Expansion Joint

Looking for expansion - you would be hard pressed to beat cork

Get the cork out - we've got an expansion joint to make!

An expansion joint is needed where two structural concrete slabs abut each other as it allows the two slabs to both expand at the expense of a compressible material which is laid between them.

An expansion joint can be very elaborate but, given the limitations of our contractor and his workforce, ours needed to be relatively simple. We had two main slabs - the double reinforced slab upon which our outdoor swimming pool is built immediately abutting a single reinforced slab carrying the path which goes around the house. The concrete mix was different, the reinforcement was different and the loading conditions were different therefore the two slabs would behave very differently when subjected to the fierce summer heat or the freezing winter nights that we also get. The summer is the bigger problem for concrete because it expands and if it is prevented from doing so, it will crack. Water then gets into the cracks, corrodes the reinforcement and freezes in winter causing the concrete to spall and the self-destruction process to begin again but a faster rate.

There are various proprietary products which are marketed as expansion joints but, ultimately the function is to absorb the compressive force exerted by heat-expanded concrete and to protect itself from the elements. With that in mind, a popular material is cork which is strong in compression (thereby allowing it to absorb a high level of movement from the slabs). In order to stop the filler being attacked by the elements, a layer of hot bitumen is poured over it. The thickness of the expansion joint is typically 2 to 3 cm.

Despite my careful explanations, our contractor assured us that the local builders merchants recommended polystyrene. Although this is sometimes used, it is very much a poor man's alternative as polystyrene does not have the strength nor compressibility of cork - its primary function being insulation. Eventually he purchased some cork tiles which, although not completely correct for the job should resist the movement.

As a final measure, we put in two joints - one on either side of the slotted drainage channel which runs the length of the slab carrying the pool and sits between that and the footpath. At the time of writing they are not quite finished as we are still awaiting the hot-poured bitumen to seal them. For some reason this is proving a problem.

Don't forget that if you have two different reinforced slabs abutting one another then you need an expansion joint.