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The American Concrete Pavement Association (ACPA) gives a general overview of using pervious concrete pavement for stormwater management (ACPA 2009). Pervious concrete is described as a mix of "specially formulated hydraulic cementitious materials, water, and uniform open-graded coarse aggregate (e.g., ASTM C33 Size numbers 5, 56, 67, 8, and 89)," which, when designed and installed properly, has a void space of 15% or more. It is suggested that pervious concrete has potential applications around buildings, in parking lots, low volume roads, and on highway shoulders and medians. A typical cross-section is given in Figure 2-3 in which the subbase is a stone reservoir that can store a finite volume of water. Drain tiles (not shown) can be added below the pavement to convey water downstream in the stormwater management system.

Figure 2.3. Typical cross-section of pervious concrete pavement. Concrete surface layer is 15-25% voids, subbase is 20-40% voids, and subgrade is 5-20% voids (ACPA 2009).

The American Concrete Institute (ACI) states that aggregate is typically single sized or course aggregate between 3/8 inch and 3/4 inch and all aggregates should meet ASTM D448 and

C33/C33M (ACI 2010). Rounded and crushed aggregate, both normal and lightweight have been used but flaky or elongated aggregate should not be used. Aggregates should also be hard, clean, and have no coating. Portland cement conforming to ASTM C150/C150M, C595/C595M, or C1157/C1157M should be used as the main binder with supplementary materials such as fly ash, blast-furnace slag, and silica fume being acceptable, although Kevern et al. (2008) recommended that fly ash use be restricted to 10% and silica fume to 5% replacement. Water-to-cement ratios should be low and typically range from 0.26 to 0.40; Kevern et al. (2008) specifically targeted 0.32 to improve workability and density. If the water content is too high the paste may drain and

clog the pore system. Finally, water-reducing, retarding, accelerators, and air-entraining admixtures can be used but should meet all relevant requirements and ASTM standards.

Typical mix proportions for pervious concrete are 450 to 700 lb/yd³ of cementitious materials and 2000 to 2500 lb/yd³ of aggregate, at a water:cement ratio of 0.27 to 0.34, aggregate:cement ratio of 4 to 4.5:1, and a fine:coarse aggregate ratio of zero to 1:1 (Tennis et al. 2004). ACI (2010) suggested repeated trial-and-error efforts that involve developing different mix proportions under laboratory settings and testing them in the field until the desired behavior is achieved. Overall, the goal would be to obtain a balance between voids, strength, paste content, and workability.

With regards to concrete strength, ACI (2010) made some general comments but provided no specific design details. For more information on trial batch proportioning, the reader is referred to ACI (2010), which describes various methods, and Sonbei and Bassuoni (2013), which used statistical modeling to estimate the impact of design variables on resulting density, void ratio, infiltration rate, and compressive strength.

The Colorado Ready Mixed Concrete Association (CRMCA 2009) recommended that course aggregate conform to ASTM C33 and fine aggregate complying with ASTM C33 make up 4% to 8% of the total aggregate weight. The combined course and fine aggregates shall have at least 10%

passing the #4 sieve. The document noted that research has suggested that additional sand increases resistance to freeze-thaw cycles, durability, and strength while maintaining enough infiltration capacity; this result is supported by observations made in Henderson and Tighe (2012). Furthermore, CRMCA recommended that the mixture have a density of 105 lb/ft3 to 130 lb/ft3 and should conform to ASTM C29. The void content should be from 15% to 25%, the water to cement ratio shall be 0.26 to 0.35, and the cementitious content shall be from 450 lb/yd3 to 550 lb/yd3. The document also gave other specifications related to admixtures, fly ash, placing and finishing, curing, etc.

It should be noted that there have been more innovative approaches to pervious concrete mix designs that featured the use of supplementary cementitious materials and recycled aggregates.

This work included studies by Ravindrarajah and Yukari (2010) and Sata et al. (2013).

Ravindrarajah and Yukari (2010) examined the use of high-levels of fly ash to replace cement in pervious concrete and recommended that measures be taken to insure that adequate strength levels are preserved if high levels of fly ash (as much as 50%) are used. Likewise, Sata et al.

(2013) found that the use of a geopolymer concrete as a basis for pervious pavement could be used, but steps should be taken to account for significantly lower strength than would be present using conventional concretes.

