Road Surfacing Composition and Process

DESCRIPTION:

Field of the Invention

The invention relates to novel road surfacing compositions incorporating rubber crumb, and to processes in which they are used to surface or resurface roads. The term "roads" is used in this Specification to include highways, tracks, bridges, car parks and any other ground areas which may be subjected to vehicular or other heavy use traffic. The compositions of the invention can also be used to cover concrete paving slabs, and the invention therefore also relates to a process for producing the surface-covered slabs and to the resulting covered slabs.

Background Art

Asphalt is the most widely used modern surfacing composition for roads. In its simplest form, asphalt consists of stone chippings bound together by bitumen. The size of the chippings can be varied, down to a coarse sand, to control the surface texture of the laid asphalt. The aggregate, whether it is stone or sand, is preheated in an asphalt plant, mixed thoroughly with molten bitumen, and then transported to site where it is spread and rolled. From the asphalt plant, the hot asphalt generally has a workable life of from six to ten hours before it becomes too cool and stiff to lay.

Modified asphalts have been proposed in which some of the aggregate is replaced by rubber crumb obtained from recycled comminuted automobile tyres. Other modified asphalts have been proposed in which the softening temperature of the bitumen binder has been modified. So- -called "polymer modified asphalts" utilize bitumen which has had blended therein an appropriate amount of a

polymer, which is generally an ethylene vinyl acetate copoly er (EVA) or a styrene butadiene styrene copolymer (SBS). So-called "rubberized asphalt" has a proportion of rubber dissolved in the bitumen. That increases the softening point of the bitumen as well as acting as a filler. The product is, however, very expensive.

All of the above asphalts and modified asphalts employ bitumen. Disadvantages associated with the use of bitumen are that the bitumen contains aromatic oils, which are known carcinogens; and that the asphalt softens in hot weather with a resulting tendency to lose the accuracy of an initially level surface. Also there is a loss of traction or "grip" in hot weather. In cold weather conditions asphalt hardens to an extent such that it may become brittle. In cold weather it also loses its water repellency, and so offers little resistance to ice formation. In all temperature conditions, asphalt affords relatively little adhesion to the sub-base, and is prone to so-called reflective cracking when subject to traffic loads, and thermal base movement. The need for repairs or resurfacing is therefore frequent.

The laying of asphalt must be completed while the bitumen is soft enough for the asphalt to be workable. This means that the mix has a finite working life, generally from 6 to 10 hours, as it is transported from the asphalt plant, where it is mixed hot, to the final site. It cannot generally be laid in very cold conditions.

Bitumen-free surface materials are known. Bare concrete is the most obvious example, used in road construction. Rubber/rubber crumb compositions have also been proposed, however, for specialized applications other than road construction. Running tracks and sports arenas., for example, have been surfaced with compositions in which rubber crumb is bound together with a thermosetting or

chemical curing rubber composition. This surface composition is unsuitable for road surfacing because the application process involves on-site mixing in small batches in a blade or paddle mixer, followed by a rapid spreading and levelling because of the characteristics of the binder curing process. Also the material components are particularly weather sensitive. In any case, the expense of the composition would make it entirely unsuitable for use in road construction.

There is therefore a need for a road surfacing composition which is economically viable, which can be delivered premixed to a site or mixed on site and laid using techniques familiar to those used to laying asphalt, which has improved wear characteristics as compared with asphalt, and which is bitumen-free. There is also a need for a road surfacing composition which avoids as far as possible problems associated with the formation of surface ice or packed snow. An elastic or resiliently deformable surfacing composition is potentially advantageous in this regard because deformation of the road surface under traffic loads will be accompanied by fragmentation of any surface ice or packed snow. The ice or snow will then be more easily dispersible.

The Invention

The invention provides a road surfacing composition comprising crushed stone or rock, small rubber pieces (rubber dust to rubber crumb) and sand bonded together with a single component, moisture-hardening polyurethane. The composition has elastic and water-repellant properties even in freezing conditions, and the surface elasticity under traffic loads causes fragmentation of any surface ice that may have formed.

