RCC STRUCTURES IN COASTAL AREAS
The entire coastal belt of the Indian peninsula is considered as an extremely corrosive belt as per the corrosion map prepared by the Corrosion Advisory Bureau, Metal Research Committee, Jamshedpur.
Steel and other ferrous metal along the coastline are susceptible to corrosion, which is mainly initiated by chloride ions present in the sea salts. It is further sustained by the oxygen present in the atmosphere, which diffuses through the moisture film to the surface of the metal.
Corrosion of reinforcing steel is one of the most important causes of deterioration of concrete structures in coastal environment. High permeability concrete, poor design detailing and construction defects such as inadequate depth of cover allow the ingress of salt and moisture into the concrete. High concentration of salt and moisture result in accelerated corrosion of reinforcing steel thereby significantly deteriorating the concrete structure.
Concrete structures in coastal areas can be divided into two categories based on their exposure: direct and indirect. Direct exposure includes structures that are fully or partially submerged and indirect exposure includes structures that do not come into direct contact with seawater. Buildings along the coastline are examples of indirect category of exposure.
Concrete structures in coastal areas can be distinguished vis-à-vis corrosion: The submerged zone (under seawater), the splash and tide zone (intermittently wet and dry) and the atmospheric zone (well above the high tide level). Each of the above zones has very different corrosion characteristics. Corrosion below the water level is limited by low oxygen availability and conversely lowers chloride and moisture content limit the corrosion rate above high tide level. Corrosion is most severe within the splash and tidal zones where alternate wetting and drying result in high chloride and oxygen content. Atmospherically exposed structures (buildings) are subjected to corrosion from airborne salts and moisture from atmosphere. The quality of the concrete and the depth of cover play a major role in the ingress of chloride
Corrosion is commonly associated with deposition of sea salt in presence of moisture on steel and light metals. Chloride is the most significant corrosive species in the salt particles. Corrosion is influenced by the amount of salt on the metal surface and related to speed and direction of wind, distance of structure from the shore, elevation of the structures, degree of sheltering and frequency and amount of rain washing. Other factors that influence corrosion include time of wetness, relative humidity, metal surface temperature etc. Normal steel has poor resistance in coastal atmosphere and hence requires protection for durability.
To prevent the penetration of chloride ions a dense, impermeable concrete needs to be produced. This can be achieved by using Ground Granulated Blastfurnace Slag (GGBS). When GGBS cement hydrates, dicalcium silicate and tricalcium silicate are formed. However, the reaction also produces other secondary hydrates, which effectively fill the cement paste pores. This is because there is an excess of reactive alumina and silica in the material. Both these compounds are free to react with the excess calcium hydroxide, which is present in the pores of cement paste to form further calcium silicate and calcium aluminate hydrates. These block the pores and reduce the permeability of the cement paste. In addition, GGBS cements are able to bind the penetrating chlorides into chloro-aluminates far better than cement pastes containing pure Ordinary Portland Cement or Sulphate Resistant Portland Cement. This reduces the danger of chloride attack still further. Sulphate Resistant Portland Cement has a reduced binding ability because of the low tricalcium aluminate content. GGBS cement has a slower rate of hydration than Ordinary Portland Cement helps in reducing the permeability of concrete as cracking due to temperature rise is minimised.
I.S. 456:2000 recommends use of slag cement and puts a note as follows: “where chloride is encountered along with sulphates in soil or groundwater, Ordinary Portland Cement with C3A content from 5 to 8 per cent shall be desirable to be used in concrete, instead of sulphate resisting cement. Alternatively, Portland slag cement conforming to IS 455 having more than 50 per cent slag or a blend of Ordinary Portland Cement and slag cement may be used provided sufficient information is available on performance of such blended cements in these conditions.”
To achieve low permeability, concrete must be dense with a good bond between aggregate and cement paste. This can be achieved by using good standard quality materials required to make quality concrete with adequate cement content, a low water cement ratio and small sized well-graded aggregates. Admixtures containing chlorides should not be used as they promote corrosion of reinforcement.
Depending upon the environmental exposure conditions, the requirements of I.S. 456:2000 for Minimum Cement Content, Maximum Water-Cement Ratio and Minimum Grade of Concrete with nominal weight aggregates of 20mm nominal maximum size and minimum concrete cover for durability and fire resistance should be complied with.
Proper compaction of concrete is of vial importance in minimising permeability. Problems may arise when placing and vibrating techniques are incorrect, slump is too low, reinforcement is congested or form shapes are not conducive to the necessary flow of concrete during placement. Proper and sufficient curing of concrete is essential to achieve low permeability as the continued hydration of cement increases the volume of the gel and hence decreases pore spaces and blocks capillaries.
Apart from carrying out the RCC construction as per the recommended standard practices using Ordinary Portland Cement, one must be sure of taking care of the following requirements before using Pozzolona Cement (GGBS).
- Design the mix for target strength after testing the cement.
- Control the water-cement ratio as low as possible by making use of plasticizers.
- Assure profuse curing for longer time, say, 14 days.
- Wait for centering removal till the concrete attains twice the strength required to resist the stresses that will be produced in the concrete while removing the centering. This is normally 1.5 times the usual centering removal period for OPC. However, the same can be determined from cube test results.
To achieve longevity of the structure, the reinforcement also needs to be protected against corrosion.
Apart from looking at the quality of the concrete for producing long lasting RCC structures in coastal areas, it is also necessary to study the corrosion potential of soils which can be judged from the following: soil chemistry, pH value of soil, soil mineral composition, effect of ground water, subsoil temperature, microbial activity in soil oxidizing or reducing capacity of soil, Ryzner Index and results of marble tests.
The effect of corrosion potential of the soil is to either attack concrete or make it weak or to corrode the reinforcement in the structural members.
It is equally important to protect the steel reinforcement against corrosion, as it inevitably weakens concrete members, reduces load carrying capacity and safety factors. In extreme cases failure of reinforced concrete members can occur partly because of loss of strength due to reinforcement itself and partly because of the breaking up of the concrete surrounding the reinforcement.
When steel reinforcement corrodes, the corrosion product occupies more than three times the volume of the original steel, exerting great disruptive tensile stress on the surrounding concrete, leading to further cracking, more weather access and further corrosion. In mild cases, rust staining occurs whereas in more serious cases severe spalling of concrete may occur and ultimately the concrete members may fail completely.
(By Neelkanth D. Joshi who is with Joshi Consultants, consulting structural engineers.)