|FHWA > Engineering > Pavements > Concrete > FHWA-IF-07-034 > Chapter 4|
Construction of the Iowa Highway 60 Precast Prestressed Concrete Pavement Bridge Approach Slab Demonstration Project
Chapter 4. Design
The Highway 60 site conditions dictated many of the approach slab characteristics, such as the overall slab length, width, and thickness. The design of the precast panels was therefore primarily used to determine the prestress requirements for the given slab characteristics. While design considerations for PPCP are discussed more thoroughly elsewhere,(1,2) below are some of the design considerations specifically related to bridge approach slab applications.
Pavements are normally designed such that they will withstand a given number of 80-kN (18 kip) equivalent single-axle load applications (ESALs) over the life of the pavement.(15) Bridge approach slabs, however, present a unique design challenge in that a significant loss of support can be expected if voids form beneath pavement due to embankment consolidation or erosion. Traditional pavement design does not take such a significant loss of support into account. Therefore, the basis for the design of the Highway 60 project was to treat the approach slabs as a simply supported slab bridge spanning over a void in the underlying embankment extending away from the bridge abutment. Using this procedure, flexural stresses in the approach slab were determined and prestress levels were adjusted to ensure the slab had adequate flexural capacity.
Traffic loading for traditional pavement design procedures is quantified by an estimate of the number of 80 kN (18-kip) ESALs the pavement will experience over its design life. However, because the Highway 60 approach slab was designed as a simply supported slab bridge, the traffic loading normally used for bridge design was used for calculation of flexural stresses in the approach slab. As per IADOT standard practice, HS 20 loading was used for the traffic loading on the approach slab.(16)
Bridge and Approach Slab Movement
Integral bridge abutments are designed to move horizontally (and rotate) with the expansion and contraction of the bridge itself. By tying the approach slab to the abutment, this movement must be accommodated at the expansion joint at the end of the approach slab. In addition, movement of the approach slab itself, which will expand and contract with daily and seasonal temperature cycles, must also be accommodated.
The polyethylene friction-reducing material beneath the approach slab will help to reduce frictional restraint to movement of the approach slab, helping to reduce stresses in the approach slab and bridge structure. It is important that the connection between the bridge abutment and approach slab is strong enough to withstand the forces from the abutment “pushing” and “pulling” the approach slab.
Highway 60 PPCP Design
The design procedure for the Highway 60 approach slabs was based on determining the prestress required to give the approach slab the flexural capacity to act as a simple span slab bridge for a given span length. For the initial design, a span length of 4.6 m (15 ft) was used as voids up to this length were observed beneath existing approach slabs in Iowa in a recent study.(13)
Traffic loading on the approach slab was based on HS 20 loading according to the American Association of State Highway and Transportation Officials’ (AASHTO) Standard Specifications for Highway Bridges.(16) Using this traffic loading, design load moments were calculated for the following load combination:
Live load moment was calculated using the estimate provided in section 18.104.22.168 of the AASHTO specifications,(16) assuming a span length of 4.6 m (15 ft). Dead load moment was calculated for the self-weight of the approach slab with 305 mm (12 in.) thickness assuming a concrete unit weight of 2,403 kg/m3 (150 lb/ft3 ). Table 1 summarizes the moments used for design for a 4.6-m (15 ft) simple span approach slab.
Initial Flexural Design
The initial flexural design of the approach slab assumed a 4.6-m (15 ft) simple span slab bridge with 305-mm (12 in.) slab thickness. Initially, the prestressing tendons were assumed to be at mid-depth of the slab. Prestress levels in the slab were adjusted by varying the spacing of the prestressing tendons, the depth of the tendons, and the type of prestressing material (7-wire strand and high-strength bars).
The AASHTO Load Factor Design method for flexure was used to compute the ultimate moment capacity of the simply supported approach slab. The following equations from section 9.17.2 of the AASHTO specifications were used to compute flexural strength.(16) Although mild steel reinforcement was included in the precast panels, the initial design only considered the prestressed reinforcement in carrying tensile stresses since mild steel reinforcement will not be continuous through the approach slab (between precast panels).
