9.5 Road Pavement

9.5 Road Pavement

Introduction to Road Pavements

A road pavement is a layered structure built over the natural subgrade to distribute vehicular loads to the subgrade within its safe bearing capacity, while providing a smooth, durable, and all-weather riding surface. The choice and design of pavement type are critical decisions in highway engineering, directly impacting construction cost, maintenance requirements, ride quality, and service life. This section covers the fundamental types of pavements, the design philosophies and methods for flexible and rigid pavements as per prevalent guidelines, and the critical loads and stresses that govern their structural design.

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1. Different Types of Pavement

Pavements are primarily classified based on their structural behavior and material composition.

1.1 Flexible Pavements

1. Definition: A pavement structure that deflects under load and distributes stresses through grain-to-grain transfer in the granular layers. It has low flexural strength and deforms elastically under load.

2. Structural Components (From Bottom to Top):

   • Subgrade: Compacted natural soil or improved soil forming the foundation.

   • Sub-base Course: A layer of granular material (gravel, crushed stone) providing drainage, frost protection, and a working platform. May be omitted on strong subgrades.

   • Base Course: The main load-distributing layer, composed of high-quality crushed stone or stabilized material.

   • Binder Course: An intermediate layer between base and surface, sometimes used to correct profile.

   • Surface Course (Wearing Course): The top layer, directly subjected to traffic and weather. Made of asphalt concrete (Bituminous Macadam, Dense Bituminous Macadam, Bituminous Concrete). Provides smoothness, skid resistance, and waterproofing.

3. Mechanism: The wheel load stresses are highest at the surface and decrease with depth. The pavement's strength is derived from the aggregate interlock, particle friction, and cohesion provided by the bituminous binder.

1.2 Rigid Pavements

1. Definition: A pavement structure possessing high flexural strength and rigidity, typically a Portland Cement Concrete (PCC) slab. It distributes the load over a wide area of the subgrade through slab action.

2. Structural Components:

   • Subgrade & Sub-base: Similar to flexible pavements, but the sub-base (often called the Dry Lean Concrete (DLC) or Granular Sub-base (GSB)) primarily provides a uniform, stable, and non-erodible support.

   • Concrete Slab: The main structural element, usually 150-300mm thick. It is a beam of low thickness-to-length ratio resting on the sub-base/subgrade.

   • Reinforcement: May include:

     • Dowel Bars: Smooth steel bars across transverse joints to transfer load between slabs and maintain alignment.

     • Tie Bars: Deformed steel bars across longitudinal joints to prevent lane separation.

     • Reinforcement Mesh (Fabric): Used in continuously reinforced concrete pavements (CRCP) to control crack width.

3. Mechanism: The concrete slab acts as a beam or elastic plate, distributing the concentrated wheel load over a large area of the underlying layers due to its rigidity and high modulus of elasticity.

1.3 Semi-Rigid/Composite Pavements

• Definition: Pavements where a flexible surface course is laid over a cement-treated base (e.g., soil-cement, lean concrete). The base layer provides rigidity, while the surface provides flexibility and riding quality.

1.4 Interlocking Concrete Block Pavements

• Used for low-speed urban roads, parking lots, and intersections. Provides permeability and ease of repair.

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2. Design Methods for Flexible and Rigid Pavements: DOR Guidelines

In Nepal, the Department of Roads (DOR) provides guidelines for pavement design. These are often adapted from international standards like the Indian Roads Congress (IRC) codes, which are widely used and referenced.

2.1 Flexible Pavement Design (IRC: 37 Guidelines)

The most common method for flexible pavement design in the region is the Empirical-Mechanistic approach using the California Bearing Ratio (CBR).

A. CBR-Based Design Method (IRC:37-2018)

This method is widely used for low to medium traffic roads.

1. Design Inputs:

   • Subgrade Strength: Measured by the California Bearing Ratio (CBR) test on the soaked subgrade soil.

   • Traffic: Expressed in terms of Million Standard Axles (msa) over the design life. This requires converting commercial vehicles into Equivalent Standard Axle Load (ESAL) repetitions using an axle load equivalency factor.

2. Design Process:

   • The design CBR value is selected (usually the 90th percentile value from test results).

