Joints regulate the cracks that occur in concrete, which tends to be random during the curing process. These cracks are a result of the different stresses that the concrete mass will experience throughout its entire lifespan.
Fresh concrete is a mixture that is highly sensitive to changes in humidity and temperature, which cause it to expand or contract, leading to cracks.
The importance of joints in concrete isn’t because they prevent cracking, but because they control it. Therefore, they need to be located and planned correctly, as well as needing a periodic maintenance plan.
What are concrete joints?
Basically, the joints are intentional cracks in the concrete, they reveal themselves as an interruption in the concrete surface. Generally, they are built by using cutting processes, known as early-entry dry cutting or sawing and conventional wet-cutting.
Thick slabs require cutters (saws), which have diamond or abrasive toothed discs. For screeds, angle grinders with diamond discs are used.
Likewise, joints between structural elements consist of polystyrene strips, which are previously cut.
Joints absorb deformations caused by the concrete expansions and contractions. Although they are also planned to control those caused by structural movements, as well as those caused by earthquakes, although to a lesser extent.
Why do we need joints in concrete?
During its pouring, concrete is exposed to external factors that influence its curing process and its volume. This causes the concrete to change in certain ways, either losing water, expanding, or contracting.
During expansions and contractions, the concrete mass builds up stresses that cause random cracking throughout. This is unavoidable and a natural process in concrete, but it can be minimized thanks to joints.
However, these are necessary. They establish separations between slab sections, schedule when pouring activities are to be suspended, allowing the correct construction process.
Types of concrete joints
Types of concrete joints are based on the action or movement they control.
As we have just explained, concrete is subject to contractions and expansions, so each of these actions corresponds to a specific joint. Also, structural displacements need their respective joints.
In conclusion, there are three main types of concrete joints, which are:
Concrete expansion joints
The concrete expansion joints are characterized by preventing crushing and distortion between adjacent elements, caused by compression forces triggered by the concrete expanding.
This expansion, beyond being caused by temperature changes, can be produced by applying loads, differential movements of structural elements, and by the ground conditions.
The basic function of expansion joints is to isolate structural elements. That is, to restrict interactions and minimize cracking. For example, because of them it is possible for slabs, walls, pillars, etc, to move independently.
This type of joint is built by cutting into the concrete, which is sealed with a compressible material, such as an elastomer.
Contraction joints in concrete
They are responsible for reducing unpredictable cracks caused by the concrete’s hydraulic retraction. So they are placed both on the longitudinal and transverse axis of the structural element.
Contraction joints are necessary for flat and thin elements, such as paving slabs, walls, among others. They also require adequate spacing to effectively control cracking.
They are created by inserting strips into the fresh concrete, or by cutting and filling grooves in concrete.
Construction joints in concrete
Construction joints aim to join concrete elements that were poured at different times.
Construction joints require precise calculations to determine their location on an element’s surface. Essentially, these should be located wherever there is the least structural weakening.
A construction joint must stand out for its regularity and superficial smoothness, being able to cancel the unevenness effects due to the presence of coarse aggregates (gravel).
Sealing and preparing joints
Once the joints have been defined and cut into the concrete, there are three finishing methods:
The first is to leave the joints open, which is not recommended in industrial flooring, especially if they are subject to the constant repetition of loads caused by forklifts.
The second method involves filling the joints, which is a deep seal in the groove. This finish is suitable for floors that are subject to heavy traffic, with a majority of vehicles with solid wheels.
Finally, the third method is the conventional joint sealing, which differs from the previous one by partially sealing the groove, and by using not such hard sealing materials.
Partial joint sealing is recommended for floors with relatively light traffic, where most vehicles have pneumatic wheels.
Industrial flooring joints require a good seal, due to the constant traffic of machinery and the wear caused by industrial activity can accelerate the deterioration of a floor.