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Glynn Holleran, Vice President, V.S.S Asphalt Technologies, USA Dejan Ristic, Director, ITG, USA Back to VSS Technology Papers Library 1. INTRODUCTION In most countries of the world pavement maintenance is a significant issue. The Federal Highways Administration (FHWA) in USA has recently developed a policy to spend more money on maintenance as a result of a successful study during the Strategic Highway Research Program(SHRP) In developing countries, where infrastructure is being rapidly built, the need for maintenance is less obvious but as critical. In the developing countries of Southern Africa economic futures will depend on a reliable road system. The methods to achieve this must be based on sound engineering, economic, environmental and quality principles. They must be able to be carried out far into the future. The methods to both maintain pavements in good condition and to upgrade failing or under-designed pavements are available and being applied in range of countries from Russia to China to South East Asia , the middle east and Africa. This paper discusses the experience gained by Valley Slurry Seal Company (VSS) in these areas in the development of practical and cost effective solutions. This bases it's projects on Total Quality Management (TQM) planning principles and attention to technical details from raw materials , design, manufacture, applications equipment and distress analysis. The aim is always to provide the best , and most economic technical solution. In most countries environmental issues are , or are becoming, key. Thus, cold applied systems based on bitumen emulsions are a focus. This paper discusses general principles of pavement maintenance and how TQM and Quality Assurance (QA) were effectively used in difficult field situations. Emulsion manufacturing parameters and raw materials are considered, including their effect on performance. Aggregate and set control additives are discussed. The use of slurry in combination with injection patching, chip seals and base recycling is briefly discussed. 2. PAVEMENT MAINTENANCE All pavements fail. The question is only when. This is obviously an engineering question and many design manuals have been developed to address this. The purpose of preventative maintenance is not to extend a pavement structure's life , but to allow it to reach its design life. Figure 1 shows the typical failure curve.
Climate and traffic are the main enemies of pavements leading to premature failure. Water ingress into pavements is a key cause of failure. For this reason, most preventative maintenance programs are aimed at keeping the pavement sealed and water proof. Bitumen ages, and as it does it will embrittle, ravel and crack. Rutting may occur in surface layers and the surface may crack due to shrinkage or reflection of joints or existing cracks (1). As shown in figure 2 a preventative maintenance treatment can renew the surface, prevent failure mechanisms from developing , and the surface is renewed. This may be repeated to achieve the design life of the pavement structure. (figure 2). Corrective maintenance is about rehabilitation that is, what can be done after failure has occurred. Pot holes, crack propagation, and stone loss once they have occurred must be repaired before the renewing surface is applied. In this case the base may even be beyond its design life and have significant deflections. A strain alleviating membrane rehabilitation (for example using a polymer modified or crumb rubber modified bitumen) is then of use. In some instances the failure may have already led to base failure and more severe re-constructive maintenance is required.
3. MAINTENANCE PROGRAMS, THE ROLE OF TQM AND QA This will be illustrated by actual projects in Russia and China. Quality is about pre determined outcomes. In the case below a smooth, water proofing, crack sealing, skid resistant and durable surfacing that will last 5-8 years is desired. The road system around a major city in Southern Russia consists of hot mix on granular bases. Maintenance has not been carried out on many areas on a regular basis. Slurry surfacing was chosen as the method, based on studies by the local authorities and by VSS in Russia. In the Russian context ambient temperatures of +40C to -30C needed to be allowed for. This meant special attention to binder rheological properties, binder content in the mix and additives in emulsion. The chemical nature of the bitumen needed to be accommodated to ensure emulsion stability and rapid cure in the field. A quality program was developed which contained the following:
3.1 Raw Materials and Mix Design Slurry surfacing consists of four main materials; emulsion, water, aggregate and set control agents (retarders), they are mixed in precise proportions in specialty equipment.(6). 3.1.1 Emulsion 3.1.1.1 Bitumen The key factor in many developing countries is raw materials. Bitumen is a key component of the emulsion In this case the bitumen to be used was made by air blowing of a pitch residue from a wide range of local crudes. Analysis was carried out using chromatographic fractionation methods (3,4).(Figure 3) This based on the "circle of acceptance" showed that the bitumen was heavy in cracked aromatics and of relatively high asphaltene content. Emulsification , while producing an adequate quick set slurry emulsion, resulted in poor storage stability leading to product separation (creaming) when held over 10 days. The emulsion also tended to reduce in viscosity with shearing (pumping) and to coarsen and separate. The modification of the bitumen was not an option due to the lack of availability of a suitable feedstock blend material. So an emulsifier solution was sought. Non ionic and slow setting stable emulsifiers (low zeta potential) to allow better packing of the emulsifier at the bitumen/water interface and hence a more stable emulsion were tried with success.
