Quick Overview:
Tack Coat is a thin application of rapid-setting cationic bitumen emulsion applied over a primed granular layer or existing bituminous surface to ensure proper bonding between pavement layers.
Why Tack Coat Is Critical in Pavement Performance
Improper tack coat application is one of the most common causes of pavement layer slippage, delamination, and premature cracking.
Tack coat ensures shear resistance and structural continuity between successive bituminous layers.
Quick Technical Reference
- Applicable Clause: MoRTH 503 (5th Revision)
- Material: RS-1 / RS-2 Cationic Bitumen Emulsion
- Application Rate: 0.20–0.30 kg/m²
- Surface Condition: Dry, clean, dust-free
- Traffic Allowance: Only after emulsion breaks
Applicable Codes & Specifications
- MoRTH Specifications – Clause 503
- IRC: SP: 84 – Manual of Specifications & Standards
- IS: 8887 – Bitumen Emulsion for Roads
- IS: 1203 – Penetration Test (Bituminous Materials)
Materials Requirements
| Material | Requirement |
|---|
| Bitumen Emulsion | RS-1 / RS-2 Cationic, IS:8887 compliant |
| Water | Not permitted for dilution unless approved |
Equipment Required
- Calibrated bitumen pressure distributor
- Mechanical broom / air compressor
- Tray for spray rate check
- Thermometer and spray bar adjustment system
Tack Coat Application Methodology – Step by Step
- Verify underlying surface condition
- Remove dust and loose particles
- Check weather (no rain, dry surface)
- Calibrate pressure distributor
- Spray tack coat uniformly at approved rate
- Avoid streaking and overlapping
- Allow emulsion to break
- Proceed with next bituminous layer
Application Rates (Acceptance Criteria)
| Surface Type | Application Rate |
|---|
| Existing Bituminous Surface | 0.20 – 0.25 kg/m² |
| Primed Granular Surface | 0.25 – 0.30 kg/m² |
Curing & Traffic Control
Tack coat must be allowed to break (brown to black appearance).
No traffic or construction equipment shall be permitted until the surface becomes tacky and non-tracking.
Quality Control Checkpoints
| Layer | Inspection Stage | Test / Parameter | Standard | Frequency | Acceptance Criteria |
|---|
| Tack 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 |
Granular surface treated with primer:
2.5 to 3.0 Kg/10 m²
Bituminous Surface:
2.0 to 3.0 Kg/10 m² |
Common Field Problems & Remedies
- Streaking: Poor nozzle calibration → clean & adjust spray bar
- Excess tack: Over-application → recalibrate distributor
- Peeling layers: Insufficient tack or dusty surface
- Tracking: Traffic allowed before breaking
Safety & Best Practices
- Use PPE during spraying operations
- Ensure barricading and traffic diversion
- Avoid tack coat during rain or fog
- Maintain daily spray records
Frequently Asked Questions (FAQ)
What is the purpose of tack coat?To ensure proper bonding between existing and new bituminous layers.
Can tack coat be applied on wet surface?No. Surface must be dry and clean.
Which emulsion is used for tack coat?Rapid-setting cationic emulsion RS-1 or RS-2 as per IS:8887.
How long should tack coat cure?Typically 20–30 minutes until emulsion breaks.
