The present invention relates to a surface construction comprising a mix of aggregate material and bitumen binder, the bitumen being in the form of an emulsion. The mix is used, for example, in the construction and repair of road surfaces.

The use of bitumen-in-water emulsions in the road building and maintenance industry is increasing. Emulsions have an advantage over the more traditional 'hot-mix' of aggregate and heated bitumen in that they can be applied at a lower, even ambient, temperature. They also have an advantage over 'cut-back asphalts' in that they do not release organic solvent pollutants into the environment. However, emulsions have had the disadvantage that the breaking of the emulsion, that is the hardening of the bitumen following release of the water from the emulsion, has been difficult to control. The action of mixing the aggregate with the emulsion tends to set off breaking so that the bitumen has started to harden before it has been applied to the road surface. To alleviate this problem more emulsifϊer can be added to the emulsion composition, but then the bitumen tends to take a long time to set after being laid into the road, and so there is a considerable delay before the road can be opened to traffic.

Our co-pending International patent application publication no. WO 92/19683 describes a bitumen emulsion wherein the bitumen particles are relatively small and uniform in size. This emulsion has the benefit that it forms a delayed-set mix with aggregate and exhibits controlled breaking. Nevertheless, it has been found that road surfaces formed using this emulsion are susceptible in early life to some loss of aggregate material and bitumen binder, and to pitting of the surface, due to passing traffic and adverse weather conditions, especially rain. This is thought to be due to the fact that the binder is still relatively soft at this stage, even though breaking of the emulsion has occurred. The hardening of the binder can be accelerated to some extent by a high degree of compacting of the bitumen-aggregate layer, but the amount of compacting required is time-consuming and not always feasible in practice. Alternatively, a more rapid hardening can be obtained if the aggregate is carefully size-graded so that the aggregate incorporated into the mix is of a more uniform size, but this adds extra quality control and expense.

A number of references describe methods for improving the breaking time of bitumen emulsions by the incorporation of a breaking agent. European patent publication EP-A-491107 discloses mixing a neutralising water-in-oil emulsion breaking additive into a bitumen emulsion, but this has the disadvantage that the emulsion must be used shortly after the incorporation of the additive. Swedish patent publication 453760 discloses encapsulating a breaking additive into microcapsules so that the breaking additive is released in a controlled manner into the emulsion, but this is a relatively expensive procedure. UK patent publication GB-A-2167975 discloses a method for the surface coating of roads in which a cationic bitumen emulsion and an anionic breaking agent solution are applied simultaneously to a road surface by brush jets. The anionic solution is discharged onto the lower part of the brush jets at an angle of incidence between 25 and 60°. This method has the disadvantage that the application procedure is complex, probably requiring specially adapted equipment.

The present invention provides, in one aspect, a bituminous surface construction comprising :

(a) a substrate comprising a mix of bitumen emulsion and aggregate material, the emulsion being either anionically or cationally charged (this emulsion hereinafter being referred to as the "substrate emulsion" or "substrate bitumen emulsion"), and

(b) a coating on the substrate, which coating comprises a bitumen emulsion of opposite ionic charge to the substrate bitumen emulsion.

Preferably the coating is contiguous with the substrate.

The emulsion coating causes a rapid hardening of the surface of the substrate. In addition, it believed that the coating penetrates at least partly through the substrate so that at least the upper region of the substrate also undergoes rapid hardening. Thus the surface construction is sufficiently hard to be able to withstand loads, such as passing traffic, immediately or within a very short period after the application of the coating, without substantial loss of aggregate or bitumen binder, and without substantial pitting or marking of the surface. The depth of penetration of the emulsion coating may depend on a number of factors, for example the type and amount of bitumen used in the coating and substrate

emulsions, the nature, amount and size of the aggregate, and the methods of application of the substrate and coating. In general, however, the depth of penetration of the coating into the substrate is approximately equal to the diameter of the largest size grading of aggregate contained in the substrate.

The substrate bitumen emulsion may be any bitumen-in-water emulsion suitable for road laying applications. Typically this emulsion comprises 35 to 85, preferably 50 to 75, more preferably 60 to 70 wt.% bitumen, 0.05 to 2.0, preferably 0.05 to 1.0, more preferably 0J to 0.8 wt.% emulsifier, the balance being water and optionally other additives as explained in more detail below. The percentages are based on the weight of substrate emulsion.

The bitumen employed in the substrate emulsion may be any commercially- available natural or petroleum-based bitumen (also known as asphalt) suitable for use in road construction, repair or maintenance. It may be a straight-run bitumen or a blown or oxidised grade. The properties of the bitumen will be selected according to the specific application and availability, but typically these will be a penetration (according to standard test IP 49) from 20 to 500 at 25°C, preferably from 50 to 400, more preferably from 60 to 220, and a Ring and Ball softening point (IP 58) from 30 to 100°C, preferably 35 to 60°C.

