As a contractor it's important for me to ensure the desired workability is achieved. For example, if I am pouring a set of stairs and the slump comes back at 180mm...I surely want a test to tell me so I can reject the truck before I get too much into the line.
As an engineer? It's not overly important when super-p or other water reducers are added into the mix. Prior to the development of such admixtures concrete slump was an okay measure of the W/C ratio of the mix, and obviously if it's too high one would want to reject the truck on concerns of low strength. However, nowadays even if the slump comes back high (in a mix with admixtures) you don't know if it's because of the admixture or the original W/C content prior to dosage. In which case, it doesn't tell you much from a engineering properties perspective.
So I guess it depends. You should still do them. I would say you'd be crazy not to given the tester is already on-site checking air. But you can feel free to make judgement calls on if it's of concern or not when slump is outside the stated range. I do not treat it, as some engineers do, as a strict test: meaning if it fails, reject the load. As a contractor I am fine to take on the risk in certain cases (where getting the pour in place is a much larger priority) and that should be my prerogative, not the EORs.
Thanks for your thoughts. I am an engineer so I care much more about the compressive strength (fc) than I do about the slump. Our specifications do require that the slump and the air content be checked.
PEFLWI - here's a nice article/Q&A about it. Brings up Enable's points, but also brings up the question of QA - if all the concrete coming out of the batch plant is the same, it should have the same consistency and the same slump. So rather than a specific slump, you may want to do something like "slump sufficient for adequate working of the concrete; all slump tests must agree within +/- 1in" or something similar.
Our notes indicate that the contractor can choose the slump based on finish requirements and placement techniques, however the contractor specified slump has to be on the mix design and batch ticket and must test within an appropriate range of the requested.
This gives the contractor freedom to choose, but to live by his choice and prove the concrete is meeting the requested spec.
The only two exceptions to this are deep piles/caissons and masonry grout. We have a minimum slump spec to ensure reasonable self consolidation.
Yes, the slump test is important. For numerous reasons. Here are a few:
1. The slump test measures consistency and consistency. Confused? The first consistency is a measure of the viscosity or workability of the concrete. Each application has its own demand for workability, whether is it "flatwork" or cast into forms. The second "consistency" is a measure of the uniformity or constancy from batch to batch. This is important since presumably the concrete being placed in a particular application will come from a single mix design. If all of the ingredients are the same, then the slump should be relatively consistent through multiple truckloads. If not, it is a quick red flag to determine the cause of the variation. Here's an example: You are placing 80 cubic yards of site cast tilt panels. Trucks 1,2, and 3 have slumps that are +/- 1 inch of each other. Truck 4 has a slump of 3 inches greater than the others. While the compressive strength of the load might meet the project requirements, would the durability be consistent? No. Truck 4 would shrink more, it would crack more and it might not weather the same, particularly in an aggressive environment.
2. The slump test is a basic quality control, quality assurance and quality verification parameter that is easy to perform and gives a quick look at specification compliance. Specification compliance is a contractual and building code mandate that can cost a contractor a lot of money if not done.
3. Remember....good concrete is made from some recipe of water, cement, aggregates and admixtures. Bad concrete is made from the same ingredients. The difference between good and bad is how we treat the ingredients. Throwing away a slump test does nothing to further the quality of concrete....it only gives contractors and suppliers and easy out when things go wrong. If they don't have a specification to meet, then how can they have done anything wrong!
