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7-day vs. 28-day compressive strength tests

7-day vs. 28-day compressive strength tests

7-day vs. 28-day compressive strength tests

As a construction services inspection and testing firm, we routinely cast sets of four 6 x 12-inch specimens (2 tested @ 7 and 2 @ 28) and sets of five 4 x 8-inch specimens (1 @ 7, 3 @ 28, 1 to hold). Recently a client questioned why they should pay us to test the 28-day specimens when the 7-day specimen achieved the specified 28-day strength. Can we hang our hat on anything other than ACI 318 indicating a valid test requires the average of 3 4x8's, and the loose "7-day tests are purely diagnostic" comment? Thanks!

RE: 7-day vs. 28-day compressive strength tests

you should ask the building department issuing the permit what they think about it.

RE: 7-day vs. 28-day compressive strength tests

Charge a fixed fee for sampling, cylinder making and storage/transportation.  Then charge a nominal fee for breaking, recording and reporting the results.  If the client wants to save 1%-2% by using only 7-day breaks, then give that option.  You are not reporting to reviewing agency or owner, give them what they want and dispose of the unbroken cylinders, (makes coring later a profitable sure thing).

RE: 7-day vs. 28-day compressive strength tests

no one says you are required to make the 7 or 56 day cylinders. aci does state that the 28 day break are the "official" ones. break the 28 day cylinders only and if they don't come up, then coring is required unless the structural ok's something else. and ultimately, the structural engineer must direct what is required...the testing firm does not have the authority to specify the required testing...all we can do is quote aci, astm, etc. (or perhaps "suggest" something else for the eor to consider) unless we intend to take on the structural liability. besides, ibc says that you to do specific things or as directed otherwise by the eor...regardless of what the owner wants unless they can produce a piece of paper from the building official. again, only the 28 day breaks are technically required. i would point out that there will be no "heads up" if a problem with the mix happens...so if it is covered up by the time it's realized, then someone will need to uncover it.

RE: 7-day vs. 28-day compressive strength tests

ACI 318 does not appear to require the 7 day test...only the 28 day.

However, I seem to recall that the 7 day is commonly specified to provide early warning of any problems with the concrete and allow the contractor time to adjust the mix accordingly to avoid even more problems.

Thus, the 7 day provides an enormous benefit to the owner should something go wrong with the mix design.

As far as your main question - once the 7 day breaks show strength above the specified f'c, do you need to break the 28 day?  

I've not ever seen a 28 day come in lower than a 7 day so from that perspective it does seem to be an honest question.  But the question, as msucog states, should be directed by the Owner to the Structural Engineer of Record as they are the ones who are responsible for specifying acceptance criteria.

Also - technically - the building code (via ACI 318) demands tests at 28 days.  The assumption that the 7 day break will always be lower than the 28 day may not be valid in some cases...I can't think what those cases might be though.


RE: 7-day vs. 28-day compressive strength tests

keep in mind the number of specimens required. 1 at 7 days is not sufficient. if the owner is so confident that it will pass, then test only 2 (6x12) or 3 (4x8) at 28 days. if they only want 7 day tests, then make sure you've got a full set and discuss in advance what needs to happen when the results come up low.

also, i would not expect 28 day results to be lower than 7 day results...however, i could see a single cylinder being the "oddball one that broke high". i suggest you stick with 28 day breaks so that you don't end up with a slightly low 7 day break and no other cylinders to test at 28 days.

how high are the 7 day breaks coming up as compared to what is required? and how much higher are the 28 day breaks versus the 7 day breaks? what are the initial curing conditions?--just curious.

RE: 7-day vs. 28-day compressive strength tests

I suggest that if this project is going to run for a few more months it would be a benefit to your client to test at 28 days. Even based on the 7 day test results you have already your client could start working with his concrete supplier on a new mix design. No point in continuing with a mix which is passing the requirements for 28 days at only 7 days (I would expect about 70% of the 28 day strength - but I'm sure you have a lot more data than I have. Why not sell this to him as a possible cost saving strategy rather than a cost.

RE: 7-day vs. 28-day compressive strength tests

It's been answered... 28 day is the official time, 7 day gives a 'heads up' and the owner is the beneficiary!


RE: 7-day vs. 28-day compressive strength tests

Thanks All! In response to muscog's inquiries...
how high are the 7 day breaks coming up as compared to what is required?
4,000-psi curb mix exceeding 4,000 at 7-days by 680-psi to 1680-psi - 3,500-psi mix strength ranges from 2,830-psi to 3,990-psi at 7-days
how much higher are the 28 day breaks versus the 7 day breaks?
no 4,000-psi 28's tested - 3,500-psi mix strength ranges from 4,270-psi to 4,940-psi at 28-days
what are the initial curing conditions?
specimens are moved inside the job trailer immediately after casting - trailer is air-conditioned and remains between 60 and 80 degrees F. - specimens remain at initial curing location for 24 to 48 hours then are returned to lab, stripped, and placed in lime-water bath

RE: 7-day vs. 28-day compressive strength tests

Typically 70% to 75% depending on the cement type.  I'll see if I can scan a graph.


RE: 7-day vs. 28-day compressive strength tests

One thing to remember is that the cylinder break requires two samples to be considered a test by ACI. Further the test results only tell how well the batched material conforms to the design mix. It is not the strength of the concrete in the field. Maturity meter testing can provide a more accurate depection of actual strengths in the pour.
So if you do substitute the 7 day for the 28 day, you need to follow ACI in terms of number of samples, etc.

