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Bituminous bases and surfacings for low-cost roads in the tropics by L. S. Hitch and R. B. C. Russell 1 I I 1977 J Digest SR284 ~ BITUMINOUS BASES AND SURFACINGS I L S Hitch and Authors -- -- .--,, BITUMINOUS BASES AND SURFACINGS FOR LOW-COST ROADS IN THE TROPICS* by L S Hitch and R B C Russell Much of the accumulated knowledge gained from the construction of roads over the last 150 years relates to the use of mechanically stable roadmaking materials such as crushed rock. These materials are not always available, particularly in the remoter parts of many developing countries, but in recent years the development of the technique of soil stabilisation provides practical alternative roadma!yy: ‘ayers and design life in terms of cumulative Mlnlmum soaked CBR values of 80 for crushed stone bases and 100 for cement and lime stabilised material are normally specified. A problem arises, however, in the application of these criteria when constructing bases with bitumen-stabilised soils. Bituminous mixtures 13 behave viscoelastically and are temperature susceptible. In 1954 Nijboer proposed the concept of “stiffness” of a mix, which reflects loading time, acceptable strain (1 per cent max) and maximum temperature experienced in practice. It was shown that this stiffness could be expressed as a ratio of the stability to the flow value (load and strain at failure respectively) 3 14 measured in the Marshall test . A high ratio may ensure resistance to deformation under traffic but at the expense of a brittle mix. The problem of selecting an appropriate ratio for surfac+ng materials is considered later (Section 5.1). The application of the concept to bituminous stabilisation is perhaps less important and its relevance probably depends upon the type of traffic using the road and the thickness of the surfacing. The stiffness concept proposed by Nijboer is “rational”; empirical methods, notably the Marshall, Hubbard-Field and the cone penetrometer methods, have ~~so been used for the design of bitumen-stabilised soil bases. A bibliography has been compiled which provides some examples of the use of different design methods and the design criteria adopted. 3.2.2 A full-scale experiment in Africas: Abrasion-resistant sand-bitumen surfaces with relatively high bitumen contents have already been referred to in contrast to the leaner sand-bitumen materials for bases which are more common today. The conflict between abrasion resistance and stability has been recognised for many years. In 1960 the Tropical Section (now Overseas Unit) of tie T- participated in the construction of experimental sections on a road in Northern Nigeria. These sections were incorporated into a new bitumen- ~gabilised road, the construction of which has been described elsewhere . Itwa~ found that cut-ba:k bitumen with a viscosity of up to approximately 8 x 10 .centistokes at 60 C (S.125) could be mixed in a simple paddle mixer with theowell-graded sand available. The ternerature of the sand was 26-28 C and of the binder when added was 8 120 C (ie at a viscosity of approximately 200 centistokes). These findings raise the question of whether it is necessary to use sophisticated mixing plant in order to heat sand simply to enable a viscous penetration grade bitumen to be used, even though the superior st~ility of such mixes on cooling is unquestionable. The experimental mixes were laid without aeration to remove volatiles and were surface dressed before curing could occur. Materials stabilised with MC2 grade cut-back failed quickly but were the only sections to fail. The remainder have, in general, performed as satisfactorily as those with the hot mix material. The road has carried mainly light traffic in its 14 years of use and the number of heavy commercial vehicles probably does not exceed 100 per day; this volume of traffic, however, is characteristic of many developing countries. It is interesting to record that samples of bitumen removed from slabs and taken from several of the experimental sections and from the hot-mixed main contract material after 13 years were all found to have essentially the same viscosity, ie 10-20 pen. A limited number of small samples taken after 18 months showed that the cut-back bitumens had, even at that early stage, cured to the consistency of 8O-1OO pen. 3.2.3 Research at TW on bituminous stabilisation: The principal objectives of the research undertaken by the Overseas Unit were: 1. To evaluate different design methods for bituminous stabilisation. 4 2. To establish design criteria. 3. To study the behaviour of a variety of sand-bittien mixtures under different shear conditions. The first two of these have so far been studied in the full-scale ex~~riment in Nigeria and tie results have been reported in full elsewhere . Briefly, sand-bitumen mixtures were prepared using the sand from the Nigeria experi~ent and three different binders. Mixtures were cured for one year at 45 C and four tests were used go determine stability. Table 2 shows the results of stability tests at 60 C after one year and Tab&e 3 shows some inter-relations of test values for material tested at 45 C. Of the five design methods originally inclu~~d in the study, two 18 (Marshall and Htibard-Field) are well documented . Alexander and Blott proposed the use of the cone penetrometer for sand-bitumen mixtures and it has been frequently used although, as with some other methods, design criteria have been little more than recommendations. It was found that the tes”tdid not correlate well ith three other methods exmined. 1~ The deformation wheel-tracking test , designed at TRRL and used mainly for studying deformation resistance in surfacings, was the fourth method studied. Tracking rates were found to correlate well with the Marshall and Hubbard-Field methods. The fifth method (CBR) was rejected as inapplicable early in the study. The following design criteria for sand-bitumen bases for lightly-trafficked roads were deduced from the above study: Sttiility, Marshall at 600C ................. 100 kg(min) Stability, Hubbard-Field at 600C ............ 300 kg(min) In a recent investigation of the causes of some failures in a road carrying heavy traffic in the Middle East it was found that the type of sand used for the sand-bitumen base varied considerably from place to place along the road and the sand particle texture varied from very rough to very smooth. A limited initial investigation appeared to show a link between failure and bitumen content, and the low residual bitumen contents found (2-4 per cent) were especially noteworthy in relation to the fineness of the sands, most of which passed a 300 pm mesh sieve. The bitumen film thicknesses were thus minimal, and mixes were suspected of being prone to shear failure. The shear properties of the sands used in this road when stabilised with different percentages of bitumen were studied using a shear box. The effects of binder viscosity and shear rate were also studied. Fipre 2 illustrates a typical stress/strain relationship for three sands, showing how the behaviour becomes more plastic as the normal load decreases, ie witi increase in depth of cons~~ction. Inspection of site failures confirmed Prmdtl-type failure, and Fig 3 illustrates the effect of binder viscosity and shear rate on Prandtl bearing capacities for one of the sands. Calculations based upon the most severe conditions likely to b 91 experienced in a sand-bitumen base showed that the Coulomb equation becomes: 5 $= C+ 1035 where ~= shearing c = apparent and 6 = angle of tan + stress (kN/m2)2 cohesion (kN/m ) shearing resistance (degrees) . The results obtained using the shear box are in good agreement with this expression. Figure 4 shows the relationship between $and estimated bitumen film thickness (EBFT) for three sands. The above formula is much simpler to use and to express graphically than the formula derived by Prandtl: Prandtl bearing capacity = c L ‘(sin d +1) Xtan 4 tan~ (sin@ -l).e -11 where c = apparent cohesion $ = angle of shearing resistance from the Coulomb equation. It was found that all of the sands possessed optimum ~ values at an EBFT of 0.7 to where x = and s= This sands are 0.8 microns, given by x .104 EBFT‘(1OO-X)S bitumen content (% wt) Specific surface area (cm*/g). recent work has also shown that optimum # values for di~~erent related to Hveem’s centrifuge kerosene equivalent (Cm) ~,the the Efflux Rat?g ~~s~~~a~~ sand. angle of sheari measured by the Angle of Repose metiod”, and Results for three sands are plo~ted in Fig 5. ~ilst this study has been by no means exhaustive, the following conclusions relating to sand-bitumen bases for medium to heavily trafficked roads can be tentatively drawn: 1. 