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8. SURFACE DRESSING
8.1 Introduction
Surface dressing is a simple, highly effective and inexpensive road surface treatment if adequate care is taken in the planning and execution of the work. The process is used throughout the world for surfacing both medium and lightly trafficked roads, and also as a maintenance treatment for roads of all kinds.
Surface dressing comprises a thin film of binder, generally bitumen or tar, which is sprayed onto the road surface and then covered with a layer of stone chippings. The thin film of binder acts as a waterproofing seal preventing the entry of surface water into the road structure. The stone chippings protect this film of binder from damage by vehicle tyres, and form a durable, skid-resistant and dust-free wearing surface. In some circumstances the process may be repeated to provide double or triple layers of chippings.
Surface dressing is a very effective maintenance technique which is capable of greatly extending the life of a structurally sound road pavement if the process is undertaken at the optimum time. Under certain circumstances surface dressing may also retard the rate of failure of a structurally inadequate road pavement by preventing the ingress of water and thus preserving the inherent strength of the pavement layers and the subgrade.
In addition to its maintenance role, Surface dressing can provide an effective and economical running surface for newly constructed road pavements. Existing roads with bituminous surfacing, carrying in excess of 1000 vehicles/lane/day, have been successfully surfaced with multiple surface dressings. For sealing new roadbases traffic flows of up to 500 vehicles/lane/day are more appropriate, although this can be higher if the roadbase is very stable or if a triple seal is used. A correctly designed and constructed surface dressing should last at least 5 years before resealing with another surface dressing becomes necessary. If traffic growth over a period of several years necessitates a more substantial surfacing or increased pavement thickness, a bituminous overlay can be laid over the original surface dressing when the need arises.
The success of a surface dressing depends primarily on the adhesion of the chippings to the road surface, hence both the chippings and the road surface must be clean and free from dust during the surface dressing process. Inappropriate specifications, poor materials, and bad workmanship, can also drastically reduce the service life of a surface dressing.
This Road Note is a general guide to the design and construction of surface dressings in tropical and sub-tropical environments and draws attention to some of the more common mistakes that are made. It provides a framework on which the engineer can base more specific decisions made to suit particular local conditions thereby producing cost effective results. It also contains brief descriptions of certain other types of surface treatment.
8.2 Types of surface dressing
Surface dressings can be constructed in a number of ways to suit site conditions. The common types of dressing are illustrated in Figure 1.
Single surface dressing
When applied as a maintenance operation to an existing bituminous road surface a single surface dressing can fulfill the functions required of maintenance re-seal, namely waterproofing the road surface, arresting deterioration, and restoring skid resistance. A single surface dressing would not normally be used on a new roadbase because of the risk that the film of bitumen will not give complete coverage. It is also particularly important to minimise the need for future maintenance and a double dressing should be considerably more durable than a single dressing. However, a 'racked-in' dressing may be suitable for use on a new roadbase which has a tightly knit surface because of the heavier applications of binder which is used with this type of single dressing.
Figure 1: Type of surface dressings
Sand may sometimes be used as an alternative to chippings for the second dressing. Although it cannot contribute to the overall thickness of the surfacing, the combination of binder and sand provides a useful grouting medium for the chippings of the first seal and helps to hold them in place more firmly when they are poorly shaped. A slurry seal may also be used for the same purpose.
Triple surface dressings
A triple surface dressing (not illustrated in Figure 1) may be used to advantage where a new road is expected to carry high traffic volumes from the outset. The application of a small chipping in the third seal will reduce noise generated by traffic and the additional binder will ensure a longer maintenance-free service life.
Racked-in surface dressing
This system is recommended for use where traffic is particularly heavy or fast (TRL, 1996). A heavy single application of binder is made and a layer of large chippings is spread to give approximately 90 per cent coverage. This is followed immediately by the application of smaller chippings which should ‘lock-in' the larger aggregate and form a stable mosaic. The amount of bitumen used is more than would be used with a single seal but less than for a double seal. The main advantages of the racked-in surface dressing are: Less risk of dislodged large chippings. Early stability through good mechanical interlock. Good surface texture. Other types of surface dressing
'Sandwich' surface dressings are principally used on existing binder rich surfaces and sometimes on gradients to reduce the tendency for the binder to flow down the slope.
