7.12 : Impact Strength of Concrete

Impact strength in concrete is a critical measure that reflects the material's capability to endure the forces applied during pile driving and when supporting machinery foundations that experience impulsive loads. It is also essential when handling precast concrete components to prevent accidental damage. The impact strength is assessed by observing the concrete's resistance to repeated impacts and energy absorption capacity. A key indicator of significant damage to concrete is when it does not rebound after being struck.

The formulation of the concrete mix affects its impact strength, with coarse aggregates that are more angular and have rougher surfaces leading to an increase in impact strength. Such mixes indicate a stronger bond and are believed to contribute more to the material's flexural strength rather than just its compressive strength. The conditions under which concrete is stored can also alter its impact strength, with wet conditions typically reducing the strength compared to dry storage.

Improving impact strength can be achieved by selecting smaller maximum aggregate sizes and aggregates with a lower modulus of elasticity and Poisson's ratio. Additionally, for optimal impact strength, it is advantageous to use a concrete mix with a cement content not exceeding 670 pounds per cubic yard. These adjustments can make concrete more resilient to the dynamic and potentially damaging forces it may encounter.