2. 3.3 (a) For natural dry sand, an angle of shearing resistance of at least 30 degrees and/or Cm of at least 1.5 is required, and (b) for the sand-bitumen mix, ~n EBFT of O .7 to 068 microns witi a # value of at lea~~ 120QlkN/m is required at 25 C and rate of strain of 2.22 x 10 sec . Comparative criteria for the Marshall and Hubbard-Field stabilities are as follows: Marshall stability of 300 kg(min) at 60°C, or Hubbard-Field stability of 700 kg(min) at 60”c, both accompanied by an EBFT of 0.7 to 0.8 microns. The mix-in-place process It is worthwhile perhaps to consider carefully what is involved in 6 any mixing process: the requirement may perhaps be expressed in two parts: 1. the components must be brought together 2. an adequate mixing action must be provided. The first statement may seem obvious but deserves discussion. The stabilisation process involves natural soil as the major component and the stabilizing agent less than 10 per cent of the final product. It should therefore be rational to bring the stabilizing agent to the natural sand; for example, in cement stabilisation the stabiliser is very often spread on the soil and mixed in by machine in one or more passes. This type of process, mix-in-place, often requires only simple plant, eg trucks, graders and water bowsers, and high output is possible. There are several drawbacks to this process however; in particular, difficulties are often experienced in controlling bitumen content, completeness of mixing and processing depth. ~ilst these difficulties are lessened by the use of purpose-built single-pass stabilisation machines, mechanical failure usually results in complete stoppage of work. For this reason simpler multi-pass equipment has much to commend it. . Single-pass machines require the soil to be spread in a windrow such that it can be picked up by the machine as it travels slowly forward; the stabilizing binder is added to the soil and mixed within the machine and is discharged from the rear, usually again as a windrow, ready for subsequent spreading and compaction. An arrangement used on one contract in the Arabian Gulf consisted of a tractor-mounted mixer equipped with a spray bar above the tines inside the mixing hood and supplied with cut-back bitumen by a tanker which preceded it. A metered quantity of bitumen was thus delivered to the soil under the mixing hood but mixing was incomplete during the first pass: subsequent passes of the machine, without the bitumen supply, and assisted by a blade grader were necessary to complete the mixing process. In its simplest form multi-pass work must often be done using only a bitumen distributor and blade grader. 3.4 The premix process Mix-in-place work is only possible where low viscosity binders, ie cut-backs or emulsions, are to be used. If it is necessary to stabilise with penetration grade binders, premix plant is required, although techniques are now available for st~~lising with foamed penetration grade bitumen; Mobil Oil (Australia static processes and Bowering }4 describe equipment for both mobile and discusses the properties and behaviour of foamed bitmen mixtures. Several points should be made concerning the role of static premix plants for bituminous stabilisation: (a) Continuous type mixers, which are often capable of high outputs, are well suited to this type of work, ie production using a cold feed of constant gradation; materials are metered by volume. For stabilisation work the -– — - process becomes one of soil and binder only in most cases. Production of uniform quality is therefore more feasible than for surfacing materials, which demand close control. Batch-type mixers are essential for surfacing work and can naturally also be used for stabilisation. (b) Pre-mixed stabilised material can be laid by paver to uniform depth and regularity; shaping by blade grader is thus cut to a minimum. (c) Modern mixing plants are often constructed as several mobile units, cap&le of disconnection and re-assembly within a few hours. Such plants can follow the progress of work, thus keeping haul distances to a minimum. 3.5 Compaction The thickness of bitumen coating in soil-bitumen systems is relatively thin, conferring cohesion rather than providing lubrication, and in this respect such systems resemble dense continuously-graded surfacing materials, such as asphaltic concrete, in’which the interlocked mineral particles resist compaction. In common with these materials, soil bitumen is compacted most effectively by ‘the kneading action of rubber or pneumatic-tyred rollers. 4. SURFACINGS FOR LOW-COST ROADS 4.1 The role of surfacings Irrespective of its type, a surfacing is basically any treatment which can withstand the abrasive effects of traffic, and many processes will satisfy this definition. The structure of a low-cost road may take several forms: (a) a compacted and shaped in-situ soil structure; (b) a cement/lime stabilised base or a mechanically stable base; (c) a bitumen stabilised base. Surfacing must be provided in all cases and must moreover be impervious, even in areas of very low rainfall. Base materials are designed to retain sufficient strength in a soaked condition but subgrade/sub-base must be protected. It follows therefore that although pervious open-textured bituminous materials are used for resurfacing on some minor roads, in the UK for example, they must be laid over structures which are known to be adequately sealed by former surface dressings. Where such materials are used as surfacings on new roads, surface dressing is indispensable. 4.2 Surface treatments The term ‘surface treatments’ is used here to describe processes in which,bitumen is applied directly to the surface of a road. Surface treatments commonly used include: 1. Dust-laying processes 2. Priming 3. Surface dressing. Treatments 1 and 2 are described briefly and surface dressing in rather more detail. 4.2.1 Dust-laying processes: These processes are used to abate the dust nuisance resulting from loose surface mat roads in the tropics. An ~. publicatio~gi~;;~ ~~;ti~~~~3flR~d the subject; low viscosity2cut-backs of the MC230 (MCO/MCl) type may be applied at 0.55 - 1.35 l/m (0.1 - 0.25 gal/yd ) or, less preferably, crude” oil, bunker grade fuel oil, etc may be used at heavier application rates. These simple applications are not very durable and, since loose material is being treated, they cannot be considered as equivalent to prime coats. Their effectiveness will be governed mainly by the residual bitumen content of the binder and the application rate. In conclusion, dust-laying must be regarded as a palliative and not a permanent solution to the problem: the latter is obtained by more expensive methods, eg mix-in-place stabilisation of the loose material so as to provide a coherent layer of at least 4-5 cm. The so-called road-mix method of mix-in-place st~ilisation is referred to in Section 5.3. 4.2.2 Priming: A prime coat prepares the surface of a new road base for superimposition of a surface dressing or bituminous ‘premix’ surfaci~g: a low viscosity cut-b~ck, MC30 (MCO-MC1), is applied at 0.4 - log l/m (0.08 - 0.20 gal/yd ) to the brushed and slightly damp surface. It should penetrate to at least 4-5 mm leaving a matt dry surface in 24 hours. If surface dressing is to follow, binder can be applied directly to the dried prime coat. However, if a premixed surfacing is applied, a tack coat may be necessary to ensure interlayer adhesion, especially if the mix is of a dense type with no large stones to give a mechanical ‘key’ to the base. The engineer must judge the need for a tack coat in each case; a 26 useful guide on prime and tack coats for use in Africa has been published . 