Construction joints in concrete: why they leak and how they are sealed

01What a construction joint is

A construction joint is where one pour stops and the next begins: wall to raft at the kicker, wall to wall, slab to slab. Unlike an expansion joint, it is not designed to move, but it is a cold joint where new concrete meets hardened concrete — and the bond and compaction there are rarely as good as in the body of the pour.

02Why they leak

• Laitance and dirt on the first pour, so the second does not bond.
• Poor compaction at the joint, and honeycombing at the base of walls.
• A waterstop that moved, is not continuous, or is not joined at corners.
• Shrinkage and early cracking along the joint line.
• Pipe sleeves and ties crossing the joint without detailing.

03Ways to seal a construction joint

Embedded waterstops

PVC or rubber profiles cast across the joint, centre-placed or on the outer face. Continuity at junctions is essential: welded or prefabricated intersections.

Swelling strips

Strips that swell on contact with water, fixed to the first pour; they need the right position, enough cover and protection from early wetting, per the manufacturer.

Injection hoses

Perforated hoses fixed at the joint before the pour; resin can be injected through them later if the joint leaks.

Crystalline treatment

A crystalline slurry on the prepared joint face before the next pour, depending on the product.

External membrane strip

A strip of the waterproofing membrane over the joint on the positive side, as part of the external system.

04Preparing the joint

1. Plan the joint positions with the designer — not wherever the concrete runs out.
2. Remove laitance and expose the aggregate on the hardened face: water jetting, mechanical scabbling, or a surface retarder on fresh concrete.
3. Clean, then wet or treat the surface as specified.
4. Fix the waterstop or hose firmly in position and check its continuity.
5. Compact well at the joint during the next pour.

05When a joint already leaks

In finished structures, a leaking joint is traced and then injected: polyurethane resins to stop water — expanding types for active flow — and epoxy where a structural bond is needed and the joint is dry enough. Injection hoses, where installed, make this simpler. A negative-side system or a joint detail may follow.

06Construction joint or movement joint?

If a joint must open and close with temperature or settlement, it is a movement (expansion) joint and needs a system that tolerates movement — sealants, tapes or profiles — not a rigid seal.