|FHWA > Engineering > Pavements > HIF-08-009 > Chapter 4. Design|
Construction of a Precast Prestressed Concrete Pavement Demonstration Project on Interstate 57 Near Sikeston, Missouri
Chapter 4. Design
Several factors must be considered for the design of a precast prestressed concrete pavement. While a detailed discussion of these design considerations can be found elsewhere,(1,2) some of the primary design factors are summarized below.
Traffic loading is one of the governing factors for the thickness design of any pavement. Wheel loads on pavements are live loads that can fatigue pavement materials under repeated application, eventually leading to failure. For this reason, pavements are generally designed to withstand a predicted number of wheel load repetitions over the life of the pavement. In general, wheel load repetitions are quantified in terms of an 80-kN (18 kip) equivalent single-axle load (ESAL). Accurate prediction of the number of ESALs over the life of the pavement is critical to ensuring that the pavement is properly designed, even though exact predictions are seldom realized due to continual changes in traffic volume and uncertainty in actual vehicle weights that the pavement will experience over its life.
Temperature and Moisture Effects
Temperature and moisture are also critical factors for concrete pavement design. Temperature differentials and moisture gradients over the depth of a concrete pavement slab cause it to curl. This curling movement is restrained by the weight of the concrete slab and by dowels at the joints, resulting in stresses in the top and bottom of the slab, depending on the curling condition. Temperature also causes expansion and contraction of pavement slabs. This horizontal movement is resisted by frictional restraint at the pavement-base interface, inducing compressive and tensile stresses in the pavement slab depending on the relative movement. The degree of horizontal movement is dependent upon the length of the slab and the frictional characteristics between the slab and base. For (precast) prestressed pavements with long slabs between expansion joints, this factor is very significant.
As described above, horizontal slab movement (expansion and contraction) is resisted by friction between the bottom of the pavement slab and the surface of the underlying base. The interaction at this slab-support interface essentially consists of four components: friction, interlock, adhesion, and cohesion.(7) For long prestressed pavement slabs, it is essential to minimize restraint between the slab and base to reduce the magnitude of stresses that develop in the slab as a result of horizontal slab movements. Fortunately, precast panels normally have a smooth bottom surface, which helps to reduce friction and interlock between the slab and base. However, a bond-breaking, friction-reducing material, such as polyethylene sheeting, is generally required to further reduce frictional restraint while also preventing adhesion.
Prestress losses are an important design consideration for PPCP. Fortunately, most of the factors are fairly well understood and can be estimated with reasonable accuracy. In general, losses of 15 to 20 percent of the applied prestress force can be expected for a carefully constructed, post-tensioned concrete pavement.(8) These factors contribute to prestress losses:
The design procedure utilized for the Missouri demonstration project is an "equivalent thickness" procedure, wherein the precast pavement section was designed to be equivalent in terms of stresses experienced by the pavement under traffic loading to that of a typical MoDOT portland cement concrete pavement design for the specific site conditions. With this procedure, the precast pavement slab thickness can be selected based on site conditions and construction constraints, and the prestress level is then adjusted such that the stresses in the thinner precast pavement slab will be equivalent to those in the thicker conventional (cast-in-place, non-prestressed) pavement slab. This benefit from prestressing helps to ensure that adequate pavement thickness (or "equivalent thickness") is provided for traffic loading while also providing flexibility with selecting the pavement thickness.
Design for Pavement Stresses
The first step in the design procedure is to use a layered elastic analysis to determine tensile stresses at the extreme bottom fiber of both a conventional pavement slab that would normally be used for the given site conditions and the precast pavement slab. The difference in stress between these two pavement sections is considered to be the compressive stress required in the precast pavement slab from prestress to achieve thickness equivalent to the conventional pavement slab.
