Durability of Concrete Piles
Properly mixed concrete compacted to a dense impermeable mass is one of the most permanent of all constructional material and give little cause of concern about its long-term durability in a non aggressive environment. However, concrete can be attacked by sulphate and sulfuric acid occurring naturally in soils, by corrosive chemicals which may be present in industrial waste in fill materials and by organic acids and carbon dioxide present in ground water as a result of decaying vegetable matters. Attack by sulphates is a disruptive process whereas the action of organic acids or dissolved carbon dioxide is one of leaching. Attack by sulphuric acid combines features of both processes. The severity of attack by soluble sulphates must be assessed by determining the soluble sulphate content and the proportions of the various cat ions present in an aqueous extract of the soil. These determinations must be made in all cases where the concentration of sulphate in a soil sample exceeds 0.5%.
A dense, well compacted concrete provides the best protection against the attack by sulphates on concrete piles, pile cap and ground beams. The low permeability of dense concrete prevents or greatly restricts the entry of the sulphates in to the pore spaces of the concrete. For this reason high strength precast concrete piles are most favorable type to use. However they are not suitable for all the site conditions and bored cast in situ / driven cast in situ piles if adopted must be designed to achieve the required degree of impermeability and resistance to aggressive action. Neither high alumina cement nor super sulphated cement is favored for piling work. Instead, reliance is placed on the resistance of dense impermeable concrete made with a low water cement ratio. Coating of tar or bitumen on the surface, metal sheeting or glass fibre wrapping impregnated with bitumen may be adopted.
Pile caps and ground beams can be protected on the underside by a layer of heavy gauge polythene sheeting laid on a sand carpet or on blinding concrete. The vertical sides can be protected after removing the form work by applying hot bitumen spray coats, bituminous paint, trowelled on mastic asphalt or adhesive plastic sheeting.
Precautions against the aggressive action by sea water on concrete need only be considered in respect of precast concrete piles. Cast in situ concrete is used only as a hearting to steel tubes or cylindrical precast concrete shell pills. For precast concrete piles for marine condition, a minimum ordinary portland cement content of 360 kg/m3 and a maximum water cement ratio of 0.45 by weight should be adopted.