What is rutting? Causes, types and how to fix ruts in the road

Rutting is permanent deformation of a road surface in the wheel paths: two long, parallel grooves (ruts) pressed into the asphalt where most tyres run. Ruts build up gradually under repeated heavy axle loads, faster in hot weather, in a mix that is too soft or on a weak foundation. Rainwater collects in them, so aquaplaning is the main danger.

This guide explains what ruts in the road are, the three types of road rutting, how rut depth is measured and graded, and which repair suits which type, from milling and a new surface course to fibre reinforcement.

What does rutting mean? Ruts in the road explained

In road engineering, rutting means permanent (plastic) deformation in the transverse profile of the carriageway, concentrated in the wheel paths. Pavement engineers define it as a longitudinal depression in the road surface that remains after the traffic has passed, and it is typical of flexible (asphalt) pavements.

Asphalt is not a rigid material. The binder, bitumen, is thermoplastic and behaves viscously under sustained loading: every axle load that passes presses the mixture in and sideways by a minuscule fraction. One lorry makes no difference; tens of thousands of axle loads per year on exactly the same strip do. The sum of all those small, permanent deformations becomes visible as two parallel grooves in the road surface, the wheel tracks. That is why engineers speak of permanent (plastic) deformation, as opposed to elastic deformation that springs back after the tyre has passed.

The location is characteristic: precisely in the left and right wheel track of the traffic lane. The grooves are often most clearly visible after a rain shower, when the water sits in them as two glistening ribbons. Alongside the deepened tracks, the mixture may bulge up slightly: the displaced material has to go somewhere, after all.

A few terms are often used interchangeably. A rut is a single groove in the road; wheel ruts are the pair of grooves in the left and right wheel path of a lane; rutting is the damage pattern as a whole; and a rutted road is a road on which those ruts have become pronounced. The same words are used for unpaved and gravel roads, where ruts form because wheels push aside and compact the soil and gravel rather than deform an asphalt layer. A two-rut road is a simple track of that kind: two wheel ruts with a strip of grass or gravel in between.

Rutting is also different from other road damage. Potholes are local holes where the surface has broken up; ravelling is the loss of stones from the surface course; shoving and corrugation are bulges and ripples across the lane, typically where traffic brakes and accelerates; cracking is a fracture of the asphalt layer. Rutting runs lengthways along the wheel paths and is above all a change of shape, although cracks and ravelling often follow at the rut edges.

Why are ruts in the road dangerous?

• Aquaplaning: the greatest risk. A film of water sits in the ruts and cannot drain to the road edge. At higher speeds, tyres lose contact with the road surface on it, precisely on the strip where everyone drives. The risk grows with rut depth and with speed, and it is greatest when one wheel track holds water and the other does not: the uneven drag on the tyres can pull the vehicle to one side.

• Steering behaviour: deep ruts "pull" at the tyres. Especially when changing lanes or swerving, the car feels unsettled; motorcyclists and cyclists can be thrown off balance by the rut edge.

• Winter ice: water standing in the ruts freezes sooner than a well-draining surface, resulting in localised icing.

• Accelerated consequential damage: a deformed transverse profile loads the surface course unevenly, so cracking and ravelling at the rut edges occur faster and the maintenance cycle shortens.

For road authorities, rutting is therefore not just a comfort issue but above all a road safety issue, and a fixed part of the annual road inspection.

Driving on a road with ruts

On a road with ruts, adjust your speed, keep extra distance and, in rain, drive just outside the ruts where possible. Avoid abrupt steering or braking, and take extra care on a motorcycle or bicycle near the rut edge, where the surface drops away. The Dutch driving theory exam asks exactly this question, and the advice applies on any rutted road.

The rest of this article covers the other side of the same question: why ruts develop and how road authorities and contractors prevent them.

What causes rutting? The four causes

Rutting of roads rarely has a single cause. In practice the damage arises from a combination of four factors, and the effective solution depends on which factor dominates.

1. Heavy and channelised traffic

The driver behind every form of rutting is repeated axle loading. Lorry traffic weighs in disproportionately: the deforming effect of an axle load increases more than proportionally with weight, so a single heavy lorry axle loads the road surface many times more than thousands of cars. If that heavy traffic also runs in exactly the same track every time (channelised traffic, as on a bus lane or an inside lane full of lorries), all the loading concentrates on two narrow strips and rutting accelerates considerably.

Speed matters as well. Slow-moving and stationary heavy vehicles, at junctions, bus stops and loading bays, load the mix longest: the longer a wheel rests on the same spot, the more time the viscous bitumen has to give way.

