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the cold joint is a joint between old and new concrete, and it may happen without planning like in a case of concrete truck delay. the contracting joint is actually a cold joint or sometimes referred to as construction joint and usually it is designed and expected upfront.
if you have any delay in concrete you need to apply a bonding primer before you proceed with the concreting to keep the concrete intact and as one piece. and you'd better use a long pot-life epoxy bonding agent like EPOPRIME LP or NITOBOND EP, however the construction joint is the meeting of todays concrete with dry concrete from yesterday or even last week, and you'd expect it to crack, and you don't care if it does. so you make a 5mm*5mm groove and fill it with joint sealant to conceal the crack and keep it controlled underneath the contol/construction joint sealant.
