) the CAD coordinates and (b) the US coordinates slump value, the amount of reinforcement, and the loca- an occurrence of a defect. Sharing the results of such tion, frequency, and severity of the violations. analyses will reinforce the learning and feedback loop, leading to a solid and proactive defect prevention plan. 5 Discussion Based on the workmanship analysis, project manag- From the site experiment, it was observed that the ers can give clear and unequivocal instructions on the locations were generally accurate, except for the case desired workmanship. For example, a specific range where the line of sight between the US sensors was not of distances and durations of insertions can be rec- secured. The monitoring solution was able to capture ommended, with particular site conditions taken into the violations of the key operational factors reliably. account. In addition, the warning data can be used to Six warning instances were generated, and one of them, coordinate training sessions for new workers. Inform- which detected excessive horizontal movement of the ing new workers of the circumstances and locations vibrator head occurred due to the abnormal insertion where the poor workmanship is more likely to occur duration. The remaining warning instances were gener - will help them avoid the malpractices, without learning ated properly as the operators’ behavior exceeded the through experiencing defects. set thresholds of KOFs. To maximize the benefits of the monitoring and warn - This system promises multiple benefits both for pro - ing solution, this study highlights the importance of pro- ject managers and concrete workers. First, the moni- ject managers considering the following points before toring and warning solution proposed in this study they adopt this solution. First, the managers should helps achieve proactive and data-driven management clarify and share the purpose of adopting this solution of concrete work and enables real-time sensor-based with the stakeholders. Since this solution can work as a feedbacks, so as to prevent concrete quality problems data collection and monitoring tool, its application may caused by poor workmanship. The modules of the solu - prompt conflicts among the stakeholders. If it is used tion translate the raw positioning data into the infor- to clarify the responsibilities of the stakeholders con- mation of workmanship and then deliver warning cerning certain concrete defects in case of a lawsuit, its messages to the managers for prompt managerial deci- deployment may create a distrustful atmosphere within sions. With these warnings, both managers and con- the projects, therefore inhibiting its full adoption by the crete workers can take immediate actions to prevent stakeholders. For these reasons, it is suggested that this defects, rather than waiting until the formworks are solution be used as a tool to improve the internal quality stripped for inspection and repair. assurance and enhance collaborative monitoring for con- In addition, the solution can expedite the learning crete work, rather than for assigning blames to any of the process of project managers and concrete workers by stakeholders. enabling collection, analyses, and sharing of data. Since Second, it is recommended to prepare a long-term the collected data of concrete placement remain in a application plan, so as to maximize the benefits from the database, managers can perform in-depth data analysis automated monitoring of concrete work. One of the limi- to understand how a certain malpractice contributes to tations of the proposed solution in the beginning stage is Lee and Skibniewski AI in Civil Engineering 2022, 1(1):4 Page 12 of 19 Fig. 3 Procedure for processing and merging sensor data L ee and Skibniewski AI in Civil Engineering 2022, 1(1):4 Page 13 of 19 Fig. 4 Procedure for updating the concrete grid’s height with the US data of a pump hose Lee and Skibniewski AI in Civil Engineering 2022, 1(1):4 Page 14 of 19 conditions, concrete equipment, and properties of con- crete mixes, the same thresholds may or may not be able to indicate certain poor workmanship. A long-term use of this solution could allow project managers to assess whether the set values are appropriate for delivering the desired quality. This experience will ultimately enable the data-driven calibration of the threshold values. Lastly, creating a communication channel between the solution and concrete workers will help them respond promptly to warning messages. The solution designed in this study can deliver warning messages to project man- agers and concrete foremen for real-time assessment, so that they can require workers to take corrective actions promptly. Project-level decisions on how to respond to the warning messages are provided. For example, if too much concrete is poured at one location, the manag- ers may instruct the pump hose workers to reduce the pumping rate and ask the vibrator workers to insert the vibrator’s head deep enough, while share the warning information with an inspector for a thorough quality investigation on all the locations where poor workman- ship is defected. If adopted by a concrete subcontractor, this solution could be more efficient to send the warning information directly to the operators, without passing through the project managers or concrete foreman. 6 Conclusion Problems with concrete quality are commonly detected at construction sites, and they can lead construction pro- jects to delays and cost overruns. Thus, project managers have long tried to prevent such quality problems by mon- itoring workmanship. However, since visual monitoring of concrete work is time-consuming and labor-intensive, managers can only perform the monitoring tasks tempo- rarily and intermittently. Current monitoring methods rely much on the visual feedbacks from managers, a pro- cess which is error-prone and subjective. Consequently, poor workmanship may not be detected on the spot, and immediate and corrective actions are often not made during the concrete operations. The current method, which depends heavily on inspect - ing and repairing defects, is costly, may fail to observe defects, and may be too late to initiate reparative actions when defects are detected. Moreover, due to the absence of instant feedback, inexperienced