SLURRY SURFACING Russian Road Federation Conference, Tambov, 1997 Glynn Holleran, Vice President, Valley Slurry Seal Company, USA Back to VSS Technology Papers Library 1. WHAT IS SLURRY SURFACING? Slurry surfacing is an application of a graded aggregate bound by a bituminous binder. It is essentially a cold mixed asphalt with a number of advantages related to the mix design and the manner in which the binder is applied. The material is mixed and applied in a mobile unit either self propelled or truck mounted. Figure 1. Figure 1 The binder applied is a quickset, or quick traffic emulsion. In USA, as in most parts of the world this emulsion is a cationic type. This has a number of advantages over anionic types. These include rapid interaction of stone and binder, creating strong adhesive bonds, compatibility with a range of stone types and rapid setting. Slurry surfaces have low void contents and, being essentially self leveling have high stability, or deformation resistance. They can be made with different stone sizes and bitumen types that allow this property to be optimized. In thin layers slurry surfacings are very flexible, being relatively binder rich but the high surface area of the aggregate creates stable films and hence they will not bleed. 2. DEFINITION Slurry surfacing is the application of a graded aggregate, a bituminous emulsion, fillers, and water with various additives to make a cold mixed material that cures quickly into a hard wearing surface. 3. REASONS FOR SLURRY SURFACING Slurry surfacing provides: a running surface for traffic a water resistant seal Minor shape correction for lateral deformities and longitudinal ones (of frequency shorter than skid length). Rut filling A textured finish of excellent skid resistance.(figure 2). An appearance of hot mixed asphalt with low noise.(figure 3). In a planned maintenance program it can reduce life cycle costs by 38%. (figure 4). The slurry surface is laid with a continuous paving machine that meters exact levels of bitumen emulsion, aggregate, additives and water.(figure 1) 4. TYPES OF SLURRY SURFACING 4.1 Slurry Seal A thin layer of slurry surfacing, about 1 to 1.5 stones thick used to resurface a pavement. Such materials may or may not be polymer modified. If they are the modification is usually in the form of a latex whose function is to assist in creating early strength and to increase flexibility. Increases in softening point of the binder can also assist in increasing softening point of the binder. Slurry seal mixes are based on smaller stones and limited to about 8mm thick (1.5 times top size). Slurry seals are used to seal sound but oxidized pavements restore surface texture and improve skid resistance. correct raveling or loss of fines in hot mix reduce noise of large seals - cape seal. where thicker overlays are not possible due to level restrictions imposed by kerb and channel. seal minor surface cracks improve ride. 4.2 Microsurfacing A layer of slurry surfacing capable of correcting minor shape deformities and usually containing a modified binder. This type of material is able to be used as rutfilling. This may be laid in multiple stone thicknesses. These may be made with larger stone up to 10mm top size. The polymer modification allows the use of larger stone without the danger of raveling and the higher cohesion means that thicker layers can be supported without deformation. Polymer modification for such mixes can include EVA and latex modification, this provides superior rut resistance. Layers of 50-75mm are possible. Such materials are usually very quick traffic, that is they build up cohesion very rapidly. Microsurfacing is used for: Rutfilling Widening shoulders Minor shape correction. All slurry seal applications. 4.3 Mixes and applications Mixes can be specified by application type as above or more specifically as in figure 5. This of course means that the contractor and the customer must agree on the problem at hand and the treatment chosen according to experience. Other designations currently in use are: By Grading There are two ways in which mix types have been classified in U.S.A to date. These are based on the aggregate grading used in the mix. They are basically as a result of different overseas methods of expressing essentially the same thing. These are: ISSA (International Slurry Seal Association) type I, II, III. The gradings are shown in figures 6. The main purpose of the gradings are to give guide, the only way to ensure performance is consistency and a smooth grading curve. There are also designations based on top size, this is used in Australia, Germany, Spain and France. By Emulsion Type Slurry surfacings are also often specified by their emulsion type. Anionic or cationic. Quick set, quick traffic, slow set. This designation is fraught with danger however as an emulsion that may be slow set with one aggregate is quick set with another. A quick traffic polymer system may be slow set with the wrong mix design. By mix characteristics This is by far the best method as slurry surfacing mixes are complex