to accumulation of laitance. Weak concrete used as a binding layer beneath pile caps is also
vulnerable to sulphate attack when the resulting expansion of the blinding concrete could lift
the cap; hence the quality of blinding concrete should match the structural quality.
Pile caps and ground beams can be protected on the underside by a layer of heavy gauge
polyethylene sheeting (designated APM3) laid on a sand carpet or on blinding concrete.
The vertical sides can be protected after removing the formwork by applying hot bitumen spray
coats, bituminous paint, trowelled-on mastic asphalt, or adhesive plastics sheeting. The recom-
mendation for placing a membrane between floors and fill, or hardcore containing sulphates,
should be considered for the undersides of slender pile capping beams, or shallow pile caps.
Coatings of tar or bitumen on the surface of precast concrete piles do not give adequate
protection against sulphate attack since they are readily stripped off by abrasion as the piles
are driven down in all but the softer soils. Protection can be given to the pile surface by metal
sheathing or glass fibre wrapping impregnated with bitumen, but the latter is likely to be
torn when piles are driven into gravelly or stony soils. If a sacrificial layer of concrete
(APM4) is added to friction piles, consideration must be given to the effects of sulphate and
thaumasite attack causing expansion and reducing frictional resistance.
The use of high-alumina cement (BS915 under revision as prEN 14647) or supersulphated
cement (BS4248) for high sulphate concentrations is referred to in Special Digest 1. The
latter cement is attacked by ammonium sulphate to which high-alumina cement alone is
resistant. Also, there is some experience to indicate that supersulphated cement has less
resistant properties to attack by magnesium sulphate than those of sulphate-resisting cement.
Neither high-alumina cement nor supersulphated cement is favoured for piling work. In
any case, approval of the use in structural concrete of the former type has at present been
withdrawn from codes of practice in Britain and in some other countries. Structural concrete
is deemed to include all concrete in foundations. The withdrawal of approval has been due
to the property of the cement to ‘chemical conversion’(10.6)which results in a serious loss
of strength. While this reduction of strength may not be critical in the case of foundations
subjected to relatively low levels of stress, the conversion is accompanied by a marked
reduction in the sulphate-resisting properties of the cement. Conversion is particularly liable
to take place in warm and damp conditions. These may occur in piles above water level in
marine structures, and in large-diameter bored and cast-in-place piles where the heat of
hydration of the cement is dissipated only slowly. The use of this cement also causes serious
practical difficulties in placing the concrete in pile shafts due to its rapid setting.
Supersulphated cement is costly and difficult or impossible to obtain in many countries
including the UK. It has a low heat of hydration and is therefore rather slow to harden. This
makes it unfavourable for use in precast concrete piles because of the long period required
between casting and driving. Special care is required when using this cement in cold weather.
Table D3 in Special Digest 1 provides for the use of Portland cements incorporating ground
granular blastfurnace slag (ggbs) or pulverized fuel ash (pfa – now designated ‘flyash’in BS
8500) and for a variety of Portland cement–pozzolanic combinations mixed on site to give
enhanced sulphate-resisting properties. Concrete containing ggbs is now recommended in
place of sulphate-resisting cement to combat thaumasite attack in the UK.
The leaching of concrete exposed to flowing river or groundwater containing organic acids
or dissolved carbon dioxide was mentioned at the beginning of this section. Organic acids are
present in run-off water from moorlands, and in groundwater in peaty and lignitic soils. The
recommendations for concrete exposed to acid attack as determined by the pH value of the soil
or groundwater are covered by the ACEC Tables in Special Digest 1. Good quality concrete,
made with any of the tabulated cements and non-degradable aggregates, is essential.
490 The durability of piled foundations