Application Sheet Crack Treatment

Before applying a coating, be it cement, resin, linoleum or parquet, it is necessary to repair any existing cracks and fissures; however, before doing so, it is important to investigate the reason for the existence of the crack to prevent its reappearance over time.

Foreword

Knowing that if the causes are not removed, cracks may reappear and subsequent repairs may be subject to new cracking states is of fundamental importance. For example, if a crack is caused by a tree root that is forcing the slab above it causing it to break, the only possible solution is to remove the root. Hence the importance of identifying the nature of the cracking in order to assess whether it is possible to carry out a definitive or at least lasting work for a specific period of time.

Classification

The causes of cracks in a concrete structure can be several: cracks caused by hygrometric and/or plastic shrinkage, by lack of or incorrect curing, by lack of construction joints, by thermal expansion, by structural failure, etc. Simplistically, these events can be grouped into three categories:

• Stable cracks, i.e. no longer subject to movement (e.g. plastic shrinkage cracks);
• Dynamic cracks (presence of roots, thermal expansion etc.);
• Cracks from missing joints.

Stable cracks

These include cracks caused by hygrometric and/or plastic shrinkage, structural failure cracks and cracks caused by incorrect curing. What to do? In such cases it is possible to saturate the crack by working as follows:

• Slightly open the crack by means of a small grinder and vacuum the residue of dust;
• Consolidate the base thoroughly by using Evercrete Pavishield;
• Fill the crack with Ercole and, fresh on fresh, embed an alkali-resistant mesh;

After at least 24 hours, the entire area can be covered with the desired coating.

Dynamic cracks

Unlike the previous category, the nature of these cracks can be the most varied. It is therefore necessary to identify their actual causes . In the case of thermal expansion, for example, it will be required to create an expansion joint. As it is well known, the crack does not generally follow a straight line. What should be done in such cases? First, the two loose parts are rigidly glued together . Usually, epoxy products are used and/or the cracks are "seamed” (cuts are made across the crack and bars are inserted into these, which are then sealed to make the two parts integral). Then a new joint is created by cutting the plate over its entire thickness (or at least 2/3 thereof). The new joint must be filled with an elastic sealant (polyurethane or polysulphide) and will remain "visible" on the final coating.

Cracks due to lack of joints

They can be considered a subcategory of the previous one. Sometimes they are caused by the particular geometry of the surface (imbalance between the two dimensions, L-shapes etc.), sometimes by a poor choice of substrate materials (cement screeds made on site). In the case of an internal surface not subject to thermal expansion or other possible movements of the substrate, the cracks can be "sealed" in a rigid manner (Ercole, epoxy mortars, staples, etc.) and then the surface is coated with the desired layer. On the other hand, in the case of an external surface subject to thermal shocks or a surface subject to possible movements, it will be necessary to provide for the creation of a surface joint.

Cracks on coating

As mentioned above, adhesion coatings (resins, cement-based trowels, etc.), such as Ecobeton coatings, follow exactly the movements of the substrate and consequently reflect all its movements. In other words, if there is a crack on the coating, then it is definitely a consequence of a crack on the underlying support. This is a physical issue, as thinly laid continuous coatings have negligible plastic or hygrometric shrinkage.

There are coatings that have good elasticity, and can somehow withstand small movements of the substrate. This property is called crack bridging ability (EN 1062-7 standard), however it is often observed that this ability is not long-lasting, i.e. sooner or later the crack is reflected even on coatings of great elasticity, especially if the crack is generated by dynamic movements. The problem of cracking can therefore be traced back completely to the substrate, whatever it may be. The categories of cracks found in concrete were described above. Let's take a closer look at the main factors responsible for coating cracks:

• The hygrometric shrinkage of the substrate (screeds), especially the most common cement screeds, suffer from shrinkage due to the shrinkage of the cement. This contraction does not occur freely, but it is hindered by friction on the substrate and all the other elements of the construction (columns, walls, etc.). The tensions generated during this contraction cause the screed to crack when the structure has not yet reached its maximum tensile strength. This phenomenon is especially important in the first 90 days after casting, after which it becomes less dangerous. After three months, in the absence of other phenomena, you can safely coat over the cracks that will have formed. When time is of the essence, it is strongly recommended to make cuts in the floor that can guide the crack through an organized pattern: these control joints can then be sealed with elastic materials (polyurethanes or polysulphides) and will be part of the floor design.

• The phenomenon referred to in point 1 also occurs in the presence of patches on the floor such as tracks to allow the passage of systems or the joining of floors made of different materials or floors laid at different times.

• One of the major contributors to cracking of supports (mainly external) is thermal expansion. It can also occur in indoor rooms with underfloor heating. In these cases, in addition to the contraction, concrete undergoes an expansion that will push it by putting pressure on the elements surrounding it. In order to contain this movement, it is necessary to provide an expansion joint of variable size depending on the size of the surface. Unlike the movement of shrinkage, which is exhausted over time, the movement of expansion will be present throughout the life of the structure. This joint must remain visible on the surface.

• The fourth cause is due to structural movements : in floors, or on soils that have not been properly compacted and prepared, it is not uncommon to find subsidence and unforeseen movements leading to cracks in the substrates . From an engineering point of view this is widely expected, from an aesthetic point of view it is less pleasing. In this category we can also include all the "disturbing" elements that are embedded in the screeds, such as pipes, various systems, manholes and gutters that create discontinuity or even bold floor geometries containing edges or jumps in section. In these cases, repairing the coating will not be an easy task and may require a crack sealing work using a Bonding Agent-cement grout (in a 1:1 ratio) and a subsequent application of a new finishing coat (Microbond or Ercole as appropriate).

As you can imagine, the substrate plays a key role and in order to avoid unpleasant surprises, it is necessary to carefully evaluate type, geometry and time of curing. Taking shortcuts can only increase the risk of incurring unpleasant sometimes irreparable blemishes.