Types of road asphalt, and why some pavement lets water through

Asphalt that lets water through on purpose is pavement designed not to leave rainwater on the surface. Rain soaks into the gaps between the stones, making splashing less likely, as well as hydroplaning, where tyres ride up on water and the steering stops working. Ordinary road surfaces are dense-graded asphalt, with stones packed tightly together, and rain runs across the surface towards the gutters at the edge.

Enlarged cross-sections of asphalt compared. Left, dense-graded: stones large and small packed with no gaps, and rainwater flows sideways across the surface. Right, porous: gaps between the stones, and rainwater soaks in — Fig. 1 Dense-graded asphalt sheds water across the surface; porous asphalt lets it in

How do asphalt and concrete differ?

Road pavement comes in two main kinds, asphalt and concrete. By road length, concrete makes up only about 4.5% (end of March 2024, calculated from Table 3 of MLIT, Road Statistics Annual Report 2025).

Cross-sections of asphalt and concrete pavement compared. Asphalt pavement has five layers from the top: surface course, binder course, upper base, lower base and subgrade. Concrete pavement has three: concrete slab, base and subgrade, with joints in the slab — Fig. 2 Cross-sections of asphalt and concrete pavement (thicknesses are schematic)

Asphalt pavement sets quickly but is more affected by heat. Concrete pavement takes longer to set but resists wear and is less prone to rutting (dips worn where tyres run) (Source: MLIT road consultation office).

	Asphalt pavement	Concrete pavement
Time to set	Quick	Takes time
Typical design period	10 years	20 years
Main uses	Ordinary roads	Roads with many heavy vehicles, tunnels

The design period is the number of years expected before vehicle loads cause cracking; the table shows the figures the national design standards treat as typical (Source: MLIT, Design standards for road pavement). Where repair work is hard to carry out, as in tunnels, longer-lasting concrete is chosen.

How many layers is a road?

Asphalt pavement has a surface course that vehicles drive on and a binder course beneath it, both made of asphalt, supported below by base courses of crushed stone and similar. At the bottom, the subgrade is the ground the pavement sits on. In the standard cross-section of the national design standards, the surface course is dense-graded asphalt and the binder course is coarse-graded asphalt.

Concrete pavement lays thick concrete slabs on the base, with joints at regular intervals (Fig. 2).

Road asphalt is crushed stone, sand and stone powder bound with asphalt. Its formal name is asphalt mixture, and the industry calls it "gozai". Construction uses hot mix laid warm, and some repair materials can be used at normal temperatures (Source: Japan Asphalt Mixture Association). Of the old asphalt pavement removed in works, 99.5% is recycled (fiscal 2018, MLIT).

What kinds of asphalt are there?

An asphalt mixture's properties change with how stones of different sizes are combined. On roads around you, you mainly see these three.

Dense-graded: Stones large and small packed with no gaps. Used for the surface of ordinary roads
Open-graded: Leaves many gaps. Used for anti-skid and permeable pavement
Porous: Especially many gaps. Used for the surface of drainage pavement and quiet low-noise pavement

The number after a name, as in "dense-graded asphalt mixture (13)", is the size of the largest stone in mm. The F in "13F" means extra stone powder, used mainly in cold regions (Source: Japan Asphalt Mixture Association, Types and features of asphalt mixtures).

What kinds of water-permeable pavement are there?

They split into three by where the water goes: drainage, permeable and water-retaining.

Cross-sections of three kinds of water-permeable asphalt pavement. Drainage pavement: rain soaking into the surface course flows over a waterproof binder course to the gutter. Permeable pavement: rain soaks through to the ground below the pavement. Water-retaining pavement: water held in the gaps of the surface course evaporates — Fig. 3 Drainage, permeable and water-retaining pavements send the water to different places

Drainage pavement: Water that soaks into the surface course flows to the gutter over a waterproof layer beneath it. It also makes traffic quieter.
Permeable pavement: Lets water through to the ground below the pavement, returning rainwater to the soil and reducing sudden runoff.
Water-retaining pavement: As water held in its gaps evaporates, it takes heat with it, lowering summer surface temperatures by about 10 to 20°C (Source: Japan Road Contractors Association, Water-retaining pavement).

Tyres riding up on water is called hydroplaning. Besides preventing it, drainage pavement scatters headlight glare less on wet nights, making the road easier to see (Source: Japan Road Contractors Association, Porous asphalt pavement).

When a carriageway is made water-permeable, the standard right after construction is that at least 1,000 ml (300 ml on some road types) soaks in through a 15 cm circle in 15 seconds (Source: MLIT, Technical standards for pavement structure).

Drainage pavement can, however, deteriorate from the binder course below because of the water soaking in. How water relates to pavement damage is explained in Why do potholes form?

What other pavements are there?

Some pavements are designed for particular local problems such as heat, rutting, darkness or ice (Source: Japan Road Contractors Association).

Heat-shielding pavement: A resin that reflects infrared is coated on the surface, lowering summer daytime surface temperatures by 10°C or more
Semi-flexible pavement: A special cement grout is soaked into open asphalt so ruts form less easily. Used at intersections, bus terminals, toll booths and the like
Light-coloured pavement: Pale stones brighten the surface, making places such as tunnels easier to see
Ice-inhibiting pavement: Anti-icing agents or rubber are mixed in, so slopes and intersections in cold regions freeze less

How to tell them apart in town

Appearance gives only a rough guide. Black with a fine-textured surface is dense-graded; rough with small visible holes is drainage or another open pavement. Whitish with joints at regular intervals is concrete.

They're easier to tell apart in the rain. On sections of drainage pavement, water doesn't pool on the surface as much and cars throw up less spray.

FAQ

Q. Why is the road surface in tunnels whitish?

Because it may be concrete pavement or light-coloured pavement using pale stones. Concrete suits places where repairs are hard to carry out, and light-coloured pavement makes the surface look brighter under the lights.

Q. Why is the carriageway slightly tilted?

To send rainwater to the gutters at the edge of the road. Paved carriageways normally have a sideways tilt (cross slope) of 1.5 to 2% (Road Structure Ordinance Art. 24). On pavement that lets water through, the tilt can be reduced or left out as long as drainage isn't affected.

For road users: read next Why do potholes form? / Yellow lines vs white lines on Japanese roads

For road managers I am developing MICHIKARTE, a service for recording pavement damage with a smartphone and sharing it on a map.

Last checked: September 27, 2026

What types of asphalt are used on roads? And why some pavement lets water through on purpose