7 days concrete strength 3

[Nkamu]

Hi All,

I got concrete crushing strength of 18 MPa for 7 days and 52 MPa for 28 days test results. These results are completely confusing me. The mix design target strength is 30 MPa and the concrete has been placed on the bridge deck, what should I do with this results? What measures should I take to get approval of the work done?

 

[Ron]

Nkamu...you'll have to give us a bit more information.

Does the mix design have flyash or some other constituent that would retard the early strength gain? The proportion of 7-day strength to 28-day strength is off....it is only about half of the expected for a conventional concrete mix design.

Getting 52 MPa when 30 was designed indicates to me that perhaps the wrong mix design was delivered to the site. The quality control process needs to be a bit better than that, either at the site, the batch plant or both.

Is the concrete acceptable? Well, from a strength standpoint, probably. From a strain compatibility, shrinkage, and uniform stress distribution standpoint, probably not.

Find out why there is such a discrepancy in the strength parameters.

 

[Nkamu]

Thank you Ron for responding to my question. Anyway I am still looking for the source of this big discrepancy in the concrete results.

 

[Ron]

Nkamu...you need to give us a bit more information to be able to provide you with evaluation or advice. We can't help you determine the source of the discrepancy without additional information...anything at this point would be a guess.

 

[cvg]

did you actually conduct trial mix design or did you just specify a mix?

did you require the contractor to use a specific mix or did you just specify concrete must have min strength of 30?

do you have the scale readings for water, cement, agg and admix that were used in your bridge deck concrete?

and finally, do you have confidence that the compressive strength tests were done accurately?

 

[mnsameti]

I think it is better you ask for taking at least 3 core , then after
getting the cores test result you can decide better

 

[dik]

Best to decide how you will deal with core test results before you take them...

Dik

 

[msesi]

The concrete strngth of seven days must be 75percent of 30MPa minimemu required for 28days ie must be 22.5MPa.If it is different the mix design is improper or no enough cement in it.Please redesign again.

 

[GavL]

Take 2 more core results and evaluate the average of the three according to what the code you are following specifies.

With regards to the discrepancy, it is as stated above more information is needed, your mix design may be incorrect, certain admixtures could of been added without approval or incorrectly.

Also ensure the concrete compression machine is properly calibrated.

 

[Ron]

msesi...perhaps you should explore concrete technology a bit more.

First of all, the 7-day percentage is a "rule of thumb" that is highly variable. To reject a concrete mix design that doesn't meet 75% of f'c at 7 days is irresponsible.

Second, it is generally taken as a 70% rule of thumb, applying only to conventional concrete mixes that contain cement, water and aggregates, without significant admixture enhancement.

With current technology, and the use of slag cements, pozzolanic materials, admixtures and other enhancements, it is common to see mix designs that achieve only 50% at 7 days but still reach f'c at 28 days.

 

[Hoaokapohaku]

Do the specifications have a required minimum water/cement ratio? When this is the case, a specification may force a design that produces much higher compressive strength than the specifications otherwise require. The specified w/c ratio may be specified in order to fulfill some other performance requirement besides compressive strength, such as impermeability. Or, the specification may be a boiler plate based on design requirements to achieve a certain strength using aggregates in another locality. In my location, we have no natural sand or gravel. All of our aggregates are 100% crushed basalt, hard with angular faces. Compressive strength 180% of design minimum is common when a minimum w/c ratio is specified. As others have pointed out, use of certain admixtures and/or fly ash can greatly alter the expected ratio of 7 to 28 day results.

 

[aaronc]

msesi, I would agree with Ron above, the 75-85% strength at seven day is a specification for stripping forms, and preloading (IBC). second - the average f'c is calculated with three samples broke at 28 day with no single break falling below design by more than 500 psi, ASTM C 39, ACI 318. The mix designs and noted specifications are a minimum requirement. Third coring is an option but you need to obtain a minimum of three cores for one test area. The average of the cores is equal to at least 85% of f'c. With out looking at the mix design and batch ticket for your project it is hard to determine if there is an actual problem. I hope this helps.

 

[runner44]

Sounds like you're getting a bit worked up over nothing. I've never heard of a bridge deck being rejected for too-strong of concrete. Cracking, too weak, segregated mix, sure. . .too strong? no.

 

[Nkamu]

Thank you all guys for your good contribution. I would like to inform you that, we have resolved the problem by making 3 cores from the bridge deck and crushed them. Crushing strength obtained were well above 30 MPa. Hence the problem has been resolved. Thank you very much!