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G. Holleran. Vice President Valley Slurry Seal Company Back to VSS Technology Papers Library1. INTRODUCTION Roads are an important resource for the country’s economy, no matter what that country may be. The economics for the use of different surfacing types varies from country to country but in all cases the need for economic construction and maintenance techniques are obvious. In many countries, such as South Africa, New Zealand and Australia surface treatment methods have become road surfacing methods. In these countries roads are constructed from granular bases, sometimes stabilized with lime, cement and or asphalt emulsion, and place on natural or stabilized subgrades, depending on traffic. In USA and Europe it is more likely that such treatments would be called surface dressings and placed as a protective layer over hot mixed asphalt pavements or as a rehabilitation for PCC surfaces. It is useful to review what we expect a road to do to be able to consider alternative treatments. Figure 1 shows a typical road surface.
Roads are in essence mechanisms by which stress applied from moving wheel loads are transferred to the earth. To operate effectively they must not crack, rut or wash away. The distress of road surfaces generally take one of these forms. (see figure 2)
Road surfacings are applied to: a) To make the road surface waterproof. b) To control dust. c) To make a smooth and non skid surface. d) To stop disintegration of the pavement. a) and d) relate to preserving the structure of the pavement, b) and c) relate to safety and comfort of the motorists and people in the vicinity. In instances where the road surface or structure has failed then the rehabilitation method must address and correct the failure mode. Techniques to achieve these ends are many. They range from hot mix overlays, polymer modified membranes as stress absorbing seals or interlayers, recycling of distressed pavement layers with emulsions or foamed asphalt, chip seals and slurry seal and microsurfacings. This paper discusses a method that combines the properties of a chip seal with that of a slurry. This method is referred to as Cape Seal. The desirable properties of chipseals are:
The desirable properties of slurry seal are:
Combination of these properties allows an economic, smooth and hard wearing surfacing that can address many of the failure modes and thus be an effective maintenance, rehabilitation and construction method. Thus a Cape Seal May be defined as the application of a chip seal followed by one or more layers of asphalt rich slurry seal. ( figure 3).
2. HISTORY The development of Cape Seals is disputed. The concept of filling the gaps between the stones in a chip seal to improve the ride and extend life of chip seals has been applied in a number of countries over the years (1). The process is similar in purpose and design to macadam, but then all aggregate asphalt mixes in whatever combination or method are similar in principle. In California for example it is claimed that slurry type materials made from premixes of aggregate and emulsion were placed over chipseals in order to overcome problems of premature failure by aging or stone loss as early as the 1930’s. South Africa The first Cape Seals as such were not based on slurry at all. They began in South Africa with the development by the Cape province, centered around Cape town in South Africa, of a process of applying a hot premix of crusher dust and asphalt over a 19mm seal and was first specified in 1950.( 2). This was largely done to improve durability of the existing single and multicoat chip seal methods and was only used initially on new constructions, and then with traffic restricted to 300 heavy vehicles per day. Hot mix was used as wearing course on more heavily trafficked roads. This appears to have become a modern Cape Seal in which a 19mm chipseal was coated with two layers of slurry seal and a 13mm chipseal coated with a single layer of slurry around 1957. The impetus for this was mostly to improve workability of the aggregate mix, preventing balling at high asphalt contents. In this process the first layer of slurry has been generally applied by hand to work the material down into the matrix. (3). It should be pointed out that chip seals in South Africa are generally prepared with single sized aggregate rather than the graded aggregates used in USA. Australia In Australia Cape Seals date back to the early 60’s where anionic slurry was applied over large stone chip seals to improve ride and to increase durability. Again this was used for new constructions. More recently the process has been used in an indirect way in which slurry is used as a rehabilitation method over chip seals as a means of replenishing binder, as the seal has aged. Slurry in this instance is considered as a void filler. Any size stone from 7-20mm may be used in the chip seal and type I or type II USA The first projects using " Cape Seal" in the USA represent a technology transfer from South Africa, a little like the transfer that has occurred in recent years of stone mastic asphalt. This appears to have been a result of an ISSA paper by Robin Campbell in 1977. John Huffmann, then at the asphalt Institute, also contributed to the introduction. The use of Cape seal in these situations was mainly as a maintenance process over existing pavements. A number of projects were carried out in Northern California on farm arterials with chip seals applied using a 6-7mm top size stone and a type II slurry and on city streets and major arterials using 9-10mm top size aggregate and type II slurry. In this instance heavier traffic generally requires a larger stone in the chip seal, the slurry remains unchanged as it is essentially a void filler. Type I slurry has also been used for this purpose. The market did not build much from this level in the years following, principally because the application of chip seal and were considered separate processes. This could lead to technical problems of aggregate overspread in the chip seal, asphalt emulsion levels being too high in either the chipseal or the