SOME ASPECTS OF THE SCIENCE OF ROAD BUILDING AND THE ROLE OF RESEARCH (Part 2)

By P. J. RIGDEN

Bases and sub-bases 

On lightly trafficked rural roads, bases of natural gravels are often satisfactory; sometimes they need to be stabilized by the addition of a small percentage of hydrated lime (to reduce the effect of clays present) or by a small percentage of cement. The latter may be Portland cement or a blend of Portland cement and ground blast furnace slag. On all the more important roads it is common practice here to employ a crusher-run stone base which meets certain specifications for grading and crushing strength of the stone. A suitable quantity of fine material must be present to bind the whole together. Crusher-run quartzite rock from the mines along the Witwatersrand provides one of the principal sources of stone for the base in the Transvaal. Bases are usually built between 4 in and 8 in thick, depending on the class of road. 

Sub-bases, because they are subjected to smaller stresses, do not have to meet such high requirements as bases and are often built of unstabilized natural gravels, including shales, sandstones, granites, dolerites, depending on the area. Sometimes, on the more heavily trafficked roads, sub-bases must also be stabilized with lime or cement to meet strength requirements. 

SOME PROBLEM IN SOUTH AFRICA 

Elastic deflection under traffic 

It was pointed out that the sub-grade soil is protected from damaging stresses by the superimposed pavement. This pavement may itself, however, undergo a certain amount of elastic (recoverable) deflection under passing traffic and if this deflection is excessive it can cause cracking of the bituminous surfacing by fatigue and is a progressive deterioration in the strength of the base layer. As elastic pavement deflection is not covered by the C.B.R. design method, other methods have been developed to measure this deflection. Fig. 13 shows a deflection beam used by the Institute to measure directly the deflection developed in the road surface under a heavy wheel load. Good roads deflect less than 20/1 000 in under a 9 000 Ib wheel load. Research is in progress to establish criteria for permissible deflections of foundations of various types of road in South Africa, such that no progressive damage is caused under the repetition of traffic loads. 

The effect of stronger bases 

In some parts of the country (e.g. Natal) materials available for bases, such as crushed sandstone, are not of ideal quality and, under heavy traffic, have given trouble in the form of loss of stability and deformation of the surface. Such materials are today often stabilized, that is strengthened, with the addition of 4 or 6 per cent of Portland cement. However, this is not necessarily the only way to improve the strength of such bases and increasing use is being made overseas of stone bases stabilized with bitumen (or tar)-the so-called black bases. Such base layers are now recognized in some countries to have load-spreading properties equivalent to those of a greater thickness of unbound granular base or even of a cement-bound base. The numerical equivalence is still a matter of some disagreement but the NIRR is planning to experiment in this matter in the near future on heavily trafficked roads. 

Materials for foundations 

This leads to the problem of materials in general a problem which occurs over the whole country to a varying extent. In some areas, good natural materials for the base and sub-base layers are almost non-existent and lower grade weathered rocks, for example, decomposed dolerites, have to be stabilized by the addition of lime or cement. The Institute has done a lot of research on the engineering properties of these weathered rocks, particularly in relation to the climatic environment in which they occur, but much more research of this kind is needed, for example into materials like the shales, the decomposed granites, sandstones, tillites, and limestones. 

Moisture conditions in the field 

In many areas of South Africa and South-West Africa soils in the natural state are in a desiccated condition throughout a large portion of the year. When a surfaced road or a building is built over such soils a process of readjustment of the moisture profile starts in which the moisture content in the profile slowly increases to a new equilibrium. This may take a number of years 'to occur. In many areas of the Orange Free State and Southern Transvaal, for example, the soil profile includes clays, which increase in volume with moisture increase and heave upwards causing cracking of buildings and roads, as well as serious deterioration in the riding quality of roads. 

The Institute has been studying the movement's inroads which occur over such profiles for a number of years and has been experimenting with methods for overcoming this problem by accelerating the heaving by wetting up the profile prior to road construction. The effect is often most marked over culverts where water usually concentrates, as illustrated in Fig. 14 which shows measurements made on the National road near Britstown. A typical expansive soil profile north of Pretoria is shown in Fig. 15. 