concrete workers Fig. 5 Procedure for updating vibration status of concrete internal may go through frequent trials and errors in learning points to improve their workmanship. The workers may forget the details of their movements when they are able to see the result of their work, which is after the fresh concrete the lack of knowledge on determining threshold values. has hardened and the formwork is removed. Obser- Project managers may not know the best performing vations and experiences of managers about concrete thresholds and may simply follow the values suggested work are often forgotten and distorted, and therefore, in Table  3. However, since projects have different site L ee and Skibniewski AI in Civil Engineering 2022, 1(1):4 Page 15 of 19 Table 4 Conditions for violation of KOFs Key operational factor (KOF) Conditions for violation 1 Uniform placement The height difference of concrete between two adjacent placement grids is greater than l m layer 2 Continuous place- A pump hose does not come back to the grid within t seconds after it leaves ment 3 Avoiding excessive The height of the discharging point of a pump hose from the surface of fresh concrete exceeds l m for consecutive t seconds free-fall height 4 Vertical drop of The angle of the sensor installed at a pump hose exceeds n degrees from the vertical axis for consecutive t seconds concrete 5 Avoiding direct The angle of the sensor installed at a pump hose exceeds n degrees from the vertical axis, and the line that connects two sensors intersects with a vertical form within l m from the main placement on vertical sensor for consecutive t seconds forms 6 Avoiding direct place- The line that connects two sensors intersects with a congested area within l m from the main sensor for consecutive t seconds ment on reinforcing bars 7 Appropriate vibration The vibrator fails to stay in l m from a cube point for consecutive t The vibrator head is submerged into concrete for less than t seconds or more than t seconds 1 1 2 duration seconds within t seconds after concrete is poured 8 Appropriate insertion The insertion depth (Z value of the vibrator head) is at least l m higher than the average depth of the recent depth insertions 9 Appropriate distance The horizontal distance between two consecutive insertions is at least l m lower or higher than the average between insertions horizontal distance of the recent insertions 10 Inserting vibrators N/A close to forms 11 Continuity between placement and vibration 12 Avoid touching forms The vibrator head stays within l m from a form face for consecutive t seconds with a vibrator 13 Appropriate penetra- For the insertions with t seconds or longer, the insertion speed is lower than v m/s, or the withdrawal speed is higher than v m/s 1 2 tion and withdrawal speeds of vibrators 14 Appropriate penetra- The angle of the vibrator head exceeds n degrees from the vertical axis for consecutive t seconds while it is submerged in concrete tion and withdrawal angles of vibrators 15 Avoiding dragging a For the insertions with t seconds or longer, the cumulative distance of a vibrator’s horizontal movement exceeds l m while the vibrator is inserted vibrator Lee and Skibniewski AI in Civil Engineering 2022, 1(1):4 Page 16 of 19 Fig. 6 Warning images for KOFs #1 and #5 Fig. 7 Installed ultrasound and electromagnetic sensors Fig. 8 (a) Observed and actual vibrator insertion locations and (b) occlusions at the site L ee and Skibniewski AI in Civil Engineering 2022, 1(1):4 Page 17 of 19 Table 5 Warning instances during the application, results of data validation, and their impacts on the quality KOF # Warning statistics Description Results of investigating the data and Impact on the quality video 1 - Number of warning instances: 1 There was one placement grid where the A large pour is suspected to be triggered Considering the concrete’s slump value - Number of placement grids: 2 amount of placed concrete was larger than by the placement method using a concrete (15.2 cm), the water cement ratio (0.4), and - Height difference: 0.33 m the nearby grids bucket the simple shape of the slab, the violation - Duration: 22.5 s is expected to have a minor impact on the concrete quality 7, 8, 9, 10 - Number of warning instances: 1 The warning module found that two con- Two nearby concrete points did not have a These points were in the middle of the slab, - Number of undervibrated internal points: 2 crete points among two hundred ones were vibrator submerged within 1.3 m for 10 min where the concrete can flow and fill from vari- - Location: center grids suspected to be undervibrated after the placement ous directions, so the warning was not critical to the quality of concrete 13 - Number of warning instances: 1 One quick withdrawal of the vibrator was The observed withdrawing speed was Since the quick withdrawal of the vibrator did - Withdrawal speed: 0.66 m/s detected 0.66 m/s, which is minimally faster than the not occur frequently, this warning is expected threshold of 0.5 m/s to be non-critical to the quality 14 -Number of warning instances: 2 There were two violations observed with The head angle was between 35°–40° from This case is expected to be non-critical for - Average degrees tilted: 39° a tilt head while it was inserted into the the vertical line, lasting for 2 s the quality, because the tilt of the vibrator concrete occur neither for a long time nor in a frequent manner 15 - Number of warning instances: 1 There was one warning of excessive horizon- From investigating the video, the researchers This was a false positive, and it does not - Duration: 9.6 s tal movement of the vibrator determined that this was a false alarm, which reduce the quality of the concrete - Cumulative horizontal movement: 1.0 m occurred due to the cumulated positioning errors during the prolonged insertion Lee and Skibniewski AI in Civil Engineering 2022, 1(1):4 Page 18 of 19 Received: 13 May 2021 Accepted: 18 March 2022 opportunities to share knowledge and analyze workman- Published: 18 August 2022 ship may be lost. 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AI in Civil Engineering – Springer Journals

Published: Aug 18, 2022

Keywords: Real-time monitoring; Concrete quality; Placement and vibration; Workmanship

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