There are cracks in my concrete slab. Should I be concerned?
When you see a crack in a concrete slab or wall, a natural reaction is to be a little concerned. You may wonder whether this could be a structural issue or just an aesthetic concern.
Recently, news channels have shone a spotlight on how dangerous concrete cracks in building structures can be. Reports of cracking concrete slabs and columns have sparked public concern about the structural stability and quality of construction in high-rise buildings.
Not only are people worried about their safety, but they are concerned about the possibility of devaluation of their property investment in addition to the cost of potential extensive repairs.
The good news
The good news is that concrete cracks are very common and most of them do not affect structural stability and can in most cases be repaired, if required.
Cracks in concrete are of common occurrence and they develop when stresses in the concrete exceed its strength. Cracks are often caused by normal shrinkage of the concrete when hardening and drying and can range from being a non-structural and unsightly crack, to being detrimental to the structural integrity and safety of a building.
As a general recommendation, cracks are best to be properly diagnosed and remediated at an early stage to help protect the integrity and longevity of an asset.
The object of any repair is to improve the durability and wear of a surface, reduce permeability and prevent a corrosive environment developing around the reinforcement steel in the concrete.
Identification is critically important
While most cracks do not affect structural stability or durability, identifying the status of cracks is critically important.
The most common types of cracks are organised in the following 7 groups:
1) Plastic Settlement – Position on diagram: A, B and C
Plastic settlement cracks occur on the surface before the concrete has set. These types of cracks are typically mirroring the pattern of the restraining elements.
Plastic settlement cracks are often identified in deep section pours, such as on top of beams and columns. Waffle slabs – where there are changes of depths – are also prone to plastic settlement cracks.
2) Plastic Shrinkage – Position on diagram: D, E and F
Plastic shrinkage cracks may form in a ransom pattern or they can appear in an almost parallel pattern. The cracks are often almost straight, raging from 25 mm to 2 metres but are usually 300 to 600 mm long. They can range from 1 to 3mm wide at the surface and are superficial.
Plastic shrinkage cracks mainly occur in concrete elements with a high surface to volume rate, such as in slabs and pavements.
3) Early Thermal Contraction – Position on diagram: G and H
Early thermal contraction cracks in thick concrete walls can present as vertical cracks spaced about 1,5 to 3 metres apart. The width of the crack is typically wider near the base of the wall to floor joint than it is further up the wall. Early thermal contraction cracks may also present horizontally along the lower section of a thick concrete element such as abutments.
Early thermal contraction cracks are common in cantilever wall often used in reservoirs, dams, concrete tanks, retaining walls, bridge abutments and basements. These cracks are also common in other large pours that exceed 2 metres, such as pavements.
4) Long-term Drying Shrinkage – Position on diagram: I
Long-term drying shrinkage cracks typically run along ineffective joints in thin concrete slabs or walls.
5) Crazing – Position on diagram: J and K
Crazing cracks are characterised by the development of fine random cracks of the concrete caused by shrinkage of the surface layer. The cracks are typically shaped like irregular hexagon patterns not more hand 50 to 100 mm across. They are rarely more than 3 mm deep and are more noticeable on overfloated or steel-trowelled surfaces.
6) Corrosion of Reinforcement – Position on diagram: L and M
The most common location one can find corrosion of reinforcement are in columns and beams with the primary cause being lack of concrete cover to protect the reinforcement and/or poor-quality concrete.
Corroding reinforcement is also commonly found in older precast concrete where excess calcium chloride has been added to the concrete mix to accelerate the hardening of the concrete.
· Flaking, spalling concrete
· Cracks typically run lengthwise directly below or above the reinforcement
· Often associated with porous or shallow concrete cover at corner of columns and beams
· Rust stains leaking out of the concrete
· Bubbling, blistering, plating of concrete render
· Leaks in overhead concrete surfaces
· Rusting and exposed reinforcement pushing concrete to become loose
7) Alkali-aggregate Reaction – Position on diagram: N
Deterioration of concrete structures due to Alkali-aggregate Reaction (AAR) is not prevalent in Australia.
Alkali-aggregate Reaction (AAR) has two forms:
1. Alkali-Carbonate Reaction (ACR) – a very rare occurrence in concrete.
2. Alkali-Silica Reaction (ASR) – not common in Australia, but cracking of concrete bridges in North-Queensland have been identified as being caused by ASR, as well as a dam in Victoria and a bridge in Perth.
Typical areas where ASR occur, are in concrete structures such as piers, bridges, retaining walls, dams, near joint edged in pavements, highway structures, median barriers or columns that are subject to capillary action.
Alkali-Silica Reaction (ASR) in concrete takes place between the highly alkaline cement paste or alkalis present in the environment (sea spray or groundwater) and non-crystalline silicon dioxide. The reaction forms an alkali-silicate gel that swells as it absorbs surrounding moisture. Over time, the reaction-caused expansion gradually leads the concrete to crack. This expansion process takes from 5 to 20 years before cracking may lead to failure of the concrete structure.
It is hoped that this article will provide practical information and act as a general guide to identifying the 7 most common concrete crack types. The aim of the content is to help determine whether a particular crack poses structural issues that would require immediate action, involving comprehensive repairs, or if it is perhaps a non-structural crack that may require maintenance, minor repairs or preventative treatments.
There are different types of defects in concrete that can form for various reasons, both during and after construction. In this article we touch on some common passageways for water ingress in concrete structures before focusing on the identification of the 7 most common types of concrete cracks.
Author: Waterstop Solutions QLD
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