Basis of Design

  • Design of flexible pavement for satisfactory functional and structural performance of the pavement during its life period. It is observed that variation in surface profile causes roughness to the pavement. and poor quality and workmanship of bituminous or cementitious material cause cracking of layers. Hence these bituminous layer and granular layers are the main indicators of the functional and structural performance of the pavement.

Performance of the pavement

  • It is explained by performance models. there is two type of models.
  • (i) Purely empirical (only based on past experience)
  • Mechanistic-empirical (most of the current design of flexible pavement is based on this model). Hence we will discuss the only mechanistic-empirical type pavement model. Distress or performance (ie stress, strain, and deflection ) calculated by specific theory and It explains in terms of mechanistic parameters.
  • The critical mechanistic parameter is identified and controlled to a limiting value in the design process.

Theory for the analysis of Pavement

  • Analysis of pavement is based on the “Linear elastic layered theory ” in which the pavement is modeled as a multi-layer. The subgrade is considered to be semi-infinite, and the sub-base, base and paving layer is considered to be infinite in the horizontal extent and finite in the thickness.
  • The design of flexible pavement inputs is elastic modulus, Poisson’s ratio and thickness of each layer by which IITPave software calculates the stresses, strains and deflection produced by a load applied at the surface of the pavement.

The major concern to be taken into Considered in Design of flexible pavement

  • The critical mechanistic parameter for controlling sub-grade rutting is the vertical compressive strain on top of the sub-grade.
  • Horizontal tensile strain at the bottom of the bottom bituminous layer has to be controlled so it can control the bottom-up cracking in the bituminous layer.
  • It is suggested If we provide a cement-treated base (CTB). It controls the fatigue cracking (bottom-up), tensile stress, tensile strain at the bottom of CTB.
  • Due to repeated application of traffic load, High Temperature, and heavy load in the bituminous pavement are the main cause of rutting in the bituminous layer. As we know that the stiffness of the bituminous of reduces at high temperature and proportion of plastic deformation out of the total deformation will be larger under higher temperature and application of higher load.
  • Water (moisture ) damages the bituminous layer.
  • Age hardening of bitumen at the top of the bituminous layer causes brittle cracking.
  • we shall use a suitable binder and mixes.
  • For the satisfactory performance of bituminous pavement, the magnitude of distress shall be within acceptable limits during the service life period.
  • Hence we shall select the pavement section in such a way that they satisfy the limiting stresses and strains prescribed by the performance model.
  • we shall select the suitable mix design as per the suggestion of guidelines

Guidelines for Design of Flexible Pavement

Sub-grade rutting criteria

  • Critical or failure in the rutting condition of the pavement is to be considered when the average rut depth of 20 mm or more. It is measured along the wheel path.
  • Hence we shall find out the equivalent number of standard axle load (80 KN ) repetitions that can serve by the pavement before the rut depth of 20 mm or more.
  • IRC 37-2018 suggests two-equation for calculation the equivalent number of standard axle load with 80% and 90% reliability Levels.
Subgrade rutting equation
  • The shape of the contact area of the tyre is assumed to be circular.
  • The uniform vertical contact stress shall be considered as 0.56 MPa.
  • But for fatigue damage analysis of cement treated bases , The uniform vertical contact stress shall be considered as 0.80 MPa.
  • The materials are assumed to be isotropic.
  • The layer interface condition was assumed to be fully bound for all the layers of the pavements.

Fatigue cracking criteria for Bituminous Layer

  • These type of cracks are interconnected cracks which appear on the surface.
  • Critical or Failure of the pavement is to be considered when these cracks appear on the 20 percent or more than the paved surface area of the section.
  • Hence we shall find out the equivalent number of standard axle load (80 KN ) repetitions that can serve by the pavement before the cracked surface area of 20 % or more occurs.
  • IRC 37-2018 suggests two-equation for calculation the equivalent number of standard axle load with 80% and 90% reliability Levels.
fatigue cracking equation for bituminous layer
  • Resistance to fatigue cracking can be increase by providing rich bituminous mix in the bottom-most. As we know larger binder mix increases the thickness of film on the aggregate and result increase in tensile strain, makes the mix moisture damage resistance and reduces the aging of the binder.

