Concrete @ your Fingertips

External paving - BS 8500 requirements

Designated concretes PAV1 and PAV2 are suitable for “House drives and domestic parking” and “Heavy duty external paving” respectively. These designations give the concrete the necessary level of freeze-thaw resistance for conditions experienced in the UK.

For designed concretes subject to freezing and thawing, exposure classes XF3 “Horizontal concrete surfaces, which are exposed to freezing whilst wet” and XF4 “Horizontal concrete surfaces, such as roads and pavements, exposed to freezing and de-icing salts either directly or as spray or as run-off would be applicable. In summary, nominally flat pavements which can be highly saturated without de-icing salts (exposure class XF3) and with de-icing salts (exposure class XF4).

The limiting constituent proportions (cement, w/c ratio, air entrainment) and strength for PAV1 are slightly more onerous than that given for XF3. For PAV2 they are considerably more than that required for XF4.

The reason for this is that BS 8500:2023 Table A.9a Note ^C) recommends the more durable PAV2 for external pavements. Cross referencing this note to Table A.14 Note ^A) it is assumed that PAV2 is used in “heavy duty external paving” where reinforcement is embedded. By inference PAV1 is normally intended for unreinforced applications.

When considering exposure classes for designed concrete it is necessary to review all exposure and in-service effects; in the case of reinforced external pavements, both freeze-thaw resistance (XF) and corrosion induced by chlorides (XD). The limiting constituent materials for the appropriate exposure class XD3 are dependent on depth of cover to reinforcement and cement type.

PAV2 combines exposure classes XF4 with XD3 to give a durable reinforced concrete pavement.

A warning is given in cl 4.4 stating cements or combinations containing higher proportions of additions might not be suitable for the wearing surfaces of pavement concrete in XF4 exposure class due to the possibility of surface scaling in the top few millimetres and therefore Table A.15 Note ^F) advises that PAV 1 or 2 has at least a BS EN 197 Portland cement clinker (K) proportion of 45%.

It is important to note that high levels of curing is important particularly if flood pour technique is employed as well as when medium to high levels of cement replacement is used for concrete subject to freeze-thaw exposure.

Acknowledgement: The Concrete Society