Q: The contractor who plans to build the foundation walls for our house makes a practice of using concrete with a slump of 6 ± 1 inch. I've argued that this would be too wet, but he says that his trade association approves this practice and that stiffer concrete can't be consolidated properly. He says he needs this kind of slump to avoid honeycombing.
He also says he orders a mix that is designed to give the required strength at this slump. Does he know what he's talking about?
A.: If the contractor does order concrete to achieve the required strength at this slump, he is taking the right approach. In recent years the Poured Concrete Wall Contractors Association of America has advocated wetter mixes for residential wall construction in order to facilitate consolidation by puddling and without a vibrator. This approach is also being taken by ACI Committee 332 in its new "Guide to Residential Cast-in-Place Concrete Construction," with the provision that the concrete be proportioned to meet the required strength at this slump and not some lower slump. The contractor should use job practices that prevent the segregation that can occur more easily in wet mixes.