Disappearing movement joints by Tremco illbruck

Simon Foy, specification manager, Tremco illbruck explains why each expansion joint is likely to be unique, and solves the issue of disappearing movement joints.

It makes sense to install a waterproof joint, in particularly on a podium area or where there is risk of water damage beneath.

Many places have computer hubs or switch rooms and it is a worry that the cleaning machine might dump hundreds of litres of water that will cascade down through the movement joint.

Sometimes the architect will have a movement joint and a paved area, and there is a need to waterproof underneath.

Buildings may also be fire rated, and movement joints need to be considered as a package with the rest of the installation.

The subcontractor who does the waterproofing should also take on board the movement joint, since it is part of the waterproofing job, to guarantee the integrity of the entire waterproofing system.

Switched on materials suppliers offer movement joints that can do three jobs in one - solve problems involved in movement, waterproofing and fire-rating.

If there is a static 30mm gap that needs to be fire rated for 90 minutes, there are a range of suitable products including Compriband Super FR and expanding polyurethane gun foams.

People who don't have to think about movement joints everyday may easily forget the fact that gap dynamics change; a 30mm gap may become 40mm as the structure moves.

When that happens, it is likely that a fire resistant product may not perform as well as it says it will, unless it is specifically designed as an expansion joint and fire rated as such.

To avoid 90 minute fire resistance dropping down to an hour, questions need to be asked when considering expansion joints, including 'what movement is going to happen?' The biggest potential gap then needs to be properly fire rated.

which is usually not a problem when the expansion joint is in closed position as denser materials tend to be more fire resistant.

Another problem is that the fire protection membrane can literally disappear - well, that's what some of the specs say.

For example, a 30mm construction gap that was supposed to be able to shrink to 0mm.

What was supposed to happen to the membrane? The answer is to have a slightly bigger gap, perhaps 50mm, giving the joint room to contract into and making sure it doesn't get squeezed to nothingness.

A further issue is waterproofing.

With a Superspan joint, you waterproof on the surface, so any water will then flow away to drains.

With a membrane in concrete, the movement joint sits on the waterproofing so you're now dealing with the water at a lower level.

Can it drain away? Does it need to drain away? At one large airport they have a specially made joint so that the top of can be lifted off leaving the legs behind.

Any trapped water can then be vacuumed out.

If a waterproof joint is required, managing the water must be considered.

Just a bit of spillage and the building may be able to live with it, as it will most likely evaporate at some time.

Should water deposition be a regular occurrence, there could be flooding over a period of time or the water could become foul.

It would then require managing and getting rid of.

If it's waterproof because of the weather, surface mounting is probably the most suitable option so it drains away.

If it is just the odd spillage, there may be a membrane system.

If it is fire-rated with a decent gap and central movement, the system can go into the structure.

If it is beyond that then it should sit on, or underneath the structure.

Some architects will blanket spec something, but if time is taken to understand a design, some expansion joints may have to be high spec while others can be made of more basic materials.

To select the right joint, the movement, the loading, the construction gap and the floor make up has to be addressed, as well as waterproofing and fire-rating requirements.

You can specify the unique joint elements that are right for that particular job only once you know what those should be.

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