When a concrete structure is heavily reinforced, the very dense rebar network can block the contraction movement of the protecting concrete cover located above the external layer of reinforcement bars due to the natural drying shrinkage process. As a consequence, a network of fissures with the characteristic honeycomb pattern also typical for the cracks resulting from the expansive chemical reactions (ASR, DEF, ESA) forms. 
The formation of fissures in the concrete cover above the reinforcement bars represents a preferential pathway for the ingress of water and aggressive agents such as carbon dioxide| (lowering of pH around the rebar) and chloride anions (pitting corrosion) into concrete. The physical formation of cracks therefore favors the chemical degradation of concrete and aggravates steel corrosion. Physical and chemical degradation processes are intimately coupled, and the presence of water infiltrations also accelerates the formation of expansive products of harmful swelling chemical reactions (iron corrosion products, ASR, DEF, ISA, ESA). 
Different approaches and methods have been developed to attempt to quantitatively estimate the influence of cracks in concrete structures on carbonation and chloride penetration. Their aim is to avoid underestimating the penetration depth of these harmful chemical agents and to calculate a sufficient thickness for the concrete cover to protect the rebar against corrosion during the whole service life of the concrete structure.