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This report is an archived publication and may contain dated technical, contact, and link information
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Publication Number:  FHWA-HRT-09-040    Date:  May 2014
Publication Number: FHWA-HRT-09-040
Date: May 2014

 

State of The Practice and Art for Structural Health Monitoring of Bridge Substructures

CHAPTER 3. VOIDED SHAFT THERMAL MONITORING

The first remote monitoring effort conducted during this study involved the thermal monitoring of a drilled shaft. Florida’s bridge substructures have continually grown in size due to the high demand of larger bridges to accommodate the growing population. Typically, drilled shafts were not considered to behave as a mass concrete element due to their smaller size (usually no greater than 4 ft (1.22 m) in diameter). However, with the increase in size of today’s bridge foundations to accommodate longer spans with reduced numbers of collision-prone piers, common sizes of drilled shafts are larger and act as mass concrete elements (such as the 9-ft (2.7-m)-diameter shafts for the Ringling Causeway Bridge in Sarasota, FL). Until recently, these larger diameter shafts have slipped through the concrete specifications without special review for mass concrete effects. Aside from the more widely recognized differential temperature concerns, an equally important issue is the high temperatures that occur during the curing of mass concrete elements. Therein, the delayed ettringite formation can lead to long-term durability reduction where internal cracking initiates in regions that experience elevated curing temperatures.

To combat mass concrete effects in large diameter drilled shafts, researchers at the University of South Florida (USF) in Tampa, FL, proposed and constructed a drilled shaft with a full-length centralized void to mitigate the mass concrete effects exhibited by the foundation element. Benefits from this approach were twofold: (1) to eliminate mass concrete effects in large diameter drilled shafts and (2) to reduce the concrete volume and cost required to construct these foundation elements.

This chapter focuses on the remote thermal monitoring procedure that was used for the research conducted on the USF voided shaft research project. Of particular interest is the installation and instrumentation of the drilled shaft, the thermal monitoring procedure and a review of its efficacy, and the results from the remote thermal monitoring system and its individual parts. More emphasis is placed on the actual monitoring procedure than the results from the voided shaft; however, these thermal results are presented in a summary.

TEST SPECIMEN INSTRUMENTATION

The testing site for the thermal monitoring of the voided shaft was in Clearwater, FL, at the equipment yard (see ©2008 google®figure 18).

The figure shows a locator map to the voided shaft testing site is located in Clearwater, FL, close to I-275 in Pinellas County.
©2008 Google®
Figure 18. Illustration. Map of voided shaft testing site.(8)

Prior to the construction of the drilled shaft, the instrumentation for the thermal monitoring was put into place. The first step was the instrumentation of the rebar cage that was installed in the shaft. The reinforcement cage was built using 36 longitudinal bars with 26 #5 stirrups at 12 inches (304.8 mm) on the center. The cage was equipped with nine Schedule 80 polyvinyl chloride (PVC) pipes for thermal testing, which were 26 ft (7.93 m) long and 2 inches (50.8 mm) in diameter (see figure 19). On three of these tubes at 120-degree spacings from each other, thermocouples (TCs) were placed at the top, middle, and bottom of the tubes to provide readings from around the shaft. The inner steel casing (needed to provide the central void in the shaft) was outfitted with three crossbar supports welded to the interior of the casing, which allowed for a central tube to be run through the center of the void for thermal integrity testing (see figure 20). TCs were also placed at the top, middle, and bottom of each side of the inner casing spaced 120 degrees away on the crossbars and attached to the top, middle, and bottom of the central tube (see figure 21). More TCs were placed at the top, middle, and bottom of the outside of the inner casing (see figure 22). In the surrounding soil, ground monitoring tubes were installed at distances corresponding to fractions of the shaft diameter (D); 0.25D, 0.50D, 1D, and 2D away from the edge of the shaft (see figure 23). TCs were also installed with the tubes at these locations.

This photo shows a voided shaft reinforcement cage. The reinforcement cage used for the demonstration voided shaft is nominally 8 ft (2.44 m) in diameter, which provides 6 inches (152.4 mm) of clear concrete cover. The cage had 36 longitudinal bars with access tubes installed between every fourth bar. Clear spacing between bars and access tubes provide adequate concrete flow during concreting.
Figure 19. Photo. Voided shaft reinforcement cage instrumentation.

This photo shows the installation of center tube supports into the voided shaft center casing. The casing used to provide the void at the shaft center was equipped with centralizing struts welded to the inside of the casing. The struts were welded at several locations along the length of the casing and provided support for a single access tube at the shaft center.
Figure 20. Photo. Voided shaft center casing center tube supports.

This photo shows voided shaft thermocouple (TC) wires inside the centered access tube in the supporting struts. The orange wires are part of the preparation for shaft construction.
Figure 21. Photo. Voided shaft TCs installed in center casing.

