Achieving durability for concrete structures:
The durable concrete structures are to be conceptualized, specified, designed, contracted, constructed and maintained. To achieve the design life of the structure at reasonable maintenance cost, the following aspects are required to be taken care of:
i) The structure should also be designed for environment loads and detailing done.
ii) Selection of proper construction materials.
iii) The concrete should be impermeable, with designed crack width due to shrinkage,thermal forces or loads.
iv) The good construction practices should be followed.
v) Selection and application of correct coatings to reinforce bars and concrete surface.
vi) Planned maintenance of concrete structures should be followed.
vii) Proper specifications for construction materials and structural components should be prepared.
Selection of durable construction materials:
Cementitious material: The cementitious material should be properly selected depending upon nature of work, prevailing environment etc. The terminology cement, cementitious material, blended cements, cement + mineral admixtures have been used and can be used to mean one and the same thing.
Aggregates: Aggregates should confirm to specification as laid down in IS:383. The coarse aggregate should be well graded and free from Alkali reactivity. The sand should be free from silt, salt, moisture and gravel and its fineness modulus should be between 2.6 and 2.9. The sand dredged from sea bed may contain large amount of chlorides and other impurities. Thus, it is preferable to avoid use of sand dredged from sea bed. If at all it is used, it should be used only after washing and drying. Washed sand should be tested for salt content, silt, moisture etc. and conform to IS:383. The water used for washing of sand also needs to be tested, prior to its use. The designer should take account of grading after washing of sand.
Water: Water used for mixing and curing shall be clean and free from injurious amount of oils, acids, alkalis, salt, sugar, organic materials or other substances that may be deleterious to concrete or steel. Potable water is generally considered satisfactory for mixing concrete. However, it should be tested for quantities of solids, which should not exceed value as laid down in IS: 456 “Code of Practice for Plain and Reinforced Concrete”.
Admixtures: Chloride free admixtures like super plasticizers etc. may be used to improve the quality of fresh concrete to achieve desired workability at low water cement ratio. Admixtures containing chlorides may increase total chloride content in concrete, making it corrosion prone. Thus chloride free admixtures only should be used.
Steel Reinforcement: Steel should confirm to IS 432/1786 for RCC work and IS 1785/6003/6006/14268 for PSC work. Increase in Carbon content in steel increases corrosion proneness of steel. Therefore, it is necessary that the steel supply is taken only from the reputed firms. The test certificate are necessary for each lot. Independent test checks from each lot is essential to ensure quality as per relevant IS Standard. It is harmful to use Corroded / Pitted steel should never be used.