4.3 Surface dressing Surface dressing, also known as a ‘spray and chip’ process, is a surfacing treatment of the highest importance not only in developing countries but also in many industrialised countries of Europe. It can be used as an economical first surfacing, either as an improvement of an existing road or as part of the initial construction of a new road. Further surface dressings can be applied as a maintenance process: the life of all forms of premixed bituminous surfacing is extended by the p~~}~~~~g:~~ of surface dressing as a maintenance treatment. Some references are given to the extensive literature on surface dressing. A surface dressing seals the road structure against surface water and presents a rugous stone layer to vehicle tyres. ,The base is thus protected against attrition but the single layer of stone chippings applied cannot restore riding quality to an irregular pavement or contribute significantly to pavement strength. 4.3.1 Surface dressing design: Although the most important feature of surface dressing is the provision of a continuous impervious film of binder, the success in the design of the dressing begins with selection of the appropriate sizes for the stone chippings, which protect the film from damage by vehicle tyres. Selection depends upon the weight and nature of traffic and penetrability (softness) of the existing surface. That choice having been made, an appropriate thickness of sprayed binder can be selected. The following stages are necessary: Select a nominal chipping size for the job; the most frequently used sizes are 14 mm (approx % inch) and 10 mm (approx % inch). The former size is preferable for roads carrying large numbers of heavy commercial vehicles and the latter for lightly-trafficked roads often used by fast-moving vehicles. Extreme cases are soft bituminous surfaces carrying very heavy vehicles (20 mm) and very hard, eg concrete, surfaces (6 mm); these cases will seldom be encountered in developing countries but are included for completeness. Table 4 shows the recommended nominal size of chippings in relation to hardness of surface and traffic loads. Table 5 gives the rates . Road Note ~9~7four nominal of spread fo sizes of chippings when using cut-back binder. in which this Table appears, stresses the need to consider the shape and ~pecific gravity of chippings in addition to size when selecting the rate of spread. Jackson30 discusses the effect of the shape of chippings and provides a design method based upon the Average Least Dimension of chippings. His procedure may be summarised as follows: (a) Measure the least dimension of approximately 200 chippings to determine the Average Least Dimension (ALD) of the stone. Chippings of a given nominal size can have different ALDs. These differences affect the application rates of chippings and binder. Refer to Figure 6; take the intercept of ALD and line AB and read off appropriate rate of spread from upper scales. (b) Select from Table 6 an appropriate constant from each of the four sets of conditions listed. Sum the four constants to obtain an overall factor and refer to Figure 6, take intercept of ALD and overall factor line determined above and read off binder application rate from lower scales. 4.3.2 Choice of stone: Aggregate should be clean dust-free and should 31 comply with requirements of the type given in BS 63 . Chippings should be roughly cubical; flaky and elongated stones tend to be broken under the roller or to be picked out by traffic. Rounded aggregates require more binder than ngular ones and offer less skid resistance. value (PS”)32 Polished stone is the main factor which affects the skid resistance of surface dressings and recowended values for different site conditions in the UK have been published (see Table 7). Aggregates of high crushing value sometimes show a tendency to polish. resistance; 34aggre9ates Low strengt may abrade and develop surfaces of poor skid such aggregates are particularly susceptible to crushing when rolled with a steel-wheeled roller, and pneumatic or rubber-tyred rollers are preferable. Occasionally coarse sand is the only cover aggregate available and can be used for seals under light traffic. In other situations, such unconventional aggregates as lightly broken cockle shell or coral have been successfully used. Some authorities specify different stone sizes for the first and second layers of double surface dressings, eg 14 mm for the first layer and 10 mm for the second; other users occasionally reverse the order. 10 Experience at TRRL has shown that there appears to be no advantage in changing the size of stone for the second layer. When applying a double surface dressing to concrete, however, a small size, eg 6 mm, would be used for the first layer followed by a 10 mm chipping for the second layer. The sprayed binder film rapidly wets the surfaces of clean chippings. Dusty chippings are less easily wetted and adhesion will be delayed. Traffic then ‘whips off’ the chippings: disastrous failure is especially likely to occur if rain falls within two or three hours of chipping. Where water to wash the chippings clean is in short supply, an alternative often used is to pre-treat the chippings with a very small amount (0.5 per cent by weight) of bitumen: these are referred to as ‘lightly coated’ in order to distinguish them from the ‘precoated’ chippings applied to rolled asphalt (see Section 5.1). Chippings must be capable of being spread by standard gritting machinery and must therefore not be tacky and liable to agglomerate. In the UK a light coating of bitumen ortar is applied to chippings in most surface dressing jobs, and abost always when treating heavily-trafficked roads which must be opened to vehicles immediately upon completion of the process. Coating temperatures at the mixing plant are somewhat higher than normal in order to produce a lacquer on the chippings; this ensures rapid adhesion to the film of sprayed binder. If coating plant is not available, a light spray of diesel oil, kerosene or very fluid cut-back can be used. The above treatments are not recommended when surface dressing with bitumen emulsion. The above reference to pre-treating chippings is included for completeness, although it is appreciated that such treatments may not be feasible within the context of low-cost roads and are, indeed, seldom necessary provided that attention is paid to correct choice of binder (see below), aggregate size and construction procedure. 4.3.3 Choice of binder: Selection of binder is often dictated by availability; nonetheless, an understanding of binder behaviour can prevent disasters. The simple rule fo~ surfacedr ~ssing binders is that they should have a viscosity of between 10 and 5 x 10 centistokes at the prevailing road temperature. At higher viscosities, stone will not be wetted by the binder and will be lost by whip-off: at lower viscosities wetting will occur but the binder will be too fluid to hold the stone. Figure 7 shows the viscosity/road temperature relationship for a wide range of binders. It will be clear that for most conditions in the tropics suitable surface dressing binders will be MC3000 (MC4/MC5) or soft penetration grades up to 80/100 grade. The use of a more viscous bitumen than 80/100 is not recommended and should not be necessary. Bitumen of 60/70 penetration grade is generally available in the Arabian Gulf area and can be cut-back with diesel at a rate of 5 gal/100 gal of 60/70 pen to give a bitumen in the rang~ 300-400 pen suitable for surface dressing at road temperatures of 40-65 C approximately. Occasionally chippings have to be spread manually instead of by mechanical gritters; in such cases it may be advisable to use a slightly less viscous binder than that theoretically required. 11 The role of bitumen emulsion in surface dressing should be discussed at this stage. The usefulness of the commonest and cheapest form of bitumen emulsion - anionic emulsion - is limited by several features: 1. Poor adhesion 2. The effective to dry dusty surfaces. bitumen content (seldom exceeding 55 per cent). 