Several studies have investigated pervious concrete properties, and some of them have been reviewed in ACI (2010) recommendations. These studies, however, are not all conclusive and cannot be solely used to design mixes. Rather they show general trends. For example, a study by Meininger (1988) showed a drop in compressive strength from over 5000 psi at an air content between 5% and 10% to just over 1000 psi at an air content between 25% and 30%. Flexural

strength can range from 150 to 550 psi (Tennis et al. 2004). Results cannot be applied universally, however, because the tests only investigated two aggregates sizes at a range of aggregate gradations and compaction efforts but did not investigate the impact of a host of other variables. Other summaries showed an increase in compressive strength with unit weight

(Mulligan 2005), a drop in air content with an increase in water:cement ratio (Meininger 1988), a drop in flexural strength with an increase in air content (Meininger 1988), and an increase in flexural strength with an increase in compressive strength (Meininger 1988).

Crouch et al. (undated) investigated three gradations of crushed limestone and two gradations of gravel in the laboratory to determine the impact of the aggregate and the compaction effort on the compressive strength of pervious concrete. Four field samples were also obtained and tested.

The following conclusions were reached:

1. For a constant paste amount and character, effective air void content appeared to be a function of three factors: 1) compactive effort, 2) aggregate particle shape and surface texture, and 3) aggregate uniformity coefficient. Smoother and rounder aggregates, for the same compactive effort, resulted in lower voids and void content decreased as the uniformity coefficient increased,

2. For a consistent paste amount and character, the compressive strength of pervious concrete appeared to be a function of 1) effective air void content, and 2) gradation fineness modulus. As void content and aggregate fineness modulus increased the compressive strength decreased,

3. A low cementitious content, a uniform aggregate gradation, and high compactive effort can produce pervious concrete with permeability values higher than 142 in/hr and compressive strengths greater than 3000 psi.

The freeze-thaw durability of pervious concrete can be a major factor in its overall performance, especially given that studies such as Kevern et al. (2009) on the thermal profile of pervious concrete pavements have found that pervious pavements can demonstrate a more rapid heating and cooling cycle as compared to traditional concrete pavements. Schaefer et al. (2006)

investigated various mix designs in order to develop a pervious concrete mix with sufficient infiltration capacity, strength, and freeze-thaw durability. Various concrete mixes with different sizes and types of aggregate, binder content, and admixtures were investigated and evaluated.

Aggregates of river gravel and crushed limestone were also investigated. River gravel sizes were 0.5 inch, 0.375 inch, and no. 4 size (100% passing the 0.375 inch sieve and 100% retained on the no. 4 sieve). Crushed limestone (0.375 inch) and pea gravel were also included in the research.

Schaefer et al. (2006) measured the porosity, permeability, strength, and freeze-thaw durability of all the mixes that were investigated. The following results/conclusions were obtained:

1. Mixes with only a single size aggregate have high permeability but insufficient strength,

2. Addition of a small fraction of sand to the mix increased strength and freeze-thaw resistance. It also lowered permeability,

3. Adding sand and latex to the mix increased strength (compared to mixes with a single sized aggregate) but mixes in which only sand was added had a higher strength,

4. Mixes with a small percentage of sand showed 2% mass loss after 300 freeze-thaw cycles, 5. Low compaction reduced compressive strength, split strength, and unit weight but

increased permeability,

6. A binder to aggregate ratio of 0.21 and a water:cement ratio of 0.27 was determined to be the optimum in terms of strength, permeability, and void ratio,

7. In terms of seven-day strength, the optimum latex content was determined to be 10%, 8. Mixes with larger aggregate size had higher void ratios,

9. Aggregate with higher abrasion resistance resulted in high strength concrete,

10. The compressive strength and unit weight decreased linearly as the void ratio increased, 11. Permeability increased exponentially as the void ratio increased, with a rapid increase in

permeability at void ratios greater than 25%,

12. At regular compaction energy, mixes with void ratios between 15% and 19% had seven-day compressive strengths ranging from 3,300 to 2,900 psi, permeabilities ranging from 135 to 240 in/hr, and unit weights from 127 to 132 pcf. The split strength was about 12%

of the compressive strength,

13. A mass loss of about 15% indicated a terminal serviceability for a pavement.

Yang et al. (2006) expanded on the durability study conducted by Schaefer et al. (2006) to examine the influence of moisture conditions on freeze-thaw durability, with a positive correlation between saturation levels during curing and freeze-thaw durability. In a laboratory study simulating field conditions on pervious concrete specimens, Yang (2011) found that the inclusion of polypropylene fibers increased durability and that the application of salt to specimens decreased durability.

A recent laboratory test conducted by UCPRC (Jones et al. 2010, Li et al. 2012) found a clear relationship between aggregate grading, cement content, water-to-cement ratio, and strength and permeability. All specimens tested exceeded permeability requirements, suggesting that

adjustments can be made to optimize mixes while still maintaining adequate permeability.

