Strength of aggregate is important when high strength concrete is required at the end of hydration (at least maximum portion of hydration is completed). But aggregate strength is not important in case of high early strength concrete. There are some accelerating admixtures of high early strength concrete. High early strength gaining cement may be enough to reach quick hydration to reach quick strength but ultimate strength is more or less same as that of normal concrete.
But good quality of aggregate is a must for producing good concrete. To have good bond strength (between the matrix and coarse aggregate), the particle dimension should be equal. So shape of particles is found to be important. As particle shape again depends on parent rock i.e. type and bedding of parent rock are also important. The production process is also important as shape depend on type of crusher say impact crusher usually produces less quantity of flaky or elongated particles.
We know flaky and elongated particles produces weak concrete even though parent rock has good mineralogical history. When shape is concerned, we can include gravel has satisfactory shape to produce high performance concrete. But if surface texture of these aggregate is very smooth the bond strength may be a critical limitation.
After strength, durability is a great concern; may be a primary concern when freezing-thawing exposure or other hostile environment or manmade conditions prevail around concrete member. In this regard soundness of aggregate is very important; we have discussed many posts about soundness of coarse and fine aggregate in our previous posts.Now come to the point of grading of aggregate particles which is very important property to produce a high performance concrete and its economy too. The uniformity in grading is essential in this regard. The cleanliness of particles is also important which means absence of dust adhered to particles (this is also a parameter of having good bond strength).
Effect of modulus of elasticity of coarse aggregate on strength of this concrete has not yet been established. But a disputable thing is that as concrete behave as a monolithic material, aggregate having low value of modulus of elasticity will yields less bond stress with matrix. In case of high performance concrete, it is considered beneficial. Here low modulus of elasticity of aggregate means values near modulus of elasticity of hydrated cement paste.
A greater entire surface area of aggregate having smaller maximum aggregate size likewise means that bond stress will be lower; thus, bond failure is also unlikely to occur.
As a consequence, in compression strength tests, failure plane passes through coarse aggregate and through hydrated cement as well. Formation of cracks, in flexure tests, through coarse aggregate is also observed on high performance concrete.We can concluded from above behavior that bond strength comparable to (not lower than) tensile strength of aggregate.