2. Heat: a soft road surface deforms faster

Bitumen loses stiffness as the temperature rises. The softening point of standard road bitumen lies around 45–55 °C, and that level is genuinely reached in the Netherlands: at 30 °C air temperature, black asphalt heats up to around 50–60 °C at the surface. Under those conditions the same traffic presses the softened mixture in noticeably faster. Hot summers are therefore the period in which rutting grows hardest. How temperature, soft asphalt and fibres interrelate exactly is covered in our article on heat, road surface temperature and rutting.

3. The mixture: binder, stone skeleton and compaction

Not every asphalt mixture is equally resistant to permanent deformation. Pavement references name the classic mix-design risk factors: a binder that is too soft or too generously dosed, excess mineral filler, too few crushed (angular) aggregate particles, for example round natural sand instead of crushed material, and air voids outside the design range. A weak stone skeleton, in which the stones do not interlock sufficiently, leaves the load to the softer mortar.

Execution counts too: a surface course insufficiently compacted at installation is "re-rolled" by traffic in the first years, with an early rut as the result. This is densification rutting, one of the types described below. Mixtures with a strong stone skeleton, such as stone mastic asphalt (SMA), are designed precisely to transfer the loading through stone-on-stone contact rather than through the mortar.

4. The subgrade and foundation

Rutting does not have to originate in the asphalt layers themselves. If the foundation or the (in the Netherlands often soft) subgrade sinks away beneath the wheel track, the whole package sinks with it. This structural type of rut can be recognised by its shape: a wide, gently sloping trough across the full width of the wheel track, without bulging alongside. A narrow, sharply defined rut with raised edges points instead to deformation in the upper or surface course. That distinction is essential for the approach: a surface-course problem can be fixed by milling and a new, deformation-resistant surface course; a foundation problem cannot.

Types of rutting in asphalt: mix rutting, structural rutting and densification

Road rutting is classified by the layer in which the deformation sits, and that classification decides the repair. Pavement engineering distinguishes three main types: mix rutting (plastic flow in the asphalt layers themselves), structural or subgrade rutting (the foundation, subbase or subgrade gives way) and densification (further compaction of asphalt that was not fully compacted at laying). A fourth type, wear rutting, is caused by abrasion from studded winter tyres and mainly occurs where those tyres are common, for example in the Nordic countries.

On site, the shape usually tells them apart, as described under the subgrade cause above. Raised ridges along the rut edge point to mix rutting: surface rutting in the upper asphalt layers, where the displaced material has been pushed sideways. A wide trough without ridges, often with cracking, points to a structural problem lower down. A uniform depression in the wheel paths early in the service life, with little or no bulging, points to densification. Wheel-path rutting from studded tyres shows as worn, roughened tracks without deformation of the layers below.

In road construction, the rutting resistance of an asphalt mix is checked at the design stage with the wheel-tracking test (EN 12697-22): a loaded wheel runs repeatedly over a specimen at a set temperature, and the rut depth is recorded.

Where does rutting appear first?

Rutting on roads is rarely spread evenly across a network. It concentrates where heavy traffic is slow, stationary or channelised:

• Roundabouts: the combination of slow-moving heavy traffic and twisting, shoving forces in the curve makes roundabouts the classic rutting location. Fatting-up and ravelling also occur faster here.

• Bus lanes: buses are heavy and run almost perfectly channelised in the same track; at stops, braking and pulling away are added on the same few square metres time and again. The approach for local authorities is covered in our article on fibre reinforcement for bus lanes and municipal roads.

• Junctions and traffic lights: stationary and slowly accelerating heavy traffic loads the mixture for long periods in one spot, exactly the loading pattern bitumen copes with worst.

• Motorways and trunk roads: mainly the inside lane with concentrated lorry traffic; on the outside lane rutting is rare.

• Business parks and loading bays: heavily laden axles standing still for long periods at high summer temperatures produce the same effect as a junction, but daily. More on rutting in car parks and loading areas.

• Construction roads: heavy site traffic on a temporary pavement ruts quickly; the options are set out under fibre-reinforced construction roads.

How is rutting measured? Rut depth and severity levels

The measure is rut depth: the depth of the groove relative to the original transverse profile. Classically it is measured with a straightedge laid across the traffic lane and a wedge or dial gauge at the deepest point. At network level it is nowadays done on the move: survey vehicles with laser profilometers record the transverse profile across entire road sections and deliver rut depths per hectometre.

In the UK the same principle applies: on English local roads, SCANNER survey vehicles record rut depth as one of the parameters in the road condition indicator.

