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Precast Concrete Panel Systems for Full-Depth Pavement Repairs: Field Trials
Chapter 3 Michigan Field Study
The test sections for the Michigan field study are located along I-675 in Zilwaukee and M-25 in Port Austin. For the existing portland cement concrete (PCC) pavement, cross section structural details, traffic, and number of panels installed are summarized in Table 2. The site was selected in concert with the Michigan Department of Transportation (MDOT's) Bay and Cass City Transportation Service Center (TSC) personnel.
Precast Panel Mixture Design and Fabrication Details
The precast PCC panels were fabricated by the contractor and transported to the project site. The typical PCC mixture design for this study is summarized in Table 3.
The contractor was responsible for documenting the fresh and hardened concrete properties. Typical PCC concrete properties are summarized in Table 4. The average 28-day compressive strength based on 18 specimens was 30 MPa (4,300 lbf/in²).
All panels were 1.8 m (6 ft) long, 3.7 m (12 ft) wide, and 250 mm (10 in.) thick. The precast panels were fitted with three dowel bars 38 mm (1.5 in.) in diameter, 450 mm (18 in.) long, and spaced at 300 mm (12 in.) on center along each wheelpath. Perimeter steel was included (#5 bars) to resist handling and transportation stresses. Steel mesh (10 mm [0.375 in.] in diameter placed at 150-mm [6-in.] intervals and held together with 6-mm [0.25-in.] ties) was placed at the panel mid depth to resist the potential of early-age cracking. The panels were wet-cured for 7 days using wet burlap covers. The 20 precast panels were stockpiled at the ready-mix concrete supplier's yard. Eight panels were installed at the I-675 site, and 12 were installed at the M-25 site. The typical structural details are illustrated in Figure 1.
Figure 2 summarizes the sequence of the fabrication process followed by the contractor. The standard hardware for lifting the slabs is visible in the figure. Figure 2 also illustrates the location and placement of dowel bars and temperature steel.
Figure 1. Structural details of the doweled precast panel.
Figure 2. Fabrication of the doweled precast panels.
Prior to the removal of the candidate panels, a distress survey was conducted in accordance with the protocol laid out in the LTPP distress identification manual (FHWA 2003). Typical distresses observed during the survey included mid-panel transverse cracks with associated spalling and deteriorated joints with spalling and asphalt patch deterioration. Examples of the typical distresses are shown in Figure 3. During the field visit the distress information was recorded on a distress documentation form (a sample form is presented in Appendix A).
Figure 3. Typical distresses.
The sequence of operations for offsite activities and onsite activities for the Michigan field study are listed below:
*Step 8 of the construction process does not exist if the precast panel grade adjustment is done using high-density polyurethane (HDP) foam. In that case, after final cleaning and grade adjustment of the base, the precast panel is placed and portholes for injecting the HDP are drilled. The slab elevation adjustment is achieved by injecting the HDP foam.
Descriptions of the activities listed above are presented in the following section of the report. (The types of distresses addressed by the repair strategy and the panel fabrication process, illustrated in Figure 2, are presented above and will not be discussed further).
Sawcutting Panel Boundaries and Removal
The candidate repair sections were identified and marked by the MDOT personnel (Bay City and Cass City TSC). The slab boundaries were outlined by the contractor and sawcut. The limits of the pavement area to be removed were sawed in the transverse direction. Following the sawcutting operation, the lift hooks were inserted and the distressed slabs were removed using a front-end loader. During this process, the outlines for the dowel slots in the adjacent panels were also cut. This concrete was later carved out using pneumatic jackhammers. Figure 4 illustrates the slab sawing and removal process.
Figure 4. Sawcutting of slab boundaries and slab removal.
Initial Base Preparation
For all the panels the aggregate base was excavated 38–50 mm (1.5–2 in.) below the bottom of the existing slab to accommodate the thicker, 250-mm (10-in.) precast panel. At this point all concrete debris from the slab removal operation was removed. Dewatering of the base was not required at any of the project sites. Figure 5 illustrates the base preparation activity.
Figure 5. Initial cleaning and preparation of the base.
Preparation of Load Transfer Slots
As shown in Figure 6, there are three dowel bars in each wheelpath. The dowel slot cutting and preparation include initial grooving to the required depth with a concrete saw; jackhammering of the concrete to carve out the dowel slot; air cleaning of dowel slot to remove debris and any loose concrete pieces; and sandblasting of the dowel slots. The dowel slots were approximately 100 mm (4 in.) wide and 133 mm (5.25 in.) deep (base of the slot cut). Figure 6 illustrates the slot cutting and preparation process.
Figure 6. Preparation of the load transfer slots.
The dowel slots were placed at 300 mm (12 in.) on center. The slots are 375 mm (15 in.) from the nearest longitudinal edge (shoulder or centerline). Figure 7 shows a schematic cross section of the dowel assembly.