ACI (2010) reported that void content is highly dependent on aggregate gradation, cementitious material content, water:cement ratio, and compactive effort. It is also stated that a range of porosities can be achieved by blending two different size aggregates. If this is done, however, the larger aggregate should be less than ~2.5 times the size of the smaller aggregate or else the smaller aggregate may fill in the voids and reduce permeability.

The pore size in pervious concrete is also an important parameter as it affects properties such as permeability and sound adsorption. Larger sized aggregate produced larger pores and increased

permeability. Pore structure also impacted pervious concrete properties. Low et al. (2008) determined that aggregate size, aggregate-cement (A/C) ratio, and water-cement ratio greatly impacted pore structure. In addition, percolation rate (or permeability) is directly related to the porosity and the pore size of pervious concrete. It has been reported (Meininger 1988) that a porosity of 15% is required to achieve a permeability of 1 cm/s.

Tests have shown that entraining air in the cement paste can improve durability. Wanielista and Chopra (2007a) also determined other relationships such as unit weight as a function of strength and porosity and permeability as a function of A/C ratio, among others. Wanielista and Chopra (2007a) also investigated existing pervious concrete systems to gather information regarding long-term performance and vitality. Wanielista and Chopra (2007a) reached the following conclusions:

1. An A/C ratio less than 5 in combination with a water:cement ratio from 0.35 to 0.39 resulted in the highest compressive strength without jeopardizing permeability, 2. Higher A/C ratios did not have enough cement,

3. Higher water:cement ratios eliminated void spaces,

4. The energy applied to the pervious concrete was 1,544 kN-m/m³ (modified Proctor).

Higher compaction energy did not reduce permeability but it increased compressive strength,

5. The compressive strengths obtained would support traffic loads up to 40 tons.

Toughness, as measured by ASTM C1399, can be improved by adding synthetic fibers. One study (SI Concrete Systems 2002) found that fibers 1.5 to 2.0 inches in length were most effective in increasing toughness. Shrinkage, which is typically around 200 x 10^-6, is about one-half of what typically occurs in conventional concrete (Tennis et al. 2004).

For quality control, Tennis et al. (2004) recommend using unit weight or bulk density because other properties, such as slump and cylinder strength tests, don't have much meaning for pervious concrete. Strengths are a function of void content and placement methods and it's difficult to accurately represent field placement in a cylinder test. Unit weights are expected to be 70% of traditional concrete mixes.

The voids in pervious concrete can provide freeze-thaw resistance if these voids drain before freezing. Air entrained in the paste can also improve freeze-thaw resistance. Placing the pervious concrete over at least 6 inches of drainable rock base is recommended in freeze-thaw

environments (Tennis et al. 2004).

Laboratory tests were performed by Wanielista and Chopra (2007a) who investigated the effect of varying components of pervious concrete on strength and, by using the test results and studying existing pervious concrete pavements, determined the traffic loads and volumes that pervious concrete can withstand. They used aggregate with a specific gravity of 2.36 and a unit

weight of 147.5 lb/ft³. Concrete cylinders with different properties and different permeability were constructed. Water:cement ratios were varied from 0.32 to 0.52 by weight, aggregate cement ratios varied from 4 to 7 by volume. Resulting permeability values ranged from zero to 2688 in/hr and specific gravity values ranged from 1.95 to 2.36.

In an effort to develop preliminary specifications for pervious concrete, the Maryland

Department of Transportation conducted investigations to enhance the structural performance and durability of pervious concrete (Amde and Rogge 2013). This was accomplished through testing different admixtures (cellulose fibers, a delayed set modifier, and a viscosity modifier).

Specimens developed from different mix designs were tested for density, void content,

compressive strength, split tensile strength, permeability, freeze-thaw durability, and abrasion resistance. Freeze-thaw testing was performed at 100%, 50%, and 0% saturation. The cellulose fiber admixture had the greatest impact on concrete durability due to the fibers ability to help hold the aggregate/paste mix together. Both abrasion resistance and freeze-thaw durability increased with the addition of cellulose fibers, although the result regarding abrasion is contradicted by Wu et al. (2011) and should thus be considered with caution. The delayed set modifier reduced permeability because more concrete paste settled to the bottom and developed a less pervious layer. The viscosity modifier resulted in a mix that was easier to handle but had little other impact.

Prior to construction, the subbase must be smoothed and compacted. Compaction to a density of 90% to 95% is often recommended but it is noted that increased compaction decreased

permeability.