mixtures of materials, it is important to have all components in correct proportion, correct grading and correct emulsion to have a successful job. Mix characteristics have evolved over a period of time to allow a specification that will give good performance. As mechanical testing is improved and materials, particularly different polymer types, are developed then the range of performance will improve. ISSA specifications for slurry seal and slurry surfacing have been the model mostly. Guide specifications for slurry and microsurfacing are attached. It should be noted that: all existing specifications are a mixture of recipe and measurement of physical properties based on experience. Setting time or traffic time has been the major emphasis of many design methods, rather than final product characteristics. It has not been reflected yet in many specifications. ISSA have spent a lot of effort in attempting to simulate field properties with test methods, and these, though still empirical, do give a good indication of field performance. Such tests include, wet track abrasion for fretting resistance, cohesion testing for screwing resistance (power steering) and wheel tracking for rolling traffic. Other field type test include adhesion based tests that give information about propensity to strip. For example a design would include grading binder content. wet cohesion (indication of traffic time). wear resistance (wet track abrasion). aggregate quality. adhesion Figure 8 shows property requirements. 5. DESIGN 5.1 Design Objectives A pavement system consists of two elements, the pavement structure and the pavement surface. Slurry Surfacings, both seals and micro surfacing are designed to restore the surface element only. The technique will not repair deficiencies in the base. For example, traffic induced rutting caused by low voids in hotmix or low stability can be repaired, rutting caused by base collapse cannot. The design objectives are: Prevention or control of weathering of the surface. Repair of weather or wear damage. Improvement of friction characteristics. Improvement of surface drainage problems caused by rutting and cross slope deficiencies. Compared to hotmix slurry systems are significantly more complex. They are chemical systems with multi-components. The binder must harden by a combination of chemical reaction and evaporation. The mechanical properties are determined by how well this process proceeds. 5.2 Materials Basic Considerations- Components and Complexity There are many thousands of aggregates, asphalt is made by different methods and from different crude's, and even the water changes from place to place. This means there are millions of possible combinations, especially when you throw in the different emulsifier types. In some other countries the complexity is reduced by using only a few aggregate types. In U.S.A we do not have that luxury. Obviously not all combinations can be tested, the mix design procedure is thus a systems approach. It examines the characteristics of the aggregate and then chooses the bitumen/ emulsion chemistry to suit. Additives and modifiers are added to change the mix characteristic and the final result is a working mix. The questions that must be answered in any mix design are: WILL IT MIX? WILL IT SET? WILL IT LAST? WILL IT BE SAFE? WILL IT PERFORM? The ISSA via extensive field experience and theoretical understanding have developed a set of design methods and criteria that may be used. It begins with aggregate assessment and emulsion assessment. Aggregate The main requirements of the aggregate are gradation and sand equivalence. Clay content is also checked by use of the methylene blue (ISSA ) test, The performance of the mix in both setting and mechanically is dependent on the gradation. The gradation is chosen depending on the job type. It must be then established that the aggregate is sound and passes all the required statutory tests (comes from an approved quarry). The chemistry of the aggregate is extremely important but this is checked later in the process. Emulsion The emulsion is chosen to be either a quickset or quick traffic, this depends on the application. The binder in the emulsion is chosen for the softening point (must be >60oC) and general elastic properties. The binder usually used should give excellent elasticity but also rapid cohesion build up and high softening point. In some cases a latex modified emulsion may be used. The nominal set time of the emulsion is also checked at this stage. This is to ISSA tests and the emulsion is identified as quick set or quick traffic. Interaction At this stage of the design potential interactions must be considered. - Aggregate Chemistry. Almost all aggregates have a negative charge, this will vary according to the aggregate chemistry. For siliceous it will be high and for calcareous it will be lower. Even limestone has a weak negative charge, but only when wetted. This has an effect on interaction. Other factors that are very important for reactivity include absorption, chemical reactivity of the