A*s = Area of prestressing steel
Variations of both prestressing strand configurations and high-strength prestressing bars were used for the capacity analysis. Table 2 summarizes the flexural capacity of the approach slab with these varying configurations. Any of these configurations will provide the necessary flexural capacity for a 4.6-m (15 ft) simply supported approach slab.
Final Flexural Design
While the prestressing configurations presented above will provide the necessary flexural capacity for a 4.6-m (15 ft) simply supported approach slab (using only prestressing steel to carry tensile stresses), the prestressing required is significantly more than that used for previous projects, and IADOT believed it to be excessive for this application. Therefore, a standard 610-mm (24 in.) monostrand tendon spacing, using 15-mm (0.6 in.) Grade 270 7-wire strand, was specified for the final prestressing configuration.
Using this standard spacing of 610 mm (24 in.), the allowable span length was back-calculated based on the flexural capacity of the slab. The calculated allowable span length considering just the prestressing steel for carrying tensile stresses is approximately 1.9 m (6.1 ft). When also considering the contribution of the mild steel in the bottom of the precast panels (25-mm [No. 8], Grade 60 reinforcing bars at 305 mm [12 in.] on center in the longitudinal direction) for carrying tensile stresses, the spanning ability of the approach slab increases to approximately 5.7 m (18.8 ft). The mild steel reinforcement in the precast panels may or may not contribute to flexural capacity since it is not continuous through the approach slab (i.e., it is isolated in each individual precast panel). If a void were to form directly beneath one of the individual precast panels, the mild reinforcement would likely give the approach slab this additional flexural capacity; but if a void formed beneath multiple panels, the reinforcement may not provide this additional capacity.
It should be noted that designing an approach slab for flexural capacity is believed to be a very conservative approach as voids as large as 4.6 m (15 ft), while they have been observed, are likely very rare. Additionally, failure of an approach slab due to exceeding the flexural capacity would not likely have catastrophic consequences on the safety of the motoring public, as it could on a bridge.
Transverse prestress was specified the same as the longitudinal prestress, with monostrand tendons spaced at 610 mm (24 in.) on center over the length of the approach slab. However, because the transverse post-tensioning tendons follow the contour of the crowned pavement cross section (figure 8), there was the potential for the prestress force to cause uplift of the precast panels during stressing, hinging about the longitudinal joint. A calculation of these uplift forces, which are resisted by both the weight of the precast panels and the horizontal component of the prestressing force, revealed that the total uplift force was only 32 percent of the resistance to uplift, and therefore would not present a problem.
Slab Movement Analysis
Normally, the expansion and contraction movement of a precast post-tensioned pavement slab is a governing factor in determining how long each post-tensioned section of panels should be. For the Highway 60 approach slab, however, the length of the approach slab was predetermined, and the slab movement analysis was only used to ensure that the expansion joint at the end of the approach slab was adequate. By tying the approach slabs to the integral abutments of the bridge, the approach slabs will be “pushed” and “pulled” by the abutments with movement of the bridge itself. IADOT estimated the total movement of each end of the bridge to be approximately 33 mm (1.3 in.).
Expansion and contraction movements of the approach slab itself were calculated using a methodology originally developed for cast-in-place post-tensioned pavement.(11,17) This methodology takes into account the slab geometry (length, width, and thickness), concrete properties (modulus of elasticity, coefficient of thermal expansion, creep and shrinkage), prestress (and prestress losses), slab–base frictional resistance, and local temperatures for summer and winter conditions. Both long-term (seasonal) movements as well as short-term (daily) movements of the slab are taken into account. Table 3 shows the results of the slab movement analysis for the Highway 60 bridge approach slabs. The values shown in this table represent the maximum anticipated movement of the free end of the approach slabs at the centerline and short and long edges of the approach slabs. This movement is additive to the 33 mm (1.3 in.) anticipated from bridge movement. The expansion joint at the free end of the approach slab, therefore, should be able to accommodate the “Total Movement” of up to 44 mm (1.74 in.) shown in Table 3.