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Compressive Strength Of Concrete

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On November 1st Miner Testing Laboratories was contacted by Giggum, Gougem, and Giggum architect, Arch E. Tech and Possum Grape Sand and Gravel, Inc. superintendent Rocky Clay in order to test sample of their 4200 psi concrete. The concrete was mixed at the Miner Testing Lab previously and was allowed to harden for 21 days. The compressive strength was determined by crushing 6 inch diameter cylinders and the flexural strength was determined by breaking a 6x6x24 inch beam in a beam breaker. The technicians also found the Modulus of Elasticity by use of various strain gauges on six inch cylinder and also the use of the beam breaking machine.


The following testing procedures were used:


The test began by removing two cylinders from the cure room. One was capped with sulfur caps and other was test with neoprene caps. The cylinders were measured for diameter two time and the average was then taken of the two values. The technicians also made sure the specimen stay moist all the way up to the test. The specimen was wrapped in a shroud to minimize flying debris and then put in the crushing device. The neoprene caps had to be put in place before the shroud was put on and then put into the machine. The cylinder was then applied with a steady load from the top until it failed. They were then unloaded and the type of failure was noted and also the final load at failure was recorded.


This test was run according to the ASTM C 78 specs. It began with the removing of the beam from the cure room. The beam was keep moist at all time when out of the cure to ensure a proper test. The corners of the beam were then smoothed with a rubbing stone. The beam was then marked all the way around at 3, 9, 15, and 21 inches. These marks aid in the alignment of the beam in the beam breaker. The beam was then loaded using leather shims between rollers of the machine and the concrete. Once the beam broke it was measured along it fractured face. Three measurements were taken along the width and depth of the fractured face. These were then averaged and used in the calculations. The final load was also recorded in the data sheets.


This test was run according to the ASTM C 469 specifications. First a six inch cylinder was fit with strain gages to measure both axial and normal deformation. The cylinder was then loaded in the same machine we used before to break the beams. The machine was then programmed to apply 60 percent of the ultimate load that was found by breaking the cylinders. The cylinder was then preloaded to remove any possible residual stress. It was then back off and then the final loading was applied. The strain than measured the deformation of the cylinder and from that data and other data we had already found the Modulus of Elasticity was found. вЂÑ"


For a compressive strength of 4200 psi we found the design w/c to be 0.59. You can find this on the Compressive Strength vs. w/c plot. No data on this plot was forecasted therefore it is pretty trust worthy. The beams and cylinders were tested at 21 days instead of 28 days, so the concrete still has an opportunity to gain strength.

The strengths seemed to have some correlation with the slump. It seemed the lower the slump the higher the compressive strength got. However an even more accurate correlation is the one between the strength and the w/c itself. The data clearly shows the lower the w/c ratio the higher the strength. During the test the 21 day compressive strengths were much higher than the 7 day tests. This shows that the concrete was curing and hydrating properly. The ratio between the 7 and 21 day compressive strengths show the percentage of ultimate strength gained in 14 days. These values ranged from 54-94%. This means the concrete has gained 54 to 94 percent of its ultimate strength. The high ratios would be good for concrete that must receive load soon after it is poured.



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