Consistency, plastic consistency, and workability

Q. What is meant by the terms “consistency,” “plastic consistency,” and “workability” as applied to concrete mixtures?

A. Consistency is the relative mobility or ability of freshly mixed concrete to flow. It includes the entire range of fluidity from the driest to the wettest possible mixtures. Plastic consistency indicates a condition where applied stress will result in continuous deformation without rupture. A plastic mixture possesses cohesion and does not crumble. It flows sluggishly and without segregation. Workability is the property of freshly mixed concrete that determines the ease with which it can be mixed, placed, consolidated, and finished to a homogeneous condition. It is synonymous with placeability. It involves not only the concept of a consistency of concrete, but also the condition under which it is to be placed — size and shape of the member, spacing of reinforcing, or other details interfering with the ready filling of the forms. For example, a stiff mixture with large aggregate that is workable in a large open form would not be placeable in a thin wall with complicated reinforcing details. Also, a mixture that appears too stiff may flow readily when it is vibrated with the proper equipment having the needed frequency and amplitude.

References: SP-1(02); ACI 238.1R-08; SP-233

Topics in Concrete: Admixture; Concrete Fundamentals; High Performance Concrete; Testing of Concrete