Cracks in concrete, what to do?

Why does concrete crack?

How to eliminate the formation of cracks in concrete?

Why do concrete cracks form?

How to minimize the occurrence of cracks in concrete?

Recognizing that there is a potential for cracking in concrete floors or concrete structures, combined with the right design, it is possible to minimize the appearance of cracks in concrete, increase the service life and aesthetics of the structure.

• It is clear that concrete is one of the main technologies in construction, being a durable and strong building material. 

• Although concrete has great compressive strength, its tensile strength is only about one-tenth that of compressive strength.

• Therefore, concrete can "crack" and is potentially prone to cracking unless steps are taken to minimize the factors that can cause concrete to crack.

• The formation of cracks in concrete affects not only the aesthetic appeal of the object.

• Cracking of concrete can have a detrimental effect on the durability of a concrete structure. 

• The problem of concrete cracking has acquired an important characteristic, with an emphasis on solidity in concrete construction, in terms of service life and operation, the likelihood of repair or replacement of concrete.

Read also, " Warehouse Concrete Floor Crack Repair " or " Warehouse Concrete Floor Repair ", " Concrete Floor Crack Repair with Epoxy Adhesive "

 

Cracking of concrete due to exposure to alkalis

Why does concrete crack?

Cracks in concrete appear for only one reason, when its own tensile strength is greater than the net tensile stresses induced in its matrix.

There are several factors that cause tensile stresses in concrete, and therefore, in order to reduce the possibility of cracking in concrete, it is important that the fundamental cause of its cracking is correctly identified.

This requires knowledge of several criteria, including:

• specificity of environmental impact, during and after construction;

• quality and requirements for concrete mix;

• structural loads;

• cracks can occur when the concrete is still in a plastic state or, as usual, in a hardened state.

Consider the different stages of concrete and the possibility of cracking at all stages of its hardening.

Plastic concrete

In ductile concrete, cracking can occur when the rapid evaporation of water from the surface of the concrete causes the surface to dry out.

Factors that affect the rate of water evaporation:

• high ambient temperature and concrete mix;

• low atmospheric relative humidity;

• high wind speed. 

The cracks that form are classified as crack cracks or plastic shrinkage cracks, depending on their appearance.

A type of cracking called settling cracking can occur right above the reinforcing steel, where the steel restrains downward movement or settling of hard materials in ductile concrete.

Such formation of cracks in concrete can be minimized by simply adding polymer, synthetic fibers.

• Microfiber, reinforces concrete when it does not have the required tensile strength.

• Polypropylene fibers also help to minimize settling of hard materials, thereby reducing cracking. 

• In addition to these synthetic microfibers, an evaporative agent , membrane former orvarnish ( acrylic, sodium silicate, lithium silicate ) must be applied to the concrete surface.

• Lac-membrane former, in adverse weather, helps to minimize the rapid loss of moisture. 

• Thus, minimizing the possibility of cracking.

 

 Shrinkage cracks in concrete

Hardened concrete

There are several factors that cause tensile stresses to occur in hardened concrete.

• The main reason is related to the structural loads that are expected. 

• Secondly, tensile stresses can be induced in concrete, due to containment of volume changes, either in compression or expansion. 

• Potentially destructive volume changes can be caused by drying shrinkage, thermal expansion and contraction, carbonation shrinkage, corrosion of built-in fittings, freezing and thawing, in cold weather regions, and numerous chemical processes.

 

Calculated cracks above reinforcement

The main types of concrete cracking

Cracks in concrete can be described in several ways.

These descriptions may be used alone or in combination to describe the exact nature of the fracture.

That is: surface appearance, orientation, depth, width, and movement characteristic of a fracture. 

The surface appearance of the crack(s) can provide important information as to the cause of the cracking.

• Cracks in concrete, in the form of a map or drawing, are a series of interconnected and evenly distributed short cracks that subdivide the affected area of ​​the concrete surface into smaller, irregularly shaped pieces. 

• The cracks in the map may be relatively shallow in depth, or they may be deeper and throughout the matrix, as in the case of cracks due to the reactivity of the alkaline aggregate. 

• Conversely, the cracks may be isolated or individual in nature, generally in the same general direction as in the case of plastic shrinkage cracks. 

• As such, the terms diagonal, longitudinal, transverse, vertical, and horizontal are often used to describe the orientation of isolated or individual fractures.

Knowing  the depth and width of a crack in concrete is  useful for evaluating the potential impact of a crack on concrete durability and determining an effective repair strategy.

As a rule, surface and shallow cracks will not have a huge impact on the durability of concrete, compared to partial depth and through cracks, which will provide free access of aggressive agents, into the concrete cavity, to the full depth. 

As for the width of the crack, they can be determined:

• as hairy or fine in nature when the width is less than 1 mm;

• average from 1 to 2 mm;

• wide, if the width is more than 2 mm. 

• structural cracks will typically be isolated, wide and deep in nature.

Active and dormant cracks in concrete are used to describe  the movement or mobility of a crack.

• Motion performance is an important criterion in determining which crack repair to use. 

• As the name suggests, active cracks in concrete are those for which the underlying factor or mechanism that led to the formation of cracks is still present, resulting in continued crack movement. 

• On the other hand, dormant cracks in concrete are those that no longer move or have little movement because the underlying mechanism is no longer causing them to form.

