Dimensional stability: what movement joints are used for
To understand how to use movement joints correctly, we must first understand the underlying causes why a floor not only moves but displays changes in dimensional stability.
First of all we address the issue of dimensional stability: a crucial factor is the temperature which affects the volume of materials subjected to thermal shock. Both at superficial and structural level, the materials react to thermal shock by expanding and retracting in volume and these physical variations must not be blocked but accommodated.
Another important factor to take into account is movement: a floor, both structurally and at intermediate level (screed), is made with a grid system to imbue the body with an elastic structure. For these reasons floor beams and slabs are armed during casting with a cross-linked galvanized iron mesh to make the structure both resistant and elastic and thus able to withstand the flow of its own weight but also that of movement from people and things on the surface.
Of course it goes without saying that the screed must display the same characteristics at intermediate level and, in addition to the electro-welded meshed frame, its elasticity is guaranteed thanks to the fractionation technique. With installed screed, this operation involves actual checkerboard type fractionation using cutting flex to divide the screed into many small squares independent of one another.
The size of fractions must be calculated based on a series of factors (type of structure, maximum range of load, type of flooring, whether high or low frequency of passage, etc …) but usually the frame types range from a measure of 3X3 m to 5X5 m.
Now as you can imagine, as it rises to the surface level, each layer should be prepared with the same characteristics and then to work in harmony with the rest. At surface level, ceramic flooring must be the expression of what was created on underlying layers.
Theoretically and practically speaking, the screed fraction should be retraced by the ceramic flooring and so the size of the tiles has to be such that you can work this way, working to follow the perimeter and not overlap the gap.
What are movement joints?
Here is where we talk about movement joints that represent the elastic link between one part and the other. The grid formed by installing the movement joints absorbs the expansions and contractions of the various parts making up the floor. The result is the creation of a fractionated surface, but also connected together by a cross-linked elastic grid: the expansion joint, precisely.
How do you choose the joint? This is often the part that is considered more difficult, but that is actually very simple. The movement joint has to be selected according to the type of fractionation so we talk of a consequential choice.
For small gaps (es. 3×3 mq): with a tile format not more than 30X30 cm coextruding pvc joints are ok NE; if the surface is bigger however movement joints of the type NA in aluminium and rubber.
Large fractionations (Eg. 5×5 sqm): for tile sizes exceeding 30X30 cm or with unequal sides to measure (type 45X90 cm or 60X90 cm) or for marble slabs to be polished the following are suitable joints NTA / STA anodized aluminum Epdm vulcanized high performance.
These are two simple examples that demonstrate how everything is all decided once you get to the flooring surface. For this reason, the product selected must accommodate and respect the characteristics of the structure on which it will be applied.