The next step is to analyze stresses in the precast prestressed pavement slab independently from traffic loading. These stresses are caused by environmental effects, which result in horizontal expansion, contraction, and slab curling. During this analysis, prestress levels in the slab are adjusted such that the worst-case stress condition never exceeds the compressive stress required from the layered elastic analysis, described above. This analysis is performed using the computer program PSCP2, developed by The University of Texas at Austin specifically for analyzing prestressed concrete pavements. A more detailed description of this program can be found elsewhere.(9,10)
It should be noted that this design procedure essentially only considers worst-case stress conditions. The procedure assumes applications of design wheel loads from the layered elastic analysis under the most extreme stress condition in the prestressed pavement slab, which would occur only once or twice per day. For this reason, the design procedure is considered to be relatively conservative. However, the procedure is based on sound prestressed pavement design procedures using analysis software that has been calibrated with actual performance data. With continual monitoring and collection of long-term performance data, it will eventually be possible to evaluate how conservative this design procedure truly is.
Design for Slab Movement
In addition to pavement stresses, slab movement must also be considered to ensure that the expansion joint is adequate to accommodate expected slab movements. After each section of precast panels is post-tensioned together, the panels essentially act as a continuous slab, expanding and contracting with daily and seasonal temperature changes. If large movements (75-100 mm [3-4 in.]) are expected, an armored expansion joint, similar to those used for bridge decks, may be needed. If smaller movements (less than 25 mm [1 in.]) are expected, plain dowelled expansion joints may be adequate.
The amount of slab movement is dictated by a number of factors including prestress levels, prestress losses over time, concrete material properties (coefficient of thermal expansion, creep, and shrinkage), frictional restraint at the slab-base interface, and climatic conditions experienced by the pavement over time. Fortunately, the PSCP2 design program mentioned previously, takes all of these factors into account in computing the expected slab movement over the life of the pavement. At this stage in the design procedure the slab length (between expansion joints) selected during the preliminary project layout can be adjusted if necessary to meet expansion joint width limitation requirements.
Interstate 57 PPCP Design
Traffic volumes were used to determine the thickness of a conventional jointed concrete pavement that might normally be constructed on I-57. Table 1 shows the traffic information for the northbound lanes of I-57 as provided by the MoDOT Transportation Planning Office.
Based on the recommendations of section 6-03 (Pavement Structure Design) of the MoDOT Project Development Manual(11) for interstate pavements with over 50 million ESALs, this section of I-57 is considered a "Heavy Duty Pavement" and should be designed for 35-year ESALs. Using the American Association of State Highway and Transportation Officials' (AASHTO) recommended design lane traffic distribution for highways with two or more lanes in each direction, the design lane ESALs should be 80 to 100 percent of the total ESALs for the northbound lanes, or between 130,687,000 and 163,359,000 ESALs.
The pavement support structure beneath the precast pavement section was assumed to be the same as that which would normally be constructed beneath a cast-in-place pavement. Figure 6-03.12 of the MoDOT Project Development Manual (11) (shown below in Table 2) requires the use of either a 455-mm (18 in.) rock base or a stabilized permeable base with drainage system on a 100-mm (4 in.) Type 1 base beneath all heavy-duty pavements. Figure 9, below, shows the resilient modulus and Poisson ratio values used for the layered elastic analysis based on recommendations from the 1993 AASHTO Design Guide.(12)
For the slab thickness of a conventional pavement, the table from Figure 6-03.12 of the MoDOT Project Development Manual (Table 2) was used. This table calls for a 380-mm (15 in.) pavement thickness for interstate pavements with more than 100 million ESALs for a 35-year design, when tied shoulders are provided. Although in practice MoDOT rarely constructs pavements of this thickness, a 380-mm (15 in.) slab thickness was used for design comparison purposes.
To determine the properties of the subgrade underlying this section of I-57, MoDOT conducted falling weight deflectometer (FWD) and dynamic cone penetrometer (DCP) testing, and collected soil samples for laboratory testing. Soil sampling revealed silty and sandy-silty soils. The original 100-mm (4 in.), dense-graded, granular base had become homogeneous with the underlying subgrade material. FWD testing revealed resilient modulus values ranging from 55 to 83 MPa (8,000 to 12,000 lbf/in2) when calculated from deflections 455 mm (18 in.) away from the load. Based on these tests, a subgrade resilient modulus of 69 MPa (10,000 lbf/in2) was used for the layered elastic analysis.
