Prime Coat and Tack Coat in Road Construction: How Much per m²

What is the difference between prime coat and tack coat?

A prime coat is sprayed on a granular base, such as WMM (wet mix macadam) or WBM (water bound macadam), before the first bituminous layer goes on. A tack coat is sprayed on a surface that is already bound, such as an old bitumen road, a fresh DBM layer or a concrete slab, just before the next bituminous layer.

The jobs are different. IRC:16 lists four for priming: plug the small voids at the top of the base, glue down the loose stone particles, seal the surface against water, and help the bitumen above stick to it. The primer has to sink in, about 10 mm deep, to do that. A tack coat is not meant to sink in at all. It is a very light film whose only job is to stop two layers sliding over each other, so that they work as one.

IRC:16 is clear that one cannot stand in for the other. A primed base still gets a tack coat before the bituminous layer if the design calls for it.

Source: MoRTH Specifications for Road and Bridge Works (Fifth Revision), clauses 502 and 503; IRC:16-2008.

Which bitumen is used for prime coat and tack coat?

Most Indian road jobs use bitumen emulsion for both coats. Emulsion is bitumen broken into tiny droplets and held in water, so it can be sprayed cold. When it touches the road the water leaves, the droplets join up, and the brown liquid turns black. Road engineers call that the emulsion “breaking”.

IS 8887, the Indian standard for cationic bitumen emulsion, sets five grades and says which job each one suits:

• SS-1 (slow setting): recommended for prime coat, fog seal and crack sealing. It breaks slowly, which gives it time to soak into the base.
• RS-1 (rapid setting): recommended for tack coat. It breaks fast, so the paver can follow soon after.
• RS-2, MS and SS-2: surface dressing and cold mixes, not prime or tack work.

RS-1 has to leave at least 60 percent bitumen when the water is driven off, under IS 8887:2018. The rest is water.

There are two other options. For priming, MoRTH allows medium curing cutback bitumen to IS 217, which is bitumen thinned with a solvent: MC-30 for WMM and WBM, MC-70 for stabilised soil bases. For tack coat, it allows VG-10, the softest paving grade, heated in a bitumen boiler until it is thin enough to spray. Rapid curing cutback RC-70 is kept for sub-zero sites and emergencies. VG-10 and the other grades are explained in the guide to bitumen grades used in road construction.

How much prime coat goes on a square metre?

It depends on how open the base is. A tight, well-rolled WMM drinks less than a loose gravel or soil base. MoRTH Table 500-3 sets the rate of spray for SS-1 emulsion, and Table 500-4 for cutback:

Source: MoRTH Specifications for Road and Bridge Works (Fifth Revision), Tables 500-3 and 500-4.

IRC:16 adds a heavier class for gravel bases and crushed rock bases, at 1.2 to 1.5 kg/m² with MC-250 cutback.

The table gives a band, not a single number, because the right quantity is decided on the road. Both documents say the same thing: spray the most the surface can absorb without primer running off the low edge. IRC:16 notes that site crews call those run-off streaks “fingers”. The site engineer fixes the final figure after a spraying trial, aiming for that 10 mm soak.

One detail worth knowing if you price or bill this work. MoRTH clause 502.8 pays for prime coat on the basis of 0.6 kg/m², or whatever the contract states, and then adjusts the payment up or down for the quantity the engineer actually approves after the trial.

How much tack coat goes on a square metre?

Much less, because it only has to wet the surface, not soak into it. MoRTH Table 500-5 and IRC:16 Table 3 set these quantities:

Source: emulsion column, MoRTH Table 500-5 and IRC:16-2008 Table 3; VG-10 column, IRC:16-2008 Table 3.

A concrete slab takes the most because it is smooth and dense, with nothing for the bitumen to key into. That matters when an old cement concrete road gets a bitumen overlay. If the contract names no tack coat quantity, MoRTH says Table 500-5 applies, and its payment clause works off 0.2 kg/m² unless the contract says otherwise.

Spray quantity by surface, emulsion, kg per m² (MoRTH Fifth Revision)

Bar length is the top of each band against the largest figure, 1.2 kg/m². The primer bars are three to four times the tack bars.

How are prime coat and tack coat sprayed?

With a bitumen pressure sprayer, often called a bitumen distributor: a heated or insulated tank on a truck or trailer, a pump, and a spray bar with a row of nozzles across the back. Both MoRTH and IRC:16 want a self-propelled or towed pressure sprayer for the main work. How much lands on each square metre depends on three things the operator controls: nozzle size, pressure at the spray bar, and forward speed. A faster pass puts down less.

Hand spraying with a pressure hand sprayer is allowed only for small patches, corners the distributor cannot reach, and narrow strips (for primer, IRC:16 puts that at strips under 3 m). Pouring primer from a perforated can is not allowed. IRC:16 allows a can only for tack coat on patchwork, never on major road works.

