In normal concrete structures, 28-day test results are often adopted.
However, in the construction of high-rise buildings using “High strength
concrete”, compressive strengths based on 56 or 91-day compression test
results are commonly used instead.
Dear Abbas,
High-strength mixes are commonly proportioned to meet their design strengths at 56 or 91 days instead of 28 days, since the full strength is ordinarily not needed as early as 28 days. Indeed, after 28 days, it was noticed that the compressive strength keep increasing. So at 91 days for example, the increase is up to 20%. The splitting tensile strength is even higher. Often we test at 28, 56, 91 (multiples of 7).
Best wishes
20% increase at 91 days, i think it is a good percent for enhancement strength of concrete.
Yes Abbas 20% increase at 90 days is a good percentage of increment.
Just remember to use the properties for the actual loading age when calculation earlier load effects.
dear Dr. Amjad,
Please check the pdf you sent before. I think it is not the file you mentioned. It happened all the times if you are using Windows. Sometimes the O/S changes the filename when you copy it. :)
I have an old one
ACI 363R-92 (Reapproved 1997)
Please read
5.11-Strength gain with age
In my oppinion the real and neccesary age for any concrete is 28 days, I thuk that to stablish the compression resistance to 56 or 91 days is only to quarantir this result, But the normal consideration is to 28 days.
It was not mentioned here that the period after which we should test the strength of concrete depends on the type of cement used. Some of them, for example those with significant share of ground granulated blast-furnace slag, develop their strength slower. In those cases the 28-days strength is not representative.
I hope I have helped. :)
So, in international standards, can we adopt the strength of concrete gain with age?
You may use 90 day strength, but be aware that the whole age adjustment is assumed to be for "Normal" cements (Old Type A).. The use of slags & etc changes the rates of strength gain. But so does the use of accelerators, higher or lower temperature curing, retarders and so forth. To do justice to your mix, you should adjust curing and age of test depending on all these parameters. To get an earlier idea of the strength at any given age, you can plot 1/strength against 1/time for constant temperature curing. Ages of 3, 7 and 14 days will provide a very good estimate right our to 1 year - but at least 3 samples for each age should be used.
If you are using high temperature acceleration (e.g. steam curing), then you should use Arrhenius Equation and add a temperature factor into it. For ease, you can use maturity meters to check the in place curing, or use in place temperature gauges and a plotter (digital or analogue) to get the appropriate real curing.
There are plenty of papers on this, including C Rajesh & NJ Carino, Malhotra M, N J Carino, and NJ Carino & RC Tank.
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