BITUMINOUS COATED MATERIALS

Abstract: The present invention provides a method of producing a storable bituminous coated material suitable for use as foothpath and/or carriageway material in a wearing course, basecourse and/or roadbase. The method comprises providing coarse and fine aggregate particles and mixing at least one of the coarse and fine aggregate particles with hot bitumen or bitumen emulsion in which the bitumen is a hard grade bitumen, and mixing the coated coarse and fine aggregate particles, in the absence of soft grade bitumen, with a slow breaking bitumen emulsion in which the bitumen is a hard grade bitumen.

Description: BITUMINOUS COATED MATERIALS The present invention relates to the production of storable bituminous coated materials suitable for use as footpath and carriageway materials, in both surface and base layers. A number of different processes have been proposed in the past for the production of bituminous coated materials which have a reasonable storage life i.e. remain workable for several days, but can also bear traffic within a short time, usually a few hours, after laying down and compacting. A significant problem with previously proposed material of this kind, though, has been that they do not develop full strength and stiffness for a very long time (measured in years) - if at all. Other materials that have previously been produced suffer from certain defects in performance e.g. reduced plastic stiffness leading to inadequate traffic bearing performance (i.e. susceptible to deformation), when compared to conventional 'hot mix' materials e.g. macadams complying with BS 4987 Part 1: 1993 and the HAUC Code of Practice Specification for the Reinstatement of Openings in Highways, A Code of Practice, Appendix A.10, Table A10.1 - Highway Authorities & Utilities Committee (HAUC) . It is an object of the present invention to avoid or minimise one or more of the above disadvantages. The present invention provides a method of producing a storable bituminous coated material suitable for use as footpath and/or carriageway material in a wearing course, basecourse and/or roadbase, which method comprises the steps of: providing coarse and fine aggregate particles, mixing at least one of the coarse and fine aggregate particles with hot bitumen or bitumen emulsion in which the bitumen is a hard grade bitumen, so as to produce aggregate particles coated with hard grade bitumen, and then mixing the coarse and fine aggregate particles, in the substantial absence of soft grade bitumen, with a slow breaking bitumen emulsion in which the bitumen is a hard grade bitumen. As used herein the expression "hard grade bitumen" indicates a bitumen which has a high viscosity and resistance to penetration and is substantially free of viscosity reducing components such as flux oils, and is distinguished from "soft grade" bitumen which includes such a viscosity reducing component (usually in order to increase and/or prolong the workability of the bitumen) . Such bitumens are sometimes referred to as "straight run" or "penetration grade" bitumen and generally have a penetration grade in the range from 15 to 450 pen, preferably from 50 to 300 pen, most preferably 100 to 200 pen. In general such bitumens typically have viscosities in the range from 700 to 20,000 cencistokes (cSt) measured using a suitable Kinematic Viscometer at a test temperature of 100°C As indicated above it is important that the bitumen should be substantially free of viscosity reducing components such as flux oils. The bitumen may nevertheless include other components for modifying its other properties e.g. elasticity, without significantly reducing its viscosity. In particular there may be used so-called polymer modified bitumen (PMB) which generally comprises "straight run" or "penetration grade" bitumen in intimate admixture with a suitable polymer. Various polymer modified bitumens are well known in the art and suitable polymers include one or more of the following: ethyl vinyl acetate (EVA) copolymer using any of a wide range of vinyl components; styrene butadiene styrene (SBS) or styrene iso-butylene styrene (SIS), or styrene ethylene butadiene styrene (SEBS), linear and/or branched, block copolymers; styrene butadiene rubber (SBR) latex, Atactic Polypropylene (APP) ; natural or synthetic rubber (crumb or latex form) ; polybutadiene (Bd) ; derivatives of Methyl Methacrylates (Acrylics) ; etc. The amount of polymer included in the bitumen may be varied within wide limits depending on inter alia the increase in elasticity and associated improvement in durability, required and cost thereof. In general there is used from 1 to 10% w/w of the polymer modifier, preferably from 3 to 5% w/w. It will be appreciated that the polymer additive generally increases the final hardness of the modified bitumen. Accordingly where a particular degree of final hardness is required, say 50 pen, then it will be necessary to use a somewhat less hard grade of bitumen, say 100 pen, to which the polymer is added, which should