- Laying and compaction of HBM layers, including any reworking and re-use, whether constructed in one or more lifts, must be carried out without segregation, drying out or before setting.
- The construction period, in degree hours, is the summation of the products of the average air temperature above 3 °C (temperature T in °C) and time for each period (time t in hours): i.e. construction period limit = Σ(T.t). The air temperature during the interval, t, must not fluctuate by more than 4 °C. The construction periods are summarized in Table A10.8 and Table A10.9.
Table A10.8 Construction period for HBM when one binder constituent only or binder constituents added at the same time Binder Construction period (°C·h) Other constituents, combinations, possibilities not listed below in this table Producer to determine and evidence during the mixture design procedure CEMI used as part of the mix 35 Lime with GGBS 200 Lime with SiFA (SiFA both as a binder constituent and as aggregate) 850 Lime alone 1700 GBS alone, ASS alone, GBS + ASS 3000 Table A10.9 Construction period for HBM when one binder constituent only or binder constituents added at the same time Binder Construction period (°C·h) Other constituents, combinations, possibilities not listed below in this table Producer to determine and evidence during the mixture design procedure SiFA followed by CEMI 35 after cement addition GGBS followed by CEMI 35 after cement addition but within 850 of GGBS addition Lime followed by CEMI 35 after cement addition but within 1700 of GGBS addition Lime followed by GGBS 200 after GGBS addition but within 1700 of lime addition Lime followed by SiFA 850 after SiFA addition but within 1700 of lime addition GBS + ASS 3 000 after final addition
- The minimum layer thickness or lift thickness for layers constructed in two or more lifts is 100 mm. Where the total thickness laid exceeds 1000 mm, the minimum strength class requirement of C3/4 applies to the top 1000 mm only, with a minimum of C1.5/2 below this depth.
- Making-up the surface level of the layer after initial compaction is not permitted for single lift working or for the uppermost lift in multiple lift working.
- For multiple lift working, fresh HBM must not be laid on HBM that has been allowed to dry. The temporary intermediate surfaces of lifts must be kept moist.
- The face of previously compacted HBM or other material must be vertical and straight before butting fresh material against it.
- Compaction to refusal of the HBM must be carried out in accordance with the method for granular and cement bound materials in Table A8.1.
- After compaction, the surface must be closed and free from cracks, loose or segregated material, visible voids and other defects. All defective areas must be rectified within the construction period for the binder stated in Table A10.8. If rectification is not completed within the construction period, the defective area must be removed to the full thickness of the layer, and new mixture laid and compacted.
Cold and wet weather working
- HBM must not be laid on a frozen surface.
- In the case of heavy or persistent rain, works must cease and laid material must be compacted immediately.
Curing, protection and overlay of HBM
- Unless overlain immediately, the upper surface of the HBM must be cured to prevent loss of moisture by applying a bituminous emulsion spray complying with BS 434-1 at a minimum rate of 0.5 l/m2 to produce an even and continuous coverage of bitumen. The surface must be free of loose material and standing water before spraying. The curing membrane must be protected from any damage until placement of the overlaying layer.
- Overlaying of the HBM is permitted at any time for HBM with IBI category IPI25. Alternatively, overlay is only permitted once the material has gained sufficient strength to enable compaction of the overlying layer.