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Name: Ken  Day
Heading: 1.1.1RE:Breaking Beams
On a recent job involving an Hanger ramp to the airstrip, the lab of record took 6x6x36 beams and will break half at 7 days and return the remainder in the cure tank and break the other half at 14 or 28 days. I looked in ASTM C 31 and could not find any reference one way or the other. Is this a proper proceedure?

Marc Robert

The ASTM does not tell when a sample should be broken but how it should be broken. You will need to look in your project specifications for when samples should be broken.

Rod Higley

In the 1980s I was with the Australian Airfield Conconstruction Authority and saw test results from several airfields. I came strongly to the view that, while flexure strength is what matters, compressive tests are a better QC tool because the former are too subject to error.(So prove the mix on lab flexure tests and establish an equivalent compressive result for QC - Bryant Mather agreed with me on this).

Now to answer your question, I agree with Rod, but the QC authority need to establish conversion factors for the particular mix and then convert results to 28day strength. It is certainly good to get early results rather than find a problem only after 28days. I suggest that if the QC authority does not also take compression specimens then you take them yourselves so as to be able to establish that the mix didn't change (IF it didn't)when their results fail from time to time. The main, but not the only, factor in defective results seemed to be great sensitivity to early curing (immediately after casting and at all times to test). Splitting tensile strength is also more reliable than beam strength but not quite as reliable as compression, however it does measure tensile strength which does not have the same relation to compressive for different mixes (eg crusher aggregate has a higher tensile to compressive ratio than gravel).
Ken Day
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