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Interpolating 28 days concrete strength 3

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LOKSTR

LOKSTR

Structural
Apr 15, 2005
122
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
 
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Why would you want to interpolate back?

Usually you would just use ACI Chapter 5 for acceptance of concrete cores.

See also this FAQ: faq507-1575


 
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JAE,

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.
 
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Unless a strength at a later date was specified, as JAE noted, going to an earlier date is not productive. The lab samples will give an indication of the long term behaviour of the concrete, unless it froze on site. At 5 deg, strength may be a bit delayed, but the end product should not have suffered any long term effects.

Dik
 
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Agree with JAE and dik...waste of time to try to estimate backwards, particularly since curing conditions were so vastly different. If the cores reach 85 percent of the design strength, call it a day.

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.
 
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PCA, "Design and Control of Concrete Mixtures" has several curves for estimated concrete strengths.
 
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ACI209 has strength models but I wouldn't attempt to back-calculate 28day strength. As long as the cylinder test come up to strength I would accept that.
 
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LOKSTR...the lab tests are not intended to represent the in-place strength of concrete. They are intended to represent the capability of the MIX DESIGN. Nothing more, nothing less. If you want in situ strength, cores are the better representation.

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.
 
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I think the bigger question here is, was the construction specifications met? Did the concrete meet the required strength at 28 days? If it wasn't meet, then why? Addition of mix water at the site, cooler curing conditions, etc. Obviously some of the possible reasons will likely effect the long term performance of the concrete. It all comes down to the owner getting what they paid for by holding a contractor to specifications.

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.
 
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Splitrings...it was already stated that the 28-day tests in the lab met the specification. That's all that's required of the mix design. The in-place conditions will be different than the laboratory tests...always! Sometimes better, sometimes worse...that's why you need to core the concrete if you want a reasonable representation of the in situ strength.
 
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Ron...I did miss that there were cylinders taken. Pretty unlikely that the construction specifications stipulated an in place strength requirement.
 
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28 day strength is a bit arbitrary, at this stage the concrete has gained most of it's final strength (maybe 95%) and is used as a good guide that the concrete has gained it's design strength (of course 28 day strength is used by various codes as an accepted norm)

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.
 
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Yesterday we received test results ( 53 days strength)of some cylinders that were "field cured". The results are ok.Although the gain of strength is bit slower than normal.

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.
 
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Lokstr...you have to look at more than if they were exposed to the same temperatures. The mass of a cylinder specimen is extremely small compared to the mass of the structure. Field "curing" is by no means a greater representation of what's in the structure than the lab cured specimens, unless special procedures were followed to maintain the specimens in the same location and conditions as the curing concrete...and even then, the masses are not comparable.
 
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Ron,

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.

 
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I'm not a structural, but managed enough projects to know that cylinders do not represent the actual strength of the slab or beam regardless of how big they are or how they were stored. They are only a quality control method used to help make a decision if the concrete slab might be acceptable. In addition to the cylinders, you need to look at the entire record:

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?
 
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LOKSTR...by definition in ACI 318, a "strength test" is the average of two laboratory cured specimens tested at 28 days or the time stated by the design determination f'c. Field cured specimens are not an alternate for the determination of the concrete strength, but are used to evaluate the adequacy of curing and protection of concrete in the structure. They are compared to the laboratory cured specimens and the "official" strength for these purposes, but are not intended to supplant the proper determination of the concrete strength under ACI 318, Section 5.6.

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.
 
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What is the underlying question - is it a matter of payment? i.e., the contractor did not do certain things according to the specification and, as a result, trying to back-calculate in order to strip away some of the cost? The question is: is the concrete good (does it make the specified grade - even after 53 days)? and if so, is that acceptable? On our contract here, the time limit is 90 days for the strength requirement - due to fly ash addition that is pretty substantial. I have a feeling that the concrete NOW has the strength and as a result the project is not "suffering" - but someone wants a claw-back . . .
 
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Ron,

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.
 
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LOKSTR the field curing as you indicated "can" give an indication - but don't forget a cylinder 150mm diameter does not have the same "warmth" around it as the slab in place (due to the mass) - so it would be hard to argue that the field cured samples are "really" representive of the slab. As you indicated the test results of the field cured are actually cores of hardened concrete and the results are more than the specified characteristic strength. So what is the problem - other than deducting funds from the contractor for not following the specified curing process?
 
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