ACI Factor of Safety Based on Design Strength?
ACI Factor of Safety Based on Design Strength?
(OP)
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?
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?





RE: ACI Factor of Safety Based on Design Strength?
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.
RE: ACI Factor of Safety Based on Design Strength?
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.
RE: ACI Factor of Safety Based on Design Strength?
RE: ACI Factor of Safety Based on Design Strength?
Check out Chapter 5 for acceptance of concrete and below is an FAQ that deals with this topic.
FAQ507-1575: How to Evaluate Concrete Tests per ACI 318