|<< Previous||Contents||Next >>|
Construction of the California Precast Concrete Pavement Demonstration Project
Chapter 6. Pavement Construction
The week prior to installation, a pre-construction meeting took place at the office of the installation contractor. Representatives from Yeager Skansa, Inc. (installation contractor), Caltrans, Pomeroy Corporation (precast fabricator), Dywidag Systems International (post-tensioning contractor/supplier), and The Transtec Group, Inc., were present at the meeting. The purpose of the pre-construction meeting was to ensure coordination of the installation effort. The precast fabricator was responsible for delivery of the panels to the site, the installation contractor was responsible for panel installation, and the post-tensioning supplier was responsible for temporary post-tensioning during installation and permanent post-tensioning and grouting after installation. Caltrans was responsible for oversight and inspection of the construction process.
The precast pavement test section was constructed over embankment fill behind a new retaining wall as part of the widening of I-10. The LCB and aggregate base were placed well in advance of the precast panels. Due to unforeseen changes in the project schedule, the actual test site was not where it was originally proposed, and therefore strict tolerances on smoothness were not enforced during the placement of the LCB. While this was not desirable, it provided a good indication of the smoothness that could be expected for a typical LCB. Any obvious "high spots" in the LCB were ground off before the panels were installed.
The polyethylene sheeting (friction-reducing material) for each panel was rolled out just before the panel was placed to minimize any damage to the sheeting caused by foot traffic or construction vehicles and equipment. The surface of the LCB was cleared of loose rocks and debris before the polyethylene sheeting was unrolled.
Transportation to Site
The panels were transported to the job site on flat-bed tractor trailers. Due to weight restrictions, only one panel could be transported on each truck. The panels were shipped from the fabrication plant in Perris approximately 97 km (60 mi) to the project site in El Monte. The panels were supported on the truck with two-point supports, as they had been supported at the precast plant. The panels were strapped down using nylon straps to prevent damage during shipment, as shown in figure 24.
Figure 24. Photo. Panels were shipped to the job site one per truck.
Precast panel installation took place on the nights of April 12 and 13, 2004. Installation was completed before 5 a.m., and post-tensioning was started immediately after panel installation was completed each night.
Panel installation took place at night so that two of the four existing lanes of I-10 could be closed to allow the trucks delivering the precast panels to stage outside of the construction barrier. The lane closures commenced at 10 p.m. and were removed by 5 a.m. A 90.7-Mg (100-ton) crane stationed on the LCB was used to lift the panels off of the trucks, swing them over the construction barrier, and set them in place. The different steps of the installation process are shown in figures 25 through 29.
Each panel was lowered into place at a "nose down" angle to facilitate mating the panel with the adjacent panel already on the ground. The panels were lowered into place with the female keyway as the leading edge, mating to the adjacent panel. The panels were classed using a mark on the surface of each panel, directly above one of the post-tensioning ducts. This prevented any misclassment that may have been caused by classing the panels by their ends. A slow-setting epoxy adhesive was applied to the keyways prior to installing each panel. Although the purpose of the epoxy was both to provide lubrication for assembling the panels and to seal the joints between panels, it proved to be too thin to provide lubrication and did not seal joints that were open more than 3 mm (1/8 in.).
As each panel was installed, it was temporarily set approximately 0.3 m (1 ft) from its final position while temporary post-tensioning strands were fed from the panels already in place into the new panel. The panel was then lifted slightly, moved into place, and the temporary post-tensioning strands were stressed to pull the panels together as tight as possible. The lifting lines were then removed, the temporary post-tensioning anchor was removed, and the process was repeated with the next panel.
Figure 25. Photo. A precast panel is lifted off the truck and over the concrete barrier and set in place.
Figure 26. Photo. Workers detach the lifting lines from a precast panel. Note the polyethylene sheeting, rolled out just prior to placement of each panel.
Figure 27. Photo. Lowering a panel into place. Each panel was lowered at a nose-down angle to facilitate mating it with the panel already in place.
Figure 28. Photo. Workers apply epoxy to the joint prior to assembling the panels.
Figure 29. Photo. Workers use temporary post-tensioning to pull the panels together as they are assembled.
During the 1st night of panel installation, 16 panels were set in place in approximately 5 hours. During the 2nd night, the process was more efficient, and the remaining 15 panels were installed in just over 3 hours. Efficiency was improved the 2nd night by adding a second set of lifting hooks and applying the epoxy to the panel while it was on the truck waiting to be installed. Overall, the average placement rate was approximately 15 minutes per panel, including the time necessary to move the crane (after every 3 panels) and apply temporary post-tensioning.