The weight ratio of rock : rubber pieces : sand :

polyurethane ^' is preferably substantially 1 :1:1:1. The rubber pieces preferably include both pieces of 1 mm and less, and pieces of 2 mm and more.

In a preferred form, the invention provides a road surfacing composition comprising:

[A] from 10% to 50% by weight of rubber granules of 2 mm to 8 mm mesh size

[B] from 0% to 10% by weight of rubber granules of a size from dust to 2 mm mesh size

[C] from 6% to 40% by weight of crushed rock of 2 mm to 20 mm mesh size

[D] from 10% to 50% by weight of sand and

[E] from 8% to 30% by weight of a single component, moisture curing polyurethane binder comprising: a polyol in admixture with a di-isocyanate , or a prepolymer obtained therefrom, optionally in combination with one or more polymerization catalysts and/or additives.

Further discussion of preferred to optimum parameters of the various components is as follows:

Rubber granules

Two size ranges of rubber granules are specified. The larger sized granules are preferably in the size range of 2 mm to 4 mm, and advantageously comprise from 10% to 25% by weight of the composition. The smaller sized granules, from dust to 2 mm mesh size, may be omitted entirely (the 0% lower limit) but preferably comprise from 2% to 10% by weight of the composition. The rubber granules may be recycled rubber crumb for example from comminuted automobile tyres, or may be granules or EPDM rubber. The latter permits the final road surfacing to be self- -coloured in colours other than black.

Crushed Rock

Hard aggregate of crushed granite or similar hard rock having ah angular or subangular shape is preferred. The particle size is preferably from 2 mm to 13 mm mesh size, more preferably from 2 mm to 6 mm mesh size. Although the polyurethane binder contributes very significantly to the oustanding strength of the final road covering, the interlocking action of angular stone particles under compaction adds substantial further structure strength and stability. The crushed rock preferably comprises from 15% to 35%, more preferably from 20% to 31%, by weight of the composition.

Sand

The sand used should preferably be relatively clean, uniform in composition and with a high silica content. Sand with a low iron content is preferred, so as to avoid redox reactions. The type of sand, by way of its composition and surface area, determines the optimum proportion of polyurethane binder that is used. The sand preferably has a mesh size of less than 2 mm, and preferably comprises from 25% to 40% by weight of the composition.

Polyurethane binder

The optimum amount of polyurethane binder composition used, within the stated limits, is such that all particles in the composition are evenly covered with the binder. In that way the polyurethane binder acts as a bridge within the construction between all particles. Free or surplus polyurethane in excess of the amount needed to encapsulate the particulate ingredients of the composition is preferably kept to a minimum.

The polyurethane binder may be, for example, a mixture or prepolymer of a polypropylene glycol and diphenylmethane di-isocyanate , together with optional additives and/or

catalysts as discussed below. Such a mixture is moisture- -curing, and yet has a high tolerance of excess water. Therefore it can be laid on road surfaces which are not totally dry, although very wet surfaces should be avoided. Similarly laying in rain should be avoided, as the curing process involves the release of carbon dioxide which could become trapped under a cured surface skin, causing blistering, if the surface were to be exposed to excessive moisture immediately after laying.

Instead of polypropylene glycol , the polyol component of the polyurethane binder may be a polyester based polyol, although that would tend to be more viscous at room temperature .

