Earthwork and Paving (C-12) · Lesson 6 of 6

Tack Coat: Residual Rate, Spray Rate, and Dilution

"Tack Coat: Residual Rate, Spray Rate, and Dilution" is lesson 6 of 6 in the Earthwork and Paving (C-12) Learning Hub. It opens with "Why and Where Tack Coat Goes" and works through 5 short steps, then a worked example you can check yourself against.

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Steps in this lesson

  1. Why and Where Tack Coat Goes
  2. The Spec Is a Residual Rate
  3. From Residual Rate to Spray Rate
  4. Dilution Rules
  5. Quantity: Area x Spray Rate, Paid on Residual

The worked example in the course applies this lesson to a realistic job in 6 steps, from "Area in square yards" through "Paperwork".

What you'll be able to do

  • Explain the main idea of "Tack Coat: Residual Rate, Spray Rate, and Dilution" in plain language you can use on the exam.
  • Apply this step with confidence: "From Residual Rate to Spray Rate".
  • Apply this step with confidence: "Dilution Rules".
  • Apply this step with confidence: "Quantity: Area x Spray Rate, Paid on Residual".
  • Test yourself on the worked example and the quick-check quiz inside the classroom.
Tack Coat: Residual to Spray RateMILL-AND-OVERLAY TACKNEW HMA OVERLAYMILLED (PLANED) PAVEMENTTack coat: 0.05 gal/sq yd residual minimumDISTRIBUTOR LOAD: 360 gal of 60/40 mix216 gal emulsion144 gal waterAdded water is 0.67 : 1 of the emulsion(limit is 1 : 1 by weight)Emulsion is about 57% asphalt residueFROM RESIDUAL RATE TO GALLONS180 x 120 = 21,600 sq ft = 2,400 sq ydSpray = 0.05 / (0.57 x 0.60) = 0.146Round up: 0.15 gal/sq ydDiluted = 2,400 x 0.15 = 360 galEmulsion = 360 x 0.60 = 216 galWater = 360 x 0.40 = 144 galRESIDUAL CHECK216 x 0.57 = 123 gal asphalt123 / 2,400 = 0.051 gal/sq yd (OK)Spray 0.15 gal/sq yd of 60/40 diluted CSS-1h = 360 galOrder 216 gal of emulsion and add 144 gal water (report weights)Residual 0.051 gal/sq yd clears the 0.05 minimum for planed pavementSources: 2025 Caltrans Std. Specs 39-2.01C(3)(f), 39-2.01A(3)(j), 94-1.02C (57% min residue)CC
The diagram for this lesson, exactly as it appears in the Learning Hub.

The first steps of this lesson

Step 1: Why and Where Tack Coat Goes

Tack coat is a thin film of asphalt that bonds a new HMA layer to the surface below it. 2025 Caltrans Standard Specifications section 39-2.01C(3)(f) says to apply a tack coat to existing pavement including planed (milled) surfaces, between HMA layers, and to the vertical surfaces of curbs, gutters, and construction joints. Apply it in 1 application before placing HMA, at the minimum residual rate shown in the section's table for the condition of the underlying surface.

Step 2: The Spec Is a Residual Rate

The contract does not tell you how much liquid to spray; it sets the minimum residual rate - the asphalt left after the water in the emulsion is gone. In the 2025 table in section 39-2.01C(3)(f), for CSS-1/CSS-1h, SS-1/SS-1h, and QS-1h/CQS-1h asphaltic emulsion the minimum residual rates are 0.02 gal/sq yd over new HMA (between layers), 0.03 over concrete pavement and existing asphalt concrete surfacing, and 0.05 over planed pavement. For CRS-1/CRS-2 and QS-1/CQS-1 emulsion the rates are 0.03, 0.04 and 0.06. For asphalt binder and PMCRS-2/PMCRS-2h emulsion they are 0.02, 0.03 and 0.04.

The remaining 3 steps, the worked example and the quiz are in the course. This is a preview. The full lesson, with the step-by-step explanations, a worked example and a quick-check quiz, is inside the classroom.

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