Cold joint
A cold joint is a structural discontinuity in concrete caused by a pour being stopped and restarted after the first batch has begun to set. A defect, not a feature.
Ask Chalkline about this →A cold joint is a plane of weakness in a concrete element that forms when a pour is stopped and restarted after the previously placed concrete has begun to set. The fresh concrete does not bond properly to the hardened face, leaving a structural discontinuity and a potential path for water ingress.
On residential slabs, cold joints form when the concrete truck runs out before the pour is complete and the replacement truck arrives too late, or when a pour is suspended for too long due to weather, equipment failure, or scheduling error. Concrete readymix has a limited workable life after batching (typically 90 minutes from the plant), so timing the arrival of each truck is critical.
Cold joints are a construction defect. On structural elements, a repair or structural assessment by an engineer is required. The goal on residential slabs is to complete the pour in a single continuous operation. Where a planned construction joint is required (for a very large slab or staged pour), it is detailed by the engineer using dowel bars or saw-cut isolation joints.
Category: Structural.
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Last updated: 2026-05-07. Verified: 2026-05-07. Quarterly review for currency.