What is the meaning of slump and what does it tell me about my concrete?
Introduction | Slump and Vibration | Error in Slump Tests
Introduction
Slump is a relative measurement in concrete consistency. It is not an indicator of quality of the material. The slump of a mix with the same aggregate, cement and water can vary significantly by adding an admixture. The admixture does not reduce the quality of the material.
Slump and Vibration
Vibration is not allowed in the slump test procedures under ASTM, AASHTO or other test procedures. This includes tests on any material - there is no distinction made between standard concrete, high-early strength concrete, econocrete, etc. Standard procedure is to tamp the material 25 times in three lifts.
The logic of allowing vibration is a matter of opinion. Vibration would generally be acceptable, as long as it is done for all similar material. Obviously, one would not want to vibrate a slump cone on some batches and tamp others. Slump is a relative measure of concrete consistency, not a measure of quality or strength.
The only deviation from tamping was found in a textbook entitled "Concrete" by Young and Mindess (concrete experts). The quote is as follows: "Concretes with slumps less than 25 mm (1 inch) should be tested by another procedure, preferably on involving vibration, since vibration will be needed to consolidate such stiff concrete."
Error in Slump Testing
In a study done by ASTM in the early 1960's, it was determined that measured differences in slump of greater than 2 inches from the same batch are considered excessive. Less than 2 inch differences are considered to be typical for sampling, testing and material variation.
What does this mean for slump measurements that are low by 1/4 inch?
First of all, by all ASTM and AASHTO standards, the measurement of slump is to the nearest 1/4 inch, which is a tight measurement for such a crude test. It is widely accepted that the test procedure itself will not be accurate to within more than 1 inch. The following quote is also from "Concrete":
"Although the (slump) test is reasonably reproducible with a skilled operator, it is quite sensitive to variations in test procedure, and it is not uncommon for different operators to achieve values more than 25 mm (1 inch) apart for the same concrete."
For example, location of measurement can easily produce a difference of 1/4 inch or more. (The measurement should typically be made to the center of the top of the slumped concrete). Other variability can be induced, such as the time the test is taken, the quickness in pulling the slump cone from the material, the tamping, etc.
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.