Layered Elastic Analysis
Figure 9 shows the pavement structures and loading condition used for the layered elastic analysis. A 380-mm (15 in.) slab thickness was analyzed representing the conventional pavement, and 200-mm (8 in.) and 280-mm (11 in.) slab thicknesses were analyzed for the precast pavement. The 200-mm (8 in.) and 280-mm (11 in.) slab thicknesses represent the precast panel thickness at the edges of the traffic lanes and at the pavement crown, respectively, as determined by the pavement cross section shown in Figure 7. Because the shoulders were integral with the main lanes for the precast pavement panels and tied shoulders would be provided for conventional pavement, only interior slab loading was considered in the analysis. The wheel loads represent the loading from dual wheels on an 89-kN (20,000 lbf) single axle.
Figure 9. Illustration. Layered pavement structure used for elastic layered analysis of slab stresses under traffic loading.
The computer program BISAR (Bitumen Structures Analysis in Roads)(13) was used for the layered elastic analysis. BISAR permits the degree of slip between the pavement and underlying base to be varied anywhere from no slip (full bond) to full (frictionless) slip. Because polyethylene sheeting will be provided as a bond breaker beneath the precast pavement, a full-slip condition was analyzed for the precast (200-mm [8 in.]) pavement slab and a fully bonded condition was assumed for the conventional (380-mm [15 in.]) pavement.
Table 3 shows the results from the layered elastic analysis. The stresses shown are for the bottom fiber tensile stress directly beneath each of the loads. The "Difference" in stresses provided in Table 3 is the compressive stress required in the precast pavement from prestress, which is determined from the PSCP2 Stress Analysis, described below, for both 200-mm (8 in.) and 280-mm (11 in.) thicknesses.
Using the stresses from the layered elastic analysis described above, the next step was to use the PSCP2 computer program to predict stresses in the I-57 precast pavement over the life of the pavement. This analysis was used to adjust the required prestress in the pavement such that the minimum compressive stress at the bottom of the pavement will be at least that shown as the "Difference" in Table 3.
The inputs used for the PSCP2 computer program for this analysis are summarized below in Table 4. The PSCP2 program was used to analyze pavement stresses during summer and winter climatic conditions. The pavement slab temperatures (at mid-depth) and top-bottom slab temperature differentials used for these climatic conditions in the PSCP2 analysis are shown in Table 5. The mid-depth slab temperatures shown are those predicted for the climatic conditions near the project site (Sikeston, Missouri) for typical summer and winter conditions. The ambient temperatures used for prediction are based on the historical highest average high temperature of 33 °C (91°F) for summer months and lowest average low temperature of -4.4 °C (24 °F) for winter months. The following four scenarios were considered in the PSCP2 analysis:
Prestress levels in the pavement slab were increased and decreased by adjusting the spacing of the longitudinal post-tensioning tendons. The critical location for analysis of bottom-fiber stresses was found to be at mid-slab at the 45-year analysis period. The PSCP2 analysis revealed that 760-mm (30 in.) longitudinal strand spacing was adequate for both the 200-mm (8 in.) and 280-mm (11 in.) slab thicknesses. The bottom fiber compressive stresses at the early-age (30-day) and 45-year analysis periods at mid-slab exceeded the "Difference" values from Table 3 for the four climatic analysis scenarios presented above. Although 760-mm (30 in.) strand spacing was adequate, the final tendon spacing was set at 610 mm (24 in.) to "standardize" strand spacing for this and future projects.
Slab Movement Analysis
The PSCP2 program was also used for the slab movement analysis to predict expansion joint widths over the life of the pavement, such that the expansion joints could be designed to be adequate for those movements. The slab movement analysis also revealed what width the expansion joints needed to be set at initially to prevent them from closing completely or opening too wide.
The same inputs, temperatures, and climate scenarios presented above were used for the slab movement analysis. However, for this analysis, only the 610-mm (24 in.) tendon spacing was analyzed, as this was used as the final design. Essentially, only the joints located between adjacent PPCP sections (joints 2, 3, and 4) were analyzed because twice the movement occurs at these joints as at the joints at the ends of the section. The PSCP2 slab movement analysis indicated a maximum joint closure (assuming construction in winter) of 15 mm (0.58 in.) and a maximum opening (assuming construction in the summer) of 45 mm (1.76 in.). Based on this analysis, the expansion joints (and seals) were required to accommodate a maximum "stretch" of 45 mm (1.76 in. ) and maximum "compression" of 15 mm (0.58 in.).
