A clamp comprising: a first arcuate portion; a second arcuate portion hingedly connected to the first arcuate portion; and a ratchet mechanism for locking the first and second arcuate portions in a closed position; wherein the ratchet mechanism comprises a linear rack provided with the second arcuate portion and a pawl provided with the first arcuate portion for receiving the linear rack; wherein the linear rack comprises one or more asymmetrical teeth and the pawl comprises a finger which is configured to engage with the teeth of the linear rack. The pawl is formed by a module which is detachably connected to the first arcuate portion