How do roads respond to hot weather?

In very hot weather, road surfaces can become warm, tacky, and soft. HGVs and heavier loads, particularly those concentrated over specific parts of the lorry (think steel coils, for example), press into the sticky surface, creating pressure.

Most British roads (around 96%) are made from tarmac, also known as asphalt or bitumen. The remaining 4% tend to be concrete, although these are rarer, and nearly all motorways or A-roads you drive on will be asphalt.

When the weather is hot, the roadway can experience ridges and large bumps, which can develop into dangerously deep potholes, traffic risks and obstructions.

In this guide, Henry Williams explains the cause of poor road surface conditions in hot weather, the benefits of asphalt over other alternatives, and the preventative steps your contractor can take to stop roads from responding adversely to hot weather.

Why Do Roads Become Damaged in Hot Weather?

Tarmac is normally stable, firm, and solid, but during a heatwave, which has become increasingly normal for British summers, temperatures of 30°C or above can mean the road becomes soft.

It is important to remember that a black road surface will normally be much hotter than the air around it. On a typical warm day where the temperatures are in the mid-20s, the road can be as hot as 50°C. Black absorbs heat, and the road will be hottest in the afternoon when it has been soaking up the direct sunlight for several hours.

The bitumen in the tarmac mix begins to rise to the top of the road surface as it softens, which is why the road becomes sticky.

Of course, some roads are made from alternative types of tarmac, and older sections of the M25 are still concrete – although when the surface degrades, it will be replaced with tarmac.

The Impact of Melting Road Surfacing

In most cases, a slightly tacky road surface will be a minor nuisance, but if left to become very soft and pliable, the effects can be more serious.

Softer roads make braking and steering more difficult and pose a particular safety risk to motorbikes. Only the very upper surface course of the road melts, around three to five cm deep, so the deeper road surface remains stable – the primary issue is that the road surface may not be suitable for drivers.

Bitumen can be removed from tyres with a soapy solution, but in higher-risk scenarios, such as an airport runway, it may be considered too dangerous to remain in use.

Deterioration on runways, and sections of the surface that lift after experiencing the pressure of an aeroplane wheel, must be repaired quickly before the runway can be reinstated.

Drivers should be advised if the roadway conditions are sub-optimal and drive at slower speeds, with gradual braking and steering. If the tyres become completely coated in tarmac, which then sets, they may require replacement.

Softened roads are more likely to occur on rural and less heavily used routes. Motorways and those with over 1,200 daily vehicle users are normally built with three layers, comprising a surface course, binder course and base layer, in total around 17 to 22 cm thick.

Country roads are typically built with two tarmac layers and without harder-grade bitumen, so they are more exposed to melting and softening in the summer.

The Pros and Cons of Tarmac Roadway Surfacing

Concrete was used to build many of the major motorway networks, and the fact that some stretches remain standing demonstrates the longevity of this multi-purpose industrial material. The problem is that concrete is more brittle than tarmac.

While tarmac can become warm and soft in extreme heat, concrete cracks under pressure and can fracture during especially hot spells. Loose chunks of concrete, cracks in the road and surface debris can be far more hazardous to vehicles than a sticky road surface.

A cracked concrete road will inevitably worsen, where rainwater permeates through the cracks, freezing in the winter and causing the road to lift upwards, creating significant damage.

Why do we predominantly use tarmac for road surfaces, both on public highways and private access routes or industrial estates? It has several advantages over other alternatives:

• Tarmac provides skid resistance and reduces water spray in wet weather.
• Smooth, even tarmac reduces surface noise.
• Most tarmac mixtures are designed to last for 40 years or more.
• The material is low-cost and efficient to lay, minimising disruption to road users and providing cost efficiencies.

Sustainable tarmac production is an increasing focus, where the materials can be recycled and reused in laying new road surfaces, providing an environmental benefit alongside safety advantages.

Downsides include susceptibility to damage from very high temperatures and prolonged exposure to UV light. However, with regular sealants (normally applied every three years) and pothole repairs, a tarmac road is one of the best solutions.

How to Prevent Tarmac Roads from Degrading in Warm Weather

One of the quickest and simplest solutions to protect road surfaces when hot temperatures are forecast is to use a granite dust mixture, similar to the gritting process used to lay salt during icy conditions.

The difference is that granite dust can absorb the softer bitumen which rises to the surface of the tarmac, reducing the sticky quality of the carriageway and ensuring stable driving conditions.

Materials such as sand can also be used, which do a comparable job of absorbing excess tar and stopping the road from melting. Roads without shade and that are south-facing are most likely to require treatment.

Laying dust is an effective way to maintain safe roadways even when high temperatures are expected over a prolonged period. Some modern carriageways are also spread with modified tarmacs, which, although more expensive, are ideal for roads with heavy traffic or a large proportion of HGVs.

For example, industrial parks or commercial estates may benefit from a tarmac road surface engineered with polymer-modified binders, laid within hot-rolled asphalt, which increases the point at which the roadway will soften to roughly 80°C.

Other options include surface systems, which contain the same polymer-modified binders and can protect older roadways from deteriorating.