On a site where there was severe contamination with acid industrial waste the authors
advised a protective scheme for the foundation piling consisting of precast concrete shell piles
coated externally with bitumen over the portion of the shaft within the fill. As a safeguard
against partial stripping of the bitumen by bricks and concrete in the fill, the concrete shells
were regarded as sacrificial. The main load-bearing element consisted of a PVC sleeve (weight
800 gm/m^3 ) lowered down the shells after completion of driving onto a concrete plug in
the lower part of the pile. The PVC sleeve was then filled with concrete. A flexible PVC
membrane (Bituthene sheeting) was provided beneath the pile caps. This was lapped and
bonded to the PVC pile sleeve.
10.3.2 Concrete piles in marine structures
Precautions against the aggressive action by seawater 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 piles, where the tube or shell acts as the protective
element. A rich concrete, well-compacted to form a dense impermeable mass, is highly
resistant to aggressive action and, provided a cover of at least 50 mm is given to all
reinforcing steel, precast concrete piles should have satisfactory durability over the normal
service life of the structures they support.
When the disintegration of reinforced concrete in seawater does occur it is usually most
severe in the ‘splash zone’and is the result of porous or cracked concrete caused by faulty
design or poor construction. Evaporation of the seawater in the porous or cracked zone is
followed by the crystallization of the salts and the resulting expansive action causes spalling
of the concrete and the consequent exposure of the reinforcing steel to corrosion by air and
water. The expansive reaction that occurs when corrosion products are formed on the steel
accelerates the disintegration of the concrete. Freezing of seawater in porous or cracked
concrete can cause similar spalling. However, where concrete piles are wholly immersed in
seawater there is no degradation of properly made and well-compacted concrete.
In an extensive review of literature and the inspection of structures which had been in the
sea for 70 years, Browne and Domone(10.10)found no disintegration in permanently immersed
reinforced concrete structures even though severe damage had occurred in the splash zone.
They concluded that corrosion of the steel cannot occur with permanent immersion because
the chloride present is restricted to a uniform low level and the availability of oxygen is low.
Although seawater typically has a sulphate content of about 230 parts per 100 000, the
presence of sodium chloride has an inhibiting or retarding effect on the expansion caused
by its reaction with ordinary Portland cement. The latter material is, therefore, quite
satisfactory for the manufacture of precast concrete piles for marine conditions but to avoid
disintegration in the splash zone the concrete should have a minimum cement content of
360 kg/m^3 and a maximum water/cement ratio of 0.45 by weight. Special Digest 1 does not
provide recommendations for concrete exposed to seawater, but reference should be made to
BS6 349-1: 2 000 Marine Structures. Air entrainment of concrete as a safeguard against frost
attack on piles above the water line is unnecessary if the water/cement ratio is less than 0.45.
The concrete in precast piles should be moist-cured for 7 days after the removal of the form-
work (with a further 10 days exposure to air in order to be classified as ‘surface carbonated’).
Great care should be taken in handling the piles to avoid the formation of transverse cracks
which would expose the steel to corrosion in the splash zone. Coatings on precast concrete
piles to protect them against deterioration in the splash zone are of little value since they are
soon removed by the erosive action of waves, and by abrasion from floating debris or ice.
The durability of piled foundations 491