What Is Pervious Concrete in Construction?
What Is Pervious Concrete in Construction?
Pervious concrete is a porous type of concrete that is used in flatwork applications and is designed to allow water to pass through it, reducing runoff and allowing for groundwater recharge. It is made by using large aggregates with minimal fine aggregates, and the concrete paste coats the aggregates, creating pores that allow water to pass through the concrete slab.
Pervious concrete is often used in parking areas, areas with light traffic, residential streets, pedestrian walkways, and greenhouses. It is a sustainable construction material that is used to protect water quality as part of low impact development techniques.
Characteristics and Features of Pervious Concrete
Pervious concrete is a type of concrete that contains minimal fine aggregate and has a higher water-to-cement ratio than standard concrete. It is made up of cement, coarse aggregate (usually 9.5 mm to 12.5 mm in size), and water, with a small amount of sand added to increase strength.
The water-to-cement ratio for pervious concrete is typically between 0.28 and 0.40, and it has a void content of 15 to 25%. The correct amount of water in the mixture is important, as too little water may cause surface failure and a low water-to-cement ratio will increase the strength of the concrete, but a proper water content is necessary to achieve a wet-metallic appearance.
To ensure the correct water content, the mixture should be checked on site using a Rapid Air system, which involves staining the concrete black and examining it under a microscope.
Pervious concrete is typically used for flatwork and is poured over riser strips that keep the screed (the top surface of the concrete) 3/8 to 1/2 inches (9 to 12 mm) above the final pavement elevation.
Mechanical screeds are preferred to manual ones, and after screeding, the concrete is compacted to improve the bond and smooth the surface. However, excessive compaction can result in higher compressive strength but lower porosity and permeability.
Jointing pervious concrete is similar to jointing other types of concrete slabs. Joints can be tooled with a rolling jointing tool before curing or saw cut after curing. Curing typically involves covering the concrete with plastic sheeting within 20 minutes of pouring, but this can generate a lot of waste that ends up in landfills.
Alternative methods for curing pervious concrete, such as using preconditioned absorptive lightweight aggregate or an internal curing admixture, have been developed to reduce waste generation.
What Are The Pervious Concrete Problems?
Pervious concrete mix design for surface-wearing courses must be strong and durable enough to withstand specific site-specific loading and environmental conditions.
In the past, there have been two main problems that have hindered the use of pervious concrete in the United States:
- Strength tests have shown that pervious concrete has lower strength than is necessary for certain applications, and
- There have been concerns about the freeze-thaw durability of pervious concrete. However, research by the National CP Tech Center has indicated that well-designed pervious concrete mixes can achieve sufficient strength, permeability, and freeze-thaw resistance to be used in cold-weather climates.
- Pervious concrete paved surfaces, such as parking lots, driveways, sidewalks, and streets, can have a negative impact on water quality and the environment. When more land is pervious concrete paved over, more rainwater falls on impervious surfaces rather than being absorbed into the soil, leading to problems such as unstable ground due to erosion, flash floods, water table depletion, and pollution of waterways. Rainwater running across pavement can also pick up pollutants like oil, grease, deicing salts, and chemical fertilizers.
