Difference in strength between plant and job site specimens

[sapperDAK]

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?

 

[concretemasonry]

What is the size (# of cylinders) of the study and over how long a period?

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

 

[cvg]

all good questions, but why would it only affect the 5000 psi mix?

 

[cvg]

maybe they have changed the admixtures they are using on this mix.

 

[testingonetwothree]

So you're saying that other mixes are being produced within the same time frame as the 5000 psi mix and none of them show any differences in strength?

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.

 

[Rjeffery]

I had a case where the contractor was ordering short loads of high strength concrete (less than 3 yards) and the strengths were much lower than when the full load was ordered. It was determined that the short load was diluted by the wash water that remained from the previous "Harry Homeowner" mix but there was much less dilution when a full load was ordered...

The best way to test something is to squeeze it, slowly, until it breaks!

 

[msucog]

how much of a reduction are you talking about? also, are the supplier and testing lab making the same size cylinders at the same time (on the site) and storing in similar conditions? also, is the mix for something different than the other mixes (being pumped for slab, wall, etc)?

 

[cvg]

...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.

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...