'Pad coats' are used where the hardness of the existing road surface allows very little embedment of the first layer of chippings, such as on a newly constructed cement
value (AAV). Appendix gives recommended values of PSV and AAV for various road and traffic conditions in Britain and provides an indication of the required aggregate properties.
The nominal sizes of chippings normally used for surface dressing are 6, 10, 14 and 20 mm. Flaky chippings are those with a thickness (smallest dimension) less than 0.6 of their nominal size. The proportion of flaky chippings clearly affects the average thickness of a single layer of the chippings, and it is for this reason that Jackson (1963) introduced the concept of the 'average least dimension' (ALD) of chippings.
In effect, the ALD is the average thickness of a single layer of chippings when they have bedded down into their final interlocked positions. The amount of binder required to retain a layer of chippings is thus related to the ALD of the chippings rather than to their nominal size. This is discussed further in the next pages where guidance is given on the selection of the appropriate nominal size of chipping and the effect of flakiness on surface dressing design.
The most critical period for a surface dressing occurs immediately after the chippings have been spread on the binder film. At this stage the chippings have yet to become an interlocking mosaic and are held in place solely by the adhesion of the binder film. Dusty chippings can seriously impede adhesion and can cause immediate failure of the dressing.
The effect of dust can sometimes be mitigated by dampening them prior to spreading them on the road. The chippings dry out quickly in contact with the binder and, when a cutback bitumen or emulsion is used, good adhesion develops more rapidly than when the coating of dust is dry.
Most aggregates have a preferential attraction for water rather than for bitumen. Hence if heavy rain occurs within the first few hours when adhesion has not fully developed, loss of chippings under the action of traffic is possible. Where wet weather damage is considered to be a severe risk, or the immersion tray test, which shows that the chippings
have poor affinity with bitumen, an adhesion agent should be used. An adhesion agent can be added to the binder or, used in a dilute solution to pre-coat the chippings. However, the additional cost of the adhesion agent will be wasted if proper care and attention is not given to all other aspects of the surface dressing process.
Improved adhesion of chippings to the binder film can also be obtained by pre-treating the chippings before Spreading. This is likely to be most beneficial if the available chippings are very dusty or poorly shaped, or if traffic conditions are severe. There are basically two ways of pretreating chippings: Spraying the chippings with a light application of creosote, diesel oil, or kerosene at ambient temperature (NAASRA, 1986). This can be conveniently done as the chippings are transferred from stockpile to gritting lorries by a belt conveyor or. Alternatively, they can be mixed in a simple concrete mixer. Pre-coating the chippings with a thin coating of hard bitumen such that the chippings do not stick together andcan flow freely. Chippings which are pre-coated with bitumen enable the use of a harder grade of binder for construction which can provide early strong adhesion and thus help to obtain high quality dressings. The binder used for pre-coating need not necessarily be the same kind as that used for the surface dressing; for example, tar-coated chippings adhere well to a sprayed bitumen film. Pre-coating is usually undertaken in a hot-mix plant and the hardness of the coating, and thus the tendency for the chippings to adhere to each other, can be controlled by the mixing temperature and/or the duration of mixing; typical coating temperature are about 1400C for bitumen binders and 1200C for tar binders. Table 1 indicates the amount of binder recommended for lightly coating chippings.
It helps to strengthen the roadbase near its surface by binding the finer particles of aggregate together. If the application of the surface dressing is delayed for some reason it provides the roadbase with a temporary protection against rainfall and light traffic until the surfacing can be laid. The depth of penetration of the prime should be between 3-l0mm and the quantity sprayed should be such that the surface is dry within a few hours. The correct viscosity and application rate are dependent primarily on the texture and density of the surface being primed. The application rate is. However, likely to lie within the range 0.3-1. kg/m2. Low viscosity cutbacks are necessary for dense cement or lime stabilized surfaces, and higher viscosity cutbacks for untreated coarse-textured surfaces. It is usually beneficial to spray the surface lightly with water before applying the prime coat as this helps to suppress dust and allows the primer to spread more easily over the surface and to penetrate. Bitumen emulsions are not suitable for priming as they tend to form a skin on the surface. Low viscosity, medium curing cutback bitumens such as MC-30, MC-70, or in rare circumstances MC-250, can be used for prime coats (Asphalt Institute, 1983). The relationship between grade and viscosity for cutback primes is shown in Table 2.