Final longitudinal post-tensioning began following placement of the last panel each night. The post-tensioning strands were precut to length and delivered to the site prior to panel installation. The strands were uncoiled, fed into the post-tensioning ducts from the central stressing pockets, and pushed by hand to the anchors in the joint panels, as shown in figure 30. An anchor chuck was threaded onto the end of each strand from the access pockets in the joint panels, and the wedges were seated around the strand. A "dogbone" or "ring anchor" coupler (shown in figure 5) was threaded onto the ends of the two strands in the central stressing pockets, and wedges were seated around each of the strands, as shown in figure 31. A monostrand stressing ram was then used to tension the strands from the stressing pockets. The stressing ram tensioned both strands simultaneously by gripping one strand while reacting against the other strand by pushing against the coupler, as shown in figure 32. Each tendon was initially stressed to 20 percent of the ultimate load, each strand was then marked, and then the tendon was stressed to the ultimate load. The elongation of the tendon was then determined by measuring the distance the mark on each strand had moved when stressing it from 20 percent to the full load.
Post-tensioning strand coated with a fine-grit-impregnated epoxy was used for the longitudinal tendons. Epoxy-coated strand provides an extra layer of protection against corrosion, which is particularly critical at the joints between panels. The grit in the epoxy helps to provide better bond between the strand and the grout. This strand requires special wedges for the post-tensioning anchors, specifically designed for epoxy-coated strand. Previous testing by the strand supplier demonstrated that grit-impregnated strand would not damage the post-tensioning ducts as it was fed through them.
Post-tensioning (stressing) required approximately 5 to 10 minutes per tendon, barring any problems with equipment. Stressing began with the strands at the center of the slab (centerline of the pavement) and alternated outward to the strands on either side of the centerline. In total, 12 tendons were stressed for each slab, 24 tendons in all.
Figure 30. Photo. Workers feed the post-tensioning strands into the ducts at the central stressing pockets and push them through the ducts to the anchors.
Figure 31. Photo. A "dogbone" coupler is used to splice the post-tensioning tendons together in the stressing pockets. The stressing ram tensions both tendons at the same time through the coupler.
Figure 32. Photo. A monostrand stressing ram is used to tension each of the post-tensioning tendons.
Expansion Joint Seal
A recess for the expansion joint seal (figure 18) was cast into the joint panels. This recess was widened prior to installation of the seal by sawcutting. A preformed elastomeric seal was then installed in the joint. The seal was selected such that it could accommodate the anticipated total expansion joint movement. The seal was installed after completion of post-tensioning, and was installed slightly lower than the finished pavement surface to permit diamond grinding.
To ensure that the finished precast pavement would expand and contract outward from the middle of the slab (at the central stressing panels), it was necessary to anchor the center of each slab to the base/subbase. These mid-slab anchors were created by drilling two holes, 38 mm (1 1/2 in.) in diameter, a minimum of 200 mm (8 in.) into the base/subbase at each of the central stressing pockets; and inserting bars, 25 mm (1 in.) in diameter, into the holes, as shown in figure 33. The bars were grouted in-place in the holes with nonshrink grout, with a minimum of 100 mm (4 in.) of the bar protruding into the stressing pocket. After the central stressing pockets were patched, the mid-slab anchor bars restricted the middle of the slab from movement.
Figure 33. Illustration. Mid-slab anchor at the central stressing pockets.
After completion of post-tensioning and after the mid-slab anchor bars were drilled and grouted in place, the stressing pockets (central stressing panels) and anchor access pockets (joint panels) were filled and finished flush with the pavement surface. Although a fast-setting concrete could have been used to permit traffic onto the pavement as soon as possible, a "pea gravel" mixture was used to fill the pockets, as shown in figure 34. The patches were given a surface texture similar to the surrounding concrete, and two coats of curing compound were applied.
Figure 34. Photo. Workers fill the post-tensioning pockets with a "pea gravel" concrete mixture after post-tensioning.
Surface Repairs and Diamond Grinding
Any major damage to the surface of the pavement that could affect ride quality or pavement durability was required to be repaired prior to opening the pavement to traffic. Minor spalling at the joints between panels was repaired at the discretion of the engineer on site. The only major distresses that occurred resulted from the panels coming together too quickly during assembly, causing spalling of the joint or corner breaks. Only one major corner break that required repair was encountered during construction; it is shown in figure 35. Had any cracking occurred during panel installation, the cracks would have been sealed using an approved method such as epoxy injection.
To achieve the Caltrans ride quality requirements, the finished surface of the precast pavement was diamond ground in the traffic lanes. Ride quality after diamond grinding was measured by the contractor using a California profilograph. Four separate passes were made with the profilograph across the width of the traffic lanes, and the trace was recorded on an automated profilogram. The profile indexes, measured for each of the four passes using a 5-mm (0.2 in) blanking band, were 15.6 mm/0.1km (9.9 in/mi), 9.7 mm/0.1km (6.2 in/mi), 1.9 mm/0.1km (1.2 in/mi), and 11.7 mm/0.1km (7.4 in/mi).
Figure 35. Photo. Corner break that occurred during panel installation-the only major distress encountered.
Grouting was essentially the final major step in the construction process, and took place after the central stressing pockets and anchor access pockets were patched. In preparation for grouting, the exposed longitudinal edge of the slab was backfilled and the bottom of each of the expansion joints was sealed with expansive spray foam to prevent grout from leaking into the joint from beneath the slab.
