An expansion joint, also known as a movement joint, is designed to absorb the natural expansion and contraction of materials as the temperature drops or rises. They are also installed to absorb vibration, hold parts together or allow those parts to move as the ground settles or if there is an earthquake.
Thankfully, there are few earthquakes in the UK, however, expansion joints are an important element in construction using concrete for all the other reasons.
What is an expansion joint in concrete?
As with wood, concrete can expand and shrink in heat. There is also shrinkage after the concrete has hardened. By placing expansion joints in a grid form across an expanse of concrete pouring, in positions where cracks can be expected to occur, cracks, if they occur, are held below the surface, remaining superficial and mostly invisible.
Without expansion joints, it is contact with soil and brickwork that is being relied on to contain the concrete shrinkage. However, this also means tensile pressure is put through the concrete as it shrinks, which can cause cracks. Also, if placed against inflexible materials, concrete has nowhere to go when or if it expands due to heat and moisture, again potentially damaging the concrete. An expansion joint that can add flexibility and buffer the movement in the concrete is therefore the best solution.
There are two types of expansion joint filler, foam and fibreboard. When working with concrete you should use fibreboard, which offers flexibility across the surface of concrete, allowing movement that will prevent cracking.
SITEWORX Fibreboard Expansion Joint Filler comes impregnated with bitumen, to aid moisture resistance.
Remember, any cracking can cause structural problems in your build. However, more likely, it will look particularly unsightly. This obvious appearance of cracks could compromise the end finish of your build.
How to use expansion joints?
Laying a concrete driveway is a typical project often requiring the use of expansions joints, depending on the size of the area to be covered.
Before pouring the concrete it is important to make sure the fibreboard strips are the same height as the depth of the concrete to be poured. Expansion joints should be positioned in a vertical position along the edge of the driveway where the drive abuts the brick of a garden or house wall. Additional joints should be included through the concrete slab area, spaced at approx. 30 times the slab thickness, e.g. for a 100mm thick slab, this means they would be spaced approx. every 3 metres. The spacing of the joints can be varied slightly to make it more practical or for aesthetic appearance.
SITEWORX Fibreboard Expansion Joint Filler comes in strips ranging from 100 to 200 mm wide or as whole sheets, in 12 or 19mm thicknesses.