Pervious concrete is a versatile no-fines material that requires special construction techniques and tools, but they aren’t difficult. The project will be less than satisfactory if it is attempted to construct a pervious pavement utilising normal concrete putting techniques and procedures.
The “no-slump” property necessitates either tailgating or conveyor belt placement; pervious concrete cannot be pumped. Although pervious concrete is a “no-slump” material, it requires enough water to bind the cement-aggregate matrix together without exuding paste. The construction of a sound, compact, and properly cured surface is essential for long-term serviceability.
The first stage, as with any pavement, is to lay down a solid foundation. Pervious concrete does not require a solid base because it is a stiff pavement, but it should be homogeneous in nature and non-expansive. Proof-rolled crushed rock serves as such a foundation while also storing storm water. In the practise of pouring pervious concrete, four alternative compaction techniques have arisen. Two of the variations compact the surface with rollers, while the other two employ vibratory plate compactors.
To ensure the optimum placements, the type of construction method used should fit the mix design and consistency.
Cross-rolling with a salt roller and longitudinal rolling with a larger steel roller are two roller compaction techniques. Small riding rollers, similar to those used to lay asphalt, can be employed, but they are rarely used. The salt roller is a three-inch-diameter solid steel roller with a long pole and a polished smooth surface.
The longitudinal roller is typically made of ten-inch diameter schedule 40 steel pipe with welded-on ends, bearings, an access port for filling with water, and handles to allow it to be manually pulled along the forms.
The most common placing method is longitudinal rolling. Because cross-rolling produces less compactive force, it necessitates a “wetter” mixture, which reduces porosity. Roller placements produce the flattest surfaces, but they are more susceptible to surface unravelling than plate compacted placements.
Vibratory plate compactors can be used directly on pervious surfaces, or 34″ plywood can be laid down on the surface and the compactor run over it. If the mixture is a 3/8″ pea gravel mix, using the plywood is suggested because direct plate compaction of such mixtures might result in over-compaction and a loss in void space. When a plate compactor is used directly on the surface, larger crushed aggregate, such as 12″ X 3/8″ crushed rock, can be used, providing the best combination.
The steps for pouring pervious concrete are usually as follows:
The subgrade should be compacted to a density of 90% to 95% of its theoretical density.
o If coarse aggregate is used as a basis or for extra storage, it should be proof-rolled to avoid rutting.Set forms with a removable 12″ to 34″ spacer. For cross-rolling or longitudinal rolling procedures, use the spacer. If employing plate compactor techniques, place the forms 12″-3/4″ above the desired grade and do not utilise spacers. Before pouring concrete, wet the sub-grade or coarse aggregate base. The subgrade or base should be moist to wet, but not wet, and there should be no standing water.
Fill a front-end loader bucket with concrete by tailgating, conveyor, or tailgating. o The amount of water in the concrete must be carefully regulated to avoid placing concrete that is either too dry to bond or too wet to exude paste into the cavities. o Using a vibratory screed, strike-off to the top of the spacer to give an even surface. o Traditional screeding is occasionally utilised, but it results in a damaged, uneven surface that requires extra compaction. o After screeding, fill in any low spots with fresh concrete. o Immediately after screeding, spray the surface with an evaporation retarder like Con-Film.
Remove the spacer and compact the surface using one of four methods, each needing a 10 psi compactive downward force. Multiple rolling techniques are frequently required.
If left in one area for too long, plate compactors can build up paste and seal the surface. To prevent concrete ravelling at the junction, use a concrete tamp to consolidate form edges. This is where radius edging tools come in handy.
Prevent the surface from drying off too quickly before covering it with plastic sheets.o Apply a traditional curing compound or an evaporation retarder to the surface. o Spray the concrete with fog. This necessitates the use of a genuine fogger rather than a misting system, which could wash the cement paste off the top layer and cause surface unravelling.Fig.
: Plate Compaction 3 Unless normal drying shrinkage fractures are acceptable, construct joints.o Use a salt-roller with a 1-1/2″ bevelled fin to roll joints into the plastic concrete. The most popular way is to roll the joints.
o On days with heavy wind and/or low humidity, the 20-minute time restriction may be reduced to as short as ten minutes. o Keep the plastic covering in place for at least 7 days. It’s crucial to make sure there are no gaps that allow air to get in — simply securing pieces with stakes, 2X4s, and the like isn’t enough.
If suitable construction and maintenance procedures are followed, pervious concrete can be expected to have a long and satisfactory service life (20-30 years). Before the pervious pavement is built, the landscaping and surfaces that drain onto it should be stabilised. Similarly, unless the surface is sufficiently protected from fine particulates, landscaping soil, plants, and other items should not be stored on pervious concrete.
1. Subgrade and Subbase Preparation
The uniformity of subgrade support is an important consideration when installing pervious pavement. Truck ruts and other irregularities must be smoothed and compacted prior to installation, just like any other form of pavement. For more information on how to prepare the subgrade and subbase for pervious concrete pavement performance, see Hydrological Design Considerations and Structural Design Considerations. For constant subgrade support, compaction to a minimum density of 90 percent to 95 percent of theoretical density (per AASHTO T 180) is often suggested; however, increasing the subgrade density reduces its permeability. Local geotechnical experts may be the finest source of information on subgrade soil qualities.
Because pervious pavements retain little water and have a high porosity, it’s important to keep an eye on them so they don’t dry out too quickly. To prevent water from being evacuated too quickly from the lower area of the pavement, the subgrade must be moist (without free-standing water) prior to installation. If conditions for high evaporation rates exist, this is a recommended practise for conventional concrete pavement placement, but it is even more important in pervious concrete placement because the high voids can allow for faster drying, resulting in a decrease in strength and durability under less extreme conditions.
Batching and Mixing
The unique features of pervious concrete necessitate more precise proportioning of the mixture. Pervious concrete’s water content is restricted to a tight range to ensure proper strength and permeability while also preventing the paste from flowing off the particles and closing the open structure. Because of the low paste content, adding water will have a far greater impact than in traditional concrete applications. Both water absorbed by the aggregate and excess moisture supplied with the aggregate should be carefully monitored and accounted for, since both can be harmful. Contractors and manufacturers must collaborate to ensure that the right mix is delivered to the job site.
It may be essential to make minor adjustments to the water content on the job site in order to obtain adequate uniformity; however, this should be done with caution because jobsite water additions can be difficult to control. A blend with the right amount of water will have a sheen to it. A unit weight test is required to ensure that mixture proportions are consistent. Unit weights of 100 lb/ft3 to 125 lb/ft3 (1600 kg/m3 to 2000 kg/m3) are common, and on-site measured values must typically be within +/- 5 lb of the design unit weight.
Variations in material characteristics (for example, cement setting times, strength development rates, aggregate shape, gradation, and density) will limit the usefulness of “cook book” or prescriptive mix designs, so proportions of aggregate and cement will be determined through testing and experience with locally available materials. The mixtures will very definitely be stiff. Although mixing durations may be longer than with normal concrete, conventional concrete mixing equipment is employed.
Because pervious concrete has a low water content, special care must be taken when transporting and installing it. Because of its low slump, it may take longer to discharge from transit mixers than normal concrete; however, transit mixers with large discharge apertures or paving mixers tend to unload faster. After initial mixing, a pervious pavement mixture should be drained fully within one hour. The application of retarding chemical admixtures or hydration-stabilizing admixtures can increase discharge times by up to 112 hours. In comparison to traditional pavements, high ambient temperatures and windy conditions would have more pronounced effects and should be considered.
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