Interpolating 28 days concrete strength
Interpolating 28 days concrete strength
(OP)
Is there any literature/graph/table available for converting 50 day concrete strength to 28 days strength?
The 50 day strength is based on actual Core sample of hardened concrete.
Thanks
The 50 day strength is based on actual Core sample of hardened concrete.
Thanks






RE: Interpolating 28 days concrete strength
Usually you would just use ACI Chapter 5 for acceptance of concrete cores.
See also this FAQ: FAQ507-1575: How to Evaluate Concrete Tests per ACI 318
RE: Interpolating 28 days concrete strength
The concrete was poured under cold weather conditions and the contractor didn't maintain required temperature for proper curing of concrete. The ambient temperature was +5 degrees centigrade at time of pour.
Although the lab cured cylinder test results are ok but they do not represent the actual concrete strength. By the time the core samples will be tested it will be around 50 days from the day of casting and so to ascertain 28days design strength, we need to back interpolate the test results.
RE: Interpolating 28 days concrete strength
Dik
RE: Interpolating 28 days concrete strength
Decreased curing temperatures have a marked effect on strength gain, even in controlled curing conditions such as in a laboratory moisture chamber. A few degrees cooler than "standard" will retard strength gain.
In the field, your strengths would probably be substantially lower at 28 days than that lab cured specimens. At 50 days, they are probably starting to get to the point they should have been at 28 days, but only coring will tell.
RE: Interpolating 28 days concrete strength
RE: Interpolating 28 days concrete strength
RE: Interpolating 28 days concrete strength
The PCA strength curves are generalizations...you need specifics. That can only be gained by in-place testing and a comparison to YOUR mix design's strength curves, provided by your concrete supplier. Anything else is a guess.
RE: Interpolating 28 days concrete strength
Now I agree with all the comments above, there is no way to know precisely what that strength was at 28 days. Any means of back calculating a strength would only a guess and a really rough guess at that.
RE: Interpolating 28 days concrete strength
RE: Interpolating 28 days concrete strength
RE: Interpolating 28 days concrete strength
In the case of high strength concretes (say 80MPa or higher) heat of hydration and cracking is a serious concern. In this case it is not unusual to ask for a relaxation and allow for testing of the "28 day" cubes at 56 days. This allows for the best possible use of cement replacements and slower gain of strength. But 80MPa at 56 days satisfies the design requirements as well as 80MPa at 28 days.
RE: Interpolating 28 days concrete strength
We now need to verify if they were exposed to same temperature as the in-situ concrete or not. If not, we will go for core sampling.
RE: Interpolating 28 days concrete strength
RE: Interpolating 28 days concrete strength
Isn't it a standard acceptance to take average cylinder test results for particular area to represent the strength of actual slab or beam? Yes, it is very small in mass compared to actual area, but that is standard as per ACI.
RE: Interpolating 28 days concrete strength
mix design
cement, aggregate and admixture certs or test results
fineness and temperature of the cement (did you check that?...)
slump tests
delivery tickets with amount of water added
air content
admixtures content
actual w/c ratio
measured concrete temperature
elapsed time from the mixer to the forms
vibration and placement
curing
weather conditions during pour and curing
special measures taken if it was hot or cold
finishing, including saw cuts
anything else I missed?
RE: Interpolating 28 days concrete strength
If field cured specimens test to less than 85% of f'c, then additional means of concrete curing and protection are to be employed.
cvg gave a good list of things to look for and check. Follow it.
RE: Interpolating 28 days concrete strength
RE: Interpolating 28 days concrete strength
The field cure samples are having more than 85% strength of lab cured cylinders( and also more than required 45MPa fc'),so results satisfies requirements for section 5.6.
BigH,
The question is acceptance of concrete slab due to non conformity by contractor for not keeping slab temperature as per cold weather concreting requirements. The 28 days strength of Lab cured cylinders are ok, so it confirms the mix was ok. To verify if the cold weather had any affect on the slab, "field cured" cylinders are tested.
RE: Interpolating 28 days concrete strength