Testing concrete strength with a laboratory compression test.
Q: We are experiencing low compressive strength test results on some concrete we supplied to a job. We know it is due to poor testing of the cylinders. How do we convince the contractor and engineer where the issue is?
A: This is a universal concern in the concrete industry. I have worked in 10 different states and two provinces in Canada and this has come up everywhere. I also can speak from experience on both sides of this argument, having worked with a concrete producer and a testing lab. I have witnessed poor testing from the producer's point of view and the testing lab side.
My words of wisdom: Be careful where you point your finger, as it can very quickly come back to point at you. Correlations are easy to draw when you look at data, but it may be completely inaccurate. Testing labs have called me, indicating that there are several projects experiencing low strengths and they all have my product in them. This is a very dangerous statement, as the testing lab is also a common denominator in this scenario.
My advice would be to start by taking a good look at your operations. If you can't find anything, you can hire someone to review the operations with you. If there is something wrong with the production or materials used, you need to know this. Accusing the testing lab incorrectly will not bode well for future discussions.
If you are convinced there is no issues with the product, ask the testing lab for information, including curing temperatures, how they transport the cylinders, and whether you can witness them conducting the testing.
Ask for the lab's quality control manual. If you're still not satisfied, hire another lab to test side by side. Do not do this out of spite. Indicate you need to protect your interests and the continued success of the project.
As a senior technical service engineer, I would be called to help with low-strength issues routinely. I have been the guy pounding his fist on the table, saying we will take cores and make this problem go away, proving the testing lab was at fault. Be present through the whole ordeal, and witness the cores being taken, prepared, and tested as they fail to meet specifications also.
If you want your ego deflated quickly, that will do it. On the opposite side, I've seen cylinders dug out of the dumpster and retested, achieving an additional 1000 psi. I also have seen cylinders buried in the back fill of foundations, frozen cylinders, and cylinders too hot to touch.
Each issue is different. I have witnessed blatant disregard for testing protocol, and have been able to point them out, although I've also witnessed many issues in the production of concrete. I've seen cement being loaded into the fly ash silo, contaminated stockpiles, admixture dosing mistakes, and three-hour haul times. Errors in testing do occur, but issues also exist in the materials, batching, and handling of concrete.
So, we all need to stop and get the full story. Recently, I dealt with an issue where strengths were maxing out at seven days and one set decreased in strength at 28 days. One might think the set that decreased in strength is a blatant testing error, and other sets look like the cylinders were cured in hot boxes which were too hot.
We can't jump to this conclusion because there are other issues which would cause good seven-day strength and poor 28-day strengths. Air entraining issues will do this, as will dirty aggregates. As the strength of the paste increases over time, air issues or dirty aggregates may cause the weak link in the chain.
In dealing with this recent issue, both of these causes were to blame. The coarse aggregate was very dirty, and the sand had a very fine gradation. These issues with the aggregates also caused clustering along with high air contents. Yes, there were testing issues, but there were also real issues with the concrete.
Try not to be adversarial in these situations. This will shut the process down. I have had more success working with all parties to resolve the issues and the project has continued.
Defensive mechanisms are easy to turn on but are difficult to turn off. You will need to work with these same companies on future projects. Build a relationship with the local lab and it will be more responsive to your requests and needs.
Contributed by Braun Intertec. Visitwww.braunintertec.com.