What are construction joints?

Construction joints are used in slabs-on-grade to allow slab movement at transitional load bearing points from one concrete slab to the next.
Traditional construction joints are considered worldwide as the Achilles’ heel of concrete slab placement (roads, airports, industrial and commercial floors). Concrete slabs-on-grade are subject to different internal and external forces and pressures. Joint deterioration occurs through load bearing weight transferring from one side of the joint to the other causing micro-shocks at the point of load cycle. In summary, a true construction joint must allow relative horizontal transition which is typically due to the thermal expansion and contraction or humidity induced shrinkage. At the same time it must not allow relative vertical transition and relative rotation due to traffic movement and load bearing weights.

Concrete floor slab movements

Figure 1 summarises the relative movements that a construction joint is subject to in a slab-on-grade.

Traditional methods for a construction joint
The most common solution for a construction joint is the one referred to as a dowel joint system. It is constructed with steel rods approximately 400mm long and 20mm in diameter; they are placed through the concrete form work every 600mm perpendicular to the plane of the slab. Ensuring the correct placement of doweled joints requires a lot of time and effort. Often, through the lack of care and placing the steel dowels unevenly in spacing or at the incorrect angle, joint failure is inevitable causing long term damage to the joint and slab-on-grade (Figure 5).

Cracks on concrete floors

Even if the dowel joint is positioned correctly they are usual knocked out of place and moved during the concrete placement (Figure 4). The fact that the steel dowels have moved will greatly reduce the support that they were intended to give to the joint The dowel bars that are misplaced or out of line now increase the risk of cracking the actual concrete (Figure 6). An irregular surface between the two sides of the construction joint will produce uneven load transfer and slab movement, which in turn produces weak points allowing cracks to appear in the slab (Figure 7).

Where to place the steel mesh or welded steel
When concrete slab-on-grade needs to be re-enforced with steel mesh it is important that the mesh is placed in the correct area of the concrete slab. Placement of the steel mesh through the construction joint is a very common mistake and can potentially cause cracks and failure (Figure 8a).

Where to place the steel mesh or welded steel.

Correct placement for the steel mesh should be placed one third of the slab’s thickness starting from the upper section of the slab-on-grade (Figure 8b).

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