Additives

Additives in the mixture are important to control the chemical reaction, to desensitize the components of the mixture, and to improve bonding and adhesion. In ideal conditions of temperature and moisture the additive may be reduced significantly in amount or even omitted. A very small amount of an amine-terminated silane (preferably less than 0.5% by weight and advantageously less than 0.14% by weight of the polyurethane binder) is advantageous for a number of reasons. In the first place it accelerates the curing process. Secondly it acts as an adhesion agent which helps the chemical bonding that takes place between the stone and sand on the one hand and the polyurethane binder on the other. Ionic bonds are formed, both between the stone and sand in the composition and the polyurethane and in many cases between the polyurethane and the ground surface (scraped or worn asphalt or concrete) on which the composition of the invention is laid. Thirdly the additive reduces the amount of carbon dioxide given off during curing, by acting as a water-scavenger and offering extra amino groups for the cross-linking. A suitable amine-terminated

silane is that sold by Union Carbide Inc under the Trade Mark UNION CARBIDE ISO-A1100.

Catalyst

The addition of a catalyst to the polyurethane binder, to assist curing, may be advantageous. A tin carboxylate, such as tin dilaurate , is preferred. Dibutyltin dilaurate, for example, is sold under the Trade Mark DABCO T12CL by Air Products and Chemicals, Inc. The amount of catalyst needed is not large; less than 0.1% by weight of the polyurethane binder is generally sufficient. The catalyst accelerates the curing process very considerably, and the optimum amount of catalyst varies depending on the laying conditions, and in particular the ambient temperature and relative humidity of the weather. On a very hot, humid day for example it may be preferred to avoid the use of a catalyst altogether so as to counteract a naturally shortened curing period.

It may be desirable to use a catalyst containing no metal, in which case an oligomeric diamine such as polytetramethyleneoxide-di-p-aminobenzoate is suitable. Such a diamine is sold under the Trade Mark VERSALINK P1000 by Air Products and Chemicals Inc.

Mixing and Laying Procedure

The ingredients of the composition are preferably premixed in a pug mill type asphalt batch plant, but could if desired be mixed cold at the laying site. Prior to mixing the asphalt plant should be cleaned as far as possible with hot, dry, abrasive aggregate mixed for several minutes. The residue should be examined and further mixings take place until it can be osberved that there is not any significant levels of bitumen.

Sand and aggregate can be fed into the mixer at

temperatures up to 150°C, subject to the degree of moisture content of the aggregate. Predried aggregate or aggregate with a moisture content below 5% would require relatively low feed-in temperatures. The rubber crumb and rubber dust is fed in at ambient temperature.

It has been found advantageous to distribute the additives and catalyst for the polyurethane binder through the solid components of the composition before the liquid polyurethane binder is added to the mixture. This ensures uniform distribution of the catalyst and good intimate contact between the additive or additives and the solid particulate matter, and is preferably achieved as follows. The liquid additive or additives and the catalyst are first intimately mixed with a small quantity (preferably a few kilograms) of dry sand and rubber dust. The quantity of liquid so added does not noticeably affect the free-flowing behaviour of the sand and rubber dust premixture , which is then introduced into the mixing box of the pug mill and distributed, together with the additive(s) and catalyst, throughout the mix. Catalyst and additive free polyurethane, already premixed to the required formulation, is free flowing and is fed into the mixing box without any preheating or need to be maintained at other than ambient temperature.

The polyurethane is initially very fluid and of low viscosity. It has good coating ability but preliminary cure reactions will become apparent as a result of the moisture content within the other ingredients and general humidity. The viscosity will increase as a result and thorough mixing should continue to ensure homogeneity.

Trial batches can determine the optimum mix time, which will again be dependent upon the ingredient proportions affected by the sand type and- the general efficiency of the plant being used and, of course, the size of the

batch. Generally a brief mix of up to 2 to 3 minutes will be required.

The manufactured material should be dropped onto a clean, flat storage platform or delivery vehicle, the deck of which may be lightly coated with a solvent or paraffinic oil to prevent excessive adhesion. Alternatively it has been found perfectly feasible to store the composition in clean dry polythene bags which are filled and sealed immediately after mixing. Because the polythene bags exclude atmospheric moisture which is essential for curing the polyurethane binder, the mix has an extended storage life before curing takes place. Bagged compositions have been stored for up to 30 days without ^' deterioration, and have cured satisfactorily by applying a fine mist spray of water after spreading.

