RESULTS

Compressive Strength

The unit weight of the hardened concrete was calculated to be 148 psi . E(ACI) was 23.91 x 10^6. The load was applied continuously and without shock and the maximum load sustained by the non silica fume cylinder was161800 lb. and the compressive strength was 5628 psi. The maximum load sustained by the silica fume containing cylinder was173100 lb. and the compressive strength was 6061 psi. Both cylinders failed in a cone failure at 45 degrees. Failure mode was across the paste and aggregate. A graph of the load vs. deformation and stress vs. strain curve can be seen in the Appendix link. See Appendix also for the Mohr’s circle for the compressive loading of the cylinders.

The next two cylinders, one silica fume and one non, were loaded with 40% of the crushing load. The chord modulus of elasticity was calculated from the graph to be 8138994 psi for non silica fume and 688060 psi for silica fume.

Flexural Strength

The flexural test was performed on the large beam (6x6in). A 3 point bent test and the maximum load sustained was10430 lbs. The maximum flexural strength was 7581.2 psi. The modulus of rupture was calculated to be 1411.76 psi. The breaking occurred in both the paste and the aggregate.

Once the beam was broken in half a shear test was performed on it. The first half broke under a maximum load of 20,390 lbs but this broke under flexural conditions. The other half of the large beam was used and it broke under shear with a maximum load of 28,770 lbs, which corresponds to a stress of 771.96 psi.

The small beams (3x3) were broken under four-point load. The maximum load on the first of the small beams was not calculated. No measured load had been applied to the beam. The second small beam did not contained silica fume and broke at a maximum load of 1088.5 pounds with a corresponding stress of 121.76 psi. The third small beam, which did have silica fume, broke at a maximum load of 1199 pounds that matches strength of 136.63 psi. The modulus of rupture (four point load) was calculated to be 735.44 psi. The flexural strength was calculated based on the ACI formula and found to be 344.8 psi in the silica fume and 336 psi in the non-silica fume beam. As in the large beam, the breakage was due to both the paste and the aggregate.

Tensile Strength

A tensile strength test on the non-silica fume cylinder sustained a maximum applied load of 38 000 lbs. This matches an ultimate strength of 1 330 psi. While a cylinder with silica fume sustained a maximum load of 39 000 lbs, the failure occurred across the paste and the aggregate. The proportion of coarse aggregate fractured during the test was estimated to be 85%. The tensile strength was calculated by the ACI formula based on the compressive strength and was 389 psi in the silica fume cylinder and 375 psi in the non-silica fume cylinder. A graph of Mohr’s circle for tensile loading can be found in the Appendix link.

Schmidt-Hammer Test

The Schmidt-Hammer test was used to predict the strengths of the concrete cylinders before the destructive tests were performed. The Schmidt-Hammer test found the hardness of the silica fume cylinder to be 5 640 psi and the non silica fume cylinder to be 3 860 psi with corresponding compressive strengths of 6 061 psi and 5 628 psi respectively.