Structural cracks in concrete (a detailed study)

Structural cracks in concrete can result from a design defect or other causes. Let's see how this happens.

Structures are designed to withstand the loads imposed on them and serve the purpose for which they were built.

Designs are based on the design loads specified in the relevant guidelines. In special cases, if there are no specifications, actual loads must be taken into account with appropriate modification factors to take into account different actions such as sudden shocks, moving loads, etc.

Failure of a structure may be due to applied load exceeding the design load, inadequate construction practices, errors in structural details, design errors, etc.

Additionally, sudden outages or evacuation warnings may occur. This may depend on the type of construction.

In summary, if there is a possibility of increasing concrete stress or if the concrete of the structural elements has not yet reached its breaking stress ( 0.0035 according to BS ), failure cannot occur suddenly as the load on the structure increases. increases. Sufficient warnings are issued before failure.

Warnings come in the form of excessive deflection or increase in crack width.

An increase in crack width is always an indication of a defect. In such cases, the cause must be carefully investigated and necessary measures taken.

Structural cracks can be examined in three different areas.

• Cracks due to unintentional overloading
• Crack formation due to creep effects
• Cracks due to design loads

Cracks due to unintentional overloading

Can we consider them as structural cracks?

To determine design loads, designers follow guidelines or consider actual loads for the design if specific standards for load evaluation do not exist.

However, after construction, most people do not think about the load capacity or know what loads will be applied.

For example, the design of the storage area may be based on the owner's specifications, as this specific information may not be available.

In some cases, owners provide incorrect information to planners to achieve a worse cost structure, which is not good practice. Based on the information provided or available details, plans are created and construction can follow the project drawing.

When operating these types of buildings, the load is greater than expected during construction and cracks and structural failures can occur.

Failure is reported in the form of cracks and when this is noticed, immediate action must be taken depending on the extent of the damage. In serious cases, the load must be reduced or removed and remedial work carried out as recommended by a civil engineer with appropriate experience and knowledge.

In many cases, the load on a structure may be greater than the design load.

The problem may be caused by optimistic planning, design load reductions, design load calculation errors, etc. Furthermore, the customer can add loads greater than those specified to the designer in their calculations.

Crack formation due to creep effects

When concrete is subjected to compressive stresses, it deforms instantly.

If the load on concrete is maintained for a long period of time, the concrete will undergo additional deformation even without increasing the load. This time-dependent expansion is called creep.

In other words, creep is the time-dependent increase in deformation in a solid under constant or controlled stress. The following figure shows the variation in deformation under constant stress.

Furthermore, the deformation of structural elements under continuous loading is called creep, which gradually generates excessive stresses due to deformations. The increased tension causes the concrete to crack.

The following factors are considered the main cause of creep

• Amount of voltage applied
• Charging duration
• Type of stress (axial, torsional, bending stress, etc.)
• Water-cement ratio
• Aggregate influence
• Healing conditions
• temperature
• Relative humidity
• Title composition
• Chemical additives

Cracks due to structural loads

Cracks due to design loads can be considered as structural cracks and can be avoided with due attention to design at the design stage.

To obtain a safe and economical structure, the calculation of design loads must be carried out correctly.

Overestimation increases the security structure.

However, there will be an increase in construction costs, which none of the financiers expect.

In the past, major structural failures have occurred repeatedly around the world due to errors in estimating design loads. There are guidelines for this purpose, but they are not applicable in all cases. The loads specified in the standards are considered typical. The designer must review the requirements and select appropriate design loads.

Increasing loads beyond what the designer expects will cause the structure to fail.

Increased load causes warnings to be issued in the early stages.

The most commonly observed warning sign of failure is the formation of cracks in structural elements.

If the loads are exceeded, cracks form on the surface, e.g. B. in a beam at the bottom of the beam, and the crack width increases continuously. As the load gradually increases, the crack width also increases.

With a sudden increase in load, sudden cracks in structural elements also occur. In this case, the necessary measures must be taken immediately to deal with the additional loads acting on the structure.

After removing all loads, the cracks must be repaired as recommended by the structural engineer, as long as the bearing capacity of the structure is not compromised due to excessive loading.

Furthermore, there are cases in which the designer did not take into account certain loads due to negligence or because he did not foresee such loads.

For example, wind load must be taken into account when designing in an area where very high wind loads can affect structures. Otherwise, an error may occur.

If the designer does not anticipate an accident due to the nature of the structure, he may not take this into account in his planning.

All of the above causes can initially lead to cracking in structural elements before failure occurs.

Once the problem is visible/observed, necessary actions must be taken and resolved by following appropriate procedures.

Designers must take special care to avoid such errors in their designs. The following table shows the types of structural cracks that can occur in various structural elements.

The following table lists the various causes of cracks and the extent of the cracks (depth/width).