1. Introduction to Road Pavements 1. History of Roads. In ancient times there was nothing more than a sparse network of tracks for humans to use in order to reach the feeding and drinking places. These tracks differed only slightly from the tracks made by any other mammal for the same purpose, except that obstacles e.g. boulders, were removed from the more important routes and thorn bushes trimmed back by humans. More elaborate lines of communication than these simple tracks, did not appear until the increasing number of people in certain areas and the social structure and organisation of communities demanded more permanent contact between communities. Roads thus appeared when groups of people started to interact with each other by travelling, doing business, fighting, etc. This occurred around 3500 BC with the invention of the wheel and development of chariots and wagons. The earliest records of paved roads for wheeled traffic date from about 2200 BC in Babylonia (modern Iraq), in Crete from about 1500 BC and in Egypt from about 540 BC. In Europe the first substantial roads were built by the Romans – a network of more than 100 000 kms of road was built between 400 BC and 400 AD. The Roman roads were cambered to shed rainwater and were constructed on a foundation of large stones with a wearing course of smaller stones and gravel, constrained between raised stone kerbs. The Romans were the best road builders of the remote ages. Conquests achieved through war games were one of the reasons for this. The Romans needed a good network of roads to control their conquered subject-nations. The army needed to be able to move fast in order to quell any revolting groups. The Roman roads were cobbled with a base system that was dependent on the subgrade. They developed a three/four layer system of: • toplayer • base/subbase sometimes with a stabilised material in the base • subgrade. As soon as the Roman Empire collapsed, the roads structures began to degenerate. Napoleon was responsible for the construction of a considerable network of roads in Europe in the late 18th and early 19th centuries. In 1747 the “Ecole des Ponts et Chaussess” was founded in Paris, France. In 1765 Tresaguet developed the Roman road structure further. His basic principle was to construct the first layer with big blocks and then to place little rocks in between. By doing this, he attempted to ensure that the first layer was consistently exposed to compressive stresses, in order to achieve better load spreading on the subgrade. At the same time, in about 1810 in England, people such as Telford and Metcalf made valuable developments, including: • design of drainage, • design of the road camber, and • active and regular maintenance.

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Telford and Metcalf found that through drainage design and the inclusion of a crossfall, that maintenance could be substantially reduced and the required layer thickness dramatically reduced. In Great Britain, the Industrial Revolution required a road building programme to satisfy the need for the movement of materials and goods and many kilometres of road were built by various means. During this time John Macadam (1756 – 1836) invented a method of road building as follows: after careful preparation and draining of the roadbed (or subgrade), he laid a 25cm layer of stone (a size that could fit in a man’s mouth), followed by a surfacing of smaller stones. This type of roadway was ideal for animal drawn wagons and coaches, and was cheap to build. John Macadam’s roads lasted well under traffic and many British roads were “macadamised”. They were a good solution in the nineteenth century for iron rims i.e. treads. However, the invention of motorised transport (Siegfried Marcus invented the first car with traction in Vienna) and rubber tyres (Dunlop in 1888) changed the requirements once again. Speeds increased making safety an important consideration. Rubber-tyred wheels “sucked” the dust from the road surface, loosening the stones and causing blinding clouds of dust. Hence in the early part of the 20th century, tar was spread over the road surface to hold the stones in place and to prevent dust. Sand and stone and tar formed a “surface dressing”. Later the “tarmacadam” surface of stone coated with and rolled to a smooth surface was used, hence the term tarmac. Today we still make waterbound macadam and penetration macadam (using bitumen-emulsion slurry) and a variation of it heat up i.e. asphalt . In South Africa the pioneer road-builder was Thomas Charles Bain (1830 – 1893), son of Andrew Geddes Bain. Thomas Bain constructed 23 major mountain roads, nearly all in the Cape Province. Some of his roads are still in use today e.g. Bain’s Kloof Pass. The book “Romance of the Cape Mountain Passes” by Dr Ross provides interesting facts about this era of road construction. Some other important developments in the nineteenth century included the train and as a result of technical breakthroughs, the steamroller. Much attention went on the development of the train and in many countries, with a focus on building new railway lines, the roads deteriorated. In 1863 Lemoine invented the two-wheel steamroller, At the same time Clark and Butler developed the three-wheeled steamroller. This made compaction of granular layers significantly easier and the quality of compaction increased,. In the figure below an overview is given of the developments. See also the added two pages in the Appendix 1A showing chronological developments.

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.

