## Modulus of Rupture

## Modulus of Rupture

(OP)

For the slender wall method (Chapter 14.8 in ACI 318-08) the modulus of rupture is listed as 7.5*lambda*sqrt(f'c).

For structurally plain concrete (Chapter 22 of the same code), it is given as 5 * lambda * sqrt(f'c).

Why are these values different? Why is the modulus stronger for the slender wall method?

For structurally plain concrete (Chapter 22 of the same code), it is given as 5 * lambda * sqrt(f'c).

Why are these values different? Why is the modulus stronger for the slender wall method?

## RE: Modulus of Rupture

Dik

## RE: Modulus of Rupture

## RE: Modulus of Rupture

I had thought about conservatism, but there are phi factors present for this. The code also requires plain concrete to be in overall compression.

To add more to this, if I pour my little beam on the job site and test it in three or four point bending I will be testing a plain concrete beam, regardless if my pour is actually reinforced. Is the resulting modulus of rupture from the lab analogous to ACI 14.8 or ACI chapter 22?

## RE: Modulus of Rupture

## RE: Modulus of Rupture

MacGregor & Wight note that the mean modulus of rupture is 8.3*sq rt f'c, so the 7.5 makes sense for deflections. 5*sq rt f'c is closer to the lower bound value, which would make sense for strength calculations. MacGregor & Wight have a 3 page explanation of it if you're interested.