charged species, ion exchange between the emulsifier and the aggregate, porosity and density. Aggregates make up about 75% of the slurry mix volume. Chemical reaction between the aggregate and the emulsion determine directly: adhesion, mix stability, compatibility, set, and cure. The aggregate may be evaluated with a standard emulsion to determine mix times, cohesion and compatibility. If the aggregate fails it may need to be rejected or emulsion changed. - Emulsion Chemistry The role of the emulsifier is critical and complicated. It must produce a stable and fine emulsion. That is the emulsion must be able to coat the aggregate evenly and react. The emulsion, if coarse can trap water and cause longer set times. The emulsifier reacts with the aggregate surface so must be compatible with the aggregate. In the applications here the emulsifier is cationic. This produces a positive surface charge on the emulsified particles. The reaction with the aggregate is a physiochemical one. Slurry emulsifiers of this type can react by ion exchange at the surface or electrochemically to neutralize charge. Other reaction paths involve depleting the emulsion of water and destabilizing it. The control of the reaction rate controls the setting. This is achieved by additives. Emulsion may be evaluated with a standard aggregate for properties such as mix stability, set and compatibility. A boiling test is also used to measure adhesion Additive requirements can also be evaluated. The level of emulsion used is set by experience or may be determined on a surface area basis, using a standard film thickness of 7.5 microns. If the emulsion fails it may need to be reformulated or the aggregate or additive system changed. - Additives/ Fillers The ability of a slurry seal system to be chemically modified is a very important requirement to modify the mixing, setting and performance properties. Additives can improve strength, improve adhesion or cohesion, alter the charge of the aggregate, slow down or speed up the setting or curing time or extend the mixing time. Additives to aggregate alter the gradation and act in the same manner as fillers in asphalt. e.g. lime, cement, gypsum etc. Such additives, via their chemistry and high surface area can also modify mixing times and set. Which is to be used depends on the rest of the materials. Other additives in prewet water act on the aggregate surface and can accelerate the emulsion reaction, or more commonly retard it to allow a control mechanism. Examples of this are aluminium sulphate, ammonium chloride (usually used in conjunction with cement), emulsifiers, other inorganic salts and water soluble bases or acids. Additives may also be added to the bitumen. This may be to improve the mechanical properties or alter the emulsion properties. Typical polymers are EVA, SBS. Latex may also be added to the emulsion. (See attached ISSA paper by G. Holleran). Additives are often also used to improve the emulsibility of the bitumen to be used, eg amines, tall oil, peptizing (dispersing) agents. Additive effects are not universal, cement may be an accelerator in some systems and a retarder in others. At high pH cement can reduce adhesion, at low pH it will improve it. There is not only an optimum type but also an optimum amount. Accuracy of addition is thus also vital. The considerations above decides a job mix formula giving the emulsion type and approximate level, aggregate, and additive types. Job Mixes Once the general parameters are established and modifications are made to the aggregate or emulsion the job mix may be carried out. Aggregate The specific gravity and bulking factor are determined. This allows adjustments to be made for aggregate water content in the field. Emulsion The binder content is determined as well as storage stability and binder characteristics. Interaction The interaction is first checked between the aggregate and the emulsion. This is done by the ISSA split cup consistency test, split cup compatibility test, and the ISSA mixing test. We now know that it will mix! Once the mix has passed these requirements it can be evaluated as either a slurry seal mix or micro surfacing mix. (ISSA). For a slurry surfacing mix the method goes directly on to determine optimum additive levels using the cohesion test (ISSA), optimum binder content using wet track abrasion and loaded wheel testing. The final mix can then be characterized by establishing working ranges and cross checking back with specification testing. For a micro surfacing mix there are several other steps. Microsurfacing System Integrity Specification evaluation begins with a measure of the system integrity.