DESIGNING THE MIX
After gathering information about job failures from around the country, Kevin MacDonald, vice president of engineering services for Cemstone, a ready-mix producer in Medota Heights, Minn., designed the pervious concrete mix. By understanding the nature of pervious concrete job failures, he was able to avoid others' mistakes.
Some failures were the direct result of poor curing, or no curing at all. “Curing is important. You have to remember that the surface area of pervious is huge — everywhere inside the slab as well as on the top,” MacDonald says. “In addition, cement paste layers are very thin so they are very susceptible to moisture loss.”
Pervious projects have failed because shale aggregates, present in the concrete, absorbed water and broke under freeze/thaw conditions, even below the surface. Cement paste and sand in the fresh concrete of another project sank to the bottom of the placement and filled the void spaces. Other pervious slabs failed because mixes included a blend of coarse aggregate sizes, reducing void spacing.
When MacDonald designed this project's pervious mix, his priorities included: finding the right coarse aggregate, providing the correct amount of cementitious paste, and preventing the paste from moving after concrete placement. In terms of hardened characteristics, MacDonald says the internal void ratio should be in the 40% range and he considers flexural strength to be more important than compressive strength. The Shoreview mix included:
- Cement content in the 550-pound range; 100 pounds consisting of fly ash
- 100% crushed bedrock aggregate to 3/8-inch clear size — 100% retained by the 3/8-inch sieve and 100% passed by the ½-inch sieve; as well as minimum deleterious aggregate content
- A hydration-stabilizing admixture to help the concrete remain plastic during placement and slow the reaction to stabilize water afterward (early stiffening causes flow problems and tempts workers to add water onsite)
- A 0.30 water-cement ratio
- An air-entraining admixture to help disperse cement
- A viscosity-modifying admixture to reduce the rundown of paste to the bottom after placement.
The void spacing in pervious concrete is important, notes MacDonald, and the large aggregate gradation is the most important factor. Technology to keep cement paste evenly dispersed is the next consideration. If it slumps down to the bottom of a placement, void spacing is diminished and the pavement is at risk.
To achieve quality pervious concrete, point-to-point contact between aggregates is also important, which occurs with good compaction immediately following placement. This is probably achieved best with vibratory screeds or surface compaction provided with heavy weights. Void spaces are provided by the mix design so overcompaction isn't a risk. “You can't reduce void spaces beyond what large aggregate sizes permit,” says MacDonald.