Top 8 Proven Steps for Prime Coat over Wet Mix Macadam (WMM)

1. Scope of Prime Coat

Prime Coat is applied over a prepared Wet Mix Macadam (WMM) base prior to laying Tack Coat and Dense Bituminous Macadam (DBM). It stabilizes the base and prevents absorption of binder from upper layers.

2. Reference Standards

• MoRTH Specifications – Clause 502 (5th Revision)
• IRC: SP: 84 – Manual of Specifications
• IS: 8887 – Bitumen Emulsion

3. Materials Used

Cationic bitumen emulsion SS-1 or MS-1 conforming to IS:8887, approved by the Engineer with valid test certificates.

4. Equipment Required

• Self-propelled bitumen pressure distributor
• Mechanical broom or power sweeper
• Air compressor for dust removal
• Thermometer and spray gauge

5. Surface Preparation

The WMM surface must be clean, dry, and free from loose particles. Light water sprinkling may be done if dusty, followed by adequate drying.

6. Application Procedure

1. Ensure emulsion temperature as recommended.
2. Calibrate the distributor for correct spray rate.
3. Apply Prime Coat uniformly at 0.7–1.0 kg/m².
4. Avoid pooling, streaking, or overlapping.

7. Curing & Traffic Restriction

Allow the primed surface to cure for a minimum of 24 hours. No traffic or construction equipment shall be allowed during curing.

8. Quality Control & Inspection

Layer	Inspection Stage	Test / Parameter	Standard	Frequency	Acceptance Criteria
Prime Coat	Inspection at Source	Quality of Binder	IS: 1201 to IS: 1220	1 Sample / Lot	Bitumen Emulsion complying with IS:8887 & Cutback Bitumen complying with IS:217
	In-process / Final Inspection	Binder Temperature	IRC:16	At regular close intervals	20°C – 70°C for Bitumen Emulsion 50°C – 80°C for Cutback Bitumen
		Rate of Spread of Binder	IRC:16	Minimum Three Tests per Day	As per Table No. 300-3 & 500-4 of MoRTH 5th Revision

Quick Reference – Prime Coat

• Clause: MoRTH 502
• Material: SS-1 / MS-1 Bitumen Emulsion
• Rate: 0.7–1.0 kg/m²
• Surface: Clean, dry WMM
• Curing: Minimum 24 hours

FAQs – Prime Coat over WMM

1. Why is Prime Coat required? It seals WMM pores and improves bonding with bituminous layers.
2. Can Prime Coat be applied on wet surface? No, surface must be completely dry.
3. Is traffic allowed after priming? No traffic until full curing.
4. What comes after Prime Coat? Tack Coat as per MoRTH Clause 503.

After Prime Coat curing, proceed with Tack Coat Application followed by DBM Construction.