We have all noticed magnificent terraces covered at great expense, expensive tiles carefully chosen or even beaches of stamped concrete swimming pools of the most beautiful effect, but whose appearance was spoiled by the appearance of random cracks, certainly most often. without consequences on the solidity of the work, but of the worst aesthetic effect. There is reason to be dissatisfied in these situations, because the appearance of these defects is highly predictable and easily preventable during the casting of the concrete slab serving as a support.
These ruptures sometimes come from a failure to respect the recommendations of the unified technical document (DTU), dealing with the implementation of concrete structures. The instability of the ground, the thickness of the concrete slab, the choice, the dosage or the mixing of the ingredients, the unsuitable climatic conditions during the pouring can be in question, but the most frequent explanation is in theabsencel’insufficiency or the bad positioning des joints of dilation.
What is an expansion joint used for?
Curiously, the meaning of the term “joint”, common in building and public works (BTP), takes on a completely opposite meaning to the definition generally given to this word. Indeed, the seal is commonly a part ensuring a volume continuity confined, passing between two elements of an assembly. To make expansion joints and other joints in concrete, on the contrary, a break in continuity is deliberately created in the material, in order to create an empty space, sometimes filled with a compressible and extensible product. The goal is to avoid the anarchic cracks, caused by mechanical stresses, in pressure or in tension, during changes in volume, under the effect of internal and external temperature variations of the material. The temperature of our concrete slab, for example, can go from – 30°C in winter, to + 70°C at the peak of the summer sun. This delta represents a dimensional amplitude of several centimeters for a large structure, explaining the interest of the split. Temperature heterogeneity et in the internal structure of the material, is at the origin, during these movements, of distortions leading to ruptures.
We can say that the expansion joint is a deliberate and programmed cracking, which can be located in strategic places, to preserve aesthetic harmony in the architectural ensemble.
The different construction expansion joints
In construction, it is customary to call “expansion joint”, all the interstices ensuring vertical and horizontal movements between built elements. In reality, expansion joints have different names depending on their positioning. They do not all have the same function, but are complementary and manufactured using similar techniques. These subtleties are more often confused, even ignored by the professionals themselves.
Types and orientation of joints relative to concrete plates:
1 – The separation joint
The uncoupling joint (or expansion), is an expansion joint, in the literal sense of the term. It allows the free movement of built elements, both in terms of horizontal only on the plan vertical. It can be useful at the junction of walls and ceilings, but becomes mandatory around the slabs large areas. These spaces must be provided at the junction of the slabs with the foundation footings or with any other contiguous autonomous element, such as manholes, columns, steles, machine supports, etc. The expansion joint must allow thermal movement of the slab, walls or ceilings, without exerting pressure on neighboring elements.
The expansion joint is made during casting, by interposing a strip of compressible material (polystyrene, dense foam, etc.) 8 to 12 mm thick, depending on the surface and the thickness of the structure. Take care scrupulously that it extends, without interruption, on the field of the slab. There are also specialized manufactured seals ensuring at the same time thesealing assemblies (roof joints, facade joints, etc.).
2 – The withdrawal joint
The withdrawal seal (or rupture joint, or contraction joint…) is the weapon anti-fissurations most commonly used in the construction industry. This is the typical splitting joint for large surface slabs. It allows the precise positioning of the foreseeable rupturescaused by the shrinkage of the material during drying and/or by thermal and hydrometric movements throughout the life of the structure.
It is possible to spare this seal by interposing a compressible strip during casting or by laying prefabricated joints specialized metal or PVC (seals with insert, car park seals, seismic seals, etc.). Wedged on studs, these joints can be used as point of support for dressage rules. The simplest, fastest, most aesthetic and least expensive method, however, remains to pour the slab over its entire surface, then to split, after the concrete has set (24 to 48 hours), by sawing with the aid of diamond disc.
When should a contraction joint be provided?
In the absence of advice to the contrary from a specialized design office, comply with the following recommendations:
- The slabs delimited by the joints should ideally have a surface square or rectangularthe length of the short sides of which does not exceed half the length of the large ones.
- The joints between plates must be in alignment. A “T” shaped joint causes cracking of the plate on which it ends.
- The space between 2 joints must not exceed 25 times the thickness of the slab, for aggregates less than 20 mm. Spacing can be extended up to 36 times the thickness, when the nature of the work allows the use of larger aggregatesbut may in no case exceed 4.5 m,
- each inside corner must be the origin of a joint arranged perpendicularly.
- the depth of the joint is, at least, equal to 1/4 the thickness of the slab, or 1/3 for slabs of strong thickness.
3 – The construction joint
The construction joint is intended to secure the slab pouring restarts, during a prolonged stoppage of the concreting work, such as the planned future addition of a terrace slab or a balcony. The goal is no longer to provide an expansion space, but to avoid a crack and a breaking point by lack of cohesion of the material.
This joint is made by providing notched or twisted concrete reinforcing rods, on hold. They are arranged at regular intervals, perpendicular to the break line and according to a sampling sufficient to allow the recovery of charges. For unreinforced slabs, a smooth iron is sufficient. If these inserts have not been provided, when, for example, casting stops unexpectedly, the insertion of studs fixed on the dry part of the slab by chemical sealingis possible.