nevertheless be substantially free of viscosity modifiers such as flux oils. With the method of the present invention there is provided a bituminous coated material which can be stored typically for from 7 to 14 days while remaining substantially workable, and when laid and compacted will develop a high degree of stiffness comparable with that of traditional, non-storable, hot bitumen coated macadam products, certainly within 2 years, generally within 1 to 6 months, and usually within 3 months of laying and compacting. The hot bitumen or emulsion coating of the aggregate may be applied to either or to both of separate (uncoated) fine and coarse aggregates. Pre-coated aggregates are then stored at ambient temperature, usually in a stockpile, and allowed to equilibrate with ambient temperature. Moreover it is possible for one fraction to be coated using hot bitumen and the other using bitumen emulsion. In general it is preferred that the application of the hard bitumen coating to the coarse and/or fine aggregate particles be effected by mixing hot bitumen with hot aggregate, in a generally conventional manner, usually at temperatures in the range 150 to 180°C, in order to secure good adhesion and binding of the bitumen coating to the aggregate particles. Where bitumen emulsion is used to coat the aggregate particles it is important that there should be used an emulsion which is not too reactive and does not break too quickly resulting in preferential binding to smaller aggregate particles leaving larger ones uncoated. It should be borne in mind that the coated aggregate particles cannot be used in the final mixing stage until this emulsion has broken and the bitumen becomes bound to the aggregate particles. It is important that the bitumen should be well bound to the aggregate particles in order to avoid stripping thereof from the aggregate particles upon compaction of the bituminous coated material when it has been laid. Various bitumen emulsions suitable for use in coating of the coarse aggregate particles are known in the art and are readily available including commercially available types such as K 140, K 160, K 170 and K 360 (where K indicates a cationic emulsion; the first digit indicates reactivity and speed of breaking with 1 quick, and 3 slow; and the remaining two digits indicate the percentage content of bitumen) . In general a slower breaking emulsion is required for pre-coating fine aggregate particles and a faster one for pre-coating coarse aggregate particles. In accordance with the present invention there is used a slowbreaking emulsion in the final mixing. The speed of breaking of a bitumen emulsion is conveniently expressed in terms of a "Break Index" such as the Quartz Filler Index defined under International Standard DIN 52047. This is determined by progressively adding quartz powder, with mixing, to a lOOg sample of the emulsion, the break index being given by the (minimum) number of g of quartz powder added to obtain breaking of the emulsion. Conventional bitumen emulsions such as K 170 and K 360 typically have break indexes of 80 and 120 respectively and can be used to coat coarse aggregate particles, breaking within a few hours in the case of K 170 and in under 1 day in the case of the slower K 360. Such emulsions break too quickly to be suitable for use in coating fine aggregate particles or for use in the final mixing. To coat the fine particles and for the final mixing stage requires a slow breaking emulsion having a high break index substantially greater than 120, preferably at least 140, for example in the range from 160 to 180. In general bitumen emulsions comprise around 40 to 70%, preferably 55 to 65%, w/w of bitumen and 60 to 30%, preferably 45 to 35%, w/w aqueous phase, (said percentages exluding the emulsifier component), and an emulsifier. In accordance with the present invention there is used an emulsifier which provides a highly stable, slow breaking, bitumen emulsion having a high breaking index as indicated above. A suitable emulsifier that may be mentioned is that available from AKZO NOBEL Ltd. of Littleborough, UK under the name Redicote EN4868 or C629. The amount of emulsifier used will depend on inter alia the amount and grade of bitumen used but typically could be around 1.0% w/w in the emulsion. In other respects the slow-breaking bitumen emulsion used in accordance with the present invention is generally conventional and the aqueous phase will normally contain salt(s) such as calcium chloride to avoid swelling of the bitumen droplets thus maintaining emulsion stability, and the pH may be adjusted (e.g. by adding an acid such as HC1) in order to maintain optimum performance of the emulsifier. Thus, for example, where the emulsifier is Redicote EN 4868 there may be present around 0.2% w/w of HC1 in the emulsion to provide a pH less than 7.0 and typically between 4.0 and 2.0. A suitable amount of salt is around 0.1% w/w of calcium chloride, in the emulsion. Thickening agents may also be present in the emulsion's aqueous phase