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Concrete for manhole surrounds and kerb backing

Traditionally, where aggressive ground conditions (e.g. sulfates) are present, the concrete for manhole chamber surrounds and kerb backing were specified as a standard mix (now standard prescribed concrete) using Sulfate-resisting Portland cement (SRPC) to BS 4027 as opposed to Portland cement (CEM I) to EN 197.

BS 4027 was withdrawn in 2012, consequently ready-mixed concrete suppliers use a combination cement (fly-ash or ground granulated blastfurnace cement blended or combined with Portland cement) where CEM I is not considered suitable to provide sulfate resistance. Factory produced cements incorporating fly ash or ggbs suitable for sulfate conditions are also available as CEMII/B-V + SR and CEMIII/A + SR, respectively. SRPC is now been reintroduced within EN 197 as CEM I-SR0 and 3 and is one of the many cements and combinations allowed in BS 8500.

In all situations, the ground conditions must be assessed in accordance with BS 8500. This will generate a design chemical class (DC) and a suitable concrete with a minimum cement content, cement or combination cement type and maximum water:cement ratio. At this point a designed concrete or possibly a designated concrete could be specified to meet the necessary performance requirements.

A standardised prescribed concrete (ST) has a minimum cement content, which can be a combination cement although the proportions are unspecified, but no water:cement ratio restriction. Therefore on this basis, a basic ST concrete cannot in theory be used for manhole chamber surrounds where sulfates are present. However if sufficient controls can be placed on the mix proportions (cement quantity and type, and water:cement ratio) to ensure that it will be equivalent to the concrete to satisfy the DC requirements then it may be possible to use a ST concrete for the applications above.

Backing for kerbs is not generally considered to be in a sulfate environment and therefore a ST concrete will generally still be appropriate.

Acknowledgement: The Concrete Society