ABSTRACT
It is generally known that the major causes of failure in asphalt pavement is fatigue cracking and rutting deformation, caused by excessive horizontal tensile strain at the bottom of the asphalt layer and vertical compressive strain on top of the subgrade due to repeated traffic loading. In the design of asphalt pavement, it is necessary to investigate these critical strains and design against them. This study was conducted to develop a simplified layered elastic analysis and design procedure to predict fatigue and rutting strain in cement-stabilized base, low-volume asphalt pavement. The major focus of the study was to develop a design procedure which involves selection of pavement material properties and thickness such that strains developed due to traffic loading are within the allowable limit to prevent fatigue cracking and rutting deformation. Analysis were performed for hypothetical asphalt pavement using the layered elastic analysis program EVERSTRESS for four hundred and eighty pavement sections and three traffic categories. A total of Ninety predictive regression equations were developed with thirty equations for each traffic category for the prediction of pavement thickness, tensile
(fatigue) strain below asphalt layer and compressive (rutting) strain  on top the subgrade. The  regression equations were  used  to develop  a layered  elastic  analysis  and  design program, “LEADFlex”. LEADFlex procedure was validated by comparing its result  with that  of EVERSTRESS and  measured field data. The LEADFlex-calculated and  measured horizontal tensile  strains  at the  bottom of the  asphalt layer  and  vertical  compressive strain  at the top of the subgrade were  calibrated and  compared using  linear  regression analysis.  The  coefficients  of  determination R  were  found  to  be  very  good.  The calibration  of  LEADFlex-calculated  and  measured  tensile  and  compressive strains resulted in minimum R? of 0.992 and 0.994 for tensile (fatigue) and compressive (rutting) strain  respectively indicating that  LEADFlex is a good predictor of fatigue  and  rutting strains  in cement-stabilized lateritic base for low-volume asphalt pavement. The result of this  research will  enable  pavement  engineers to  predict critical  fatigue  and  rutting strains  in low-volume roads  in order to prevent pavement failures.
CHAPTER
INTRODUCTION
1.1 Background of Study
Since the early 1800′ s when the first paved highways were built, construction of roads has been on the increase as well as improved method of construction. The need for stronger, long-lasting and all-weather pavements has become a priority as result of rapid growth in the automobile traffic and the development of modern civilization. Since the beginning of road building, modeling of highway and airport pavements has been a difficult task. These difficulties are due to the complexity of the pavement system with many variables such as thickness, material technology, environment and traffic. Most attention has been given to material technology and construction techniques and less was given to material properties and their behaviour. Terzaghi was the first to introduce the concept of subgrade modulus and plate load test to pavement studies. Given the load (traffic) and the measurement of deflection under this load, the carrying capacity of a pavement could be determined. Several other soil tests were developed, such as the California Bearing Ratio (CBR), the triaxial test and the unconfined compression test.
Several theoretical developments followed in the different parts of the world, In Europe, for flexible pavements, Shell adopted Burmister’s theoretical work to model and analyze the pavement as an elastic layered system involving stress and strain (Claussen et al,
1977). In North America (USA), a comprehensive set of full-scale road tests were launched. The American Association of State Highway Official [AASHTO, 1993) introduced its first guide in 1972 which was revised in 1986 and 1993. From these two agencies, a conclusion can be drawn that the trend in pavement engineering was either empirical or a mechanistic method. An empirical approach is one which is based on the results of experiments or experience. This means that the relationship between design inputs (loads, material, layer configuration and environment) and pavement failure were arrived at through experience, experimentation or a combination of both. The mechanistic approach involves selection of proper materials and layer thickness for specific traffic and environmental conditions such that certain identified pavement failure modes are minimized. In mechanistic design, material parameters for the analysis are determined at conditions as close as possible to what they are in the road structure. The mechanistic approach is based on the elastic or visco-elastic representation of the pavement structure. In mechanistic design, adequate control of pavement layer thickness as well as material quality are ensured based on theoretical stress, strain or deflection analysis. The analysis also enables the pavement designer to predict with some amount of certainty the life of the pavement.
It is generally accepted that highway pavements are best modeled as a layered system, consisting of layers of various materials (concrete, asphalt, granular base, subbase etc.) resting on the natural subgrade. The behaviour of such a system can be analyzed using the classical theory of elasticity (Burmister, 1945). This theory was developed for continuous media, but pavement engineers recognized very clearly that the material used in the construction of pavements do not form a continuum, but rather a series of particular layered materials.