From the delivery vehicle, the material can be loaded into a standard or mini paving machine with the screed bars set to the required depth. The minimum permitted depth may be dictated by the paving machine or by the size of stone particles used in the composition, and the maximum permitted depth may be dictated by the need for atmospheric moisture to permeat all through the laid composition to effect curing. That in turn will be affected by the ambient relative humidity and by the degree of compaction after spreading. In general, laid depths of 10 mm to over 30 mm are easily attainable, with about 30 mm providing a reasonable general purpose covering. It is of course not necessary to spread the composition by a paving machine: it could alternatively be raked flat by hand.

Whilst it is not essential for the road surface to be completely dry, there should not be standing water or water of any measurable quantity. A small degree of moisture, however, will enhance the bottom layer cure and

promote steadfast adhesion. The road surface is preferably pre-coated with a . primer of a polyurethane or epoxy resin adhesive.

Damaged road areas should be repaired to level prior to laying. Road surface planing can take place to level in preparation for laying operations as required.

Edge detail may be manipulated by hand manually if required.

The composition may be laid as a porous, water-permeable surface covering or as an impermeable surface covering. For the former, a good proportion of larger sized crushed rock (component [C] ) and the omission of the smaller sized rubber granules (component [B] ) can produce a suitable porous texture. The resulting road covering rapidly drains and disperses surface water leaving the road clear of standing water even in very heavy rainfall. It minimizes splashback even in torrential rain. The surface is applied on top of a porous or impermeable base constructed to a crossfall to allow accumulated water to drain to the edge.

In the majority of applications, however, it is anticipated that the composition will be laid as a water- -impermeable road covering, in which case reasonable cross falls should always be recognized given the impermeability and high water repellancy of the finished surface material. As such, good drainage and soakaways are important to prevent back-ups of standing water.

Rolling of the surface with a roller but avoiding waving effects will assist in providing mechanical interlocking of the aggregate and rubber components within the binder, and assist surface bonding.

The high proportions of larger size aggregate and rubber crumb help to give a surface texture to reduce wet weather skid. The asperities of the texture can be supplemented by further texturing operations. Brush techniques may be possible which will be across the road from crown to edge and generally in accordance with the water run off gradient. Also, surface dressings can be applied if required, with a thin coating of binder (bituminous or polymeric) and broadcast with stone chips.

The surface itself creates a different road noise to standard asphalt and this is further enhanced by any surface texturing operations. The road noise can thus assist in alerting drivers of dan-gers areas, junctions, etc. The noise and vibration absorbing features of the surface provide further benefits.

Generally, after 6 to 8 hours, the surface is moderately load bearing and can be walked upon for further work operations to take place, such as road marking operations. The material will generally be cured after 24 hours and capable of withstanding vehicular traffic, subject to suitable curing conditions of temperature, humidity and correct mixing.

The material should not be laid in severe cold conditions as the application and proportions of catalyst to counter the slow cure rate in extreme cold can pose too great a difficulty to handle efficiently. However a successful trial was carried out when the road and air temperatures were about 0°C, and the subsequent overnight temperature dropped to about -8°C. Laying of asphalt would have been impossible in those conditions, but the road surface laid according to the invention was properly laid and suffered no significant adverse effects from the relatively low temperatures.

T e composition of the invention, when laid and cured, provides a very durable road surface which has a high inherent structural strength, bonds well to the surface on which it is laid (particularly to concrete) and has an outstanding stability through extremes of temperatures. It shows no serious adverse performance characteristics either at temperatures so cold that asphalt would tend to become brittle and crack, or at temperatures so hot that asphalt would tend to deform or creep under traffic loads. It is far less dense than asphalt and is therefore extremely suitable for covering the road surfaces of bridges and elevated highways. In that respect its physical adhesion to concrete is of immense advantage.

Another potential use for the composition of the invention is in the manufacture of surface-coated paving slabs which can be pre-coated with the composition of the invention and then handled and laid as conventional concrete paving slabs.

