Basic piling methods - Piling Engineering - page 93

and hence cracking occurs, leading to spalling and an accelerated rate of attack. In
order to cause corrosion, sulphates must be in solution, and therefore the solubility
of the particular sulphate in question has a considerable bearing on the severity of
attack. Groundwater mobility (and therefore soil permeability) is a relevant factor.
The sulphates commonly encountered are those of calcium, magnesium and sodium.
Various standards are available for assessing the aggressiveness of the ground-water,
based on its sulphate concentration. Commonly used guidance in the United Kingdom
is that contained in BS 8500:2002. This document gives general guidance regarding
the sulphate resistance of buried concrete. The document has been augmented by BRE
Special Digest 1 Concrete in aggresive ground (SD1: 2005). The BRE Digest takes
into account the type of site (i.e. 'Greenfield' or 'Brownfield'; Nixon et al. , 2003) and
whether the groundwater is mobile or static. Analysis for magnesium, chloride, nitrate
and total sulphur together with sulphate and pHwill formpart of the assessment. Dete-
rioration of concrete by thaumasite is a form of particularly damaging sulphate attack.
It is discussed by Crammond and Nixon (1993). Special considerations that may apply
to buried concrete in the Middle East are discussed by Fookes and Collis (1975).
3.7.2.2 Chloride attack
Chlorides are encountered in particular in Middle Eastern soil conditions but could
well also be present in industrial waste. Concrete itself is not usually subject to attack
by chlorides in solution, but at even very low concentrations, chlorides can cause
accelerated corrosion of steel reinforcement with consequential bursting of the con-
crete as the products of corrosion are formed. The corrosion can then progress at an
enhanced rate. Protection of the reinforcement is therefore important, and increasing
the cover of dense concrete is one way of improving matters. Stainless steel, galvanized
or epoxy-coated reinforcement provide alternatives. A further measure is the use of the
protective membranes around the pile shaft. Bartholomew (1979) considers permanent
sleeving of pile shafts is necessary under tropical conditions for chloride concentra-
tions exceeding that of sea water (3% salt by mass). Further detailed discussion of the
corrosion of reinforcement steel in concrete (not specifically in piling applications) is
available in the a publication by the ACI Committee 222, 1996, 'Corrosion of Metals
in Concrete'.
3.7.2.3 Acidic groundwater
This condition, represented by a low-pH value, is most frequently met in peaty soils,
but can also be present because of industrial contamination. In the former situation, the
condition may not be severe. Reinforced concrete piles extracted from peaty Thames
Alluvium at Littlebrook Power Station after approximately 40 years in the ground did
not exhibit any obvious deterioration (author's observations). In the latter situation of
industrial contamination, aggressive conditions can be extreme and localized, perhaps
affecting only a small number of piles. In this case the maximum likely concentra-
tions should be determined, and an assessment made of the distribution and extent
of the contamination which can then be assessed using the criteria in the BRE Special
Digest 1 ( loc. cit. ). External protective measures to a pile shaft may be necessary and
the selection of the pile type may need to be carefully considered.