Pile Design and Construction Practice, Fifth edition

The durability of piled foundations 493
of 0.0l mm per year. No precautions are required for such low rates of loss of thickness.
Where piles in land structures are extended above ground, mild steel thickness losses of
0.2 mm per year were measured over a 10-year period in a marine environment. Morley
considered that a more usual figure for the UK would be less than 0.1 mm per year. For steel
bearing piles in natural soils, BS 8002 and BS 8004 advised a maximum corrosion allowance
of 0.015 mm per year per side where no other corrosion protection is required; this is
consistent with corrosion rates derived from Eurocode EC3-5 (Piling). The long-term corrosion
rate of piles in normal atmospheres in urban conditions given in EC3–5 is 0.01 mm per
year per side and for coastal areas 0.02 mm per side per year. In areas where localized conditions
give rise to more aggressive microclimates the greater allowances in BS 8004 may be
needed. Paint treatment (10.14) would be a suitable precautionary measure for the exposed steel
provided that it is accessible for maintenance. If the aesthetic appearance of the steel is important,
Arcelor (10.15) suggest application of coating systems using zinc silicate epoxy primer and
aliphatic polyurethane topcoat. Where the water table is shallow the pile cap can be extended
down to a depth of 0.6 m below water level to protect the steel of the piles.
Morley (I0.13) reported a corrosion rate of 0.05 mm per year for steel piling immersed in
fresh water except at the waterline in canals where the rate was as high as 0.34 mm per year.
This locally higher corrosion zone may be due to abrasion by floating debris or to cell action
between parts of the structure in different conditions of oxygen availability. The pH range
of fresh water has little effect on corrosion, but to reflect the variability due to potential
pollution, the corrosion rate allowances derived from EC3-5 are approximately 0.02 to
0.05 mm per year per side. Corus suggest (2.4) that glass flake epoxy coating with nominal
dry film thickness of 400 �m be used for piers and jetties to extend the time to the first
maintenance period to beyond 20 years. An alternative for shorter maintenance periods, in
both immersed and atmospheric exposures (10.15) , is a polyamine-cured epoxy with dry film
thickness of 300 �m. The coatings must be applied over blast-cleaned steel. Isocynate-cured
pitch epoxy and cheap coal tar coatings are no longer recommended and are being phased
out for health and safety reasons.
Paint coatings are not generally satisfactory for protection against bacterial corrosion.
Any pinholes in the coating or areas removed by abrasion serve as points of attack by the
organisms. Cathodic protection (see Section 10.4.2) is effective but higher current densities
are required than those needed to combat normal corrosion in aerobic conditions.
Where steel piles are buried in fill or disturbed natural soil, the thickness of metal in a
bearing pile should be such that the steel section will not be overstressed due to wastage
of the metal by corrosion over the period of useful life of the structure. Taking a figure of
0.08 mm per year as a maximum in the range established by the US Bureau of Standards for
disturbed ground, a steel H-pile with web and flange thicknesses of 15.5 mm exposed to the
soil on both sides will lose 50% of its thickness in a period of 48 years, although there may
be localized areas of deeper pitting. Long-term corrosion allowances for service periods up
to 100 years provided in EC3-5 for non-aggressive and aggressive non-compacted fills are
approximately 0.02 mm per year per side and 0.06 mm per year per side respectively.
In compacted fills these figures may be halved. Marsh and Chao (10.16) have refined the
contamination guidelines so that more accurate long-term corrosion allowances can be made.
Protection coating of piles in severely contaminated ground should resist abrasion, impact,
and acidic attack using, for example, (10.15) a polyamide-cured epoxy system with increased
chemical resistance and a nominal dry film thickness of 480 �m onto blast-cleaned
surfaces. Protection should extend to around 0.6 m below water table.