Higher compressive strengths than required
Higher compressive strengths than required
(OP)
Hi Guys,
I have a concern regarding excessive compressive strength of concrete for flat slab structure. We have designed one concrete structure (2 storeys) and we are getting concrete test results of almost 65MPA (9427psi) against 35MPa (5000psi) as specified per drawings for slabs.
My question is, does this higher compressive strength than required per design, harmful in anyways to the conrete behaviour in terms of brittleness, resilience, cracking etc.
Appreciate for your input.
I have a concern regarding excessive compressive strength of concrete for flat slab structure. We have designed one concrete structure (2 storeys) and we are getting concrete test results of almost 65MPA (9427psi) against 35MPa (5000psi) as specified per drawings for slabs.
My question is, does this higher compressive strength than required per design, harmful in anyways to the conrete behaviour in terms of brittleness, resilience, cracking etc.
Appreciate for your input.






RE: Higher compressive strengths than required
It seems that an 80% overage does raise a flag, however.
RE: Higher compressive strengths than required
RE: Higher compressive strengths than required
Any thoughts on this?
Regards,
Lutfi
RE: Higher compressive strengths than required
If your development lengths are all still based on the lower prescribed f'c, then it all should work fine.
Minimum reinforcement in beams and slabs is based upon equation 10-3 in ACI 318 and that includes an f'c parameter - but it also says you need not go over 1.33 x As(required) so that lets you off the hook.
Beyond that - yes, richer mixes sometimes shrink more but with an elevated, formed system the shrinkage stresses don't build up as much.
RE: Higher compressive strengths than required
RE: Higher compressive strengths than required
It appears that it more like a shrinkage issue in the earliest setting time of concrete than any design issues. Concrete strength exceed with the time anyways for a certain period of time.
RE: Higher compressive strengths than required
I ended up approving the submittal, as the structures are pretty insignificant and I really didn't have a reason to reject it.
But the whole design theory of ultimate strength of concrete is based on a well controlled yield point, giving sufficient warning of overload. There are calculations that adjust phi based on whether the section is tension or compression controlled. It's never a net negative to have stronger steel, but it makes me a little uneasy.
Anyone else seeing this trend?
RE: Higher compressive strengths than required
RE: Higher compressive strengths than required
Regards,
Lutfi
RE: Higher compressive strengths than required
In 21.1.5, there's restrictions on strength of reinforcing and ratio of yield vs. tensile strength, but it's only in certain seismic design categories.
I suspect that someone had a batch of reinforcing that couldn't be used in California because of 21.1.5, so they dumped it in Arizona.