Road Construction Materials in India: The List and the Tests Each Must Pass

What materials are used in road construction?

A bitumen road uses soil, granular material, crushed stone, fine aggregate, filler and bitumen. A concrete road swaps the bitumen for cement, adds steel bars at the joints and may add fly ash, admixtures or fibres. Water goes into almost every layer, for compaction or for mixing and curing. The table lists each material, where it ends up and the standard the MoRTH book points to.

Brick metal, kankar and crushed concrete are the cheap local options, and the book allows them only in the lower sub-base, where the load reaching them has already been spread out. The order in which these materials go down, and how thick each layer is, is covered in our guide to road construction layers in India.

Which soil can go into the subgrade?

Most local soil can, but not all of it. The subgrade is the compacted earth the whole road rests on, and MoRTH clause 305 rules out six kinds of fill for embankments: material from swamps, marshes and bogs; peat, logs, stumps and other perishable matter, along with organic soils; material that can catch fire on its own; frozen material; clay with a liquid limit above 50 and a plasticity index above 25; and soil carrying salts that leach out.

Two of those terms need a plain meaning. The liquid limit is the water content at which soil stops behaving like a stiff paste and starts to flow. The plasticity index is the range of water content over which it stays mouldable, like potter’s clay. A high plasticity index means a lot of clay, and clay swells when wet and shrinks when dry.

Swelling clay, the black cotton soil of much of central India, gets its own rule. If its free swelling index goes above 50%, it cannot be used as fill at all. If it is below 50% and is used, the subgrade and the top 500 mm (0.5 m) of embankment under it still have to be non-expansive soil.

Size and weight are checked too. Stones in the soil should not ordinarily be larger than 75 mm in the embankment or 50 mm in the subgrade. And the soil has to be dense enough when compacted in the laboratory:

Density alone is not enough. The subgrade must also reach the design CBR (California Bearing Ratio, the soaked strength test for soil). IRC:37-2018, the design code for bitumen roads, bases the pavement thickness on a 4-day soaked CBR and asks for an effective subgrade CBR above 5% on roads carrying more than 450 commercial vehicles a day. Soil that falls short is either replaced or stabilised with cement, lime or fly ash, which is how the cement treated sub-base and base come about.

Fly ash and pond ash are allowed as embankment fill. MoRTH requires them to follow the environment ministry’s guidelines and asks for a soil cover of at least 500 mm on the sides so the ash does not wash out.

What goes into the granular sub-base?

The granular sub-base, or GSB, is the first layer of stone over the soil. MoRTH allows natural sand, crushed gravel, crushed stone, crushed slag or a mix of them. The material is graded, meaning sized from coarse to fine in set proportions, and has to pass four checks from Table 400-2:

• Aggregate impact value no more than 40%.
• Liquid limit no more than 25.
• Plasticity index no more than 6, so very little clay.
• CBR of at least 30% at 98% dry density, unless the contract says otherwise.

If the stone soaks up more than 2% of its weight in water, it has to pass a wet impact test (IS 5640) as well. Kankar, brick ballast and laterite always take the wet test, because soft stone can look fine dry and crumble when soaked. Two of the gradings in the table, V and VI, are open enough to drain water and are used as a combined sub-base and drainage layer. Where the soil below is soft and wet, a fabric is often laid under the GSB to keep the two apart; see geotextile and geogrid in road construction.

How strong does road aggregate have to be?

Stronger the higher it sits, because the top layers take the tyres directly. Four tests decide it, and they are worth knowing by name because they appear on every lab report a contractor receives.

The Los Angeles abrasion test tumbles a sample of stone with steel balls in a rotating drum and measures how much of it wears away. The aggregate impact value drops a heavy hammer onto a cup of stone and measures how much breaks into fine pieces. In both tests a lower figure means tougher stone. The flakiness and elongation indices measure the share of flat chips and long, thin pieces, which snap under a roller; the two figures are added and the total is capped. Water absorption shows how porous the stone is.

For WMM, DBM and BC the stone has to pass either the abrasion test or the impact test, not both. The WMM figures match IRC:109-2015 and the DBM and BC figures match IRC:111-2009, the IRC’s own codes for those layers.

The impact limit tightens as the layers rise

Each bar is the largest share of stone that may break up in the aggregate impact test, so a shorter bar means a stricter rule. The surface layer (BC) allows 24%, the dense bituminous macadam (DBM) under it 27%, the wet mix macadam (WMM) 30% and the granular sub-base (GSB) 40%. Source: MoRTH Specifications for Road and Bridge Works, Fifth Revision, Tables 400-2, 400-12, 500-8 and 500-16.

The bituminous layers have three more checks. Soundness, which soaks the stone in a sulphate solution for five cycles, allows a loss of no more than 12% with sodium sulphate or 18% with magnesium sulphate. The bitumen coating must stay on at least 95% of the stone surface in the stripping test (IS 6241). And for the wearing course, IRC:111 asks for a polished stone value of at least 55, a measure of how well the stone keeps its grip on tyres after years of traffic polish it smooth.

What are stone dust and filler used for?

They fill the gaps between the larger stones. Fine aggregate for bituminous mixes is crushed or natural material that passes a 2.36 mm sieve and stays on a 75 micron sieve, and it must be clean and free of clay and organic matter. The MoRTH clause for bituminous macadam bars natural sand from the binder course.

