PERVIOUS CONCRETE

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Pervious concrete:

Pervious concrete Pervious concrete is also called as “no fines” or porous concrete.

Introduction:

Introduction Pervious concrete is a special type of concrete with a high porosity used for concrete flatwork applications that allows water from precipitation and other sources to pass directly through, thereby reducing the runoff from a site and allowing ground water recharge. DEFINITION:- The high porosity is attained by a highly interconnected void content .

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pervious concrete has little or no fine aggregate and has just enough cementitious paste to coat the coarse aggregate particles while preserving the interconnectivity of the voids . Pervious concrete is traditionally used in parking areas, areas with light traffic, residential streets, pedestrian walkways, and Greenhouses. It is an important application for sustainable construction and is one of many low impact development techniques used by builders to protect water quality. INTRODUCTION

Drawbacks of Impervious Concrete:

Drawbacks of Impervious Concrete Pervious concrete Impervious concrete

Pervious Concrete pavements:

Pervious Concrete pavements 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. Typical pervious concrete pavement has a 15-25% void structure. It is consequently lightweight, with a density of 1600 to 1900 kg/m 3 .

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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.

Summary of Applications:

Summary of Applications Pavement Edge Drains Swimming pool decks Tree gates in sidewalks Alleys &driveways Tennis courts

Engineering Properties:

Engineering Properties

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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.

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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.

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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. 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.

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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.

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Abrasion Resistance Definition : The ability of a material to withstand mechanical action such as rubbing, scraping, or erosion, that tends progressively to remove material from its surface. Such an ability helps to maintain the material's original appearance and structure . 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

MIX DESIGN & PLACEMENT:

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

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Material Proportions (kg/m 3 ) Cementitious Materials 270 - 415 Narrowly graded aggregate (gravel/ crushed stone) 1190 - 1480 w/c ratio 0.25 – 0.34 ( with chemical admixtures) 0.34 - 0.40 ( without chemical admixtures) Cementitious materials/Aggregate ratio 1:0.21 – 0.25 Fine aggregate: Coarse aggregate ratio 0 to 1:1 Polypropylene fibres (optional when no fine aggregate is present) 0.1% by volume or 0.9 kg/m 3 Typical Mix Proportions for Pervious Concrete

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Transportation A pervious pavement mixture should be discharged completely within one hour after initial mixing. The use of retarding chemical admixtures or hydration-stabilizing admixtures may extend discharge times to 1½ hours or more. 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

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Placement and Consolidation Sub-base preparation and forms should be double-checked, prior to placement. 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

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Placement & strike-off using vibratory screed

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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. Plastic Sheeting for curing

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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.

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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.

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Benefits of Pervious Concrete 1 . Reduces storm water runoff 2 . Eliminates need for detention ponds and other costly Storm water management practices. 3. Replenishes water tables and aquifers. 4. Allows for more efficient land Development. 5. Minimizes flash flooding and Standing water. 6 . Prevents warm and polluted water From entering our streams. 7. Mitigates surface pollutants. 8. Salmon Friendly

Disadvantages:

Disadvantages 1 . Many pavement engineers and contractors lack expertise with this technology. 2. Porous pavement has a tendency to become clogged if improperly installed or maintained . 3. Porous pavement has a high rate of failure . 4. There is some risk of contaminating groundwater, depending on soil conditions and aquifer susceptibility . 5. Some building codes may not allow for its installation . 6. Anaerobic conditions may develop in underlying soils if the soils are unable to dry out between storm events. This may impede microbiological decomposition.

Thank q:

Thank q by G. Sravan Kumar

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