Road Materials and section information

1) Embankment

Embankment

The embankment or subgrade forms the foundation of the pavement, consisting of compacted natural soil or fill material. Its purpose is to provide a stable platform with adequate strength (minimum CBR of 8% for major roads per IRC:37). Poor subgrade can lead to pavement failure like rutting or cracking.

• Materials: Locally available soil, stabilized if needed (e.g., with lime or cement for low CBR soils).
• Thickness: 300–500 mm compacted, in layers of 150–250 mm.
• Specifications: As per MoRTH Section 305 and IRC:36. Compaction to 97–100% of maximum dry density (Proctor test, IS:2720 Part 8). Minimum CBR: 5–8% depending on traffic.
• Construction: Excavate unsuitable material, compact in layers using vibratory rollers. Slope for drainage (2–4% camber).

2) Granular Subbase (GSB)

GSB Layer

GSB acts as a drainage layer and distributes loads from the base to the subgrade, preventing capillary rise of water that could weaken the structure.

• Materials: Crushed stone, gravel, or natural aggregates meeting grading requirements (Grades I–VI per MoRTH Table 400-1). Liquid limit <25%, plasticity index <6%.
• Thickness: 150–300 mm, laid in 100–150 mm layers.
• Specifications: MoRTH Section 401, IRC:109 for alternatives. Compaction to 98% density. Minimum CBR: 30%. For drainage, use open-graded material in lower layers.
• Construction: Spread, moisten, and compact with vibratory rollers. Ensure proper cross-fall for drainage.

3) Granular Base (Grading I & II or WMM)

WMM befor compaction	WMM after compaction	WMM + Primer +DBM

The base course provides structural strength and uniform support for bituminous layers. WMM is a mechanically stable mix, superior to traditional Water Bound Macadam (WBM) due to better interlocking.

• Materials: Crushed aggregates (40–75 mm down), water, and fines. Optimum moisture content (OMC) for mixing: 4–6%.
• Thickness: 200–300 mm, in 100–150 mm layers (max 250 mm with vibratory roller per MoRTH).
• Specifications: MoRTH Section 406, IRC:109-2015. Grading as per Table 400-13; compaction to 100% density. Minimum soaked CBR: 80–100%.
• Construction: Mix at plant, spread with paver, compact immediately. Avoid segregation.

4) Prime coat:- IRC:15

Primer Coating Process	Locations	Primer Coating Process

     

Prime coat seals the granular base, prevents moisture ingress, and promotes adhesion to the bituminous layer. It penetrates 8–10 mm into the base (per IRC:16).

• Materials: Low-viscosity bitumen emulsion (RS-1, SS-1 per IS:8887) or cutback bitumen (MC-30/70 per IS:217). Avoid on wet surfaces; emulsions preferred in damp conditions.
• Application Rate: 0.6–1.2 kg/m² for WMM/GSB (MoRTH Section 502, IRC:16-2008). Spray uniformly with distributor.
• Specifications: IRC:16-2008 (not IRC:15; possible typo in original). Cure for 24 hours before overlay.
• Construction: Clean surface, apply at 60–80°C for cutbacks. No traffic until cured.

5) DBM/MPM/BM/BUSG

DBM laid	Fresh DBM layer

 The binder course provides bulk strength and resistance to rutting. DBM is dense-graded for high-traffic roads.

• Materials: Bitumen (VG-30/40 per IS:73) or modified bitumen (IRC:SP:53), aggregates (clean, durable).
• Thickness: 50–150 mm per layer (MoRTH Section 505 for DBM).
• Specifications: MoRTH Sections 504 (BM), 505 (DBM). Marshall stability >9 kN for DBM. Lay with paver for uniformity.
• Construction: Apply tack coat first (0.2–0.4 kg/m²), then hot mix at 140–160°C, compact to 98% density.

BM work using stringline and paver finishing	Tack Coat

 Equivalency factor for various layers need to be calculated.

6) BC / Carpet + seal coat / Mastic /SMA / Micro surfacing

Fresh BC Mix	BC mix After 1 year traffic movement

 

Mastic Asphalt	Micro surfacing	SMA

 The top layer provides skid resistance, waterproofing, and a smooth surface. Choices depend on traffic, climate, and maintenance needs.

• Materials: For BC: VG-30/40 bitumen, dense-graded aggregates. For seal coat: Emulsion or bitumen with chips. SMA: High bitumen content with fibers for rut resistance. Micro-surfacing: Emulsion-based slurry for thin overlays. Mastic: Dense, void less mix for heavy loads.
• Thickness: BC: 30–50 mm; Carpet: 20–25 mm + seal; Micro-surfacing: 6–12 mm.
• Specifications: MoRTH Section 509 (BC), 510 (Seal Coat), IRC: SP:79 (SMA), IRC: SP:81 (Micro-surfacing). Fine aggregates: Sand equivalent ≥60 (IS:2720 Part 37). Binder: Paving bitumen (IS:73) or modified (IRC: SP:53). Tack coat binder: Rapid-setting cationic emulsion (IS:8887).
• Construction: Apply tack coat, lay hot/cold mix, compact. For seal coat: Apply immediately after carpet; no traffic until laid.

Q) After how much time sealcoat must be done??
Ans: - The seal coat shall be applied immediately after laying the premix carpet and rolled.
No traffic shall be allowed on the road until the seal coat has been laid.
Binder for premix carpet: -
1) Paving bitumen --> IS:73
2) Modified bitumen --> IRC: SP:53

Fine aggregates for Carpet should have sand equivalent value as min 60, as per test method in IS:2720 (Part 37)

Binder for tack coat --> Rapid setting cationic bitumen emulsion--> IS:8887

In snow bound areas where emulsion or Paving bitumen is not feasible, cut back bitumen conforming to IS:217 or IS:454 can be useful.

Additional considerations: Use polymer-modified bitumen for high-traffic or rainy areas to enhance durability. Regular maintenance like crack sealing extends life. Environmental note: Recycle aggregates and use warm-mix asphalt to reduce emissions.