Concrete Strength Issues

[strguy11]

I have a project where the Contractor is having issues getting concrete strengths that are required. Their submitted mix design for 4000 psi includes 395# cement, 169# slag, 1220 lbs fine agg, 1800# coarse agg + water reducing admixtures. Water cement ratio = 0.45

When the mix design was reviewed, test reports from previous projects in 2013 were sent in and these showed that at 28-days, this mix achieved around 6500 psi at 28 days. On this job, the same concrete mix is barely reaching 4,000 psi, and in some cases only reaching about 3800 psi. The concrete is being cured using a 7-day wet cure.

We have had an inspector go into the batch plan un-announced to inspect the plant and they indicate that the plant has passed certification. Batch tickets provided to the special inspector indicate the correct amount of cement has been used.

I am at a loss to understand why the strengths are so low compared to the historical tests. Any thoughts? The cement concrete seems right to me.

 

[Ron]

The character of the slag cement could have changed as could the fineness of the portland cement. Both will cause strength gain delays. Do a bit deeper comparison of the cements used then and now. Strength gain delay is common with GGBFS cement.

 

[strguy11]

Thanks for the information. How long does it typically take for the strengths to increase? We have had some cores taken at the 45 day mark and they are not that much higher than the 28 day tests (maybe a 10% increase) but still lower than the specified 4000. The sources of both the cement and slag are listed as the same, but I will try and determine if the character of the slag has changed.

 

[JAE]

I'd continue to investigate and get your strengths where they ought to be.
But don't forget to follow ACI's acceptance procedures:
faq507-1575

 

[Ron]

If the sources of both the slag cement and the primary cement are the same, I would suspect that you might only have slag cement in the mix. This will cause VERY slow strength gain and it might not achieve strength in any reasonable time period.

Agree with JAE...follow ACI procedures for evaluation.

 

[strguy11]

As a follow up, we are now getting 56 day breaks on cylinders and they are showing an average of 4500 psi, which is above the f'c. Appears we are having some lag in strength increase, but we have asked the concrete supplier to adjust mix to get the 28 day strengths up where required. Thanks all for the input.

 

[msquared48]

Sounds like Ron gave you very concrete information.

Mike McCann
MMC Engineering

 

[conceng]

Was there any plastic testing performed on the concrete? What was the air content, slump, density, temperature? These are also very key in investigating low strength results. Also a good idea to weigh the cylinders when strength appears to be an issue. The more data the better.


Greg

 

[BigH]

The OP gave lbs of cement and lbs of slag - so it appears that the slag is added separately from the cement? We are getting a lot of Portland Pozzolan cements over here in Asia - and becoming more difficult to get OPC. Problems do occur due to varying actual percentages in the PPC cements and also sources of the slag/fly ash. On the previous testing (not yours) do they have any strengths beyond 28 day?, say 56 or 90? so that you could tell what kind of strength gain you are really getting.

 

[dik]

We used to have issues in Winnipeg in the late spring and summer with low strengths. This was attributed to a lack of fineness in grinding the cement clinkers into powder. With the high demand, less time was spent grinding.

The mix design appears to give a much higher strength than is specified. You may want to check a couple of cylinders at 56 and 90 days and see if there is an sdded strength gain.

Dik