The denomination of the different crack types corresponds with their causes or appearance.
Flexural crack / Bending crack: Flexural/Bending load acting on a structural concrete element produces two stress zones: the compression zone in which bending generates compressive stress, and the tension zone in which convexity generates tensile stress. As concrete reacts not very flexible to tension, so-called flexural or bending cracks are caused within the tension zone that develop perpendicular to the structural element axis from the outside to the inside and end in the area of the zero line (no compressive or tensile stress).
Hairline crack: Hairline cracks are very fine cracks as thick as human hair.
Longitudinal crack: Longitudinal cracks in reinforced concrete develop parallel to the axis of the reinforcement bars. In masonry or tilework, longitudinal cracks may form parallel to the joints.
Craquelure / Spider crack: Finest cracks forming a network pattern on the surface, low crack depth.
Surface crack: Surface cracks are wider than hairline cracks or craquelure. The crack depth does not affect the constructional function of the structural element.
Shrinkage crack: In fresh concrete, shrinkage cracks are caused by volume reduction during the hydration process (plastic shrinkage); in older concrete by dehydration/drying out. Shrinking cracks may cross the complete structural element. The formation of shrinkage cracks in fresh concrete is termed plastic shrinkage cracking.
Shear crack: Shearing forces acting additionally to flexural load generate shearing load. Shear cracks form when the shearing stress exceeds the load capacity of the structural (concrete) element.
Settlement crack: Settlement cracks are caused when the ground below a structural element/building subsides inconsistently, e.g. due to building activities (tunnels), traffic loads, earthquakes, or in case of ignoring calculated loads, e.g. heightening, annexes to existing buildings.
Separation crack: Separation cracks cross the complete structural element dividing it in two or more parts.
Bond crack: Bond cracks in reinforced concrete develop due to lacking bond with the surface of reinforcement (bars, mats) and propagate correspondingly.