Changes in technology

Maximum aggregate size

Traffic Vehicle technology

Motorisation Equipment The Wheel 2000 Mesopotamian 500Roman 500 BC Persian BC AD

Modern Middle ages

1800 2000 Time AD AD

Figure 1.1 Conceptual chronological developments impacting on road technology Prior to the early 1920s the thickness of pavements was based purely on experience. The invention of the car and the introduction by Henry Ford of his Model T-Ford in 1908 gave a strong impulse to look at design of roads more seriously. The traction of a car i.e. friction between tyre and road, causes damage to the surface and unpaved roads could not cater for this. Twenty million Model T Fords were sold between 1908 and 1927.This resulted in the use of the following in the 20th century: • empirical design systems i.e. experience- and observation-based designs, • mechanistic design systems (linking performance to critical pavement properties and failure mechanisms), and • empirical-mechanistic design systems. In Scotland and Ohio expensive solutions were found through experimentation. At the same time experiments were being undertaken to investigate tar and split (aggregate) or with natural asphalt. Skid resistance and a lack of bond to existing layers, proved problematic. Typical problems of the time can be seen in the figure below.

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Figure 1.2 Impassable unsurfaced roads in the wet (TRB) From the pre-second world war history of roads, the growing importance of roads is apparent. In the twentieth century, the car took the lead as main mode vehicle of transport. After the second world war, growth in traffic loads, tyre pressures and higher speeds necessitated the development of pavement technology beyond empiricism.. After the Second World War, the growth in traffic, loads and tyre pressures, and the higher speeds necessitated the development of pavement technology beyond empiricism or designs based on experience only. Functional performance had to be defined, being the basis of the service that is provided to the road users in relation to the cost. This is indicative of fitness for use. Performance also needed to be better understood and predictable. This required knowledge of structural behaviour and pavement distress in relation to time. This motivated the AASHO road test. The AASHO Road Test took place in Ottawa, Illinois about 100km SW of Chicago between 1956 and 1958. It was an enormous effort to systematically quantify the complex interaction between road deterioration, traffic and composition of the pavement structure on a closed loop test track with trucks. AASHO stands for American Association of State Highway Officials and later became AASHTO (Highway and Transportation). The aims of the AASHO road test are still very relevant: • developing satisfactory pavement design procedures to meet the growing demands of traffic, • aid legislators in setting user taxation and control of vehicle size and weight. The cost of the AAHSO road test was $29 million in 1954 (or about $300 in 1996). For the first time the relationship between performance and loading was investigated. The two main findings were: • Present Serviceability Index (PSI). The test was the first in which many facets of pavement condition and its progressive change with time (deterioration) were

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•

defined and quantified in a comprehensive index, the PSI. The .PSI was statistically related to roughness, rutting and cracking by the subjective PSR determined from users of the road. Load equivalency. The load equivalency factor relates the number of load repetitions of a given axle load to the equivalent number of 80kN loads. It can be used for comparing the damaging effect of different loads. Damaging Effect = [P/80]n

Where, P = axle load in kN 80 = Equivalent Single Axle Load (ESAL) in kN n = damage exponent , depending on pavement type (4 is commonly used for granular pavements). More details will follow. In the modern world it is well known that apart from social factors such as transport to hospitals, quick access to a fire and emergencies, visiting friends and tourism, a good road system is the backbone for all kinds of economic activity. John F Kennedy said: “ It is not America’s strong economy that gave us our good roads but rather our good roads that gave us our strong economy!” The Land Registration and Consolidation Programme that was started in the Kikuyu areas of Kenya in the 1960s ensured that each farmer should have a road access. This was to ensure that the farmer could reach his farm with commodities such as tree seedlings, building blocks and fertiliser. It also meant that the farm produce, in the form of bags of maize or coffee, could be got out to the markets or for processing at the factory. Rural road construction was a major factor in the Kenyan small-holder tea growing expansion that took place mainly after independence. Tea is one of Kenya's leading exports and Kenya shares the position of top black tea exporter in the world with Sri Lanka. The number of tea growers expanded from only 19,775 in 1964 to 273,000 by 1995. This expansion continues up to the present and one of its main features is the improvement of rural roads. Tealeaves have to be in the tea factory within a few hours of plucking or they spoil, so any dislocation in transport is disastrous. Each tea grower has to have ready access to an all-weather road whereby he/she can deliver the crop in time to be picked up daily by one of the 600 trucks owned by the Kenyan Tea Development Authority (KTDA). Road development in new tea areas has been undertaken using a number of World Bank and bilateral loan schemes. The attraction of such loans is that they can be justified because of the increased national income from tea. Construction of rural roads is only one aspect to pavement engineering. Another aspect is maintenance. All roads have to be cared for and without this maintenance lose their intrinsic value. In Madagascar in the 1990’s, the road repairs on the 18,000 km of the network that are almost impassable, was proceeding at a mere 900 km per year. Trips take at least twice as long as they should in Madagascar, because of the need for drivers to constantly dodge potholes. In many cases there are no signs of work being undertaken anywhere along the roads. In order to fund new road construction, developing countries often take out loans from institutions such as the World Bank. Perhaps all loans for new roads should be backed up with a parallel loan for repairs to existing roads. Roads are important assets to a country and need a maintenance programme to ensure that the longest

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life is achieved. In developing countries, initiatives such as local community programmes for filling potholes in return for donations deserve to be taken more seriously. With a little organisation and training of the manpower, and supplies of cement, these methods could prove to be effective for emergency repairs. It can be seen that roads are sources of both revenue and employment. For a general cross section of the pavement structure and the terminology actually used, see TRH4, figure 9.

