Mix, Forms, Admixtures
Mix design. Pervious mixture suppliers must employ high quality control. Pervious mixes contain Portland cement, a nominal 3/8″ or larger, Florida limerock or granite aggregate, admixtures and minimum water as designed. For the best percolation to be achieved the aggregate must be washed before mixing. Almost all fine aggregate is eliminated from the mix to provide the necessary voids to allow the maximum penetration of water. Typically Pervious Concrete has about 70% of the density of standard concrete paving mixtures. The amount and timing of the addition of water to the mix is critical. Too much water will carry the cement slurry to the base of the pavement creating a barrier that prevents storm water from passing into the sub-base and substrate. Too little water prevents bonding of the aggregate to the cementitious mixtures.
Forms. If you scan through most pervious concrete web pages you will see crews working with wood forms, usually 2″ x 4″s. What is wrong with that picture?
- 1. They don’t stand up well to the vibration of compaction.
- 2. They warp and sag, making straight edges difficult or impossible.
- 3. They waste lumber. That means removing trees that could be removing CO2 from the atmosphere.
- 4. They probably don’t have the forms trailers needed for straight steel forms, as pictured below.
Admixtures. Over the past few years a few new trade names have been given to a a combination of ingredients that we have used in our proprietary Pervious mixtures for many years. A few new ones are under evaluation. But a word of caution: Some think that these ingredients will substitute for the methods necessary for successful Pervious placement.