3.1.1.2 Polymer Modification Polymers were also considered to address the temperature extremes (high and low). These included crumb rubber, Styrene butadiene block copolymer (linear and radial) preblended with the bitumen prior to emulsification , EVA preblended with the bitumen prior to emulsification , Styrene butadiene random copolymer in latex form co milled with the emulsion, neoprene in latex form co milled with the emulsion , and crumb rubber dispersed in a solvent (RG-1) post- added to the emulsion were tried. Fibre was included in one experiment. Figure 4 shows the Dynamic Shear Rheometer test results (SHRP method) on these binders. Bending beam Rheometer testing was not carried out but results from other studies indicate good low temperature performance by each of these systems. Mix results are shown in figures 5, 6, and 7. The elastomeric materials appear to perform best.
3.1.1.3 Manufacturing In this study manufacturing parameters were also considered. These parameters have been extensively discussed elsewhere (2,6,7) The key properties of an bitumen emulsion are the stability in storage, the stability ,coating and adhesion in mixing, and the rate of curing. Many of these properties are functions of the particle size and particle size distribution of the emulsion. This in turn is a function of the type and grade of the bitumen, the equipment used for emulsion manufacture ,and the chemicals used for stabilization. The particle sizing is particularly important. As the particle size becomes more mono-disperse and finer the viscosity, break rate and adhesion will all be optimized. However this limits the bitumen content of the emulsion and can effect cure rate.In general ,a narrow distribution of fine particles in the 1-5 micron range, has been found by VSS to give the best overall properties.Figure 8 shows mean particle size created by manufacture under different conditions with different equipment. Clearly use of a high shear mill set at a tight gap- like the Charlotte mill - will produce narrower distributions.
3.1.2 Aggregate Aggregate for example is critical to the performance of a slurry seal or microsurfacing. Important aggregate properties are hardness, durability and resistance to polishing. In both cases, hot mix is successfully made and laid and these properties were not at issue. However , grading and sand equivalent (SE) were significant issues. Neither project had an aggregate that could meet the base microsurfacing SE and both had problems with oversize. That is material outside the upper limits of the ISSA type II (6mm top size). In this case aggregate passing 25mm in excess of 3% was measured. Pre-washing of aggregate to remove clay materials was considered but facilities were not available. Screening of oversize requires a special screen and will occur next season. 3.1.3 Set Control Agents In many situations weather can be changeable and field adjustments must be made to maintain traffic times. In general set control agents are materials such as sodium or calcium salts, chlorides, borax or surfactants These provide a prophylactic action on the aggregate, that is they form a barrier to the interaction of the emulsifier and the aggregate. In many microsurfacing systems cement is used or sometimes lime. This acts as an accelerator by increasing pH and destabilizing the cationic emulsion. By careful choice of emulsifier system and set control agent with respect to aggregate chemistry, the set can be controlled to allow acceleration or retardation of the cure time. Figure 9 shows the effect of traffic time on additive level for three different ambient temperatures for the Russian system.
4. SLURRY IN COMBINATION WITH OTHER TREATMENTS 4.1 Injection Patching with Slurry Overlay Slurry surfacings have been found to have lifetimes of 5-10 years (2). However the performance depends on the base. Surface preparation is key to successful jobs. In a study by SHRP, and picked up by many DOT's in USA , injection patching was examined (8). The three methods used were the traditional throw and roll, semi- permanent method (where the hole is prepared and tacked), and the spray injection method. In USA State Highway agencies spend $400 million per year on patching. (Cities and counties almost double that). This study found that the injection patching method not only provided permanent (7 years) patches but also did this at a much lower cost. On DOT work alone this would be $89 million per year saving. Injection patching is performed with a specialty machine (see figure 10) that:
Slurry surfacing can be applied within 48 hours (or faster in high temperature conditions).
4.2 Modified Cape Seals Modified Cape seals were originally created to allow slurry surfacing application on cracked pavements. The technique was developed based on Strain Alleviating Membrane (SAMI) technology used often in rehabilitation with hot mix overlays. The pavement is assessed for failure mode. The system is suitable for block cracking and fatigue cracking where cracks are 5mm or less in size. In cases where larger cracks exist they must be filled first. An bitumen rubber chip seal is carried out as follows: The binder, consists of 18-20% rubber (80% ground tyre rubber of a suitable grading with 20% natural crumb(passing a 1.16mm sieve). A heavy aromatic extender oil is often used at levels of up to 3%. The binder is blended in a low shear system with a mixing auger or a propeller mixer. It is "reacted" with the bitumen for about an hour and used within a few hours of manufacture. The Figure 12 shows a micrograph of the finished binder. It will be noticed that large chunks of rubber are present, the conjecture (9,10) is that the large chunks act as crack blunting sites, analogous to butyl rubber in high impact polystyrene.
The binder is sprayed at 195-215C at a rate of up to 2.5lt/m2 by a calibrated sprayer- often with a fumes extraction system. The aggregate (10-16MM) is precoated with hot bitumen (60pen) and applied at a temperature between 45-60C via an automated spreader. The rolling is carried out almost immediately after spreading. Pick up is avoided by use of water. Sweeping to remove loose chips is carried out soon after. The slurry course is applied within 2 days and is usually a polymer modified microsurfacing that covers the entire bitumen rubber seal, filling all voids. Such systems have been recorded to last in excess of 8 years with little crack reflection. Such systems have been successfully applied to residential streets, transport yards and highways. 5. CONCLUSIONS
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