What is the design of pavements?

Pavement is a structure that is made up of different layers such as sub-base course layer, base course layer, binder course, and a surface course formed with different materials such as gravel, above the subgrade soil constructed to act as a path or carriageway enabling the movement of vehicles. The surface of the pavement should possess characteristics such as adequate friction, light reflection, and drainage. The pavements are classified based on structural design into rigid pavements, flexible pavements, composite pavements, and unpaved pavements. The process of determining a suitable pavement type, its constituent layers, and its thickness is called pavement design.

Factors affecting the design of pavements

  • Design wheel load.
  • Axle load.
  • Subgrade soil.
  • Vehicle speed and contact pressure.
  • Materials used for constructing the pavement structure.
  • Environmental factors such as rain, temperature, snow, and frost.

Flexible pavements

Flexible pavement is a pavement layer made up of aggregates and bitumen or asphalt that has been heated and mixed appropriately before being poured and compacted on a granular layer substrate. A bituminous surface course is laid over a base course and sub-base course in a standard flexible pavement. One or more layers of bituminous or Hot Mix Asphalt (HMA) may make up the surface course. Because these pavements have a low flexure strength, they deform when loads are applied to them. The joint action of the multiple layers of the pavement gives flexible pavements their structural strength.

An image showing different layers of flexible pavements.
CC BY-SA 4.0 | Image credits: https://en.wikipedia.org | Pirehelokan

Design of Flexible pavements

Empirical Method

The design procedures for flexible pavements are empirical design, limiting shear failure methods, limiting deflection methods, regression methods, and mechanistic-empirical methods.

Empirical pavement design methods are based on the physical characteristics of the subgrade soil. The AASHTO (American Association of State Highway and Transportation Officials) guide 1993 gives the empirical pavement design formula as follows,

log10W18=ZR×S0+9.36×log10SN+1-0.20+log10ΔPSI4.2-1.50.40+1094SN+15.19+2.32×log10MR-8.07where,W18 =Predicted number of 80kN ESALs (equivalent single axle load)ZR=Standard normal deviateS0=Combined standard error of the traffic prediction and performance predictionSN= Structural Number (an index which indicates the required pavement thickness)ΔPSI=Difference between the initial design serviceability index and the design terminal serviceability index.MR=Subgrade resilience modulus (in psi)

The design procedures include equations that were created using data from the AASHO Road Test, which included particular pavement materials and roadbed soil. First, determine the inputs (ZR, S0, ΔPSI, and MR). Assume a Structural Number (SN). The equivalency factor is determined by solving the equation for each load type for the assumed SN. The traffic count for each load type is estimated and is multiplied with ESAL to obtain the total ESALs of the entire pavement design period of the asphalt pavement. Then the assumed SN is substituted into the equation to find the total ESALs the pavement will support during its pavement design life.

Mechanistic-empirical design

In this method, empirically determined equations that compute the number of loading cycles to failure, describe the link between physical processes and pavement failure.

Rigid pavements

The pavements that possess flexural strength are called rigid pavement. It is made up of plain or reinforced cement concrete and pre-stressed concrete, so it is also called concrete pavement. The major difference between the rigid and flexible pavements is that flexural stress in the rigid pavement is due to wheel loads of vehicles but in the flexible pavements, the stress is created due to temperature changes.

Design of rigid pavements

Empirical method for rigid pavement design

1993 AASHTO guide for Rigid Pavement Structural Design gives the empirical equation for the concrete pavement design.

Assume a slab depth (D) for the concrete pavement, the equivalency factor for each load type is determined by solving the ESAL equation using the assumed slab depth (D) for each load type. The traffic count is determined for different load types for the whole design period and is multiplied with ESAL to obtain the total ESAL. The slab depth value which is assumed is inserted in the equation to determine the ESALs that will be supported by the pavement during its entire design life. If both the obtained ESALs are close to each other, the slab depth can be used, otherwise, the entire procedure is repeated.

