huangyhg

low 7-day concrete breaks
does anybody know of any requirement that f'c @ 7 days must be at least 75% f'c? on the particular job i'm working on (washington dc), we've had about 50% f'c after 7 days for several spread foundations...obviously~generally speaking~high early strength isn't as important with foundations as it is with elevated slabs. however, low early breaks are often an indicator of low 28 day breaks, but not always. the owner of this building is very upset about the low 7 day breaks, but i'm not aware of any aci *requirements* or recommendations with regards to f'c @ 7 days, so i really can't come down on the gc until the 28 day breaks are in, right? the owner is convinced that he's not getting what he's paid for
any thoughts?
thanks in advance...
the owner paid for f'c = that specified at 28 days, not the 7 day break. the 7 day breaks are usually provided so as to allow intermediate information regarding form removal, application of loads, etc. as well as what you indicate - an indication of the expected 28 day strength.
just review chapter 5 of the aci 318 and jump on the 28 day tests as soon as they come in.
along with jae's comments, there are wy too many variables that could affect break strengths for cylinders.
1. concrete mix additives (fly ash)
2. admixtures
3. cylinder preparation
4. curing technique
5. temperature
wait for the 28 day breaks and follow aci.
50% sounds low at 7 days. it is likely that 28 days may not meet specified f'c. i agree with jae and sperlingpe to wait for the 28 day test result.
below is my old response to another thread. hopefully it is of some use to you. based on the formulas, 7 day stregnth is 70% for type i cement and 80% for type iii cement.
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the rate of curing depends on many factors but assuming type i cement and moist cured at 70 degree fahrenheit, try the following equation:
f'c(t) = f'c(28) * { t / (4 + 0.85t) }
where f'c(t) represents the compressive strength at age t and f'c(28) represents the specified 28 day strength.
for type iii cement, use (2.3 + 0.92t) in the denominator.
jtdpsu...i agree with the above posts. jae's comment on the 28-day criteria is exactly the case and is something that most owner's/others don't understand. the strength gain between 1 and 28 days can vary all over the place, influence by many of the factors noted by sperlingpe.
one common reason for strengths between 50 and 60% at 7 days is the use of gbfsc (slag cement such as newcem or others). this material has a delayed early strength gain and many suppliers use it as a substitute for portland cement. fly ash and admixtures can also cause this.
get the supplier to provide strength gain curves for each mix design. this will help you evaluate early compressive strength indicator results.
just out of interest - what are you going to do if the 28 day results are low?
here in the uk we tell the contractor that strength is low and he gets extra cubes (cylinders in us) tested at 56 days which usually demonstrates a continued strength gain. i've always felt cheated by this as the extra strength gain was going to happen even on concrete with the full 28 day strength - as that designer, that is part of my factor of safety. it seems to me that what we are really doing is accepting a lower specification...
the 7 day strengths do appear low to someone who has not seen the records of strength development of previous batches. it may be that there is a genuine concern that the 28 day strengths will be low and the owner may be worried that concrete is continuing to be placed in the intervening 21 days with no attempt to invetigate if there is a problem.
problems which cube or cylinder test results will show up are change in aggregate quality (possibly excess dust), out of calibration weigh gear, defective admixture dosing equipment. if the 7 day results have changed dramatically the reason should be investigated.
zambo
astm c94 16.2 tests of concrete required to determine compliance with this specification shall be made by a certified aci concrete field testing technician, grade i or equivalent. equivalent personnel certification programs shall include both written and performance examinations as outlined in aci cp-1
to relieve yourself of concerns that the cylinders may be at fault ask to see the tech’s certification card and ask the contractor exactly how he is accomplishing the initial curing (first 24 hours at 60 to 80 f) of the cylinders.
but as ron wrote, your best bet for strengths between 50 and 60% at 7 days is the use of slag cement or fly ash. check the mix design.
techmaximus
if you have 3, 7 and 14 day breaks you can plot strength vs time and estimate from the curve the projected 28 days strength.
50% after 7 days does sound a little low.
in my career, i had two jobs that the 28 came lower than specified. in situation, the contractor lucked out because the lower strength percentage was with aci tolerances. however, on the second job, the contractor had to rip the entire concrete pour and start all over because he was way low and he hid the results from everyone. his thought process was that if concrete cured and construction forged ahead, client and the engineer (me) would accept the lower strength.
regards,
lutfi
can you think of anything that might have contributed to the low strength - any irregularities in the making/storing/curing/crushing of the cubes?

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