Filler is the finest part of the mix. MoRTH allows rock dust, hydrated lime or cement, all of it passing a 0.6 mm sieve. Rock dust used as filler must have a plasticity index of 4 or less. Hydrated lime also does a second job: if the stone fails the water sensitivity test, which checks whether water can strip the bitumen off, 2% hydrated lime by weight of aggregate goes into the mix. The other fix the book allows is an anti-stripping agent in the bitumen.

Concrete roads use natural sand, crushed stone sand or a blend, to IS 383, with a sand equivalent value of at least 50. That test shows how much clay and fine dust is mixed into the sand. How crusher dust and manufactured sand compare is set out in our guide to quarry dust and M-sand.

Which bitumen is used in road construction?

Paving bitumen to IS 73, sold by viscosity grade (VG-10, VG-20, VG-30 and VG-40). MoRTH leaves the choice of grade to climate and traffic, with a selection table in Section 500. The grades, the mixing and rolling temperatures, and where bitumen emulsion is used instead of hot bitumen are all in our guide to bitumen grades in road construction. The refineries and plants that supply it are listed in our post on bitumen and road material companies in India.

What materials does a concrete road need?

A concrete road, which the specification calls pavement quality concrete (PQC), needs cement, coarse and fine aggregate, water, steel bars at the joints and, often, a chemical admixture. The rules in MoRTH clause 602 are tighter than for ordinary building concrete.

Cement. Any of four types may be used: OPC 43 grade (IS 8112), OPC 53 grade (IS 12269), Portland slag cement (IS 455) or Portland pozzolana cement (IS 1489 Part 1), with a preference for 43 grade or higher. Where the soil around the road carries more than 0.5% soluble sulphates, the cement has to be sulphate resisting (IS 12330). Fly ash can replace up to 20% of the cementitious material, but only where a batching plant blends it automatically; mixing by hand on site is not allowed. Factory-made PPC used here must also hold no more than 20% fly ash. Our guide to the best cement in India explains the types.

Aggregate. Stone to IS 383 with a Los Angeles abrasion value of 35% or less, no piece larger than 31.5 mm, water absorption no more than 2% and the same 35% cap on flaky and elongated pieces. Chlorides must stay under 0.06% and sulphates under 0.25% by weight. The stone must also be free of reactive silica such as chert and flint, which can react with the alkalis in cement and crack the slab from inside.

Steel. Dowel bars, which carry load across the joints, are plain bars to IS 432 Grade I. Tie bars, which hold neighbouring slabs together, are Fe 500 deformed bars to IS 1786 or IS 432 Grade I plain bars. All of it is epoxy-coated against rust. For what Fe 500 means, see our guide to TMT bar grades.

Admixtures and fibres. Admixtures to IS 9103 may be used to improve workability or delay setting, once trial mixes prove they do no harm; our guide to concrete admixture types covers them. Silica fume can be added at 3 to 10% of the cement. Synthetic fibres, where the design calls for them, go in at 0.6 to 2.0 kg per cubic metre.

The slab sits on a dry lean concrete base, explained in our DLC guide, and the slab thickness, concrete grade and joints are covered in our guide to CC road construction.

Where do road materials come from, and how are they bought?

Stone comes from quarries and crushers, usually the nearest approved source, because every extra kilometre of haulage adds to what crushed stone costs at site. Soil comes from borrow areas or from cutting along the road itself, and MoRTH asks the contractor to use the best soil it finds for the subgrade and keep the rest for the embankment below. Where the engineer allows borrow pits beside the road, they may be no deeper than 1.5 m, and none may be dug within 10 m of the embankment. Soil and stone are minor minerals, so the state charges a royalty on them; how that works is explained in our post on royalty on earthwork.

Bitumen comes from oil refineries, cement from cement plants and fly ash from thermal power stations. All of them are bought by the tonne, and stone and sand by the tonne or cubic metre. On a government job the rates come from the state’s schedule of rates, so the same material can cost very differently in two districts. That is why this page gives no rupee figures. For tax on these materials see GST on building materials, and for how material, labour and machine costs add up over a kilometre see road construction cost per km.

Every one of these materials is moved and compacted by machines. A wheel loader such as the Volvo L120H loads stone into crushers and mixing plants. A motor grader such as the Sany STG170C-10 spreads and levels the granular layers. A soil compactor such as the Escorts EC 5250 compacts soil, GSB and WMM, and a tandem roller such as the HAMM HD 99i VV finishes the bituminous layers. Compare live soil compactors, tandem rollers and motor graders and connect with a dealer in your state.

The tests on this page are run on samples from each source before the source is approved. For a concrete road, MoRTH asks for the source and its test data 45 days before the trial length is laid, so a quarry that fails is changed before any layer goes down. The full sequence of road topics is collected in the road construction guide.

The two specifications quoted most on this page are public: the MoRTH Specifications for Road and Bridge Works, Fifth Revision (clauses 305, 401, 406, 505, 507 and 602) and IRC:111-2009, Specifications for Dense Graded Bituminous Mixes.

Specification figures on this page are quoted from the MoRTH Specifications for Road and Bridge Works (Fifth Revision, 2013), IRC:37-2018, IRC:109-2015 and IRC:111-2009, as named beside each figure. Codes are revised, so confirm the edition named in your contract. Machine names are taken from our listings at the time of writing; 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. Material approval and mix design are decisions for the project engineer. DesiMachines is not liable for decisions taken on the basis of information that may have changed after publication.