5. Design of Pavements

5.1 General Approaches to Pavement Design

In its simplest form the design approach consists of the following steps:

• define the loading and environmental conditions;
• select materials with the appropriate properties for use in the pavement;
• select the thickness of pavement based on empirical rules or on a stress – strain analysis of the pavement structure; and
• adjust the initial design using different materials until a satisfactory design emerges.

5.2 Types of Pavements

• Flexible pavements consist of materials such as gravels and loams overlain by bituminous layers. They are so named because they flex under the actions of traffic and rebound when traffic loads are removed.
• Rigid pavements on the other hand exhibit a slab action when loaded. A typical type of rigid pavement is the cement concrete pavement.

5.3 Design Considerations

5.3.1. Subgrade Evaluation

The performance of a road pavement is influenced to a great extent by the subgrade support. This subgrade support is dependent upon soil type, material density and moisture content. The method of subgrade support determination most commonly used is the California Bearing Ratio (CBR). The aim of the subgrade evaluation is to determine, for design, a subgrade CBR value at density and moisture conditions which are expected to prevail in service.

5.3.2 Pavement Environment

The environmental factors which significantly affect pavement performance are moisture and temperature.

Pavement and subgrade moisture conditions exert a major influence on the performance of roads. In pavement design it is important to be able to recognise ways by which moisture may enter the pavement or subgrade and to determine measures needed to control moisture movement. Moisture changes usually result from one or more of the following effects:

• seepage from higher ground near the road pavement;
• fluctuations in water table level;
• infiltration of water through the surface of the road pavement and shoulders; and
• transfer of moisture in liquid or vapour states.
The temperature environment has a major influence on the performance of asphalt pavements. Asphalt becomes stiff and brittle at low temperatures while it is soft and visco-elastic at higher temperatures. Permanent deformation in asphalt at higher temperatures may occur, although this is generally considered as a mix design problem and not a pavement design problem.

5.3.3. Design Life

The design life, or design period, is the length of time (in years) before it is anticipated that rehabilitation of the pavement will be necessary to restore shape, repair other forms of distress or to provide additional pavement strength. The factors which influence the selection of the design period include:
• subgrade type and environment effects;
• maintenance strategies of the road organisation;
• possibility of reconstruction being required for road geometry or traffic capacity reasons;
• funding consideration; and
• type and cost of rehabilitation anticipated at the end of the design period.
Typical design periods are:
• New granular pavements 20 to 25 years
• New rigid pavements 20 to 40 years
• Asphalt overlays 10 to 15 years
• Granular overlays 10 to 20 years

5.3.4 Available Materials

Pavement materials can be classified into four categories according to their fundamental behaviour under the effects of applied loadings.
• Unbound granular materials behave according to their shear strength characteristics, developed by particle interlock. Under repeated stresses deformation occurs primarily through shear and partly through desensification.
• Cemented materials (cement or lime stabilised granular materials) are semi-brittle materials which under repeated flexure form initial cracks which then propagate through the material.
• Bituminous materials are visco-elastic bound materials. Under repeated stresses they may either crack and/or deform.
• Cement concrete (or simply concrete) refer to aggregate bound hydraulically with Portland Cement. Concrete can be used as a sub-base in either flexible or rigid pavements and as a base in rigid pavements.

5.3.5 Construction and Maintenance Factors

Several construction and maintenance factors must be taken into account in pavement design because they can influence the type of surfacing which is adopted, the base and sub-base material requirements or the choice of pavement type. These factors include:
• Extent and Type of Drainage;
• Use of Boxed Construction;
• Availability of Equipment;
• Use of Stage Construction;
• Use of Stabilisation;
• Social Considerations;
• Construction Under Traffic; and
• Maintenance Strategy.

5.3.6 Design Traffic

The major features of traffic that influence pavement performance are:
• the number of axle passes;
• the axle loadings; and
• the axle configurations.
For all pavements, performance is influenced by only the heavy end of the traffic spectrum. No account need be taken of cars and light commercial vehicles, as far as pavement design is concerned.

The design traffic is expressed in terms of the number of standard axle load repetitions (in one lane) which are equivalent in destructive effort to the total number of repetitions of the actual axles using the pavement during the design period. The standard axle is a single axle with dual wheels that carries a load of 80kN. The design traffic is given in terms of the Equivalent Number of Standard Axles (ESAs).

5.3.7 Whole-of-Life Economic Evaluation

It is essential in the evaluation of pavement designs that all costs occurring during the life of the facility be evaluated. When making economic comparisons this has not always been understood or practiced by pavement designers because comparisons were often made over a fixed, equal design period. Whole-of-life (life-cycle) costing should account for all costs and benefits associated with the planning, design, construction, maintenance, rehabilitation and use of the pavement over its whole life.

5.4 Flexible Pavement Design

Early methods developed an empirical relationship between pavement thickness and soil properties such as grading and/or Atterberg limits. Sometime later correlation studies were carried out between pavement thickness and soil strength with the typical strength test used being the California Bearing Ratio. These investigations resulted in CBR – Traffic – Thickness Charts being produced. The major limitation with empirical methods was the dangers which existed where the methods had to be extrapolated beyond the observed conditions upon which the methods were formulated. However, empirical pavement design methods served for a number of decades in a reasonably satisfactory way and are still used to some extent today.

