Pervious concrete uses the same materials as conventional concrete, with the exceptions that the fine aggregate typically is eliminated entirely, and the size distribution (grading) of the coarse aggregate is kept narrow, allowing for relatively little particle packing. This provides the useful hardened properties, but also results in a mix that requires different considerations in mixing, placing, compaction, and curing. The mixture proportions are somewhat less forgiving than conventional concrete mixtures—tight controls on batching of all of the ingredients are necessary to provide the desired results. Often, local concrete producers will be able to best determine the mix proportions for locally available materials based on trial batching and experience.
Cementitious materials
As in traditional concrete, portland cements and blended cements may be used in pervious concrete. In addition, supplementary cementitious materials (SCMs) such as silica fume, fly ash, and ground-granulated blast furnace slag may be used. These influence pervious concrete performance (setting time, rate of strength development, porosity, permeability, etc.). Testing materials beforehand through trial batching is strongly recommended so that properties that are important to performance can be determined.
- Silica fume is a byproduct of silicone production. It consists of superfine spherical particles which significantly increase the strength and durability of concrete. Used frequently for high-rise buildings, it produces concrete that exceeds 20,000 psi compressive strength. Silica fume can replace cement in quantities of 5-12%.
- Fly ash is the waste byproduct of burning coal in electrical power plants; it used to be landfilled, but now a significant amount is used in cement. This material can be used to replace 5-65% of the Portland cement.
- Blast furnace slag is the waste byproduct of steel manufacturing. It imparts added strength and durability to concrete, and can replace 20-70% of the cement in the mix.
Aggregate
Fine aggregate content is limited in pervious concrete, and coarse aggregate is kept to a narrow gradation. Commonly-used gradations of coarse aggregate include ASTM C 33 No. 8 (⅜ in. to No. 16), and No. 89 (⅜ in. to No. 50) sieves [in metric units: No. 67 (19.0 to 4.75 mm), No. 8 (9.5 to 2.36 mm), and No. 89 (9.5 to 1.18 mm)]. ASTM D 448 also may be used for defining gradings. A narrow grading is the important characteristic. Larger aggregates provide a rougher surface. Recent uses for pervious concrete have focused on parking lots, low-traffic pavements, and pedestrian walkways. For these applications, the smallest-sized aggregate feasible is used for aesthetic reasons. Coarse aggregate size 89 (⅜-in. or 9.5-mm top size) has been used extensively for parking lot and pedestrian applications, dating back 20 years or more in Florida.
Both rounded aggregate (gravel) and angular aggregate (crushed stone) have been used to produce pervious concrete. Aggregate for pavements should conform to ASTM D 448, while ASTM C 33 covers aggregates for use in general concrete construction. As in conventional concrete, pervious concrete requires aggregates to be close to a saturated, surface-dry condition, or close monitoring of the moisture condition of aggregates should allow for accounting for the free moisture on aggregates. It should be noted that control of water is important in pervious concrete mixtures. Water absorbed from the mixture by aggregates that are too dry can lead to dry mixtures that do not place or compact well. However, extra water in aggregates contributes to the mixing water and increases the water-to-cement ratio of the concrete.
Water
Water-to-cementitious ratios (w/cm) between 0.30 and 0.34 are used routinely with proper inclusion of chemical admixtures, and those as high as 0.35 to 0.40 have been used successfully. The relation between strength and w/cm is not the same for pervious concrete, because unlike conventional concrete, the total paste content is less than the voids content between the aggregates. Therefore, making the paste stronger may not always lead to increased overall strength. Water content should be tightly controlled.
Water quality is discussed in ACI 301. As a general rule, water that is drinkable is suitable for use in pervious concrete. Recycled water from concrete production operations may be used as well, if it meets provisions of ASTM C 94 or AASHTO M 157. If there is a question as to the suitability of a water source, trial batching with job materials is recommended.
Admixtures
Chemical admixtures used in pervious concrete provides many benefits to performance and durability. Commonly used admixtures are:
Air-Entraining Admixture- Hydration-Controlling Admixture
- Internal Curing Admixture
- Mid-Range Water-Reducing Admixture
- Rheology-Controlling Admixture
- Viscosity-Modifying Admixture.