This photo shows additional thermocouples (TCs) mounted to the exterior of the central casing attached to welded tabs at the desired locations. The exterior of the casing represents the inner-most portion of the concrete that was to be placed around the casing. All TC wiring was extended to the top of the casing for later installation to the data collection system.
Figure 22. Photo. Voided shaft TCs on outside of center casing.

This photo shows a drill rig used to install access tubes in the ground around the proposed location of the voided shaft, which was to be constructed later. These access tubes in the soil were installed at various distances from the edge of the proposed shaft location several weeks prior to shaft construction.
Figure 23. Photo. Voided shaft ground monitoring tube installation.

TEST SPECIMEN CONSTRUCTION

The voided shaft was constructed at the test site on September 25, 2007. The entire construction process was broadcast via webcam from the USF geotechnical Web page for those who were unable to visit the construction site. Records of the construction sequence, thermal testing, and long-term thermal monitoring were posted and updated every 15 min to http://geotech.eng. usf.edu/voided.html. A 9-ft (2.7-m)-diameter drilled shaft with a 4-ft (1.2-m)-diameter central void was constructed. The first step was the excavation; an oversized surface casing 10 ft (3.05 m) in diameter and 8 ft (2.4 m) in length was embedded 7 ft (2.1 m) into the soil. Excavation was carried out in the dry condition with a 9-ft (2.7-m)-diameter auger for the first several feet. After which, polymer slurry was introduced into the excavation for stabilization. The excavation proceeded without issue to a depth of 25 ft (7.6 m) (see figure 24). A cleanout bucket was used to scrape the bottom of the excavation of debris immediately after the auger and then again after a 30-min wait period.

This photo shows the excavation of the shaft that was completed with a truck-back drill rig equipped with a 9-ft (2.75-m)-diameter auger and surface casing to support the upper soils. Around the surface casing, the access tubes previously installed were painted flourencent orange to minimize inadvertent disturbances.
Figure 24. Photo. Excavation for voided shaft.

The reinforcement cage was picked at two locations to avoid excess bending (see figure 25). Locking wheel cage spacers were placed along the length of the reinforcement cage to maintain 6 inches (152.4 mm) of clear cover (see figure 26). The reinforcement cage was hung in place during the pour so that the finished concrete was level with the top of the cage (see figure 27).

This photo shows the reinforcement cage being picked up by two boom trucks. The reinforcing cage was carefully picked up from four points and two boom trucks to minimize lifting distortion.
Figure 25. Photo. Picking of reinforcement cage for voided shaft.

This photo shows the placement of the reinforcement cage into the voided shaft. The was held as close to center as possible prior to lowering. Wheel spacers were installed to aid in centering and providing the correct concrete cover.
Figure 26. Photo. Placement of reinforcement cage for voided shaft.

This photo shows the self-weight of the cage supported by chains, which helped maintain the correct cage elevation and prevented the bottom of the cage from resting on the soil, thereby providing the minimum steel cover at the toe of the shaft. Beams capable of support the cage self-weight were placed across the top of the surface casing to assure the chains were vertical.
Figure 27. Photo. Hanging of reinforcement cage for voided shaft.

The central casing used to create the full-length void had a 46-inch (1,68.4-mm) outer diameter steel casing that was 30.5 ft (9.3 m) long. It was set into the center of the excavation with a crane (see figure 28 and figure 29). The self weight of the steel casing penetrated the soil to about 3 to 6 inches (76.2 to 152.4 mm). This prevented the concrete from entering the void area. To prevent the top of the inner casing from shifting during the initial concrete pour, a back-hoe bucket was used to hold the top of the casing steady (see figure 30). A double tremie system was used to place the concrete on opposite sides of the excavation (see figure 31). Concrete specifications were a standard 4,000 psi (27,560 kPa) with an 8-inch (203.2-mm) slump and #57 stone mix design. During the concrete placement, concrete level at three points around the shaft was measured to ensure that the concrete was flowing around the void and through the reinforcement cage. The temporary surface casing was removed after final concrete placement (see figure 32 and figure 33).

This photo shows central casing with the centralized access tube being lifted from two opposing holes in the casing near the top. Because the casing was much stiffer than the cage, it was lifted with only one boom truck.
Figure 28. Photo. Picking of central casing for voided shaft.

This photo shows the placement of the central casing into the voided shaft. The inner face of the voided shaft was formed by the central casing installed in the excavation after the cage. The weight of the casing was sufficient to cut into the soil at the bottom of the excavation approximately by 6 inches (152.4 mm) as indicated by the lengths premarked on the side of the casing.
Figure 29. Photo. Placement of central casing for voided shaft.

