- 29 - PROTEA ASPHALT'S EXPERIENCE IN BITUMEN-RUBBER J D DACHTLER INTRODUCTION Bitumen-rubber is a blend of bitumen and rubber crumb, this normally being produced from pneumatic tyres. It should be stressed that the product being discussed in this paper bears little relationship to the product used 40 years ago; the products in current use contain larger quantities of rubber crumb and are produced under carefully controlled production conditions.,.,, The main countries where bitumen-rubber is used are Australia and Southern USA; it is interesting to note the similarity in climatic and road infrastructure between these two areas and South Africa. All three areas are essentially dry, have high road temperatures, and do not have the full depth asphalt roads found in Europe or cooler wetter climates. The bitumen-rubber technology used by Protea Asphalt is based on the Sahuaro Petroleum and Asphalt Company, Phoenix, Arizona technology, and has been developed by protea to allow for the bitumen, rubber crumb, and climatic conditions, specific to South Africa. Bi tumen-rubber in South Africa is currently produced by three companies, and since its introduction to the South African market two years ago, its use has rapidly Expanded. Protea Asphalt are active in three main areas, these being surface dressing, crack sealing and bitumen-rubber premix.
r, - 30 - SURFACE DRESSING WITH BITUMEN-RUBBER Bi tumen-rubber is used in surface dressing as a Stress Absorbing Membrane (SAM) or a Stress Absorbing Membrane Interlayer (SAMI). The particular technical characteristics which make the material suitable for SAM or SAMI applications are its elastic recovery, good stone retention and resistance to bleeding; it is only when these particular characteristics are fully utilised that bitumen-rubber is being used at its maximum cost effectiveness. The roads in RSA are currently suited to rehabilitation techniques utilising the SAM or SAMI technique. Protea Asphalt produce bitumen-rubber by means of an inline blending unit, coupled to the distributor. The whole plant is mobile, allowing the bitumen-rubber to be manufactured close to the site where the binder is to be applied. In producing the binder, the following points have particular significance to the successful application of the binder. - Rubber morphology - Bitumen type - Reaction time - Reaction temperature In the production process the rubber partially reacts with the bitumen; some of the rubber is dissolved in the bitumen, whilst the remaining rubber particles expand and retain bitumen on their surface. The mechanism of this reaction is currently being studied, but it is thought to be a combination of absorption and occlusion. If the rubber particles are examined under a microscope, the nature of the rubber surface can be studied in detail. Rubbers that are ground at ambient temperature have a very spongy surface and these have a morphology suited to bitumen-rubber. Rubber that has been cryogenically ground, fractures more cleanly, and as a result the surface is less spongy; such rubbers will have a greater bulk density Prate a Asphalt's experience in bitumen-rubber
- 31 - ~ than the "spongy" type of rubber. Bulk density coupled with particle size distribution are the main criteria used by Prate a to control the quality of the rubber used in the manufacture of bitumen-rubber. During the production process the state of the reaction is determined by viscosity. The bitumen is absorbed into the rubber surface and the viscosity increases until a maximum viscosity is reached. The time period over which this maximum viscosity occurs is dependant upon reaction time, reaction temperature, bitumen type and rubber morphology. Once the optimum production condi tions have been determined in both laboratory and the field, it is important that the materials remain uniform, and that this uniformity is maintained by quality control of both the raw materials and the finished product. In maintaining the uniformity of the product, the following factors are determined in the laborato~y and field. Rubber Bitumen Reaction gradation, bulk density, ratio of rubber types asphaltene and aromatic content penetration index viscosity, temperature and time CUrrent experience indicates that if these factors are maintained within defined limits and if any fluxing material is added at a constant rate, the resulting product is uniform, and has optimum viscosity and elasticity characteristics. When bitumen-rubber is applied to the road, the method of application is extremely important. The binder distributor used by Protea is manufactured by Bearcat and has been specially designed for spraying bitumen-rubber. A mechanical agitation system is fitted; this ensures that the product is homogeneous within the tank, and improves the heat transfer characteristics, thus minimising any thermal degradation caused by local overheating.
