Pervious, Porous Concrete Pavement Mix and Mixtures

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. Table 3 provides typical ranges of materials proportions in pervious concrete, and ACI 211.3 provides a procedure for producing pervious concrete mixture proportions.

As in traditional concreting, portland cements (ASTM C 150, C 1157) and blended cements (ASTM C 595, C 1157) may be used in pervious concrete. In addition, supplementary cementitious materials (SCMs) such as fly ash, pozzolans (ASTM C 618), and ground-granulated blast furnace slag (ASTM C 989) may be used. Testing materials beforehand through trial batching is strongly recommended so that properties that can be important to performance (setting time, rate of strength development, porosity, and permeability, among others) can be determined.

w Silica fume is a byproduct of silicone production. It consists of superfine

w Fly ash is the waste byproduct of burning coal in electrical power plants;

Figure 4. Pervious concrete is made

with a narrow aggregate gradation,

but different surface textures can be

obtained through the use of different

maximum sizes. The concrete in the

box contained a 1/4-in. (6.5-mm) top

size, while that below used a larger

top size, 3/4 in. (20 mm).

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. 67 (¾ in. to No. 4), 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)]. Single-sized aggregate up to 1 inch (25 mm) has also been used. 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. Figure 4 shows two different aggregate sizes used in pervious concretes to create different surface textures.

Generally, A/C ratios are in the range of 4.0 to 4.5 by mass. These A/C ratios lead to aggregate contents of between about 2200 lb/yd³ and 3000 lb/yd³ (1300 kg/m³ to 1800 kg/m³). Higher A/C ratios have been used in laboratory studies, but significant reductions in strength result.

Both rounded aggregate (gravel) and angular aggregate (crushed stone) have been used to produce pervious concrete. Typically, higher strengths are achieved with rounded aggregates, although angular aggregates are generally suitable. 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-to-cement ratios between 0.27 and 0.30 are used routinely with proper inclusion of chemical admixtures, and those as high as 0.34 to 0.40 have been used successfully. The relation between strength and water-to-cement ratio is not clear 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. The correct water content has been described as giving the mixture a sheen, without flowing off of the aggregate. A handful of pervious concrete formed into a ball will not crumble or lose its void structure as the paste flows into the spaces between the aggregates. See Figure 5.

Water quality is discussed in ACI 301. As a general rule, water that is drinkable is suitable for use in 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.

Chemical admixtures are used in pervious concrete to obtain special properties, as in conventional concrete. Because of the rapid setting time associated with pervious concrete, retarders or hydration-stabilizing admixtures are commonly used. Use of chemical admixtures should closely follow manufacturer’s recommendations. Air-entraining admixtures can reduce freeze-thaw damage in pervious concrete, and are used where freeze-thaw is a concern. ASTM C 494 governs chemical admixtures, and ASTM C 260 governs air-entraining admixtures. Proprietary admixture products that facilitate placement and protection of pervious pavements are also used.

	Proportions, lb/yd³	Proportions, kg/m³
Cementitious materials	450 to 700	270 to 415
Aggregate	2000 to 2500	1190 to 1480
Water:cement ratio*** (by mass)	0.27 to 0.34	--------------
Aggregate:cement ratio*** (by mass)	4 to 4.5:1	--------------
Fine:coarse aggregate ratio**** (by mass)	0 to 1:1	--------------

* These proportions are given for information only. Successful

mixture design will depend on properties of the particular

materials used and must be tested in trial batches to establish

proper proportions and determine expected behavior. Concrete

producers may have mixture proportions for pervious concrete

optimized for performance with local materials. In such

instances, those proportions are preferable.

** Chemical admixtures, particularly retarders and hydration

stabilizers, are also used commonly, at dosages

recommended.

by the manufacturer. Use of supplementary cementitious

materials, such as fly ash and slag, is common as well.

*** Higher ratios have been used, but significant reductions in

strength and durability may result.

**** Addition of fine aggregate will decrease the void content and

increase strength.