3. Suitable source of supply, transportation and storage. Conditions in the Middle East are predominantly dry and dusty and do not therefore favour the use of anionic emulsions, which are better suited to cool and damp conditions. For this reason special cationic emulsions of higher viscosity and containing 7Q-75 per cent bitumen have been developed in recent years, for example, for surface dressing in the early spring months in the UK. It is stressed, however, that these emulsions are sprayed at 2G approximately 80”c; requires heating to equipment is lost. surface dressing in break occurs: cover lost, whilst excess 650per cent cationic emulsion as used in South Africaa” 70 C before spraying and the advantage of use in unheated The low viscosity of some emulsions is a disadvantage in that they tend to drain from the crown of the road before aggregate is therefore poorly held at the crown and is binder accumulates at the edges and leads to bleeding. Apart from considerations of bitumen content, the grade of base bitumen present in an emulsion is imp~~tant and may need to be varied according to season and traffic conditions . Finally, coagulation can occur when some emulsions are transported or stored in unsuitable conditions. It is tempting to assume that when hot binder is not available, bitmen emulsion applied at ambient temperature is a simple alternative. Indeed, emulsions are often used for successful surface dressings - but the Engineer should be aware of the problems cited above that can arise. Careful attention to technique of application is needed. 4.3.4 Construction procedure: C s uction methods for surface dressing 97,56 are adequately described elsewhere ; attention to detail and good planning will ensure success. Excellent equipment is now available for surface dressing. Recent developments in the UK have included the following: 1. Constant-rate-of-spread distributor (binder metering pump coupled to distributor transmission). 2. Extending spray bar (spray width adjustable whilst spraying: for use with 1 above). 3. Forward-control, self-propelled metering gritter (tows chipping lorry; operator has excellent view of work). The process selected should be suitable for the function expected of it; it is appreciated that cost considerations often affect the choice. Generally a double surface dressing is preferred on new bases; single dressings are sometimes used however, particularly on lightly-trafficked roads and as temporary seals. Single dressings are used for road maintenance and are particularly valuable for treating badly polished slippery surfaces / often found at road junctions, traffic lights, etc, in cities. The use of adhesion agents in surface dressing (and bituminous premix) overseas is sometimes advised. These are correctly used to prevent traffic damaging a new surface dressing when rain may be expected to fall within two 12 or three hours of construction, ie before chippings are thoroughly wetted by the binder and orientated into a matrix by traffic; fast vehicles are particularly damaging in these cases. tien needed, these agents are usually blended with the binder in small amounts, often only 1-2 per cent by weight, but are rapidly decomposed by heat and the treated binder must be used within 1-2 hours of blending. Some agents in solution can be sprayed on to the aggregate before chipping commences or even directly on to the sprayed binder film. Normally, however, provided that clean chippings are used with the appropriate grade of bitumen, adhesion agents are not required. Men it is difficult to obtain dust-free chippings, the lightcoating treatment for chippings described earlier (section 4.3.2) may be an economical alternativeto the use of adhesion agents. 4.3.5 Surface dressing as a maintenance treatment: Surface dressing is undoubtedly the cheapest and most widely used process for the maintenance of road surfacings generally. A new road surfaced with a dense and expensive wearing course will require maintenance, sometimes within the first three years. The effects of traffic, climate and constituent materials, often combined with those of errors in mix production, can produce several conditions: Condition Causes (a) Lean ‘hungry’ surface (b) Fatting up or bleeding (c) Surface hard but polished Bitumen content too low Poor mix design Poor process control Binder hardened and embrittled Binder content too high Poor mix design Poor process control Exposed hard stone polishes under traffic Result Surface frets; stone lost, ravening may occur (damage to windscreens) Possibility of excess binder on surface; surface slippery. Pushing and/or rutting Skidding accidents Skidding accidents, especially when wet These conditions occur regularly and can usually be treated by surface dressing, provided that local depressions, pot-holes, etc are rectified beforehand; it is customary in the UK to burn off any excess binder from bleeding areas using specialised equipment. Surface dressing maintenance treatment providing a waterproof seal, additional and an anti-skid surface at a cost which is 20-30 per cent of thinnest practicable premix overlay. Rolled asphalt, which is dealt with in Section 5.1, is a premix used. in the UK for the surfacing of all main roads and is a valuable cover aggregate that of the dense bituminous has a life of 20-25 years. The layer of pre-coated chippings (20 mm) applied as an antiskid measure during laying may remain effective for some 10-15 years (more or less depending on numbers of heavy vehicles) . Thereafter surface dressing is done at 4-5 year intervals. Traffic on motorways canbe exceptionally heavy and surface dressing is then required more frequently. 13 4.4 Slurry seals These materials may be considered as intermediate between surface treatments and premixed materials with respect to cost and performance. They are basically mixtures of fine aggregate, water, bitumen emulsion and occasionally cement. Where new seals have to be opened to traffic soon after laying, chemicals are added to break the emulsion within a controlled period. Some UK treatments can be trafficked within 2-3 minutes of laying; a typical composition would be: combined aggregate (5 mm down) 70-80% weight water 7-lo% weight emulsion 13-20% weight Anionic or cationic emulsions may be used. Anionic slurries may be premixed in a static mixing plant, but, because emulsions in cationic slurries break relatively quickly, these slurries have to be prepared in a purpose-built mixing and laying machine. Slurry seals are of fluid consistency when applied and can thus penetrate and seal cracks and surface voids. The resulting layer, some 3-4 mm thick, improves the riding quality of the road to some extent depending upon the laying technique. This can range from simple squeegees to modern mixerspreader units; costs are higher than for surface dressing but may be justified for old badly-cracked or lean bituminous surfacings. In South Africa slurry seals are sometimes applied to new surface dressing, thus furnishing a superior type of seal, often referred to as ‘cape seal’. They are particular ~y useful for re36 sealing aerodrome runways and standard reference works are avail~le. The runways of two international airports in the Arabian Gulf area have been successfully re-sealed using the slurry seal process. One of these was badly cracked and abrasion loss was occurring; in the touch-down areas particularly, disintegration was imminent due to embrittlement of the binder. A double seal, 2 x 3 mm, was applied at these areas and a single seal over the major portion of the runway. The treatment has been entirely successful at both airports. 5. PREMIXED BITUMINOUS SU~ACINGS AND T~ LOW-COST ROAD In the context of low-cost roads, premixed bituminous surfacings would appear initially to be both unnecessary and economically unjustified. The following points, however, should be considered: (a) New roads are invari~ly found to generate traffic; the estimated daily usage is often exceeded early in the life of a road. . (b) The low-cost road of today can become the important route of tomorrow. ~~~~~~ ‘o what were formerly inaccessible areas encourages development. has concluded from available evidence that most forecasts of r ral Y traffic in developing countries are subject to considerable error: a - 50 per cent confidence interval is likely. The design history of many low-cost roads in developing countries, notably those in Africa, may be summarised simply as follows: 14 (i) Initial construction Subgrade CBR = 25 (rein) (100 mm (rein)sub-base, CBR = 25 (rein),where subgrade has CBR less than 25 at equilibrium moisture content) . 