In the particular county where I work, precast, prestressed concrete (PPC) bridges make up about 75% of the bridges on the local system. These bridges are simple to construct, cost effective, and very durable. I believe they are a great option for traversing small streams or grade separations. One of the biggest enemies to any concrete bridge is salt. In this part of the country, we do not use much road salt, so the degradation of the concrete is a very small issue. The bridges are inspected every two years, and the deterioration of the beams is always checked. If salt were to decay the concrete causing the exposure and degradation of the prestressing cables, then there would be major issues. In areas where much road salt is used, precast, prestressed concrete beam construction may not be the best option.
For the most part, the construction of the PPC deck beam bridges is very simple. Its pretty much a block of concrete over piling, with precast, prestressed beams on top. There aren’t many pieces to the puzzle. Soil borings are taken to determine the depth of the bedrock that the piling should rest upon. Hydraulic surveys are done, taking into account the design year of the high water elevation. There are a few factors, including average daily traffic, that determine if the flood year to be considered is the 15 year high water, 20 year high water, etc. One hundred year high water elevations are also determined. Keeping these records is vital, as these elevations can be useful in determining the base flood elevation in the future if a floodplain is near the bridge. These type of bridges can also be used for railroad crossings. One of the major design concerns is for seismic activity. Depending on which region the bridge will be in determines the design parameters. In many cases, there is no need to purchase additional right of way, unless the road will be raised to an extent that would require the slopes be rebuilt to safer design criteria, or if the road needs to be realigned.
Once the plans have been approved, and the job has been bid, the construction is pretty straightforward. Any old structure is removed, and piling is driven. In cases where bedrock is shallow, piling may have to be drilled into the rock, or a spread footing wall may be built. The abutments are really just big blocks of concrete. No matter if the bridge is 25 feet long or 80 (about the limit of single-span bridges of this type), the abutment designs are very similar. They are reinforced with steel reinforcement bars. In the cases of multi-span bridges, concrete piers are built. Riprap can then be placed in necessary places before the beams are put on. In areas of high velocity flows, it is important that the riprap be big enough not to wash away. I have seen 150 pound riprap wash as far as 50 feet downstream. After the abutments have cured for a week, the beams are placed on. The first beam is always a nail biter. If the bridge was not laid out properly, this is where it will show. The beams are put on bearing pads, which are really just pieces of neoprene. Most of our bridges are 24′ wide, which may be either 6 four foot wide beams, or 8 three foot wide beams. The beams are connected laterally so they act as one unit. They are also ‘pinned’ into the abutments with large reinforcement bars the are set down into holes drilled into the abutments. At this point any bridge rail may be installed, as well as pouring the last of the concrete which will act as a back wall. The beams have ‘keys’ which will then need to be filled with grout. One thing that we have always done, is place ‘flowable fill’ behind the abutments. This keeps the dirt from settling, and creating a future speed bump. Flowable fill is really nothing more than sand with some cement in it. It goes in like quicksand, and gets hard like concrete. But instead of having a strength of 3000 psi plus, its strength is more like 300 psi. It can easily be dug out if necessary. The road can then be reconstructed with rock, asphalt, or oil and chip, depending on the existing surface. Besides some tidying up and other item such as seeding if necessary, that is about it. Where time constraints have been an issue, we have torn out a bridge and built a new one in two weeks-start to finish-before. The only difference was a fast-setting concrete was used.
For many bridge replacements, in particular on local agency systems, I believe prestressed precast concrete deck beam bridges are a logical and economical choice. When all factors are considered, in areas that are not susceptible to deterioration due to road salt, PPC bridges are a good fit due to their cost, quick construction time, and durability.