A considerable programme of basic research into the factors governing the movement and distribution of moisture in soils under various field conditions is in hand in NIRR in collaboration with the Soils Division of the National Building Research Institute. As has been indicated, a better understanding of the moisture regime in soil profiles under roads is vital in considerations of the bearing strength of the subgrade and hence to the whole question of pavement design itself, and to the important matter of adequate provision for sub-soil drainage. 

The natural soil 

Much more needs to be known about the natural soil on which the road is built (the sub-grade). Apart from the small static load provided by the weight of the pavement layers, the loading provided by traffic is essentially dynamic, that is, transient in character. At a point on the road surface, the vertical stress imposed by a passing car wheel may only last 5 milliseconds (vehicle moving at 60 miles/h). The corresponding stress at the top of the sub-grade will be much smaller but will be imposed for a longer period because of the load-spreading effect of the depth of pavement. 

Research is in progress to measure sub-grade soil strength under repeated loading conditions, instead of a steady load, in order to simulate more closely traffic loading. Important differences in strength are often found under static and dynamic loading for the same stress magnitude. A view of a battery of repeated load test equipment in the NIRR is shown in Fig. 16. Tests are often taken to 1 million repetitions of the load before completion. 

Another approach is to use vibration methods of test to examine the elastic properties of pavement layers and subgrade layers, and this has been in hand in the NIRR for a number of years. Sinusoidal forces are applied to the surface of the road or pavement layer under test and the magnitude of the resultant displacement and its phase is recorded. The mechanical impedance of the road structure can then be calculated as a function of the frequency of the applied loading. 

The other technique is to pick up the displacement at various distances from the point of load application and so study the propagation of elastic waves through the construction. This also yields information about elastic parameters of the various layers. A photo of the apparatus currently in use in NIRR is shown in Fig. 17. These studies are now yielding significant results relevant to the evaluation of the strength of existing roads and runways, as well as to the design of new road foundations. 

Control on construction 

This, again, is a fruitful field for research as the need today is usually for more rapid methods of control of constructional operations which demand a minimum of manpower for their operation. An important example of work in this field is on the problem of controlling the density to which constructed layers of a road foundation are compacted. Density, as indicated earlier, is an important determinant of soil strength and generally on construction is a most important factor to control. 

Existing methods usually involve digging a test hole to the full depth of the layer, weighing the material removed and finding the volume of the hole by backfilling with sand. The NIRR has been working for a number of years to perfect the development of an instrument employing radio-isotopes for this purpose. A source of gamma rays (cobalt 60 or radium) is used to irradiate the layer at the test point and the amount of back-scattered radiation is detected and measured. Over a certain range of density, the back-scattered radiation is inversely proportional to the density. A source of fast neutrons (radium-beryllium) is also used in the instrument to measure the moisture content, using the principle that hydrogen atoms slow down fast neutrons to thermal neutrons and these are detected and counted and give a reading proportional to the amount of hydrogen, and hence to the amount of water present. Apart from enabling rapid checks of density and moisture content of a layer to be made (2 minutes for each measurement), this equipment has also proved useful in studying the performance of heavy compaction equipment during construction, thus enabling optimum use of compaction plant to be achieved. An example is shown in Fig. 18. 

THE BITUMINOUS SURFACING 

The bituminous surfacing of a road pavement may vary, depending upon the class and importance of the road, from a very light single coat surfacing of t in or less in thickness up to multi-layered pre-mix surfacings totalling 4 in or more in thickness. 

All such surfacings are composed essentially of aggregate, that is to say, crushed natural rock, slag, sand, the mechanical properties of which play an important part in the behaviour of the surfacing. The proportion of bituminous binder in the surfacing lies normally in the range of 5 to 8 per cent by weight. Its function is to bind all the aggregate particles together and to the underlying surface; although forming a relatively small portion of the whole, the bituminous binder plays a dominating role in the behaviour and properties of the surfacing. 

Main functions of the surfacing 

Before discussing the different types of surfacing in common use, it will be useful to review briefly the principal functions which a good surfacing should fulfil. 

1. It must provide strength right at the road surface to resist the disruptive stresses imposed both in horizontal and vertical directions by the moving traffic. As indicated before, relatively high shear stresses occur near the surface under a loaded area and in the surface at the edge of the area. Without the protection of a surfacing, an unbound granular stone base would soon be broken up by traffic (as happens on gravel roads). 

