Slipform paving is defined as a process used to consolidate, form into geometric shape and surface finish a PCC mass by pulling the forms continuously through and surrounding the plastic concrete mass.
Slipform paving is most appropriate for larger jobs that require high production rates. Particular advantages of slipform paving are (ACPA, 1995):
- Uses low-slump PCC. Low-slump PCC (on the order of 0 – 75 mm (0 – 3 inches)) is necessary so that the fresh PCC is able to hold its shape once the slipform paver has passed. Low slump PCC can be made with less water and usually has higher compression and flexural strengths than comparable high slump mixes.
- High productivity. Large jobs generally require high production rates in order to be profitable. Slipform paving production rates are typically in the range of 65 – 100 m3/hr (85 – 131 yd3/hr) for mainline paving. That translates into between 70 – 90 m/hr (230 – 300 ft./hr) of 3.66 m (12 ft.) wide, 250 mm (10 inch) thick PCC surface course.
- Smooth riding surface. Automation and computer control allow slipform pavers to produce very smooth riding surfaces (IRI on the order of 0.90 m/km or less).
This section presents PCC placement, consolidation, finishing and curing as it is typically done in slipform paving. Most often, these steps are accomplished by three pieces of equipment: the placer/spreader (used for rough placement), the concrete paver (used for final placement, consolidation and initial finishing), and the texturing and curing machine. These machines usually travel together in series down the length of the project.
Although not always used, placer/spreaders are quite common. They place a metered supply of PCC in front of the paver using a series of conveyor belts, augers, plows and strikeoff devices (see Figures 1 and 2). Using a placer/spreader allows the contractor to receive material from transport vehicles and place a uniform amount of PCC in front of the entire paver width, while minimizing segregation.
The paver usually performs screeding, consolidation and initial finishing. A typical track-mounted, self-propelled paver operates at speeds between 1 and 2.5 m/minute (3 and 8 ft./min) (ACPA, 1995). Some pavers are equipped to place reinforcing steel (if needed), dowel bars and tie rods as well. Figure 4 shows the basic slipform paving process as it occurs underneath the paver. First, an auger spreads the PCC in front of the strike off plate. Second, the strike off plate (screed) removes excess portions of the auger-placed PCC and brings the slab near its final elevation. Third, the PCC is consolidated by a group of vibrators. Fourth, a tamper (typically operating between 0 and 150 strokes per minute), if present, pushes large aggregate particles below the slab surface. Finally, the profile pans level off the slab at the right elevation and provide initial finishing. The remainder of this section describes this process in more detail.
Figure 4. Typical slipform paver operation schematic.
Slipform pavers first use an auger to perform any final material spreading and then strike off the PCC at the correct elevation using a simple strike off plate, or screed.
After screeding, the paver consolidates the fresh PCC using a series of vibrators (Figure 5). Typically, the most effective vibrator position is after the strike-off mechanism and at the final slab elevation. Depending upon mix design and slab depth, vibrators are usually set in the 7,000 – 9,000 vibrations per minute (VPM) range. Vibrators are positioned next to one another such that their influence zones overlap by about 50 – 75 mm (2 – 3 inches) at normal paver speed (ACPA, 1995). Gaps between the influence zones (caused by incorrect vibrator settings or excessively fast paver operation) can cause segregation (ACPA, 1995). Most pavers use fully adjustable vibrator spacing to account for different conditions and mix types, while still providing adequate influence zone overlap.
Initial finishing is accomplished by extruding the PCC mass through a moving form made up of the base course (bottom), the side forms (vertical edges of the paver) and the profile pan (flat paver pieces mounted behind the vibrator) (Figure 6). Extruding PCC through the resulting rectangular shape provides the final slab dimensions and also serves to imbed larger aggregate particles below the surface, which results in a smooth finish. Some pavers are also equipped with a hydraulic tamper bar (sometimes called a “jitterbug”), located just behind the vibrators. By moving up and down, the tamper bar is thought to (ACPA, 1995):
- Assist in consolidation and finishing by tamping large aggregate particles below the slab surface.
- Keep the large aggregate moving in an area where it may have tendencies to stop or stick.
- Keep the material moving around the vibrators so as not to collect and cause flow problems.
However, a tamper may not be necessary on many jobs. Although it forces the coarse aggregate away from the surface, making finishing easier, it can also creates a mortar-rich surface layer which could scale or craze (USACE, 1995). Usually, a tamper is not necessary with a well designed PCC mixture, however, it may be helpful when finishing a harsh, low-slump mixture.
Additional finishing, when needed, occurs just behind the profile pan and is usually accomplished using simple floats (Figure 7). Microtexturing is usually accomplished by dragging a section of burlap (Figure 8) or artificial turf (Figure 9) behind the paver.
Texturing and Curing Machine
The texturing and curing machine follows the paver and is used to impart macrotexture (usually by dragging a tined instrument across the fresh pavement – Figure 10) and apply a curing membrane over the pavement. Sometimes the paver is equipped with a tining machine, while a separate machine is used for applying the curing membrane. Although it used to be quite common, slipformed PCC pavement is rarely if ever water cured due to the high material and labor costs. Figures 11 and 12 show curing machines in operation.
Curing is typically done once finishing of an area is complete and the original wet sheen has nearly disappeared. On tined pavements, curing is usually specified to occur in two passes, one forward and one in reverse, to ensure both sides of the texture ridges are coated with curing membrane.
- American Concrete Pavement Association (ACPA). (1995). Construction of Portland Cement Concrete Pavements. National Highway Institute Course No. 13133. AASHTO/FHWA/Industry joint training. Federal Highway Administration, Department of Transportation. Washington, D.C.↵
- U.S. Army Corps of Engineers (USACE). (30 June 1995). Engineering and Design – Evaluation and Repair of Concrete Structures. EM 1110-2-2002. U.S. Army Corps of Engineers. Washington, D.C.↵