Guidelines for Gap Graded Wearing Course with Rubberised Bitumen-(Bitumen Rubber)
2015 Edition

IRC SP 107 advises using crumb rubber derived from discarded vehicle tyres, ensuring wire contamination is below 0.01% and fabric content under 0.95%. The gradation should comply with specified sieve passing percentages, such as 100% passing 1.18 mm, 90-100% passing 0.6 mm, and 45-100% passing 0.425 mm sieves. Particle size is ideally finer than 1.18 mm with a median near 425 microns. Rubber content in the binder should be at least 15% by weight, commonly reaching 20% or more to achieve desired viscosity. Finer rubber particles reduce the required proportion. This ensures a flexible, durable binder suited for heavy traffic conditions.

According to IRC SP 107 Clause 7.1, the optimal mixing temperature for rubberised bitumen mixtures lies between 160°C and 170°C, while compaction should occur between 140°C and 150°C. The use of warm mix additives is permitted to lower these temperatures. During pumping, the binder temperature must be maintained at 180°C to 190°C for adequate flow. Compaction should be executed promptly after laying using only steel wheel rollers. Additionally, avoid placement when ambient temperatures are below 15°C or when wind speeds exceed 40 km/h to ensure proper mix performance.

Applying gap grading to rubberised bituminous wearing courses optimizes aggregate size distribution, creating increased void spaces that accommodate higher volumes of bitumen-rubber binder. This results in thicker binder films enveloping aggregates, which improves durability and fatigue resistance. The high viscosity of the rubberised binder (exceeding 1500 cP at 177°C) combined with crumb rubber content around 20% introduces elasticity, enhancing rutting resistance. The carbon black in crumb rubber helps protect the binder from ultraviolet degradation, extending pavement life. Furthermore, the gap-graded surface texture reduces tyre-pavement noise, contributing to quieter roadways. These factors collectively provide longer-lasting pavements compared to dense-graded mixes.

To ensure consistent quality of the bitumen-rubber binder as per IRC SP 107, it is vital to control input materials by using uniform crumb rubber and appropriate base bitumen grades (VG30 or VG10 depending on conditions). Precise measurement of crumb rubber and bitumen proportions must be maintained. Mixing should occur at 190°C with crumb rubber dry and free-flowing; calcium carbonate powder may be added to improve flow. The reaction phase requires agitation at 190°C for 60 minutes. Post-reaction, the binder should be stored at 180-190°C if used immediately, or cooled to 130-140°C for longer storage, avoiding temperatures above 160°C for over 10 hours. Documentation of mixing parameters and routine testing of viscosity, softening point, and elastic recovery are mandatory to minimize variability.

Rubberised bitumen mixtures should not be directly laid over pavements exhibiting cracks wider than 12.5 mm, as stated in Clause 7.1 of IRC SP 107. For such cracked surfaces, a Stress Absorbing Membrane Interlayer (SAMI) constructed using crumb rubber modified bitumen (CRMB) or polymer modified bitumen (PMB) per MORTH specifications should be applied prior to the rubberised overlay to mitigate reflective cracking. The application of RB mixtures is suitable for new or existing bituminous layers with minimal cracking. Precautions include maintaining mixing temperatures between 160-170°C, compaction temperatures of 140-150°C, avoiding placement when air temperature is below 15°C or wind exceeds 40 km/h, and restricting compaction to steel wheel rollers only. Additionally, applying a sand blotter at 1 kg/m² before opening to traffic helps prevent mix pickup.