Standard Specifications and Code of Practice for Design and Construction of Surface Dressing
2005 Edition

The standard specifies physical properties including maximum limits on abrasion (Los Angeles value up to 40%), aggregate impact (max 30%), combined flakiness and elongation index (up to 30%, or 25% for low category roads), and minimum retained coating values of 95%. Water absorption should not exceed 1% except for slag aggregates. Grading requires the use of single-sized aggregates selected according to traffic intensity and road surface type, with nominal sizes and gradation limits defined in related tables. Aggregates must be hydrophobic with low porosity, and dusty aggregates should be cleaned or wetted before use.

Quantities are determined by considering voids at different stages: 50% immediately after spreading aggregates, 30% after rolling, and 20% following traffic exposure. Aggregate volume per square meter for a single coat is computed using the Average Least Dimension (ALD) of aggregates adjusted for 10% loss due to whip-off. Bitumen quantity accounts for 75% occupancy of the 20% void space after traffic. Design involves calculating ALD from median sieve size and flakiness index, summing weighting factors related to traffic, surface condition, climate, and aggregate type, and then referencing design charts to establish application rates. Adjustments are made based on traffic speed and binder type.

Surface dressing should be applied in dry weather with temperatures above 16°C for penetration bitumen and 10°C for bitumen emulsions. The base must be cleaned of dust and repaired as necessary. Mechanical sprayers ensure uniform binder distribution, while mechanical grit spreaders apply aggregates evenly—preferably equipment that synchronizes both operations to prevent excess binder use. After spreading, immediate rolling is required to embed aggregates without damaging them. Traffic controls include closing sections equal to a day's work and providing diversions, safety barriers, and signage.

Penetration grade bitumen requires application at temperatures of 16°C or higher on dry surfaces, whereas bitumen emulsions can be applied at temperatures as low as 10°C and tolerate damp aggregate surfaces (without standing water). Emulsions necessitate higher binder quantities due to residual content after curing and do not require pre-coating of aggregates. Penetration bitumen occupies 75% of the void space post-traffic, while emulsions initially occupy the full aggregate thickness before curing. These distinctions affect application procedures and material quantities.

Surface dressing must be performed in dry, clear weather with no rain, fog, or dust storms. The minimum temperature should be 16°C for penetration bitumen and 10°C for bitumen emulsions. Surfaces must be dry for penetration bitumen and can be damp (without standing water) when using emulsions. Adverse weather conditions delay work to ensure proper binder adhesion, curing, and long-term durability.