Salt damage and corrosion of reinforced concrete
Bangladesh is going through rapid infrastructural development. We often hear about a 100-year design life of megastructures, yet significant structural repairs are required even before the structure is 20 years old. Annually, considerable expenditure goes to only on repairing existing structures, much of which could be saved if they had been built differently.
Primarily, we are concerned about the strength of the structure and tend to ignore its durability. If we look forward to making sustainable cities in Bangladesh, we need to address the issue of durability and make our structures perform.
The structures in coastal areas of our country are subjected to various types of hazards such as, torrential rainfall, high wind speed, extreme temperatures, cyclone, flood, coastal inundation and salinity. In order to make climate-resilient structures, it is essential that we keep this geographical diversity of Bangladesh in mind.
“In reinforced concrete, all common forms of serious deterioration due to salt damage are as a result of water ingress. Therefore, if the concrete could be kept inherently dry, most corrosion issues would disappear.”
In present times, when construction is on the boom in coastal areas of the country, ensuring the durability of structures should be a significant priority. Saline water is hostile to ordinary concrete structures, particularly to the steel inside, making it highly susceptible to corrosion. Unfortunately, in our country, the corrosion factor remains neglected in all infrastructural development projects.
In reinforced concrete, all common forms of severe deterioration due to salt damage are as a result of water ingress. Therefore, if the concrete could be kept inherently dry, most corrosion issues would dissipate. The externally visible impact of rising damp and salt attack, known as efflorescence, can be seen on the external surface of walls. It causes blistering of paintwork and damage to the wall plaster.
Internal deterioration in concrete structures is due to the combination of various factors. Permeable concrete in the foundation allows salt-water to enter its pore spaces and cause physical damage to it. The periodic wetting and drying of concrete foundations may cause rising of damp where the groundwater is drawn into the brick, stone or cement by capillary action.
Corrosion of steel is the critical cause of deterioration in concrete. When steel corrodes coming in contact with this moisture, the resulting rust occupies a higher volume than the steel. This expansion eventually causes cracking, delamination, and spalling of the concrete material. Once the steel-concrete debonding occurs in critical parts of the structure, it leads to an ultimate capacity reduction.
Mongla Port Municipality gives an alarming picture of corrosion of reinforced concrete. Several concrete structures show clear signs of salt damp. The discarded labour hospital, not even 20 years old, has exposed steel on the structural members. Despite the risk of collapse, it is still in function. Damp patches of efflorescence disfigure the interior walls of most of the houses and government offices.
The port city is undergoing massive development in the present days. This issue needs to be addressed for sustainable planning for the city. Any infrastructural developments must address the corrosion factor for the durability of the structure. Megaprojects like Rampal Power Plant should take adequate corrosion-resistant measures for ensuring long-term durability.
Most often, the use of unsuitable or salt contaminated raw materials like water, aggregate, and cement has been a significant cause behind this problem. Clean water is a vital ingredient of concrete as it actively takes part in the chemical reaction with cement. Lack of freshwater is a significant drawback for construction in Saline prone regions like Mongla.
Typically, locally available saline water is used in concrete mixing, which increases the risk of corrosion in steel. Brick chips are widely used for concrete production in place of stone aggregate in developing areas due to its high availability and low cost. The resulting structures have a lower strength, high water absorption, and high permeability characteristics compared to standard concrete—making it more susceptible to salt attack.
Bangladesh National Building Code (BNBC) refers to various aspects of construction in coastal areas but has a little give away about corrosion of structures in these areas. Therefore, broadening the scope of the building codes, specifically the zone-specific construction codes by incorporating standards to address the corrosion issues and ensure longevity.
It is essential that we look ahead into long term planning. From the beginning of the construction process, measures should be taken to ensure the durability of concrete in a hostile saline environment. Even though concrete structures cannot be prevented entirely from deteriorating, adequate measures can significantly extend the life of such structures.
Sumaiya Binte Anwar is a Research Officer at International Centre for Climate Change and Development.