To set the initial joints widths at an optimal width, Table 6 was provided in the Job Special Provisions. The contractor was required to adjust the expansion joints following post-tensioning to these widths, based on the prevailing ambient temperatures during construction.
Transverse Prestress Requirements
Transverse prestress has been found from previous demonstration projects to be dictated by lifting and handling stresses for the precast panels. Because the "slab length" in the transverse direction is relatively short (11.6 m [38 ft]), stresses generated from the combination of wheel loading and slab expansion and contraction are much lower than those resulting from lifting the precast panels. The governing condition for calculation of lifting stresses is during stripping of the precast panels from the forms when concrete strength is the lowest and "suction" forces create higher lifting stresses.
Transverse prestress requirements were calculated based on the flexural stresses in the precast panels during lifting of the precast panels as they were removed from the forms. Estimated locations of the lifting anchors (which were later confirmed by the fabrication plant) were first determined by locating the lifting points where bending stresses would be minimized. Because of the asymmetrical shape of the panel cross section, the lifting points were different distances from the ends of the panels. Bending stresses were then computed (using an additional 1.3 multiplier for the concrete unit weight to account for suction forces), and compared with allowable stresses. Allowable tensile stresses were limited to 3√f'ci, where f'ci is the concrete compressive strength at the time of prestressing, or 1.2 MPa (177 lbf/in2) for f'ci = 24.1 MPa (3,500 lbf/in2), as per ACI 318-95 requirements.(14)
Pretensioning tendons were assumed to be 13-mm (0.5 in.), Grade 270, low-relaxation strands. The tendons were assumed to be tensioned to 80 percent of the ultimate strength of the strand, and a lump-sum loss of 310 MPa (45,000 lbf/in2) was assumed for calculating the total effective prestress in each strand.(15) Based on these criteria and the difference between calculated lifting stresses and permissible lifting stresses, eight pretensioning strands were required for each precast panel. This level of transverse pretensioning provides an effective prestress of approximately 1.1 MPa (160 lbf/in2) at the pavement crown (thickness = 275 mm [10 7/8 in.]), 1.5 MPa (218 lbf/in2) at the lane edges (thickness = 200 mm [8 in.]), and 2.1 MPa (310 lbf/in2) at the outside shoulder edge (thickness = 143 mm [5 5/8 in.]). For the Joint Panels, 12 strands were provided, 6 on each side of the expansion joint, to give them additional rigidity due to the number of pockets contained in these panels.
The overall concept and many of the design details for the I-57 demonstration project were based on the original FHWA feasibility study(1) and adaptations of the details used for previous demonstration projects in Texas(2) and California.(3,4) However, several unique design details were developed and implemented for this project based on job-specific constraints and to simplify certain features of the PPCP system.
Keyways along the joints between individual precast panels are a fundamental feature of the PPCP system, helping to ensure vertical alignment between panels as they are assembled. The keyway dimensions specified for the I-57 demonstration project, which were based on those used for previous demonstration projects, are shown below in Figure 10.
Figure 10. Illustration. Keyway dimensions for I-57 precast panels. (Note: 1 in. = 25.4 mm)
For fabrication and installation purposes, horizontal keyways that are parallel to the bottom of the precast panels are preferred. Therefore, the keyways did not follow the "crowned" shape of the panels, but were kept parallel to the bottom of the panel. The crowned cross section was achieved by varying the depth of the vertical face above the keyways. As discussed previously, due to the thickness of the panels at the edges of the shoulders, it was necessary to discontinue the keyways in the shoulders. The keyways extended a minimum of 0.3 m (1 ft) into each shoulder, however, to ensure that the top surfaces of adjacent precast panels matched in the traffic lanes. Figure 11 shows these discontinuous keyways for precast panels in storage at the fabrication plant.
A flared or trumpeted opening was provided at the end of each post-tensioning duct to accommodate slight misalignment of the ducts on site. For the recessed keyway, a recess around each post-tensioning duct was provided to receive compressible foam gaskets. The compressible foam gaskets were installed prior to setting each panel in place and helped to minimize grout leakage from the post-tensioning ducts during the tendon-grouting operation.
