COMPONENTS OF RIGID PAVEMENT

 Before designing the rigid pavement i.e. CC pavement, we first need to have thorough knowledge about its components. The rigid pavement is composed of 

1. Concrete slab
2. Granular sub-base and base layer
3. Soil subgrade and
4. Joints in pavement

Let's get started with the lower most to upper most layer:

A. Soil subgrade:  

The soil subgrade for rigid pavements is composed of either natural or selected soil sourced from designated borrow areas, with specific thickness and density requirements. This layer serves as the foundational support for all other layers of the concrete pavement (CC) and the traffic loads above it. If the subgrade experiences settlement or deformation due to insufficient compaction or other factors, various types of failures can occur in the rigid pavements. However, the compressive stress that the rigid pavement transmits to the subgrade is minimal, which means there is no necessity to account for an allowable vertical strain value in the design process, unlike in flexible pavements.

To evaluate the strength of the soil subgrade for rigid pavement design, the 'plate bearing test' is commonly used, employing a large diameter plate. The capacity of the subgrade to support loads is measured through the 'modulus of subgrade reaction,' denoted as K, according to Westergaard's analysis method for rigid pavements. 

B. Granular sub-base and base layer:

The granular sub-base (GSB) layer plays a crucial role in rigid pavement as it acts as an efficient drainage system, helping to prevent premature pavement failures caused by excess moisture in the subgrade soil. When subgrades contain fine particles like silt or clay, the presence of excessive moisture can lead to early failures due to a phenomenon known as 'pumping and blowing,' which is covered in Chapter 10 of 'Highway Maintenance.' To mitigate this issue, crushed stone aggregates are preferred for the GSB layer because of their high permeability, which allows effective drainage.

For optimal drainage, coarse-graded aggregates with a minimal amount of fine particles (preferably less than 5% passing through the 0.075 mm sieve) are recommended. A well-functioning drainage layer beneath the cement concrete (CC) pavement provides several benefits:

• Increased service life and better performance of the pavement.
• Prevention of early failures, such as 'pumping and blowing' of the rigid pavement.
• Protection of the subgrade from frost action, particularly in areas prone to freezing conditions.

Base course

a. Granular Base Course for Low to Moderate Traffic Roads:

Typically, granular materials such as crushed stone or gravel are used as a base layer beneath concrete pavements in low-volume roads or roads with moderate traffic loads. These materials provide adequate support to the pavement by distributing the load evenly and enhancing drainage, which helps maintain the durability of the concrete in less demanding traffic conditions. This makes them a cost-effective choice for roads that experience lighter traffic, as they offer sufficient structural stability without the need for more complex and expensive base materials.

b. Dry Lean Concrete (DLC) or Lean Cement Concrete (LCC):

For roads subjected to heavy to very heavy traffic loads, a stronger base layer is necessary to ensure durability and longevity. In such cases, Dry Lean Concrete (DLC) or Lean Cement Concrete (LCC) is commonly used, as it provides a solid and stable foundation for the concrete pavement. These roads are typically designed with a lifespan of 30 years, but with regular maintenance, they can last up to 40 years or even longer, offering a long-term solution for high-traffic conditions.

Benefits of DLC

• Uniform Support: DLC ensures a flat, even surface for the pavement.
• High Modulus of Subgrade Reaction (K value): This means the base can resist deformation, providing greater durability.
• Excellent Working Platform: The DLC layer makes it easier to lay the Pavement Quality Concrete (PQC) slab accurately, using a sensor-paver.

c. Separation Layer:

Before the Pavement Quality Concrete (PQC) slab is laid on top of the Dry Lean Concrete (DLC) base, a separation layer, or membrane, is placed over the DLC. This separation layer serves a crucial function by preventing bonding between the PQC and DLC layers. By doing so, it allows for flexibility within the pavement structure and helps to minimize stress, reducing the likelihood of cracks or other forms of damage over time. This ensures a more durable and long-lasting pavement.

C. Cement Concrete (CC) Slab or Pavement Quality Concrete (PQC) slab

For highways subjected to heavy to very heavy traffic loads, the IRC recommends using an M-40 cement concrete mix, which is prepared in a suitable mixing plant and has a minimum flexural strength of 45 kg/cm². This high-strength concrete is essential for CC pavements, as the pavement slab must withstand both the heavy traffic loads and the warping effects caused by temperature differentials between the top and bottom surfaces of the slab during the daily temperature cycle. These variations lead to flexural stresses, which the CC pavement must endure. While steel reinforcement, if provided, is typically placed at the mid-depth of the pavement slab, it does not contribute significantly to resisting the flexural or tensile stresses caused by heavy traffic or warping. Hence, using a high-quality concrete mix with superior flexural strength is critical for constructing the Pavement Quality Concrete (PQC) slab in such pavements.