The emulsifier contained in the substrate emulsion may be cationic or anionic, but is preferably cationic as it is generally cationic bitumen emulsions that are employed at the current time in the road industry. Any suitable cationic emulsifier may be used; examples include hydrochlorides of fatty amines, fatty amido-amines, ethoxylated amines, imidoazalines, quaternary ammonium salts, and mixtures thereof. Preferably the cationic emulsifier is a diamine, for example tallow diamine hydrochloride, and more preferably is a mixture of a diamine with a quatenary ammonium salt. When a cationic emulsifier is employed, it is preferred to include an acid in the bitumen emulsion to counteract the alkalinity of the emulsifier. Generally acid is added to adjust the pH of the emulsion to approximately pH 3. Typically from 0.05 to 0.5, preferably from 0J to 0.3, wt.% acid based on the total weight of the emulsion is added. Suitable acids include inorganic acids, for example hydrochloric acid.

If the substrate emulsion is anionic, any suitable anionic emulsifier may be employed. Examples include carboxylic fatty acid salts; organo-sulphonate compounds, e.g. sulphonated fatty acid salts, petroleum sulphonates, lignosulphonates, alkylsulphonates.

arylsulphonates and alkylarylsulphonates; organo-phosphates, e.g. alkylphosphates; and the inorganic bases, e.g. alkali metal or alkaline earth metal hydroxides, carbonates, borates and the like. Preferred anionic emulsifiers include salts of carboxyUc fatty acids and salts of sulphonated fatty acids, for example sodium salts of alkylsulphonates.

Other additives known for use in bitumen emulsions may also be incorporated in the substrate emulsion. These include, for example, certain polymers, mineral salts, organic solvent cut-backs and fluxes.

Preferably the substrate bitumen emulsion is a delayed-set emulsion wherein the bitumen in the emulsion preferably has a mean particle size of 3 to 9 μm, more preferably 5 to 8 μm. Also it is preferred that the standard deviation of this mean is less than 0.3, more preferably less than 0.2, for example from 0J to 0.2.

The substrate emulsion may be prepared by any conventional bitumen emulsification process, for example by colloid milling, rotary mixing or static mixing, which may be high or low shear. Preferably however, it is prepared by a static mixing process which comprises

a) feeding the bitumen into a first static mixer at a temperature above 50°C;

b) introducing water under pressure into the first static mixer, the pressure being sufficient to prevent substantial vaporisation of the water, the weight ratio of water to bitumen in the first static mixer being between 1:3 and 1:35;

c) introducing the emulsifier into the first static mixer;

d) mixing the components in the first static mixer, and then passing the resultant mixture from the first static mixer into at least one other mixer in which the temperature is preferably below the boiling point of water;

e) introducing water into the other mixer(s) in an amount such that the total amount of water present in the resulting emulsion is from 14 to 65% by weight based on the total weight of the emulsion; and

f) passing the mixture through the other mixer(s) and removing the resulting bitumen emulsion.

Preferably the said other mixer is also a static mixer.

To facilitate the introduction of the emulsifying agent into the first static mixer, the emulsifier, in part or in total, may be mixed with the water to be added to the first mixer to form an aqueous solution or "soap". If desired, the bitumen may be premixed with the water and/or the emulsifying agent prior to introduction into the first mixer.

The bitumen is preferably introduced into the first static mixer at a temperature of 100 to 200°C, more preferably 120 to 190°C, and at a pressure of 2 to 80 bars, more preferably 10 to 30 bars. The water or soap is preferably introduced into the same mixer at a temperature of 20 to 70°C, more preferably 30 to 50°C, and also at a pressure of 2 to 80 bars, more preferably 10 to 30 bars. The temperature in the first mixer is usually 70 to 250°C, more preferably 100 to 150°C. A high temperature faciliates the emulsification of the bitumen and water, and the relatively high pressure prevents the water from evaporating.

It is preferred to pass the resultant mixture from the first static mixer directly into the other mixer, which is also preferably a static mixer. The remainder of the water is introduced into this other mixer, preferably cold, for example 5 to 20°C, so that it cools the emulsion enabling the resultant emulsion to exit the other mixer below the boiling point of water. It is also advantageous to operate this other mixer at a reduced pressure relative to the first mixer so that the emulsion emerges from the other mixture at atmospheric pressure.

If other additives are to be included in the emulsion, they may be added at any stage during the process, although are preferably added to the first static mixer to ensure thorough mixing, and conveniently are added at the same time as the emulsifier. If they are water soluble, as in an inorganic acid andor a metal salt, they may be added to the water- emulsifier soap.

