Ideally, specification formats should follow either a prescriptive approach or a performance approach and not a mixture of the two. In the prescriptive format the concrete producer is told how to design the mixture with parameters such as a minimum cement content, maximum w/c, admixtures quantities, or in some cases batch quantities of concrete ingredients. The performance approach, on the other hand, would require attributes such as strength, workability and permeability.
An interesting exercise is to evaluate the specification formats from the first survey. Overall, for the 108 job specifications, 80 percent had a strength requirement, 73 percent had a specified w/c and 24 percent had a requirement for chloride ion. Only 13 percent followed a strictly prescriptive format by specifying only a w/c ratio. A strictly performance format was used in 28 percent of the specifications where strength and/or permeability were specified. Illustrative of the mixing of specification formats, 72 percent of the jobs prescribing strength and 84 percent of the jobs prescribing chloride ion permeability, also specified a maximum w/c.