Concrete is prone to spalling when exposed to temperatures between 200 degrees Celsius and 500 degrees Celsius. Spalling can be affected by internal factors such as water-cement ratio, moisture content, aggregate types, and permeability. External factors like heating rate, fire temperature, environmental conditions (temperature and humidity), and load level can also play a role.

Causes of concrete spalling

1. Freeze-thaw cycles

One of the primary causes of concrete spalling is the relentless cycle of freeze and thaw. When water infiltrates the concrete, it occupies space within the material. During freezing, this water expands as it turns to ice, exerting pressure on the concrete. When the ice thaws, the water contracts, leaving behind voids and cracks. Over time, these cracks propagate, causing chunks of concrete to break off. Climate and geographical factors play a significant role in the severity of freeze-thaw damage.

2. Corrosion of reinforcement

Reinforcement bars play a crucial role in enhancing the structural strength of concrete. However, when moisture and oxygen reach the reinforcement, a destructive process known as corrosion begins. As the reinforcement rusts and expands, it exerts pressure on the surrounding concrete, leading to cracks and spalling. This hidden enemy can weaken structures from within, compromising their load-bearing capacity.

3. Alkali-silica reaction (ASR)

The seemingly innocuous interaction between certain aggregates and alkalis in cement can result in a destructive chemical reaction called alkali-silica reaction (ASR). This reaction produces a gel-like substance that swells upon absorbing water, causing concrete to crack and spall. Identifying ASR-induced spalling can be challenging, but it's essential for preventing further deterioration.

4. Chemical exposure

Concrete surfaces are often subjected to various chemicals, ranging from acidic solutions to sulphates. These substances can react with the concrete, weakening its structure and leading to spalling over time. This is particularly evident in industrial environments where chemical exposure is commonplace. Using chemical-resistant materials and implementing proper storage and handling practices can mitigate this cause of spalling.

Effects of concrete spalling

The consequences of concrete spalling are far-reaching and can significantly compromise the overall integrity of a structure. Firstly, the structural stability of the affected concrete is compromised, reducing its load-bearing capacity and potentially jeopardising the safety of occupants. Secondly, the aesthetic appeal of the structure takes a hit, as spalling leads to an unsightly appearance that diminishes the value of the property. Furthermore, spalled areas become more vulnerable to additional deterioration, accelerating the cycle of damage.

Prevention and remediation

1. Quality concrete mix design

The foundation of preventing concrete spalling lies in the mix design. Ensuring the proper ratio of aggregates, cement, water, and additives is crucial. Additionally, using appropriate aggregates and cement types for specific environments can minimise the potential for spalling.

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