150 mm Base a) Stabilised, soaked CBR = 100 (rein)* or b) Mechanically stable, soaked CBR = 80 (rein). Double surface dressing (*Does not apply to bitumen-stabilised sand) (ii) Strengthening overlay 50 mm (rein)bituminous premix. It is imperative that the benefits expected at Stage (ii) should be realised in full, particularly in view of the high cost of bituminous materials. Bituminous overlays often fall short of their objectives for various reasons, among which are: (a) inappropriate premix type; (b) inadequate materials and manufacturing equipment; (c) inadequate control of manufacture. The characteristics of different mix types and their application are discussed briefly. 5.1 Bituminous pretix types Bituminous mixes may be considered as being of two main types, the performance of which depends upon: (i) the mechanical interlock of aggregate (ii) the stiffness of a bituminous mortar. ‘escrfied in Bs ~~,,~~se ‘ate Type (i) comprises t rials known generally as macadams, eg as ; bitumen of very high viscosity is not required in these mixes since stability is obtained by the interlock of particles which must, however, be l~ricated during the compaction process. The well-known ‘asphaltic concrete’ is a very dense continuously-graded macadam. The Marshall mix design method is generally used to determine the ‘optimum’ binder content, which must reflect the condition of optimum packing of aggregate (and therefore optimum lubrication) together with optimum cohesion. This is the condition for maximum stabi~~ty which is often given attention at the expense of flexibility. Nijboer introduced the idea of mix ‘stiffness’ and it was subsequently concluded that an could be calculated as follows: Stiffness = Marshall st~ility (lb) Marshall flow (0.01 in) = 1.2 x tyre approximate value pressure (lb/in2) 15 Certain-assumptions and approximations were made in arriving at this simple relationship; nonetheless, it appears to have been supported reasonably well by the results of field and laboratory tests. Experience at T~L is that mixes produced in the developing countries often have an unnecessarily high stiffness. Marshall design criteria for road surfacings in the tropics have been calculated for three types of traffic on the basis of assumed tyre pressures, permissible flow values and the stiffness calculated as shown above. The results are given. in Table 8. Type (ii) mixes are bituminous mortars, ie sand is coated with a very viscous bitumen which has been further stiffened by the addition of filler. Stone is usually added as an extender and enhances stability. In the UK these stone-fi~~ed mortars are known as rolled asphalts, and are described in BS 594:1973 . The gradings of stone and sand are specified separately and a discontinuity appears in the overall grading: rolled asphalts are therefore often referred to as ‘gap-graded’ mixtures. Compositions have evolved from experience and the resulting recipes consistently define materials with lives of 20-25 years, requiring only routine re-texturing by surface dressing. Initially, rolled asphalts containing less than approximately 45 per cent stone are given, as an anti-skid measure, an application of coated chippings during laying. 5.2 over Ex~rience and research Macada and rolled asphalt compositions for use in the UK have evolved many years of experience without the need for mix design procedures. In contrast, asphaltic concrete has come into general use in the developing countries, especially in the tropics: this is understandable because of high road temperatures. It is usually found that asphaltic concrete correctly designed, mixed and laid performs well under fairly continuous traffic, the tyres of which tend to keep the surface ‘closed’. Troubles are common when such surfacings are very lightly trafficked, particularly by fast moving cars. Aggregate particles in the surface tend to be picked out instead of being re-embedded into the matrix, for which slow, high-pressure tyres are required. Picking can develop rapidly into severe ravening: this can and does occur very easily with materials made to specifications which result in low ‘optimum’ bitumen contents. Table 9 contains two recommended specifications for asphaltic concrete. The aggregate particles in asphaltic concrete are very thinly coated: mixes tend to be brittle and to develop cracks which are not self-healing. In recent years the Overseas Unit, TM, has been examining the relative performances of different premix types in tropical and sub-tropical conditions. At one experimental site in the Arabian Gulf the performance of asphaltic concrete, surface dressing, open and dense macadams and rolled asphalts is being compared. ~ilst there is no noticeable deterioration of any of the sections after three years of use, it is encouraging to note that the rolled asphalt sections show no signs of rutting. A second nd more extensive 41 group of experimental sections constructed in Turkey is devoted almost entirely to rolled asphalt surfacings but is relatively new: further opportunities for trials are being sought, the objective being to est~lish suitable surfacing specifications for given environments. Gap-graded compositions are already in4~ se in South Africa and Table 10 shows a specification used in Natal . 16 tiere dotit exists as to a suitable surfacing specification, a ‘recipe’ such as the ‘Group 3 (14 mm) wearing course shown in Table g can be adopted: if necessary this would be sealed with a surface dressing. For lightlp trafficked roads it will be advisable to use binder contents at the higher end of the appropriate range. Compared with aspha,ltic concrete such a mix will have higher voids and will be permeable; however~ the aggregate will be well-coated and the risk of bleeding (resulting from poor production control) will be lessened. The risk of picking and abrasion will be small and the mix will have greater flexibility than asphaltic concrete, cracking being therefore 1~gs likely. Table 9 includes three further specifications taken from BS4987 . 5.3 Construction tectiiques Excellent construction equipment is available for all bituminous road processes. Recent developments in surface dressing equipment were mentioned earlier (Section 4.3.4) ; this equipment is relatively cheap compared with sophisticated premix plant. Bitumen distributors can be used for work other than surface dressing and priming. mere expensive mixing plant is not availabl~3it is often possible to manufacture premix by the so-called ‘road mix’ process: 1. Aggregates are windrowed on a suitable mixing site; this is often a new base, old road or a levelled piece of ground which has been sealed with a bitumen spray. 2. The required quantity of cut-back bitumen is sprayed by a distributor in several applications and a blade grader mixes the material between applications. . 3. The mixed material is then spread or, if necessary, aerated for some time to remove volatiles before spreading. A range of gradations can be mixed by this technique which is limited only by the stiffness of the resulting mix; for this reason the cut-back grade will be determined by prevailing aggregate temperatures. The process is used extensively in bituminous stabilisation for bases and will doubtless be applicable to remote areas in the developing world. Control of mix proportions in this process is poor. Nonetheless, some examples of effective application have been seen in the Arabian Gulf area. Available premix plant varies in complexity from simple paddle mixers producing approximately 10 tph to fully automatic asphalt plants producing 25o tph. Simple mixers have no aggregate drying unit and are restricted to mixing at ambient temperatures; thus bitumen-stabilised soils can be produced and occasionally coated materials, for which a cut-back bitumen or slowbreaking (stable) emulsion must be used. Aggregates must be pre-wetted when mixing with emulsion, which must be particularly stable when large amounts, of fine material are present. The production of well-controlled premix requires appropriate mixing plant. For high-grade surfacings, particularly those like asphaltic concrete for which binder content control is so critical, only a weigh-batch type plant is permissible. Continuous mixers are susceptible to variations in 17 hot bin level and serious departures from the design grading and bitumen content may occur with these plants, which are better employed for preparing single-size aggregate mix”tures or for soil stabilisation. Weigh-batch plants normally have four hot aggregate bins and a filler bin; all ‘fractions are pre-weighed before discharge to the pugmill. Binder proportioning is almost invariably by weigh-batching, which has been found to be more accurate than volumetric devices. In the UK it is customary to produce macadams in batch-heater type mixers; stone fractions are pre-weighed in the cool state and then combined before loading to the batch-heater. This system is much simpler than that of the more familiar asphalt plant and is well suited to macadams and their lower drying/heating demand. Dense mixtures such as rolled asphalt and asphaltic concrete require much higher heat inputs for drying and involve higher temperatures since the binders used are more viscous than in macadams. The continuous dryers in asphalt plants are better suited to these requirements and, although macadams can be produced effectively with such plants, it is not operationally feasible to produce dense mixtures using batch-heater plants. 