Should we worry when we discover cracks in the slab on the floor of our basement? If cracks are found, it is not necessarily a serious problem. It is possible that they are only shrinkage cracks, that is to say cracks caused by the drying, more or less rapid, of the concrete. When setting, the concrete undergoes a slight shrinkage.

This is more of a concern when the cracks are deep and continuous. It is therefore important to follow the evolution of the cracks in order to establish an appropriate diagnosis.

Here are some examples of deficiencies causing cracks in the slab on the ground:

  • Pyrite - It can cause normally star-shaped cracks in the concrete slab on the ground. The presence of needle-like crystals intertwined on the concrete near the cracks is an obvious sign that there is pyrite. However, if this index is not present, it does not mean that there is absence of pyrite. The only method to accurately check the presence of pyritic shalex (pyrite) in the embankment, under the concrete floor, is to have this embankment analyzed by a specialized laboratory.
  • An uplift caused by hydrostatic soil pressures - In the event that the slab is lifted, star-shaped cracks can also be found. When we see efflorescence (white powder) on the slab on ground, this indicates that the cracks are due to hydrostatic pressure under the slab. Poor foundation drainage can cause hydrostatic pressure under the slab on grade if the land drainage directs water into the building. This situation places an abnormal strain on the drainage of the foundations. Finally, an unsuitable French drain (crushed, negative slope, etc.) can also generate hydrostatic pressure under the slab.
  • Sulfation of concrete - The sulfuric acid produced in the soil can come into contact with the underside of the concrete slab causing the decomposition as well as the friability of the slab concrete on the ground. The latter then loses its bearing capacity and has significant swelling and cracks. Le National Building Code recommends the installation of a polyethylene vapor barrier between the backfill and the slab. The vapor barrier thus protects the concrete against sulfur attacks.
  • The subsidence of the embankment supporting the slab on the ground - It can be caused by a poorly compacted embankment or by the migration of the soil under the slab. This situation is often encountered on land with a steep slope, such as at the edge of a river. Water flowing below and around the building transports the earth away, resulting in subsidence of the embankment that creates stress in the concrete slab, creating cracks.
  • The absence of control joints (between the slab and the foundation wall) - The lack of control joints between the foundation walls and the concrete slab means that the concrete slab can barely move horizontally, causing long cracks in the slab.
  • The lack of control joints (between the slab and the elements crossing the slab) - Cracks near columns and components inside the slab (plumbing, retention pit, columns, etc.) are also generated by a slab that can not move horizontally in these areas. So, this stress appears in the form of cracks.
  • Control joints - Missed or poorly made control joints can cause cracks in the floor slab. A control joint must have a depth equal to one quarter (¼) of the thickness of the slab. In addition, it must be done near the columns and elements that penetrate the slab.
  • An overloaded slab - This is so when applying a load greater than the bearing capacity of the slab. Cracks will then appear, but what further aggravates the situation is the lack of drainage foundations. The reason is that at this moment the soil - below the slab - becomes soggy and loses its bearing capacity. Then, the load applied on the slab can push the latter and create an internal stress that causes cracks in the slab on the ground.
  • Plastic shrinkage cracks - They appear when surface water evaporates faster than penetrant water. This results in shrinkage and surface tension stresses cause small, irregular cracks to appear. The cracks are generally short, parallel and spaced 30 to 90 mm apart.
  • Warping slabs - Concrete foundation slabs tend to lift at joints and along their perimeter, which sometimes leads to loss of contact between the slab and the foundation material. The main cause of warping of a slab is the differential shrinkage that occurs when the exposed surface shrinks relative to the non-moving core. This removal is usually due to drying, but it can also be caused by carbonation concrete surface or, in the case of a mixture of high-strength cement-rich concrete, internal drying (autogenous shrinkage) occurring during the hydration of the cement paste.
  • The lifting of the slab caused by the action of freezing and thawing - This situation can occur when there is insufficient insulation under the concrete slab in an unheated area, such as a garage with no heating. The action of freezing and thawing produces significant cracks in the concrete slab.
Cracks in the slab on the ground

Crack in the slab on the ground

Star-shaped crack in the slab on ground

Star-shaped crack in the slab on ground