Figure 11. Photo. Discontinuous keyways are shown for the precast panels in storage at the fabrication plant.
The expansion joint is a critical design aspect of the PPCP concept. These joints essentially experience impact loading from traffic since they are typically open wider than conventional pavement joints. For the demonstration project in Texas, a steel-armored joint was required to accommodate joint movement of 75 to 100 mm (3 to 4 in.). For the demonstration project in California, the expansion joint was not expected to open more than 13 mm (0.5 in.), and therefore a plain (nonarmored) dowelled joint was adequate. As described above, up to 50 mm (2 in.) of movement are anticipated for the I-57 demonstration project. While this would normally necessitate the use of an armored joint, diamond grinding of the final pavement surface was anticipated, and there was a concern with diamond grinding over an armored joint.
The solution employed for the I-57 project was a dowelled, header-type, expansion joint, as shown in Figure 12, which is commonly used for bridge joints. The header material at the top of the joint, which can be diamond ground, gives the joint better ability (over a plain concrete joint) to withstand impact from heavy truck loading. This joint detail uses a poured silicon seal with the expansion and compression capacity discussed above. The recesses for the header material were cast into the precast panels and later filled in the field after the expansion joint width had been adjusted. The dowels, spaced at 305 mm (12 in.) on center, provide load transfer across the joint and are epoxy coated for corrosion resistance. Expansion caps are provided at the ends of the dowels to permit movement of the dowels within the joint panels. The dowels were spaced such that they would not coincide with the post-tensioning anchor pockets.
Figure 12. Illustration. Expansion joint detail for the I-57 demonstration project (Note: 1 in. = 25.4 mm).
In an attempt to simplify the PPCP system by reducing the number of different types of precast panels, end stressing was used in lieu of central stressing. This eliminated the central stressing pockets and the central stressing panels. It required, however, special attention to the process for post-tensioning the tendons.
To accommodate end stressing, anchor access pockets were specified such that they were large enough to accommodate the stressing ram and would permit the post-tensioning strands to be fed into the ducts from the pockets. Figure 13 shows the stressing pockets that were developed by the post-tensioning supplier (Dywidag Systems International) to meet these requirements. The intent was to minimize the size of the stressing pockets as much as possible. As such, a pocket that could accommodate a "banana nose" post-tensioning ram was used. The pockets featured a sloped face in the back of the pocket to accommodate the ram. Because of the variable thickness of the precast panels, the dimensions for each pocket were different and required pocket formers to be fabricated for each individual pocket.
Figure 13. Illustration. Typical post-tensioning pocket cast into the joint panels (Note: 1 in. = 25.4 mm).
To transfer the post-tensioning force from the post-tensioning anchors back to the expansion joint opening, stirrups were provided, as shown in Figure 14. These stirrups confined the concrete between the post-tensioning anchors and the expansion joint, transferring prestress force through the joint panels. The stirrups wrapped around the longitudinal reinforcing bars located in front of the post-tensioning anchors as well as the pretensioning strands and reinforcing bars located near the expansion joint. The stirrups (No. 4, Grade 60 reinforcement) were spaced at 100 mm (4 in.) on center across the width of the joint panels, except where post-tensioning blockouts were located. This stirrup detail was used successfully on the previous demonstration project in California.
As discussed in Chapter 2, it was necessary to anchor the center of each post-tensioned section of precast panels to the underlying base so that they will expand and contract outward from the center, ensuring uniform expansion joint widths and preventing the pavement sections from "creeping" in the direction of traffic under traffic loading. For this purpose, Anchor Panels were located at the center of each post-tensioned section of pavement. A series of sleeves, 100 mm (4 in.) in diameter, were cast into these panels to receive dowel pins, which were drilled and grouted into the underlying base/subgrade after installation of the panels, as shown in Figure 15. While this procedure did create another type of precast panel, the anchor sleeves could have simply been drilled into standard base panels on site if necessary.
Figure 14. Photo. Joint panel prior to concrete placement, showing post-tensioning pockets, anchors, stirrups, and dowels.
Figure 15. Illustration. Mid-slab anchor detail (Note: 1 in. = 25.4 mm).