This photo shows a back-hoe holding the central casing in place. In lieu of welded centralizing struts from the central casing to the surface casing, the contractor opted to secure the top of the central casing with a back-hoe bucket continually manned by an operator. The base of the casing had already been secured by the self-weight cutting into the soil.
Figure 30. Photo. Holding of central casing steady for voided shaft.

This photo shows the pouring of the concrete around the central casing. Two tremies were placed at roughly opposite sides of the annular region formed between the cage and the central casing. This allowed two concrete trucks to supply two sources of concrete.
Figure 31. Photo. Double tremie concrete placement of voided shaft.

 

This photo shows two boom trucks lifting the outer steel casing from the voided shaft. To provide minimal disturbance to the freshly poured shaft, the trucks were used to extract the temporary surface casing. The trucks provided a more concentric removal that was minimally affected by boom deflection during loading.
Figure 32. Photo. Voided shaft outer steel casing removal.

This photo shows the finished shaft with the access tubes and the reinforcing cage stickups surrounding the central casing. The centralized access tube can also be seen, as well as a mounting pole for the data collection system.
Figure 33. Photo. Final voided shaft at ground level.

MONITORING SYSTEM INSTRUMENTATION AND PROCEDURE

Once the construction of the voided shaft was complete, all of the TC wires were accessed through the tubes so they could be attached to the data collection system. The remote monitoring system was composed of several parts: A Campbell Scientific, Inc.® CR1000 data logger, an AM25T 25-channel multiplexer, a Campbell Scientific, Inc.® Raven100 CDMA AirLink cellular modem, PS100 12-V power supply and 7-Ahr rechargeable battery, a 12W Solar Cell panel from Unidata, and a large environmental enclosure to protect all the materials from the elements (see figure 34 through figure 38). The total cost of the system including all equipment and ongoing services was approximately $4,500. The TC wires were connected to the multiplexer because there were not enough channels on the CR1000 to read all of the TCs. The multiplexer was then connected to the CR1000 (see figure 39). LoggerNet, the remote monitoring and data collection software from Campbell Scientific, Inc.®, was used to program the CR1000 for remote monitoring and data recovery. The data collection system was equipped with the solar panel to help sustain the battery voltage (see figure 40). The system was programmed to wake up every 15 min, take a temperature reading, record it, and go back to sleep. The Raven100 modem was programmed to wake up once every 60 min and transmit the collected data back to the host computer for processing, which was stationed in the Geotechnical Research Group at USF. Sideline measurements of ground temperature for a companion study were taken 1D and 2D away from the shaft via an OMEGA® OM-220 data logger that collected data at the same rate as the CR1000; however, the data were simply stored, and a site visit was required to collect that data. The remote system’s battery voltage was also monitored and sent to the host computer along with the thermal data so that the power consumption could be tracked.

The figure shows the Campbell Scientific Inc. CR1000 data logger. It is relatively small (approximately 4 inches (101.6 mm) tall and 8 inches (203.2 mm) long) and has screw terminal connections for instruments, power in and out, and digital inputs and outputs. Communication between the data logger and the cellular modem was provided by the serial RS-232 port.
Figure 34. Photo. Campbell Scientific, Inc.® CR1000 data logger.

The figure shows the AM25T 25-channel thermocouple (TC) multiplexer that connects via screw terminals to the Campbell Scientific Inc. CR1000 data logger. Screw terminals line both the upper and lower edge as shown for connection to as many as 25 TCs.
Figure 35. Photo. AM25T 25-channel multiplexer.

The figure shows the Raven100 CDMA AirLink cellular modem that connects to the Campbell Scientific Inc. CR1000 data logger using a null-modem serial interface cable (not shown). The face of the modem was equipped with seven light-emitting diode indicators that provide information concerning the status of the power, available network service, available signal strength, communication activity, etc. The device could be either configured for static or dynamic Internet protocol connections.
Figure 36. Photo. Campbell Scientific, Inc.® Raven100 CDMA AirLink cellular modem.

The figure shows the PS100 12-V power supply which came complete with a 7 amp-h rechargeable battery. It was capable of recharging the battery using alternating current or direct current sources typical of land power or solar panel. An external deep cell battery (not shown) was also maintained by the PS100 through screw terminals dedicated for that application.
Figure 37. Photo. Campbell Scientific, Inc.® PS100 12-V power supply with rechargeable battery.

The figure shows an ENC12x14 environmental enclosure that was selected to house the entire remote data collection system. It  contains the following basic components: data logger, power supply/battery, cellular modem,and multiplexer.
Figure 38. Photo. Campbell Scientific, Inc.® ENC12x14 environmental enclosure.