slurry, slurry being overapplied and totally covering the chip seal, or excessive chip loss in the chip seal before slurry application. These sometimes lead to poor surface finish, poor skid resistance, bleeding and a general perception that Cape seals were not as advertised! This problem was largely rectified in California by the emergence of contractors that carried out both chip and slurry work. In 1984 an upswing in use began with Northern Californian cities such as Salinas and Sacramento. Here the process is used for arterial roads and residential areas. Currently about 15% of surface dressing is carried out in cities using Cape seal and about 5% of work in counties. The numbers are somewhat obscured by the fact that many counties do their own chip seals and slurry may or may not be applied as a part of a Cape Seal. Developments that have assisted in the increase in Cape Seals relate to the use of modified binders to promote crack resistance and increase stone retention. This will be discussed later. Cape seals fill the gap in the cost effective rehabilitation of pavements between straight surfacings such as slurry and chip seals and hot mixed asphalt. Cape seals are viewed as direct alternatives to more costly overlays. 3. DESIGN OF CAPESEALS Design methodology must take into account three distinct aspects a) The chip seal design b) the slurry design c) the combination effects to achieve the final performance. To achieve this requires a clear picture of what the expected outcome is to be. This is a single layer of aggregate placed shoulder to shoulder on a film of asphalt with a layer of slurry filling the interstitial voids of the chip seal and just leaving the tops of the stone exposed. ( figure 3). The following design methodologies have been used in South Africa , Australia and are similar to the AEMA guidelines in USA. 3.1 Chip Seal Design Rational Approach a) Materials. The materials used for making a chip seal are key to its performance. They must be carefully chosen and understood. The basic materials are: i) Binder ii) Cutters/ Fluxes iii) Aggregate. i) Binder The main purpose of the binder is to provide waterproofing and adhesion to the aggregate, The binder may be rapid setting emulsion, hot asphalt, cutback, modified emulsion or modified asphalt ( polymer or scrap rubber). The choice depends on the pavement condition, traffic conditions and the availability of local binders. For example hot cutbacks may be prohibited for use. In instances where the existing pavement is cracked or, in the case of a new construction where lime or cement treatment have been used and shrinkage cracks may occur then polymer modified binders should be chosen. In instances of high traffic count where chip seals may be a problem in early life then a latex or polymer modified binder is called for. ii) Cutters and fluxes The use of cutter and flux pertains to hot systems where viscosity must be lowered to ensure that wetting of the stone occurs, particularly at lower pavement temperatures. In general the use of cutters and fluxes in Cape Seals is to be avoided as cutter and flux will be retained in the lower layers and could lead to bleeding at elevated temperatures. iii) Aggregate. The main purposes of the aggregate are:
The main aggregate properties required are:
3.1.2 Application Rates: To determine the application rates for the chip seal requires a knowledge of the job to be done. The seal is designed as a standard single coat seal. Binder is applied at the lighter end of the range. 3.1.3 Road Data The following road data is required. a) Location b) Existing Surface c) Traffic
d) Turning Movements e) Topography
f) Climate. 3.1.4 Material Data a) Size of aggregate (s) b) Type c) ALD d) Loose Density e) Porosity. Available seals are 20mm, 16mm 14mm, 10mm, 7mm and 5mm. 3.1.5 Aggregate Application This may be estimated from the ALD of the stone. It assumed that the average compacted depth is equal to the ALD. The theoretical spread rate is then given in m 2/ m 3 by: (1000/ALD ) X [(100-VL) /(100 -VC) ] Where : VL = Void Volume In Loose Bulk aggregate VC = Void Volume in Compacted Layer. Figure 4 shows typical application rates.
Generally spread rates close to the theoretical minimum are found for larger size aggregates and heavier rates to the smaller ones. Allowances must also be added for whip off (5%) and stockpile wastage (5%). Varying application rates may be required for 5-7mm depending on the seal objective. For example lighter spread rates would be required if the stone were used as a pinning coat with low application of binder. In South Africa the CPA spread rate curve ( figure 5 ) is used. This gives a shoulder to shoulder spread.
3.1.6 Binder Application This discussion refers to bitumen binders, not polymer modified, manufactures recommendations must be followed in these cases. Usually polymer modified binders require allowances based on their high viscosity. For SBS and Scrap rubber these allowances can be as high as double. This is to ensure that voids are filled. The design objective is 70% of voids filled in the final seal .The amount of binder required will depend on the size, shape, and orientation of the aggregate particles. Also important is the existing surface , i.e. whether embedment can occur, whether binder can be absorbed. Orientation and embedment are functions of traffic. In light traffic situations for example, there is little reorientation after seal completion. In heavy traffic areas embedment, aggregate wear and reorientation will change the required binder content. Cutter is not taken into account in these determinations so must be allowed for in determination of the final binder content. Also the binder is calculated as at 15oC, allowance must be made for volume expansion when hot. a) Basic Voids factor. Design is based of filling voids with bitumen and how this voids filled changes with time. The Voids factor ( VF) when multiplied by the ALD of the stone gives the basic application rate. Basic Application Rate = VF X ALD. Figure 6 shows voids factors for different traffic loadings. The lower level is for the better quality aggregates.
