Pervious Concrete

2. Pervious Concrete • A mixture of coarse aggregate, Portland cement, water and little or no sand. • Controlled amounts of water and cementitious materials are used to create a paste that forms a thick coating around aggregate particles without flowing off during mixing and placing. Using just enough paste to coat the particles maintains a system of interconnected voids. The result is a very high permeability concrete that drains quickly. • Typical pervious concrete pavement has a 15-25% void structure. It is consequently lightweight, with a density of 1600 to 1900 kg/m3.

3. Pervious Concrete Pavements • Primary use of pervious concrete is in pavements. • The compressive strength of pervious concrete is limited since the void content is high. However, compressive strengths of 3 to 30 MPa are typical and sufficient for many applications. • Its low paste content and low fine aggregate content make the mixture harsh, with a very low slump. This also leads to increased traction for vehicles and prevents driving hazards like hydroplaning. • Also referred to as porous concrete, permeable concrete, no-fines concrete, gap-graded concrete, and enhanced-porosity concrete. • By capturing storm water and allowing it to seep into the ground, porous concrete is instrumental in recharging groundwater, reducing storm water runoff, etc.

4. Asphalt Pervious Concrete Comparison of asphalt and pervious concrete surfaces in a case study from USA. Water stagnant on asphalt surface while pervious concrete allows the water to flow through.

5. Other applications that take advantage of the high flow rate through pervious concrete include drainage media for hydraulic structures, parking lots, tennis courts, greenhouses, and pervious base layers under heavy-duty pavements. • Its high porosity aides it to be used in buildings as a thermal and sound insulator.

6. Engineering Properties

7. Fresh Properties • The plastic pervious concrete mixture is stiff compared to traditional concrete. When placed and compacted, the aggregates are tightly adhered to one another and exhibit the characteristic open matrix. • For quality control and quality assurance, unit weight or bulk density is the preferred measurement because some fresh concrete properties, such as slump, are not meaningful for pervious concrete. • Concrete working time is typically reduced for pervious concrete mixtures. Usually, one hour between mixing and placing is all that is recommended. This can be altered using stabilizers & retarders.

8. Hardened Properties Density • The density of pervious concrete depends on the properties and proportions of the materials used, and on the compaction procedures used in placement. In-place densities in the order of 1600 to 2000 kg/m³are common, which is in the upper range of lightweight concretes. Compressive Strength • Pervious concrete mixtures can develop compressive strengths in the range of 3 MPa to 30 MPa, which is suitable for a wide range of applications. Typical values are about 20 MPa. • Drilled cores are the best measure of in-place strengths, as compaction differences make cast cylinders/cubes less representative of field concrete.

9. Flexural Strength • Flexural strength in pervious concretes generally ranges between about 1 and 4 MPa. Factors which influence the flexural strength are the degree of compaction, porosity, and the aggregate-to-cement (A/C) ratio. Shrinkage • Drying shrinkage values in the order of 0.002 have been reported, roughly half that of conventional concrete mixtures. • The material’s low paste and mortar content is a possible explanation. Roughly 50% to 80% of shrinkage occurs in the first 10 days, compared to 20% to 30% in the same period for conventional concrete. Because of this lower shrinkage and the surface texture, many pervious concretes are made without control joints and allowed to crack randomly.

10. Durability : Freeze-Thaw Resistance • Freeze-thaw resistance of pervious concrete in the field appears to depend on the saturation level of the voids in the concrete at the time of freezing. • In the field, it appears that the rapid draining characteristics of pervious concrete prevent saturation from occurring. • Anecdotal evidence also suggests that snow-covered pervious concrete clears quicker, possibly because its voids allow the snow to thaw more quickly than it would on conventional pavements. • When the large open voids are saturated, complete freezing can cause severe damage in only a few cycles.

11. Sulfate Resistance • The open structure of pervious concrete makes it more susceptible to acid and sulfate attack over a larger area than in conventional concrete. Abrasion Resistance • The open & rough structure of pervious concrete, abrasion and ravelling of aggregate particles on the surface can be a problem. Thus, highways are generally not suitable for pervious concretes.

12. MIX DESIGN& PLACEMENT Grading Pervious Concrete Samples with different Quantity of water content – too little, medium & too much water.