This function is complicated by the requirement that the surfacing resists the deforming effects of traffic at high road temperatures during summer and also the tendency to brittle fracture at low temperatures during winter. 

2. When the surfacing is thick enough and has a high modulus of elasticity, it can contribute significantly to the strength of the whole pavement as a load-bearing structure. This was illustrated in Fig. 10. The light surfacings in common use in South Africa do not contribute strength in this manner. 

3. The surfacing must retain enough flexibility during its life to withstand the repeated small elastic deflections which are imposed on it by heavy wheel loads, without cracking. The thicker the surfacing the smaller is the limiting repeated deflection it can withstand. 

4. The surfacing should provide a waterproof cover over the pavement layers below. 

5. It must be durable under the effects of traffic and the action of the weather, i.e. the deteriorating influences of heat, strong sunlight, oxygen in the air, and water. 

6. It should have and retain a safe, non-skid riding surface. This depends on the composition of the surfacing, but particularly on the properties of the aggregate; some road stones, for example, tend to polish under heavy traffic and produce a surface that is slippery when wet. 

7. The surfacing should confer good riding properties on the road, i.e. produce a smooth ride for the motorist. The ability to surface to do this varies with composition and thickness. Light surface treatments will only reproduce the profile of the base on which they are played, while the thicker pre-mix surfacings can be laid by paving machines to a uniform surface profile. 

The main requirements of a successful bituminous surfacing can be met, in principle, by providing one or more layers of crushed stone of size ranging from sand up to about ¾ in provided this is bound together with a suitable adhesive. As such large quantities are required for road surfacing the adhesive must obviously be cheap, and the only materials available in large quantities to satisfy these requirements are bitumen and tar. 

Bitumen (or asphalt, in American terminology) is the by-product or residue from the refining of crude petroleum oil, or it may occur naturally in association with mineral matter, as in a lake asphalt or a rock asphalt. In South Africa, at the present time, the main sources of bitumen are the oil refineries in Durban operating on crude oil shipped down from the Middle East oil fields. 

Tar is the by-product of the destructive distillation of organic matter like coal and wood. In this country, the main sources of tar are coke ovens (at steelworks) and municipal gasworks. 

Some properties of bituminous binders 

The tars and bitumens used for road surfacing work are essentially viscous liquids and are characterized and specified by viscosity or consistency, a large number of such grades being in common use. As the viscosity is dependent on temperature all testing has to be done at accurately specified temperatures. Once in the road surface, the bitumen must possess a minimum viscosity to ensure that stone is held in place under traffic in surface treatments and that adequate stability in pre-mix surfacings is ensured. To ensure adequate wetting of aggregate particles during application to the road or in a mixing plant, the bitumen is normally heated to a temperature of 250°F or higher at which it has a suitably low viscosity. The so-called cutback bitumens, often used in surface spraying operations, contain a proportion of volatile oils which soon evaporate on the road, but which ensure low viscosity to promote good initial adhesion at the time of construction. In pre-mixed surfacings, it is more common to use the so-called straight run bitumens which do not undergo initial hardening by loss of oil. 

The fluid, cutback bitumens are tested in a viscosimeter which consists of a cup having an orifice in the bottom through which a standard quantity of the bitumen at the specified temperature is allowed to flow. The viscosity is specified in seconds of efflux time. The harder grades of bitumen are tested by a simple penetration test in which a needle of the standard area is allowed to penetrate the bitumen sample at 25°C for 5 seconds under a 100-gramme load, the penetration is expressed in tenths of a millimetre. Commonly used grades have penetrations of 80 to 100 and 150 to 200. 

Because the viscosity changes so rapidly with the change in temperature it is usual to relate log. viscosity with a log. temperature as indicated in Fig. 19. It will be seen that a typical 80/100 pen. bitumen varies in viscosity over the range of surface temperature (0°C to 60°C) often encountered on the road in South Africa from 109 poises to 103 poises. 

During their life on the road, these binders undergo slow hardening as a result of weathering (mainly oxidation and a slow loss of oils) so that their original viscous character may be gradually lost, especially at winter temperatures when more brittle characteristics can develop, resulting in cracking of the surfacing and loss of stone. A discussion of this subject is, however, beyond the scope of this paper. Tars are more prone to deterioration in this way than bitumens and are consequently used mainly in the lower layers of a surfacing where they are protected from the direct action of the weather.