Fatigue performance for Cement Treated base.

In the case of Pavement with CTB , Design of flexible pavement is carried out as per equation 3.5, 3.6 and 3.7

Steps for the design of flexible pavement on IITPave Software

  • To design of flexible pavement
  • Run the IITPave Software
  • Input the Name of Work
  • Select the carriageway width after construction from the dropdown menu
design of flexible pavement
  • Select the classification of the road from the dropdown menu. after selection of classification of the road, it will automatically fill the design life of the road
  • As per IRC 37 clause no 4.3.1 page no 14. NH , SH and urban road shall be designed for 20 years, and for other categories of the road shall be designed for 15 years. Pavement for very high-density corridors and expressway shall be design for 30 years
  • input the initial traffic as per census in both direction
  • Input the traffic growth rate as per past trends, if it is less than 5 percent. then input 5 percent traffic growth
  • select traffic census year from the drop-down menu
  • Select terrain condition from drop-down menu
  • select date of start of construction and construction period from the drop-down menu.
design of flexible pavement
  • Select YES for different CBR in calculating 80/90 percentile CBR from the drop-down menu and then input minimum 4 nos of CBR values with seperating comma
  • The effective sub-grade CBR should be more than 5% for roads estimated to carry more than 450 commercial vehicles per day (CVPD)(two way) in the year of construction
  • Input the CBR of existing subgrade as per lab report
  • Select YES for whether stage construction involved from the dropdown menu. it will automatically fill the life of the bituminous layer is 5 years.
  • Select NO for whether an axle load survey conducted. if it is conducted than you have to provide VDF value from the axle load survey.
  • Then click on Load previous design to get the values of 90th percentile CBR and 80th Percentile CBR
  • As per IRC Code, 90th percentile subgrade CBR is to be used in the calculation for the design of flexible pavement such as Expressway, NH, SH, and Urban roads
  • And, 80th percentile subgrade CBR is to be used in the calculation for the design of flexible pavement for other types of roads.
  • input the rate of BM and DBM
  • then click OK
design of flexible pavement
  • Next page Layer properties open
  • Select pavement Composition type from drop down menu. it will automatically fill the calculation data.
  • You can choose grade of bitumen, IRC suggest to use of higher grade of bitumen. But you can choose as per site condition.
  • You can provide the layer thickness of BM or DBM and GSB layer . But IITPave automatically suggest the layer thickness
  • Then Click OK
  • Click on Export Result in PDF
design report
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IITPave Result

Analysis of the Design of Flexible Pavement report IITPave Page no 6

  • Z represents the depth of the location measured from the pavement surface.
  • R represents the radial location measured from the center of the circular contact area of the road
  • The mechanistic parameters are
  • Vertical stress ( SigmaZ),
  • Tangential Stress ( SigmaT),
  • Radial Stress ( SigmaR)
  • Shear Stress (TaoRZ)
  • Vertical Deflection (Dispz)
  • Vertical Strain (epz)
  • Horizontal tangential strain (epT)
  • Horizontal radial strain (epR)
design report
  • The Horizontal tensile strain (epT) computed ( 0 mm radial distance) at the bottom of the bituminous layer: 0.0003494 is the largest
  • The Horizontal tensile strain (epT) computed ( 155 mm radial distance ) at the bottom of the bituminous layer: 0.0003343
  • Horizontal radial strain (epR) computed ( 0 mm radial distance) at the bituminous layer : 0.0002294
  • Horizontal radial strain (epR) computed ( 150 mm radial distance) at the bituminous layer : (-)0.00002526
  • The Vertical compressive strain (epZ) computed ( 0 mm radial distance) at the interface between the subgrade and granular layer : -()0.0004823 is the largest
  • The Vertical compressive strain (epZ) computed ( 150 mm radial distance) at the interface between the subgrade and granular layer : (-)0.0005230
  • Positive stresses and strains are tensile whereas Negative stresses and strains are compressive

Thanks for reading