Workability is a general term used to describe the basic rheological aspects of fresh PCC (e.g., PCC in a wet, plastic state). Workability is instrumental in the proper placement and compaction of fresh PCC. In general, excessively stiff (or harsh) fresh PCC can be difficult to place and compact resulting in large void spaces and a honeycomb-like structure that can quickly fracture and disintegrate. This is especially true in and around reinforcing steel. Pavement PCC, especially that used for slip form paving, is usually quite stiff and must be vibrated into place. Excessively fluid fresh PCC is easy to place but may not be able to hold the coarse aggregate in place resulting in segregation and bleeding.
The slump test (see Figure 1) is the most common test for workability. The slump test involves hand placing an amount of fresh concrete into a metal cone and then measuring the distance the fresh PCC falls (or “slumps”) when the cone is removed.
The slump test is meant to be a basic comparative test. Variation in slump measurement on the same PCC can be as much as 50 mm (2 inches). The American Concrete Pavement Association (2002) says the following about slump:
“The bottom line is that the slump test is useful only as a comparative tool. If changes in slump are greater than 2 inches on a given job, one can conclude that there was likely a change in the mix. Variation in slump less than 2 inches is more than likely from a combination of the testing and typical concrete variability. No conclusion can be drawn from slump tests to the quality of the material. Strength measurements must be used to indicate quality.”
The standard slump test is:
- AASHTO T 119, ASTM C 143: Slump of Hydraulic Cement Concrete