Specified Compressive Strength VS Average Compressive Strength?

[1rv4n]

Hi, I desperately need your help.

I'm currently doing cylindrical compressive strength on concrete with according to ASTM procedure. As far as I know, the minimum compressive strength of the concrete on 7th day should be at least 65-70% from the strength. In my calculation, I'm using concrete with fc'= 3500 psi. So, if I'm testing the specimen, the 7th day's concrete strength should be at least 70% from 3500 psi.

But, on ACI318 table 5.3.2.2 "Required Average Compressive Strength When Data are not Available to Establish A Sample Standard Deviation", there is required average compressive strength (which is fcr' = fc' + 1200 psi). The owner's supervisor told me to use that value (fcr') because the strength in my design (which is 3500 psi) is the specified compressive strength (fc'), while in the test, I should use the average compressive strength(fcr').

The value of 1200 psi is given through standard deviation which I really not familiar with. So, in testing the concrete, should I make a concrete mixture to achieve the value fcr' or fc'?

Can someone help me on this case please?
Thank you very much.

 

[ishvaaag]

Yes, you should target the value of the specified strength plus the standard deviation, hence you should have 65% of fcr' in your 7 days' tests.

By making series of tests with the trial mix prior to starting concrete production you may get bigger assurance and economy since you will get a more accurate statement of the standard deviation, so you can finetune your trial mixes for target strengths.

I attach one Mathcad worksheet I did on the matter as per 318 95, the basic ideas should stand more or less the same.

 

[ishvaaag]

The actual Mathcad 2000 Professional Worksheet that allowed for the fc calcs as per ACI 318 95

 

[Ron]

Assuming you are in the US, since you reference ACI 318...

You should not base your mix design assumptions on a 7-day strength. That is for indicative purposes only and is only a "rule of thumb".

The design compressive strength is to be reflected in the mix design. This is done by testing two specimens at 28 days and taking the average of those two specimens.

The value of 1200 psi is given through standard deviation

....not true. The value of 1200psi is a "safety factor" to use when you have no statistical data from which to compute a standard deviation.

If you have no historical data on the mix design, such as at least 30 consecutive tests of the mix, you cannot compute the standard deviation, s. Therefore you must move on to the table that requires using f'c+1200 psi for your design. The actual test value in checking f'c+1200 psi is obtained from the average of two specimens tested at 28 days.

In summary, if you have no historical data on the mix design, your mix design must be based on a compressive strength of 4700 psi, not 3500 psi. The mix design results are then tested by doing compressive strength tests on at least 2 specimens tested at 28 days. The designer would still use f'c of 3500 for his computations in the design, you are just using the 1200 psi buffer to make sure that the 3500 psi requirement is met at 28 days.

 

[dcarr82775]

I agree with Ron. You need to shoot for a 28 day strength of 4700 psi in your mix design.

The 7 day strength percentages are just rules of thumb so view them in that light. So long as it is in the ballpark of 60-70% (of 4700) at 7 days the mix will probably be ok. If it comes in well under that it could mean there is a problem. Of course if you have a fair amount of fly ash in the mix it tends to gain strength a little slower so that needs to be kept in mind.

 

[DST148]

Reference ACI Concrete Knowledge Center: Results from the 7-day tests mentioned are usually not used for acceptance purposes, and thus there isn't a percent of the specified compressive strength that the cylinder must meet in order to pass the compressive strength test.

The 7-day test result is used to monitor early strength gain and is often estimated to be about 75% of the 28-day strength (Kosmatka, Kerkhoff, and Panarese, “Design and Control of Concrete Mixtures,” PCA, 2002). Neville (Properties of Concrete, 4th Ed.,” Prentice Hall, 1995), however, suggests that if the 28-day strength is to be estimated at 7 days, a relationship between the 28-day and 7-day strengths has to be established experimentally for the given concrete. For this reason, he states that various expressions for the ratio of the two strengths (expressions that were discussed in the previous edition of his book) are no longer thought to be reliable.
Regardless of the reliability of the estimate for 28-day strength, 7-day strength test results are useful to the contractor and concrete producer as an early warning signal. With today's fast-track concrete-placement schedules, it's essential for the contractor and concrete producer to know when 7-day test results are low. Then suitable steps can be taken promptly to adjust batch quantities, improve quality control procedures at the job site, and ensure that sampling, molding, and testing of the cylinders are being done in accordance with ASTM applicable standards.