Softening can take place when an aggregate contains weak or porous particles. In addition, aggregates composed of, or containing large proportions of clay minerals are also soft and porous. When such rocks are present in hardened concrete, the volume of the concrete will change more on wetting and drying than similar concrete containing non-swelling aggregate. They will facilitate the ingress of aggressive substances such as chlorides. Aggregate softening will lead to pop-outs on the surface and the formation of ‘Isle of Man’ crack pattern.