The following Examples illustrate the invention.

Example 1

A composition according to the invention had the following content by weight:

2 mm to 4 mm rubber granules 20%

2 mm to dust rubber granules 3%

2 mm to 6 mm crushed rock 28% sharp sand 30% polyurethane binder 19%

The rubber was recycled rubber crumb obtained from the comminution of automobile tyres. The crushed rock was granite. The sand was a clean sharp silica sand with a high silica content • and a low iron content. The polyurethane was a polypropylene glycol/diphenylmethane di-isocyanate prepolymer, which is a single component,

moisture curing rubber.

The composition was mixed in a pug mill type asphalt plant which had previously been thoroughly cleaned to remove bitumen residues. The sand and crushed rock had been pre-dried and stored outside, and were cold-mixed with the rubber crumb in 2000 kg (approx.) batches. A few kilograms of the sand and rubber crumb dust were separately mixed together, and about 1.4 grams of dibutyltin dilaurate and about 3-0 grams of an organo aminosilane were added to that mixture and thoroughly dispersed through the mixture by mixing. The resulting premixture was added to each 2000 kg batch of solids, to obtain a substantially uniform distribution of the catalyst (0.07% by weight) and additive (0.15% by weight) over the surface of the solids in the mill. The polyurethane binder was then added at ambient temperature and mixed with the solids in the mill for a total mixing time of about 90 seconds. No catalysts or additives were added to the polyurethane binder, other than those already added to the batch of solids in the mill.

After mixing, the composition was transported to the intended laying site, which was an asphalt-covered road the surface of which had been planed. On site, the ground temperature was about 0°C. The surface had been primed with concentrated polyurethane.

The composition was transferred to a mini paving machine. The composition was then immediately spread by the paving machine to a depth of 15 mm, and rolled to a compact finish.

After 8 hours the road was firm enough to be marked out with white lines, and after 16 hours was suitable for use by vehicular traffic.

A core sample section was cut and lifted from the road surface after three days and examined for bonding strength compaction and evenness of cure. The core sample was judged to be very good in all of these respects.

Example 2

Using the same techniques of mixing as described in Example 1, the composition was varied as follows:

2 mm to 4 mm rubber granules 12% by weight

1 mm to dust rubber granules 12% by weight

2 mm to 6 mm crushed rock 28% by weight sharp sand 28% by weight polyurethane binder (as Example 1) 20% by weight

dibutyltin dilaurate 0.06% by weight organo aminosilane 0.10% by weight

The polyurethane binder and mixing sequence was the same as in Example 1.

The mix was transported to site and then spread on an asphalt-covered road, the surface of which had been planed and primed with polyurethane. The mixture was spread manually by raking and looting. The material was then rolled to a compact finish.

After 8 hours the road surface was firm enough for moderate loads and after 24 hours was judged to be firm enough for vehicular traffic. A sample was cut and lifted from the road after several months and showed no discernible signs of wear.

Example 3

Example 2 was repeated but using the following formulation for the composition :

3 mm to 8 mm rubber granules 12% by weight

2 mm to dust rubber granules 12% by weight

5 mm crushed rock 8% by weight

13 mm crushed rock 10% by weight sand 38% by weight polyurethane binder 20% by weight

dibutyltin dilaurate 0.06% by weight organo aminosilane 0.10% by weight

The performance characteristics of the road coverings of Examples 2 and 3 were equal to those of Example 1. In particular, excellent adhesion was noted between the polyurethane binder and the crushed rock of the aggregate, and a better interlocking of the crushed rock particles due to their larger particle size.

Example 4

A porous road surface was prepared as follows.

Mixing and laying techniques of Example 1 were used to create a mix with the following composition:

2 mm to 8 mm rubber granules 22% by weight

5 mm crushed rock 16% by weight

13 mm crushed rock 15% by weight sand 30% by weight polyurethane binder 17% by weight

dibutyltin dilaurate 0.06% by weight organo aminosilane 0.15% by weight

The road covering after laying was found to give a good water permeability, draining and dispersing rainwater even in heavy rain. The subsurface on which the road covering was laid had itself a good crossfall to assist the water dispersal.

