Restraint refers to the resistance to movement imposed on concrete by adjacent structures, older concrete, internal aggregates, or embedded elements like rebar. As concrete hydrates and loses moisture, it naturally attempts to contract or shrink. However, when surrounding elements prevent this free contraction, it generates tensile stresses within the material.
Restraint is considered the most common cause of cracking because it transforms simple volume reduction into structural failure. While concrete is strong in compression, its tensile strength is relatively low (typically around 2 MPa). When the tensile stresses induced by restraint reach levels of 2-3 MPa, they exceed the concrete’s capacity to hold itself together, resulting in the formation of cracks. In roofing applications, this is particularly prevalent because structural supports, formwork friction, and reinforcement grids provide significant external and internal resistance to the slab’s natural movement.
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