In the world of reinforced concrete, few things are more impressive than watching a building rise gradually: floor by floor, slab by slab, like a well-executed score. But behind that harmony lies a key technical detail that ensures the solidity and durability of the entire structure: construction joints.
These are the areas where concreting is interrupted—because the day ends, because the truck didn't arrive on time, or because the project is designed to be executed in phases—and must be resumed later. If not treated correctly, they can become the invisible crack that, over time, causes problems. But when executed properly, they allow the building to remain a single solid and continuous block of concrete.
What exactly is a construction joint?
A construction joint is the plane that separates two concrete pours made at different times. It is not a crack or a defect: it is a planned or inevitable interruption of the construction process. The key lies in how that surface is prepared and treated before continuing the concreting, to achieve that both parts work together as if they were one.
How to achieve concrete continuity between phases
Professional practice recommends a series of essential steps to ensure that monolithic union:
1 Joint planning
Before concreting, it is advisable to define where the interruption will occur. The ideal location is where stresses (especially shear) are minimal. For example, in a beam, it is usually placed near 1/4 of the span—where shear and bending stresses are moderate; in walls, at mid-height; and in slabs, in compression zones.
2 Preparation of the old concrete surface
When it's time to continue, the surface of the already hardened concrete is lightly chipped until the coarse aggregate is visible. This achieves stronger mechanical adhesion. Then any residue of dust, grease, or release agent is cleaned and it is saturated with water, preventing the substrate from absorbing water from the new pour.
3 Application of a bonding bridge
Just before pouring the new concrete, a cement slurry or epoxy adhesive is applied. This material acts as a "glue" between the old and new concrete, ensuring chemical adhesion in addition to mechanical adhesion.
4 Reinforcement continuity
The steel bars are the true connecting thread between the two phases. They must be extended or spliced so that they cross the joint, ensuring structural continuity. If "starter bars" were left, they are cleaned and straightened before the new pour.
5 Careful concreting and vibration
During the pouring of the new phase, vibration must pay special attention to the contact zone, to avoid air pockets and achieve perfect anchorage between the two concretes.
A common example: phased slabs
Imagine a floor slab that cannot be concreted in a single day. It is decided to divide it into two halves. When resuming work:
- • During the first pour, starter bars are left protruding from the concrete, properly aligned and protected against corrosion.
- • Once the first section has hardened, the edge is chipped until the aggregate is visible and thoroughly cleaned.
- • The surface is moistened and the slurry or epoxy adhesive is applied just before the new pour.
- • If starter bars were not left, post-installed bars in drilled holes with epoxy resin can be used, ensuring the necessary anchorage.
- • Finally, the continuity reinforcement is placed and the second half is poured, ensuring vibration in the joint zone.
In this way, the joint does not represent a weakness but a controlled connection that allows the transmission of forces and maintains the monolithic behavior of the assembly.
What happens if it's not done correctly
A poorly treated joint may seem harmless at first, but over time it can cause:
Consequences of a poorly executed joint
- Visible or internal longitudinal cracks.
- Water leakage, especially in retaining walls or basements.
- Reinforcement corrosion due to moisture ingress.
- Loss of stiffness or load-bearing capacity in critical areas.
That's why each joint must be seen as a line of responsibility, not as a line of separation.
In summary
The key is not to see construction joints as an interruption, but as a controlled transition. When properly planned and executed, the building maintains its structural continuity and durability over the years.
Ultimately, the true mastery of the engineer lies in achieving that what is executed in parts behaves as a whole: a continuous, solid, and durable structure, the result of careful planning and respect for the principles of good concreting.