Final answer:
The minimum compressive strength of 2500 psi for concrete is to ensure durability and reliability under load. Concrete's compressive strength is high, but its shear strength is much lower, necessitating this minimum standard to avoid premature failure.
Step-by-step explanation:
The reason for setting the minimum compressive strength for concrete at 2500 psi is to ensure that the concrete elements have sufficient strength to withstand the loads they will be subjected to without failure. Concrete exhibits high compressive strength, which refers to its ability to resist forces that would reduce the material's size. While concrete has a high ultimate compressive strength of approximately 20 × 106 N/m², its ultimate shear strength is significantly lower, only about 2 × 106 N/m², which could lead to chipping or failure in shear before reaching the ultimate compressive strength.
Moreover, while the decrease in volume under high pressure, around 500 atmospheres, is measurable, it still remains small due to the material's low compressibility. This inherent property of solids and liquids makes them highly resistant to compression. The presence of reinforcement within concrete is critical as it enhances tensile strength, allowing for a greater range of strain before reaching the breaking point, which is important since concrete is weak in tension.