Westergaard's formula for pavement design

The modulus of subgrade response, the radius of relative stiffness, and the radius of wheel load distribution are utilized in the rigid pavement design, which is based on Westergaard's analysis. A mixture of load stress, frictional stress, and warping stress is considered in critical design.

The subgrade of the pavement structure acts as a barrier against slab deflection. The deflection tendency of the slab is determined by its flexural strength, which is determined by the stiffness of the subgrade. The slab's bending as a result of the subgrade pressure is a measure of its magnitude. As a result, the rigid pavement's pressure deformation is a function of the subgrade slab's relative stiffness.

Westergaard defined the radius of relative stiffness by the following formula,l=Eh312K1-μ214where,l=Radius of relative stiffness in cmE= modulus of elasticity of cement concrete in kg/cm2μ=Poisson's ratio for concrete=0.15h= Slab thickness in cmK=Subgrade modulus or modulus of subgrade reaction in kg/cm3

The interior, edge, and corner, are the locations where the conditions of slab continuity do not exist and these locations are termed as critical load positions. The cement concrete slab is assumed to be homogeneous and to have uniform elastic properties with vertical sub-grade reaction being proportional to the deflection. The stresses at the interior, edge, and corner regions are found using formulas. The temperature stresses are developed in cement concrete pavement due to variations in slab temperature. Daily variation resulting in a temperature gradient across the thickness of the slab is called warping stresses, and seasonal variation resulting in overall change in the slab temperature results in frictional stresses. From this, the combination of the stresses is found out. Then the expansion joint and contraction joints are provided in the rigid pavement. The purpose of the expansion joint is to allow the expansion of the pavement due to a rise in temperature with respect to construction temperature, provided along the longitudinal direction. The purpose of the contraction joint is to allow the contraction of the slab due to a fall in slab temperature below the construction temperature. After this, dowel bars are provided in the pavements. The purpose of the dowel bar is to effectively transfer the load between two concrete slabs and to keep the two slabs at the same height. The dowel bars are provided in the direction of the traffic (longitudinal).

Context and Applications

This is an important topic in Highway and Transportation Engineering and is taught in the following fields of study.

  • Bachelors in Civil Engineering
  • Masters in Transportation Engineering
  • Masters in Highway Engineering

Practice Problems

1. Which of the following parameters are important in highway pavement design?

  1. Axle load
  2. Subgrade soil
  3. Climatic factors
  4. All the above

Answer: Option d

Explanation: All of the above are important factors in pavement design. The axle load is important in deciding the pavement thickness. The sub-grade soil should have sufficient strength to take up the loads from the vehicle. The climatic factors such as rain help to decide upon the factors such as drainage.

2. The longitudinal depression or deformation in any of the layers of the asphalt pavement due to continuous vehicle movement is called as

  1. Rutting failure
  2. Potholes
  3. Cracks
  4. None of the above

Answer: Option a

Explanation: Consolidation or lateral displacement of the materials owing to traffic loading causes permanent deformation in any of a pavement's layers or subgrade for asphalt pavement.

3. The asphalt layers tend to become brittle and break at ________.

  1. intermediate temperature
  2. low temperature
  3. high temperature
  4. None of the above

Answer: Option b

Explanation: Asphalt layers lose rigidity and stiffness in extremely hot weather. Asphalt layers become brittle and shatter when temps drop below freezing.

4. What does WBM stand for in pavement design?

  1. Water Bound Macadam
  2. Water Bound Modulus
  3. Water Based Macadam
  4. Water Based Modulus

Answer: Option a

Explanation: WBM is Water Bound Macadam. It is the layer of the roadway built with broken stone aggregates bound together by stone dust and water applied during construction.

5. What is the design life of a flexible asphalt pavement structure?

  1. 15 years
  2. 20 years
  3. 50 years
  4. 60 years

Answer: Option a

Explanation: The structural design period of flexible pavements is generally considered to be 15 years and it can be increased in the long term with the use of quality materials and proper maintenance.

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