In the Dutch methodology for rational road management (CROW), rutting is one of the fixed damage patterns assessed during periodic inspection. The measured rut depth, together with road type and speed regime, determines when a measure is needed: what is acceptable on a 60 km/h access road is a safety risk on a 100 km/h through road because of aquaplaning. The rut depth measurement thus feeds directly into the multi-year maintenance plan; more on that in our article on asphalt maintenance and extending service life.

How deep is a deep rut? Severity classes

To grade severity, many pavement condition surveys use the three classes of the Pavement Condition Index (PCI) method, based on the mean rut depth (see the table below). The classes describe the condition of the road, not a legal limit. Very shallow ruts, up to about 8 mm (1/3 inch), are often left untreated according to US pavement guidance.

There is no single universal intervention level: road authorities set it by road type and speed, because the aquaplaning risk rises with speed. Whichever classification is used, the measured depth and its growth over successive surveys show whether a section can wait or belongs in next year's maintenance programme.

How to fix rutting: repair options by rut type

The fix depends on the type of rutting and the layer in which it sits (see the types table above). Shallow ruts are monitored. Filling ruts with a thin levelling or surfacing layer restores the transverse profile, but only buys time while the cause is still active. Mix rutting is solved by milling out the deformed layer and laying a new, deformation-resistant surface course; structural rutting only by reconstruction including the foundation. Prevention works through the choice of mixture and, where the loading is heaviest, reinforcement. In practice that gives three routes, from preventive to curative.

Mixture choice and binder

The first line of defence is a deformation-resistant mix design: a strong stone skeleton (SMA or stone-skeleton mixtures), crushed aggregate and a binder that suits the loading. Whether a mix really resists deformation is verified in the laboratory with the wheel-tracking test before it goes on the road. On heavily loaded locations, polymer modified bitumen (PMB) has traditionally been specified, remaining stiff and elastic even at higher temperatures. PMB has drawbacks, however: a higher production temperature, more difficult recycling and a higher environmental impact. The trade-off is set out in our explanation of what fibre-reinforced asphalt is and in the comparison of aramid fibres vs PMB.

Fibre reinforcement: holding the mixture in place mechanically

Fibre reinforcement tackles rutting through a different mechanism from PMB: not a tougher binder, but a three-dimensional network of strong fibres that spreads the load of each passing axle over a larger volume and so counters local, permanent deformation. The fibres retain their strength even when the bitumen softens on a hot day: aramid fibres have a melting point above 450 °C and a tensile strength of 3,000 MPa.

The dosage is strikingly small: around 0.05%, or roughly 500 grams of aramid fibres against rutting per tonne of asphalt, depending on mixture and application, without any change to the asphalt mix design or the equipment at the plant. AsphaltX is a blend of Twaron aramid and polyolefin fibres of about 19 mm, suitable for porous asphalt, SMA and dense asphalt, and it is included on the OPWA list.

The proof comes from practice: in the European FIBRA research on the A73 near Roermond, the aramid mixture met all Dutch requirements at a 15–20 °C lower production temperature than the PMB reference; the full results are in our case study of the A73 trial section. Schagen Infra chose fibre reinforcement for the most rutting-prone location there is, a roundabout with heavy, twisting traffic: in the renovation of the N711 roundabout in Flevoland, 1,585 tonnes of asphalt were reinforced with 400 kg of AsphaltX, with an MKI score of €8.50 and around 70% lower environmental impact as an added result.

More background on the working principle, mixture types (porous asphalt, SMA, dense asphalt) and tendering can be found on our pillar page on fibre-reinforced asphalt.

Milling, a new surface course or reconstruction

Once the rut is there, superficially "filling it up" is rarely durable: the cause sits in the layer beneath. For rutting in the surface course, the standard measure is milling and applying a new, deformation-resistant surface course. That is also the natural moment to lay the new layer fibre-reinforced, at limited additional cost within the major maintenance already planned. If the deformation sits deeper (foundation or subgrade), only a partial or full reconstruction of the road structure helps, including tackling the foundation layer.

At extreme locations such as bus stops, some road authorities switch to a concrete surface altogether; see our article on white topping with fibre concrete.

Preventing rutting on your next project

For road authorities and contractors the practical conclusion is: fight rutting at the moment a new layer is coming anyway. On risk locations (roundabouts, bus lanes, junctions, heavily loaded lanes), choose a deformation-resistant mixture and consider fibre reinforcement as a recyclable alternative to or complement of PMB. The dosage of 500 grams per tonne requires no change to mix design or execution, and the performance has been demonstrated in practice on the A73, the N711 and the provincial road N337, where monitoring showed 50% less rutting and 30% less cracking than on the reference section.

Want to know what fibre reinforcement means for your road section, roundabout or business park? Request a quotation or put your situation to us via contact: we will think along on mixture type, dosage and the substantiation towards your client or specification.