Construction joints are used when laying cement or concrete and can double as contraction joints to prevent cracking. Bulkheads are used to create the edge of the joint and can act as screed rails. The design depends on the load, with load transfer devices used for heavy traffic. Contraction joints guide cracks along a predetermined line and can be formed by cutting a groove or embedding plastic strips.
When laying cement or concrete, a mason sometimes can’t lay the entire slab all at once. Weather, time constraints, or lack of materials could cut your project midway. When this happens, the builder must leave a uniform stopping point, called a construction joint. This joint is basically a straight line in the concrete. When done correctly, a construction joint can double as a contraction joint, a joint placed in the concrete to prevent the slab from cracking.
The edge of a construction joint is created using a bulkhead. Bulkheads can be made of various materials, from plastic to steel to preformed concrete. During the pouring and leveling process, the bulkheads can also act as screed rails. The concrete is poured between the screed rails, the tops of which are then used as a guide when leveling the concrete. Normally, the bulkheads and screed rails are removed after the slab is completed.
The design of a construction joint depends on the load that the concrete should bear. For foot traffic or small vehicle traffic, a simple butt joint is simpler. A butt joint consists of two concrete slabs laid a few millimeters apart and delimited by bulkheads. If heavy vehicle traffic is expected, the construction joint should be reinforced with a load transfer device.
Load transfer devices prevent concrete slabs from shifting under heavy loads. Moving can cause uneven slabs and cracks in the joints. Dowels, load plates or slots can be embedded in concrete to act as load transfer devices. These fixtures are laid perpendicular to the construction joint, extending into both slabs across the joint. Their purpose is to evenly distribute the load between the slabs.
In many cases, construction joints can also act as contraction joints. As concrete dries, it shrinks, causing cracking. While cracking is nearly impossible to prevent, it can be controlled. Uncontrolled cracking can cause an uneven surface, subject to increased wear over time and water infiltration, which can damage the substrate. Contraction joints are inserted into the slabs to guide the cracks along a predetermined line.
Contraction joints can be formed by cutting a groove in the slab or by embedding strips of plastic into the slab. The purpose of the joint is to weaken the slab along the approved line so that the slab will crack there instead of elsewhere. To be effective, the contraction joint must be at least a quarter of the thickness of the slab deep.
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