Surface concrete loss in pieces of various sizes is called spalling and is caused when expansive forces inside and near the surface of concrete act along a weak plane or create a weakened plane. The expansive force can be caused by the stress of corrosion of reinforcing steel or imbedded metal items. Corrosive oxidation (rust) causes expansion which in turn creates additional stress. Internal expansion can also be caused by moisture absorbed by porous aggregate that expands and contracts in thermal cycles.
In the above picture of an example of spalling, note the unbroken, protruding pieces of limestone aggregate amid the crumbled matrix of the concrete.
This is to be contrasted with the following picture (which should not necessarily be considered to be an example of spalling) in which the limestone aggregate has been sheared, as might result from a blow with a hammer.
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