Can concrete be prevented from cracking under the effect of climatic hazards? Indeed, variations in temperature and hygrometry end up causing the expansion and retraction of the concrete. To prevent this slight movement of the concrete surfaces, the expansion joint is the solution. What is it, what is it for, how to implement it? We tell you everything.
What is a concrete expansion joint?
Building professionals know it: it is essential to integrate expansion joints in a concrete construction. Their function is to absorb variations in the size of the concrete linked to the instability of temperature, humidity, wind or vibrations of mechanical origin. Slabs or walls can crack or even break.
- The expansion joint is space left between two concrete slabs or structures. Thanks to this interstice of about four centimeters, the concrete can “breathe” without undergoing excessive stress. The expansion joint is made over the entire thickness of the paving or concrete wall.
- This space is filled with flexible and resistant materials, which may experience compression. Most often rubber, silicone, aluminum or steel are used.
- Expansion joints have the effect of division of the surface into several independent zones. This stabilizes the entire structure.
- Seals remain effective in the temps. They protect the concrete construction from possible cracks, thus preserving its integrity.
- The expansion joints are covered with a joint coverwhich will need to fit into this space.
- The DTU (Unified Technical Document), which applies to building works contracts in France, specifies their spacing. Concrete expansion joints are usually placed every 20 or 30 meters.
- The installation of seals is essential in the case of grandes surfaces concrete, at least 40 square meters. It is also recommended for buildings adjoining. Small buildings are not affected.
- We can manufacture expansion joints or obtain joints profiles made of PVC, waterproof and solid. These protect the structure from water infiltration – and corrosion. They are removable.
- Expansion joints must be able to resist not only to meteorological hazards and vibrations, but also to aggressive products: diluted acids or bases, hydrocarbons, de-icing salts, etc.
What are the different types of concrete joints?
In the realization of concrete structures, the use of joints is done according to various techniques, each one giving the expansion joint a specific name.
Professionals use these PVC profiles, before pouring, to subdivide a large concrete slab into compartments close to the square.
Once poured, concrete shrinks when it dries, which can cause cracks or breaks. Control joints are used to counter this phenomenon. These are saw cuts made every 15 or 20 meters, depending on local weather variations.
Deferred keyed joints
This fairly simple technique consists of pouring a large concrete slab into several smaller pieces. Scrap strips are left at the ends. Ninety days later, casting is completed by filling these strips. The slab thus produced will undergo less pressure.
It happens that certain structures are subjected to different weights. The concrete foundations are fragmented, so as not to undergo the constraints of differential settlements. Stability assured.
Between two adjoining buildings, there is a great risk of collision under the effect of the expansion of the concrete and the movements of the ground. Seismic joints of at least six centimeters, perfectly straight, are made over the entire height of the building. The two buildings are thus protected from any mutual impact.
If by misfortune a construction site has to be stopped, stopping the concreting requires a construction joint – or pouring stop joint. This technique makes it possible to isolate two concrete cast at two different periods.
Isolation (or separation) joints
They are put in place to isolate a slab from certain obstacles that could hinder its retraction movement. This is the case of a post, a door threshold, a wall, etc.
The expansion joint
It absorbs the natural deformations of the concrete. In the family of concrete joints, it is essential: by allowing the concrete to vary freely as it deforms, the expansion joint protects the structure.
When to use an expansion joint for concrete?
Expansion joints are not made as soon as a concrete surface is poured. For small structures, they are not essential. In general, individual houses do not need it.
- Expansion joints play a role as soon as a concrete surface exceeds 40 square meters or when a slab measures more than 15 meters. They are compulsory in the case of large structures (buildings, sidewalks, car parks, esplanades, etc.) or adjoining buildings.
- Certain criteria are decisive for the construction of expansion joints. In particular the local climatic hazards and the configuration specific to the site.
How to make an expansion joint for concrete?
After the theory, the practice. The installation of joints is done in stages:
- Grid the surface, so as to identify the spaces where the joints will be placed.
- Position PVC profile joints on a polyane film. This kind of plastic sheet placed under the slab solves various problems: sealing, insulation and soundproofing. This is done the day before pouring.
- Fixer joints with cement pads.
- When everything is dry to flow the concrete.
Some masons prefer to cut the concrete with a grinder to create the space themselves, rather than using PVC profiles.
The finishing aesthetics is perfect when the top of the joint is covered with a compressible material (foam or flexible sealant for example) or a joint cover.
What is the distance between 2 expansion joints for concrete?
According to the DTU (Unified Technical Document), the number of expansion joints to be provided depends on the concrete surface and the expansion coefficient.
- For a terrace, a joint must be provided if it is at least 30 meters long.
- On a slab of 50 square meters, it is planned to lay two expansion joints.
- A yard of at least 60 m2 can have three joints.
- For a wall, the maximum distance between two joints is 20 to 50 meters (depending on the region).