Concluding Part
But as they are used with water, they greatly decrease in strength in the long run. The sub-florescence (crystallization under the surface) is one of the most dangerous forms of crystallization, which causes weakening of the materials within and a material may seem as if it is damaged, but no signs are visible on the surface. Sodium chloride (NaCl) and sodium sulfate (Na₂SO₄) are the most common damaging salts found in brick masonry. These salts travel through moisture and accumulate inside pores. As they crystallize, they generate pressure strong enough to disrupt the microstructure of bricks and mortar. When moisture repeatedly enters and evaporates from masonry walls, cycles of dissolution and crystallization accelerate worsening and fall off.
The problem of degradation of structure is not only natural but also technological. In coastal areas, there are scarcity of fresh water for daily use. Extra cost and effort are involved for fulfilling the requirement of fresh water for construction. Saline or partially saline water is mostly used in concrete mixing and curing, either deliberately or due to lack of alternatives in coastal areas.
Many local builders use saline pond water (70%) and river sand (60%) for construction and thereby unintentionally accumulating salt into the structure. Brick aggregates in the coastal belt naturally contain salinity and require extensive soaking to reduce salt content. This practice increases the initial salt content of concrete and shortens its service life and sustainability of structures. Besides, the use of low-quality materials, improper mix design, insufficient curing and lack of corrosion-resistant techniques further worsen the condition.
Mitigation strategies: Salinity is a silent but formidable opponent of the concrete structures of the coastline of Bangladesh. Its effects are not easily observable, but after a certain period of time, it undermines safety, functionality and economic worth. To solve this problem, it demands modernization of the construction process, professional staff and good government policies. The strength of the structures and buildings in the coastal areas of Bangladesh also depends on the quality of the construction, materials and water incorporated in the mix and curing.
The site exploration, the design of the foundation based on the soft coastal soil and good construction habits are the concern of the Bangladesh National Building Code (BNBC). It gives advice on the design of wind loads to resist the cyclonic force and advises for corrosion protection. The code emphasizes superior reinforced concrete structures measures. The code suggests a reduced ratio of water to cement, higher concrete cover to reinforcement and the other cementitious materials. Code also suggests increase of fly ash or slag in cement and a tight control of the chloride content of materials.
The concrete structures along the coastline of Bangladesh are growing
faster. The structures are ports, jetties, cyclone shelters, bridges,
embankments, housing, industrial and power structures. Most of these
constructions are spotted with signs of decay within a short period
after being built
The American Association of State Highway and Transportation Officials (AASHTO) recommends that the concrete water must be clean and free of any disastrous salts, acids and organic materials. Ideally, the mixing water pH must be 6 to 8.5 to have safe construction. One of the factors that determines the quality of the mortar is the compressive strength, which can be significantly affected by the conditions on the coast. Although saline water can raise the concrete strength in the early ages to a certain extent, research has found that the overall concrete strength of the saline water concrete over the long-term is lower than that of the freshwater concrete. This implies that there are short-term gains at the cost of long-term sustainability.
Field and laboratory studies have been undertaken in Bangladesh to understand the scope of salinity damage. There were also efforts to carried out to determine the source of construction materials and the knowledge of the people on salt-related deterioration. Experiments in a lab, such as full-immersion weathering tests were conducted to determine the effect of varying salt levels and pH on mortar strength. These experiments were conducted to identify the salinity damage rate exhibited following the construction of the building.
The most harmful salts on compressive strength are found out to be the chloride and sulfate ions. Studies have shown that the replacement of about 25-35 percent Portland cement with fly ash (mass of the total cementitious materials) can enable even a significant improvement in the concrete durability in the marine environment. The reduced water-cement ratio (less than 0.40-0.45) is less likely to be penetrated by ions. The measures to prevent salt effect, also include the use of waterproofing materials to prevent the diffusion of groundwater and washing of the building materials with ion-free water prior to their use. Building in the salty environment should be done with low-permeability concrete, added cementitious material like fly ash or slag, corrosion inhibitors and preferably, the reinforcement should be epoxy-coated or made of stainless steel. Good curing methods and protective finishing can also make sure that the penetration of salt is very minimal.
Way Forward: The concrete structures along the coastline of Bangladesh are growing faster. The structures are ports, jetties, cyclone shelters, bridges, embankments, housing, industrial and power structures. Most of these constructions are spotted with signs of decay within a short period after being built. These decay lead to high maintenance and restoration expenses and cost in terms of revenue of the country. Such losses exert a strain on the investment in infrastructure both in the public and private sectors in a developing country like Bangladesh. Economically, it is better to spend more money during construction for sustainable concrete structures in coastal areas.
The initial costs of construction can be expensive due to the improved construction materials and technologies. It is economically beneficial to save on repair and replacement costs. More importantly, it preserves national resources such as ports, power generation plants and industrial estates that are crucial in the development of the economy. Proper planning and innovation, climate-resilient design, investment in durable structures and intense quality control can help to build sustainable infrastructure in coastal areas of Bangladesh which will offer success and safety of construction industry for future generations.
The writer is serving in Border Guard Bangladesh