   • Based on the design CBR and the cumulative ESALs (in msa) over the design life, the total pavement thickness is read from design charts (e.g., IRC:37 charts).

   • This total thickness is then composed into individual layer thicknesses (Granular Sub-base, Base, Binder, Surface) as per specifications. Minimum layer thicknesses are specified to ensure constructability.

3. Mechanistic-Empirical Refinement (for high-traffic roads): For roads with >30 msa, IRC:37 recommends a more advanced approach where:

   • Pavement materials are characterized by their resilient modulus (Mr).

   • Critical stresses and strains (tensile strain at the bottom of the bituminous layer and compressive strain at the top of the subgrade) are computed using multi-layer elastic theory software.

   • The design is checked against failure criteria:

     • Fatigue Cracking: Controlled by limiting horizontal tensile strain at the bottom of the bituminous layer.

     • Rutting: Controlled by limiting vertical compressive strain at the top of the subgrade.

2.2 Rigid Pavement Design (IRC: 58 Guidelines)

The design of jointed plain concrete pavements (JPCP) is governed by IRC:58.

1. Design Inputs:

   • Subgrade Strength: Expressed as the Modulus of Subgrade Reaction (k) determined from a plate bearing test, or estimated from CBR value ($k \approx 10.2 \times CBR$ for CBR ≤ 5%).

   • Concrete Properties: Modulus of Elasticity ($E_c$), Poisson's ratio ($\mu$), flexural strength (modulus of rupture, $f_{cr}$), and coefficient of thermal expansion ($\alpha$).

   • Traffic: Cumulative number of repetitions of Equivalent Single Axle Load (ESAL) during design life.

   • Design Factors: Reliability, load safety factor.

2. Design Output - Slab Thickness:

   • The main output is the required slab thickness (typically 150-300mm).

   • Thickness is determined using Westergaard's stress analysis and fatigue considerations. IRC:58 provides design charts relating slab thickness to flexural stress, which is a function of wheel load, slab thickness, and subgrade modulus (k).

3. Joint Design: A critical part of rigid pavement design.

   • Transverse Joints: Spaced at 4-5m intervals to control cracking from temperature and moisture shrinkage. Include contraction joints (with dowels) and expansion joints (with dowels and compressible filler, less common now).

   • Longitudinal Joints: Separate traffic lanes.

4. Reinforcement Design: For JPCP, only temperature reinforcement in the form of tie bars and dowel bars is designed. For CRCP, continuous steel reinforcement is designed to hold cracks tightly together.

2.3 DOR Practices in Nepal

• The DOR typically follows the IRC guidelines (37 and 58) for design.

• For flexible pavements on strategic roads, the Asphalt Institute (MS-1) or AASHTO methods are also sometimes referenced.

• Local material availability, climate (especially monsoon and freeze-thaw cycles in hills), and construction quality control are major considerations integrated into the design process.

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3. Loads and Other Factors Controlling Pavement Design

3.1 Traffic Loads

1. Static Loads:

   • Magnitude: Defined by axle configurations (single, tandem, tridem) and legal axle load limits (e.g., 10 tons for single axle, 16-18 tons for tandem axle in many countries).

   • Contact Pressure: The tire pressure (around 550-850 kPa) distributed over the contact area (assumed circular for design).

2. Dynamic/Varying Loads:

   • Impact Loads: Caused by vehicle suspension and road roughness. Typically accounted for by an impact factor (e.g., 10-25% increase over static load).

   • Repetition: The number of load applications over the pavement's design life is more critical than a single heavy load. Pavement failure is a fatigue phenomenon.

3.2 Environmental Factors

1. Temperature:

   • Flexible Pavements: High temperatures soften asphalt, leading to rutting; low temperatures make it brittle, leading to cracking.

   • Rigid Pavements: Daily and seasonal temperature variations cause expansion and contraction, leading to joint opening/closing and curling stresses.

2. Moisture/Precipitation:

   • Water infiltration weakens subgrade and unbound layers, leading to loss of support and premature failure. Proper drainage is paramount.

   • Freeze-thaw cycles in cold climates cause severe damage.

3. Frost Action: In freezing climates, water in the subgrade freezes and expands (frost heave), and thaws to create a weak, saturated layer.

