Concrete is a widely used construction material all over the world. Buildings, bridges, tunnels and all of our infrastructures are mostly made of concrete. We may plan a structure for a 50 to 100-year service life. But for many reasons, concrete structures may deteriorate before their expected service life comes to an end. Deterioration of structures may lead to casualties and may also damage equipment, vehicles, and other properties. For example, a concrete chunk may fall on someone when he/she is sleeping in bed, working in office, sitting under the roof of a school or walking under a bridge. It may also fall on vehicles when they pass under a bridge, tunnel, etc. Reports on such accidents can be found regularly in our daily newspapers. For example, two teachers and four students were injured as chunks of ceiling plaster fell on them in a classroom of a government primary school in Brahmanbaria’s Nabinagar upazila on April 11, 2019. We need to understand the reasons behind this problem and take countermeasures to avoid such casualties.
Concrete is said to be the heaven for steel because steel in concrete is protected against corrosion due to the high alkaline nature (pH of which is more than 13) of concrete. But, atmospheric carbon dioxide causes reduction of alkalinity of concrete and therefore steel bars in concrete start to corrode. Corrosion of steel bars can also start due to chlorides in the marine environment. When steel transforms into rust, it increases in volume up to 6 to 8 times. For example, if 1cm3 of iron is transformed into rust, its volume will increase to 6-8cm3. Due to the increase in volume, cracks will form over the concrete surface. With the formation of more rust, the crack will gradually become wider and finally lead to falling of concrete. The weight of a small chunk of concrete, such as 5 inch by 5 inch by 1 inch will be about 1kg. If it falls from a height of 10ft on our body, it can cause significant casualties. It can also damage our industrial instruments, vehicles, and other properties. Considering health and safety reasons, we need to take this issue very seriously.
When we observe cracks on the surface of concrete, the reasons behind this observation should be investigated. If cracks formed after several years of construction and cracks run along the reinforcement, these symptoms indicate that the cracks are formed due to the formation of rust over the steel bars. If we hear a dull sound after striking the surface of concrete by a hammer, it indicates formation of a significant amount of rust. It also indicates the possibility of falling of concrete chunks at any moment. The load carrying capacity of a structure may also be reduced. In such a situation, it is necessary to consult with an expert for necessary steps regarding repair or retrofitting of the structure.
Dr Md Tarek Uddin, PEng. is Professor, Department of Civil and Environmental Engineering, Islamic University of Technology (IUT).