in order to increase 'bitumen' film thickness on the aggregate particles to enhance durability. Typical thickeners are based on Hydroxy Ethyl Cellulose and its derivatives. One example is V100 available from CECA, France. The amount of bitumen emulsion used is generally chosen so as to provide a bitumen content in the final bituminous coated material mix of from 3% to 9% preferably from 5% to 7% w/w. Using a bitumen emulsion containing around 60 to 65% of bitumen, there is generally used around 4 to 8%, preferably from 5 to 7%, w/w of the emulsion relative to the final mix. In a preferred aspect the present invention provides a method of producing a storable bituminous coated material suitable for use as a footpath and for carriageway material, which method comprises the steps of: coating a coarse aggregate fraction with hot bitumen of a hard grade and allowing the bitumen coated coarse aggregate to cool; coating a fine aggregate fraction with hot bitumen of a hard grade and allowing the bitumen coated fine aggregate to cool; and mixing the bitumen coated coarse and fine aggregates with a bitumen emulsion based on a hard grade bitumen. As used herein the expression "coarse aggregate" generally indicates an aggregate having a particle size substantially within a range of from 3.35 mm to 28 mm diameter, and "fine aggregate" generally indicates an aggregate having a particle size less than 3.35 mm, usually from 3.35 to 0.001 mm, diameter. It will be understood that the relative proportions of the coarse and fine aggregates can be varied. Nevertheless it is important that their proportions are more or less carefully controlled in order to obtain a suitable void space within the laid and compacted material, which should generally be in the range from 4 to 16% v/v when the bitumen content is restricted more or less to that sufficient for binding the aggregate. In general for wearing course macadams the coarse fraction will constitute from 50 to 70% of the total aggregate, preferably there is used from 60 to 55% of coarse aggregate with from 40 to 45% of fine aggregate. The ratios of coarse to fine aggregates will vary for basecourse and roadbase layers as indicated in BS 4987. The amount of bitumen used for pre-coating the aggregate particles is generally selected so as to be sufficient for substantially coating the exposed surface of the particles but not so great as to cause substantial bonding between particles in the stockpile during storage resulting in loss of workability of the mass of coated aggregate particles. In practice there will generally be used from 0.1 to 3, advantageously from 1 to 2%, preferably around 1.5%, w/w of bitumen with the coarse aggregate and from 0.1 to 5, advantageously from 1 to 3.0%, preferably around 2.5%, w/w of bitumen with the fine aggregate. The amount of bitumen and bitumen emulsion in the final mix is generally chosen so as to be sufficient to bind the aggregate particles together, without however excessively reducing void volume and stiffness of the laid and compacted material. The amount of bitumen in the final mix will of course depend on the amount used in pre-coating of the aggregate particles together with the amount of slow breaking bitumen emulsion used in the final mix. As already noted the amount of bitumen and or emulsion used with each of the coarse and fine fractions can be varied considerably, as can the relative amount of the two fractions. The bitumen content of the slow breaking emulsion for the final mix can also be varied. Thus the amount of bitumen emulsion required will also vary correspondingly for any given required final mix bitumen content. The latter can vary typically from 3 to 8% w/w depending upon inter alia the application with, for example, lower contents being preferred for roadbase and higher contents being preferred for wearing course. Further preferred features and advantages of the invention will appear from the following detailed examples given by way of illustration. Example 1 - Preparation of Macadam Wearing Course Hot coarse aggregate (1200 kg, granite, Gritstone, Slag, Limestone or Basalt) chippings having a nominal particle size of 6mm was mixed with hot molten bitumen (18 kg) at a temperature of 150 to to 180°C in a conventional batch plant mixer for less than 1 minute to provide a fully bitumen coated material containing approximately 1.5% w/w of bitumen. The bitumen used was a straight run grade bitumen having a penetration grade of 200 and a typical viscosity of 85 cST at 160°C. The bitumen was obtained from Lindsey Oil Refinery of Immingham, Humberside and was a straight run penetration grade with no added flux oil. The coated aggregate was allowed to cool by storing at ambient temperature for typically 1 to 4 days prior to use. Hot fine aggregate (800 kg, Limestone, Gritstone, Basalt, Slag or Granite having a particle size of 0 to 3.35 mm was mixed with hot molten bitumen (20kg) as before to provide a fully coated material containing approximately 2.5% w/w of ' bitumen. It was then stored at