13. Typical Mix Proportions for Pervious Concrete • Cement may be replaced by about 10-30% of fly ash, 20-50% of blast furnace slag and 5% of silica fume • Addition of fine aggregate will decrease the porosity and increase strength

14. Transportation • A pervious pavement mixture should be discharged completely within one hour after initial mixing. Its very low slump may make discharge from transit mixers slower than for conventional concrete; transit mixers with large discharge openings or paving mixers tend to provide a faster unloading time. The use of retarding chemical admixtures or hydration-stabilizing admixtures may extend discharge times to 1½ hours or more. Placement and Consolidation • Pervious concrete mixtures cannot be pumped, making site access an important planning consideration. Prior to placement, the sub-base preparation and forms should be double-checked. • Placement should be continuous, and spreading should be rapid. Mechanical vibrating, laser screeds and manual screeds are commonly used, although manual screeds can cause tears in the surface if the mixture is too stiff. • Consolidation is generally accomplished by rolling over the concrete with a steel roller, which compacts the concrete to the height of the forms. Because of rapid hardening and high evaporation rates, delays in consolidation can cause problems

15. Curing As pervious concrete pavements do not bleed, they can have a high propensity for plastic shrinkage cracking. In fact, “curing” for pervious slabs and pavements begins before the concrete is placed - the sub grade must be moistened to prevent it from absorbing moisture from the concrete. After placement, fog misting followed by plastic sheeting is the recommended curing procedure, and sheeting should remain in place for at least seven days. Placement & strike-off using vibratory screed Plastic Sheeting for curing Joints

16. Construction Inspection and Testing • Slump and cylinder/cube strengths are not meaningful. Strength is a function of the degree of compaction, and compaction of pervious concrete is difficult to reproduce in specimens. • Instead, a unit weight test is usually done for quality assurance, with acceptable values dependent on the mix design, but generally between 1600 and 2000 kg/m³. Post-Construction Inspection and Testing • After seven days, cores can be taken and measured for thickness and unit weight as quality assurance and acceptance tests. A typical testing rate is three cores for every 75m³.

17. Maintenance • Maintenance of pervious concrete pavement consists primarily of prevention of clogging of the void structure. • Cleaning options may include power blowing and pressure washing. Pressure washing of a clogged pervious concrete pavement has restored 80% to 90% of the permeability in some cases. • Pervious concrete in freeze-thaw environments must not become fully saturated. Saturation of installed pervious concrete pavement can be prevented by placing the concrete on a thick layer of 200 to 600 mm of open-graded stone base.

18. REFERENCES • Construction Specifier Magazine, http://perviouspavement.org/PDFs/ A%20NCSU%20Structural%20Design%20-%20Permeable%20Pavements.pdf • "NC State University Permeable Pavement Research and Changes to the State of NC Runoff Credit System", From the 8th International Conference on Concrete Block Paving, Nov’ 2006, http://perviouspavement.org/PDFs/ncsu_study.pdf • "Structural Design of Permeable Pavements Worksheet"  North Carolina State , University Department of Biological and Agricultural Engineering • "Hydraulic Design for Permeable Pavement Worksheet"  North Carolina State , University Department of Biological and Agricultural Engineering         • “Infiltration Opportunities in Parking-Lot Designs Reduce Runoff and Pollution"  By Betty Rushton, Southwest Florida Water Management District. An analysis of the treatment train used in the parking lot of the Florida Aquarium in Tampa. From the 7th Biennial Stormwater Research and Watershed Management Conference in Florida. • "Mix Design Development for Pervious Concrete in Cold Weather Climates" National Concrete Pavement Technology Center - Final Report, February 2006.

19. "Oil Retention and Microbial Ecology in Porous Pavement Structures" Coventry University, School of Science and The Environment. • "Heavy Metal Retention Within a Porous Pavement Structure"      Department of Civil Engineering, Urban Water Management - University of Essen. • N. Subramanian, The Indian Concrete Journal, Vol. 82, Dec’2008. E- BROCHURES : • Concrete in Practice (CIP): Pervious Concrete • "Concrete Parking Areas-- They're GREEN“ • "When it Rains, It Drains" (4-page Pervious Concrete brochure) • Freeze-Thaw / Pervious Brochure (17 pages) WEB SOURCES: • NRMCA'sConcreteAnswers.org • RMC Research And Education Foundation • www.perviouspavement.org • www.youtube.com – pervious concrete installation – 1 min 48 sec • Video.google.com – pervious concrete – 41 sec