Modeling the uncracked pavement as a layered system, the following assumptions are usually made:
1. Each layer is linearly elastic, isotropic and homogenous, hence are not stressed beyond their elastic ranges.
2. Each layer (except the subgrade) is finite in thickness and infinite in the horizontal direction.
3. The subgrade extends infinitely downwards
4. The loads are applied on top of the upper layer
5. There are no shear forces acting directly on the loaded surface
6. There is perfect contact between the layers at their interfaces.
Because of assumption (1), the constitutive relationship for such material involves variables such as the modulus of elasticity (E) and the Poisson’s ratio (v), Elastic constants or bulk modulus (K) and shear modulus (G). While some authors; (Domaschuck and Wade, 1969); (Naylor,1978); (Pappin and Brown,1980); (Bowles,1988) feel that K and G are preferable to E and v to characterize earth materials, it is customary to use E and v in all geotechnical and pavement engineering computations. Because of the transient or repetitive nature of loading in pavement engineering, the elastic modulus can be replaced by the resilient modulus (M,). The resilient modulus is defined as the recoverable strain divided by stress.
1.2 Definition of Problem
Road failures in most developing tropical countries have been traced to common causes which can broadly be attributed to any or combination of geological, geotechnical, design, construction, and maintenance problems (Ajayi, 1987). Several studies have been carried out to trace the cause of early road failures, studies were carried out by researchers on the geological (Ajayi, 1987), geotechnical, (Oyediran, 2001), Construction (Eze-Uzomaka, 1981) and maintenance (Busari, 1990) factors. However, the design factor has not been given adequate attention. In Nigeria, the only design method for asphalt pavement is the California Bearing Ratio (CBR) method. This method uses the California Bearing Ratio and traffic volume as the sole design inputs. The method was originally developed by the California Highway Department and modified by the U.S Corps of Engineers (Corps of Engineers, 1958). It was adopted by Nigeria as contained in the Federal Highway Manual (Highway Manual-Part 1, 1973). Most of the roads designed using the CBR method failed soon after construction by either fatigue cracking or rutting deformation or both. In their researches (Emesiobi, 2004, Ekwulo et al , 2009), a comparative analysis of flexible pavements designed using three different CBR procedures were carried out, result indicated that the pavements designed by the CBR• based methods are prone to both fatigue cracking and rutting deformation. The CBR method was abandoned in California 50 years ago (Brown, 1997) for the more reliable mechanistic-empirical methods (Layered Elastic Analysis or Finite Element Methods). It is regrettable that this old method is still being used by most designers in Nigeria and has resulted in unsatisfactory designs, leading to frequent early pavement failures. In Pavement Engineering, it is generally known that the major causes of failure of asphalt pavement is fatigue cracking and rutting deformation, caused by excessive horizontal tensile strain at the bottom of the asphalt layer and vertical compressive strain on top of the subgrade due to repeated traffic loading (Yang, 1973; Saal and Pell, 1960; Dormon and Metcaff, 1965; NCHRP, 2007)). In the design of asphalt pavement, it is necessary to investigate these critical strains and design against them. There is currently no pavement design method in Nigeria that is based on analytical approach in which properties and thickness of the pavement layers are selected such that strains developed due to traffic loading do not exceed the capability of any of the materials in the pavement. The purpose of this study therefore is to develop a layered elastic design procedure to predict critical horizontal tensile strain at the bottom of the asphalt bound layer and vertical compressive strain on top of the subgrade in cement-stabilized low volume asphalt pavement in order to predict failure modes such as fatigue and rutting and design against them.
1.3 Research Justification
A long lasting pavement can be designed using the developments in mechanistic-based method (Monismith, 2004), hence, the transition of structural design of asphalt pavements from the pure empirical methods towards a more mechanistic-based approach is a positive development in pavement engineering (Brown, 1997; Ullidtz,
2002). The mechanistic-based design approach (Layered Elastic Analysis and Finite Element) is based on the theories of mechanics and relates pavement structural behaviour and performance to traffic loading and environmental influences. The CBR design method developed by the California Highway Department has since been abandoned for a more reliable mechanistic approach. Therefore the need to develop a layered elastic analysis has become necessary in order to evaluate the response of asphalt pavement due to traffic loading. Since the failure of asphalt pavement is attributable to fatigue cracking and rutting deformation, caused by excessive horizontal tensile strain at the bottom of the asphalt layer and vertical compressive strain on top of the subgrade, in the design of asphalt pavement, it is necessary to investigate these critical strains and design against them. The layered elastic analysis approach involves selection of proper materials and layer thickness for specific traffic and environmental conditions such that certain identified pavement failure modes such as fatigue cracking and rutting deformations are minimized. The use of the layered elastic analysis concept is necessary in that it is based on elastic theory(Yang, 1973), and can be used to evaluate excessive horizontal tensile strain at the bottom of the asphalt layer(fatigue cracking) and vertical compressive strain on top of the subgrade (Rutting deformation) in asphalt pavements. The major disadvantage of the CBR procedure is its inability to evaluate fatigue and rutting strains in asphalt pavement and its use in Nigeria should be discontinued. In the final analysis, the research will go along way in proffering solution to one of the factors responsible for frequent early pavement failures which have been attributed to unsatisfactory designs. The research will also be a noble contribution to the review of the Nigerian Highway Manual proposed by the Nigeria Road Sector Development Team in 2005.
