CE 223 - HIGHWAY & RAILROAD ENGINEERING FINALS - AY 2024-205
Types of Flexible Pavements
• Conventional flexible pavements are layered
systems with high quality expensive materials placed
on top where stresses are high, and low quality cheap
materials placed in the lower layers.
• Full-depth asphalt pavements are constructed by
placing bituminous layers directly on the soil sub-
grade. This is more suitable when there is high traffic
and local materials are not available.
• Contained rock asphalt mats are constructed by
placing dense/open graded aggregate layers between
two asphalt layers. Modified dense graded asphalt
concrete placed above the sub-grade will significantly
reduce the vertical compressive strain on soil sub-
grade and protect from surface water.
Typical layers of a flexible pavement
• Seal Coat: A thin surface treatment used to waterproof
the surface and provide skid resistance.
• Tack Coat: A very light application of asphalt, usually
asphalt emulsion diluted with water. It provides proper
bonding between two layers of the binder course and
must be thin, uniformly cover the entire surface, and
set very fast.
• Prime Coat: An application of low viscous cutback
bitumen to an absorbent surface like granular bases,
on which the binder layer is placed. It provides bonding
between two layers. Unlike tack coat, prime coat
penetrates into the layer below, plugs the voids, and
forms a watertight surface.
• Surface Course: Directly in contact with traffic loads,
made with high-quality materials (dense graded asphalt
concrete). Provides friction, smoothness, drainage, and
prevents excessive water penetration. Must resist
distortion, provide a smooth, skid-resistant surface, and
be waterproof.
• Binder Course: Provides bulk to the asphalt structure
and distributes load to the base course.
Made with aggregates having less asphalt, less costly
than surface course.
• Base Course: Located beneath the surface and binder
courses, provides load distribution and aids drainage.
Composed of crushed stone, slag, or other materials.
• Sub-Base Course: Located beneath the base course,
provides structural support, improves drainage, and
prevents fines intrusion. Not always needed, especially
over stiff sub-grades, where a sub-base may be
omitted.
• Sub-Grade: Natural soil layer prepared to support the
stresses from upper layers. Must be compacted to the
optimal density and moisture content to prevent
overstressing.
Rigid Pavements
Rigid pavements have sufficient flexural strength to transmit the
wheel load stresses to a wider area below. Compared to flexible
pavement, rigid pavements are placed either direct prepared sub-
grade or on a single layer of granular or stabilized material. Since
there is only one layer of material between the concrete and the
sub-grade, this layer can be called as base or sub-base course.
Types of Rigid Pavements
• Jointed Plain Concrete Pavement:
o Constructed with closely spaced contraction joints.
o Dowel bars or aggregate interlocks are used for load
transfer across joints.
o Joint spacing typically ranges from 5 to 10 meters.
• Jointed Reinforced Concrete Pavement:
o Reinforcements don't significantly improve structural
capacity but increase joint spacing to 10 to 30 meters.
o Dowel bars are required for load transfer.
o Reinforcements help keep the slab together even after
cracks.
• Continuous Reinforced Concrete Pavement:
o Joints are completely eliminated through
reinforcement.
Typical layers of a rigid pavement
• Surface Course – Made of Portland Cement Concrete
(PCC), provides a strong and wear-resistant layer.
• Base Course (Optional) – Supports the concrete slab
and improves load distribution.
• Subbase Course – Provides additional support,
drainage, and prevents subgrade movement.
• Subgrade – The prepared natural soil that supports the
pavement structure.
Composite Pavements
Composite pavements are pavement structures that combine
multiple layers of different materials, typically incorporating
both flexible and rigid characteristics to optimize
performance. These pavements are designed to take
advantage of the benefits of both rigid and flexible pavement
systems, offering improved durability, load distribution, and
cost-effectiveness.
Key Features of Composite Pavements:
Hybrid Structure: Typically consists of a rigid base layer
(such as Portland cement concrete or cement-treated base)
overlaid with a flexible asphalt layer.
Load Distribution: The rigid layer provides strong structural
support, while the flexible layer improves surface
performance.
Enhanced Durability: Prevents issues like rutting and
cracking by utilizing the strengths of both pavement types.
Improved Ride Quality: The asphalt layer provides a smoother,
quieter driving surface.
Reduced Maintenance Needs: The top layer can be
periodically resurfaced to extend pavement life without
reconstructing the entire structure.
Typical layers of a composite pavement
Surface Course – Asphalt or concrete overlay for improved
performance and durability.
Existing Rigid/Flexible Pavement – Acts as a strong base
layer for resurfacing.
Base and Subbase Courses – Improve load-bearing capacity
and drainage.
Subgrade – The natural soil layer that provides foundational
support.
Pavement Materials
1. Soil - refers to the natural or compacted ground that
serves as the subgrade, providing foundational support
for all pavement layers. Its strength and stability
influence the pavement’s performance and durability.
2. Aggregates - are crushed stone, gravel, sand, or recycled
materials that provide strength, stability, and durability to
the pavement structure. They form the bulk of the base,
subbase, and surface layers, ensuring load distribution
and resistance to wear.
3. Bitumen - A viscous, black, and sticky material derived
from crude oil, used as a binder in asphalt pavements.
