ACI Factor of Safety Based on Design Strength?

[Timoteo811]

Do the ACI load factors depend on the late gain strength of concrete?
Specific question:
Both Field and Labortory test come back below strength at about 85% of fc'(28 day). Testing at 56 days shows that the strenth is about 90% of desing strength.
Knowing that there is still some design strength to be gained and the it is not a construction loading issue the concrete is accepted.
Is this concrete more prone to failure since it did not achieve the design strength at 28 days but rather after 56 days?
Also if cores are used to determine the acceptance of the placed concrete, do I understand it correctly that the samples only need to acheive 85% of the desing value to be "full strength" per ACI?

 

[JedClampett]

If I understand your question, no, ACI doesn't consider any value of strength except the 28 day values. Judgements are made all the time by engineers that if the 28 day strength isn't achieved, but it happens later, the structure is still acceptable.
I would be worried about a concrete mix that can only reach 90% of its design value at 56 days. Some investigation is in order. Most concrete is 20% (or more) over at 28 days. You're likely to have other issues (shrinkage, deflection, etc.) that are only loosely related to strength. There's something wrong, like old cement.

 

[Ron]

Agree with JC....

Basis of design is f'c, which is defined as the strength at 28 days.

Judgment of acceptance should be based on more than just the lab data. Field investigation is in order. If you can't resolve, then recommend an in-place load test.

If the concrete is in an exposure that durability is at issue, it will be seriously compromised (even moreso than strength), based on the results you have noted.

 

[Ron]

Correction....f'c is not necessarily defined at 28-days, but is generally considered to be such and is the common basis for design. 56-day, 90-day and other defined time strengths can also be used for design, but should be considered on the front end, not forced for compromise after the fact.

 

[JAE]

If you are at 85% of f'c you may not necessarily be outside the required parameters specified by ACI.

Check out Chapter 5 for acceptance of concrete and below is an FAQ that deals with this topic.

faq507-1575