What characteristic of concrete makes it weak in tension and shear?

Concrete is primarily composed of cement, water, aggregates, and sometimes admixtures. While this composition gives concrete its excellent compressive strength, it inherently lacks characteristics that provide significant tensile strength. The material tends to form micro-cracks internally, which can severely limit its ability to withstand tension and shear forces.

Concrete's composition does not include materials that enhance its ductility or tensile strength; instead, it relies on steel reinforcement to handle tensile loads. Unlike some other materials that can deform under tension and shear, concrete tends to crack and fail when these forces are applied beyond its limited tensile capacity. Understanding the material composition allows for better design choices that accommodate these inherent weaknesses, such as using steel reinforcement bars (rebar) to enhance tensile strength where needed.

The other options do not directly address the core issue of tensile and shear strength in concrete. High weight and low density are not critical factors influencing tensile weakness, while internal cracking, although related, stems from the composition and structural behavior of the concrete itself rather than being a standalone characteristic.