Durability of Building Foundation
A foundation is that part of the structure which is in direct contact with ground. Therefore foundations are subjected to attack by chemicals present in soil, water and by mechanical abrasion and erosion.
The extent of deterioration depends on the concentration of chemicals, the level of and fluctuations in the ground water table or the variation in tidal and river level or on climatic conditions.
The only way to increase the durability of foundation is to protect the foundation material from the above mentioned attacks.
There are 4 types of material commonly used as foundation material, such as
- Timber foundation
- Metal foundation
- Concrete foundation
- Brick work foundation
Let us discuss methods of protecting different foundation material from deterioration.
1. Protection of Timber Foundation
Timber piles are liable to fungal attack and termites, if kept in moist conditions. The protection of timber is best achieved by pressure treatment with coal tar creosote of the copper arsenic type waterborne preservative.
Molluscan and crustacean borers which inhabit saline or brackish water attack on timber piles in marine structures.
Protection can also be given by concrete before jacketing piles in driving, or by gunite mortar after installation.
Concrete can also be used in land foundations either in composite concrete-timber piles, or in deep pile caps down to ground water level.
2. Protection of Metal Foundation
Steel piles can be given impervious coating of bitumen, coal tar pitch, synthetic resins. These coatings are not effective and get partially stripped off. Generally, sufficient cross-sectional area of steel should be provided to allow for wastage over the useful life of the structure while leaving enough steel to kep the working stresses within safe limits. In sever condition; it may be necessary to adopt a system of cathodic protection.
3. Protection of Concrete Foundation
Concrete foundations below ground level are liable to attack by sulphates in the soil or ground water. In sea water because of the inhibiting effect of the chlorides the sulphates do not cause an expansive reaction to normal Portland cement concrete. However, it is good practice to use sulphate resisting cement in reinforced concrete structures immersed in sea water.
High alumina and super sulphate cements give protection in acidic ground conditions where pH values can be as low as 3.5. However, high alumina cement concrete cannot be used where alkalis are present in strong concentrations.
In severe conditions of exposure, a dense concrete mix should be used with water-cement ratio of less than 0.5. If the ratio is in between 0.5 to 0.6, there is a risk of frost attack and above 0.6 the risk becomes progressively greater.
4. Protection of Brick Work Foundation
High absorption bricks are liable to frost disintegration. They can absorb sulphates or other aggressive substances from the soil or from filling in contact with the brick work. In sulphate bearing soils or ground water, the brick work mortar should be a 1:3 (cement:sand) mix made with sulphate resisting cement or in severe conditions with super-sulphated cement.