There are two basic methods of building concrete pavement: fixed-form paving and slipform paving. Fixed-form paving requires the use wooden or metal side forms that are set up along the perimeter of the pavement before paving. Slipform paving does not require any steel or wooden forms. A slipform paving machine extrudes the concrete much like a caulking gun extrudes a bead of caulk for sealing windows. In general, slipform paving is preferred by contractors for large paving areas where it can provide better productivity with less labor than fixed-form paving.
All slipform machines use the same principle — extrusion. The manufacturers provide a variety of sizes for everything from municipal curb and gutter to airport work. Some machines are also equipped with automatic finishing equipment and equipment to automatically insert dowel bars into the pavement at transverse joints. These devices are called Dowel Bar Inserters or DBI’s.
While paving, slipform paving machines are equipped with sensors to follow stringlines that are put into position along either side of the paving area. The stringlines control the paver direction and surface elevation. All slipform machines also are equipped with vibrators to help consolidate the concrete and ease the progress of paving by making the concrete more fluid. The vibrators are located toward the front of the machine ahead of its profile pan. The profile pan is the part of the paver that actually extrudes the concrete creating the final shape of the slab.
After the fixed-form or slipform equipment passes, most contractors have crew members use hand-tools to further finish the slab. These operations are called: finishing, floating or straightedging.
The entire set of paving and placing machines and activities is called the paving train. On a highway project the typical paving train consists of a spreader or belt placer, slipform paver, and curing and texturing machine. Smaller paving projects may use only the slipform machine.
Advantages of Slipform over Pre-Cast:
- Less time consuming - one slipform crew can install hundreds to thousands of feet per day
- More cost effective - the installation rate allows costs at 30-40% less than precast
- No precast to order and re-handle on the jobsite several times
- No chasing down that last piece of radius
- Slipforming allows a radius as small as 24"
- Concrete is poured directly on subgrade, minimizing settlement from individual curb placement