Chapter 7, Problem 7.39QP

To evaluate the effect of a certain admixture on the flexure strength of concrete, two mixes were prepared, one without admixture and one with admixture. Three beams were prepared of each mix. All the beams had a cross section of 4 in. × 4 in. and a span of 12 in. The third-point loading flexure strength test was performed on each beam after 7 days of curing. The loads at failure of the beams without admixture were 6044, 5071, and 5934 lb, while the loads at failure of beams with admixture were 7299, 7305, and 6983. Determine:

1. a. The modulus of rupture of each beam in psi.
2. b. The average moduli of rupture of the beams without and with admixture.
3. c. The percent of increase of the average modulus of rupture due to adding the admixture.