Part- 01
Bangladesh’s vast coastline extending over 710 kilometers and covers about one-third of the nation’s total land area, is a blessing as well as challenge for concrete structures.Coastline supports trade, fisheries, ports and livelihoods for millions of populations. Coastal area includes districts like Khulna,Satkhira, Bagerhat, Patuakhali, Barisal, Chattogram, Cox's Bazar etc. These areas are characterized by rivers,mangrove forests, estuaries, tidal plainsand numerous islands locally known as Chars.
Saline water in coastal areas have adverse effect on agriculture and livelihood of people. Saline water intrusion in coastal areas is generated by a complex interaction between freshwater flow coming from upstream river systems and saltwater flow coming from tidal force of the ocean. Besides, sea-level rise, tidal surges and cyclonic flooding (Sidr, Aila) intensify salt intrusion into both groundwater and soil.Several other parameters like upstream river discharge, salinity magnitude, tidal excursion, residence time etc. playdynamic role in determining the intrusion process of salinity.Coastline exposes the country’s buildings andstructuresto most violent enemies that is salinity.
In recent years, engineers and researchers have raised serious concerns about how saline environments are rapidly deteriorating concrete structures in coastal regions, causing economic losses and threatening long-term sustainability of structures.The coastal areas of Bangladesh are facing growing pressure in maintaining durable and sustainable infrastructure. Sea coasts expose buildings to a harsh maritime environment. In these regions,especially in districts like Khulna, Bagerhat and Satkhira, structures habitually suffer from peeling plaster, cracked concrete, corroded steel reinforcement and the gradual breakdown of building materials. A major reason behind these problems is salinity intrusion from the Bay of Bengal. The impact of saltwater is observedalmost everywhere in coastal areas. The effect of climate change has also impacted the salinity of land and water, as well as resulted in a dire threat to the construction industry.
The intrusion of the salinity of the structures and buildings occurs primarily under the influence of seawater infiltration into the building materials as well as because of the activities of tides, storm surges and the gradual rise of the sea level. The groundwater content of the coastal region in most cases is high in salts, particularly chlorides and sulfates. The salts are readily accessible to the concrete via the pores and cracks. They get access to the reinforcing steel bars and cause corrosion. This is gradual but constant and in course of time it deteriorates the structure internally and without any obvious external evidence.The structure loses its load-bearing capacity through cracking, spalling and loss of its load-bearing capacity.
In Bangladesh, where the coastline is a crucial economic and social factor, overcoming the issue of salinity is the major step towards the sustainability of infrastructure and its stability in the long term.Developing and adopting improved construction techniques and selecting appropriate materials can help to protect buildings and structuresfrom premature decay from salinity.
How salinity weakens concrete structures: Concrete, by nature is a porous material. In normal conditions,porosity is manageable. However, in a marine or saline environment, it becomes a trail for harmful ions. Chloride ions are particularlyhazardous as they break down the protective oxide layer on steel reinforcement. Once corrosion starts, the steel expands, creating internal pressure in the concrete. This leads to cracks and consequently causes the concrete cover to fall off, exposing more steel and accelerating corrosion.Sulfates, on the other hand, react with cement compounds to form ettringite, a mineral that expands in volume and creates internal stresses, leading to cracking and weakening of the hardened cement paste.
Saline water contains various dissolved salts that chemically react with cement hydration products in concrete and mortar. The salt can reach the pores of such materials and crystallize, developing internal pressure that, in turn, leads to cracks and damage on the surface. Salty water has an average of 35 grams of dissolved salts per liter. The saltsdiffer in composition with regard to the location. Chloride and sulfate ions are very high in seawater compared to river water and are very aggressive to cementitious materials. The magnesium compounds, e.g. brucite, can be leached in very alkaline solutions in the pores of the cement and this further weakens the internal structure of the material.
Moisture also leads to structural decay. Wind-induced rain, water vapor and tidal flooding, which render them more permeable, increase the likelihood of corrosion, efflorescence and mould growth of structures. Some researchers implied that it was only salts that might have a slight increase in the initial strength of bricks.
(To be continued)
The writer is serving in Border Guard Bangladesh
