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