What is a 'cold joint' in concrete?

A cold joint forms when one batch of concrete begins to set before the next batch is placed against it, so the two layers never intermix and no continuous paste matrix forms across the interface. The result is a plane of weakness running through the element and, in water-retaining or below-ground concrete, a direct path for water to track through. A cold joint is distinct from a planned construction joint: a construction joint is designed, set out and detailed with a waterbar or prepared surface, whereas a cold joint is an unintended consequence of the pour going wrong. The usual causes are delays between deliveries, a pump breakdown mid-pour, hot weather accelerating the set of the placed concrete, over-large pour areas that outrun the placing rate, and insufficient revibration of the previous layer before it stiffens. Concrete generally remains workable enough to knit for a limited window after placing, and once the surface has stiffened past the point where a poker will penetrate and reliquefy it, a cold joint is already forming. Structurally, a cold joint reduces shear transfer across the plane and can compromise a section designed to act monolithically, though in many elements the loss is tolerable and the real problem is durability rather than capacity. Water tracking the joint carries chlorides and carbon dioxide to the reinforcement, so corrosion and subsequent spalling often follow along the line of the joint years later. Visible signs are a distinct horizontal line on the struck face, sometimes with honeycombing, laitance staining or damp patches following it. Repair depends on what the joint is doing. Where the issue is water ingress, the joint is drilled at intervals and injected with low-viscosity polyurethane or epoxy resin to fill and bond the interface and seal it against further leakage. Where structural continuity is required, the joint may need to be cut out and reinstated, or strengthened. MPS Concrete Solutions injects and seals cold joints in basements, tanks, car parks and retaining walls, working from the accessible face without excavation.