# 46 to 1113 as the specimen changed from low strength

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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 strength of the concrete. The value of the flexural strength could reflect the value of the tensile strength of the concrete. As discussed from the compressive strength and tensile strength relationship shown above, it is reasonable to have such result. Flexural strength will not change significantly as the composition of the concrete varies while compressive is likely to change significantly. Hence, the ratio of compressive to flexural strength increases when the specimen type changes from low-strength to high-strength.
4.2 Fracture Surface As 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 through and around the aggregates which exhibits a relative tortuous fracture surface. For the high-strength concrete specimens, however, they fracture through the aggregates which exhibits a relatively flat fracture surface. The difference of the fracture surface for low and high strength concrete is due to the difference between bond difference. High- strength concrete tends to have higher strength bonds between cement paste and aggregates, 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. For the low-strength concrete, on the other hand, the bond strength between cement paste and aggregates 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 a tortuous fracture surface. 4.3 Modulus of Elasticity There are two methods for us to determine the modulus of elasticity for our testing concrete 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 the appendix. The modulus of elasticity for 100mm diameter (low strength) from slope and equation methods were 5057ksi and 4256ksi separately. For 100mm diameter (high strength), the modulus was 5788ksi and 5696ksi separately. The modulus of elasticity for 150mm diameter (low strength) from slope and equation methods were 4506ksi and
3958ksi separately. For 150mm diameter (high strength), the modulus was 5282ksi and 5638ksi separately. The equation to approximate the modulus of elasticity is for 28-days-concrete with normal weight (2400kg/m^3). The specimen we tested in the lab didn’t precisely fulfill the conditions listed above, and hence, some difference between two methods is likely to exist. As we can observe from the results shown above, high-strength concrete’s modulus of elasticity had a smaller difference between different methods while low-strength ones had a relatively higher difference. This is probably due to the density of high-strength concrete is closer to 2400kg/m^3 and casting time closer to 28 days. For low-strength
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