Also, cracks can be identified in relation to the  potential cause leading to the formation of cracks.

These terms include:

• plastic shrinkage; 

• shrinkage on drying;

• thermal;

•  d-cracking; 

• structural and accounting.

Influence of design decisions on cracking

• Inappropriate structural design will obviously result in failure and cracking of concrete elements under stress during service. 

• This includes incorrect selection, detailing of reinforcement, limitation of elements subject to volume changes caused by temperature, humidity, and incorrect foundation design. 

• In addition, incorrect detailing, such as the inability to provide adequate shrinkage seams (cut seams), in one form or another.

• Failure to provide the necessary reinforcement, in repeated corners, can lead to cracking of concrete floors.

 

 Seam design error

Influence of applied materials and concrete laying technologies on cracking

In hardened concrete, it is important that the concrete, by its nature, does not exhibit high curing shrinkage, and that measures are taken to minimize the possibility of cracking from other causes such as: 

• corrosion of built-in fittings;

• chemical reactivity of the aggregate or sulfate exposure. 

• low concrete shrinkage on drying, can be achieved initially by limiting the water content of the concrete. 

• A significant reduction in drying shrinkage can be achieved by adding specialized admixtures to the concrete mix to reduce shrinkage or cracks. 

 

Cracking due to improper compaction and vibration of the concrete mix

These additive technologies are increasingly being used in fluid retention designs: 

• bridge structures;

• in parking structures and slabs on the ground;

• and also in other areas where reduction of formation of cracks in concrete is required.

With regard to cracking of concrete floors or structures, for reasons other than drying shrinkage, low concrete permeability (water tightness) and the addition of the necessary constituent elements that increase the durability of the structure are necessary.

Low permeability concretes can be effective in improving the durability of concrete exposed to chlorides and sulfates or other corrosive substances.

What can be done to reduce the permeability of concrete?

• Low permeability of concrete can be achieved with hydrophobic additives to reduce the water content of the mixture, pozzolans (such as fly ash, silica, metakaolin) and slag cement. 

• The durability of concrete in chloride environments can be further improved by using anti-corrosion rebar additives that effectively delay corrosion of embedded rebar or prevent corrosion throughout the life of the structure. 

• In addition to these measures, the concrete mix must not have high residuals, free calcium oxide or magnesium oxide, or other materials that may cause expansion at a later age. 

• As far as possible, the use of potentially reactive aggregates should be avoided or, if their use is unavoidable, steps should be taken to suppress the reactivity of alkaline aggregates. 

• These measures include the use of appropriate pozzolans, slag cement or lithium-based additives. 

• In regions with cold weather, concrete that will be exposed to the elements must be entrained with air using an appropriate air-void system ( air-entrainment additives).

• Also, the use of materials with different temperature coefficients of thermal expansion and contraction should be avoided.

• Re-diluting concrete with water, i.e. adding water at the job site beyond the calculated water content, is not recommended. 

• Mainly due to the concomitant decrease in the compressive strength of the concrete. 

• Thus, the amount of shrinkage during the maturation of concrete increases, due to additional water added to the concrete mix. 

• The combination of lower strength and increased dry shrinkage increases the likelihood of concrete cracking. 

• Softening can be corrected with workability additives or hydration control additives.

Other construction methods that affect the likelihood of cracking include:

• improper or insufficient curing of concrete;

•  poor quality soil compaction (soil shrinkage);

•  incorrect formwork;

•  poor sealing and erroneous connection.

 

Shrinkage cracks in concrete floors

Who is responsible for the formation of cracks in concrete?

The information provided in this article gives a general idea that minimizing the potential for cracking in concrete is a shared responsibility of the design engineer, concrete manufacturer and contractor.

Responsibility of the design engineer

It consists in the correct design and detailing of the structure, as well as the development of clear and concise specifications regarding the use of proper construction methods and the use of the correct materials for concreting.

Responsibility of the concrete producer

• It is important to understand the impact of materials used in concrete production on the likelihood of concrete cracking. 

• It is also the concrete manufacturer's responsibility to use materials that meet the project's specifications. 

• In addition, the concrete manufacturer should suggest the use of alternative materials, where appropriate, as a solution to potential cracking problems. 

Some of these solutions include using:

• shrinkage reducing additives to minimize shrinkage on drying, especially in boards;

• additives that prevent corrosion to slow down the onset of corrosion of reinforcement in aggressive environments;
  
  
• pozzolans, slag cement or lithium-based additives to inhibit alkali-silica reactivity if the use of reactive aggregates cannot be prevented.

Contractor's responsibility

• Compliance with good construction practice is mandatory for the contractor to ensure that the design, structural detailing and concrete mix provided, are not compromised.

• For this, soil preparation must be conscientious and according to technology, and the formwork must be properly designed and installed. 

• Most importantly for the contractor, water should not be added to the concrete, as additional water will increase shrinkage and the potential for cracking, in addition to reducing strength. 

• Proper concrete compaction, curing and provision of appropriate joints where necessary are also essential requirements.

 

Installation of industrial floors

Concrete does have low tensile strength and can be prone to cracking.

However, with proper design and detailing, the use of good concreting materials, including special softening materials where necessary, and adherence to best construction practices, the likelihood of concrete cracking can be minimized.

"Reduce cracking in concrete floors, effectively"

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