r p! - 32 - The pump and spray bar are designed to spray highly viscous materials. This has certain technical advantages in that whilst the amount of diluent is kept to the absolute minimum, relatively low spray temperatures of l60-l75 0 C can still be used. These factors are particularly beneficial to the mechanical properties of the bi turnen-rubber, since they result in better elastic recovery I and reduce harmful thermal degradation, thus permitting longer storage life. Load cells are fitted to the spray tank so that immediate distribution rates can be determined by mass, this having obvious technical advantages in spray rate determination, since any temperature/volume effects are eliminated. The special design of the spraying system allows a triple overlap to be achieved, even though an extremely high viscosity binder is used. This is important in the successful application of bitumen-rubber since the common problem of poor transverse distribution (roping) is eliminated. As Protea' s rubber-bitumen is both as elastic and adhesive binder, loss of chippings due to poor adhesion, or brittle fracture are unlikely events, providing the stone is precoated and the binder sprayed at road temperatures in excess of 20 0 C. To achieve maximum benefit from bitumen-rubber, it is essential that high quality stone is used. The use of stone with a flakiness index of 20 per cent rather than the normal 25 per cent is preferred. The stone should be applied directly after application of the binder and followed by pneumatic tyred rolling as quickly as possible. It is important to match the relative speeds of the binder distributor and the self-propelled chip spreader to achieve a continuous application process.
- 33 - Providing due care and attention is paid to both production and application, bitumen-rubber is an extremely durable binder, and will prove very cost effective in the rehabilitation of trunk roads, where the objective is to extend the road life prior to complete reconstruction..
- 34 - A P PEN D I X A SPECIFICATIONS USED BY PROTEA ASPHALT IN THE PRODUCTION OF BITUMEN-RUBBER 1. RAW MATERIAlS 1.1 Bitumen The grades of bitumen used are 60/70; 80/100 or 150/200 pen, as per SABS 307. The actual grade selected is based on: (a) The prevailing climatic conditions in the area where the material is to be used.' (b) The reaction with the available rubber granules as pre-determined by the laboratory. 1.2 Rubber The granulated rubber used meets the following specifications: (a) Vulcanised rubber content 80 per cent m/m (b) Natural rubber content (including polyisoprene) as a percentage of total hydrocarbon rubber : 30-70 per cent m/m
~ " 35 - ~ (c) Gradation Percentage passing (m/m) sieve Size (mm) Type I Type II TYl?e III 2,36 100 2,00 95-100 1,18 100 100 0,60 0-10 60-90 95-100 0,30 0-5 0-20 30-60 0,18 0,5 15-35 0,075 0-10 The type of rubber selected is based On laboratory tests to determine the reaction between the rubber and bitumen. (d) Maximum fibre lengt.h 6mm ee) specific gravity 1,19.:!:: 0,05 (f) The granulated rubber shall be free of loose fabric, wire and other contaminants. 1.3 Extender oil Depending on the fractional composition of the bitumen, the addition of a bitumen modifier with the following properties may be required:! Viscosity (c.s at loooc) 5-25 Flash Point c 200 min. Aniline Point 25-60 Saturates (m/m) 25 max. 1.4 Diluent In order to adjust the spray viscosity if required, a diluent may be added. The diluent shall be kerosene according to CKS 78 or CKS 215.."
0_ 36-3. MIXING PROCESS The rubber granules are dispersed in t.he bitumen by means of a special in line blender. The blended product is then reacted to the required specifications. During the reaction the product is continuously agitated by an auger mixer situated at the bottom of the spray tank. The blending and mixing are carried out wi thin the following limits: Percentage rubber (by mass of blend) o Reaction temperature ( C) Reaction time (hours) o Final reaction viscosity (c.p at 165 C) Diluent (% m/m) 18-27 170-210 0,5-4 6000-15000 o - 6 4. SPRAY SYSTEM AND APPLICATION The binder distributor has a number of unique features specially incorporated for the successful spraying of bitumen-rubber. o Spray temperature C Spray viscosity cps 160-170 1000-6000