Mastic Asphalt: What It Is, How It Is Made and Laid, and Where It Is Used

What is mastic asphalt made of?

Mastic asphalt is a mixture of four things: a hard bitumen, fine aggregate, limestone filler and a measured amount of coarse stone. MoRTH clause 516.1 describes it as a mix in such proportions as to give a plastic and voidless mass that can be trowelled and floated hot.

The bitumen is the first difference you notice. Ordinary road bitumen in India is a VG grade, and the VG-30 used on most highways must show a penetration of at least 45 at 25 °C. Penetration is how far a standard needle sinks into the bitumen in five seconds, so a bigger number means a softer binder. The binder for mastic asphalt is a hard paving or industrial grade with a penetration of only 15 ± 5 and a softening point of 65 ± 10 °C (MoRTH Table 500-39). In cold regions, MoRTH allows VG-40 instead. How the VG grades compare with each other is set out in the guide to bitumen grades for road construction.

The fine aggregate is crusher dust, natural sand or a mix of both, the part that passes the 2.36 mm sieve and stays on the 0.075 mm sieve. The filler is limestone powder finer than 0.075 mm with at least 80 per cent calcium carbonate, tested to IS 1514. The coarse aggregate is clean, hard, cubical stone of 13 mm nominal size.

The binder content is high. MoRTH clause 516.3.2 puts it at 14 to 17 per cent by weight, and its Table 500-42 applies that range to the mastic itself, before the coarse stone is stirred in. Compare that with bituminous concrete, where Table 500-17 asks for a minimum of 5.2 or 5.4 per cent of the total mix. The extra bitumen is what fills every gap between the grains. Nothing is left for a roller to close up.

The finished mix is checked with a hardness number, measured by the method in IS 1195. MoRTH asks for 50 to 70 at 25 °C when the mastic is made, before the coarse stone goes in, and 10 to 20 at the time of laying, after it has been added.

Where is mastic asphalt normally used?

On Indian roads, mastic asphalt is used as a single wearing course in two kinds of places. The first is bridge decks. Water that soaks through a road surface on a bridge reaches the concrete deck and the steel inside it, and a voidless layer keeps it out. MoRTH clause 2702.1 lists two mastic options among the four bituminous wearing coats for bridges, covered further down this page.

The second is any stretch where traffic brakes, turns and stands hard on the surface. MoRTH Table 500-40 names junctions and toll plazas as heavily stressed areas and gives them a thicker layer, 40 to 50 mm, with a little more coarse stone.

Mastic is also used well away from traffic. The Bureau of Indian Standards has separate standards for bitumen mastic in buildings: IS 1195 for flooring, with IS 1196 as the code for laying it, IS 3037 for waterproofing roofs, IS 5871 for tanking and damp-proofing, and IS 9510 for an acid-resisting grade used on factory floors. These are different products from the road mix, so check which standard a quotation is written against. For roof and basement work, the other options are compared in the guide to waterproofing chemicals.

How is mastic asphalt different from bituminous concrete and stone matrix asphalt?

The three are easy to mix up because all three can be the top layer of a road, and because stone matrix asphalt is also called stone mastic asphalt. The difference is in how each holds together and how it is placed.

Bituminous concrete is a dense stone mix with a small amount of bitumen. It is laid by a paver and rolled until it reaches its target density. Stone matrix asphalt (SMA) has a skeleton of coarse stones touching each other, with the gaps filled by a paste of bitumen and filler. Cellulose fibres are added so the bitumen does not drain out of the stones on the way to site. It is also paver-laid and rolled. Mastic asphalt has no skeleton at all: the stones float in the bitumen paste. That is why it can be poured, and why rolling would do nothing.

A rolled surface like BC or SMA is the normal choice for kilometres of open road, because a paver and two rollers cover ground far faster than a gang with floats. Mastic asphalt earns its place on short, wet or high-stress stretches. How the rolled layers stack up underneath, from soil to wearing course, is shown in the guide to road construction layers.

If your work is on the rolled layers that sit beside or under a mastic deck coat, compare tandem rollers for bituminous work and talk to a dealer before the job starts.

How is mastic asphalt made?

Mastic asphalt is cooked, not mixed in a hot mix plant. MoRTH clause 516.4.4 calls for an approved mechanically agitated mastic cooker, which is a heated vessel with paddles that keep the mix turning. The cooking runs in two stages.

1. The limestone filler goes into the cooker alone and is heated for about an hour.
2. Half of the bitumen, heated separately to 170 to 180 °C, is added and mixed in.
3. The fine aggregate and the rest of the bitumen follow, and the batch cooks for about another hour.
4. In the second stage the coarse stone is added, and the whole batch is cooked for a total of about three hours.

At no point may the contents of the cooker go above 210 °C. That limit appears three times in the clause.