Table 2 : Kinematic viscosities of current cutback binders
Bitumens for surface dressings
The correct choice of bitumen for surface dressing work is critical. The bitumen must fulfil a number of important requirements. They must: be capable of being sprayed; 'wet' the surface of the road in a continuous film;
not run off a cambered road or form pools of binder in local depressions; 'wet' and adhere to the chippings at road temperature; be strong enough to resist traffic forces and hold the chippings at the highest prevailing ambient temperatures; remain flexible at the lowest ambient temperature, neither cracking nor becoming brittle enough to allow traffic to 'pick-off' the chippings; and resist premature weathering and hardening. Some of these requirements conflict. hence the optimum choice of binder involves a careful compromise. For Example, the binder must be sufficiently fluid at road temperature to 'wet' the chippings whilst being sufficiently viscous to retain the chippings against the dislodging effect of vehicle tyres when traffic is first allowed to run on the new dressing.
Figure 2 shows the permissible range of binder viscosity for successful surface dressing at various roads about 250C and 500C, normally being in the upper half of this range unless heavy rain is falling. For these temperatures the viscosity of the binder should lie between approximately l04 and 7 x l05 centistokes. At the lower road temperatures cutback grades of bitumen are most appropriate. Whilst at higher road temperatures penetration grade bitumens can be used.
The temperature/viscosity relationships shown in Figure 2 do not apply to bitumen emulsions. These have a relatively low viscosity and 'wet' the chippings readily, after which the emulsion 'breaks', the water evaporates. and particles of high viscosity bitumen adhere to the chippings and the road surface.
Depending upon availability and local conditions at the time of construction, the following types of bitumen are either commonly used in the tropics or are becoming so: Penetration grade. Cutback. Emulsion. Modified bitumens.
Figure 2: Surface temperature/choice of binder for surface dressings
If split application of the binder is used care must be taken with the following: The rate of application of chippings must be correct so that there is a minimum of excess chippings.
The second application of binder must be applied before traffic is allowed onto the dressing. For a single seal it will be necessary to apply grit or sand after the second application of binder. Cutback bitumens Except for very cold conditions, MC or RC 3000 grade cutback is normally the most fluid binder used for surface dressings. This grade of cutback is basically an 80/ penetration grade bitumen blended with approximately 12 to 17 per cent of cutter.
In some areas of the world the range of binders available to the engineer is restricted. In this situation it may then be necessary to blend two grades together or to 'cut-back' a supplied grade with diesel oil or kerosene in order to obtain a binder with the required viscosity characteristics. Diesel oil, which is less volatile than kerosene and is generally more easily available, is preferable to kerosene for blending purposes. Only relatively small amounts of diesel oil or kerosene are required to modify penetration grade bitumen such that its viscosity is suitable for surface dressing at road temperatures in the tropics. For example Figure 3 shows that, for the road temperatures prevailing during trials in Kenya, between 2 and 10 per cent of diesel oil were required to modify 80/100 pen bitumen to produce binders with viscosities within the recommended range for use (Figure 2). Figure 4 shows the temperature/viscosity relationships for five of the blends made for these trials.
