The time is coming when the word slump will describe the exploits of a major-league slugger and not the workability of fresh concrete.

Some of the concrete industry's "standards" have become more like traditions than useful measurements. The longevity of the 70-year-old-plus slump test, for example, is a great testament to the test's simplicity and the industry's ability to remain stagnant.

Acceptance of rheological measurement will require contractors and engineers who truly value the benefits of performance-based standards. Workability based on slump produces concrete to meet a specification. Workability based on yield stress and viscosity, in contrast, produces concrete that meets a specified performance level.Sophisticated contractors and producers recognize the need for rheology right now.

A widely accepted mathematical equation used to predict concrete placement productivity states:

Q=P/p, whereQ is the delivery output of concrete in cubic yards/hr,
P is the power required by the placing unit (whether mechanical or manpower), and
p is the placing pressure (see reference).

Lowering the placing pressure increases the output of concrete. Yield stress would serve the function of placing pressure, and the producer could design the concrete mix accordingly.

In other words,
yield stress = placing pressure = P/Q.

The measurement units, pressure and time, are fundamental to engineering. It might be possible to determine the onsite manpower requirement according to the yield stress and delivery output, i.e., pQ=P. With this concept in mind, a producer and contractor could work together to design a mix that is not only economical in regard to the per-yard material cost, but also the per-yard installation cost.