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Some would suggest aesthetics as the answer of the question. Others may consider safety on the contrary because chamfered edge is less liable to damage to the concrete structure and any objects hitting on it. However, the main reason of the provision of chamfer is to make formworks easier to pull out after concreting operation.
It is not uncommon that concrete is getting adhered to formwork and tore away during removal of formwork. In fact, the formation of a sharp nice corner is practically difficult and the concrete at corners is easily chipped and broken into pieces during the removal of formwork. Hence, the provision of chamber could protect the corner from chipping when striking formwork.
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