Pervious concrete is a versatile no-fines material that requires unique, but not difficult, construction techniques and tools. If an attempt is made to construct a pervious pavement using conventional concrete placing techniques and procedures, the project will be less than satisfactory.
The “no-slump” characteristic requires direct placement by either tailgating or conveyor belt; pervious concrete cannot be pumped. While pervious concrete is a “no-slump” material, it must be placed with enough water to bind the cement-aggregate matrix together without exuding paste. Long-term serviceability depends upon constructing a sound, compact surface which has been thoroughly cured.
As with any pavement, the first step is to develop a sound base. Being a rigid pavement, pervious concrete does not require a strong base, but it should be uniform in character and non-expansive. Proof-rolled crushed rock provides such a base while also providing additional storm water storage. There are four different compaction techniques which have emerged in common practice placing pervious concrete. Two variations use rollers to compact the surface and two other variations use vibratory plate compactors.
The type of construction method utilized should match the mix design and consistency to ensure the best placements. Roller compaction techniques include cross-rolling with a salt roller, and longitudinal rolling using a larger steel roller. Use of small riding rollers, as used to place asphalt, is feasible but rarely utilized. The salt roller is a solid steel roller perhaps three inches in diameter, polished smooth, and fitted with a long pole.
Fig.: Crossroller
The longitudinal roller is usually made from ten-inch diameter schedule 40 steel pipe, with ends welded on, bearings, an access port for filling with water and handles to allow pulling it along the forms manually.
Fig.:Roller Compaction
Longitudinal rolling is the most common placement technique. Cross-rolling provides less compactive force, and therefore requires a “wetter” mixture, reducing porosity. Roller placements generally provide the flattest surfaces, but are more prone to surface unraveling than plate compacted placements.
Fig.: Plate Compaction
Vibratory plate compactors can be used directly on the pervious surface, or ¾” plywood can be placed on the surface and the compactor run over the plywood. Use of the plywood is recommended if the mixture is a 3/8” pea gravel mix, as direct plate compaction of such mixes can cause over-compaction and reduction of void space. Use of a plate compactor directly on the surface can allow for the use of larger crushed aggregate, such as a ½” X 3/8” crushed rock, which provides the best combination of both void content and strength. Plate compactor placements, especially without plywood use, tend to be wavy, but have little surface unraveling.
The following steps are normally utilized for placing pervious concrete:
· Subgrade should be compacted to 90% to 95% of theoretical density.
o If coarse aggregate is used for base or additional storage, it should be proof-rolled to eliminate any rutting.
Fig.: Base Prep
· Set forms with a ½” to ¾” removable spacer. Use the spacer for cross-rolling or longitudinal rolling techniques. Set the forms ½”-3/4” above desired grade and do not use spacers if using plate compactor techniques.
· Moisten the sub-grade or coarse aggregate base prior to placing concrete. Subgrade or base should be very moist to wet, but with no standing water.
Fig.:Plate Compaction
· Place concrete via tailgating, conveyor, or tailgating into a front-end loader bucket.
o Water content must be tightly controlled to prevent placing concrete that is too dry and will not bond or, too wet and will exude paste into the voids.
o Strike-off to top of spacer using a vibratory screed in order to provide an even surface.
o Conventional screeding is used sometimes, but will produce a torn, uneven surface, and needs extra compaction.
o Add fresh concrete to any low spots visible after screeding.
o Spray the surface with an evaporation retarder such as Con-Film immediately after screeding.
· Remove spacer and compact surface by one of the four methods, each requiring a compactive downward force of 10 psi. Rolling techniques usually require multiple passes to properly compact the surface:
o Cross-rolling with a salt roller
o Longitudinal rolling with a steel pipe roller
o Plate compactor used directly on the fresh pervious concrete.
o Placement of sheets of 3/4" plywood on the fresh pervious concrete and running the plate compactor over the plywood, then hand tamping out any ridges left at the seams of the plywood.
o Plate compactors can bring up paste and seal the surface if left in one spot too long.
· Consolidate form edges with concrete tamp to prevent raveling of concrete at the joint. Radius edging tools work well for this.
· Keep the surface from rapidly drying prior to covering with plastic sheeting.
o Spray the surface with conventional curing compound or evaporation retarder.
o Fog the concrete. This requires a true fogger – not a misting, which can wash the cement paste off the top layer and cause surface unraveling.
Fig.: Plate Compaction3
· Construct joints unless normal drying shrinkage cracks are acceptable.
o Roll joints into the plastic concrete a salt-roller to which a 1-1/2” beveled fin has been welded. Rolling joints is the most common method utilized.
Fig.: Rolled Joints
o Saw cut joints in the hardened concrete using either a dry-cut or wet-cut saw.
o If saws are used, expose only the amount of concrete surface needed to make the sawcut joint.
o Sawcut joints must be made before the concrete develops drying shrinkage cracks.
· Cover the pervious concrete with minimum 5-mil plastic sheeting within 20 minutes of placement to prevent loss of surface water.
o Pervious concrete has a limited amount of free water
o It is essential that this water be conserved to enable cement hydration to occur.
o The 20-minute time limit may be reduced to as little as ten minutes on days of high wind and/or low humidity.
o The plastic sheeting should remain in place for at least 7-days. Making sure that there are no gaps to let air in is critical – just weighing portions down with stakes, 2X4s, etc. is not sufficient.
Pervious concrete can be expected to have a long and satisfactory service life (20-30+ years) if proper construction and maintenance procedures are followed. The landscaping and surfaces draining onto the pervious pavement should be stabilized before the pavement is constructed. Likewise, landscaping soil, plants, and other materials should not be stored on the pervious concrete unless the surface is well protected from intrusion of fine materials. Maintenance generally consists of sweeping and possibly vacuuming the pervious pavement. Many pervious concrete placements have received no maintenance in their twenty-plus year lifetimes.