Pile Design and Construction Practice, Fifth edition

The durability of piled foundations 489
continue into permeable soil deposits which are not naturally sulphate-bearing. An account
of the distribution of sulphates in various ground conditions in Great Britain is given by
Bessey and Lea (10.8) . Methods of analysis to determine the sulphate content and pH-value of
soils and ground waters are set out in BS 1377 and by Bowley (10.9) in BRE Report 279 and
are critically reviewed by Eglinton (10.6) .
A dense, well-compacted concrete provides the best protection against the attack by
sulphates on concrete piles, pile caps and ground beams. The low permeability of dense
concrete prevents or greatly restricts the entry of the sulphates into the pore spaces of
the concrete. For this reason high-strength precast concrete piles are the most favourable
type to use. However, for the reasons explained in Chapter 2, precast concrete piles are not
suitable for all site conditions and the mixes used for the alternatives of bored and castin-place
or driven and cast-in-place piles must be designed to achieve the required degree of
impermeability and resistance to aggressive action.
In British practice recommendations for the types of cement and the mix proportions are
given in BRE Special Digest 1: 2005 (2.6) . There are several significant changes compared
with the previous BRE recommendations, mainly designed to harmonize with the new
British and European standards and to take account of research into combating the thaumasite
form of sulphate attack on concrete. The five classes of severity of attack (‘Design Sulphate’
classes DS1 to 5) have been retained from which are derived the new ‘Aggressive Chemical
Environment for Concrete’ (ACEC) classes (AC1 to 5) for natural ground and brownfield
sites, subject to certain conditions (e.g. pH should be greater than 2.5). The AC classes
provide for adjustment from one DS class to another depending on the conditions of
exposure, the pH and mobility of groundwater, and other environmental conditions. For a
given AC class a ‘Design Chemical’ (DC) class is derived for the intended working life,
either 50 or 100 years, together with recommended ‘additional protective measures’ specific
to highly aggressive ground types. Concrete mixes are then tabulated to suit the DC class
giving a wide selection of free-water/cement ratios and aggregate sizes down to 10 mm and
the appropriate cement and cement combinations in accordance with BS EN 197-1 and
BS 8500-2. The use of sulphate-resisting Portland cement is covered in BS 4027.
The workability of the BRE suggested cast in-situ concrete mixes may in some cases be
too low for placing for bored and driven small diameter cast-in-place piles. Slightly modified
mixes are given for certain precast products, including surface-carbonated precast concrete,
which would be suitable for precast piles. As it is not possible to cover in this text the
various comments and qualifications to the recommendations given in Special Digest 1, it is
important to follow the step by step approach to determine the appropriate concrete quality
for a particular assessment of ground conditions. It should be noted that the Digest does not
purport to assess and advise on the use of sand–cement grouts in minipiles and around
the permanent sleeves to piles.
Mixes suitable for concrete in pile caps, ground beams and blinding concrete depend on
the size shape and amount of reinforcement of the members which govern the workability
requirements. Footnotes to the Special Digest 1, Table D1, provide for modifications to the
DC class depending on the size of a structural member.
Generally, no additional protection measures (APMs) are necessary where the groundwater
is considered ‘static’, but other conditions may over-ride this (e.g. thickness of
concrete section). When in doubt the ‘mobile’ groundwater condition should be used. For
example, it would be unwise to assume a static groundwater table at a shallow depth for castin-place
concrete piles where the concrete may be weaker than in the body of the pile due