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What Is Tack Coat? Why It's Essential for Paving
Tack coat is a thin, uniform application of asphalt emulsion sprayed between pavement layers during asphalt paving. It serves as the "glue" that bonds the new asphalt overlay to the existing surface, preventing delamination (layer separation) that causes costly failures.
Without proper tack coat, asphalt layers can slip under traffic loads — especially on grades, curves, and intersections. This is why every state DOT specification requires tack coat between all lifts. Our tack coat calculator helps engineers and contractors determine the exact quantity needed.
Common Tack Coat Emulsions
- SS-1 / SS-1h: Slow-setting anionic emulsion. Most widely specified for tack coat. Dilute 1:1 with water before application. $2.50–$4.00/gal.
- CSS-1 / CSS-1h: Slow-setting cationic emulsion. Better adhesion in some aggregate chemistries. $3.00–$4.50/gal.
- RS-1: Rapid-setting emulsion for quick break time. Used when overlay follows immediately. $3.00–$4.00/gal.
- Trackless tack: Polymer-modified non-tracking emulsion. Won't stick to truck tires. $4.00–$6.00/gal. Ideal for areas with traffic during paving.
- PG-based tack: Performance-graded binder heated and sprayed. Used by some DOTs as alternative to emulsion.
Tack Coat Application Rate Table
Application rates are specified as residual asphalt (the asphalt that remains after water evaporates). With 1:1 dilution, spray double the residual rate:
| Surface Type | Residual Rate (gal/yd²) | Residual Rate (L/m²) | Spray Rate (1:1 diluted) | Coverage per 55-gal drum |
|---|---|---|---|---|
| New HMA on HMA | 0.04 | 0.18 | 0.08 gal/yd² | 690–1,375 yd² |
| Existing smooth asphalt | 0.05 | 0.23 | 0.10 gal/yd² | 550–1,100 yd² |
| Milled surface | 0.07 | 0.32 | 0.14 gal/yd² | 390–785 yd² |
| Concrete / PCC base | 0.06 | 0.27 | 0.12 gal/yd² | 460–920 yd² |
| Bridge deck | 0.08 | 0.36 | 0.16 gal/yd² | 345–690 yd² |
Always verify application rates with your project specifications and local DOT requirements. Rates shown are AASHTO/ASTM guidelines.
How to Apply Tack Coat Properly
- Clean the surface: Sweep or blow off dust, dirt, and debris. Tack coat won't bond to contaminated surfaces.
- Dilute if needed: Mix SS-1 or CSS-1 with equal parts water (1:1 ratio) for uniform coverage. RS-1 is typically sprayed undiluted.
- Set distributor: Calibrate the spray bar to achieve the specified residual rate. Verify with paper test squares.
- Spray uniformly: Apply in a thin, even coat. Avoid puddles, missed spots, or over-application (which causes slippage).
- Allow break/cure: Wait until the emulsion "breaks" — changes from brown to black. SS-1: 15–60 min. RS-1: 5–15 min. Trackless tack: 10–30 min.
- Pave promptly: Overlay with hot mix asphalt within 24 hours. Don't let vehicles drive on uncovered tack coat.
Common mistakes: Over-application (causes fat spots and slippage), spraying on wet/dirty surfaces, paving before break, and driving on tack coat.
Tack Coat vs Prime Coat vs Seal Coat
| Property | Tack Coat | Prime Coat | Seal Coat |
|---|---|---|---|
| Purpose | Bond between asphalt layers | Seal granular base | Protect finished surface |
| Where applied | Between HMA lifts | On aggregate base | On top of finished pavement |
| Rate | 0.04–0.08 gal/yd² | 0.15–0.50 gal/yd² | 0.10–0.20 gal/yd² |
| Material | SS-1, CSS-1 emulsion | MC-30/MC-70 cutback | Coal tar or asphalt emulsion |
| Cost | $0.03–$0.08/ft² | $0.10–$0.20/ft² | $0.15–$0.35/ft² |
Need to calculate prime coat for your base layer? Use our Bitumen Calculator which includes prime coat estimation. For surface sealcoating, see our Chip Seal & Sealcoating Calculator.
Tack Coat FAQ
What is tack coat application rate?
The standard tack coat application rate is 0.04–0.08 gallons per square yard (0.18–0.36 litres/m²) of residual asphalt. For diluted emulsion (1:1), the spray rate doubles. Milled surfaces need the higher rate; smooth asphalt needs less.
What is tack coat in road construction?
Tack coat is a thin layer of asphalt emulsion sprayed between pavement layers to create a bond. It prevents delamination and is required by all state DOTs between asphalt lifts and between existing pavement and overlay.
How to calculate tack coat quantity?
Tack coat quantity = Area (yd²) × Application Rate (gal/yd²). For 1,000 yd² at 0.05 gal/yd² residual using SS-1 diluted 1:1: Total = 1,000 × 0.05 × 2 = 100 gallons. One 55-gallon drum covers 550–1,100 yd².
How much does tack coat cost?
Tack coat emulsion costs $2.50–$4.50/gallon. Applied by contractor: $0.03–$0.08/ft². For a typical driveway of 600 sq ft, material cost is $5–$15.