The process is preferably a two-mixer process, but more than two mixers may be used. These additional mixers may be static mixers or other types of mixers, such as a colloid

mill. Examples of suitable static mixers are those known as Sulzer or Kenics mixers, which are well-known to those skilled in the art of static mixing.

The aggregate material mixed with the bitumen emulsion to form the substrate may be any type of aggregate material suitable for the required application, provided it is compatible with the ionic nature of the substrate bitumen emulsion. Usually the aggregate is either siliceous or calcareous stone. Typically the proportion of aggregate to bitumen emulsion in the substrate is from 6:1 to 40: 1, by weight.

The emulsion coating may be any bitumen-in-water emulsion that is capable of being applied as a coating. Preferably this emulsion comprises 30 to 80, more preferably 35 to 70 wt.% bitumen, 0.05 to 2.0, more preferably 0J to 1.0 wt.% emulsifier, and 18 to 69.95, more preferably 29 to 64.9 wt.% water. Additional emulsion additives may be included if desired.

The bitumen of the coating emulsion is preferably a relatively hard, low penetration bitumen, preferably having a penetration (IP 49) of from 20 to 200 at 25°C, more preferably from 50 to 150, and most preferably from 80 to 120, and a Ring and Ball softening point (IP 58) of from 30 to 100°C, more preferably 35 to 60°C.

The emulsifier of the coating emulsion is anionic if the substrate bitumen emulsion is cationic, and, conversely, is cationic if the substrate emulsion is anionic. Suitable emulsifiers are as given above for the substrate emulsion. Preferably the substrate emulsion is cationic and the emulsion coating is anionic. It has been found that an effective, easy-to-apply anionic emulsion is one that conforms to class Al-40 of British Standard BS 434.

In use, the substrate bitumen emulsion and aggregate are preferably mixed, using conventional mixing techniques, before being applied to the road surface. However, in an alternative application method, the substrate emulsion may be applied to the road, e.g. sprayed, and then aggregate laid on top of the emulsion layer, followed by the emulsion coating. The emulsion coating may be applied using any conventional coating technique, for example spraying or painting.

Preferably the bitumen emulsion-aggregate substrate is compacted after laying, according to conventional road laying techniques. This compacting should preferably be carried out after the emulsion coating has been applied to the substrate. Thus in another aspect the present invention provides a method for accelerating the hardening of a bitumen emulsion in a bitumen emulsion-aggregate substrate which comprises applying a coating of a bitumen emulsion to the surface of the substrate, the bitumen emulsion in the coating being of opposite ionic charge to the bitumen emulsion in the substrate, and compacting the substrate. Beneficially, this coating/compacting process is repeated one or more times. In addition the bitumen emulsion-aggregate substrate may be compacted prior to the first coating of the anionic bitumen emulsion.

It has been found that a particularly high quality surface construction, i.e. one that sustains little stone loss and surface damage, is obtained if the emulsion coating is applied both before and after compaction. In fact, this method has been found to be beneficial even if the emulsion coating has the same ionic charge as the substrate bitumen emulsion, i.e. both are cationic, ionic, amphoteric or non-ionic. Thus in a further aspect the present invention provides a method for forming a bituminous surface construction which comprises applying a first coating of a bitumen emulsion to a substrate comprising a mix of bitumen emulsion and aggregate material, compacting the substrate, and applying a second coating of a bitumen emulsion to the compacted substrate. Preferably, however, this method of application is employed and the substrate emulsion and emulsion coating are of opposite ionic charge.

The bituminous surface construction according to the invention is especially suitable for road construction and repair. The bitumen-aggregate substrate as defined herein usually forms what is known as the wearing course, that is the top layer, or series of layers, of the road. The emulsion coating is applied as a top coating of this wearing course. Alternatively, the coating may be applied onto the bitumen-emulsion-aggregate substrate when employed in other layers of the road construction such as the base layer. The coating provides a hardened surface more able to withstand the weight of road construction vehicles and machinery.

The surface construction of the invention may also be employed for other purposes, for example in the construction and repair of pavements, or any other macadam surfaces.

This invention shall now be illustrated by the following Example and with reference to the accompanying drawings in which :-

Figure 1 is a graphical representation of daily stone loss from four bituminous surface patches; patch 1 is comparative and patches 2-4 are according to the invention: and

Figure 2 is a graphical representation of cumulative stone loss from the four bituminous surface patches as described in Figure 1.