After several weeks of vehicular traffic, the surface was examined and was seen to be generally good.

Example 5

In this Example a different mixing technique was used. Instead of mixing the components in an asphalt mixer, they were batch mixed on site in relatively small quantities to resurface an asphalt driveway of which the asphalt surface was over seven years old.

The site was first prepared by roughly cleaning the old surface of moss and dirt by scraping and brushing, using a spade and a wire brush. A primer was then applied over the surface at a coverage rate of 50 grams per square metre. The primer consisted of 93% by weight methylated spirit, 2% by weight of water and 5% by weight of an organo aminosilane. The primer dried rapidly in the cold (3°C) but windy conditions.

The surface composition was mixed in three batches in a large polypropylene drum. The ratio of components was as follows :

rubber crumb 23.5% by weight

2 mm crushed rock 24.5% by weight

5 mm to 6 mm crushed rock 2.5% by weight high silica sand 29.5% by weight polyurethane binder 20.0% by weight

dilbutyltin carboxylate 0.08% by weight organo aminosilane 0.17% by weight

The rubber crumb was predominately in the size range of 2 mm to 6 mm, although they contained a small amount (less than 10% by weight) of smaller particles, down to dust.

The rubber crumb and 2 mm and 5-6 mm crushed rock was fed

into the drum. The sand was added at about 150°C, and the contents of the drum were rapidly mixed with an electric spiral mixing rod. The organo aminosilane was added, and a slight haze or smoke was observed over the drum as the additive contacted the hot sand. The tin dilaurate catalyst was then added, by which time the temperature of the mix had fallen to 50°C. During mixing, care was taken continually to bring the sand to the top of the drum from the bottom.

It was observed that the sand and aggregate were not totally dry at the start. Small lumps of compacted sand and small portions of damp aggregate indicated significant moisture content in places. When thoroughly mixed, this was not noticeable and the overall mixture appeared dry.

The polyurethane binder was added while the mixed solids were still warm, and the mixture was warm and fluid and mixed easily. It took about 2 to 3 minutes for the polyurethane binder to become distributed through the entire mix.

The first mixed batch was poured over the prepared asphalt surface and roughly raked and looted, and left for an hour to settle, to cool and to be exposed to the atmosphere. During this time the second and third batches were similarly prepared and spread in adjacent bays. The coverage rate was about 17 kg per square metre at a minimum depth of 10 mm and an average depth significantly higher.

After about 1.5 hours the material was trowelled smooth with a minimal degree of compaction. It was stiff and rather difficult to work manually due in part to the low air temperatures (2 to 3°C) and in part to the fact that it was raining slightly. After a further 3 hours there

were signs of gas evolution with slight surface blistering. At this stage the covering was compacted by rolling using a hand roller the surface of which had been smeared lightly with a paraffinic oil to reduce sticking. The weather deteriorated into steady rain, and about 5 hours from the first mixing the surface was therefore covered with polythene sheeting to avoid too rapid a cure.

After a further 13 hours the polythene sheeting was removed. An area that had been left uncovered held a pool of surface water which was swept away. The surface beneath that surface water showed no obvious signs of deterioration. The surface was firm enough to be walked upon. After 20 hours a car was driven over the area without any sign of tyre depressions. After 40 hours a heavy car was driven over the area a number of times without any marks or damage to the surface. Repeated use of power steering to turn the vehicle wheels while the car was stationary caused no damage at all to the surface.

Example 6

A composition was prepared on a small batch scale from the components: rubber crumb 25% by weight sand and small aggregate 55% by weight polyurethane binder 20% by weight

organotin catalyst 0.08% by weight amine-terminated organosilane 0.14% by weight

The rubber crumb was primarily from 2 mm to 4 mm mesh size with a small proportion (no more than 10% by weight) of 2 mm to dust content. The sand and aggregate mixture contained about 50% by weight of 2 mm to 6 mm particles.