The blending process is not difficult but it must be undertaken with great care by staff who are properly trained. A convenient method is to pump the required amount of cutter (e.g. diesel oil) into the distributor whilst simultaneously pumping in hot bitumen. Before pumping in the cutter, sufficient bitumen should be pumped into the distributor to enable the cutter to discharge below the surface of the bitumen. Because of the fire risk, all the burners must be extinguished and naked lights and smoking prohibited during this operation. Polymer modified bitumens
Polymers can be used in surface dressing to modify penetration grade, cutback bitumens and emulsions. Usually these modified binders are used at locations where the road geometry, traffic characteristics or the environment, dictate that the road surface experiences high stresses. Generally the purpose of the polymers is to reduce binder temperature susceptibility so that variation in viscosity over the ambient temperature range is as small as possible. Polymers can also improve the cohesive strength of the binder so that it is more able to retain chippings when under stress from the action of traffic. They also improve the early adhesive qualities of the binder allowing the road to be reopened to traffic earlier than may be the case with conventional unmodified binders. Other advantages claimed for modified binders are improved elasticity in bridging hairline cracks and overall improved durability.
Examples of polymers that may be used to modify bitumens are proprietary thermoplastic rubbers such as Styrene-Butadiene-Styrene (SBS), crumb rubber derived from waste car tyres and also glove rubber from domestic gloves. Latex rubber may also be used to modify emulsions. Binders of this type are best applied by distributors fitted with slotted jets of a suitable size.
Rubber modified bitumen may consist, typically, of a blend of 80/100 penetration grade bitumen and three per cent powdered rubber. Blending and digestion of the rubber with the penetration grade bitumen should be carried out by experienced personnel prior to loading into a distributor. This must be done in static tanks which incorporate integral motor driven paddles. The blending temperature is approximately 2000C.
Cationic emulsion can be modified in purpose made plant by the addition of three per cent latex rubber. One of the advantages of using emulsions is that they can be sprayed at much lower temperatures than penetration grade bitumens, which reduces the risk of partial degradation of the rubber which can occur at high spraying temperatures.
Bitumen modified with SBS exhibits thermoplastic qualities at high temperatures while having a rubbery nature at lower ambient temperatures. With three per cent of SBS, noticeable changes in binder viscosity and temperature susceptibility occur and good early adhesion of the chippings is achieved. SBS can be obtained in a carrier bitumen in blocks of approximately 20kg mass. The blocks can be blended, at a concentration recommended by the manufacturer, with 80/100 penetration binder in a distributor. In this procedure it is best to place half of the required polymer into the empty distributor, add hot bitumen from a main storage tank and then circulate the binder in the distributor tank. The remaining blocks are added after about 30 minutes and then about 2 hours is likely to be required to complete blending and heating of the modified binder. Every effort should be made to use the modified bitumen on the day it is blended.
Adhesion agents Proprietary additives, known as adhesion agents. are available for adding to binders to help to minimize the damage to surface dressings that may occur in wet weather with some types of stone. When correctly used in the right proportions. These agents can enhance adhesion between the binder film and the chippings even though they may be wet. The effectiveness and the amount of an additive needed to provide satisfactory adhesion of the binder to the chippings in the presence of free water must be determined by tests such as the Immersion Tray Test which is described in Appendix C.
Fresh hydrated lime can also be used to enhance adhesion. It can be mixed with the binder in the distributor before spraying (slotted jets are probably best suited for this) or the chippings can be pre-coated with the lime just before use. by spraying with a lime slurry. The amount of lime to be blended with the bitumen should be determined in laboratory trials bitt approximately 12 per cent by mass of the bitumen will improve
Figure 5: Outline procedures for design of surface dressings
Guidance on the selection of chipping size for single surface dressings, relating the nominal size of chipping to the hardness of the underlying road surface and the weight of traffic expressed in terms of the number of commercial vehicles carried per lane per day. These recommendations are shown in Table 3.
Road surface hardness may be assessed by a simple penetration probe test (TRL. 1996). Alternatively the hardness of the existing road surface may be made on the basis of judgement with the help of the definitions given in Table 4.
Although the recommendations for the selection of chipping size were developed for conditions in the United Kingdom they have been found to be applicable to roads in tropical and sub-tropical countries.
Table 3 Recommended nominal size of chippings (mm)
If larger sized chippings are used than is recommended in Table 3 then the necessary bitumen spray rate, required to hold the chippings in place, is likely to be underestimated by the design procedure described in Section 5. This is likely to result in the 'whip-off' of chippings by traffic early in the life of the dressing and also to have a significant effect on the long term durability of low volume roads.
