The primary mode of transportation in our everyday life is our road network that connects various services and destinations. The roads we use play an integral part in the growth of our economy, and render a social benefit to society. In order for a road to perform a satisfying service, it requires a pavement design, construction and routine maintenance in a sustainable and feasible manner. Therefore it is crucial for a pavement design to address the need of a road.
Pavement design is a discipline in engineering that produce fit for purpose designs, based on scientific knowledge and experience. In this literature review, the focus is on materials that are chosen for perusal in low-volume roads. A low-volume road is also classified as a category D road in some manuals, and the growth on such roads can be very unpredictable.
The need to design and construct a pavement depends on various facets such as traffic volumes, availability of material and the growth of the population in an urban area.
The objective of a pavement structure design is to determine in a layer system the material combination that will provide sufficient strength to resist the shear pressure of traffic loads. The prime function of a pavement structure is to distribute the applied load to the sub-grade. Therefor a broad understanding of the performance, characteristics, and properties of materials is crucial in the pavement design phase of a road, in terms of structure, materials and layer thickness.
In South-Africa, there are three pavement design systems utilized to maintain the good quality of the country’s road network.
The three main design principles utilized for pavement design systems is;
An empirical design is a design based on knowledge gained from road tests and past experience. It is a method where numerous observations were made to establish the relationship between input and outcome variables of a process. There is no need to determine a scientific basis between the variables and outcome, but the limitations need to be acknowledged in the design.
One of the many design methods that make use of the empirical pavement design method is The American Association of State Highway and Transportation Officials (AASHTO, 1993) design. The AASHTO method that was derived from the (AASHO, 1961) method. The late 1950s AASHO Road Test is a determination of the performance equations of a pavement structure of known thickness under moving loads of known frequencies. The extensive test was performed in Ottawa, Illinois. It should be acknowledged that the development of the original equations took place under given factors e.g. climate, pavement materials and subgrade soil, hence the enhancement in the empirical performance equation to accommodate other regions in America. The AASHTO pavement design uses the empirical equations to relate pavement characteristics with pavement performance (Pavement Interactive, n.d)
A disadvantage of the empirical design method will be the reliability of the data obtained through extensive road test that is only applicable to conditions form which they were collected. New enhancements in materials, construction methods and improved traffic characteristics cannot be worked into empirical pavement design procedures, if used in the design principle it can inflict a pavement structure with low service life and expensive maintenance and rehabilitation needs. The advantage on the other hand is that the empirical design is based on actual simulation data and simple to apply. Many low-volume roads have been success stories of the design and constructed data obtained. When designing within the condition limits the expected results is reliable and the principle can be executed with confidence.
The Mechanistic design method deals with the pavement structure that is subjected to a load. More often the pavements are analysed mechanistically, considering the pavement structure as a mechanism. In order for an engineer to comprehend the behavior of the pavement and materials the concept of material mechanics need to be understood in materials (SAPEM 2013: 26).
The South African Mechanistic Design Method (SAMDM) utilize the linear elastic theory to calculate the theoretical stresses and strains at various positions in a pavement layer (THR 4, 1996: 50). The stress and strain relationship is also an indication of the materials stiffness. The maximum stress a material can withstand is also referred to as an ultimate stress and measure of the materials strength (SAPEM 2013). In other words the layered elastic model, the pavement structure layers consist of different materials. The elastic component refer to the pavements ability to regain its shape after deformation occurred when load is removed. In essence the mechanistic design is simply a model to determine reaction of the pavement to traffic load (David E. Newcomb et.al.). The same principle applies when designing bridges and building. Equations are used to determine the deflections and strain resulting from load imposed on columns and beams, in pavements the equations is more complex but the concept remains the same.
There are various other conditions that also need to be considered when using the SAMDM such as, traffic loading that varies during pavement structure service period, critical point and how the pavement response to load in the Mechanistic approach, the climate and environment that affects the service life of the pavement structure, and the transfer function that use equations to determine if the pavement structure will last its service life before fatigue or rutting occurred. The Mechanistic design is well suited for the design of low-volume roads because it can predict the performance of the material in conjunction with the applied load.
