Pavements form the basic supporting structure in highway transport. Each layer of pavement has a number of functions to perform which must be duly considered during the design process. Different types of footpaths can be adopted depending on the traffic requirements. Due to improper design of the sidewalls, the pavement wears out quickly which also affects the ride quality.
Sidewalks can be classified into two parts on the basis of structural performance,
- flexible sidewalks and
- hard pavement
In flexible sidewalls, wheel load is transferred by grain-to-grain contact of the aggregates through the granular structure. Flexible pavement, which has low flexural strength, acts like a flexible sheet (such as bituminous road).
In contrast, in rigid pavements, the wheel load is transferred to sub-grade soils by the flexural strength of the pavement and the pavement acts like a rigid plate (such as cement concrete roads).
Apart from these, mixed sidewalks are also available. A thin layer of flexible pavement over a rigid pavement is an ideal pavement with the most desirable characteristics. However, such pavements are rarely used in new construction due to the high cost and the need for complex analysis.
1. flexible pavement
Flexible sidewalls will transmit wheel load stress to the lower layers grain to grain transfer through the points of contact in the granular structure (see image shown below).
The wheel load acting on the pavement will be distributed over a wider area, and the stress decreases with depth. Taking advantage of these stress distribution characteristics, flexible pavement normally consists of multiple layers. Therefore, the design of flexible pavement uses the concept of layered system, A typical cross section of a flexible pavement is shown below.
Based on this, flexible pavement can be constructed in multiple layers and the top layer needs to be of the best quality to maintain maximum compressive stress, apart from wear and tear.
Lower layers will have less stress and lower quality materials can be used. Flexible pavement is manufactured using bituminous material. These can be either in the form of surface treatments (such as bituminous surface treatments commonly found on low volume roads) or, asphalt concrete surface courses (commonly used on high volume roads such as national highways). ). Flexible pavement layers reflect the deformation of the lower layers at the surface layer (for example, if there is a fluctuation in the sub-grade it will be transferred to the surface layer). In the case of flexible pavement, the design is based on the overall performance of the flexible pavement, and the stresses produced must be kept well below the allowable stresses of each pavement layer.
Flexible pavement types
The following types of construction have been used in flexible pavement:
- traditional layered flexible pavement,
- full depth asphalt pavement, and
- Containing Rock Asphalt Mat (CRAM).
traditional flexible pavement Layered systems with high quality expensive materials are laid on top where stresses are high, and low quality cheap materials are laid in the lower layers.
full depth asphalt pavement Bituminous layers are created by placing them directly on the soil subgrade.
It is more suitable when there is high traffic and local content is not available.
included rock asphalt mats Constructed by placing a dense/open graded composite layer between two asphalt layers. Modified dense graded asphalt concrete placed over the sub-grade will reduce the vertical compressive stress on the soil sub-grade and protect against surface water.
Reasons for failure of flexible pavements
The major flexible pavement failures are fatigue cracking, ratting, and thermal cracking.
fatigue breakdown The reason for flexible pavement is the horizontal tensile stress on the floor of asphalt concrete. The failure criterion is related to the allowable number of load repetitions for the tensile stress and this relationship can be determined in the laboratory. fatigue test On samples of asphalt concrete.
Rating Occurs only on flexible sidewalls as indicated by permanent deformation or rut depth along the wheel load path. Two design methods have been used to control routing: one to limit the vertical compressive stress at the top of the subgrade and the other to a tolerable amount (typically 12 mm).
thermal cracking It includes both low-temperature cracking and thermal fatigue cracking.
2. hard pavement
Rigid sidewalls have sufficient flexural strength to transmit wheel load stress over a wider area below. A typical cross section of a rigid pavement is shown below.
Compared to flexible pavements, rigid pavements are laid either directly on a finished sub-grade or over a layer of granular or immobilized material. Since there is only one layer of material between the concrete and the sub-grade, this layer may be called a base or sub-base course.
In a rigid pavement, the load is distributed by slab action, and the pavement behaves like an elastic plate resting on a viscous medium (see figure below).
Rigid pavement is manufactured by Portland Cement Concrete (PCC) and must be analyzed plate theory Rather than layer theory, assuming an elastic plate resting on a sticky foundation. The plate theory is a simplified version of the layer theory which treats the concrete slab as a medium thick plate that is level before loading and remains flat after loading. Bending of slabs due to wheel load and temperature variation and resultant tensile and flexural stresses.
types of hard pavement
Rigid pavements can be classified into four types:
- Combined Plain Concrete Pavement (JPCP),
- Combined Reinforced Concrete Pavement (JRCP),
- Continuous Reinforced Concrete Pavement (CRCP), and
- Pre-stressed concrete pavement (PCP).
composite plain concrete pavement Plain cement are concrete pavements constructed with closely spaced contraction joints. Dowel bars or aggregate interlocks are commonly used for load transfer across joints. They generally have a combined distance of 5 to 10 metres.
composite reinforced concrete pavement: Although reinforcements do not significantly improve structural capability, they can increase joint spacing by 10 to 30 m. Dowel bars are required for load transfer. Reinforcement helps to hold the slab together even after cracks.
continuous reinforced concrete pavement: Complete elimination of joints is achieved by reinforcement.
Causes of failure of hard pavements
traditionally fatigue breakdown Rigid pavement has been considered to be the major or sole criterion for design. The allowable number of load repetitions due to fatigue cracking depends on the stress ratio between the flexural tensile stress and the solid modulus of rupture.
of late, pumping recognized as a critical failure criterion. Pumping is the evacuation of a soil solution through the joints and cracks of the cement concrete pavement, which occurs during the downward movement of the slab under heavy wheel loads.
Other major types of hazards in hard pavements include: faulty, spelling, and worsen.
Dr. Tom V Matthews (IIT Bombay)