Difference in strength between plant and job site specimens
Difference in strength between plant and job site specimens
(OP)
At a project that has been on-going for over a year, the testing agency recently reported a reduction in 28-day strength of a 5,000-psi mix. This project uses multiple strength concrete from the same supplier ranging from 3,000-psi to 7,000-psi, but only the 5,000-psi mix has demonstrated this reduction in strength. There have been no changes in the testing agency, that is, the field technician casting the test specimens is the same, the initial and lab curing techniques have not changed, and the lab strength testing personnel and equipment are no different. Additionally, the concrete supplier is showing 28-day strengths for specimens cast at the batch plant from the same mix between 750 and 1200-psi higher than the specimens cast in the field. The slumps and air content at both the plant and the site are similar for comparison samples, and the concrete temperature is predictably 8 to 10 degrees F. higher on site. Any ideas why only the 5,000-psi concrete is showing this reduction in strength?





RE: Difference in strength between plant and job site specimens
The the same technican make all the samples and travel between the plant and the site? Despite what ASTM says, a technician can have an effect on cylinder strengths - between different technicians or just carelessness.
How old was the concrete when sampled at the site?
Look at the handling and curing before transport to the lab. This is one factor that will be different, even if it meets the procedure specifications.
Dick
RE: Difference in strength between plant and job site specimens
RE: Difference in strength between plant and job site specimens
RE: Difference in strength between plant and job site specimens
Is there anything unique about the 5000 psi mix that the other mixes don't share? If so, look at that.
Also, maybe the lab got another project's cylinders mixed up with this project.
Something has changed somewhere and you have to find out what that change is.
RE: Difference in strength between plant and job site specimens
The best way to test something is to squeeze it, slowly, until it breaks!
RE: Difference in strength between plant and job site specimens
RE: Difference in strength between plant and job site specimens
I would guess that they may be adding the wrong type or amount of super P, HR water reducer, air entrainer or other such additive. Also possible - if air temperature has increased, mix may be setting up quicker and drivers may be adding water before they deliver to the site...