Figure 7. Schematic cross section of the dowel assembly.
Final Grading and Installation of the Precast Panel
This part of the installation process can be achieved by two different methods: slab grade adjustment using HDP foam or slab grade adjustment using flowable fill.
Elevation Adjustment Using High-Density Polyurethane Foam
Once the dowel slots were prepared and a final cleaning and grading of the base prepared the surface for receiving the precast panels, the precast panels were transported from the flat-bed truck to the excavation using a front-end loader. The HDP foam method of slab stabilization was used for 6 panels along the I-675 site and 10 panels along the M-25 site. Approximately 4–6 holes (16 mm [0.625 in.] in diameter) were drilled per panel to inject the foam. The polyurethane foam is made from two liquid chemicals that combine under heat to form a strong, lightweight, foam-like substance. The chemical reaction between the two materials causes the foam to expand and fill the voids. According to the manufacturer's specification, the HDP foam sets in approximately 15 minutes (approximately 90 percent of full compressive strength), and the precast panel is ready to carry load. For the purpose of slab stabilization, the foam density is about 64 kg/m3 (4 lbs/ft³) with a compressive strength range of 414 to 1,000 kPa (60 to 145 lbf/in²).
Once the slab elevations were verified and deemed acceptable, the dowel slots were grouted and the joints were sealed. Figure 8 illustrates the slab installation process.
Figure 8. Precast panel installation and stabilization using high-density polyurethane foam.
Elevation Adjustment Using Flowable Fill
Two precast panels were stabilized at each of the test sites using flowable fill. The excavation was 38–50 mm (1.5–2 in.) below the bottom of the existing slab to accommodate the thicker 250-mm (10-in.) precast panel. The flowable fill was transported to the project site in a ready-mix concrete truck and discharged directly into the excavation. Figure 9 illustrates the flowable placement and the backfilling of the dowel slots. The fill was leveled to a depth of 250 mm (10 in.) from the surface of the existing slab. The flowable fill mixture design includes 1,020.6 kg (2,250 lb) of sand, 56.7 kg (125 lb) of cement, 136.1 kg (300 lb) of water, and 118 ml (4 fl oz) of air entraining admixture. The average 28-day compressive strength ranged from 1.0 to 1.2 MPa (150–175 lbf/in²). Once the slab elevations were verified and deemed acceptable, the dowel slots were grouted and the joints were sealed.
After the slab was installed and leveled, the dowel slots were backfilled and transverse joints were sealed.
Figure 9. Flowable fill operation and dowel slot backfilling.
The construction productivity metrics include the documentation of time to complete the installation of one panel, list of possible concurrent activities, various equipment used for the installation of the panels, and panel installation crew size. The detailed panel installation activities include the following:
Typical individual times, labor requirements, and equipment needed to execute the activities listed above are summarized in Table 5. Based on the proximity of the candidate panels to each other, construction activities A2–A7 can be performed somewhat concurrently, resulting in increased productivity.
For the I-675 project, all nine panels were installed in 1 day under the same traffic control due to the close proximity of the panels. Figure 10 illustrates the relative location of the precast panels. Figure 11 illustrates the timeline (typical) for the installation of panel 5.
Figure 10. Relative distances of patches along the I-675 project.
Figure 11. Construction timeline for panel 5.
The total time required to install the panel was less than 120 minutes. The two most time-consuming activities were the preparation of the dowel slots and adjustment of the panel elevation with respect to the existing concrete pavement. The dowel slot-cutting time can be shortened by reducing the dimensions of the slots.
Patch 1 in this project had to be converted to a conventional cast-in-place, full-depth repair because the pavement at this location was superelevated and the precast panel dimensions were such that the resulting joint openings would have been unacceptable. In the future, such issues can be resolved if the contractor makes exact measurements of the panels to be replaced. This measurement data can then be used during the manufacturing of the panels. Therefore, this panel will serve as the control. The performance of the precast panels (2 through 9) will be compared with that of panel 1. Also, panels 5 and 6 were stabilized using flowable fill, whereas as the other precast panels were stabilized using the HDP foam. The impact of these two methods of slab stabilization on panel performance will be monitored and evaluated over the next 2 years as part of this study.
At the M-25 project site, 12 precast panels were installed over a period of 3 days. Contributing to the longer installation time were the distances between some of the panels and the construction interruptions due to rain. Figure 12 illustrates the relative location of the precast panels.
Figure 12. Relative distances of patches along the M-25 project.
Figure 13 illustrates the contruction timeline for panels 2 and 3. Due to the close proximity of these panels, some of the construction activities overlap. Panels 2 and 3 were stabilized using flowable fill, and the remaining panels were stabilized using HDP foam.
Figure 13. Construction timeline for patches 2 and 3.