For aggregates 7mm or less ALD determination is not usual, in this experience is used. may be used. This is only the basic application and the level must be adjusted to take into account traffic effects, surface texture and embedment and absorption. These factors particularly relates to the differences between reseals and new seals. Tables showing these adjustments are available in references 4,5,6. Effects are: i) Channeling. This is where traffic is channeled into particular wheel paths such as on tight bends, single lane bridges or confined lane widths. If this is the case the loading consideration in the AADT should be taken into account. ii) Short term Traffic Effects. These include such effects as harvest traffic, country field days etc. Low traffic initially can also be a problem ,e.g. new works. Hardening of seals can occur without the working effect of traffic, this can be overcome by the use of primer seals and deferring the wearing seal until near the date of opening. iii) Traffic composition and Grades. c) Adjustments for existing surface. i) Surface Texture. This is an area that requires significant judgment. ii) Embedment. To allow for aggregate being forced into a surface a ball penetration test is required. iii) Absorption. On pavements that are absorbent allowance is 0.0- 0.2 lt/ m 2 Absorbent aggregate allowance is around + 0.1 lt/ m2. Judgment and local knowledge are essential to ensure that adjustments are made correctly. In South Africa the top spray rates are determined by traffic only. Absorption is also taken into account. Cold net binder is determined by multiplying the ALD by the spray factor P ( see figure 7), for the traffic. If heavy construction traffic is to be on the seal this rate is reduced by 5%. If there is no prime coat in a new construction an allowance of 0.15lt/m2 is made. If the stone is likely to crush then a reduction in spray rate is made of 5%.
3.2 Slurry Design. A complete design guide for slurry is published in ISSA guidelines and the ISSA design bulletins. Type I or Type II are used. 3.3 Combination Effects. The slurry and the chipseal must complement each other, as the idea is for the slurry to fill up the voids of the seal with the stones just protruding the following allowances should be made. 3.3.1 Chip seal Use the lower end of the range for binder application , this could be as much as 10-15% lower ( on an emulsion basis). Do not allow for whip off in aggregate spread rate. Also the aggregate level may be reduced slightly below shoulder to shoulder to give a more open surface which will allow higher void levels and more slurry, forming a tighter final surface. If a greater thickness of layer is required this is best achieved by using a larger stone chip seal. Use polymer modified binders if possible to optimize aggregate retention at the lower asphalt application rate. Use of polymer modified binders will also reduce reflection cracking and , giving a more flexible seal be used to treat deflecting pavements. 3.3.2 Slurry Seal Use the highest level of emulsion in the range. The higher the asphalt content, i.e. the thicker the films of asphalt on the aggregate and hence the greater the aging resistance ( aging is a function of film thickness and voids). This also optimizes the waterproofing and flexibility. Latex modified emulsions should be used in the slurry to increase the residual viscosity of the binder and combat potential bleeding. 4. APPLICATION OF CAPE SEALS Cape seals are applied using a basically two stage process. The chip seal is applied using standard methods , the seal is allowed to cure for at least five days and then the layer(s) of slurry are applied. 4.1 Chip Seal a) Surface Preparation. The base is the key part of any pavement construction. In new constructions the base must be prepared well ahead of time, swept clean of dust and loose materials and primed with either an asphalt emulsion prime or a cutback primer. The aim of this treatment is to penetrate the surface voids , bind the surface layers and render the base water proof. If the base needs to carry traffic a layer of sand or grit may be applied at this stage. In instances where the road is only for light traffic and will not be inundated at any stage, no prime will be required. If the base is an existing asphalt or PCC surface it must be swept clean and any potholes repaired, usually it is also wise to seal cracks. The cape seal will not remove irregularities in the surface such as corrugations, ruts or waves. b) Application of Chip seal. A calibrated sprayer must be used to apply the design application of binder. The steps to do this will depend on the binder used. For an RS-1 or CRS-1 binder a tack spray of emulsion , diluted 50/50 with water is applied first. This represents a third to a half of the total emulsion application. Remember that the design gives residual binder and the application rate of the emulsion will depend on the water content. The aggregate is then applied shoulder to shoulder in a single layer by a calibrated chip spreader. This is applied immediately after the sprayer. The aggregate is lightly broomed to ensure even distribution. The surface is then rolled in one pass of a pneumatic tyred roller The remaining emulsion is sprayed as a penetration coat , if necessary watered first . to ensure penetration. Care must be taken to ensure that the surface