Since the mid 1960’s an increasing amount of research has been directed towards the formulation of theoretical or semi-theoretical pavement design methods. These methods treat the pavement as a structure and attempt to analyse stress-strain conditions within the pavement. They are therefore referred to as mechanistic design methods.

5.5 Rigid Pavement Design

• jointed unreinforced concrete pavements;
• jointed reinforced concrete pavements;
• continuously reinforced concrete pavements;
• steel-fibre reinforced concrete pavements; and
• prestressed concrete pavements.

The amount (if any) of reinforcement required in a concrete pavement is determined by the spacing of contraction joints. This spacing can range from 4 to 7m for unreinforced concrete pavements, through 8 to 30m for reinforced concrete pavements to the elimination of joints (infinite spacing!) for continuously reinforced concrete pavements.

The design methods used for rigid pavements are based on an assessment of:

• subgrade strength (CBR or Modulus of Subgrade Reaction (k));
• predicted traffic; and
• thickness and nature of sub-base.

5.6 Pavement Rehabilitation

5.6.1 Pavement Evaluation

A number of techniques have been developed to provide information on pavement deterioration, in order that the most appropriate rehabilitation treatment will be used.
• Visual Evaluation Techniques. These methods assess severity and extent of cracking, deformation, surface texture, edge defects, shoulder condition and drainage problems.
• Deflection Testing. It has been found that the deflection of the pavement caused by a standard axle load is an indication of the rate at which permanent pavement deterioration will occur under traffic. Studies have been able to establish design deflections for particular traffic loadings. If actual deflections remain below the design deflection for a particular traffic loading then permanent deformation of the pavement remains at a tolerable level. The actual deflections of a pavement generally increase with age, and the aim of pavement investigation will be to ascertain when deflections are reaching a critical level, so that an overlay can be applied to return the pavement to an acceptable state. It should be noted that the critical state for deflections will generally occur before any visually identifiable deterioration of the pavement is noticed, or before riding quality is affected. Three devices are commonly used in Australia for the measurement of pavement deflection. These are:
• Benkelman Beam. A long thin beam supported on the pavement, whose tip is placed between the tyres of a standard loaded axle, and which measures the rebound of the pavement as the axle is moved away.
• Deflectograph. Essentially a set of Benkelman beam like devices suspended below a truck. The beams sit on the pavement as the truck moves slowly forward. The beams are then lifted and slide forward beneath the truck to be repositioned on the pavement again.
• Falling Weight Deflectometer. A trailer device which applies a dynamic load to the pavement by release of a suspended weight. Pavement response is measured by a load cell and series of geophones.
• Ride Quality or Roughness. This is the term given to variations in the longitudinal profile from a uniformly shaped surface. The two most common types of devices for the measurement of roughness are the Road Response Measurement Type (such as the Bump Integrator and the NAASRA Roughness Meter), and the Laser-based type.
• Surface Texture. Surface texture may be assessed with the Sand Patch Test, or through non-contact laser-based testing.
• Skid Resistance. Methods currently used include The Pendulum or Portable Skid Resistance Tester, and the vehicle mounted SCRIM (Sideways force Coefficient Routine Investigation Machine)

5.6.2 Rehabilitation Treatments

• Overlay and Resheet Techniques. An asphalt overlay or granular resheet involves the addition of material over the existing pavement, thereby increasing the finished surface level. A fundamental concern then in the investigation of one of these treatments is level control. Controls on the thickness and finished level of such a treatment is governed by kerb and channel levels, existing pavement shape, height of overhead structures and dead weight considerations on bridge decks.
• In Situ Stabilisation. Stabilisation of road making material is defined as the process of improvement of a material to increase and maintain its load bearing properties. In the case of pavement rehabilitation, stabilisation is used to improve the load bearing properties of one or more layers of the pavement structure. The main forms of stabilisation are (1)Mechanical, where various construction materials are blended to produce one material with desired properties (e.g. the blending of a gravel deficient in fines with a sand may produce a satisfactory base material); and (2)Chemical, where chemical reaction is used to produce a satisfactory material (e.g. lime stabilisation of a clayey gravel may produce an acceptable material).
• Recycling. Asphalt may be recycled using either a hot process or a cold process. Cold recycling involves removing the existing asphalt, carting it to a mixing plant, adding rejuvenating agents, and returning it to the site for cold laying. Hot asphalt recycling uses a machine to heat the asphalt in situ, remove it by milling, mix in rejuvenators and other additives, and then relay it.
• Deep Lift Recycling. Deep lift recycling of pavements was introduced in the early 1990s. The process consists of the milling, mixing (usually with the addition of new material or chemicals) and relaying of pavement material in a single thick layer (perhaps in the order of 300 to 400 mm thickness).

LINKS TO SITES ON STABILISATION

Page last modified 24 June 2002.