The installation with adhesive involves the preliminary construction of a suitable screed that in a second time will be covered by a thin layer of adhesive (3 - 5 mm) and by the chosen stone material.It is indispensable to have a perfectly prepared screed for this installation system
which requires a perfectly flat surface and the scrupulous respect of the useful installation dimensions, in fact the thin layer of adhesive is not able to compensate for any irregularities in the foundation. The screed must guarantee a cohesive tensile strength ≥ 1.0 N/mm2, must appear compact, flat, smooth on the surface, free of cracks, clean, dry
and what is very important it must be dry.
For the drying it is necessary to wait at least one week for each cm of thickness of the screed with a minimum time of 28 days; the residual humidity in the screed before applying the adhesive must be < 4.0% for cement screeds; < 0.5% for anhydrite screeds.
It is necessary to scrupulously respect these times and these prescriptions since all the foundations shrink with a very rapid decrease in volume in the first weeks and a consequent "shrinkage" of the screed of about 1mm/linear meter or even more. The duration of shrinkage is greater for high thickness since the quantity of water contained that must evaporate is higher.The main advantages of this system depend on the properties of the adhesive used, which if of good quality guarantees:
The two-stage installation method using:
- greater adhesion on non-absorbent or sensitive materials;
- greater adhesion on large and reduced thickness materials (from 1 to 2 cm);
- flexibility in processing times, with the possibility of intervening even partially by stopping and resuming installation operations;
- reduced binder setting times and therefore rapid commissioning with pavement walkability already after one or two days from the end of the installation, a fundamental feature for shops, offices, accommodation facilities and in general for all those sites where it is essential to precisely coordinate the sequence and optimize the times;
- possibility of bonding stone elements directly onto an existing paving.
The installation system involves the application of paving or wall covering elements using a thin layer of adhesive spread out evenly both on the installation surface and on the back of the stone element (double spreading method). This is a method developed in the field of ceramics that in recent decades is also experiencing success in stone installation.For paving applications it is necessary to have stone materials with constant thickness and particularly accurate installation foundations
especially as regards the dimensions, consistency and regularity, the adhesive bedding layer allows indeed very small compensations.
It is essential to choose the correct type of adhesive to achieve a correct installation. The presence on the market of multiple adhesives requires adequate and constant information and it is necessary to scrupulously follow the installation instructions prescribed by the manufacturing companies. The adhesive will be selected depending on the climatic conditions and placement (indoors or outdoors), the intended use, the foundation composition. The manufacturers are present on the market with products that guarantee an increased duration of the setting over time, elasticity, impermeability, resistance to temperature changes, as well as considerable resistance to tearing values.
Today it is possible to distinguish three large families of adhesives among the ones most widely used for paving and wall stone covering:
ADHESIVES BASED ON NON-MODIFIED HYDRAULIC BINDERS
These are white or grey powder adhesives to be mixed with water at the time of use, composed of cement, fillers, synthetic resins. These adhesives have the ability to retain the water needed to hydrate the cement that they contain even when they are used in a thin layer, without wetting the base or the material to be installed. This type of adhesive is not recommended for installing stone materials as it is not always able to guarantee sufficient adherence, especially when the base of the elements to be bond is smooth and with low absorbency capacity.
MODIFIED CEMENT ADHESIVES
These are white or grey powder adhesives to be mixed with water at the time of installation (one-component) or with synthetic latex (two-component). These are hydraulic adhesives to which additives are added during production (with specific powders) or during installation (with dispersing liquids) to polymers that provide greater adhesive power and greater elasticity. They are suitable for installing stone materials both indoor and outdoor, on absorbent and non-absorbent foundations, in case of stresses due to expansion or operating movements, for applications subjected to rain or permanent water (e.g. swimming pools) frost, high temperatures, or with significant daily temperature changes.
BICOMPONENT ADHESIVES BASED ON REACTIVE RESINS
These are two-component adhesives in the form of pastes or thick liquids to be mixed with a reagent at the time of use; they consist of resins (polyester, polyurethane, epoxy) and a hardening product. The two components react chemically, regardless of contact with the air or the degree of absorption of the stone material or of the foundation. They are used when it is necessary to achieve high resistance values or special performances as a result of particular uses or applications in critical conditions (for example on highly deformable structures such as metal stairs, subject to high concentrated loads such as storage warehouses with transit of forklift trucks, on immersed surfaces made of synthetic materials such as prefabricated pools).
Stuccoes or coloured resins are usually used to seal the joints of the stone elements installed with adhesive. The exposed surface must be kept perfectly clean from any adhesive or stucco residues using a damp sponge.