7 days concrete strength
7 days concrete strength
(OP)
Hi All,
I got concrete crushing strength of 18 MPa for 7 days and 52 MPa for 28 days test results. These results are completely confusing me. The mix design target strength is 30 MPa and the concrete has been placed on the bridge deck, what should I do with this results? What measures should I take to get approval of the work done?
I got concrete crushing strength of 18 MPa for 7 days and 52 MPa for 28 days test results. These results are completely confusing me. The mix design target strength is 30 MPa and the concrete has been placed on the bridge deck, what should I do with this results? What measures should I take to get approval of the work done?





RE: 7 days concrete strength
Does the mix design have flyash or some other constituent that would retard the early strength gain? The proportion of 7-day strength to 28-day strength is off....it is only about half of the expected for a conventional concrete mix design.
Getting 52 MPa when 30 was designed indicates to me that perhaps the wrong mix design was delivered to the site. The quality control process needs to be a bit better than that, either at the site, the batch plant or both.
Is the concrete acceptable? Well, from a strength standpoint, probably. From a strain compatibility, shrinkage, and uniform stress distribution standpoint, probably not.
Find out why there is such a discrepancy in the strength parameters.
RE: 7 days concrete strength
RE: 7 days concrete strength
RE: 7 days concrete strength
did you require the contractor to use a specific mix or did you just specify concrete must have min strength of 30?
do you have the scale readings for water, cement, agg and admix that were used in your bridge deck concrete?
and finally, do you have confidence that the compressive strength tests were done accurately?
RE: 7 days concrete strength
getting the cores test result you can decide better
RE: 7 days concrete strength
Dik
RE: 7 days concrete strength
RE: 7 days concrete strength
With regards to the discrepancy, it is as stated above more information is needed, your mix design may be incorrect, certain admixtures could of been added without approval or incorrectly.
Also ensure the concrete compression machine is properly calibrated.
RE: 7 days concrete strength
First of all, the 7-day percentage is a "rule of thumb" that is highly variable. To reject a concrete mix design that doesn't meet 75% of f'c at 7 days is irresponsible.
Second, it is generally taken as a 70% rule of thumb, applying only to conventional concrete mixes that contain cement, water and aggregates, without significant admixture enhancement.
With current technology, and the use of slag cements, pozzolanic materials, admixtures and other enhancements, it is common to see mix designs that achieve only 50% at 7 days but still reach f'c at 28 days.
RE: 7 days concrete strength
RE: 7 days concrete strength
RE: 7 days concrete strength
RE: 7 days concrete strength