is not too wet. In South Africa it was common practice to wait 24 hours , back roll with a flat steel roller then apply the slurry. Alternatively the surface could be blinded with sand or grit, broomed and rolled, then re rolled. This way the surface could be left for several weeks, natural whip off revealing sufficient voids for the slurry application. In the USA it is common to allow 5-7 days before slurry application. This is reduced to 48 hours after a hot applied crumb rubber seal. Petromat or other geotextile may also be used in a base seal. If CRS-2, PMCRS-2 or hot binders are used no tack coat is required and the binder is applied in a single spray with the chips spread directly and rolled. The chip seal is swept within 24 hours to remove loose aggregate. The chip seal is allowed to cure well before slurry application. This is important. 4.2 Slurry Application Before the surface is slurried all loose chips must be removed along with any debris or dust. This will ensure that the slurry adheres to the chip seal . The aim of the slurry application is to fill the surface voids of the chipseal. The slurry provides a dense high asphalt filler that protects the chip seal. Extensive work carried out in Australia by the Australian Road Research Board examined the aging of seals showing that there is a distress viscosity at which asphalts crack and lose aggregate. Cape seals alleviate this by creating a physical barrier. For this physical barrier to work requires the slurry to be worked down into the chipseal. In South Africa this is done by hand , at least on the first application. Machine application then follows. This would seem to be too labour intensive for most places and in USA the application is exclusively with machine. In this instance care must be taken to thoroughly wet down the pavement with the fog spray ensuring that the slurry can flow into the voids. The slurry itself may have 20-30% total fluids ( emulsion plus water). This will create a dense packing of the mix where voids content is 2-3% max. This, as the voids are not interconnected- as shown by low air and water permeability- gives good sealing and, as the stones protrude still macrotexture is high and hence skid resistance is also good. Care must also be taken to ensure that the stones from the chip seal are protruding from the slurry and not totally covered. Excess slurry may lead to loss of skid resistance under heavy traffic loads, deformation in the form of rutting or corrugations. The chip seal is the traffic loading element in a cape seal, not the slurry. The slurry mix should be slightly richer than a standard mix in most cases to increase durability. This only is the case if the bottom seal has allowed for the extra binder in the system Normal practice for slurry operations in trafficking are followed. Microsurfacing mixtures may be used in this application. They do have some advantages over standard slurry mixes in that aging resistance is higher and the binder content can be increased without causing subsequent bleeding. 5. ADVANTAGES AND PERFORMANCE OF CAPE SEALS Current Cape seal usage guidelines are shown in figure 8 and 9. Cape seals have the following advantages:
5.1 Durability They are durable. Aging resistance is a function of voids content and film thickness . Figures 10 and 11 show this effect. Cape seals reduce voids to 2-3%, increasing the durability, especially compared to a chip seal where void level is high. This can give seal durability of 12-15 years ( 2). This is compared to seal lives of 5-8 years.
5.3 Skid resistance They are skid resistant. Figure 12 shows skid resistance for common road surfacings. The scrim results give a sideways coefficient of friction as 0.8 to 0.9 at 80 km/hr. This compares to a recommended minimum of 0.5- 0.6 for hot mix. c) They can , with correct selection of binder alleviate ( 7) cracking except alligator and block cracking. This is because these modes of cracking are normally due to structural problems in the base and cape seals add no structural strength. However basically sound pavements that are deflecting significantly can be treated with such seals as they can be designed to be flexible enough to move with the deflection. The fatigue resistance may be improved by the use of a highly modified PBA (3-5% polymer) in the chipseal and a latex modification in the slurry ( 2-5%) ( 8).
5.4 Restoration They can be used to restore a weatherproof smooth surface caused by raveling or abrasion. 5.5 Noise Reduction They have less noise than a chip seal ( figure 13).
5.6 Cost Effective They are cost effective. In instances where the choice is a thin lift of hot mix or a slurry or chip seal they will give many of the benefits of the overlay at a lower cost. Indicative costs are shown in figure 14.
6. CONCLUSIONS Cost and ape seals have a long and successful history. They began as an alternative to hot mix on availability grounds in South Africa and as a troubleshooter in USA. They have progressed real to a alternative to thin overlays where the hard wearing cost effectiveness of a chip seal needs to be combined with the smooth durability of a cape seal. Cape seals are an excellent weapon in the armory of the road designer. 7. BIBLIOGRAPHY
Back to VSS Technology Papers Library Copyright © 2000 Valley Slurry Seal Co All Rights Reserved. |
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