Which of the following is true about the stress-strain behavior of concrete?

The correct answer highlights that concrete stress varies linearly with strain up to the concrete's specified compressive strength, denoted as f'c. This linear relationship occurs within the elastic region of the material, where the stress and strain are directly proportional. As long as the applied stresses remain below this threshold, the concrete behaves predictably and can be modeled using linear elasticity principles.

Beyond the linear portion, concrete begins to exhibit non-linear behavior, typically showing a parabolic increase in stress with strain until it reaches its ultimate compressive strength. This non-linear behavior is a fundamental characteristic of concrete as it approaches failure, encompassing both cracking and eventual failure under large strains.

In the context of the other choices, while concrete does have some limit where it exhibits linear behavior, it is not entirely accurate to claim that concrete stress is linear at all strain levels, as it transitions to non-linear behavior after reaching f'c. Additionally, concrete is a material that is inherently weak in tension, as it cannot sustain tensile stress adequately, which is why reinforcement such as steel is often incorporated within concrete structures to handle tensile forces. Lastly, the suggestion that concrete exhibits constant stress regardless of strain does not align with how concrete behaves under load; stress continuously varies with increasing strain until