The figure shows an entire system, which contains the basic components. The null modem cable (white) is shown connected to the CR1000 (top). The cellular modem (gold) is shown on the right, while the multiplexer is near the bottom of the enclosure. Thermocouple (TC) wires were fed through the bottom of the enclosure and directly to the multiplexer. A putty-type sealant was used to keep out moisture. A desicant pack was also enclosed to absorb trapped moisture once the enclosure was closed.
Figure 39. Photo. TC wire connection from AM25t 25-channel multiplexer to CR1000 data logger.

This photo shows a remote monitoring system enclosure for the voided shaft mounted to a pole, along with a wide band cellular antenna (top) and a solar panel trained on the predominant direction of the sun for that location. Thermocouple wiring can be seen running into the bottom of the enclosure.
Figure 40. Photo. Remote thermal monitoring system for voided shaft.

RESULTS AND CONCLUSIONS

Overall, the system worked well. At one point during the monitoring period, there was a cellular timeout, and the modem stopped transmitting the data to the host computer. This was fixed by a site visit to reset the modem, and the problem did not occur again. However, the main problem that was encountered was an issue with power usage. At the beginning of the monitoring procedure, the Raven modem was left on and sent back data every hour, which used a large amount of power, and the system lost power after a few hours (see figure 41). The monitoring procedure was revised so that the modem would go to sleep and only wake up once every hour to transmit the collected data. Even with this alteration, the battery was still losing power relatively quickly. Once the battery voltage dropped below 11.6V, the data collection system had approximately 8 h of life before it quit. Due to this large amount of power usage, three site visits were required to recharge the battery. These visits are seen in the plot of the battery voltage over time (see figure 42). In order to provide a completely remote unit, a larger solar cell was recommended because the 12W did not provide enough power to make the system fully remote.

The figure shows a line chart with battery voltage level on the y-axis from 11 to 12.4V and the date on the x-axis from September 25, 2007, to October 9, 2007. Almost immediately after construction, the battery voltage dropped drastically. This was due to a continuous connection between the cellular modem and the host server. Below 11V, the system is programmed to hibernate and protect the collected data until power can be restored. The following trend (after charging) shows a much slower voltage decay but indicated that the solar panel was insufficient in size/capacity to maintain the system power demand.
Figure 41. Graph. Battery voltage of thermal monitoring system as of October 8, 2007.

The figure shows a line chart with battery voltage levels on the y-axis from 11 to 12.4V and the date on the x-axis from September 25, 2007, until December 24, 2007. The battery voltage of thermal monitoring system from the time of construction to the end of monitoring shows three separate occasions where the system voltage was critically low and five occasions where a site visit was required to recharged the system over the 3-month period. Times where the rate of voltage decay is less can be attributed to highly solar days and vice versa.
Figure 42. Graph. Battery voltage of thermal monitoring system as of December 14, 2007.

Originally, the data collection period was supposed to last until the temperature in the shaft had reached equilibrium. However, in reviewing the data, the temperatures recorded from the soil surrounding the shaft were increasing while the temperatures within the shaft had reached equilibrium (see figure 43). Therefore, data collection continued for another period of time until it was determined that the temperatures both in the shaft and in the surrounding soil had reached equilibrium. From the final temperature plot, it is evident that the temperature in the soil 1D away from the shaft was the last to eventually reach equilibrium. It can also be seen that the temperature in the soil at 2D away from the shaft was affected only slightly by the heat coming from the shaft (see °f = 1.8(°c) + 32 figure 44).

This figure shows a wireframe contour with temperature on the y-axis from 70 to 140 ï‚°F (21.11 to 60 ï‚°C) and the dates of the data collection on the x-axis from September 25, 2007, to November 24, 2007. Each thermocouple (TC) is represented by a line of a different color. The data from all TCs within the voided shaft are shown for the entire monitoring period. The peak temperature measured was 138 ï‚°F (58.58 ï‚°C). Elevated soil temperatures continued to be present even after 3 months. The daily temperature flunctuations are shown.
°F = 1.8(°C) + 32
Figure 43. Graph. TC data from voided shaft as of November 12, 2007.

This figure shows a line chart with temperature on the y-axis from 60 to 140 ï‚°F (15.56 to 60 ï‚°C) and the dates of the data collection on the x-axis from September 25, 2007, to December 4, 2007. The daily average thermocouple (TC) data for all locations within the shaft and surrounding soil are annotated with a cross section view of the shaft. The distances away from the shaft edge are denoted in terms of the diameter (D). The 0.25D, 0.5D, 1D, and 2D temperature traces represent 2.25, 4.5, 9, and 18 ft (0.61, 1.37, 2.75, and 5.49 m), respectively.
°F = 1.8(°C) + 32
Figure 44. Graph. Final average TC data for all locations.