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Construction of the Iowa Highway 60 Precast Prestressed Concrete Pavement Bridge Approach Slab Demonstration Project
Chapter 6. Approach Slab Construction
Pre-construction meetings with the installation contractor were an essential part of this project, particularly since the contractor had not constructed a project of this nature previously. Several pre-construction meetings and conference calls were conducted to work out the installation schedule and other construction details such as materials to be used and coordination with the post-tensioning subcontractor. Dixon Construction of Correctionville, Iowa, constructed the bridges for Highway 60 and was responsible for installation of the PPCP approach slabs.
The Highway 60 approach slabs were constructed over a layer of crushed limestone aggregate that was placed over the bridge embankment fill. The base was graded as closely as possible to the required cross slope with a bulldozer, then fine-graded by hand. A tripod-mounted rotating laser with x- and y-axis tilt control was used to fine-grade the base to the proper cross slope and elevation. A portable plate vibratory compactor was used to consolidate the coarse aggregate base to the proper elevation. Because it was important for the precast panels to rest on the paving notch at the bridge abutment, the aggregate base was graded slightly lower than the paving notch. Over the aggregate base, the single layer of 0.15-mm (6 mil) thickness polyethylene sheeting (friction-reducing material) was rolled out just prior to placement of the panels. Figure 27 shows the base-grading operation.
Precast panel installation for the south and north approach slabs took place the weeks of August 28, 2006, and September 4, 2006, respectively. Panels were shipped to the jobsite one panel per truck due to the weight of each panel. Panel installation required between 15 and 30 minutes per panel, depending on how much fine-grading of the base was required to set the panels at the correct elevation.
Prior to installing the panels, neoprene pads, 13 mm (1/2 in.) thick, were placed over the paving notch and against the vertical face of the abutment. The neoprene pads provided more uniform support for the precast panels on the paving notch and will permit some degree of hinging action at the abutment–approach slab interface as the abutment rotates over time. The skewed panels (1A and 1B) were installed first into their final position, aligning the longitudinal joint with the centerline of the roadway. The panels for each section of panels (A and B) were then installed separately. A gap was initially left between each of the panels for applying the joint epoxy and installation of the foam gaskets around the duct openings.
After all four panels of a section were temporarily in place, the post-tensioning strands were fed from the end of the approach slab (panels 4A and 4B) through the panels to the anchors at the abutment end. Epoxy was then applied to the keyways of each joint just prior to the crane moving each panel into its final position. Temporary post-tensioning (using two of the strands) was used to snug the panels together to seat the keyway in the epoxy. Slight tension was maintained on the lifting lines during the temporary post-tensioning process. The sequence was then repeated for the adjacent section of panels. Figures 28 and 29 show the installation of the skewed panels at the bridge abutment and a finished approach slab prior to post-tensioning, respectively.
The epoxy used for the panel joints was a high-viscosity, gel-paste epoxy, suitable for bonding hardened concrete to hardened concrete. The epoxy had a pot life of 45 minutes and a 1-day compressive strength of 62 MPa (9,000 lbf/in2). The epoxy sealed the joints between panels to prevent water from infiltrating the embankment and helped prevent leakage of the tendon grout. The epoxy also helped to compensate for unevenness in the keyway surfaces. By applying a layer of epoxy 3 mm (1/8 in.) thick to the keyways and squeezing the panels together lightly, the epoxy filled any irregularities in the keyway surface, ensuring full contact between panels to eliminate stress concentrations. Figure 30 shows epoxy being applied to a keyway before the panel was pulled into its final position.
Temporary post-tensioning was used to snug adjacent panels together as each was installed. This process helped to seat the keyways together, squeezing out excess epoxy. Two strands, located at approximately the quarter points, were used for temporary post-tensioning. Only enough pressure to pull the panels together was applied so that spalling would not occur if there were any unevenness in the joint. This pressure was maintained long enough for the epoxy to reach an initial set. The temporary post-tensioning was then released, and the next panel was pulled into place. After all four panels were installed, the two temporary post-tensioning strands were tensioned long enough for the epoxy in all of the joints to reach a final set. Figure 31 shows the application of temporary post-tensioning to one panel.
