Cold Joint in Concrete: Causes, Problems and Correct Treatment

A cold joint is a weak plane in concrete. It forms when a new layer of concrete is placed against a layer that has already started to set, so the two layers do not blend into one solid mass.

This is different from a planned construction joint. A construction joint is placed where the engineer approves it and is prepared properly. A cold joint is an unplanned joint, usually caused by a delay or a stop in the pour.

Common causes of cold joints

• Breakdown of the concrete plant, mixer or pump during the pour.
• Late arrival of ready-mix trucks, or traffic delays.
• Not enough labour or equipment to place concrete in time.
• Running out of concrete before a section is finished.
• Rain stopping the work.
• Poor planning of the pour sequence in large slabs and walls.
• Hot weather that makes concrete set faster than expected.

Why cold joints are a problem

• Lower bond and tensile strength across the joint.
• Leakage. Water can pass through the joint in basements, water tanks and retaining walls.
• Visible lines on the surface that spoil appearance.
• Corrosion of reinforcement, because moisture and air can reach the steel.
• Possible cracking along the joint under load.

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How to avoid cold joints

• Plan the pour. Know how much concrete is needed and how fast it can be placed.
• Check the plant, pump and a spare pump before starting.
• Keep enough labour and vibrators, including one spare.
• Place concrete in layers and vibrate each layer into the one below while it is still plastic.
• Use a set-retarding admixture in hot weather or for long pours.
• Do not let the interval between layers go beyond the initial setting time. A simple check is whether the vibrator needle can still sink into the lower layer under its own weight.
• Plan the stopping points in advance, and put the construction joints where the engineer approves, usually at low-shear locations.

Treatment of a cold joint

If a delay happens, treat the stop as a construction joint. Follow IS 456 requirements for construction joints and the instructions of your engineer.

• Remove laitance and loose material from the old concrete surface. Do this when the concrete is hard enough, using a wire brush or light chipping to expose clean aggregate.
• Clean the surface with water and air. Remove dust and standing water.
• Keep the old surface wet but with no free water (saturated surface dry).
• Apply a bonding coat, such as a cement slurry or an approved bonding agent, just before placing the new concrete.
• Place the new concrete and vibrate it well, so it merges with the old concrete along the joint.
• Cure both sides well. See our guide on curing of concrete.

For water-retaining structures, add a waterstop at the joint. If a cold joint has already hardened and shows leakage, repair it with crack injection using epoxy or polyurethane grout, as your engineer advises.

A cold joint can be avoided with good planning. Keep the concrete moving, and keep the equipment ready.