( compatibility/ adhesion) The Schulze Breuer test employs pills of slurry that are soaked for six days then subjected to abrasion in tubes filled with water. A boiling test is also included. Weight loss is recorded. The test then provides a classification that may be used in specification. Also wet track abrasion loss is measured after one and six days. This gives an idea of the wear performance. We know now that the mix will stay together once set and will perform as a surfacing. Cohesion To answer whether the mix will set to the required level in the required time we have previously looked at simple mix time tests. In the cohesion test samples of the mix are cured in pats for various periods of time and then subjected to a standard shearing stress. The torque can be related to the cohesion build up. For microsurfacing lower limits of 12 kg/cm are set for 30 minutes set and 20 for 60 minutes. The additive type and level can be confirmed at this stage. We now know that it will set in a reasonable time. we do not know the field set time exactly as conditions will effect this but this can be adjusted with additive level. It is useful to also take cohesion measurements at a range of temperatures and humidities. This will give allow the set times to be adjusted with additives. Binder Content The loaded wheel test is done at 3 binder contents to establish a graph of sand adhesion versus bitumen content (figure 9). The Wet Track Abrasion is similarly done at 3 binder levels, including a 1 day soak test (6 day for microsurfacing). The results are plotted as abrasion loss versus bitumen content. By combining the two graphs an optimum bitumen content range and hence emulsion range can be calculated. These two tests help answer the question, "will it perform? Water content The water content is based on total fluids contents of around 20?25% This is determined in the mixing tests. It is also necessary to consider the water quality. Hard water contains calcium ions that act as a stabilizer. This can retard set. The way around this is to do the design with the water to be used locally. If too hard to obtain a good mix then water softener may be required. Conclusions - Final Mix The performance of the mix in terms of the main requirements is assured by control of the emulsion, aggregate and the testing discussed. Will It Mix? is answered by mixing tests, consistency tests and compatibility tests. Will it Set? is answered by mixing tests and cohesion tests. Will It last? is answered by rolling wheel and wet track tests as well as integrity tests an adhesion tests. Aggregate soundness tests are also important. Is it Safe? is answered by using only safe additives and practices. Will it perform? is answered largely by experience that the testing does have a relationship to the performance. Skid resistance via pendulum testing or scrim machine. Noise by trials. rut filling by stability testing. Sealing by boiling tests and permeability testing. Safe running surface by early cohesion results and resistance to flushing. Combination of wet track and rolling wheel for rutting. 6. APPLICATION Application is by specialized equipment. Surfaces are prepared as for chip sealing. Though and preparation must be given to ensuring good stockpiling procedures, supply of potable water and supply of additives. The VSS Macropaver ® provides state of the art metering, mixing and spreading to ensure that the mix design is delivered to the road. Information on this equipment is available from ITG. VSS can supply specialist advice on application methods. 7. TROUBLE SHOOTING 7.1 Common Problems Flushing Surface A flushed surface can be caused by too high a binder content in the design, a lack of or loss of fines in the gradation or attempting to lay too thick a layer in one pass. This results in segregation of the mix and a floating of the binders with binder to the surface. Other causes of such segregation are too much water or poor grading. Premature setting Of The mix This will,lead to a poor mat surface of low cohesion and raveling. It is caused by poor aggregate of high clay content and low sand equivalent, inadequate emulsion, excess of fines, emulsion hot (> 30C), a dusty or wet base, insufficient additive, base at too high a temperature, insufficient water or too much cement or other fines added. Colour Differences This may be caused by too much water, clay in the aggregate, varying emulsion rates or equipment problems such as a worn screed or pump failures. Peeling This is caused by clay in the aggregate, inadequate emulsion or dust on the pavement. Poor Surface texture This is caused by inadequate emulsion, lack of fines or too much cement. It can also be due to dragging of partially broken slurry by the spreader box. Raveling caused by lack of binder, poor grading, the slurry being laid at too low a temperature or at too low an application rate. Such problems are best addressed by improving mix design and paying better attention to application. 8. CONCLUSIONS Slurry Surfacings are an effective method of surfacing. Microsurfacing is an effective method of filling ruts and smoothing surface imperfections. They may be used on all types of road surface. Slurry surfacings can be designed to meet the objectives of surface requirements. They require careful laboratory testing, good equipment and careful attention to detail in application. Back to VSS Technology Papers Library Copyright © 2000 Valley Slurry Seal Co All Rights Reserved.