Concrete Strength

Gnanchandra

We have a concrete pour(M30) where we have collected cubes 150x150. At 7 days the strengths were good but at 28 days the cube strengths are lower than the 7day strengths.
Our engineer has asked for concrete core collection.Accordingly,we have collected two cores from the concrete structure. The core sizes are 70mmx150mm. When tested,one of the cores exhibited low strength.

What is the relationship between core dia / length with the concrete cube strengths? I would like to get some information on this to show the strength of the structure is as per design(M30).One sample strength is 85% of the cube strength but another sample strength is 75% of cube strength.What is the exact correction factor that is to be applied to confirm whether the core strengths are within allowable limits.

I shall greatly appreciate if any one could give some references of codes(ASTM) or ACI.

Kanding

I imagine you have this issue resolved by now, but if not, here are my thoughts.
Typical test cylinders are 6” diameter x 12” long. There is no general relationship that I am aware of between specimen size and maximum compressive stress. For beam and column design, for example, there is no adjustment made whatsoever to max. stress (f’c) and the shape/size of the compressive stress block. Were the cubes taken from the pour or cut from the cured concrete? Where were the core samples taken? A difference of 15-25% doesn’t surprise me.
However, one issue to consider is, if the cylinder diameter is small relative to the largest aggregate size, I would expect a reduction in strength. Your 7 cm diameter test cylinders appear small. What is your largest aggregate? I would think if the aggregate is larger than about 2 cm you may experience a reduction in cylinder strength. I don’t think ACI 318 specifies cylinder size, but I am not sure about ASTM. You may want to get the details on how the compressive test was conducted and how force (what the test machine measures) was converted to stress. It may be as simple as a calculation error because of using the incorrect cross-sectional area.
It seems very odd to me that the 28 day strength is lower than the 7 day. Did the concrete experience excessive heat or cold during the curing process?
These are some of the issues that I would raise with your engineer if you haven’t already.
Good luck.

Chris Johnson

Any strength less than specified or what was ordered falls back on the concrete supplier.

Rather than loose sleep over this I would be contacting there quality control and batch department and let them know what is going on.

Assuming your mix design is 3000 psi (approx 20 mpa) or higher engineer approval of the mix design should have been obtained prior to pour, this removes the onus of responsibility from you and leaves it between the engineer and the ready mix plant.

Any failure below design strength should be removed and replaced, unfortunately there is a cost involved with this, fortunately you will not have to bore the cost, the ready mix operation is insured for these failures...you did keep your tags from the pour...right?

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Chris

concretemasonry

First, you test the compressive strength by either cubes or cylinders, which ever was used in the mix design strength. Trying in to intermigle sampling and testing method for the two types of samples (cores and cubes) gets complicated by the aggregate size also. There is no cook-book formula, but you need a structural/concrete engineer to sort out the mess.

Second, you have a problem with the sampling or handling/testing of the samples. Both your 7 day and 28 day tests should have been from the same sample and load out of the truck/mixer. Concrete does not lose strength going from 7 to 28 days, so it must be a sampling problem, handling problem or a testing problem if they were actually from the same batch.

Are you testing individual samples or set of samples (3 or more samples per set).? Most standrds require sets since an average of one is meaningless.

Give your specifications to a concrete engineer along with the test results.

If you only have individual samples, you have nothing meaningful. - Is it a major application (several hundred cubic yards or meters) or just incidental work?

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Dick

Engineer, designer and consultant recently active domestically and internationally on construction and design in about 35 countries.