RE: 7-day vs. 28-day compressive strength tests

Wouldn't the 28-day breaks be required (even if the 7-day breaks are above the required strength), just to get appropriate data to the mix supplier.  Since you need to design the mix to have a 28 day strength that is some number greater than f'c, I would think that the 28-day breaks are critical since that is what the ready mix suppliers are designing to.  I doubt that they want to start making mixes so strong that they are over the 28-day strength at 7 days as a rule.  
I understand that this may end up falling on the ready mix supplier and not on the owner, I just wanted to ask the question.
I guess I'm just asking this - aren't the requirements for a mix design to include the most recent 28-day breaks?  If you do away with 28-day breaks then that would skew the mix design in the future, wouldn't it?

RE: 7-day vs. 28-day compressive strength tests

28 day strengths are the ones... On a recent project, the contractor was not doing additional testing if the 7 day ones were greater than specd. I just cautioned him that although it was likely that the conc was OK, it didn't conform to the specifications.


RE: 7-day vs. 28-day compressive strength tests

building code requires the 28 day break set...just follow it. it a very small amount of money ($20) to break the two (or 3 for 4x8's) 28-day breaks. the contractor doesn't set the requirements so if it's their suggestion, it should be ignored. if the eor absolutely says they only want two (or three) cylinders at seven days, then document it and let them pay the price when the breaks come up low. be sure to point out that the directive does not satisfy ibc 2006 (assuming in usa) and don't put out any paper saying they satisfy building code requirements.

RE: 7-day vs. 28-day compressive strength tests

zambo is right. there is no need to go all the way to do the 28th day test when the 7th day strenth is greater than the 28th. its just an extra cost to the client for the over designed mix (consider the cement content. i say 7 day is ok if greater than the specified 28 day strength.

RE: 7-day vs. 28-day compressive strength tests

extra cost to the client? what extra cost? $10-20 per set of cylinders? as soon as you need to core, you're well in to several hundred dollars just to show up and core one location.

i urge caution casting and breaking only the 7 day breaks. the 7 day strength can be greatly effected by the initial curing conditions and if you don't plan to go to 28 days, then the only other option is to automatically core if the 7 day breaks (assuming you're using two or three specimens as required for the "official" set) doesn't come up okay. i would do away with the 7 day breaks and shoot for the 28 day breaks.

i say cutting the testing will save no money in the long run. look for cost savings in other places.

RE: 7-day vs. 28-day compressive strength tests

pipelineTZM:  What you say may make rational sense, but from a contractual point of view, only the 28-d strength is regarded as the acceptance criteria unless there is a specific clause in the contract that waives this and makes the 7-d strength the acceptance strength.

RE: 7-day vs. 28-day compressive strength tests

msucog, bigh

I get your concerns. But I still believe that the strength of concrete at 28-day will never be less than the 7-day and if the 7-day is greater than the designed or required strength then the 28-day results will just satisfy your curiosity on the maximum strength attainable by that designed mix.

And one thing, I flipped things earlier, I meant "extra cost to the Contractor or whoever is supplying the concrete"

RE: 7-day vs. 28-day compressive strength tests

From a legal and code perspective, the only "official" test is the 28-day strength.  It could be argued, in the event of an issue, that if only the 7-day test had been done, there was no official test.  

Keep in mind that the 28-day test is to validate the design mix.  It has little to do with the actual in-place concrete strength.  In situ strength is an assumed extrapolation of the design compressive strength and will vary significantly depending on ambient conditions.

Further, there are numerous reasons that you should charge for the 28-day specimens, tested or not.  A majority of the time/effort was spent in obtaining proper specimens for testing.  The administration of all of the specimens occurs at the same time (logging/marking/staging in curing room) when they get to the lab.  Pulling the specimen and performing the test is only one relatively small aspect of the process.

As for validity, the 28-day test not only verifies the design compressive strength, it validates the 7-day test.  To delete the 28-day test prevents the use of a moving average as required for evaluation of strength on larger projects.  

Suppose the 7-day specimen got mis-marked and was from a project that had 5000 psi concrete.  At 7-days, it would likely test above the required 28-day strength for a 3000 psi mix.  Further, lets assume that the actual 3000 psi mix was slow to gain strength and actually exhibited 7-day strength of 1300 psi and shoring removal was done based on the erroneous, mis-marked compressive strength test.  A failure occurs...would you want to not only explain why the mis-marked specimen occurred but also that the contractor relied on an erroneous result to remove shoring?  Now you go from a simple administrative error to professional negligence.  THIS SCENARIO HAS OCCURRED MORE THAN ONCE!

Bottom line...don't compromise the integrity of the testing process for the sake of the client saving a few bucks.  You should get a reasonable fee for making/breaking the concrete.  Don't low-ball it.  It hurts the profession and allows an influx of cost-cutting measures that compromise the professionalism of engineering.

RE: 7-day vs. 28-day compressive strength tests

This is simple, you do as your client directs.  Express your concerns if they are not following the Specifications or Building Codes.  Only do this once, more then that and you will probably not remain their testing firm.  They can deal with the Building Officials if the concrete test reports are not accepted.  

On a side note, we routinely except 7 day breaks and have never been questioned by a Building Official.  However, we fully expect 7 and 28 days breaks when specified.   

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