Pervious Concrete

Installation Process

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The pervious concrete installation process consists of four basic steps:

Mixing

Placing

Compaction

Curing

The creation, placement, and curing of concrete are all done on-site, rather than in a factory under uniform conditions. Although pervious concrete can be mixed by the same suppliers and delivered by the same trucks as dense concrete, its unique physical characteristics require a contractor with specialized experience. The structural dissimilarities between pervious and impervious concrete call for different installation processes, neither of which is more difficult or time-consuming than the other. However, the quality and performance of pervious concrete are dependent upon the installer’s familiarity with the constructional impact of its properties. This can be gained only through hands-on experience with the substance, which is not a commonly-used building material in all locales.

To ensure quality control , NRMCA has established a certification program for contractors who place pervious concrete pavement. The contractor certification includes a written exam and components of performance and work experience. This project is funded by the Concrete Advancement Foundation and is implemented in cooperation with state ready mixed concrete associations.

Mixing

Pervious concrete is made up of cement, course aggregates, and water. The mixing process requires tight control of ingredient proportions, more so than that of conventional concrete. Only a narrow range of water content provides sufficient levels of both strength and porosity, so measurements must be exact.

Pervious concrete generally has a void content of 15-25%, with a .35-.45 water to cementitious materials (w/cm) ratio. Since this small proportion of water evaporates quickly, the mixture should be discharged completely within one hour of its initial mixing. This time frame can be extended by 50% or more with the inclusion of moisture-sustaining admixture chemicals in the mix.

Placing

A stone reservoir is installed as a subbase for the pavement, creating large air voids and allowing for further water infiltration and storage. Prior to concrete placement, this layer is compacted to 92-96% of its maximum density, so as to provide maximum support while maintaining permeability. The level of compaction depends on the soil properties, pavement design, and infiltration requirements.

Soil with high clay content has low permeability, requiring a thicker stone reservoir to compensate for decreased water retention. Engineering fabrics are often used to separate the soil and stone layers. The subbase must be kept moist in order to prevent the lower portion of the pavement from drying out too quickly. The longer the drying time, the greater the strength and durability of the concrete.

Because of its thick, rocky consistency, discharge from concrete mixers is slower for pervious concrete than for dense. Since it cannot be pumped, it must flow, usually requiring worker assistance. The pouring of the mixture into the framework should be rapid and continuous. Once it’s in place, a vibrating screed is used to level off the fresh pavement, resulting in a smooth final surface. The vibrations increase the compaction, and therefore density, of the concrete.

Vibrating screeds also allow for the use of extremely thick, low-slump mixtures, which produce maximum strength and durability. Manual screeding methods lack the force necessary to smooth over such thickness.

Compaction

The final compaction or consolidation of the poured concrete mixture is accomplished with a steel-pipe roller immediately after screeding. This step must be completed within fifteen minutes of placement in order to avoid the hardening effects of evaporation. In the placement of conventional concrete, troweling operations are performed after compaction, resulting in a smooth surface finish. However, since this step tends to close off the uppermost voids of porous concrete, it is skipped in pervious installation. This sacrifice leaves the pavement fully permeable, and the rougher surface finish actually improves traction.

    Although pavement-cracking does occur, it is not abundant, since pervious concrete contains minimal amounts of water. When cracks do arise, they do not significantly compromise the pavement, either structurally or aesthetically. However, if the prevention of random pavement-cracking is desirable, control joints can be placed soon after consolidation as a remedy. These can be formed with a rolling joint tool, or less ideally, with sawing equipment at a later time. Since pervious concretes shrink much less than impervious ones while drying, their individual joints tend to be larger in width. Control joints at ¼ the thickness of the slab generally provide good control of cracking, and are installed to line up with the joints of adjoining pavements at recommended intervals of twenty feet.   

Curing

The final installation step is the protection of the pavement by curing. This is essential because the open pore structure and faceted surface of pervious concrete create a high potential for evaporation. Moisture is vital because it acts as the catalyst for the cement’s chemical reactions, which produce the concrete’s strength. The purpose of the curing procedure is to allow the concrete to gradually harden over an extended period of time, building up its maximum strength capacity through steady, controlled evaporation. Curing should begin as soon as possible after the placement, compaction, and jointing of the pavement. The process involves first fog-misting the concrete with a curing compound, and then covering it with plastic sheeting for a minimum of seven days. This maintains the pavement’s moisture, allowing it to induce hardening for an extended period of time.

    Once the curing process is complete, pervious concrete can function well with minimal maintenance. Due to its considerable strength and durability, it requires fewer repairs than most other building materials, such as asphalt. In order to prevent the void structure from becoming clogged with debris, it may be necessary to regularly vacuum or pressure-wash the pavement. The latter method has been shown to restore as much as 80-90% of the concrete’s original permeability.