Finding New Cracks in Concrete

Hotline Question

A crane sitting on an existing concrete slab on grade lifted loads heavy enough to crack the slab. There may have been a few cracks in the slab before it was overloaded, but I want to prove damage to the slab by showing the new cracks. Is there any way of doing this?

Hotline Answer

To make all cracks more visible, wet the concrete. As the surface dries and lightens in color, the cracks stay dark, making them more visible. Cracks caused by overloads in the center of the panel typically have a circular or curved shape. This differentiates them from some drying-shrinkage or thermal-contraction cracks that often run parallel to contraction joints.

Cracks are likely to appear under the crane's outriggers. If you can measure the distance between outriggers for the crane that was on your slab, you may be able to correlate the spacing of circular cracks with outrigger spacing. Also, if you have any photos showing the crane on the slab, these may help to connect the crane position with the load-crack location. If you know where the crane entered and exited the slab, look for cracking at these locations.

The wetting technique will also reveal existing cracks. If the slab is several years old, the new surfaces exposed by these cracks may have carbonated by now, whereas crack-face surfaces of newer cracks may not have carbonated. A petrorapher may be able to analyze several cores from cracks you suspect were caused by the overload. If the cracks are recent in origin, the concrete may not have carbonated to a great depth at the crack faces. Concrete at existing crack faces should be more deeply carbonated.

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