ACI Load Factor for f'c(90-days) 1

[cliziagatto]

Hello all!

Due to temperature problem we need to work with Low-Heat (LH) cement but is impossible to obtain the needed 28-days concrete resistance. For this reason we are re-designing with 90-days concrete resistance. But, what about the Load Factor in ACI 318? This load factors are calibrated for 28-days or we can assume the same for our case? And regarding durability (our project is a power house in "wet" conditions)?

thank you in advance!

 

[JAE]

The load factors (safety factors applied to loads) is essentially independent of the resistance side of the equation.

The standard 28 day strength is used as a measure of normal concrete as its 28 day strength usually gets fairly close to the long term strength gained by the concrete over time.

If you are using a mix that develops the required compressive strength of the concrete at a slower rate, this would not affect the load factors, or really the design. What it would affect is the timing of form removal, reshoring, application of loads to the structure, etc.

The load factors are not calibrated to 28 days. They are calibrated to the phi factors to achieve a probability of failure.

 

[cliziagatto]

Ok Thank you JAE,

I understood the load factor take into account the variability in load effects only.

The strength-reduction factors (resistance factors) take into account the variability in resistance. Is there some changing in variability due to slower strength rate?
Now, Should I change the strength-reduction factors?

Thank you

 

[cliziagatto]

And what about durability?

 

[JAE]

I would think that slowing down the strength gain on concrete does not mean there is higher variability in f'c values.

So the statistically driven safety factors (load factors and phi) shouldn't change.

Durability is a long term issue so whether you gained your specified f'c in 28 days or 90 days doesn't seem to matter to the concrete properties once cured.

 

[cliziagatto]

Thank you JAE.

Enlighting.

 

[dik]

I'm not a big advocate of spec'cing concrete strengths at longer time periods, except, in some cases high fly ash mixes.

I have a pre-conceived notion of what 25 MPa @ 28 days will do... that I don't have with 30 MPa at 56 days or that sort of ilk.

Dik