I take exception to the National Ready-Mixed Concrete Association's P2P program (which aims to change the specification of concrete mixes from prescription- to performance-based) when it comes to slab-on-ground mixes. There has been a lot of progress in recent years achieving concrete that exhibits less shrinkage, cracking, and curling than gap graded, over-sanded, over-cemented mixes. ACI Committee 302, ACI Committee 360, and several national consultants have had great success in developing well-graded, lower paste slab mixes that include larger quantities of coarse aggregate, some with top sizes of 2 inches or greater. It would be a shame to have the industry go off in another direction.
I can sympathize with the ready-mix producers since what we are now asking for is more demanding and in many cases more costly to produce. As I work around the country, I find that many ready-mix plants have limited bins and storage space available for material. This limits them in their ability to blend two or sometimes three coarse aggregates and two fine aggregates to produce a good quality, well-graded floor mix.
The NRMCA, in my opinion, is trying to protect its members, and that is admirable. But, in this case, I believe it is wrong headed. I would guess that concrete for flatwork might be as high as 50% of the total volume, and I would suggest that it might encompass more than 80% of the problems. My gut feeling is that NRMCA should be guiding its industry in the direction of better mix designs that will result in fewer call-backs and issues among the producer, the contractor, the general contractor, and the owner, not just looking for a way for their members to control their product and cost.
More and more contractors, consultants, and engineers understand the workings of the mix: well-graded as opposed to gap-graded, aggregate shapes and sizes, and natural versus manufactured sands. It is no longer an industry dominated by the ready-mix supplier and his knowledge of the business. Don't get me wrong, the flat-work industry needs to educate the contractors, owners, engineers, and designers that there is a better way to make floor concrete, and it's a time-proven method, not smoke and mirrors. I would hope that the American Concrete Institute, the American Society of Concrete Contractors, and NRMCA will put together a joint education force to give seminars around the country to bring this important element of the business to all. For years contractors have relied on the ready-mix supplier to provide a state-of-the-art product, but in many cases that has not been the best mix for the owner's expectations.
The concrete contractor is his own worst enemy as he tells the supplier, “It had better be cheap, go in easily, and set consistently and fast.” This is the standard floor mix that producers supply for most flatwork, and it is usually produced with small #57 stone or #67 stone, 560 to 600 pounds of cementitious material, and 1300 to 1400 pounds of sand. In my opinion, you could not send a worse product to a floor slab pour. The industry (both contractors and producers) needs to educate itself and start working as a team to produce floors that exhibit good long-term performance that both can be proud of.
This will take time and energy but a lot of people in the industry are working together to make it happen, and I guarantee you it will happen as those who use it quickly become advocates. Don't let this pass you by; become proactive and start to think of the big picture. It is kind of like the stock market: Those who get in for the long haul are usually the ones who are most successful. Start to think about updating your plant so that you can become the one in your area that has accepted change and has made the commitment to make it happen.
—Joe Neuber, Neuber Concrete, Kimberton, Pa.