ambient temperature to cool for 1 to 4 days prior to use. Coated coarse aggregate (1200 kg) and coated fine aggregate (800 kg) were then placed together in a conventional continuous of batch plant. Hard bitumen emulsion (143 kg) was added in while stirring the whole mass at ambient temperature (10°C) for 1 to 2 minutes. The resulting bituminous coated material was then stored in a stockpile for 14 days, prior to delivery to site for laying and compaction. The emulsion used had the following composition: % w/w Hard bitumen (200 pen) 65.0 Emulsifier (Redicote EN 4868) 1.1 pH adjustment (HC1) 0.2 Stabilizer (CaCl 2 ) 0.1 Water to 100% Examυle 2 - Use of Macadam The Macadam of Example 1 was laid to a depth of 65 mm (i.e. 30% surcharge) over a basecourse layer (laid using a cold mix macadam) and compacted using a Vibrating Plate (e.g. Wacker Plate) to a thickness of 50 mm. After three months the resulting compacted material stiffness was then assessed by removing core samples and testing in accordance with BSI draft for development DD213:1993 using the Nottingham Asphalt Tester (NAT) . The average stiffness Modulus (6 cores) for the 6 mm dense macadam at 3 months age was measured at 614 MPa. Table A10.1 in the HAUC Specification defines the minimum Elastic Stiffness for a 6 mm nominal size wearing course including a 200 penetration bitumen as 600 MPa. Hence the cold lay material according to the present invention demonstrated an Elastic Stiffness equivalent to that of a conventional hot mix material. In contrast a conventional "Storage grade" macadam, manufactured using a combination of graded aggregate and 'fluxed' bitumen would show typical stiffness values of below 100 MPa which is not acceptable for carriageways and would have its field of application limited to footpaths • only for use as permanent trench reinstatement materials i.e. conventional 'fluxed' or cut-back macadams made using fluxed bitumens or fluxed bitumen emulsions are not suitable to be used for trench reinstatements in carriageways, being only acceptable for footpaths.

Claims: CLAIMS 1. A method of producing a storable bituminous coated material suitable for use as footpath and/or carriageway material in a wearing course, basecourse and/or roadbase, which method comprises the steps of: providing coarse and fine aggregate particles, mixing at least one of the coarse and fine aggregate particles with hot bitumen or bitumen, emulsion in which the bitumen is a hard grade bitumen, so as to produce aggregate particles coated with hard grade bitumen, and then mixing the coarse and fine aggregate particles, in the substantial absence of soft grade bitumen, with a slow breaking bitumen emulsion in which the bitumen is a hard grade bitumen. . . 2. A method according to claim 1 wherein the or each said hard grade bitumen has a penetration grade in the range from 15 to 450 pen. 3. A method according to claim 2 wherein the or each said hard grade bitumen has a penetration grade in the range from 50 to 300 pen. 4. A method according to any one of claims 1 to 3 wherein each of the bitumen components is substantially free of viscosity reducing components. 5. A method according to any one of claims 1 to 3 wherein each of the bitumen components is substantially free of flux oils. 6. A method according to any one of the claims 1 to 5 whereas at least one of the bitumen components includes a modifier which does not substantially reduce the viscosity of the bitumen. 7. A method according to claim 6 wherein said at least one bitumen components is a polymer modified bitumen. 8. A method according to any one of claims 1 to 7 wherein at least one of the coarse aggregate particles and the fine aggregate particles is coated by mixing with hot bitumen. 9. A method according to claim 8 wherein both of the coarse aggregate particles and the fine aggregate particles is coated by mixing with hot bitumen. 10. A method according to any one of claims 1 to 9 wherein said slow breaking bitumen emulsion has a high break index substantially greater than 120. 11. A method according to claim 10 wherein said slow breaking bitumen emulsion has a break index greater than 140. 12. A method according to claim 11 wherein said slow breaking bitumen emulsion has a break index of from 160 to 180. 13. A method according to any one of claims 1 to 12 wherein is used from 3 to 9% w/w of the slow breaking bitumen emulsion in the final mix. 14. A method according to claim 13 wherein is used from 4 to 8% w/w of a bitumen emulsion which contains around 60 to 65% w/w of bitumen. 15. A method according to any one of claims 1 to 14 wherein is used coarse aggregate having a particle size substantially in the range from 3.35 to 28mm diameter and fine aggregate having a particle size less than 3.35 mm diamater

Inventors: ROBINSON HOWARD LLOYD, ROBINSON, HOWARD, LLOYD
Applicants: ROBINSON HOWARD LLOYD, TARMAC QUARRY PRODUCTS LTD, ROBINSON, HOWARD, LLOYD, TARMAC QUARRY PRODUCTS LIMITED
Application year: 1996
Main classification: C08L95
IPC codes: C08J3, C08L95, E01C19