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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