One can argue why the mechanistic pavement design was developed, because the AASHO Road Test performed satisfactory. The pavement structures designed through the empirical method lasted their service life and longer. The following advantages will highlight why a new design method was developed. The mechanistic pavement design allows an evaluation of changes in vehicle loading. The impact in construction can be assessed. New materials can be evaluated through design properties. A database of materials for updating pavement design input values can be developed and as built information can become available. This design approach yields a realistic prediction of a pavement’s serviceability life based on mechanistic response to a load. The disadvantages the SAMDM poses, the damage models is outdated, the method is perceived biased towards certain pavement types, the method is only suited for computer application due to its complexity.
The various tests performed to support the Mechanistic approach is California Bearing Ratio, Triaxial and Dynamic Cone Penetration are conducted to determine the stiffness and bearing capacity of each pavement layer. The test results are used to analyse and evaluate the suitability of pavement model configurations.
The Mechanistic- empirical design method is an analysis of all rigid, flexible and block pavements. The analysis perceive the pavement as a mechanism and calculate engineering parameters of mechanics such as stress, strain and deflection by assuming the material model for pavement type (SAPEM 2014). This is performed through Pavement response model, in an attempt to model resilience response of a layer in isolation or the whole system. The correlation between the structural capacity and the engineering parameters are drawn through performance observations applying the empirical method to the mechanistic approach.
A systematic approach is applied with respect to laws of mechanics to allow a design engineer to design a pavement structure with allowable structural capacity, but in essence may not be the best performing design. Therefor a sound engineering practice is needed to link the experience with the design. In South Africa the sound engineering practice has been captured in national and industry guidelines and documents for engineer’s perusal.
The composition of the Mechanistic-Empirical pavement design method comprises of various components. To highlight the sequence and components of the method, a design commences with determining the; Pavement structure, design loading and material parameters (resilient and strength properties), Pavement (Resilient) response model, Critical response parameters, Empirical damage models, Pavement structural capacity estimate. The design variables acquire establishment through the aid of tests to determine numerous inputs. The problem is solved by the various software packages available in the pavement engineering industry e.g. ME-Pads and Rubicon Toolbox (SAPEM 2014). Therefor based on this inputs and results a more practical performance can be predicted.
The advantages of the Mechanistic-Empirical design method approach purely are: utilization for both new and existing pavement construction, accommodate the ever-increasing load types, defining the role of construction better, provide more sensible performance forecast, improve the characterization of materials, and accommodate the factor of aging on materials. The disadvantages
In addition to the study of the three main approaches in design of pavement structures one has to acknowledge the big improvements that has been made in terms of pavement design in the Pavement engineering industry. From the simple empirical method of designing in the late 1950s to a more complex mechanistic-empirical method currently. The ultimate aim is for the method adopted to give a design output that is economical and meet the traffic demand. The low-volume roads is roads that have a reliability of 50% and in terms of a low volume roads demand it can be designed using a simple method.
Pavement Interactive [Online]. [n.d.]. Available:
[2019, August 27].
Technical Recommendations for Highways 4 (TRH4), 1996, Chapter 8: Pavement
Type Selection and Structural Design, Pretoria, Department of Transport.
[2019, September 03].
South African Pavement Engineering Manual (SAPEM), 2014, Chapter 10:
Pavement Design, South Africa, The South African National Roads Agency
(SANRAL).
[2019, September 03].
AASHTO Guide for Design of Pavement Structures, 1993, Chapter 4: Low-Volume
Road Design, Washington D.C, American Association of State Highway and
[2019, September 07 ].
The AASHO Road Test: Summary Report, 1962, Chapter 5: Pavement Research,
Washington D.C, National Academy of Sciences.
[2019, September 07].
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