In conjunction with the connections to the other structural parts, with the operating conditions and with the environmental conditions, the pavings and coverings are subjected to tensions of various kind that can cause cracks and breaks in the stone elements or make them come out from their seat, with serious damage both from a functional and aesthetic point of view.In order to absorb these tensions it is necessary to prepare specific joints that allow a controlled movement between the different parts of the buildingThe main types of joints are:
On the market there are essentially two large families of flexible expansion (and compression) joints
- Structural joints
are those that repeat, on all layers of the paving, the joint present in the underlying structural support; they must be studied and executed with particular care and may require the production of special pieces;
- Desolidarizing (separation) joints
separate the paving from the parts that delimit it or pass through it like walls and columns, absorbing the different movements. They can be made up of a strip of compressible but not perishable material (polystyrene, polystyrene foam) placed along the perimeter connections with the walls and are usually hidden by the use of skirting boards or wainscots.
- Expansion (and contraction) joints
allow the paving assembly to expand as the temperature and humidity conditions change; they must be prearranged to contain pavings not exceeding 25 m² for interiors and 16 m² for exteriors. Very important especially for the installation of large slabs with reduced thickness, indoors in presence of underfloor heating or of strong sunlight as in the case of large window surfaces, outdoors in climates with strong seasonal temperature range but especially with daily temperature range; in all these cases it will be necessary to pay even more attention to the prearrangement of floor joints, always interrupting continuous surfaces greater than 7 linear metres.
These joints must be chosen knowing their performance and durability, also taking into account the installation method and texture as well as the colour and nature of the stone surface on which they will remain at sight.
- Flexible joints
allow the compensation of dimensional variations due to morphological alterations, such as vibrations and deflections. Thet are usually created by interposing a horizontal layer between the foundation and the bedding or adhesive layer, consisting of a sheath (mat) made of polyethylene with a pattern of raised channels between squares, whose elasticity and deformation in several directions guarantees the stress absorption function often in combination with desolidarizing and waterproofing functions between the layers. Particularly useful on deformable structures such as garage slabs in cement-based prefabricated units, wooden slabs or with steel beams especially for large areas, or for movable structures such as elevators or lift platforms for concealed garages.
- Shrinkage joints
are used to control and hide cracks due to shrinkage in concrete screeds and in industrial paving. When the concrete hardens c' there is always a reduction in volume that causes it to crack, cutting the floor at regular intervals, weakening the structure on precise points so that these cracks "prefer" the selected paths. These joints, appropriately designed and manufactured, are in practice an aesthetic solution aimed at obtaining an ordered surface and without cracks out of place, but also a functional solution that makes it possible to locate where, in the stone floor, it will be essential to provide suitable contraction joints. They should be designed to preferably form squares or rectangles whose width must never exceed one and a half times their length; the distance between the cuts should be from 24 to 30 times the thickness of the paving, and their depth 1/4 of the thickness. For example on a 20 cm screed we could provide 4.8-meter squares, or rectangles with maximum dimensions of 3.2 x 4.8 m and the depth of the cuts should be at least 5 cm. Little deep joints do not allow cracks to engage in the right place, with the result that they will most likely appear random on the surface.
: those in profiles to be inserted
and those in the form of sealing material to be poured
in the joint.
The latter are the most suitable for application on stone paving in case of squared formats with sawn sides and a reduced size of the joints (a few millimetres).
While for materials with irregular formats (opus, cubes, pebbles) or squared but with sheared sides that necessarily require wider joints (a few centimetres) it is customary to operate by inserting pre-filled plastic "V" profiles in the joint with silicone sealant.
Over the years, innovative methods and also simple easy-to-use expedients with low aesthetic impact have been used. These methods have shown satisfactory results for some porphyry or natural stone pavings with mortar installation and important joints sealed with cement grout, such as:
- interruption of the paving texture with strips of cubes or pebbles with bedding and sealing made of sand only;
- insertion, for cube pavings, of sequences of arches with bedding and sealing made of sand only;
- filling of joints with thickness greater than one centimetre with hot-poured elastic resin.