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strength increased from 5.46 to 11.13 as the specimen changed from low-strength to high-strength concrete. The flexural test is an indirect method to measure the tensile strengthof the concrete. The value of the flexural strength could reflect the value of the tensilestrength of the concrete. As discussed from the compressive strength and tensile strengthrelationship shown above, it is reasonable to have such result. Flexural strength will notchange significantly as the composition of the concrete varies while compressive is likelyto change significantly. Hence, the ratio of compressive to flexural strength increaseswhen the specimen type changes from low-strength to high-strength.
4.2 Fracture SurfaceAs we can observe from Figure 3-9, which shown fracture surfaces of the tensile,flexural, and compressive test, the low-strength concrete specimens have fracture throughand around the aggregates which exhibits a relative tortuous fracture surface. For thehigh-strength concrete specimens, however, they fracture through the aggregates whichexhibits a relatively flat fracture surface. The difference of the fracture surface for lowand high strength concrete is due to the difference between bond difference. High-strength concrete tends to have higher strength bonds between cement paste andaggregates, which is even higher than the fracture strength of the aggregates alone.Hence, the fracture is likely to occur through the aggregates instead of around them. Forthe low-strength concrete, on the other hand, the bond strength between cement paste andaggregates is weaker than the fracture strength of the aggregates alone. Therefore, low-strength concrete is more likely to fracture around the aggregates which result in atortuous fracture surface. 4.3 Modulus of Elasticity There are two methods for us to determine the modulus of elasticity for our testingconcrete cylinders. First is to obtain the value of the slope from the stress vs. strain curve.The second way is to use the equation shown in the sample calculation part in theappendix. The modulus of elasticity for 100mm diameter (low strength) from slope andequation methods were 5057ksi and 4256ksi separately. For 100mm diameter (highstrength), the modulus was 5788ksi and 5696ksi separately. The modulus of elasticity for150mm diameter (low strength) from slope and equation methods were 4506ksi and
3958ksi separately. For 150mm diameter (high strength), the modulus was 5282ksi and5638ksi separately. The equation to approximate the modulus of elasticity is for 28-days-concrete withnormal weight (2400kg/m^3). The specimen we tested in the lab didn’t precisely fulfillthe conditions listed above, and hence, some difference between two methods is likely toexist. As we can observe from the results shown above, high-strength concrete’s modulusof elasticity had a smaller difference between different methods while low-strength oneshad a relatively higher difference. This is probably due to the density of high-strengthconcrete is closer to 2400kg/m^3 and casting time closer to 28 days. For low-strength