5.4 1. 2. 3. 4. 5. 6. 18 Asphalt plant operation Several points can be made concerning asphalt mixing and paving plant: Where cold feeders at the mixing plant are not charged with several screened materials of different sizes, the gradations of the materials in the hot bins must be checked frequently. Too often an all-in material is cold fed; the contents of the hot bins will then depend entirely upon variations in the stockpile. Materials such as asphaltic concrete cannot be produced under these conditions. Plant output must be adequate for paver laying capacity. The latter usually exceeds production, with the result that the paver stops and restarts frequently, leaving a series of irregularities in the carpet; the problem can be aggravated when insufficient trucks are used. Mix temperatures must be checked frequently; it is not unknown for producers to keep temperatures high in order to speed up compaction, thus increasing output. Appropriate binder temperatures are given in Table 11. The paver must be well maintained, particularly the hydraulic system. Trucks must not be permitted to reverse into direct contact with the paver. The laying gang should not be allowed to back-blind the spread material; this is commonly done for little or no reason and can affect the riding quality. Hand rakes are frequently used to remove excess material at the longitudinal joint: exce=ive raking invariably segregates coarse stone, leaving open-textured areas. Roller operators can, and will, ruin any laying operation if inexperienced or badly controlled. Correct overlapping of rolled passes by a half width, staggering of stop points, cleanliness of efficiency of cleaning mats and water spray bars requiring supervision. rolls (or tyres) , are particular points There is adequate publisQe$4w$~k4~n4$he operation and maintenance of bituminous construction plant ‘ ‘ ‘ ‘ 6. ACKNOWLEDGEMENTS The work described in this Report was carried the Transport 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. SHERWOOD Ministry Research and Road Research Laboratory. 7. REFERENCES out in the Overseas Unit of P T. The properties of cement stabilised materials. of Transport, RRL Report LR205. Crowthorne, 1968 (Road Laboratory) . ROAD RESEARCH LABORATORY. Notes on the cement treatment of Ghana soils. Department of Scientific and Industrial Research, Road Research Overseas Bulletin No. 8. Harmondsworth, 1958 (Reprinted 1962) (Road Research Laboratory). ROAD RESEARCH LABORATORY. Lime stabilisation of soils for use as road foundations in Northern Rhodesia. Ministry of Transport, Road Research Overseas Bulletin No. 9. Crowthorne, 1969 (Road Research Laboratory). JACKSON G P and D BRIEN. Asphaltic concrete. London, 1962 (Shell International Petroleum Co Ltd). WILLIAMS F H P. Bitwen-sand mixes for road bases. Ministry of Transport, RRL Report LR 239. Crowthorne, 1968 (Road Research Laboratory). TILLEY A C. Stabilizing sandy soils in Nebraska. Proc. llth Nat. Asph. Conf. New York, 1938 (Asphalt Institute), pp 49-55. BECKHAM W K. Soil stabilisation with asphaltic materials in South Carolina. Proc. llth Nat. Asph. Conf. New York, 1938 (Asphalt Institute), pp 61-68. WEATHERS H C. S.tabilising sandy soils with asphaltic materials in Florida. Proc. llth Nat. Asph. Conf. New York, 1938 (Asphalt Institute), pp 43-47. SHELL CO~ANY OF EGYPT LTD. Plastic roads (Shell Co of Egypt), 2nd edition. PORTER O J. The preparation of subgrades. Wash. , 1938, 18 (2), 324-31. — T~NSPORT AND ROAD RESEARCH LABO~TORY. A and runways. Cairo, 1939 Proc. Highw. Res. Bd. guide to the structural design of bitumen-surfaced roads in tropical and sub-tropical countries. Department of the Environment, Road Note 31. London, 1977 (H M Stationery Office), 3rd Edition. 19 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 20 LIDDLE W J. Application of AASHO Road Test results to the design of flexible pavement studies. Proc. internat. Conf. struct. Design Asph. Pavements. Ann Arbor, 1963 (University of Michigan), pp 42-51. NIJBOER L W. Some considerations of the ’Marshall test method for investigating bituminous masses (in German) . Strasse und Autobahn, 1957, ~, 210-4. .. ., THE ASPHALT INSTITUTE. Mix design methods for asphalt concrete and other hot-mix types. Manual Series No 2 (MS-2). College Park, Maryland, 1969 (The Asphalt Institute) , 3rd edition. HITCH L S. Bibliography on bituminous stabilisation of soils. Ministry of Transport, Road Research Laboratory Library Bibliography NO 99/LSH. Crowthorne, 1966 (Road Research Laboratory). JOHNSON C M and J J GANDY. Construction of the Maiduguri-Bama road, Northern Nigeria. proc. Instn. civ. Engrs, 1964, 29 (6814), 1-18. — HITCH L S and R B C RUSSELL. Sand-bitmen for road bases; an examination of five methods of measuring stability. Department of the Environment, T~L Report LR 717. Crowthorne, 1976 (TransPort and Road Research Laboratory). ALE~NDER P and J F T BLOTT. Factors affecting the structural stability of sand carpets. J. Sot. them. Ind., Lend., 1945, 64, 89-101. — BROOME D C bitwinous asphalts. PRANDTL L. of plastic DEPARTMENT LABORATORY. and A P~ASE. The use of mechanical tests in the design of road surfacing mixtures. II St~ility tests on rolled J. appl. Chem., Lend., 1958, ~, 121-35. On the resistance to indentation (hardness) and the strength materials. Z. angew. Math. Mech., 1921, —1, 15-20. OF SCIENTIFIC AND INDUSTRIAL RESE~CH, ROAD RESEmCH Stresses in soils and bearing capacity of ground. Soil Mechanics for Road Engineers. London, 1952 (H M Stationery Office), pp 451-63. HANSON J M. Particle interaction and the stability of mixes. Faculty of Engineering of the University of London~ MSC Thesis. London, 1966. (Unpublished). MOBIL OIL AUSTRALIA LIMITED. The foamed bitumen process as a soil stabilisation technique. Technical Bulletin No 5. Melbourne, 1969 (Mobil Oil Australia Limited). BOWERING R H. Properties and behaviour of foamed bitumen mixes for road building. Proc. Fifth Conf. Aust. Rd Res. Bd, 1970, 5 (part 6), 38-57. ROAD RESEARCH LABO~TORY. Dust-laying on unsurfaced roads. Department of the Environment, Road Research No 14. Crowthorne, 1971 (Road Research Laboratory). earth and gravel Overseas Bulletin 26. NATIONAL INSTITUTE FOR ROAD RESEARCH. Guide on prime coats, tack coats and temporary surfacings for the protection of bases. South African Council for Scientific and Industrial Research, TRH 1. Pretoria, 1970 (National Institute for Road Research). 27. TRANSPORT AND ROAD RESEARCH LABORATORY. Recommendations for road surface dressing. Department of the Environment, Road Note No 39. London, 1972 (H M Stationery Office). 28. SWAMI S A. Australian method of surface dressing. J. Inst. Highw. Engrs, 1965, 12 (1), 22-31. — 29. NATIONAL INSTITUTE FOR ROAD RESEARCH. Bituminous surface treatments for newly constructed rural roads. South African Council for Scientific and Industrial Research, TRH3. Pretoria, 1971 (National Institute for Road Research). 30. JACKSON G P. Surface dressing. London, 1963 (Shell International Petrolew Co Ltd). 31. BRITISH STANDARDS INSTITUTION. Single-sized roadstone and chippings. British Standard 63:1951. London, 1951 (British Standards Institution). 32. BRITISH STANDARDS INSTITUTION. Methods for sampling and testing of mineral aggregates, sands and fillers. British Standard 812:1975. London, 1975 (British Standards Institution). 33. SALT G F and W S SZATKOWSKI. A guide to levels of skidding resistance for roads. Department of the Environment, TRRL Report LR 510. Crowthorne, 1973 (Transport and Road Research Laboratoq). 