All transverse and longitudinal post-tensioning tendons consisted of single, 15 mm (0.6 in.) in diameter, Grade 270, 7-wire strand. The longitudinal post-tensioning strands were fed through the ducts as the panels were installed to ensure that all strands could be inserted. After all of the panels for each approach slab were installed, the transverse post-tensioning strands were then fed through the panels and across the longitudinal joint. Figures 32 and 33 show the longitudinal and transverse strands being fed through the panels, respectively.
Longitudinal post-tensioning was completed first to ensure that any differential longitudinal movement of the sections on either side of the longitudinal joint had occurred prior to transverse post-tensioning. The longitudinal tendons were stressed only after the approach slab had been anchored to the paving notch to ensure that it did not pull away from the bridge abutment during stressing. Wedges were seated by hand into the post-tensioning anchors from the anchor access pockets. The strands were tensioned to 75 percent of their ultimate strength or approximately 1.4 MPa (203 ksi). Tensioning began with strands near the middle of each post-tensioned section and alternated out to the edges of the slab.
After insertion of the transverse post-tensioning strands, the ducts extending from each of the panels into the longitudinal joint were spliced together to create a continuous duct. The longitudinal joint was then filled prior to tensioning the strands. After the joint had gained adequate strength (approximately 3 to 4 hours after placement), the transverse tendons were partially tensioned to prevent shrinkage cracking in the joint. After the joint had cured for approximately 24 hours, final transverse post-tensioning was applied. Strand tensioning began with the tendons at the bridge abutment and progressed to the end of the approach slab.
Grouting was completed after all post-tensioning had been completed and the pockets (anchor access and instrumentation pockets) and longitudinal joint had been filled. Initially, tendon grouting was started prior to underslab grouting. However, after experiencing significant grout leakage from the tendons, the process was reversed and underslab grouting was completed first.
As discussed previously, the purpose of grouting the post-tensioning tendons is both to provide an additional layer of corrosion protection for the strands and to bond the strands to the concrete so that individual panels can be cut out and removed in the future for repairs or replacement without compromising the prestress of the entire approach slab.
A prepackaged cable grout mixture, specifically formulated for post-tensioning strands, was used for the tendon grout. Grout was pumped from the inlet port at the anchor at one end of the tendon to the port at the anchor at the other end of the tendon. Although some grout leakage was observed when grouting the longitudinal tendons, grout flow from the outlet of each tendon indicated full grouting. No leakage was observed during transverse tendon grouting. Figure 34 shows the tendon grouting operation.
Underslab grouting was used to fill any voids beneath the approach slab. Because a very course crushed stone base was used beneath the panels, it was not possible to fine-grade the base to the point that complete support was provided. The grout mixture used was a standard IADOT underslab grout mixture consisting of Type 1 portland cement, Class C fly ash, and water, according to Section 2539 of the IADOT Standard Specifications.(18) Grout was pumped beneath the approach slab through ports cast into the precast panels for this purpose. The grout was pumped at very low pressure (< 20 kPa [30 lbf/in2]) to reduce the risk of lifting the slab. Grout was pumped until it would not flow anymore (after reaching the maximum pressure) or until it began to flow out of an adjacent grout port. A rod and level was used to monitor any slab lifting during the underslab grouting operation, as shown in figure 35. The polyethylene sheeting beneath the approach slab helped to prevent the grout from bonding the approach slab to the underlying base. While some leakage through the polyethylene sheeting is likely unavoidable, it should not be significant enough to restrain movement of the approach slab.
