Workability

A mix must be workable enough to fill the form spaces completely with the assistance of a reasonable amount of shoveling, spading, and vibrating. Since a fluid or runny mix does this more readily than a dry or stiff mix, you can see that workability varies directly with fluidity. The workability of a mix is determined by the slump test. The amount of the slump, in inches, is the measure of the concrete’s workability; the more the slump, the higher the workability.

The slump can be controlled by a change in any or all of the following: gradation of aggregates, proportion of aggregates, moisture content. If the moisture content is too high, add more cement to maintain the proper water-cement ratio.

Attain the desired degree of workability by running a series of trial batches, using various amounts of fine to coarse aggregate, until producing a batch with the desired slump. Once you determine the amount of increase or decrease in fines required to produce the desired slump, alter the aggregate proportions, not the water proportion, in the field mix to conform. If the water proportion is changed, the water-cement ratio will be upset.

Never yield to the temptation to add more water without making the corresponding adjustment in the cement content. Also, make sure that crewmembers who are spreading a stiff mix by hand do not ease their labors by this method without telling you.

As you gain experience, you will discover that adjustments in workability can be made by making very minor changes in the amount of fine or coarse aggregate. Generally, everything else remaining equal, an increase in the proportion of fines stiffens a mix, whereas an increase in the proportion of coarse loosens a mix.

NOTE

Before you alter the proportions set forth in a specification, you must find out from higher authority whether you are allowed to make any such alterations and, if you are, the permissible limits beyond which you must not go.