I was in Salt Lake City last week to give a talk at the ACI chapter meeting. At the end, one of the audience members asked what I think about performance-based specifications for fresh concrete. I’m afraid I didn’t give him a very clear answer because I have mixed feelings.
While I lean towards specifying performance, including long-term performance (durability), as an engineer, I hate giving up that much control. My problem with performance-based concrete is that measuring the performance is not always that simple. There aren’t good simple field tests for measuring workability or placeability and measuring long-term performance takes, well, long term. And with a prescription spec, I can get exactly what I want.
On the other hand, many concrete producers today have the knowledge and materials to provide a great mix if the owner and contractor can be very specific in describing what they want. And producers can get a lot of help from the technical staff of their chosen admixture supplier. For example, at the Colorado Concrete Conference on September 21, Larry Kaiser with GCP Applied Technologies (formerly Grace Construction Products) was telling me about their new “control flow concrete.” This is a high-slump mix, but not as fluid as self-consolidating concrete (SCC). With a slump flow between 16 and 25 inches, this mix is achieved with a single dosage of what GCP calls Concera, which is a proprietary blend of a polycaboxylate high-range water reducer, a viscosity-modifying admixture, and other fairy dust. It ends up adding only about a $10/yard premium to the mix, as opposed to $40 to get a true SCC.
So this is the epitome of a performance mix—even the ready-mix producer doesn’t know exactly what’s in it! But GCP can show you the performance expectations in great detail. I don’t always believe the test data I see on products, but GCP is a very credible source so I expect that this mix will perform as advertised.
I think we are moving towards more performance-specified concrete and in the end that may be a good thing.
