IRC 48-1972 outlines provisional guidelines for bituminous surface dressing employing precoated aggregates, specifying the required materials, application techniques, and quality standards for both single and double coat treatments on road surfaces. This standard is designed for engineers and contractors engaged in road construction and upkeep, emphasizing enhanced bond strength and surface durability through precoating methods.
Overview
IRC 48-1972 outlines provisional guidelines for bituminous surface dressing employing precoated aggregates, specifying the required materials, application techniques, and quality standards for both single and double coat treatments on road surfaces. This standard is designed for engineers and contractors engaged in road construction and upkeep, emphasizing enhanced bond strength and surface durability through precoating methods.
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This section introduces key material specifications and aggregate criteria essential for road surface construction. Aggregates must conform to limits such as a maximum abrasion value of 35%, impact resistance capped at 30%, flakiness index not exceeding 25%, stripping value up to 25%, and water absorption limited to 1% (excluding slag). For slag aggregates, soundness loss should not exceed 12%, and bulk density must be at least 1120 kg/m³. Aggregate sizes are specified according to the treatment coat: the initial coat employs nominal 13.2 mm aggregates passing a 22.4 mm sieve but retained on an 11.2 mm sieve, while the renewal coat utilizes nominal 11.2 mm aggregates passing a 13.2 mm sieve and retained on a 6.7 mm sieve. These stipulations ensure the aggregates' quality and gradation for long-lasting pavement layers.
This part details the specifications for cover materials and aggregates employed in bituminous surface applications. It highlights aggregate sizing for different coats — 13.2 mm nominal size for the first coat and 11.2 mm for the renewal coat — along with physical requirements such as maximum abrasion and impact values, flakiness index, stripping resistance, water absorption, and soundness specific to slag. These criteria guarantee the aggregates' suitability and durability for surface dressing.
According to the standard, cover materials shall be composed of crushed stone, crushed slag, crushed gravel (shingle), or other crushed aggregates that are clean, robust, enduring, and generally cubical. They must be free from deteriorated particles, salts, alkalis, organic matter, dust, and coatings. In cases where crushed gravel is not economically viable, rounded gravel or water-washed shingle may be substituted. Aggregate nominal sizes for cover materials vary by coat type, with explicit sieve specifications. Physical properties including abrasion, impact, flakiness, stripping values, water absorption, soundness, and bulk density must comply with the prescribed limits, referencing IS standards for testing.
The binder employed in surface dressing must be selected from straight-run bitumen 80/100, road tar grades RT-3 or RT-4, or approved cutback binders, depending on climatic conditions. The binder should be heated to the appropriate temperature and applied evenly using mechanical sprayers. Application rates vary: for the first coat, straight-run bitumen ranges from 15 to 17.5 kg per 10 m², while cutbacks and road tar require 17.5 to 20 kg per 10 m². Priming coats have different specified rates. Post-application, rolling with a 6 to 8 tonne smooth-wheeled roller is performed starting from the edges inward, ensuring proper embedment without crushing aggregates. For renewal coats, the second binder application should follow immediately or shortly after the first, especially with road tar.
This section emphasizes the critical requirements for cover materials and aggregates during construction. Aggregate sizes are specified for both initial and renewal coats, alongside detailed physical test limits ensuring durability and adequate bonding. These requirements are integral to achieving a resilient pavement surface in accordance with the guidelines.
Specifications for surface finish focus on the nominal size and sieve designation of aggregates used in bituminous surface dressing. The first and renewal coats require aggregates of 13.2 mm and 11.2 mm nominal sizes respectively, adhering to precise sieve passing and retention criteria. Maintaining these aggregate gradations is essential to securing surface quality and longevity, even though explicit finishing formulas are not provided.
The standard outlines conditions for reopening treated surfaces to traffic. When using straight-run bitumen or road tar, roads can be opened the following day. If immediate traffic access is necessary after rolling, speeds must be limited to 16 km/h until full curing. Surfaces treated with cutback bitumen require closure until the binder sufficiently sets to retain cover aggregates. Proper traffic control measures, including barricades and watchmen, are mandated. Further specifications include binder application rates, rolling procedures, and surface evenness tolerances for various base types to ensure quality before traffic introduction.
Frequently Asked
IRC 48 specifies that suitable aggregates for precoating include crushed stone, crushed slag, crushed gravel (shingle), or other crushed aggregates that are clean, strong, durable, and largely cubical in shape. These must be free from broken fragments, salts, alkalis, organic materials, dust, and coatings. Where crushed gravel is not cost-effective, rounded gravel or water-washed shingle can be used. Aggregate sizes vary with coat type: first coat requires 13.2 mm nominal aggregates passing a 22.4 mm sieve and retained on an 11.2 mm sieve; the second renewal coat uses 11.2 mm aggregates passing a 13.2 mm sieve and retained on a 6.7 mm sieve. Aggregates are precoated with 0.75 to 1 percent binder by weight and cured for at least one week to ensure non-stickiness for easy spreading.
IRC 48 recommends selecting binders based on climate considerations, including straight-run bitumen 80/100 for general use, road tar grades RT-3 or RT-4 for specific climatic needs, and approved cutback binders depending on local weather and application requirements. These selections ensure optimal adhesion and durability for bituminous surface dressing.
Precoated aggregates should be free from dust and small particles and preheated to approximately 60°C, except when using certain cutbacks like RC-3. They are then coated with 0.75 to 1 percent binder by weight, heated to application temperature, and thoroughly mixed in approved equipment to achieve uniform coating. After precoating, aggregates must cure for at least one week to become non-sticky, facilitating easy spreading similar to untreated aggregates. They can be stored for several months if necessary before use.
Application rates for binders per IRC 48 are: for straight-run bitumen, 9.4 to 10.8 kg per 10 m² for priming coats and 15 to 17.5 kg per 10 m² for surface dressing coats; for cutbacks and road tars, 10.8 to 13.2 kg per 10 m² for priming and 17.5 to 20 kg per 10 m² for surface dressing. Aggregates used for precoating should have 0.75 to 1 percent binder content by weight and be screened to pass a 13.2 mm sieve while retained on a 6.7 mm sieve. Precoated aggregates require curing of at least one week prior to application.
Surface dressing is best performed when ambient shade temperature exceeds 16°C. Application should not occur if the surface or cover material is damp, or during fog, rain, or dust storms. For traffic management, the length of road treated each day must be closed with appropriate diversions, barriers, flags, signs, and red warning lights to ensure safety. Diversions should be watered to prevent dust contamination. When cutback bitumen is used, traffic must be prohibited until the surface has cured sufficiently to hold aggregates, with barricades and watchmen employed to control access.
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