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Cracks, joints and interfaces

Cracks, joints and interfaces
Steve Thornton

Published by Steve Thornton - Technical Manager - Wall and Floor

Steve Thornton has almost 30 years of experience in the flooring industry. Starting out as an apprentice floor-layer, Steve acquired a wide range of installation expertise over two decades. He moved on to project management, followed by several years as a National Technical Manager before joining Bostik as a Technical Consultant.

One of our major functions in a technical capacity is to test for moisture, which is one of the most critical things to identify prior to commencing any flooring installation. But what else do we need to identify during a site inspection? A common issue is the presence of cracks or joints in the subfloor.

There is very rarely a project inspected which does not have some form of crack, joint or interface. But what are the differences between all of these? It may seem like they are all similar, but that’s not the case – let me explain.

First, we will start with cracks. Although they can sometimes be quite small, cracks are relatively easy to identify. They will have an irregular pattern to them, much like a lightning strike. However, it may surprise you to know that there are six main types of cracks that you may find within a project:

Shrinkage cracks (sometimes called plastic shrinkage cracks) – these happen before the substrate has hardened. Water evaporates from the concrete and leaves voids behind, making the concrete slightly weaker. As the stresses build up in the slab, cracks can occur.

Expansion cracks – not commonly seen on a building site, as they generally occur outside. The concrete heats up and expands, pushing against anything in its way. When resistance is encountered, the expanding force can be enough to make the concrete crack.

Heaving cracks – again, not commonly seen on building sites, heaving is when the water in the ground freezes and thaws in a cycle. This process causes the ground to be pushed upwards. This is also caused by tree roots as they grow underground.

Settling concrete cracks (or settlement cracks) – these are caused when stresses are exerted upon the slab, either by the weight of the building/structure upon the slab or more commonly when there are voids formed below the slab. This can be the slow compression of the building materials underneath but can also be from the decomposition of organic matter beneath the building.

Overloading cracks – quite simple to explain, these are cracks that are caused by too much weight being placed on the substrate.

Finally – and perhaps most common on sites – Premature drying cracks – these form when the surface of the slab loses moisture much more quickly than the rest of the slab. When the slab is exposed to strong breezes (which carry moisture away more quickly) or very warm temperatures (which evaporates the moisture away from the surface too quickly) then the surface starts to crack. If the concrete has been mixed with too much water, then this too can contribute. These cracks look like a crazing effect on the surface. Generally, these do not cause any structural issue and do not penetrate down past the uppermost layer of the concrete.

Whereas cracks are generally not wanted, joints on the other hand are there by design and very much serve a purpose. There are a few joints commonly found on projects, and what they are for, how they were formed, and how long they have been there, really does determine what you can or can’t do with them.

Building movement joints (also referred to as expansion joints) – these are literally to allow the building to move. Due to thermal changes, building materials will expand and contract and the joints are there to allow the building to tolerate this movement without causing any damage. These joints are usually quite wide, and you will be able to see that there is a complete departure from one slab to the other. You must never fill these joints with flooring products, there are other more specialist products and profiles which are used for treating these.

Crack inducement joints (sometimes called control joints) – very aptly named. These joints are introduced into a slab very early on after it has been poured. Usually this will involve cutting a joint into the floor approximately 25% of the overall thickness. This joint acts as a weaken point. So, when stresses are exerted onto the slab during drying, the crack will naturally be formed at this point. Treatment of these joints depends at what stage the slab is at. Once the crack has formed and the slab is unlikely to move anymore, then these joints can be filled with a resin and then treated as a one continuous floor. If there is still the likelihood that movement will occur, then you should not fill them and use a suitable cover profile instead.

Day joints (sometimes called bay joints) – day joints were named because that was the actual amount of work that had been completed in a day. But more accurately, these should be called bay joints, which is the point at which the maximum size of the ‘bay’ you are laying is reached. What they effectively mean though, is that one slab stops and then another one starts. These junctions can be slurry bonded, which will usually stop this joint opening and allows the floor to be treated continuously. More commonly though, these are not bonded together, meaning that they are independent of each other and will move independently whilst drying. These joints can be treated with a resin just like the inducement joints, but again, this is done once the likelihood of movement is no longer present.

Finally, there are interfaces. These simply refer to two or more substrates meeting. Although in this instance, the adjoining substrates would be different materials. When two substrates of the same type, laid at roughly the same time meet, then they will move and dry approximately the same, meaning that treatment of this type of joint is relatively simple. But when you have different materials meeting, which have a different age, then things get a bit more complicated. When exposed to thermal changes, changes in humidity and varying resistance to being under load, then different materials can react very differently – referred to as differential movement. Where these instances exist, there is usually a much more complicated way in which these are treated. Solutions in these locations will usually involve multiple products and processes. And still, with these multiple processes and products, there will always remain a possibility that the solution may fail.

As you can see, the number of cracks, joints and interfaces encountered on site can be extensive. Treating these to provide a continuous installation of flooring, without the need for trims and profiles is possible, but not always done in the same way. Whether a new project or a refurbishment, it is always wise to have an inspection done by a member of your chosen manufacturer’s technical team. They will identify these joints and cracks and recommend the products and processes needed to treat them.