Example

A cationic bitumen emulsion having the following composition and properties was prepared. The percentages of components are weight percents based on the total weight of the emulsion unless otherwise specified :

Bitumen: Amount 64%

Penetration 25°C/10 mm/5 sec 200

Ring and Ball softening point 39°C

Emulsifier (N-tallow diamine hydrochloride) 0.6%

Hydrochloric acid 0.2%

Calcium chloride 0.2%

Water 35%

The emulsion was prepared using a two-stage static mixing process as disclosed in EP-A-283246. The two static mixers employed were both Kenics mixers of the type described in Chemineers brochure 800E published by Chemineers Ltd., 1984. The bitumen and a soap solution comprising the emulsifier, hydrochloric acid, calcium chloride and one-third of the total amount of the water were injected into the first static mixer. The bitumen was injected under a pressure of 2400 kPa and was at a temperature of 150°C. The soap solution and water were both injected under a pressure of 2400 kPa and at a temperature of 50°C. The flowspeed into the first static mixer was about 14m/sec and the flowrate about lδm^/hr. The shear produced by the first static mixer was calculated, based on the Darcy equation and taking flowspeed, flowrate and size of pipes into account, to be about 35,000 sec ^* *.

After passing through the first static mixer, the mixture was fed directly into the second static mixer into which the remainder of the water was injected at a pressure of 400 kPa and a temperature of about 18°C. The shear rate in the second static mixer was calculated to be about 5,000 sec ^* * and the flowrate was about 20m ^ per hour. After passing through the second mixer the resulting emulsion exited at atmospheric pressure and a temperature of about 85°C. The total amount of water present in the mixture was 35% by weight of the total weight of the final product.

Using a Coulter Multisizer (II), model S.STD II, the emulsion was found to contain bitumen droplets having a median particle diameter of 5.5 μm and a standard deviation of OJ8.

An aggregate/bitumen emulsion mix was prepared by combining 93.8 wt.% limestone aggregate and 6.2 wt.% of the above bitumen emulsion. The size gradings of the aggregate conformed to British Standard BS 4987 (Table 5) for a 10 mm dense graded macadam. The aggregate and emulsion were mixed together at ambient temperature using a conventional mixer and mixing was continued until the emulsion had coated the aggregate particles.

An anionic bitumen emulsion conforming to class Al-40 under BS 434 containing 40 wt.% bitumen having a penetration of 100 at 25°C and a Ring and Ball softening point of 46°C, and 0.45 wt.% sodium alkyl (Cγ) benzene sulphonate anionic emulsifier was prepared using conventional high shear rotary milling.

The following tests were performed : Four holes, each measuring lm x lm were cut into a test macadam road surface in series at a spacing of 1.5m. Each hole was laid with a lean concrete substrate to leave a hole depth of 50mm. The holes were numbered 1 to 4, with hole 1 being the first to be met by oncoming traffic.

Each hole was filled with a wearing course as described below. Compaction was carried out using a Wacker vibrotamper and each compaction step consisted of four passes of the compactor unless otherwise specified. The anionic emulsion was apphed using a small pressurised hand sprayer.

Hole 1 (Comparative) : Filled with the cationic emulsion-aggregate mix to a depth of 75 mm (25 mm above the road surface) and compacted.

Hole 2 (Invention) : FiUed with the cationic emulsion -aggregate mix, the surface sprayed with 0.5 Utres anionic emulsion, compacted, then sprayed with a further 0.5 Utres of the anionic emulsion.

Hole 3 (Invention) FiUed with the cationic emulsion-aggregate mix, the surface sprayed with 0.5 Utres of the anionic emulsion, one pass made with the compactor, sprayed with a further 0.5 Utres of the anionic emulsion and then compaction completed, i.e. a further three passes made with the compactor.

Hole 4 (Invention) : FiUed with the cationic emulsion -aggregate mix, the surface sprayed with 1 Utre of the anionic emulsion, and then compacted.

The four patches thus formed were left for half an hour, except patch 1 which was left for three days. The quaUty of the resulting surfaces was then determined by driving a variety of vehicles over the patches from time-to-time for seven days, and measuring the stone loss from each of the patches. The results are given in Table 1 below and are also given graphicaUy in Figures 1 and 2.

TABLE 1

Stone loss (g)

Day Patch 1 Patch 2 Patch 3 Patch 4

Daily Cumulative Daily Cumulative Daily Cumulative Daily Cumulative

1 1016 1016 94 94 132 132 137 137

2 1016 2032 57 151 130 262 216 353

3 837 2869 47 198 117 379 113 466

5 497 3366 24 222 39 418 75 541

6 205 3571 30 252 76 494 80 621

7 148 3719 22 274 37 531 35 656

The results show that the appUcation of an anionic bitumen emulsion coating to a wearing course of cationic bitumen emulsion-aggregate mix provides a significant improvement in the quaUty of the surface. After a week of testing the total reduction in stone loss compared to the untreated surface was over 80%, and after the first day this reduction was even more significant, at over 86%.

Although aU three methods of applying the anionic emulsions produced a significant benefit, particularly good results were obtained from Patch 2 where the anionic emulsion had been appUed twice, once before and once after fuU compaction.