Repair Effectiveness of the Precast Panels
The repair effectiveness of the panels was determined by conducting distress surveys and falling-weight deflectometer (FWD) tests according to the protocol illustrated in Figure 14. Test locations 1 through 8 allowed for the computation of load transfer efficiency (LTE) across the approach and leave joints. The LTE was computed using the following equation, where LTE:
Figure 14. Falling-weight deflectometer test locations to evaluate panel effectiveness.
Field evaluations of the precast panels were conducted in October 2003, May 2004, October 2004, and May 2005.
Field Evaluation Results
I-675 Test Site
The FWD results are presented in Figure 15, in charts (a) through (c). The dashed lines in the charts represent the minimum LTE threshold of 70 percent and a deflection ratio threshold of 3 for doweled joints. On average, the approach and leave LTEs are in excess of 70 percent (the average LTEs range from 61 percent to 90 percent) as shown in Figure 15 (a) and (b) and in Table 6. Figure 15 (c) represents the relative deflections (peak) of the joint with respect to the mid-slab deflection (peak). The approach joint deflection ratios range from 1.2 to 2.1, whereas, the leave joint ratios range from 1.3 to 2.3, indicating an acceptable support under the panel.
Table 7 displays and summarizes the condition of the precast panels as of September 2005. The performance evaluation was conducted by MDOT. Panel 2 exhibited premature cracking along the leave joint; the remaining seven panels exhibited acceptable behavior at the time of the last performance evaluation.
For panel 2, the average before-leave joint LTE (%) ranges from 63 to 75 whereas the after-leave joint LTE (%) ranges from 68 to 91. The type of distress observed in panel 2 looks like failure at the end of the dowels due to thermal movement of the pavement. The hot pour joint sealant is pushed up in the joint, indicating the pavement closing up on the leave side of the joint. Uniform restraint was not provided in the reservoir opening between the dowels, which may have resulted in high bearing stresses at the dowel ends. Another reason for this premature joint failure could be that the dowels along the leave joint were horizontally skewed as a result of the installation. This skewing could have resulted in "locking" of the joint and impeding joint movement under environmental and traffic loads.
Figure 15. Average load transfer efficiencies and deflection ratios for the I-675 test site.
a) Leave slab loading.
b) Approach slab loading.
c) Deflection ratio.
M-25 Test Site
The FWD results are presented in Figure 16 (a) through (c). The dashed lines represent the minimum LTE threshold of 70 percent and a deflection ratio threshold of 3 for doweled joints. On average, the approach and leave LTEs are in excess of 70 percent (the average LTEs range from 72 percent to 90 percent) as shown in Figure 16 (a) and (b) and Table 8. Figure 16 (c) represents the relative deflections (peak) of the joint with respect to the mid-slab deflection (peak). The approach joint deflection ratios range from 1.2 to 2.3, whereas the leave joint ratios range from 1.0 to 3.0, indicating an acceptable support under the panel.
Figure 16. Average load transfer efficiencies and deflection ratios for the M-25 test site.
a) Leave slab loading.
b) Approach slab loading.
c) Deflection ratio.
Table 9 summarizes (by way of photographs) the condition of the precast panels in September 2005. The performance evaluation was conducted by MDOT. Of the 12 panels, 10 exhibited acceptable to good behavior at the time of the last performance evaluation.
Figure 17 summarizes the structural response of the leave and approach joints for panels 4 and 9. It is evident that for both panels there was a significant drop in leave joint efficiencies from October 2003 and May 2004. The plot also shows that there is also a significant loss in relative support along the distressed joint during the same time frame. During the summer of 2004 the thumb area of Michigan experienced a series of 32 °C (90 °F) days that may have resulted in "abnormal" expansion of the pavement slabs. Such expansion could have resulted in a joint blowout. A possible contributor to joint blowout is horizontal misalignment of dowel bars. If such misalignment occurred during installation of the panels, the joint may not have been flexible enough to accommodate slab expansion caused by the high ambient temperatures. Figure 18 illustrates how dowel misalignment could reduce joint flexibility.
Figure 17. Structural responce of panels 4 and 9.
a) Leave joint.
b) Approach joint.
Figure 18. Line sketch illustrating the possible dowel skew and uneven joint opening.
The finite element model EverFE (Davids 2003) was used to approximate the effects of horizontal skew and high ambient temperatures on the tensile stresses on the precast concrete panel and vertical shear stresses in the dowel bars. Three variables were included in this limited analysis:
Figure 19 summarizes the findings of the analysis.
Figure 19. Results of the finite element analysis.
a) Effect of temperature differential on concrete tensile stresses.
b) Effect of horizontal dowel skewness on dowel vertical shear force.
Recommendations for Precast Panel Installation
Based on the experience of the Michigan field trials, the following practices are recommended for future precast panel installations for full-depth repair of jointed concrete pavements: