Recommended Practice for Sand-Bitumen Base Courses
1974 Edition

This section introduces the recommended practice for sand-bitumen base mixes and specifies essential requirements including:

• Coarse aggregates such as gravel or kankar must be clean, free from clay and fines, retained on the 4.75 mm IS sieve with a maximum size not exceeding one-third of the compacted layer thickness (Clause 4.75).
• Bituminous binders should conform to IS 73-1961 or IS 217-1961 standards.
• Selection of bitumen type depends on the sand characteristics and environmental conditions (Clause 5.1).
• Bitumen content is designed to fulfill mix performance criteria (Clause 5.2).
• Use of twin shaft paddle mixers for 1-2 minutes; binders heated to 150-165°C for penetration grade or 80-120°C for cut-back bitumen (Clause 6.3).
• Mix laid to a loose thickness of up to 15 cm, compacted to 10 cm; immediate rolling for penetration bitumen, curing followed by rolling for cut-back bitumen (Clause 6.5).
• Curing periods for cut-back bitumen range from 1 to 7 days depending on conditions.
• Quality control parameters include temperature difference limits, density above 95% of lab density, stability and voids testing, and surface irregularities within 6 mm (Clause 7).
• Wearing surface recommended is a 20 mm premix carpet following IRC 14-1977.

Mix design is based on the Hubbard Field Stability Method.

This part describes the extent of use for sand-bitumen base courses in road construction. It specifies that the sand-bitumen mixes must be designed with the Hubbard Field Stability Method (Clause 3). Aggregates should be gravel or kankar, clean, retained on 4.75 mm sieve, with maximum particle size limited to one-third of the layer thickness (Clause 4.75). Bitumen must comply with IS 73-1961 or IS 217-1961 (Clause 4.75). Bitumen selection depends on sand type and climatic factors (Clause 5.1), while quantity is proportioned to meet mix requirements (Clause 5.2). Construction procedures include heating binders to required temperatures and sand to 135-163°C if penetration bitumen is used (Clause 6.3). The mix is laid loose up to 15 cm and compacted to 10 cm thickness (Clause 6.5). Rolling and curing depend on bitumen type (Clauses 6.5, 6.8). A 20 mm premix carpet is suggested as the wearing surface (Clause 7). Quality control covers temperature, density, mix testing, and surface smoothness (Clause 7).

Design specifications for sand-bitumen mixes include:

1. Use the Hubbard Field Stability Method for mix design, targeting lower void content (Clause 3).
2. Coarse aggregates must be clean gravel or kankar, retained on 4.75 mm sieve, with maximum size not exceeding one-third of the compacted layer thickness (Clause 4.75).
3. Bituminous binders must adhere to IS 73-1961 or IS 217-1961 (Clause 4.75).
4. Bitumen type selection depends on sand characteristics and weather (Clause 5.1).
5. Bitumen content is designed to meet mix performance requirements (Clause 5.2).
6. Heating: penetration bitumen heated to 150-165°C; cut-back bitumen to 80-120°C; sand heated to 135-163°C if necessary (Clause 6.3).
7. Mix laid loose up to 15 cm, compacted to 10 cm, with uniform density and curing time based on bitumen type (Clause 6.5).
8. Quality control includes temperature difference not exceeding 14°C, density above 95% of lab density, stability and voids tests, and surface smoothness within 6 mm over 3 m (Clauses 6 and 7).

Materials used in sand-bitumen mixes must meet these standards:

• Sand: Clean dune or pit sand, free from organic and harmful materials, with less than 10% passing the 75-micron sieve by weight (Clause 4.1.1).
• Coarse Aggregates: Gravel or kankar, clean, retained on 4.75 mm sieve, maximum size not exceeding one-third of the compacted layer thickness, limited to 25% by weight (Clause 4.75).
• Binder: Paving or cut-back bitumen meeting IS 73-1961 or IS 217-1961 (Clause 4.75).
• Heating: Penetration bitumen heated to 150-165°C (viscosity 150-300 cSt), cut-back bitumen heated to 80-120°C (Clause 6.3.1).
• Mixing: Twin shaft paddle mixer with 1-2 minutes mixing time (Clause 6.3.2).
• Laying thickness: Loose up to 15 cm, compacted to 10 cm; rolling immediately for penetration bitumen, curing for 1-7 days before rolling for cut-back bitumen (Clause 6.5).
• Quality control: Temperature difference between sand/aggregate and binder not exceeding 14°C, density not less than 95% of lab design, stability and voids tests, surface irregularities within 6 mm (Clause 6).

Mix design must comply with the Hubbard Field Stability Method.

Important points in designing the sand-bitumen mix include:

• Coarse aggregates, maximum 25% by weight, must be gravel or kankar, retained on 4.75 mm IS sieve, with size limited to one-third of the compacted thickness (Clause 4.75).
• Bitumen used should be paving (penetration grade) or cut-back conforming to IS standards.
• Bitumen selection depends on sand type and local climate (Clause 5.1).
• Bitumen content is proportioned to meet performance criteria (Clause 5.2).
• Mixing in twin shaft paddle mixers for 1-2 minutes.
• Binder heating to achieve viscosity 150-300 cSt (penetration grade 150-165°C; cut-back 80-120°C); sand heated to 135-163°C if necessary (Clause 6.3).
• Laying loose mix up to 15 cm thick, compacted to 10 cm; rolling immediately for penetration bitumen, after curing (1-7 days) for cut-back (Clause 6.5).
• Quality control includes temperature difference limit of 14°C, density above 95% of lab density, stability and void tests, and surface irregularities within 6 mm (Clause 6).
• Wearing surface recommended is 20 mm premix carpet as per IRC 14-1977 (Clause 7).
• Design methods include Hubbard Field Stability or Modified Hubbard for kankar mixes.

This approach ensures a strong, well-compacted sand-bitumen base suitable for pavement construction.

Key points for construction include:

• Use clean gravel or kankar aggregates retained on 4.75 mm sieve, max size limited to one-third compacted layer thickness (Clause 4.75).
• Bitumen to comply with IS 73-1961 or IS 217-1961 standards (Clause 4.75).
• Provide grooves 8 cm wide and 5 cm deep at edges for thickened edge strength (Clause 6).
• Heating: binders to viscosity 150-300 cSt; penetration bitumen heated to 150-165°C, cut-back to 80-120°C; sand and aggregates heated to 135-163°C if required (Clause 6.3).
• Mixing in twin shaft paddle mixers for 1-2 minutes; immediately lay the mix (Clause 6.3).
• Lay loose mix up to 15 cm thick, compacted to 10 cm with uniform density (Clause 6.5).
• Rolling: immediate rolling for penetration bitumen; for cut-back, initial rolling after 1-7 days curing; final rolling with 8-10 tonne smooth drum roller until target density is reached (Clause 6.5).
• Traffic opening after 24 hours of final rolling, initially for light pneumatic-tyred vehicles; normal traffic allowed after 1 month; wearing surface to be laid within 3 months (Clause 6.8).
• Wearing surface is a 20 mm premix carpet as per IRC 14-1977 (Clause 7).
• Quality control requirements include temperature difference limits, density and mix testing, and surface smoothness (Clause 6).

This section covers the wearing surface laid over the sand-bitumen base:

• A 20 mm thick premix carpet is advised as the wearing course to provide a durable running surface and protect the base from wear (Clause 7).
• Repair any ruts or unevenness in the base with premix aggregate before laying the wearing surface (Clause 7).
• Quality control includes maintaining temperature difference between sand/aggregate and binder within 14°C.
• Field density tests should be conducted every 500 m², ensuring density is at least 95% of the lab design.
• Mix samples must be tested for stability, voids, and binder content at least once per 50 m³ or twice per shift.
• Surface smoothness is checked using a 3 m straight edge; irregularities exceeding 6 mm must be corrected.
• The base with premix carpet is suitable for daily traffic up to 150 vehicles over 3 tonnes and occasional heavy traffic of up to 1500 vehicles per day.

These measures ensure durability and functional performance of the wearing surface over sand-bitumen bases.

Quality control procedures include:

• Controlling temperature difference between sand/aggregate and binder to within 14°C during mixing (Clause 6).
• Conducting one field density test for every 500 m² or less of compacted surface, with density not less than 95% of laboratory design density (Clause 6).
• Testing mix samples for stability, void content, and binder proportion at a frequency of at least one per 50 m³ or twice per shift (Clause 6).
• Ensuring surface smoothness with a 3 m straight edge; correcting any irregularities greater than 6 mm (Clause 6).
• Materials must meet specifications, including clean aggregates retained on 4.75 mm sieve and binder heating to required temperatures (Clause 4.75, 6.3).
• Mixing times typically 1-2 minutes using a twin shaft paddle mixer (Clause 6.3).

These measures guarantee uniformity, strength, and longevity of the sand-bitumen base layers.

According to IRC 55, the sand-bitumen base course can be opened to traffic 24 hours after final rolling, initially permitting only light vehicles with pneumatic tires. Full traffic is allowed after one month. The final wearing course, typically a 20 mm premix carpet as specified in IRC 14-1977, should be applied within three months to protect the base layer and provide a smooth running surface.

Specifications include:

• Maximum 25% fines content by weight (Clause 4.75).
• Coarse aggregates retained on 4.75 mm sieve with size limited to one-third of compacted thickness.
• Bitumen conforming to IS 73-1961 or IS 217-1961.
• Heating of binders to 150-165°C for penetration grade and 80-120°C for cut-back (Clause 6.3).
• Mix laid loose up to 15 cm and compacted to 10 cm thickness (Clause 6.5).
• Density requirements of at least 95% of laboratory design (quality control).

These ensure the base achieves sufficient strength and durability before traffic introduction.

IRC 55 was ratified by the Specifications and Standards Committee (Jan 31–Feb 1, 1974), Executive Committee (May 1, 1974), and Council (May 2, 1974). Essential specifications include:

• Use of paving bitumen meeting IS 73-1961 or IS 217-1961.
• Coarse aggregates to be clean gravel or kankar, retained on 4.75 mm sieve, with maximum size up to one-third of compacted layer thickness (Clause 4.75).
• Binder heating temperatures: 150-165°C for penetration bitumen, 80-120°C for cut-back (Clause 6.3.1).
• Mix laying thickness: loose up to 15 cm, compacted to 10 cm (Clause 6.5).
• Rolling and curing procedures vary according to bitumen type (Clause 6.5).
• Traffic opening after 24 hours following final rolling; normal traffic after one month (Clause 6.8).
• Wearing surface to be a 20 mm premix carpet as per IRC 14-1977 (Clause 7).
• Quality control standards include maintaining temperature difference within 14°C, density above 95% lab density, and surface smoothness within 6 mm (Clause 7).