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What is concrete spalling?
Spalling can be defined as “breaking down materials into smaller pieces, such as chips and fragments breaking away from rocks/ores”. Concrete spalling refers to cracking, flaking, peeling and chipping on concrete surfaces, which starts to peel and pop out, and in excessive cases, resulting in exposed steel reinforcement.
Concrete spalling can effect a variety of structures such as bridges, car parks, colleges and hospitals. This can cause problems with the durability of concrete over the long-term design, in terms of exposure and corrosion to steel reinforcement. This may lead to increased costs of the maintenance of these structures through patch repairs over their lifetime or replacement of concrete elements.
What Causes Concrete Spalling?
The common causes of spalling is the presence of moisture, salts, heat and mechanical expansion within the concrete leading to cracking, peeling and chipping etc.
Concrete is a porous material and when moisture/de-icing salt is present on the surface of the concrete, this can seep into the concrete and corrosion of the steel reinforcement occurs (in the presence of water and air).
A chemical reaction between iron, water and air occurs, and iron oxide (rust) is formed. The rust has a greater volume than the original bar dimensions and this increase in volume leads to a build up of internal pressures causing spalling and delamination/cracking.
Common building elements that are at risk of spalling are basements, due to the surface being in contact with potentially saturated soil and damp/in-adequate waterproofing. This can be identified through striated lines, discolouring and coarse textures where the concrete surface can be easily removed.
Factors affecting concrete spall are the following:
Porosity of concrete
Temperature and temperature change
Expansion and contraction of the freeze/thaw cycle
Chemicals such as de-icers
The preparation of the concrete mix and location of the concrete (coastal or countryside) has an impact on spalling. Air entrainment additives should be used, which allows space in concrete for the expansion and contraction of water and ice. However, air entrainment and high water content decreases the concrete compressive strength, and this should be considered when choosing concrete mixes.
In coastal areas, the presence of salt in the air can increase spalling and corrosion of concrete. The concrete surface can become inundated with airborne salt, which can then permeate through the concrete pores and cause chloride attack to the concrete.
The design of concrete mixes in hostile environments or exposed to rainfall/de-icing salts should be designed with adequate cover to the reinforcement, reducing the amount of water/chloride passes through. Values for concrete cover for difference types of environment and conditions can be found in BS 8500, which tabulates the exposure class of concrete.
Effects of Spalling
Excessive spalling and delamination results in the steel reinforcement to be further exposed to the atmospheric element, where it will corrode even further. This will lead to a reduction of strength as the rebar continues to deteriorate. This leads to a weakened concrete that may not be adequate under loads in bridges, structure etc.
If spalling and continuous corrosion takes place without any monitoring or checks, this can potentially lead to failure of concrete elements. Regular monitoring of concrete and structures of important significance (i.e, hospitals, bridges) is required and remediation should be undertaken if required.