Finishing and Tie-in
As discussed above, the longitudinal joint was filled after completion of longitudinal post-tensioning, but prior to transverse post-tensioning. A pea gravel concrete mixture was used to fill the longitudinal joint. After placing the material, wet burlap was placed over the joint for curing. Within 3 to 4 hours after filling the joint, the transverse post-tensioning tendons were tensioned to approximately 10 percent of their final load to compress the joint to prevent cracking from shrinkage. Approximately 24 hours after filling the joint, the full and final transverse post-tensioning force was applied. Figure 36 shows the filling operation for the longitudinal joint.
At the same time that the longitudinal joint was filled, the post-tensioning anchor access pockets and the post-tensioning tendon instrumentation pockets were also filled. The same concrete mixture used for the longitudinal joint was also used for the pockets, and the pockets were also covered with wet burlap mats for curing.
Bridge Abutment Anchor
Before the final longitudinal post-tensioning was completed, the approach slab was anchored to the paving notch, as shown in figure 37. A hole 32 mm (1 1/4 in.) in diameter was drilled into the paving notch using a core drill bit that would not damage the paving notch. A grout mixture was then poured into the hole, and the anchor pins were inserted.
After installation of the panels, post-tensioning, and grouting, an IADOT standard EF expansion joint was constructed at the end of the approach slab. Epoxy-coated dowel bars, 38 mm (1 1/2 in.) in diameter, were inserted into the ends of the precast panels, and a flexible foam expansion joint was placed over the dowels to provide a joint 100 mm (4 in.) wide. The cast-in-place pavement was then placed up to the expansion joint, encasing the other end of the dowels, and the expansion joint was sealed. Figure 38 shows the end of the south approach slab prior to inserting the dowels.
The as-constructed smoothness of the approach slabs did not meet IADOT requirements for multilane primary divided highways, and required diamond grinding to achieve an acceptable level of smoothness. This was anticipated, however, as previous demonstration projects also required diamond grinding to provide a high-speed facility level of smoothness. Diamond grinding was also required for the bridge deck between the approach slabs. It is important to note, however, that the level of smoothness prior to diamond grinding was adequate for opening the approach slabs to traffic if necessary.
Both approach slabs were ground across the full width of the slab. Up to 19 mm (3/4 in.) of material was removed from some areas, particularly at the abutment where the top surface of the precast panels was slightly higher than the surface of the bridge deck. Grinding of both approach slabs took approximately 10 hours to complete. Figure 39 shows the diamond-grinding operation for the south approach slab.
Construction Issues and Challenges
Construction of the Highway 60 PPCP approach slabs presented several challenges and revealed several details that could be improved for future projects. Below are some of the key issues that were encountered during the construction process.
Paving Notch Elevation—When the bridge was constructed, the top of the paving notch was constructed 380 mm (15 in.) below the top elevation of the bridge deck, rather than 330 mm (13 in.) per IADOT bridge abutment standards. An additional 76 mm (3 in.) was subsequently added to the paving notch to accommodate the 305-mm (12 in.) thickness of the precast panels. Unfortunately, the thickness of the neoprene pad was not accounted for, and consequently the precast panels at the abutment were 13 mm (1/2 in.) higher than the bridge deck. Although this was corrected later with diamond grinding, careful attention to the paving notch elevation is needed when designing the thickness of the precast panels.
Bridge Skew—The Highway 60 bridge was designed with a 30-degree right ahead skew. However, the actual as-constructed skew was not field-verified prior to fabricating the precast panels. Consequently, the skew angle on the panels did not match exactly the skew angle of the bridge when aligning the centerline edge of the precast panels to the centerline of the roadway. This discrepancy resulted in a larger gap at one end of the joint between the bridge and abutment, as shown in figure 40. While the difference in angle for the Highway 60 project was very small and did not cause a major problem with construction, verifying the as-constructed skew angle prior to panel fabrication is critical. Also, because it is impractical to achieve a perfect match of the skew angle, a plan for accommodating a joint gap should also be considered. For the Highway 60 bridge, the additional joint width was sealed with bituminous material.
