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# 7 Days Compressive Strength of Concrete2

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## 7 Days Compressive Strength of Concrete

(OP)
Concrete usually attains about 90% of its compressive strength after 28 days. What is the approximate 7 days and 14 days compressive strenght of concrete as a percentage of its ultimate compressive strength?

### RE: 7 Days Compressive Strength of Concrete

Check out Neville's tome on Properties of Hardened Concrete.  A lot depends on whether you are talking lab cured samples or site realities; of cement grade and fineness and admixtures.

### RE: 7 Days Compressive Strength of Concrete

Agree with BigH....lots of variables.

In general, using Type I Portland cement, you can expect about 65 to 70 percent of the 28-day strength in 7 days.  Not to many consistently reliable estimates for 14-day strengths, since it isn't usually a measured parameter.

Many reactions affect the strength gain of concrete, so you really need historical data on the mix you intend to use.

### RE: 7 Days Compressive Strength of Concrete

Two of the important variables that affect the site mix is the ambient temperature and the volume of the concrete cast. Another factor is whether pfa is used. As Ron says there are many others.

### RE: 7 Days Compressive Strength of Concrete

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.

Reference document is the textbook "REINFORCED CONCRETE Mechanics and Design" Second Edition by James G. MacGregor.

### RE: 7 Days Compressive Strength of Concrete

All response posters are correct but I think Stephen is right on the money, especially when looking at in-situ strengths.  Mass traps heat and heat cures concrete, hence strength develops much quicker in a massive concrete beam than it might in a 6" cube or cylinder.  An excellent tool to estimate insitu strength of concrete is the ASTM C1074 Maturity Meter.  A logger or sensor is placed in the concrete member and it will monitor concrete temperature over a pre-formated time, i.e. 28 days, 56 days, etc.  Time and temperature is calculated using a maturity formula to estimate either compressive or flexural strengths.  According to NIST maturity is the most accurate non-destructive test method of estimating insitu strength.  Contact me for more info.
Also, keep in mind that early strength is dependent on heat of hydration of the cementitious materials in the mix design.  If you have a large quantity of fly ash or slag in the mix, calories per btu will be lower resulting in deferred strength to later ages, i.e. 28 or 56 days.

Hope this helps.  It is huge in times of cement shortages.

Fred J. Croen, RSM
Engius, LLC
Boston, MA
www: engius.com

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