The present invention relates to a method of enhancing the frictional level of a paved surface such as a highway or runway. In particular, the method enables the lifetime of a pavement to be extended before complete resurfacing is required.

In Annex 14 to the Convention on International Civil Aviation, the International Civil Aviation Organisation (ICAO) sets out the principles that cover the provision of paved runway surfaces with acceptable friction characteristics. In particular, Contracting States to the Convention on International Civil Aviation are required in Annex 14 to undertake friction testing "periodically in order to identify runways with low friction when wet" and also to define and publish in their Aeronautical Information

Publication (AIP) the Minimum Friction Level (MFL) which will require Notice to Airmen (NOTAM) advice, if reached for any given runway. States must also establish a 'Maintenance Planning Level' (MPL) of runway friction below which prompt corrective action is required.

Friction is expressed as the coefficient of friction; this is the ratio of the friction force (F) between two surfaces in contact and the normal force (N) which exists between the object resting on the surface and the surface i.e. F/N. This ratio is particularly, but not exclusively dependent, upon:

The physical characteristics of the two surfaces.

The prevailing temperature at the point of contact.

The speed of movement of the object (the tyre) over the surface.

The degree of surface friction for a specific aircraft at a given moment is directly proportional to the braking action, subject only to the activation of wheel lock-up and anti-skid protection systems, which most modern transport aircraft have. The precise texture of a pavement has a considerable effect upon friction, especially when the surface is wet.

The most important aspect of a pavement surface relative to its friction

characteristics is the surface texture. The effect of surface material on the tyre-to- ground coefficient of friction arises principally from differences in surface texture. Surfaces are normally designed with sufficient macrotexture to obtain a suitable water drainage rate in the tyre/road interface. The texture is obtained by suitable proportioning of the aggregate/mortar mix or by surface finishing techniques.

Pavement surface texture is expressed in terms of macrotexture and microtexture. However, these are defined differently depending on the context and measuring technique in which the terms are used.

Texture is defined internationally through ISO standards which refer to texture measured by volume or by profile and is expressed as Mean Texture Depth (MTD) or Mean Profile Depth (MPD). These standards define microtexture to be below 0.5 MPD and macrotexture to be above 0.5 MPD. There is no universally agreed relationship between MTD and MPD. Macrotexture is the texture between the individual stones and allows water to escape from beneath aircraft tyres. The scale of texture may be judged approximately by the eye. Macrotexture is primarily created by the size of aggregate used or by treatment of the surface. Macrotexture is the major factor influencing the tyre/ground interface drainage capacity at high speeds.

As macrotexture affects the high speed tyre braking characteristics, it is of most interest when looking at runway characteristics for friction when wet. Simply put, a rough macrotexture surface will be capable of a greater tyre to ground friction when wet than a smoother macrotexture surface.

Surfaces are normally designed with a sufficient macrotexture to obtain suitable water drainage in the tyre/pavement interface.

Microtexture is the texture of the individual stones and is detectable by touch rather than appearance. It allows the tyre to break through the residual water film that remains when the bulk of water has run off and is especially important at low speeds. On a wet surface at higher speeds, a water film may prevent direct contact between the surface asperities and the tire due to lack of drainage from the tire-to-ground contact area. Microtexture is a built-in quality of the pavement surface. By specifying crushed material that will withstand polishing, microtexture and drainage of thin water films are ensured for a longer period of time. Resistance against polishing is expressed through the polished stone values, which are in principle a value obtained from friction measurement in accordance with international standards (ASTM D 3319, CEN EN 1097-8).

A major problem with microtexture is that it can change within short time periods without being easily detected. A typical example of this is the accumulation of rubber deposits in the touchdown area which will largely mask microtexture without necessarily reducing macrotexture. Microtexture may also be lost when exposed to mechanical wear of the aggregate. Devices which detect surface friction are termed 'Continuous Friction Measuring Equipment' (CFME). Their primary application is the determination of reference friction levels on dry and artificially wetted surfaces. The latter requirement needs a controllable self-wetting capability which can deliver a water depth of 0.5mm-1 mm. These reference friction measurements allow airport operators to ensure that the range of surface frictions encountered operationally on un-contaminated runways remain acceptable most of the time.

There are currently at least eight different types of CFME, of which the 'Grip Tester' and 'Mu Meter' are in widespread use. Usually, CFME is towed behind a vehicle at a constant speed and a wheel fitted with a smooth tyre is fitted with equipment which directly measures the friction encountered. Measurements are typically output to an on board processor which, when downloaded, produce tabulations and charts showing the friction level detected. The MFL measured by a Mu-meter is 0.50 and 0.55 when measured by a Grip Tester.

