Methodology for Prime Coat over Wet Mix Macadam (WMM)

Quick Summary:
The Prime Coat is a thin application of bituminous emulsion over the prepared Wet Mix Macadam (WMM) base to promote bonding with the bituminous layer. As per MoRTH Clause 502, the primer shall be applied at a rate of 0.7–1.0 kg/m² using a mechanical sprayer on a dry, clean surface. Read complete process →

Scope

The work includes applying a single coat of low-viscosity bituminous emulsion on the prepared WMM surface before laying the Dense Bituminous Macadam (DBM) or Bituminous Concrete (BC). Application shall not be done if the surface is wet, during rain, fog, or if ambient temperature is below 10°C.

Reference Standards

MoRTH Specifications – 5th Revision, Clause 502
IRC:SP:84–2014
IS:8887 – Bitumen Emulsion Specification
Contract Agreement and Approved Drawings

Materials

The bituminous material used for priming shall be a cationic bitumen emulsion (SS-1 or MS-1) conforming to IS:8887. The grade shall be approved by the Engineer and accompanied by valid test certificates.

Equipment Required

Self-propelled bitumen pressure distributor with calibrated spray bar
Air compressor for cleaning the surface
Mechanical broom or power sweeper
Thermometer and measuring gauge for temperature and spray rate

Preparation of Surface

The WMM surface must be clean, dry, and free from dust, loose material, or standing water. The surface shall be checked for levels and irregularities before priming. If the surface is excessively dusty, lightly sprinkle water, allow it to dry, and then apply primer.

Application Procedure

Heat and mix the primer emulsion to the required temperature.
Ensure uniform pressure in the distributor before spraying.
Apply the emulsion evenly at the rate of 0.7–1.0 kg/m² as per MoRTH Clause 502.2.2.
Do not overlap or pool the material; ensure full surface coverage.

Curing

After application, the primed surface shall be allowed to cure for at least 24 hours to ensure penetration into the base course. During this period, no construction traffic or vehicular movement shall be allowed.

Quality Control & Testing

Check emulsion quality as per IS:8887 before application.
Verify application rate and spray uniformity visually and by tray test.
Confirm that the primer has properly penetrated before laying the tack coat.

Traffic Management

No traffic shall be permitted over the primed surface during curing. If required, temporary diversion roads shall be provided as per MoRTH Clause 112.

Quick Reference: Prime Coat over WMM

Applicable Clause: MoRTH 5th Revision, Clause 502
Material: Cationic Bitumen Emulsion (SS-1 or MS-1)
Application Rate: 0.7–1.0 kg/m²
Equipment: Pressure Distributor, Broom, Air Compressor
Surface Condition: Clean, dry, dust-free WMM
Curing Period: Minimum 24 hours (no traffic)
Test Standard: IS:8887 for Bitumen Emulsion

Before applying the Prime Coat, ensure the Wet Mix Macadam (WMM) layer is constructed and approved. After priming, proceed with the Tack Coat application as per MoRTH 503.