7-day vs. 28-day compressive strength tests
7-day vs. 28-day compressive strength tests
(OP)
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 5.6.2.4. 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
RE: 7-day vs. 28-day compressive strength tests
RE: 7-day vs. 28-day compressive strength tests
RE: 7-day vs. 28-day compressive strength tests
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
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
RE: 7-day vs. 28-day compressive strength tests
Dik
RE: 7-day vs. 28-day compressive strength tests
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
Dik
RE: 7-day vs. 28-day compressive strength tests
RE: 7-day vs. 28-day compressive strength tests
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
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
Dik
RE: 7-day vs. 28-day compressive strength tests
RE: 7-day vs. 28-day compressive strength tests
RE: 7-day vs. 28-day compressive strength tests
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
RE: 7-day vs. 28-day compressive strength tests
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
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
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.