We asked Bob Banka President of Concrete Management Solutions to answer some important questions in a 5 part series about Pervious Ready Mix Concrete and the best way to get the most out of a pervious concrete installation.
Question: What are the 5 most important steps for a pervious concrete installation?
Step 3: Once the decision to use pervious concrete has been made and a well written set of spec’s have been used to set a job on its way to success the next important step is to talk to your local ready mix supplier.
Questions you will want to ask are:
- Have you made and delivered pervious concrete to a job before?
- If the answer to question #1 was yes then get the address and go by and look at the job(s).
- Does the job look good?
- Does the concrete take water?
- Are there any signs of raveling?
- Is anyone at the plant NRMCA Certified in pervious concrete?
Get a copy of the mix design as well as the aggregate gradations and void content of the aggregate per ASTM C-29. If you don’t know what you are looking for, ask one of the five national pervious consultants for assistance. Too many times we see mixes that are too heavy on cementitious materials and admixtures with not enough concern being placed on the aggregate being used. More times than not, the aggregate used for everyday concrete will not make the best pervious concrete. (Hint-asphalt gradations work better)
The paste must contain enough water to bring a certain rheology to its consistency. The critical function of this is to allow the aggregate to slide together with uniform consolidation with aggregate touching other aggregate on all sides. The paste must get out of the way as we seek a point to point contact in the aggregate. It is also necessary to shape the paste coating that is able to deform and reform into the paste bridge. The shape of this paste must exhibit apparent surface tension on the paste. The paste surface should shed water rather than absorb it for freezing climates.
The toughest part of marketing pervious is the reputation for ravel. Even in freezing climates, this is traced back to a malfunction in hydration. To overcome this, the producer and installer need the free use of water. This is not possible without measuring the aggregate and moderating cementitious to allow the space for adequate water. If the aggregate voids calculate out to 39% or higher, you can expect the tasks of mixing and discharge to be much easier. Excessive fines and excessive cementitious can often bog down the placement operation which can eat up your profit.
If the mix is designed with enough space in the voids, water can be used without fear of collapsing the voids. Remember to check the chosen aggregate for voids using a standard C 29 bulk density test. It is perfect if it has 41 to 43% voids. When you add 564 to 600 pounds of cementitious you fill up approximately 23% of those voids leaving 18% +/- 3% voids.
Bob Banka is President of Concrete Management Solutions and has been involved with the development of pervious concrete for over twenty years. To learn more visit: http://www.perviousconsulting.com