Questionalble Concrete Strength. 1

[Berniedog]

On a project with footings with piers for High School Bleachers, the 7 day concrete breaks came in at 1420 psi amd 1320 psi. THE specified concrete was 3000 psi. Assuming the 7 day strength is 75% of what will be the final strenght, this concrete will be under 1900 psi. The minimum code (2006 IBC) specified concrete strength is 2500 psi.

I cannot find any literature that states what the consequesnces are for 1900 psi concrete. I checked the development length of the rebar and it is OK since I over designed the reinforcing steel some. I believe the concrete will not hold up to the freeze and thaw.

Any ideas?

 

[boffintech]

Was that a slag mix or flyash mix? If so the stated 7-day strength may be correct.

If not, what was the ambient air temp and the concrete temp as-placed on the day of placement and did the test cylinders receive proper initial curing? (I'm cynically doubtful.)

 

[boffintech]

Was that a slag mix or flyash mix? If so the stated 7-day strength may be correct. These ingredients make a mix low & slow.

If not, what was the ambient air temp and the concrete temp as-placed on the day of placement and did the test cylinders receive proper initial curing? (I'm cynically doubtful.) Failure to provide proper initial curing makes a mix appear low & slow.

 

[OHIOMatt]

I agree with boffintech, cylinders are often cured improperly and they are much more sensitive to cold weather than the mass of concrete in the gound for the footing.

I have, in the past used 56 day break results when using slag, flyash or had cold weather conditions (hopefully the testing company has kept a cylinder in reserve).

If after 56 days, you are still short of the 2,500 psi, do some NDT such as a swiss hammer and then core the areas with the lowest results and perfom a compressive test.

 

[JAE]

Check out the FAQ on Eng-Tips here:

faq507-1575

 

[Ron]

Boffintech is correct in that concrete made with fly ash or slag cement will exhibit slower initial strength gain; however, at 28 days the strength should be there without regard to the presence of slag or flyash.

You are at about 45 percent of the expected f'c at 28 days. For concrete having 30 to 40 percent ground blast furnace slag cement in the mix, I would expect the strength to be about what you are showing, maybe slightly more, but not much. Check the mix design (post it if you have a copy).

If you have a conventional mix with only portland cement, then you have an indication of low compressive strength. The result of that will be the obvious strength issues, but at least as important, maybe moreso, is that the durability will be greatly compromised. The concrete will have less resistance to freeze-thaw, it will be more permeable, will exhibit faster and deeper carbonation, and will offer less protection to the rebar from corrosion.

Don't evaluate the acceptance of concrete on strength alone. That is but one parameter, albeit the more measurable one.

 

[BigH]

We have a fairly extensive database of using Type F fly-ash. For our 20 MPa and 30 MPa mixes, the ratio of 7d to 28 d is about 0.62 and 0.65 respectively at a ratio of 50% fly ash and 50% Type 1 OPC. Specified strength is for 90 d strengths, not 28 d. For 30% fly ash to 70% OPC type 1, we found that the strength from 28 d to 90 d increased by about 30% on average - I usually used 20% for estimation purposes. This particular contract used 28 d strengths as the acceptance strength.
Concrete placing temps are in the order of 28degC; ambient air temps range from about 25degC to 35degC. We do not have cold weather - unless you call 20degC cold.
If the OP's strengths fall short of the 3000 psi needed (and he should check against the ACI critiria (which does allow one value in 3 to fall 500 psi short so long as 3 consecutive values exceed his characteristic strength, he should, in my view, determine what the "real" strength is required for the footings based on the actual loadings - and determine whether the resulting values are reasonable. He should also core and use the criteria (as, for example, per CSA). This information along with judgment to the durability issues, etc. would lead to whether to accept the footings as is (with appropriate reduction in unit rates).