I have seen multiple blog posts recently critical of the slump test. Some have even gone as far to say slump is a useless test. While I agree slump is a very old test method, it is still a powerful and worthwhile tool in the concrete industry. Others within our industry have advocated the slump test should not be conducted when running quality control or quality assurance (QC/QA) testing on a project. However, if we don’t employ the slump test, what other controls could replace it and still give the design team and end user a level of confidence that the concrete is suitable for their project without waiting for weeks to pass? The slump test has a long history of use in the construction industry and still has a significant role to play in producing quality concrete.
During concrete placements, regardless of their size, there is a need for a quick, basic, fundamental, and repeatable test method to accept or reject concrete deliveries. While there are far better test methods and more currently under development that might be useful in the future, they are often too complicated and expensive. The construction industry still needs a simple test which technicians can conduct efficiently on the job.
Ultimately, I think people are looking at the purpose of slump testing the wrong way. The slump of the concrete is actually part of the means and methods of concrete construction. For example, if you are placing concrete in a slip form paving operation, you wouldn’t deliver concrete at an 8-inch slump. Slip form paving placement requires a low slump concrete. Moreover, a low slump paving mix and 8-inch slump can both have the same harden properties.
So where exactly is the value in the slump test?
I suggest the slump range should be defined by the contractor and ready-mix supplier and not specified in the contract documents. The submitted mix design would present the targeted slump and acceptable range. When the submitted mix design is approved, this range should then be the targets used for QC/QA testing on site. The specification should spell out the requirements of mix design which are important to the structure, such as: strength, durability and architectural requirements.
I agree that slump does not directly measure the comprehensive performance of hardened concrete; however, there still is a need for a quick, inexpensive method to control concrete mixtures during construction. Until there is a cost-effective replacement for slump, the construction industry will continue to use the slump test for many years to come. Instead, specifiers should consider how slump is used and specified. Many construction-related issues could be solved by properly designing concrete mixtures to be placed at the proper slump rather than at specified slump ranges which do not allow for proper means and methods.