CBGM base in flexible and composite roads

S6.3.2

All composite roads constructed with a base of CBGM, lean-mix concrete or equivalent must be reinstated with a CBGM base.

S6.3.3

CBGM base must either be C16/20 (to Table A14 in BS 8500-1) or comply with the following:

the aggregate requirements for MCHW clause 822
particle size distribution of the mixture in accordance with BS EN 14227-1 CBGM 1 (0/20 or 0/14),
R_Rc must be ≥80 as per A10.2.14 and
the following strength requirements which are dependent on coarse aggregate type:
1. crushed rock aggregate: C8/10;
2. other sources of coarse aggregate (including gravel): C12/16

S6.3.4

Where a CBGM base is used, the reinstatement must:

be designed to be suitable for immediate overlay (see requirements in A10.2) for immediate overlay; or
have achieved a compressive strength of C3/4 (curing of samples used to determine the minimum compressive strength suitable for overlay must be representative of the site conditions) before overlay; or
be trafficked and overlaid depending on the road type, as follows:
1. in Types 0 and 1 roads, the CBGM base may either be surfaced on the same day or allowed 7 days curing. In either case, the CBGM base must be allowed 7 days curing before trafficking.
2. in Types 2, 3 and 4 roads, the CBGM base may either be surfaced on the same day or allowed 3 days curing. If the surface is placed in the same day, the road may be opened to traffic the following day.

S6.3.5

In composite roads, the reinstated CBGM base must be laid flush with the top of the existing CBGM, lean-mix concrete or equivalent.

S6.3.6

Continuously reinforced concrete bases that have been connected with dowel bars are not covered in this Code. Special conditions will apply and the reinstatement method must be agreed with the authority.