Road Construction Materials: Base Course Aggregates, Bitumen 60/70 and PMB — Quantities per km

Before a road tender is priced, someone has to turn drawings into tonnes: how much aggregate for the base, how much asphalt, how many tonnes of bitumen 60/70 or polymer-modified binder. There is no universal figure, because every road has its own width, pavement design and local density. What follows is a transparent worked estimate for one kilometre with every assumption written down, so a buyer can change the inputs and get a defensible number for his own project.

Road construction materials per km: the worked example

Assume a two-lane road with a paved width of 7.0 m, so one kilometre covers 7,000 m². Assume a flexible pavement consisting of a 4 cm asphalt surface course, an 8 cm asphalt binder course, a 14 cm asphalt base course and a 20 cm unbound crushed-aggregate base course on a prepared subgrade or frost-protection layer. These thicknesses are in the range used by national pavement design catalogues for medium traffic; a motorway or an airport apron is thicker, a rural access road thinner. Assume a compacted asphalt density of 2.4 t/m³ and a compacted unbound aggregate density of 2.2 t/m³, both typical and both to be replaced by the values from the job-mix formula and the field compaction trial.

The asphalt layers together come to roughly 4,360 t per kilometre. At a binder content of 4 to 6 % by mass, that mixture contains in the region of 175 to 260 t of bitumen. Surface courses usually carry the higher binder content and base courses the lower, so a reasonable planning figure for this cross-section is about 200 to 220 t of bitumen per kilometre, of which only the surface and binder courses, roughly 2,000 t of mixture, would normally receive polymer-modified binder if PMB is specified. Shoulders, junctions, waste and the frost-protection layer are extra.

Rule: a quantity per kilometre is only as good as the three numbers behind it: width, layer thickness and compacted density. Write them next to every figure and the estimate survives the first site visit.

Base course aggregates

Unbound base and sub-base layers use crushed rock or gravel graded to a continuous envelope, for example 0/32 or 0/45 mm, specified in Europe under EN 13242 and in the United States by state DOT gradations derived from AASHTO M147. The properties a buyer should see on the test report are the Los Angeles abrasion value (EN 1097-2 or ASTM C131) as a measure of hardness, the flakiness or shape index (EN 933-3), the fines content and plasticity, water absorption and, in frost regions, the magnesium sulfate soundness. Asphalt aggregates fall under EN 13043 and need, in addition, polished stone value for the surface course and affinity to bitumen. A quarry that cannot produce a current declaration of performance under the Construction Products Regulation is not a supplier for a public road in the EU.

Bitumen 60/70 and the European grades

Penetration-grade bitumen is classified by the penetration of a needle at 25 °C in tenths of a millimetre. The 60/70 grade, with penetration between 60 and 70, is the standard paving binder in large parts of the Middle East, Africa and South Asia and is sold against ASTM D946 or national standards. Europe uses EN 12591 grades such as 50/70 and 70/100, with softening point (ring and ball, EN 1427), penetration (EN 1426), flash point, solubility and resistance to hardening (RTFOT, EN 12607-1) as the specified properties. The two systems overlap but the limits are not identical, so a contract should name one standard and one grade and attach the test methods. Bitumen ships in heated bulk tankers, drums or jumbo bags; the delivery form changes the price and the Incoterms 2020 point at which risk passes.

Polymer-modified bitumen

PMB is penetration-grade bitumen modified with elastomers (usually SBS) or plastomers to improve rutting resistance at high temperature and cracking resistance at low temperature. In Europe it is specified under EN 14023 by a code such as PMB 45/80-65, which states the penetration range and the minimum softening point, together with elastic recovery and storage stability. In the United States the Superpave performance grade system (PG 76-22 and similar, AASHTO M320) is used instead. PMB costs more per tonne, needs heated and agitated storage, and should be reserved for the layers that benefit: surface and binder courses on heavy-traffic lanes, bridge decks, junctions and climbing lanes.

Turning the estimate into a purchase

1. Get the pavement design and the cross-sections from the engineer; do not derive thicknesses from this or any other generic table.
2. Replace the assumed densities with the job-mix formula and the trial section results.
3. Add shoulders, junctions, bus bays and a waste allowance; a few percent on asphalt and more on unbound layers is normal.
4. Specify aggregates, bitumen and PMB by standard, grade and test method, and require a certificate of analysis for each bitumen delivery.
5. Phase deliveries to the paving programme; hot-stored bitumen ages and stockpiles segregate.

The arithmetic takes ten minutes. The value lies in writing the assumptions down, because those are what the supplier, the engineer and the bank will challenge.