Can We Confidently Conquer Cracks in Concrete?
In short … yes, but the root cause has many different avenues to explore - let me explain.
Designing for Concrete
The top recommendation for the management of concrete cracking is to address it in the design phase of a project and to avoid issues with cracking in the first instance. The structural engineer should be able to specify the following to control the location and size of cracks so as not to detrimentally effect the performance or appearance of the concrete:
· Appropriate selection of reinforcement.
· Use of joints (construction, expansion, day etc.).
· Specification of Concrete classification.
· Good quality site workmanship.
All concrete cracks - appropriate engineering can determine the size and position of these cracks and, if needed, reduce them to micro-cracks that will not be easily seen. If we now consider only non-micro cracks, i.e., visible cracking, they can vary generally and can be classified as either structural, or non-structural in nature:
Structural cracks are of primary concern and can lead to issues which affect the stability and durability of a concrete structure. They can occur from incorrect design, poor construction/ workmanship or overloading of the structure.
Non-structural cracks are formed due to internal (or surface) stresses exceeding the material capacity, resulting in ‘small’ cracks appearing throughout the construction. Whilst these do not have the same implications of structural cracks, they often lead to durability issues whereby the cracks allow ingress of water (and potential corrosive products) into the concrete. These may lead to longer term issues such as corrosion of reinforcement.
To determine the category and severity of any cracking, an experienced and competent structural engineer will be able to inspect, identify and classify the severity of cracking and (if required) suggest an appropriate repair methodology.
Main Causes of Cracking in Concrete Structures
Cracks are mainly caused by:
· Movement or Settlement
· Thermal Expansion and Contraction
· Concrete Shrinkage
· Corrosion of Reinforcement
· Flexural or Shear Failure
Movement / Settlement
By nature, concrete is a cementitious medium that (like natural stone) is excellent in resisting compressive load, but poor in resisting tensile loads. The first contributing factor for concrete cracking is movement of the concrete either by settlement (usually occurring early in the lifecycle) or by physical movement (which can happen at any lifecycle stage). Both will lead to an accumulation of tensile stress in the outer fibre, which can lead to surface cracks.
Thermal Expansion and Contraction
In a similar process as movement, when concrete cures it undergoes an exothermic reaction causing the internal temperature of the concrete to reach a higher temperature than the ambient. Depending on the mass of the pour, this can lead to the external edges cooling faster than the internal mass. The resulting effect is the cooler outer edge of the concrete contracts at a faster rate than the core which generates temporary internal stresses in the face and may ultimately lead to cracking.
When concrete beings to cure, the moisture within beings to dissipate which in turn results in global shrinkage of the concrete section. If the concrete was unrestrained and free to shrink, there would be no residual stresses and the section would find equilibrium (and not crack). However, realistically all concrete structures (beams, slabs etc.) will have restraints formed by the method of construction (abutting walls, anchorage points, structural framing etc.). Therefore, to control these shrinkage cracks, adequate reinforcement and specification of control joints should be specified by the engineer.
Corrosion of Reinforcement
The reinforcement within concrete, whilst providing the section with its ‘strength’ is also the area most susceptible to attack by chlorides (i.e., salt). Normally the highly alkaline environment of concrete forms a passive protection film on the bar. However, when this film is compromised (through ingress of chlorides from cracking) the protection layer is depleted and corrosion of the steel bars commences.
The consequential corrosion product (rust) is highly expansive and causes residual tensile stresses in the concrete to increase. The resulting pressure causes the concrete to crack and over time this will lead to spalling of the cover, exposing the corroded reinforcing steel bars and accelerating the damage cycle.
Flexural or Shear Failure
In some instances, cracking can be because of failure of the concrete element or structure normally by being subjected to forces greater than can be resisted. In addition, physical damage can have occurred. Cracking of this nature will require structural assessment to determine severity and risk prior to determining remedial actions. Fortunately, these types of cracks are far less common than those discussed above.
What’s the Crack?
If you have concrete structures that are cracked or damaged, it is vital to understand their nature and whether they present a concern in either the immediate, short or long term.
The key is identifying the likely cause of the cracks and whether they are structural or non-structural. The sooner any issues are identified, the sooner remedial action can be taken to avoid costly issues further down the line. The good news is that we can assist with site inspections, retrospective engineering assessments, structural repairs and remedial recommendations!
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