chapter  1
10 Pages

- Introduction—Why a New Book on Fracture of Concrete?

Concrete cracking is primarily caused by the material’s low tensile resistance. For ordinary concrete with a compressive strength of around 40 MPa, we may expect a tensile strength of no more than 10% of the compressive, thus approximately 4 MPa is the maximum. The imbalance between tensile and compressive strength is traditionally taken care of through the use of steel reinforcement placed at those locations where the highest tensile stresses appear in the considered structure. The main reason the joint venture between steel and concrete is successful is their almost identical thermal expansion coefªcient. Moreover, the bond between steel and concrete is sufªciently good to allow for short anchorage length of the rebar. In principle the combination is ideal, yet problems arise owing to the relatively high porosity and often good permeability of the concrete cover of the steel reinforcement. Bad workmanship may do the rest, and given sufªcient amounts of water and oxygen the rebar may start to corrode once the protective oxide layer is passivated, either through the ingress of chlorides, or through carbonation of the cover concrete. Cracks may facilitate the ingress of corrosive media; therefore there is interest in reducing crack widths to a minimum, or to prevent cracking altogether. The swelling of a corroding rebar may lead to substantial increase of cracking, thereby accelerating the corrosion process, depending on the actual climatic conditions.