What's an Expansion Joint Cover Anyway?

10th February 2021

Expansion joints, EJ’s, PMJ’s, expansion joint covers, movement joint, construction joint. There are so many names for expansion joints. But really what is an expansion joint?

It’s a metal strip in the ground at your local Westfield. Well, that’s what I used to say before I really understood what they were. After it was explained to me that expansion joints weren’t just strips of metal in the ground at my local shopping centre, and that they actually served a significant purpose in construction, I found a total renewed respect for expansion joints.
You see under that metal strip (which we refer to as an expansion joint cover plate) there is actually a void (an expansion joint gap) between the two concrete slabs. The expansion joint is installed into this gap. But why is there a gap between two concrete slabs I hear you say?

Well, large buildings that have thousands of square meters of concrete are first poured with these gaps to allow the concrete to move in cold and hot weather. Crazy right, that buildings are moving all the time depending on whether it’s hot or cold! This is called thermal movement. When the building gets hot the concrete slabs expand and when the building gets cold they contract.

 So how do the concrete slabs stop busting up with the constant extracting and retracting? That’s where our expansion joint comes into play. For example, the Heard Group Medium Duty Floor Expansion Joint has two aluminium base plates held together with a stainless steel centre bar with nylon balls on each end, that allows the base plates to contract out to 30mm in hot weather and retract 30mm in cold weather, allowing for 60mm of movement (see the below side view of the Medium Duty Floor Expansion Joint and you’ll see what I mean).

The funny thing is you don’t ever see it happening because the expansion joint cover plate, the aluminium strip that sits on top of the expansion joint bases (which is attached to the centre of the stainless steel centre bars) is covering the movement of the expansion joint and the expansion joint gap making it look aesthetically pleasing!

So you see, they really are a significant bit of kit for large buildings, and not just strips of metal in the ground at my local Westfield.