Joint Spalling—The only distresses of significance that occurred during panel installation were two shallow spalls that occurred on the top surface of two of the panels on the south approach slab. These spalls occurred when the temporary post-tensioning was applied and were likely the result of slight unevenness in the top face of the keyway at the ends of these two panels. Figure 41 shows one of the spalls. Fortunately, these shallow spalls were mostly removed by the diamond-grinding process.
Transverse Duct Alignment—To align the precast panels with the centerline of the road and with the bridge abutment, adjacent sections of precast panels were offset, resulting in slight misalignment of the transverse post-tensioning ducts, as shown in figure 42. While the amount of misalignment did not cause any problems with the transverse post-tensioning system, the use of flat, multistrand ducts should be considered for future projects, since these ducts would permit up to 50 mm (2 in.) of misalignment if needed. Flat ducts will require special adapters, however, to transition the flat multistrand duct to a monostrand anchor.
Grout Leakage—As discussed above, significant grout leakage from the longitudinal post-tensioning ducts was observed during grouting of the tendons of the south approach slab. This indicates that an adequate seal was not provided at some of the transverse joints, despite the use of both foam gaskets and epoxy around the ducts. While grout leakage was substantially reduced after underslab grouting had been completed, efforts should be made to reduce grout leakage for future projects. Wider and thicker gaskets will likely help reduce leakage, as would positive duct connections.
Post-Tensioning Anchor Recess—A conical shaped recess is normally formed behind post-tensioning anchors and subsequently patched with a dry-pack concrete mortar after stressing to protect the anchor and seal it from grout leakage. This recess was not provided behind anchors for the Highway 60 approach slab panels, and consequently grout leaked from the anchors during tendon grouting. Because a bonded post-tensioning system was used, corrosion protection for the anchors is not so critical, but providing these recesses would have prevented grout leakage.
Instrumentation and Monitoring
To monitor the behavior of the PPCP approach slabs and the potential effects they have on the behavior of the bridge, both the bridge and south approach slab were instrumented by Iowa State University through a separate effort sponsored by the Iowa Highway Research Board. To compare the behavior of the PPCP approach slab with that of a typical cast-in-place approach slab, the south approach slab on the southbound Highway 60 bridge was also instrumented. While the details of the instrumentation and monitoring can be found elsewhere,(8) the following is a brief summary of the instrumentation.
The behavior of the northbound bridge was monitored through a series of strain gages and displacement transducers. Strain gages were mounted at the ends and midpoints of the bottom flange of three of the bridge girders of the south end span, as well as to three of the H-piles beneath the south abutment. Tilt meters were mounted at either end of the south abutment to monitor abutment rotation, and displacement transducers were attached to the either end of the south abutment to monitor longitudinal and transverse abutment movement. A total of 35 sensors were mounted to the northbound bridge.
Approach Slab Instrumentation
Behavior of the PPCP approach slab was monitored through crack meters mounted to the precast panels to measure joint movement between panels and between the abutment and the approach slab. Concrete strain was monitored using vibrating wire strain gages embedded in each of the precast panels of the south abutment (figure 43). Additionally, strains in the post-tensioning strands were monitored using strand meters mounted to selected longitudinal and transverse tendons after grouting had been completed. A total of 33 sensors were used to monitor the behavior of the PPCP approach slab.
To provide IADOT and other State highway agencies, contractors, and industry representatives with a better idea of the PPCP bridge approach slab application, IADOT and FHWA sponsored a showcasing workshop that was held during construction of the Highway 60 approach slabs. The workshop was held on August 31, 2006, and featured presentations by those involved with each aspect of the project and a visit to the jobsite during installation of the panels for the south approach slab. Just over 40 attendees were present at the workshop from IADOT, industry, and other transportation agencies. Figures 44 and 45 show the presentation and site visit portions of the workshop (the agenda appears in appendix B).