Improvement of skid-resistance for pavement surfaces may be obtained by surface dressing using high quality crushed aggregates and modified polymer binder for better adhesion of granularities on the surface and for minimising loose aggregates. Over time and use of a paved surface, the macrotexture erodes as the bond between the aggregate and the underlying binding agent weakens. On a runway, lose aggregate is a severe hazard as it presents not only a skid risk but the possibility of engine damage through ingestion, as well as damage and breakage to windows, runway lights and aircraft and vehicle parts as a result of airborne aggregate. Where the surface has reached the end of its life, re-surfacing is required. However, there are commercially available products that enable the life of the surface to be extended before a full re-surface is required.

An example of an asphalt preservative is Rhinophalt™ sold by ASI Solutions Ltd. Rhinophalt™ extends the life of asphalt and macadam by sealing and protecting the surface course from weathering, oxidisation and traffic use, by halting the

deterioration of the bituminous binder in the surface. Rhinophalt™ is a cold, spray applied material which is formulated chemically to penetrate and integrate with the bituminous binder in the surface course. As such Rhinophalt™ does not create an additional coating on top of the surface but coats the aggregate. Rhinophalt™ has been successfully applied to commercial airport runways and is known to be effective in extending the life of a runway, in particular by improving stone retention within the asphalt.

The drawback of any asphalt preservative is that a residual layer is necessarily applied to the top surface of the aggregate. As a result, the aggregate loses its microtexture which, in turn, reduces the friction coefficient of the surface. As a result there is a need to remove the residual layer and regain the aggregate microtexture to provide a treated surface with a friction coefficient that is above the required MFL. Indeed, application of Rhinophalt™ to the runway at Manston International Airport in Kent, U.K. saw the MFL readings on the treated surface drop as low as 0.32. Various small road sweepers were used to roughen the surface but insufficient improvements were seen with readings only rising to an average of 0.53, with the Airport engineer requesting 0.63 as a minimum MFL, an MPL of 0.57 as recommended in CAP683 and a Design Objective Level (DOL) of 0.72 or greater. It was then appreciated that a method was required to remove the residual layer of asphalt preservative from the upper surface of the aggregate, thereby leaving the asphalt preservative around and underneath the aggregate to bind the aggregate in place. As a result, the method of the present invention was devised.

Specifically, the present invention resides in a method to improve the friction level of a paved surface. The method comprises brushing the surface with one or more brushes rotating at a speed of between 400 and 900 revolutions per minute travelling over the surface at a speed of between 4 and 12 kilometres per hour. The

combination of brush revolutions and speed of travel of the one or more brushes has been found to provide the optimum condition to remove residue such as asphalt preservative from the upper surface of aggregate and to impart suitable friction qualities to the surface. Preferably, the one or more brushes rotate at a speed of between 520 and 700 revolutions per minute.

In a particular embodiment, the brushes travel over the surface at a speed of 8kph. Ideally, the one or more brushes have metal bristles or brush filaments, for example steel wire filaments. A suitable diameter for the wire filaments is between 0.3 and 0.6mm, preferably 0.45mm. This diameter provides the wire filaments with sufficient strength to cause the required abrasion as well as flexibility that merely "flicks" the upper surface of the aggregate rather than lifting it out from the binding agent.

Indeed, it is believed that the make-up of the one or more brushes, combined with the speed of travel and rotation, roughen the upper surface of the aggregate, enhancing the frictional qualities to the surface yet further.

In one embodiment, the bristles are arranged in groups of tufts. It will be appreciated that any number of groups of tufts may be organised over the area of the brush.

In one particular example, a Sicard™ snow clearing machine was either towed behind or fitted to the front of a tractor unit. The machine had an array of metal brushes covering a length of 4 metres, with each brush having a diameter of approximately 1 15cm. After brushing of an area of 98,928 square metres, friction readings ranging between 0.72 and 0.74, averaging 0.72, were achieved.

Thus, after pre-treatment with an asphalt preservative, the method removed the asphalt preservative from the top of the aggregate to expose the stone texture thereby giving a much improved friction reading.

It will be appreciated that the method may be used on an untreated surface to improve the friction either by exposing the aggregate texture or by imparting texture and roughness to the aggregate.

While the invention has been described with reference to runways, it will be appreciated that the method is equally applicable to highways and other surfaces such as running tracks and non-skid surfaces, where a degree of surface friction is required, desired and/or important.