34. HOSKING J R. The effect of aggregate on the skidding resistance of bituminous surfacings: factors other than resistance to polishing. Department of the Environment, TRRL Report LR 553. ‘Crowthorne, 1973 (Transport and Road Research Laboratory). 35. NATIONAL INSTITUTE FOR ROAD RESEARCH. The use of bitumen emulsions in the construction and maintenance of roads. South African Council for Scientific and Industrial Research, TRH7. Pretoria, 1972 (National Institute for Road Research). 36. HAWKEN N. Slurry seal - practical application and current use. Slurry seal Symposium. Rds Rd Constr., 1967, 45 (531), 60. — 37. THE ASPHALT INSTITUTE. Asphalt surface treatments and asphalt penetration macadam. Manual Series No 13 (MS-13). College Park, Maryland, 1964 (The Asphalt Institute). 38. HOWE J D G F. The sensitivity to traffic estimates of road planning in developing countries. Department of the Environment, TRRL Report LR 516. Crowthorne, 1973 (Transport and Road Research Laboratory). 39. BRITISH STANDARDS INSTITUTION. Coated macadam for roads and other paved areas. British Standard 4987:1973. London, 1973 (British Standards Institution). 21 40. 41. 42. 43. 44. 45. 46. 47. BRITISH STANDARDS INSTITUTION. Rolled asphalt (hot process) for roads. and other paved areas. British Standard 594:1973. London, 1973 (British Standards Institution). HITCH L S. Bituminous surfacings for the tropics: the laying of experimental sections on a road in Turkey. Department of the Environment, TRRL Report SR 142UC. Crowthorne, 1975 (Transport and Road Research Laboratory). GROTH P J. Overlay design in Natal. Proceedings of First Conference on Asphalt Pavements for Southern Africa. Durban, 1969 (Secretary Birds (Pty) Ltd, Durban). THE ASPHALT INSTITUTE. Asphalt mixed-in-place (Road mix) manual. Manual Series No 14 (MS-14). College Park, Maryland, lg65 (The Asphalt Institute). HATHERLY L W and P C LEAVER. Asphaltic road materials. London, 1967 (Edward Arnold (Publishers) Ltd). DEPARTMENT OF SCIENTIFIC AND INDUSTRIAL RESEARCH, ROAD RESEARCH LABOWTORY. Bituminous materials in road construction. London, 1962 (H M Stationery OffiCe). THE ASPHALT INSTITUTE. (MS-3). College Park, THE ASPHALT INSTITUTE. (MS-8). College Park, Asphalt Plant Manual. Manual Series No 3 Ma~land, 1967 (The Asphalt Institute). Asphalt Paving Manual. Manual Series No 8 Maryland, 1965 (The Asphalt Institute). 22 Process/Material Priming Tack coating Surface dressing Slurry sealing Macadams includes asphaltic concrete) Mortar type mixes (includes rolled asphalt) TABLE 1 Some features of different bituminous processes and materials Use binds surface of new base in preparation for surfacing provides bond between existing surface and bituminous premix overlay re-sealing re-texturing sealing open/’hungry’/cracked bituminous surfacings bases, surfacings bases, surfacings Binder function sealant adhesive sealant and adhesive sealant/adhesive lubricant/adhesive adhesive/sealant Binder viscosity (as constructed) low high low/medium low medim/high V. high Aggregate system nil nil single-size chippings dense, very fine angular, interlocking (very open - very dense] very dense; may include stone Properties of sand-bi~umenmixes after 1 year (+) storage at 45% stmility tests at 60 c (all compacted densities, CDM, in Mg/m ) (Sand: as for construction of experimental sections, Maiduguri-Bamaroad) Cone stability Deformation wheel tracking test Hubbard-Field Marshall Properties of recovered binder Viscosity (absolute~4 (poises x 10 ) Rate of deformation (mm/min) % Wt 3 4 Stability (kg) Stability (kg) Flow (mm) Penetration at 25°C Binder s 2.7 2.2 m CDM 1.8 1.3 Y 0.9 0.9 CDM CDM CDM at 45°C I at 600C MC2 1.91 0.O36 1.91 293 304 413 468 343 333 1.93 206 2.0 5.89 I 1.87 92 0.66 1.92 0.098 1.92 1.93 170 * 1.8 1.89 88 3 2.7 1.8 0.9 1.91 0.036 1.91 1.92 337 2.0 1.89 58 s.125 4 3 4 2.4 1.7 1.6 1.5 0.9 0.8 0.9 0.8 0.8 1.92’ 0.O38 1.92 1.93 327 256 2.0 1.91 65 18.5 I 2.2 80/100 pen 1.90 0.094 1.90 1.91 2.0 16.7 I 1.88 44 4.0 1.92 0.041 1.92 1.93 219 2.0 1.91 42 16.0 I 1.8 —.— –- ,,,,,,,,,,,,,, ,, ,, ,,, ., ,. ,,, ,,,, ,, TABLE 3 Hubbard-Field/Marshall stability ratios and cone stability/equivalent tyre_pressure (imperial) relationships; (Sand: as for construction uncured and cured material; tested at 45°C of experimental sections, Maiduguri-Bama road) Cured 1 year (+) at 45°C kinder Uncured HubbardField/ Marshall ratio HubbardField/ Marshall ratio HubbardField Equivalent tyre pre~sur (lb/in ) 41 Cone stabilit (S) Y (kg/cm ) 2.9 Binder content (% Wt) HubbardField (kg) Cone stabilit (S Y (kg/cm ) Equivalent tyre pre~sure (lb/in ) larshall (kg) Marshall (kg) content (% Wt) (kg) k 3 MC2 4 3 671 578 382 421 1.76 1.37 11.0 8.9 156 SPECI~NS UNSTABLE 7 198 79 2.51 197 44 4.48 3.0 42 4 126 1 I E 3 s125 4 3.1 3.4 44 48 3 587 473 1.24 9.6 136 4 3 637 507 425 384 1.49 1.32 9.0 128 112 F 85 80/100 3 pen 4 420 I 232 I 1.81 2.7 38 7.9 517 I 192 I 2.69 3.4 48 4 586 391 1.50 6.o TABLE 4 Recommended nominal size of chippings (millimetres) I Lane Traffic Categogy I Approximate number of commercial vehicles currently Type of surface carried per day in the lane under consideration (1) (2) (3) (4) (5) Over 2000 1000-2000 200-1000 20-200 Less than 20 Very hard (10) 10 6 6 6 Hard 14 14 10 6 6 Normal 20+ 14 14 10 6 soft * 20+ 14 14 10 Very soft * * 20+ 14 10 Note: The size of chipping specified is related to the mid-points of lane traffic category ranges 2-5: lighter traffic conditions may make the next smaller size of stone more appropriate. +At the discretion of the Engineer, 20mm chippings may be used for remedial treatment where traffic speeds are low. Very particular care should be taken when using 20mm chippings to ensure that no loose chippings remain on the surface when the road is opened to unrestricted traffic as there is a high risk of windscreen breakage. *Unsuitable for surface dressing. 26 TABLE 5 Rate of spread when using cut-back bitumen Lane Traffic Category (See Table 4) 1 2 3 Type of 4 5 Surface Chipping Binder Chipping Binder Chipping Binder Chipping Bind&r Chipping Binder Size Rate Size Rate Size Rate Size (mm) Rate (litre/m2) (mm) (litre/m2) Size (mm) Rate (litre/m2) (mm) (litre/m2) (mm) (litre/m2) Very hard I 10 1.0* 6 1.0 6 1.1 6 1.2 Hard I Not recommended 14 l.l+ 10 1.0 6 1.0 6 1.0 Normal 14 l.O+ 14 1.0 10 1.0 6 1.0 soft Conditions not 1 20# 14 0.9 14 1.0 10 1.0 Very soft suitable for surface dressing I 20# 0.9 14 0.9 10 0.8 * Rubberised cut-back bitmen only is recommended. + Evidence is being sought for successful use of bitumen in these categories. # At the discretion of the Engineer, 20mm chippings may be used for remedial treatient where traffic speeds are low. Traffic Very light Light Medium Medium Heavy Heavy Very heavy Existing surface Untreated/primed base Very lean bituminous Lean bituminous Average bituminous Very rich bituminous TMLE 6 Veh/day Constant Type of chippings 0-100 +3 Round/dusty 100-500 +1 Cubical 500-1000 0 F1aky 1000-3000 -1 Pre-coated 3000-6000 -3 6000+ -5 Constant +2 o -2 -2 Climatic conditions +6 Northern Europe (wet and cold) +2 +4 Tropical (wet and hot) +1 o Temperate o -1 Semi-arid (dry and hot) -1 -3 Arid (very dry and very hot) -2 28 PSV of aggregate necessary to achieve the required skidding resistance in bituminous surfacings under different traffic conditions I PSV of aggregate necessary Aggregate I I abrasion value not greater than 12 ; not greater than 10 , SFC = sideway force coefficient SFC values in these traffic conditions are sometimes achievable with aggregates of extreme hardness and very high resistance to abrasion, such as certain grades of calcined bauxite 29 Traffic level Light Mediu Heavy Tyre Pressure(P) rang2 (lb/in ) 40-50 60-75 80-100 TABLE 8 Suggested Marshall design criteria for road surfacings in the tropics Stiffness, S/F* = 1.2P 48-6o 72-90 96-120 Range of flow valu s -5 (in x 10 ) 8-2o 8-18 8-16 Calculated Stability range (lb) 380-1200 580-1620 770-1920 * # S/F = Stability (lb) –9 obtained in Marshall test Flow (in x 10–”) to nearest 5 Suggested range of Marshall stabi$ity I S/F Range and flow values * I -2 # lb inx10 kg lb -2 kg# kN m — — inx10 m m 7w-1200 12-20 ‘ 315-545 3.0-5.1 35-1oo 60-180 0.6-1.8 .lw-1600 11-18 455-725 2.8-4.6 55-145 100-250 1.0-2.5 1200-1900 10-16 545-86o 2.5-4.1 75-190 135-340 1.3-3.3 b -., ,,,, ,, Authority Mix type Aggregate grading (% wt passing) (total aggregate) . 