The properties of the coarse aggregate used in a concrete mixture affects the modulus for a few reasons. One property is the modulus of elasticity of the coarse aggregate. A higher aggregate modulus will result in a concrete having a higher modulus. As expected, a lightweight aggregate will have a lower modulus than the mortar paste. Conversely, a strong aggregate produces a concrete that is stronger than the mortar paste (Zhou 180). In tests, concrete containing a higher percent of coarse aggregate resulted in a higher elastic modulus. As stated earlier, this is due to the aggregate being stronger than the mortar (Cetin and Carrasquillo 256).

The particle shape of the aggregate contributes to the effectiveness of producing a high performance concrete. Crushed rock creates a much better bond between the paste and the aggregate than a gravel does. However, the aggregate-mortar bond may be more important in flexure tests than in compression tests.

The mineral makeup of the aggregate also influences the modulus of elasticity of concrete. In a study by Aitcin and Mehta, they tested four different types of aggregates: diabase, limestone, gravel, and granite. Their test results showed that the limestone and diabase aggregates gave the highest values for the elastic modulus. The gravel performed poorly because of the weak bond between the aggregate and the cement paste. The granite aggregate, on the other hand, gave the worst results because of its mineral composition. In their granite sample, they found a mineral that is unstable in moist environments which is why the granite specimen's results were poor. Figure 4 shows a sample of a pink granite. Granite has a variety of mineralogical makeups. Therefore, one granite may be much stronger than another.

Figure 4 Sample of Granite