Top 8 Proven Steps for Tack Coat Methodology | As per MoRTH Clause 503 (5th Revision)

Top 8 Proven Steps for Tack Coat Methodology (As per MoRTH Clause 503 – 5th Revision)

Quick Reference: Tack Coat Application

  • Applicable Clause: MoRTH Clause 503 (5th Revision)
  • Binder Type: Cationic Bitumen Emulsion (RS-1 or RS-2)
  • Spray Rate: 0.20–0.25 kg/m² on bituminous surface; 0.25–0.30 kg/m² on granular surface after prime coat
  • Surface Condition: Dry, clean, and dust-free
  • Equipment: Self-propelled bitumen distributor, Air compressor, Mechanical broom
  • Curing Time: Allow emulsion to break (approx. 30 min) before overlay
  • Next Layer: Dense Bituminous Macadam (DBM) or Bituminous Concrete (BC)

1. Scope

The work covers the application of a single uniform coat of bituminous binder (tack coat) over an existing primed or bituminous surface to ensure a strong bond between successive pavement layers, as per MoRTH Clause 503.

2. Reference Standards

  • MoRTH Specifications, Clause 503 (5th Revision)
  • IRC: SP: 84-2014 – Specifications & Standards for Road Works
  • IS: 8887 – Specification for Bitumen Emulsion
  • IS: 1203 – Determination of Penetration of Bituminous Materials

3. Materials

The tack coat shall consist of Rapid-Setting (RS-1 or RS-2) Cationic Bitumen Emulsion conforming to IS 8887. The material shall be approved by the Engineer based on the manufacturer’s test certificate and on-site verification.

4. Equipment

  • Calibrated bitumen pressure distributor
  • Air compressor or blower
  • Mechanical broom for cleaning surface
  • Thermometer and spray bar adjustment system

5. Preparation of Surface

The surface shall be free from dust, dirt, and loose material. If required, the surface shall be lightly swept and blown using an air compressor. The surface must be dry before tack coat application.

6. Application of Tack Coat

The emulsion shall be applied at a uniform rate of:

  • 0.20–0.25 kg/m² on existing bituminous surfaces
  • 0.25–0.30 kg/m² on primed granular surfaces

The bitumen pressure distributor shall ensure even spraying without streaking or overlapping. Spray bar height and pressure must be calibrated before use.

7. Curing and Protection

After application, allow the tack coat to “break” (turn from brown to black) — usually within 30 minutes. No traffic or construction equipment shall move over the surface until the emulsion has fully broken and become tacky.

8. Quality Control and Inspection

  • Check spray rate and coverage using tray test.
  • Ensure temperature and viscosity of emulsion are within specification.
  • Confirm uniform application and absence of streaks or missed patches.
📘 View Official MoRTH Specification (5th Revision) →

Top 10 FAQs – Tack Coat Methodology (MoRTH Clause 503)

  1. What is Tack Coat Methodology? Tack Coat Methodology is the process of applying a thin bituminous emulsion layer between pavement courses to ensure strong bonding and prevent slippage between layers.
  2. Which emulsion is used for tack coat? Rapid-Setting cationic bitumen emulsion (RS-1 or RS-2) conforming to IS:8887 is used as per MoRTH Clause 503.
  3. What is the spray rate for tack coat? 0.20–0.25 kg/m² for existing bituminous surfaces and 0.25–0.30 kg/m² for primed granular bases (MoRTH 503.4.2).
  4. How should the surface be prepared? The surface should be clean, dry, and free from dust or loose materials using a mechanical broom and air compressor.
  5. How long should the tack coat cure? Allow approximately 30 minutes for the emulsion to break and become tacky before laying the next layer.
  6. Can tack coat be applied during wet or windy weather? No. The surface must be dry, and tack coat should not be applied during rain, fog, or high winds.
  7. What is the purpose of the tack coat in pavement construction? It provides adhesion between existing and new bituminous layers, ensuring structural integrity and preventing delamination.
  8. What equipment is used for tack coat application? A calibrated bitumen pressure distributor, air compressor, and mechanical broom are essential for uniform application.
  9. What are the common defects in tack coat application? Streaking, overlapping, or non-uniform coverage — usually due to improper sprayer calibration or excessive emulsion.
  10. What comes after the tack coat? After curing, the next layer such as Dense Bituminous Macadam (DBM) or Bituminous Concrete (BC) is laid.

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