The additive and catalyst were premixed with a small quantity of the sand/aggregate before being uniformly dispersed through the mixture. After mixing, the composition could easily be spread over concrete paving slabs to a depth of 5 mm to 10 mm after compaction. The paving was first primed, however, to aid adhesion. The following four primers were used, all giving excellent results.

(a) Each concrete paving slab was primed with a light mist spray of 5% by weight of the amine terminated organosilane dissolved in industrial alcohol or methylated spirit. An application rate of less than 100 grams of the primer solution per square metre was sufficient to provide a strong bond strength between the cured composition and the concrete.

(b) Each concrete paving slab was primed with the same polyurethane binder as that of the surfacing composition, applied with a brush.

(c) Each slab was primed with the polyurethane binder diluted with 30% by weight of an organic solvent. Both (b) and (c) resulted in virtually identical adhesion of the final cured coating.

(d) An epoxy adhesive was used as primer, with excellent results.

Example 7

The following components were used to prepare a composition for temporary storage in the open air before use.

Rubber granules 2 mm to 4 mm 23.8% by weight Rubber dust 2.0% by weight

6 mm stone chippings 20.5% by weight

2 mm stone chippings (grit) 5.0% by weight

Sand • 30.0% by weight

Polyurethane binder 18.7% by weight

tin dilaurate 0.02% by weight aminosilane 0.08% by weight

The rubber, sand and aggregate were heated to 150°C in a clean asphalt mixer and thoroughly dried, and then the additive and catalyst mixed in, after prior distribution through a small quantity of dry sand. There was some smoking on the addition of the additive and catalyst, with evolution of a small amount of carbon monoxide and carbon dioxide. Then the polyurethane binder was added and the composition thoroughly mixed, before being discharged into a pile.

It was a dry summer's day with an ambient temperature of about 15°C. The composition remained workable for 8 hours without noticeable curing, but cured rapidly when spread and rolled as in Example 1 , with a fine mist spray of water being applied after spreading but prior to rolling.

Example 8

The mixing procedure of Example 7 was repeated using the same components and proportions except that the catalyst was 0.02% by weight of tin carboxylate and 0.03% by weight of tin dilaurate, and 0.14% by weight of amine- -terminated organosilane. Thorough drying of the solid components of the mixture was ensured by maintaining the temperature in the asphalt mixer at 150°C for seven minutes longer than in Example 7.

After mixing, the composition was delivered into dry polythene bags which were immediately sealed. Samples were used at intervals over a period of 30 days, at the end of which period the composition was still easily

workable. Each sample was spread, moistened with a fine mist spray of water and then rolled. In each case the sample cured quickly, achieving a strength sufficient to take pedestrian and light vehicular traffic after 15 hours at 15°C

Example 9

This Example illustrates the highly advantageous properties of the composition of the invention in conditions of snow and ice.

A road surface was resurfaced in strips according to the techniques of Example 2, with the exposed wear surface of the finished road alternating between strips of original asphalt about 300 mm wide and strips of the composition of the invention of similar width. The strips ran transversely to the road direction, so that vehicular traffic passed sequentially over the alternating asphalt and polyurethane-bound surfaces.

After a light fall of snow, the effect of traffic on the striated road surface was observed. Initial vehicular traffic tended to cause the packed snow to adhere to the asphalt surface but not to the surface of the composition of the invention. After continued exposure to traffic, the asphalt sections were fairly uniformly covered with a layer of highly compacted, firmly adherent and very slippery snow and ice. The sections surfaced with the composition of the invention were largely free from snow and ice.

The trial showed that when subjected to identical conditions of temperature, snow fall and subsequent traffic, the surface composition of the invention provided by far the superior road surface which showed relatively little adhesion to snow and-ice.