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OCR for page 100
100 capacity design procedures can be used in this loading grout at the time of load transfer. Shim stacks should be sta- direction. bilized and prevented from moving during cap setting. A minimum 500-psi strength margin between the expected compressive strength of the precast bent cap concrete and the 4.4 Construction Specifications specified compressive strength of the connection material Based on specifications from Matsumoto et al. (4) and test (grout for grouted duct and concrete fill for cap pocket) specimen fabrication and assembly, the following conclusions should be adopted to help ensure that the connection does for construction specifications of precast bent cap connections not become a weak link in the system. can be drawn: For seismic applications, the maximum thickness of a grouted bedding layer should be limited to 3 in to maintain Construction specifications are expected to help ensure that the overall integrity. For hybrid bent caps, polypropylene precast bent cap connections using precast connections are fibers should be included in the grout matrix to maintain the constructible and also to provide the expected seismic per- overall integrity of the joint at a 3 lb/cu yd fraction. formance, durability, and economy. Grout used in vertical joints of integral bridge systems should Fabrication of precast bent caps using grouted ducts and cap contain a minimum 3 lb/cu yd fraction of polypropylene pockets is feasible and relatively straightforward, facilitated fibers to ensure that the essential joint integrity is maintained by the use of readily available, stay-in-place corrugated during loading. ducts (grouted duct connection) or corrugated steel pipe Although lightweight concrete can provide significant (cap pocket connection). advantages for a precast bent cap system, its use should be Grouting or concreting of a precast bent cap connection based on relevant research, including its effect on the seis- involves procedures, operations, and equipment that may mic performance of the connection. not be familiar to the Contractor, and thus specifications To ensure appropriate mechanical properties, compatibil- include detailed provisions and commentary to ensure ity, constructability, and durability, grout for the grouted connections are made properly in the field. duct connection should be specified as shown in proposed Semi-rigid corrugated metal (steel) ducts specified per Table 8.13.8-1. ASTM A653 should be adopted, based on excellent anchor- Concrete should be sufficiently flowable to fill the pocket age between the column bar, grout, and surrounding con- and bedding layer and to flow out of air vents at the top of crete in grouted duct connections. Plastic ducts should only the bedding layer. be used based on applicable research and when approved by To accommodate fabrication and placement tolerances as the Engineer. well as grouting or concreting operations, a bedding layer Lock seam, helical corrugated steel pipe per ASTM A760, with transverse reinforcement should be used between the using a pipe thickness that provides equivalent CIP joint column top and bent cap soffit. Clear spacing between the hoop reinforcement should be adopted for cap pocket con- transverse reinforcement and the formed surfaces should nections. This pipe can effectively be used as a stay-in-place be at least three times the top size of the aggregate, to ensure form and seismic joint reinforcement, and can also provide adequate flow of grout to fill all voids. In addition, bedding excellent confinement and mechanical interlock, allowing layers greater than 3 in should be reinforced based on pro- column bars to be anchored within lengths comparable to visions established by the owner. CIP connections. Plastic pipe should not be used. Uniform spacing between hoops at the top of the column Special forming is required above and below the steel cor- and the bedding layer is critical to ensure that the system rugated pipe to form the cap pocket void through the full ductility is not compromised. Shop drawings should show depth of the precast bent cap. the intended placement of the first hoop at the top of the col- Fabrication and placement tolerances should be established umn as well as the bedding layer reinforcement. on a job-specific basis and be considered in the establish- Contractors should submit a Precast Bent Cap Placement ment of duct and pipe diameters. Plan, including (1) a description of bent cap placement, Accurate positioning of ducts and column bars may be (2) a description of the hardware and method used to hold achieved using templates and/or supplementary reinforce- the bent cap in position, (3) product information for candi- ment. Guide pipes may be used to facilitate cap setting. date grouts or concrete mixes, (4) a description of hardware Friction collars and shims may be used to support the cap and equipment for grouting or concreting, and (5) the during placement. Compressible shims should be preferred mitigation plan to repair any voids. over steel shims, where possible. Compressible shims such Contractors should submit detailed shop drawings, includ- as engineered multipolymer high-strength plastic should ing (1) the proposed construction sequence; (2) the size and have a modulus of elasticity slightly less than the hardened type of ducts or pipes, supports, tremie tubes, air vents, and