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