50 mm 37.5 m 28 mm 20 mm 14 mm 10 mm .6.3 mm 5mm 3.35mm 2.36mm 1.18mm 600 pm 425 pm 300 ~m 150 ~m 75 ~ Bitumen Content Crushed rock (% wt total mix) Limestone TABLE 9 Typical specifications for bituminous premix Shell International Petrolem Co Asphaltic concrete Binder course 100 80 - 100 60 - 80 35 - 56 28 - 44 20 - 34 15 - 27 10 - 20 5-13 2- 6 4.8 - 6.1 Hearing course 100 80 - 100 54 - 72 42 - 58 34 - 48 26 - 38 18 - 28 12 - 20 6-12 5.7 - 7.0 Group 1 Road base (40 mm nominal) 100 95 - 100 70 - 94 56 - 76 44 - 60 32 - 46 7-21 2-8 !.9 - 4.1 British Standard 4987 Group 2 Dense basecourse (28 mm nominal) 100 90 - 100 71 - 95 58 - 82 44 - 60 32 - 46 7-21 2-8 4.1 - 5.3 : ,. Group 3 Dense wearing course (14 mm nominal) lm 95 - 100 70 - 90 45 - 65 30 - 45 15 - 30 3-7 4.4 - 5.4 (A) 4.6 - 5.6 (B) 4.1 - 5.1 (A) 4.4 - 5.4 (B) ,. ,,,, Group 4 Coarse cold asphalt (10 mm nominal) 100 95 - 100 70 - 90 40 - 60 15 - 30 5-15 3-10 4.9 - 5.9 (A) 5.1 - 6.1 (B) 4.7 - 5.7 (A) 4.9 - 5.g (B) A)>300 cvpd) in one ‘B)<300cvpd) direction TABLE 10 42 Gap-graded overlay specification, Natal, South Africa BS sieve size (per cent passing) (1 in) 26.5 mm (+ in) 19 mm (+ in) 9.5 mm (3/16 in) 4.75mm (No 7) 2.36 mm (No 25) 600 pm (No 52) 300 Am (No 72) 200 pm (No 100) 150 @ (No 200) 75 Km Mix composition Coarse aggregate 100 90 - 100 60 - 80 25 - 45 15 - 30 5-15 0- 5 I Fine aggregate 100 95 - 100 70 - 80 45 - 65 25 - 45 10-20 ‘ +- Coarse aggregate (ret 7 mesh) 47 : 5% Wt Fine aggregate (7-200 mesh) 40 ~ 5% Wt Filler (pass 200 mesh) 7.5 : 2% Wt Bitumen (60/70 pen) 5.5 - 0.3% Wt Filler/bitumen ratio I between 1 and 2* Marshall StabilltY (rein) 340 kg (750 lb) Marshall flow value 2- 4.5 mm (8-18x10-2in) Voids in mix 4- 10% Vol Suitable for compacted course 30 to 65 mm (1% to 2% in) thicknesses of Sand equivalent value of >45 combined aggregates (AASHOT.176 - 65) I Aggregate crushing value (BS 812:1967) I <30 per cent , *in the authors’ view a filler/bitumen ratio of between 1 and 1.5 is likely to be the optimum ratio in most cases. 32 TABLE 11 Application temperatures for bitumens and cutbacks GRADE Cutback grades MC/RCO MC/RCl Mc/Rc2 Mc/Rc3 Mc/Rc4 Mc/Rc5 Penetration grades 400/500 280/320 180/200 80/100 60/70 40/50 30/40 Spraying Atomising Jets ‘c 50- 60 70- 85 90-110 110-125 125-140 140-155 160-170 165-175 170-190 180-200 ‘F 125-145 165-190 195-225 235-255 255-285 285-310 32O-34O 330-350 340-370 360-390 Slot Jets ‘c 35- 45 60- 70 75- 85 100-110 110-120 125-135 140-150 150-160 155-165 165-175 ‘F 100-120 140-160 170-190 210-230 230-250 255-275 290-300 300-320 310-330 330-350 Mixing ‘c 50- 65 65- 95 80-105 95-120 120-135 125-140 130-150 140-160 150-165 160-175 165-190 ‘F 125-150 150-200 175-225 200-250 250-280 260-290 270-300 290-320 30*330 315-345 330-375 Notes: (1) Owing to the inflammable nature of the solvent in RC type cutbacks the application temperatures of these grades should be kept to the lower end of the ranges quoted. (2) The temperature range given for each grade is necessarily rather wide because local climatic conditions and the type and condition of the equipment affect the optimum spraying temperature. 33 A Surface dressing Either 50mm (2in) of bituminous surfacing and 150mm (6in) of base or 150mm (6in) of base 200mm (8in) of base with surface dressing Minimum thickness of sub-base of 100mm (4in) to be used with subgrades of CBR 8 to 24 per cant. Material used in this zone to have CBR of not less than 25 Per cent 0.05 0.1 0.2 0.3 0.4’0.5 0.75 1.0 1.5 2.0 2.5 Cumulative number of standard axles in one direction ( x 106) If it is dasired to provida at the time of construction a pavement capable of carrying more than 0.5 million standard axles, the d=igner may choose either a 160mm (6in) base with a 50mm (2in) bituminous surfacing or e 200mm (Bin). base with a doubla surface dressing. For both of thasa alternatives, the recommended sub-base thickness is indicated by tha broken line. Alternatively, a base 150mm (6in) thick with a double surfaca dressing mav be laid initiallv and the thicknaee increased when 0.5 million standard axles hava been carried The extra thickness mav consist of 50mm (2in) of bituminous surfacing or at Iaast 75mm (3in) of crushed stone with a double surface dressing. The largest aggregata size in the crushed stona must not exceed 19mm (%in) and the old surface must be prepared bv scarifying to a depth of 50mm (2in). For this stage construction procedure, the recommended thicknass of sub-base is indicatad bv the solid Iina. o 4 6 8 10 12 ~ c o 14 = 16 18 20 22 Fig. 1 PAVEMENT DESIGN CHART FOR FLEXIBLE PAVEMENTS 400 350 300 250 200 150 100 50 6 — Sand A — — Sand B ---- Sand C Strain rate = 1.67 x 10-4sec-’ 629 Binder viscosity 6 x 103poiSeS / / ~ 629 (Normal pressure in kN/m2) / / I / / I /. i ~ - 315 -— ___ _ ---- —— 315 I --- ---- _ -- --- 79 0 2 4 6 8 10 12 14 Strain (per cent) Fig. 2 TYPICAL STRESS/STR&N RELATIONSHIPS FOR SAND/BITUMEN IN SHEAR BOX TEST Binder viscosity (poises) o 10 ● 10 A 6x 103 A 6X103 ❑ 106 ■ 106 Strain rate (see-l) 1.67 X 10-4 2.22 x 10-2 1.67 X 10-4 2.22 x 10-2 1.67 X 10-4 2.22 x 10-2 0 2 4 6 8 10 Binder content (per cent wt) Fig. 3 PRANDTL BEARING CAPACITY VALUES FOR TYPICAL SAND/BITUMEN 1800 1600 1400 “: ~ 1200 9 1000 800 ( 600’ [ \ \ , Sand A \ I I I I I I I I I I //- / 0 0 “ Sand B / / 0 0.5 1.0 1.5 2.0 Estimated bitumen film thickness @m) Fig. 4 RELATIONSHIP BEWEEN EBFT AND~ AT 2.22 x 10-2 SEC-~ 106 POISES 1800 1700 1600 1500 300 1200 1100 1000 900 0 Centrifuge Kerosene 1 ,,\ o 1 2 3 Centrifuge Kerosene equivalent (CKE) I 1 1 I 25 30 35 40 Degrees I 1 I I 1 1 I i I I [ 34 32 30 28 26 24 22 20 18 16 14 Efflux rate (ml/see) Fig.5 RELATlONSHIPBEWEEN *opt AND SOME PHYSICAL CONSTANTS OF THREE SANDS (DRY) Chipping application rate 45 50 60 70 80 90 100 120 150 200 yd2/ton I 1 I 1 1 I I I I I 1 I I I I 1 I I 1 [ , I I I I 1 , I 1 I 1 I I I 1 I 1 I I ‘~ I ~ -m :m 27 25 23 21 19 17 15 13 11 g 7 5kg/m2 ~> ,g g 6% 20 19 18 17 16 15 14 13 12 11 10 9 8 7 6 5 41itre/m 2J m) 5 4 3 2 1 0 9 8 7 6 W1’ 14 13 12 11 10 9 8 7 6 5 4 3 0.6 0.7 0.80.9 1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1,8 1.9 2.0 2.1 2.2 kg/m2 1 1 1 I 1 1 1 I 1 I J 9876 .5 4 i.5 3 2.5yd21gal Binder application rate (in) 1 7/8 3/4 5/8 112 3[8 1/4 3/7 6 1/8 Fig. 6 SURFACE DRESSING DESIGN CHART ❑ ✚✚✚✚✚✚✚✚✚✚✚✚✚ ✚✛✚✚❊✎✛ ❑ ~M Too stiff (poor wetting) ✎✎:~~~~:~:~:~:. Too fluid (drainage and whip-off) . . . . . . . . ................ ., . . . . . . . . ..................... . . . . . . . . ............. . . . . . . . . 10 20. 30 40 50 60 70 80 90 1 1 1 I 1 I I 50 77 100 122 140 160 180 Surface temperature Fig. 7 SURFACE TEMPERATURE / CHOICE OF BINDER FOR SURFACE DRESSING (“c) (°F) (911) Dd443233 1,000 6/77 HPLtd So’ton G1915 PRINTED lN ENGLAND -. ABSTRACT BITUMINOUS BASES AND SURFACINGS FOR LOW-COST ROADS IN THE TROPICS: L S Hitch and R B C Russell, BSC MPhil MIHE FGS: Department of the Environment Department of Transport TRRL Supplementary Report 284: Crowthorne, 1977 (Transport and Road Research Laboratory). Mechanically stable materials for road bases are often not obtainable in developing countries and the technique of soil stabilisation has therefore been developed. In the Middle East, aggregates are often scarce but oil products are readily availtile. The region has therefore provided some of the earliest examples of bituminous stabilisation, which originally consisted of thin running surfaces over compacted sand. Bituminous stabilisation can also enable local sand to be used for base construction, and various tests and design criteria have been proposed for such applications. The report describes full-scale experimental trials supported by laboratory research, which have enabled acceptance criteria for bitumen-stabilised sand bases for light/medium traffic to be proposed. Construction methods for bitminous stabilisation are also described. Details are given of methods of surface dressing, which is important both as an initial running surface on new bases and as a maintenance treatment. Premixed bituminous materials, both as bases and surfacings, might perhaps be considered as inadmissible for low-cost roads. Such roads, however, usually require progressive improvement because of the traffic growth which accompanies development. There is a growing use of strengthening overlays and the report briefly discusses premixed materials and their application. ISSN 0303-1315