In order to prevent drying, it was recommended that the subgrade be moist (but without standing water) just prior to pervious concrete placement (Tennis et al. 2004). ACI (2010), like other documents, stated a typical subgrade compaction of 90% of the Standard Proctor Maximum Dry Density in order to maintain infiltration capacity. The subgrade soil, however, should be

considered because compacting clayey soils to 90% can essentially eliminate infiltration whereas compacting some sandy soils to 100% has no impact on infiltration. Regardless of the extent of compaction specified, it is important to field test the base and subgrade after compaction to ensure that it meets the desired objectives with respect to infiltration and structural integrity.

Pervious concrete mixes should be placed as quickly as possible because they typically have almost no excess water in the mix and can dry out quickly. This can lead to reduced strength and, in the future, raveling of the concrete. Edge forms, as used with conventional concrete, should be used and concrete should be placed as close to its final location as possible to minimize

workmanship. After deposition of concrete it should be cut with a concrete hand rake to a rough elevation and care should be taken to maintain the intended voids. ACI (2010) also discussed other construction techniques such as riser strips, placing equipment, miscellaneous tools, and how to place new pervious concrete next to an existing section that has already been placed.

Construction joints, having a sawcut depth of one-fourth to one-third the thickness of the pavement should be installed (Tennis et al. 2004). A spacing of 20 ft or more being typical. It was also recommended that joints be installed in fresh concrete with special tools. A specially designed compacting roller-jointer with a blade that is at least one-fourth the thickness of the slab and has enough mass to create a clean joint is recommended. The roller should also produce a rounded edge so that square edges, which have a greater tendency to ravel, can be avoided.

Other applications, however, such as the Shoreview, MN case study presented in Chapter 8, have found saw-cut joints perform well. In addition, handling time between mixing and placement should be one hour, although this can be extended to 1.5 hours by use of appropriate admixtures (Tennis, et al. 2004).

The curing cover should be placed no later than 20 minutes after concrete placement in ideal, high humidity conditions. For other environments, the cover should be placed sooner. Cover material should be heavy-duty polyethylene that meets ASTM C171 requirements and should cover the entire width of concrete. All measures should be taken to accomplish the construction process quickly to prevent the concrete surface from drying. Concrete should be allowed 7 to 10 days to cure, depending on the use of admixtures. In cold weather, however, curing times may need to be extended. Like other documents, ACI (2010) recommended placing concrete only when temperatures are expected to be above 40°F. Curing blankets may also be used in times of cold weather.

Pervious concrete cannot be pumped, so site access is an important aspect of any job. Placement should be continuous and spreading and strikeoff should be rapid. Conventional formwork is used as is mechanical vibration and manual screeds. Manual screeds must be used with caution, however, as they can cause tears in stiff mixes. Strike off should occur about 0.5 to 0.75 inches above the desired final height to allow for compaction. Compaction is typically done with a steel roller and should be completed within 15 minutes of placement. Compaction is typically the last step as normal floating and trowel operations reduce the permeability of the surface.

Kevern et al. (2006) described methods related to pervious concrete that were used in practice and also discussed a study that determined field level checks for pervious concrete quality control and assurance. Finishing and compaction were listed as the most important steps in producing a durable concrete pavement. Typically, pervious concrete was placed and struck off 0.75 to 1 inch above forms by means of a shim and vibratory screen. After removing the shims the concrete was compacted using a weighted roller. Roller screeds were also used and discussed in Kevern et al. (2006). Other methods investigated and/or discussed included a hand-held vibrating screed to strike off the concrete and the use of an asphalt paver to place pervious concrete. It was also mentioned that a standard edging tool can minimize raveling.

Although joints can be either cut or formed, Kevern et al. (2006) listed formed joints (e.g. via a joint roller) as the preferred method. The study resulted in the following conclusions:

1. Although many methods exist to place and finish pervious concrete, the impact of construction methods on long-term durability is not well understood,

2. Compaction energy can be used to balance strength and permeability, and 3. Compaction energy plays a crucial role with respect to freeze-thaw durability.

The impact of compaction energy on pervious concrete void ratio, compressive strength, tensile strength, unit weight, and freeze-thaw durability was investigated by Suleiman et al. (undated).

Single sized crushed limestone and river gravel were investigated. The study found that

decreasing compaction energy reduced compressive strength, split strength, and unit weight but increased permeability. Furthermore, samples prepared with regular compaction energy when subject to freeze-thaw cycles experienced aggregate failure while those prepared with low

decreasing compaction energy reduced compressive strength, split strength, and unit weight but increased permeability. Furthermore, samples prepared with regular compaction energy when subject to freeze-thaw cycles experienced aggregate failure while those prepared with low