1.4 Objectives
The summary of the main objectives of the research shall be as follows:
1. Develop a layered elastic analysis procedure for design of cement-stabilized low volume asphalt pavement in Nigeria.
2. Develop design equations and charts for the prediction of pavement thickness, critical tensile and compressive strains in cement-stabilized low volume asphalt pavements using layered elastic analysis procedure.
3. Collect pavement response standard data from Literature.
4. Calibrate and verify developed equations using the collected data.
5. Develop a design tool (program) LEAD Flex for design of cement-stabilized lateritic base low-volume asphalt pavement.
1.5 Scope and Limitations
Scope:
The study is to address one of the factors responsible for frequent early pavement failures associated with Nigerian roads; the design factor, however, particular emphasis will be on the adoption of the layered elastic analysis procedure to predict critical fatigue and rutting strains in cement-stabilized low volume asphalt pavement. A design tool (software) shall be developed for the procedure. The very popular layered elastic analysis software, EVERSRESS (Sivaneswaran et al, 2001) developed by the Washington State Department of Transportation (WSDOT) will be employed for pavement analysis.
Limitations:
1. Assumption of elasticity of pavement materials
11. Assumptions of Poisson’s ratio of pavement materials
1.6 Methodology of Study
The method adopted in this study is to use the layered elastic analysis and design approach to develop a procedure that will predict fatigue and rutting strains in cement• stabilized low volume asphalt pavement. To achieve this, the study will be carried out in the following order:
1. Characterize pavement materials in terms of elastic modulus, CBR/resilient modulus and poison’s ratio.
2. Obtain traffic data needed for the entire design period.
3. Compute fatigue and rutting strains usmg layered elastic analysis procedure based the Asphalt Institute response models.
4. Evaluate and predict pavement responses (tensile strain, compressive strain and allowable repetitions to failure).
5. If the trial design does not meet the performance criteria, modify the design and repeat the steps 3 through 5 above until the design meet the criteria.
The procedure shall be implemented in software (LEADFlex) in which all the above steps are performed automatically, except the material selection. Traffic estimation is in the form of Equivalent Single Axle Load (ESAL). The elastic properties (elastic modulus of surface and base, resilient modulus of subgrade and Poisson’s ratio) of the pavement material are used as inputs for design and analysis. The resilient modulus is obtained through correlation with CBR. The layered elastic analysis software EVERSRESS (Sivaneswaran et al, 2001) was employed in the analysis.
1.7 Purpose and Organization of Thesis
The purpose of the study is to use the layered elastic analysis approach to develop procedure that will predict fatigue and rutting strains in cement-stabilized low volume asphalt pavement. The study is presented in six chapters. Chapter One introduces the research topic on the application of analytical approach in design in flexible pavement and the need to develop an analytical approach for the Nigerian (CBR) method for flexible pavement design. Chapter Two presents Literature Review on highway pavements and design of flexible pavements. The use of empirical and mechanistic (analytical) design procedure is presented in detail. Chapter Three outlines and describes in details the procedure adopted in the research including material characterization, design inputs and summary of the development of the design procedure. Chapter Four presents details of the development of the layered elastic
analysis procedure for prediction of fatigue and rutting strains in cement-stabilized low volume asphalt pavement. The developed equations, program algorithm, visual basic codes and program interface and design are presented in details in this chapter. Chapter Five will present the results and discussion of the results of the study. Effect of pavement parameters on pavement response shall be discussed in this section. Finally, Chapter Six will present the Conclusions and recommendations of the study.
This material content is developed to serve as a GUIDE for students to conduct academic research
DEVELOPMENT OF LAYERED ELASTIC ANALYSIS PROCEDURE FOR PREDICTION OF FATIGUE AND RUTTING STRAINS IN CEMENT STABILIZED LATERITIC BASE OF LOW VOLUME ROADS
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