The slump test is a commonly used tool for quality control of fresh concrete at the jobsite and for enforcement of specifications. It has limitations however, that should be recognized. Unfortunately, slump does not describe the properties of workability, pumpability, and finishability that are so important to the contractor.
AGGREGATE GRADING AFFECTS SLUMP
Concrete slump can vary even if the total water content remains constant. To understand why this is so, we need to look at the materials used to make concrete. Aggregates are usually thought of as being either coarse or fine. However, it's better to think of three sizes: coarse, intermediate, and fine. Both coarse and intermediate particle sizes are building blocks, like two sizes of field stone used in laying up a wall. They carry loads but must have something to hold them together. In fresh concrete they also need a lubricating layer to help them move into place. Fine particles combined with cement paste provide this lubricating layer that later holds the bigger particles together. Fine particles have much more surface area to be coated with paste than do coarse or intermediate particles. If the amount of fines in the combined aggregate increases, slump will decrease and more water will be needed to restore the lost slump. The additional water will increase water-cement ratio and decrease strength even though slump is kept the same.
THE IMPORTANCE OF MORTAR CONTENT
Fines and paste together comprise the mortar in a concrete mix. Mortar performs the following functions: it controls workability; provides the fat needed to assure dense formed surfaces and slabs that can be hard troweled properly; and mortar is the source of lubrication in pumplines.
CONTROL MORTAR CONTENT AND SLUMP
It is suggested that slump and mortar content should be specified when concrete is ordered. Mortar content is calculated by summing the absolute volumes of all mortar ingredients--aggregates finer than the No. 8 sieve, cement, fly ash, water and entrained air--and expressing the sum as a percentage of the total concrete volume.