Longitudinal Tendon Grouting
As discussed previously, the purpose of grouting the longitudinal post-tensioning tendons is two-fold. First, the grout provides an additional layer of protection for the tendons against corrosion. Secondly, the grout bonds the tensioned strands to the precast panels. This bonding will prevent a complete loss of prestress in the slab if one or more of the tendons are cut, either inadvertently or to remove a damaged precast panel.
Grouting was performed by the post-tensioning contractor/supplier. A high-capacity grout pump/mixer was used for both tendon grouting and underslab grouting. The grout mixture consisted of Type II/V cement and 19 L (5 gal) of water per sack. Grout was pumped into one end of each tendon, at either the joint panel or the central stressing panel, until it flowed out of each intermediate grout vent and finally out of the vent at the opposite end of the tendon, as shown in figure 36. Each intermediate vent was closed off when grout flowed out of it.
Figure 36. Photo. Grout is pumped into one end of the post-tensioning tendon at the stressing pocket until it flows out of the far end.
In some instances, grout began to leak from the joints between panels onto the surface of the pavement. In those instances, the grouting hose was moved to an intermediate grout vent and pumped until it flowed out of the end vent. If grout did not flow out of the end after a significant amount of pumping (indicating that grout was leaking from a joint to beneath the slab), the hose was moved to the end vent and pumped until it flowed from an intermediate vent. Each tendon was grouted until the inspectors were confident it was fully grouted.
Underslab grouting ensured that any voids beneath the precast panels were filled so that the panels were fully supported. Underslab grouting was completed in much the same way as tendon grouting, using the same mixer/pump and grout mixture. Grout was pumped into the inlet at the low end of each underslab grout channel until it was observed flowing either out of an intermediate vent or out of an adjacent grout channel vent.
Challenges and Problems Encountered
Due to the experimental nature of this demonstration project, which is only the second known precast prestressed pavement constructed in the United States, minor problems were anticipated during construction. This section will discuss some of these problems and the solutions that were developed.
Leveling Course Smoothness-The primary issue encountered during panel placement was the smoothness of the LCB leveling course. Because LCB is a rigid material, it did not conform to the bottom of the precast panels as the asphalt leveling course did in the Texas pilot project.(2) This rigidity resulted in a significant number of voids beneath the precast panels that required a large volume of grout to fill. Fortunately, no cracking due to the uneven leveling course was observed. Smoothness of the leveling course is an issue that must be addressed on future projects, however.
Joint Spalling/Corner Breaks-As discussed previously in this chapter, the only major distress that required repair was a corner break, which occurred as a panel was installed. This break was most likely caused by the panel impacting the adjacent panel as it was lowered into place. It is possible that debris may also have been present in the keyway, causing a stress concentration. There were also several incidences of minor spalling at the top surface of the pavement at the joints. This spalling, likewise, was attributed to the panels impacting each other during assembly. These problems can be avoided on future projects by carefully monitoring the assembly process to ensure that the keyways are free from debris and the panels do not impact each other as they are assembled.
Pinched Duct-As the post-tensioning strands were being pushed through the ducts, one of the strands could not be pushed past a certain point. Measurement of the location where the strand had stopped revealed that it was in the middle of one of the panels and not at a joint. It is believed this was caused by a pinched post-tensioning duct that prevented the strand from sliding freely. Fortunately, with enough force, the workers were able to force the strand past this point to the anchorage. This was only an isolated incident and can be prevented on future projects by carefully checking all post-tensioning ducts at the precast plant for blockages or pinched areas.
Seating of Wedges-Another isolated incident occurred during post-tensioning when a set of wedges would not seat around the post-tensioning strand at the coupler in the stressing pocket. As the operator began to release the tension in the stressing ram, one set of wedges would not seat around the strand, forcing the workers to de-tension the entire tendon and replace the wedges. Inspection of the wedges revealed that the epoxy coating had clogged the teeth, preventing them from gripping the strand. This was most likely caused by repeated seating of the wedges during the tensioning process, which required up to two resets of the stressing ram to get the full tension in the strand. This problem could be prevented on future projects by using a stressing ram with a long enough stroke to fully tension the strand with one pull (if epoxy-coated strands are used).
Stressing Pocket Dimensions-Although the workers were able to successfully tension all of the tendons, the size of the stressing pockets made post-tensioning more difficult. The width of the stressing pockets (200 mm [8 in.]) was not wide enough for the stressing ram to lie flat, and the length of the pockets required the ram to stick out of the pockets at an angle during stressing (see figure 32). While this did not cause any problems with stressing, larger pocket dimensions would have simplified the operation.
Grout Leakage-The only problem encountered during grouting was leakage of the grout onto the surface of the pavement. This was caused by an inadequate seal around some of the post-tensioning ducts at the joint between panels. Although the leakage was not a major problem (about 6 percent of the ducts leaked at a joint), it did slow the grouting process and required the excess grout to be cleaned off of the surface. It may never be possible to eliminate this problem completely, but it can be minimized by carefully inspecting the gaskets around the ducts prior to panel assembly and by ensuring that the joints between panels are closed as tightly as possible during panel installation. Epoxy applied to the keyways may also reduce the occurrence of grout leakage.
|<< Previous||Contents||Next >>|