Formação de fissuras no concreto (noções básicas, tipos e causas)

Formation of cracks in concrete (basics, types and causes)

June 6, 2024 Roberto Magalhães

Cracks in concrete are inevitable and are not taken into consideration during planning and construction. Environmental factors, structural form and construction methods greatly influence the formation of cracks in concrete. This article focuses on the types of cracks in concrete and how they form.

Cracking in concrete can be observed in three stages.

Crack formation in raw concrete (before curing)
Formation of cracks in hardened concrete
Construction practices

This article explains the different factors that influence cracking in each category. It is very important to know the environmental conditions, structural loads, special structural arrangements, special restrictions, material properties, etc. in the design and construction phases to minimize concrete cracking, which is inevitable if not much attention is paid.

Cracks in immature concrete

Immature concrete is generally called plastic concrete and is uncured concrete or concrete that is hardening. There are several reasons why concrete may crack as it hardens. Evaporation of moisture from concrete, construction movements, etc. cause cracks in the concrete. These types of cracks in immature concrete can be divided into two main categories.

Cracks in the plastic
Constructive movements

Plastic Cracks

The cracks that cause the plastic phase of concrete are called plastic cracks. Cracks can occur for two main reasons.

Plastic shrinkage
Plastic Clearance

Shrinkage cracks in plastics

Plastic shrinkage cracks occur on the concrete surface when moisture on the concrete surface evaporates faster than it gives way through bleeding. The concrete surface dries out due to evaporation and shrinks, resulting in cracks in the concrete surface. It is very difficult to control cracks in concrete by altering the concrete mix to achieve the desired bleeding rate. However, plastic shrinkage cracks can be minimized through various measures, as mentioned below.

Because the rate of evaporation affects cracking, it can be controlled in several ways. Erect windbreaks to reduce wind speed and umbrellas to reduce heat at the concrete surface and reduce evaporation.
Sprays can be used to reduce evaporation.
The temperature of the concrete can be reduced at the time of placement.
Concrete when the ambient temperature is low. If you start pouring concrete in the evening or at night, the risk of concrete cracking will be reduced.
Quickly finish the concrete surface and, if possible, place jute bags, polyethylene, etc. on top. damp to cover the surface.
Start healing as soon as possible. Membrane healing is also possible.
If necessary, accelerators can be used to reduce the concrete setting time.

Plastic settlement cracks

Plastic settlement cracks occur in uncured concrete when the settlement of fresh concrete is hampered by reinforcement or formwork. This type of crack in concrete occurs a few hours after concreting. In fresh concrete, the dense material tries to move down and the water rises. This process is blocked by the upper reinforcement or formwork and, as a result, plastic shrinkage cracks can occur. The following figure shows how cracks form.

These cracks can be observed along the reinforcement lines as shown in the figure above. The following measures can be taken to minimize cracking at the mix design and construction stage.

Mixture designs with lower bleeding behavior could be used.
Increase the shell to stem diameter ratio
When concrete is poured onto the ground, it can be done on a wet basis as this prevents water loss from the concrete.
Prevent any movement of the form by fixing it firmly
Compact the concrete sufficiently
Concreting of deeper sections can be done in advance to give them time to set and to coordinate setting with other castings.
Start healing as early as possible and do it properly

Cracks during construction movements

The effects of movement of the formwork or concrete supports can cause cracks in the concrete. This problem is more common in low-rise buildings because the skills and attention of contractors are comparatively less compared to high-rise buildings. There are two main types of movements

Formwork movement
Ground movement

If the formwork does not take sufficient care of the soil conditions at the time of concreting, cracks will occur. The consultant must also ensure that the conditions of the column are acceptable for concreting. The formwork must be checked before and during concreting to determine if there is a risk of the formwork moving.

The soil conditions must be checked and verified with the necessary tests to ensure that the soil can support the load of the concrete without movements that affect the hardening of the concrete. Concrete has a much higher density and therefore exerts a very large weight. For example, once the soil has been filled, compaction tests may be carried out to ensure that the concrete has sufficient strength.

Formation of cracks in hardened concrete

Even cracks in mature or hardened concrete due to loads, thermal effects, accidental influences, etc. Some types of cracks can be controlled by design, taking them into account in the design. For example, cracking in concrete due to delayed ettringite formation can be minimized by reducing the rise in core temperature, which is discussed in more detail in the final part of this article. However, it is very difficult to consider all factors during the design phase, and also some of the long-term chemical effects, such as alkali-silica reactions, are very difficult to control. Cracking in hardened concrete can be divided into four main categories.

Physical (disappearance of aggregates, drying shrinkage and fine cracks)
Chemical (corrosion, alkali-silica reaction and cement shrinkage due to carbonation)
Thermal (freeze/thaw cycles, seasonal temperature fluctuations and early thermal contractions)
Structural (random loads, creep effects and design loads)

Each of the above categories and their influence on cracking in various subject areas are discussed in different articles published on this website Click on it for more information and based on the main causes of cracks, the above categorization was made.

Cracking due to construction practices is also discussed in another article and can be consulted for more information.

Source: Internet