Mastic that is not needed straight away is cast into blocks of about 25 kg each, made of filler, fine aggregate and bitumen with no coarse stone. On the day of use, the blocks are remelted at 175 to 210 °C, the coarse stone is stirred in, and mixing carries on without a break until laying is finished. The stirring matters. Stop it and the heavier stones sink to the bottom of the batch. The hot mix travels to site in a towed mixer transporter that keeps stirring it and keeps it hot on the way.

How is mastic asphalt laid on a road or bridge deck?

Most of the work happens before the first bucket of hot mastic is poured.

Preparing the base. The surface is shaped to the right profile. A concrete base is power brushed until it is free of dust. MoRTH is firm that mastic asphalt must never go onto a base whose binder could soften under the heat; any such material is cut out and repaired first. A tack coat is then sprayed, as described in the guide to prime coat and tack coat.

Extra steps on a bridge deck. On an old bridge, the camber and cross fall, the expansion joints and the drainage spouts are checked first, loose parts of the joints are fixed, the old wearing coat is removed and cracks in the concrete are repaired. A new deck that was cast flat is first given its camber with concrete or a bituminous layer. (Camber is the sideways slope that sheds rain; the guide to superelevation in road design explains how that slope is set.) On bare concrete, the surface is roughened with a stiff broom or wire brush, the deck must be dry, a thin VG-30 tack coat goes on, and a chicken-mesh of 1.5 mm steel wire with 20 to 25 mm openings is fixed in place before the mastic is poured.

Spreading. The mastic is laid in one coat at 175 to 210 °C and spread evenly with wooden floats, or by machine. Wooden battens of the right depth set the thickness. Any blow holes that rise to the surface are punctured while the mix is still hot and floated over again.

Grip on the surface. A freshly floated mastic surface can be slippery. While it is still hot and soft, it is covered with stone chips of 13.2 mm or 9.5 mm size that have been coated with 2 ± 0.4 per cent of VG-30 bitumen. They are spread at 0.005 m³ for every 10 m² and rolled or pressed in while the mastic is at least 100 °C.

Joints and opening. A joint with yesterday’s work is made by pouring extra hot mix against the old edge to warm it, then trimming it flush. Traffic goes on once the new layer has cooled to the highest air temperature of the day. Finally, a 3 m straight edge laid along the road must show no dip deeper than 7 mm, and a camber template checks the same across it.

Which wearing coat goes on a bridge deck?

MoRTH clause 2702 gives five wearing coats for bridges. Four are bituminous and two of those use mastic asphalt. The project drawing names the one to build; the table shows what each means.

For every bituminous type, the deck is cleaned and tack coated first. Type 2 is a two-trade job: a paver and roller crew lays the BC, then the mastic gang tops it.

What goes wrong with mastic asphalt, and how do you check for it?

Most mastic faults trace back to heat, timing or the surface underneath. These are the checks to make on site, each tied to a line of the specification.

• Watch the cooker thermometer. The mix must stay under 210 °C at every stage, and it must be at 175 °C or more when it is laid, the bottom of the laying range.
• Keep the transporter stirring. The specification keeps mixing going until laying is over so the coarse stone stays spread through the mix.
• Look for blow holes behind the floats and get them pricked and floated over while the mix is still hot, as the clause asks.
• Do not skip the coated chips. Without them the floated surface can be slippery, which on a bridge deck is the last thing you want.
• Do not lay when the air at the surface is below 10 °C. On winter nights in north India that limit can rule out a late shift.
• Fix the deck before covering it. Cracks, loose expansion joints and blocked drainage spouts left under a fresh mastic coat will show up again.
• Keep traffic off until the layer has cooled to the day’s highest air temperature.

Which machines are used for mastic asphalt work?

The machinery list is short. A mechanically agitated mastic cooker makes the mix, and a towed mixer transporter stirs it and keeps it hot on the way to site. Laying is mostly hand work with wooden floats and battens, and the surface chips are rolled or pressed in. MoRTH also allows the mastic to be spread by machine.

The rolled layers around a mastic job need the usual road plant. Where the old surface on the approaches has to come off first, cold milling of bituminous pavement is one way to do it. The bituminous concrete under a Type 2 deck coat, and the approach roads on either side of the bridge, are compacted with a tandem roller such as the Volvo DD90 or the HAMM HD 109i VV. The tests the stone and bitumen must pass before any of this starts are covered in the guide to road construction materials.

Every other topic on how roads are planned, built and tested sits under the road construction guide. The figures on this page are from the MoRTH Specifications for Road and Bridge Works (Fifth Revision), clauses 516.1 to 516.8, Tables 500-17 and 500-38 to 500-42, Section 515 and clause 2702, with the cooker and laying method corroborated by the mixture formulations section of Wikipedia’s asphalt concrete article. Before the bituminous layers go down, compare tandem roller models and connect with a dealer.

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. Mastic asphalt figures quoted here follow the MoRTH Fifth Revision and are not a substitute for the contract specification, the project drawings or the instructions of the Engineer.