3.3 Material Properties

• Flexible Pavements: Resilient modulus ($M_R$) of each layer, bitumen viscosity and penetration.

• Rigid Pavements: Concrete's modulus of elasticity ($E_c$), flexural strength ($f_{cr}$), and coefficient of thermal expansion ($\alpha$).

3.4 Pavement Performance Criteria

Design aims to keep critical responses below thresholds that cause unacceptable levels of distress:

• Flexible: Limit rutting and fatigue cracking.

• Rigid: Limit slab cracking and faulting (difference in elevation at joints).

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4. Stress Due to Load, Temperature

4.1 Stresses in Flexible Pavements

Load-induced stresses are analyzed using Multi-Layer Elastic Theory (Burmister's model). The pavement is modeled as layers of linear-elastic materials with different moduli resting on a semi-infinite subgrade.

1. Critical Stresses/Strains:

   • Vertical Compressive Stress on Subgrade ($\sigma_z$): Causes permanent deformation (rutting).

   • Horizontal Tensile Strain at Bottom of Asphalt Layer ($\epsilon_t$): Causes bottom-up fatigue cracking.

   • Shear Stress within Layers: Can cause shear failure.

2. Factors Affecting Magnitude: Wheel load, tire pressure, thickness, and modulus of each layer.

4.2 Stresses in Rigid Pavements

Stresses are more complex due to the slab's rigidity and are analyzed using Plate Theory (Westergaard's equations) or Finite Element Methods.

A. Load-Induced Stresses (Westergaard)

Westergaard derived equations for stress due to a loaded area on a concrete slab resting on a dense liquid subgrade (Winkler foundation, characterized by modulus k).

1. Critical Load Positions:

   • Interior Loading: Wheel load far from edges/corners. $\sigma_i = \frac{0.316 P}{h^2} [4 \log_{10}(\frac{l}{b}) + 1.069]$

   • Edge Loading: Wheel load at the edge, midway between corners (most critical for design). $\sigma_e = \frac{0.572 P}{h^2} [4 \log_{10}(\frac{l}{b}) + 0.359]$

   • Corner Loading: Wheel load at the corner. $\sigma_c = \frac{3P}{h^2} [1 - (\frac{a\sqrt{2}}{l})^{1.2}]$

   • Where:

     • $P$ = wheel load

     • $h$ = slab thickness

     • $l$ = radius of relative stiffness, $l = [\frac{E_c h^3}{12 k (1-\mu^2)}]^{1/4}$. A key parameter relating slab stiffness to subgrade support.

     • $a$ = radius of loaded area

     • $b$ = radius of equivalent resisting section, $b = \sqrt{1.6 a^2 + h^2} - 0.675 h$, if $a < 1.724h$

B. Temperature-Induced Stresses

Caused by temperature differential between the top and bottom of the slab.

1. Warping Stress: When the top is hotter than the bottom (day), the slab tends to curl downwards, creating tensile stress at the bottom. At night, the reverse causes tensile stress at the top.

   • Warping stress at interior: $\sigma_{t} = \frac{E_c \alpha \Delta t}{2(1-\mu)} \cdot C_x$ (where $C_x$ is a coefficient based on $L_x/l$, $L_x$ being slab length).

2. Frictional Stress: Due to restriction of slab movement from friction with the underlying layer during expansion/contraction.

   • $\sigma_f = \frac{\gamma_c L f}{2} \times 10^3$

   • Where $\gamma_c$ = unit weight of concrete, $L$ = slab length, $f$ = coefficient of friction (0.9-1.5).

C. Combined Stresses

For design, the critical stress is often the combination of load stress and temperature stress. For example, the maximum stress in a slab is often taken as: $\sigma_{total} = \sigma_{load} + \sigma_{temperature}$ Where $\sigma_{load}$ is usually the edge stress and $\sigma_{temperature}$ is the warping or frictional stress, depending on the condition.

Conclusion: Pavement design is the art and science of balancing traffic demands, material properties, and environmental aggressiveness within economic constraints. Whether choosing a flexible or rigid solution, the designer must ensure that the calculated stresses from all sources—wheel loads, temperature, and moisture—remain within the capacity of the carefully selected materials over millions of load repetitions, thereby guaranteeing a safe and durable roadway.