Before the first pass, the contractor has to run a spraying trial to show that the machine gives an even spray within tolerance. Then the site rules are short:

• Sweep first. Power brooms or mechanical sweepers take off dust and loose stone. On WBM with soil or moorum binder, brush off about 2 mm so the primer can get in.
• Get the moisture right. Emulsion goes on a surface that is damp but has no standing water. Cutback goes on a dry surface.
• Watch the weather. No spraying in a dust storm, fog, rain or wind, or when the shade temperature is below 10°C.
• Do not heat or dilute the emulsion on site. Spraying temperature for emulsion is 20°C to 70°C, and for cutback 50°C to 80°C. VG-10 is heated in the boiler until it is runnier than 2 poise (a measure of thickness).

After the sprayer and the paver, the rollers take over. On a bitumen job that means a tandem roller on the fresh mix, models such as the Dynapac CC425 CEV-V, L&T 990HFi and Ammann ARX 90.2, often with a pneumatic tyre roller alongside.

How long should prime coat and tack coat cure?

A primed surface needs at least 24 hours, and longer if the water or solvent has not gone. Lay the next layer too early and you trap moisture under the bitumen. If some primer has not soaked in, the rule is to blot it with a light dusting of sand, using as little as possible. A primed base is closed to all traffic except the vehicles laying the next course.

Tack coat is quicker. IRC:16 says to wait until the emulsion has broken, meaning it has turned from brown to black, before the paver lays the mix on it. Until then, only plant that is needed for the work may drive on it.

How do you check how much was sprayed? The tray test

You weigh it. IRC:16 Appendix 2 describes a tray test that any site laboratory can run:

1. Take three light metal trays, 200 mm by 200 mm and 20 mm deep. Number them and weigh them empty.
2. Place them 10 m apart along the road, in the path of the distributor, between its wheels.
3. Once the distributor has covered 50 m, lift the trays, wrap each in a weighed polythene bag and weigh them again.

Each tray covers 0.04 m², so the grams the tray gained, divided by 40, gives kg per square metre. A tray that picks up 10 g shows 0.25 kg/m², right inside the tack coat band for a bituminous surface. A primer tray should gain 28 to 40 g on WMM. Three trays laid 50 cm apart across the spray width check that the bar is spraying evenly from side to side.

IRC:16 asks for one test per 1,000 m² and at least two a day. MoRTH Table 900-4 asks for three tests of rate of spread a day, plus a test of the binder for every lot and regular checks of the spraying temperature.

How much emulsion does one km of road need?

Multiply the area by the spray quantity. Take a road 3.75 m wide and 1 km long. That is 3,750 m².

• Prime coat of SS-1 on WMM at 0.7 to 1.0 kg/m²: 2,625 to 3,750 kg, so roughly 2.6 to 3.8 tonnes.
• One tack coat of RS-1 between two bituminous layers at 0.20 to 0.30 kg/m²: 750 to 1,125 kg.

The prime coat goes on once. A tack coat goes under each bituminous layer the contract asks it for, such as between DBM and BC, so count it once for every joint it bonds. Where these coats sit in the full stack, from subgrade to surface, is set out in the guide to road construction layers, and how the bitumen line fits a whole budget is in road construction cost per km.

These are the bands from the specification. The final figure on your stretch comes out of the spraying trial, so treat the sum as a planning figure for ordering, not as the quantity you will be paid for.

What goes wrong when the coats are skipped or overdone?

Skip the tack coat and the new layer can slide on the old one, because the two are not bonded and do not work as one. That is the whole reason the coat exists. Dust on the surface gets in the way of the bond too, which is why both codes put sweeping first.

Overdo the primer and it runs off the edges in fingers, or stays wet on top, and has to be blotted with sand before anything can be laid. Overdo the tack coat and you leave a soft film of bitumen between the layers, the opposite of what you sprayed it for. IRC:16 calls a tack coat “a very light application” for that reason.

Spray on the wrong surface and the coat does not work either. Cutback on a wet base, or emulsion on a base with water standing in the low spots, will not soak in as it should.

Prime coat and tack coat: what to remember on site

Prime coat seals a granular base with 0.7 to 1.2 kg/m² of SS-1 emulsion and needs a day to cure. Tack coat bonds two bound layers with 0.20 to 0.35 kg/m² of RS-1 emulsion and is ready when it turns black. Both go on with a pressure sprayer after a trial run, never in the cold, wet or wind, and both are checked by weighing trays laid in the sprayer’s path.

The quantities above are from the MoRTH Specifications for Road and Bridge Works, Fifth Revision, and the tray test and VG-10 figures from IRC:16-2008, the code of practice for prime and tack coat. Emulsion grades follow IS 8887. Every other layer and machine on a road job is mapped in the road construction guide, and the rollers that finish the bituminous layers are under compactor models and prices, including pneumatic tyre rollers such as the Dynapac CP275 and L&T 2490 HD.

Prices, specifications and features are indicative, vary by variant, location and date, and should always be confirmed with the official OEM or authorised dealer before any purchase decision. DesiMachines is not liable for decisions taken on the basis of information that may have changed after publication.