2. Types of Pavements. Pavements can be divided into 3 major types: • Flexible pavements (upper layers of asphalt) • Rigid pavements (upper layers of concrete) • Composite pavements. a) Flexible pavements. Flexible pavements consist of a number of layers. Conventional flexible pavements: surface course ( 2-5 cm ) binder course ( 5-10 cm ) base course ( 10-30 cm ) subbase course ( 10-30 cm ) compacted subgrade ( 15 cm ) natural subgrade Full depth asphalt: surface course (5-10 cm ) base course

(5-30 cm )

compacted subgrade ( 15 cm ) natural subgrade

This type of pavement structure is quite popular in areas where local materials are not available i.e. limited base and subbase aggregates available in the area. Advantages of full depth asphalt pavements can be: 1. They do not have permeable layers that entrap water (such as granular layers). 2. Time required for construction is relative low. Especially beneficial on widening projects, where adjacent traffic flow must be accommodated. 3. When placed in thick layers, the construction season is not very limited. 4. Asphalt pavements provide and retain uniformity in the pavement. 5. It seems that moisture contents do not build up in subgrades under these types of pavement structures, causing little or insignificant reduction in subgrade strength.

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b) Rigid pavements. A typical cross-section of a rigid pavement is: Concrete pavement: concrete (PCC) (15-35 cm ), also regarded as base subbase layer (15-20 cm ), usually cemented compacted subgrade ( 15 cm ) natural subgrade There are different types of concrete pavements: • jointed plain concrete • jointed reinforced concrete • continuously reinforced concrete • pre-stressed concrete Where plain concrete is used it is especially important for pumping to be taken into account. The control of pumping is paramount otherwise cavities will occur under the concrete and the loss of support will result in collapse of the pavement. Three factors must act simultaneously to produce pumping: 1. Material under the concrete slab must be saturated i.e. with free water 2. Frequent passage of heavy wheel loads 3. Material under concrete slab must be erodable. c) Composite Pavements Where components of both flexible and rigid pavements are combined in one road structure, composite pavements are created. A typical composite pavement structure would, for example, include rigid block elements that in their composite form provide a semi-rigid structure: Composite block pavement: Interlocking concrete blocks (6-8 cm ) sand bedding subbase layer (15-20 cm ), usually cemented compacted subgrade ( 15 cm ) natural subgrade Another example of a composite pavement is a concrete pavement where the concrete surfacing layer is supported by an asphalt layer.

3. Labour intensive construction. See also TRH4 par.7.5. Since the launch of the RDP (Reconstruction and Development Programme) in SA employment has become a very important issue. The road building industry has historically been labour intensive, but in the last century has moved into more mechanised construction. Particularly in the fifties and sixties the use of machines

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was found to be found to be cheaper. Incentives now exist to rediscover more labour intensive methods again. Another important factor is the health considerations in labour intensive work. Although incentives may exist to move towards labour intensive construction (LIC), the risks involved with hard labour for the health of the body should not be forgotten. In the times when LIC was intensively applied, life expectancies were short, for good reason, It is possible to create the opportunities for people to construct roads employment intensively. This, of course, places pressure on the designer. He/she must be innovative and aware of the different facets of LIC, often using locally available material. The design and planning of LIC is actually a specialist field requiring specific knowledge and skills. The important issue in all these cases is that the quality of the final product must be adequate, otherwise big investments can be wasted. Even low trafficked roads can be exposed to high axle loads and must be capable to resist these loads even is constructed using LIC.

4. Other Pavement Structures. The principles of sound pavement design systems (mostly mechanistic) can be used for any type of structure with its specific requirements. Some examples are: • Industrial areas • Harbours (storage areas, container depots, forklift pavements) • Runways, taxi ways, aprons at airports • Railways • Dykes etc The principles remain the same, but the loads, for example, are totally different from normal pavement loading. For example, industrial areas may have static loads of containers stacked upon each other.

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Appendix A1: Introduction to Road Pavements

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Figure 1.3 Evolution of Road Construction: Part A (reference unknown)

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Figure 1.4 Evolution of Road Construction: Part B (reference unknown)

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