Bitumen is a solid at road temperature, which is a problem — you cannot spread a solid over a road surface. Every grade covered in this post is an answer to the same question: how do we make bitumen workable, and which method suits which job? There are three answers. Heat it, dissolve it in a…

Bitumen is graded, bought and specified almost entirely on the basis of test results. This post covers all ten tests — what each measures, the exact test conditions, and the values you need to remember. 1. Viscosity Viscosity is the property of bitumen which resists flow due to internal friction. Absolute or Dynamic Viscosity Measured…

Bitumen is the material that turns a heap of stone into a road. It makes up only a few percent of a bituminous mix by volume, yet without it the aggregate would simply scatter under the first wheel that passed. This post covers where bitumen comes from, the six properties road engineers look for, and…

Aggregate makes up over nine-tenths of a bituminous pavement, so the road is only as good as the stone in it. But “good stone” is not one property — it is seven, and each one guards against a different way the pavement can fail. This post covers all seven, explains what damages the aggregate in…

Ask most people what a road is made of and they will say tar or concrete. Both answers are wrong by volume. A road is overwhelmingly made of stone — the bitumen or cement is only the glue holding it together. This post covers what aggregates are, the binders used with them, and the numbers that show…

Every road, however well built, ultimately rests on soil. Load passes from the tyre through each pavement layer and finally into the ground — and if that ground cannot take it, no thickness of bitumen above will save the road. This post covers subgrade soil: what it is, the properties it must have, how its…

Designing a rotary means fixing about eight quantities — how fast traffic should travel, how tight each curve should be, how long and wide the weaving section must be, and finally how much traffic the whole thing can carry. This post works through all of them in order, with every value and formula you need….

A rotary solves the crossing conflict problem elegantly — but it is not the right answer everywhere. It demands land, it slows everyone down, and it treats pedestrians poorly. This post lists the four advantages and six disadvantages, explains the reasoning behind each, and sets out when a rotary is and is not the right…

A rotary looks like a simple idea — put an island in the middle of a junction and make everyone drive around it. But there is a specific piece of engineering reasoning behind that island, and once you see it, the whole topic falls into place. This post covers what a rotary is, the manoeuvre…