PERVIOUS CONCRETE:
Pervious concrete is sometimes specified by engineers and architects when porosity is required to allow some air movement or to facillitate the drainage and flow of water through structures.
Pervious concrete is referred to as “no fines” concrete because it is manufactured by leaving out the sand or “fine aggregate”.
A pervious concrete mixture contains little or no sand (fines), creating a substantial void content. Using sufficient paste to coat and bind the aggregate particles together creates a system of highly permeable, interconnected voids that drains quickly. Typically, between 15% and 25% voids are achieved in the hardened concrete, and flow rates for water through pervious concrete are typically around 480 in./hr (0.34 cm/s, which is 5 gal/ft²/ min or 200 L/m²/min), although they can be much higher.Both the low mortar content and high porosity also reduce strength compared to conventional concrete mixtures, but sufficient strength for many applications is readily achieved. Pervious concrete pavement is a unique and effective means to address important environmental issues and support sustainable growth. By capturing rainwater and allowing it to seep into the ground, porous concrete is instrumental in recharging groundwater, reducing stormwater runoff, and meeting US Environmental Protection Agency (EPA) stormwater regulations.This pavement technology creates more efficient land use by eliminating the need for retention ponds, swales, and other stormwater management devices. In doing so, pervious concrete has the ability to lower overall project costs on a first-cost basis.
Perhaps the most important aspect in designing pervious concrete pavements for freeze-thaw areas is avoiding, or at least limiting, saturation, especially during the time of year when freezing can be expected. It is possible to design pervious concrete pavements to control the degree of saturation and the average maximum distance to a free surface. Proper subbase design and preparation are keys to pulling rainwater, ice, and snowmelt away from the pavement and ensuring suitable drainage.
Replacing as little as 7% of the coarse aggregate with fine aggregate increases the freeze-thaw resistance; however, there will be a reduction in voids of a few percent.In addition, the paste (or mortar) should be protected by using air-entraining admixtures to create a sufficient air-void system. Kevern, Wang, and Schaefer (2008) found that the coarse aggregate properties play a large role in providing freeze-thaw durability, with absorption values below 2.5% being of greatest impact. The National Ready Mixed Concrete Association (NRMCA 2004) has developed guidelines for using pervious concrete in areas prone to freeze-thaw conditions.The correct quantity of water in the concrete is critical. A low water to cement ratio will increase the strength of the concrete, but too little water may cause surface failure. A proper water content gives the mixture a wet-metallic appearance. As this concrete is sensitive to water content, the mixture should be field checked.Entrained air may be measured by a Rapid Air system, where the concrete is stained black and sections are analyzed under a microscope.
A common flatwork form has riser strips on top such that the screed is 3/8-1/2 in. (9 to 12 mm) above final pavement elevation. Mechanical screeds are preferable to manual. The riser strips are removed to guide compaction. Immediately after screeding, the concrete is compacted to improve the bond and smooth the surface. Excessive compaction of pervious concrete results in higher compressive strength, but lower porosity (and thus lower permeability).
Jointing varies little from other concrete slabs. Joints are tooled with a rolling jointing tool prior to curing or saw cut after curing. Curing consists of covering concrete with 6 mil. plastic sheeting within 20 minutes of concrete discharge.However, this contributes to a substantial amount of waste sent to landfills. Alternatively, preconditioned absorptive lightweight aggregate as well as internal curing admixture (ICA) have been used to effectively cure pervious concrete without waste generation.
The use of pervious concrete pavements provides a solution to new requirements under Environmental Protection Agency regulations that call for decreasing the amount of surface water runoff and initially treating the runoff.
Pervious concrete is made of cementitious materials, water, admixtures, and narrowly graded coarse aggregate. Very little or no fine aggregate is used in the mixture. With just enough cement paste to coat the aggregate, a system of interconnected voids (15 to 35%) is created resulting in a highly permeable concrete that drains very quickly. By allowing water to pass directly through the concrete, the amount of surface water runoff is reduced dramatically. It can also be used as part of a system to reduce the level of pollution contained in storm water that is captured in the pervious pavement.
Pervious pavements have been used for years throughout the warmer climates of the United States with excellent results. However, in climates prone to severe freeze-thaw cycles, some are hesitant to use pervious concrete pavements until it has been proven that pervious concrete can be made to resist freeze-thaw damage.
Resistance of any concrete to freezing and thawing depends on the permeability, the degree of saturation, the amount of freezable water, the rate of freezing, and the average maximum distance from any point in the paste to a free surface where ice can form safely. The rate of freezing in most applications is dictated by the local climate. Entrained air may help protect the paste as well.
concrete allows Percolation-previous concreate
0 comments:
Post a Comment