Specifications for Bituminous Macadam (First Revision)
2009 Edition

According to IRC 27 Clause 3.1.1 and associated tables, bitumen viscosity grades are recommended based on regional climate: VG-30 suits most Indian areas, replacing the older 60/70 penetration grade; VG-20 is preferred for colder, high-altitude northern regions; VG-10, formerly 80/100 penetration grade, is used mainly for spraying and in very cold climates. Table 2 further guides selection by correlating daily mean temperatures with suitable bitumen grades, ensuring optimal performance across temperature ranges.

IRC 27 specifies fine aggregates must be clean, durable mineral material passing the 2.36 mm sieve and retained on 75 microns, with limitations on rounded sand content depending on depth from the surface. Coarse aggregates should be crushed rock or gravel, with at least 90% of those retained on 4.75 mm sieve having two fractured faces. Physical properties such as cleanliness, particle shape, strength (LA abrasion or impact value), durability, water absorption, and bitumen coating retention are detailed in Table 3 to ensure suitability for base course layers between 50 and 100 mm thick.

Per IRC 27 Annex A Clause 4.2, hot mix plants, preferably batch type, must have coordinated components ensuring uniform mixing per job mix formula. Cold aggregate feeding requires a minimum of four bins with conveyors and vibrators to ensure smooth flow, plus variable speed drives with load sensors. The dryer must heat aggregates efficiently without leaving residues. Temperature monitoring instruments track aggregate, bitumen, and mix temperatures, with controls to maintain differences within 15°C. Plant calibration is essential for consistent quality, supported by regular quality control tests as per Clause 5.3 and Table 7.

IRC 27 outlines that bituminous macadam layers are laid using mechanized pavers with continuous mix supply to prevent segregation. Manual spreading is reserved for restricted zones. Compaction starts immediately after laying, using rollers in a sequence from edges to center or lower to upper edges for cambered surfaces. Rolling speed should not exceed 5 km/h, and wheels must be kept moist to avoid mix sticking. Joints require vertical cutting, bitumen coating, and overlapping of new layers by 25-50 mm with about 25% extra loose thickness, then compacted to high density. Temperature control during laying and rolling is critical to ensure proper density and durability.

Joint construction in multi-layer bituminous macadam involves cutting vertical joints through the full thickness of the underlying layer, cleaning the joint face, and coating it with bitumen or emulsified bitumen. The new layer is placed overlapping 25 to 50 mm onto the existing layer with an additional 25% loose thickness before compaction to ensure densification. Alternatively, multiple pavers may work in echelon to achieve continuous rolling. Longitudinal joints in successive layers should be staggered by approximately 150 mm, and transverse joints offset by at least 2 meters to avoid weak planes within the pavement structure.