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Bridge Substructure Systems 14

2.3.1 Precast Abutments

Precast abutments can be beneficial to rapid construction projects. One drawback to using precast abutments is connecting the abutment to the deck. If the abutment is entirely precast, an expansion joint has to be placed between the deck and the abutment. Expansion joints tend to reduce the lifespan of bridges, and integral abutments are typically preferred. Even if an integral abutment is used, precast elements can still be used for the wingwalls to reduce the amount of formwork and CIP concrete (Tokerud, 1979). A closure pour between the precast elements and the abutment will be required to achieve an integral abutment.

2.3.2 Precast Concrete Piers

Precast concrete pier systems use precast piles and a precast pier cap to create a pier. Precast piers are compatible with a number of different foundation and substructure types. For larger bridges, precast pier caps, also called bent caps or cap-beams, may be necessary. A picture of a bridge in Nebraska using precast girders and precast pier caps is shown in Figure 8. Pier caps allow twin-span bridges to use the same piers for foundations. Precast, prestressed or post-

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tensioned caps can significantly reduce the cap size, which reduces the weight of the

substructure. For extremely large bridges, pier caps may be fabricated and transported in two pieces and post-tensioned together on site (Billington et al., 1999).

Figure 8. Bridge pier cap located Lincoln, Nebraska

Individual components of the pier are connected with mild reinforcing steel splices or post- tensioning. Two types of connections are typically used: grouted joints and match-cast joints. Match-casting uses the joint face of a previously cast component as part of the formwork for the adjacent component. This results in a “perfect” fit between the two components and reduced on- site construction time, when the elements are delivered in the correct order. The drawback to this procedure is that the fabrication process is often more time and labor intensive, which can be more expensive and if the elements are placed in a different order than which they were

fabricated, the segments may not fit (Hieber et al., 2005).

There are several advantage to using precast concrete piers besides the time and labor savings in the field. As with all precast elements, fabrication of all the elements is done off-site, which allows the precast elements to be fabricated before construction at the bridge site begins.

Elements can then be stockpiled until they are needed in the field. Because precast elements are typically fabricated in a climate-controlled environment, the quality control for the material and construction of precast elements is higher than the quality control that can be maintained though all kinds of weather in the field. This results in higher durability of the precast elements

(Billington et al., 2001).

Precast elements also have the advantage of using high performance concrete, which typically increases the strength and durability of precast elements. The use of high-performance concrete also results in more slender substructure designs, as can be seen in Figure 9, which also results in significant material savings for a project (Billington et al., 2001).

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There are several key issues to consider with precast concrete piers. The first issue is the connection between the footing and the column, which will depend on the type of footing, precast or cast-in-place. There are several different types of connections available, however, the most appropriate connection will depend on project-specific conditions (Hieber et al., 2005).

Figure 9. Slender pier design of the Florida Turnpike in Miami, Florida

The next issue is the connection between the column elements. Match-cast joints are typically preferred because of problems that may occur with grouted joints. Grouted joints may lack a uniform bearing surface, which can cause edge crushing. Poor grout placement or grout quality may result in partially filled joints, stress concentrations, cracking, and corrosion of the

reinforcing steel. Shear keys can be added to increase the shear capacity of the joint. The epoxy used in the joint may also provide extra shear capacity, but typically is not designed to do so (Hieber et al., 2005).

Another issue to consider with precast pier systems is the connection between the column and pier cap or abutment. There are two types of connections used: post-tensioning bars or strands, or mild reinforcing bars in grouted ducts. A drawback to the grouted duct connection is the ducts takes up twice as much room as a normal reinforcing bar. The connection region in the column and pier cap is already congested, so fitting the ducts in can be a problem (Hieber et al., 2005). A fourth issue to consider is the connection between the pier cap elements. If the pier cap has to be fabricated and transported in several segments, the segments will have to be connected with grouted, cast-in-place concrete or match-cast connections. The last issue to consider in precast pier systems is the weight and size that can be transported, either of which can govern the size of the precast segment. Local limitations should be researched and considered during design (Hieber et al., 2005).

As with most accelerated construction technologies, there is an increased cost to precast

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fabricator. Some of the cost of precast substructures will reduce as standardized substructures are developed and the contractors involved gain experience (Billington et al., 2005).

Another precast pier system currently under investigation for use in the United States is the Sumitomo system. The Sumitomo precast structure for resisting earthquakes and for rapid construction (SPER) system uses stay-in-place precast concrete panels as formwork and as structural elements. For shorter piers, the segments are stacked on top of each other, epoxied together, and then filled with CIP concrete, creating a solid pier. For taller piers, inner and outer panels are used to create a hollow pier. For both types of piers, cross ties and couplers are used to provide transverse reinforcement. High strength bars are typically used for the transverse reinforcement to reduce congestion between the panels. CIP concrete is typically used to connect the piers to the superstructure (Russell et al., 2005).

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3. LABORATORY CONSTRUCTION AND TESTING

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