Construction slabs of horizontally poured concrete are susceptible to cracking caused by changes in temperature, as they expand and contract in the heat/cold. Unless some form of physical relief barrier is incorporated into the concrete, either during the pouring operations or afterwards, it may be prone to shrinkage or heave. Concrete will shrink slightly as it dries during the curing process.
Contraction or control joints are commonly included in concrete slabs to regulate where the concrete will naturally separate as it dries, or as the underlying sub-base shifts. Isolation and expansion joints serve a similar purpose by preventing the transfer of forces from concrete expansion or shifting to adjacent structures.
Contraction joints may be created in poured foundations – even existing ones – by sawing through the raft and fitting silicone as a semi-liquid, or inserting fibre-board.
Isolation joints are intended to create a separation between a concrete slab and other fixed – point elements such as walls, bollards, or recessed paving trays. Failure to maintain this separation can lead to cracks or heaving in the slab and potential damage to the fixed points and paving.
Expansion joints play a vital role in addressing temperature-related changes in concrete. These joints are installed at regular intervals depending on the depth and size of the construction slab. Their purpose is to allow the concrete to expand and contract naturally as temperature fluctuations occur. By providing this flexibility, expansion joints effectively mitigate the accumulation of internal stresses within the concrete, thereby minimizing the risk of cracks or structural damage.
Expansion joints may be fitted prior to pouring, by leaving a gap/fibre board against the boundary (wall etc). On a standard 100mm slab expansion joints should be installed every 3-5m. On large projects it is important to consult a structural engineer or concrete installation professional who can evaluate specific conditions and design and advise as to the correct joint installation. All construction slabs should be a minimum of 100mm deep, with reinforced mesh and a minimum c25.
Care should be taken to match the line of the expansion joint to the line of the paving above, to prevent differential in movement, the joint in the paving to be filled with a flexible silicon grout to colour match or a surface expansion joint.
In summary, isolation and expansion joints are essential in preserving the integrity of concrete structures. They efficiently manage the forces and movements caused by shrinkage, expansion, and temperature changes, ensuring the longevity and integrity of the concrete over time.