D. Joints in pavement

Rigid pavements are a crucial component of modern road construction, known for their durability and ability to support heavy loads. A significant aspect of rigid pavements is their joint systems, which are essential for managing stresses and movements within the concrete slabs. This discussion will focus on the types of joints in rigid pavements, specifically transverse and longitudinal joints, along with their characteristics and functions.

Fig: Joints in rigid pavement

I. Traverse Joint:

Transverse joints are a critical element in the design and construction of rigid pavements, particularly in managing the stresses that arise from temperature fluctuations and load conditions. Transverse joints are vertical separations in concrete slabs that are oriented perpendicular to the direction of traffic. Their primary purpose is to:

• Control Cracking: They help manage cracking due to thermal expansion and contraction of the concrete. Without these joints, the concrete could crack randomly as it expands or shrinks with temperature changes.
• Accommodate Movement: Transverse joints allow for horizontal movement within the pavement structure, accommodating shifts caused by temperature variations and load impacts.

Types of Transverse Joints

There are several types of transverse joints used in rigid pavements:

1. Contraction Joints:

• Function: These are specifically designed to create a weakened plane in the concrete slab, encouraging controlled cracking at predetermined locations.
• Placement: Typically spaced at intervals ranging from 10 to 15 feet apart, depending on the slab thickness and expected load condition. 

2. Expansion Joints:

• Function: These joints allow for the expansion of concrete slabs during temperature increases. They are crucial in preventing buckling or heaving of the pavement surface.
• Design: Expansion joints usually incorporate materials that can compress and expand, such as rubber or flexible fillers.

3. Construction Joints:

• Function: Used during the construction phase, these joints connect different sections of pavement that are poured at different times.
• Importance: They ensure continuity in load transfer between adjacent slabs while allowing for some movement.

Design Considerations

When designing transverse joints, several factors must be taken into account:

• Joint Spacing: The spacing between joints is critical for effective stress management. Common practice suggests spacing them every 10 to 15 feet, but this can vary based on specific project requirements and environmental conditions.
• Thickness of Slabs: The thickness of the concrete slab influences joint placement; thicker slabs may require different spacing or additional joint types to effectively manage stresses.
• Material Selection: The materials used for joint fillers and sealants must be durable and capable of withstanding environmental conditions such as moisture infiltration and temperature fluctuations.

II. Longitudinal Joints:

Longitudinal joints are an essential component of rigid pavements, designed to accommodate the structural and thermal dynamics of concrete slabs. Longitudinal joints are vertical separations that run parallel to the direction of traffic. They are primarily utilized to:

• Manage Warping and Settlement: These joints help mitigate warping stresses that arise from temperature changes and moisture gradients within the concrete. They also accommodate differential settlement of the pavement foundation, allowing for slight angular movements without causing significant stress concentrations in the slabs.
• Provide Structural Integrity: By tying adjacent slabs together, longitudinal joints maintain alignment and support load distribution across the pavement. This is particularly important in multilane pavements where stability is critical

Types of Longitudinal Joints

There are several types of longitudinal joints employed in rigid pavements:

1. Construction Joints:

• Function: These joints are created when new concrete is placed adjacent to previously hardened concrete. They ensure continuity between different sections of pavement laid on separate days.
• Importance: Properly designed construction joints prevent issues related to misalignment and load transfer between slabs.

2. Tie Bar Joints:

• Function: Tie bars are placed within longitudinal joints to connect adjacent slabs. They do not transfer loads like dowel bars but serve to hold the slabs together.
• Design Consideration: Tie bars must be properly anchored into the concrete to function effectively and are typically smaller than dowel bars, spaced at larger intervals.

3. Warping Joints:

• Function: These joints allow for limited angular movement due to warping caused by temperature changes or moisture differentials.
• Design Aspect: Warping joints help prevent cracking by accommodating slight movements without compromising the overall integrity of the pavement.

Design Considerations

When designing longitudinal joints, several factors must be taken into account:

• Spacing: Longitudinal joints are typically spaced every 4.5 meters (approximately 15 feet) or based on specific project requirements. Proper spacing is crucial to accommodate potential differential settlement and warping stresses.
• Material Selection: The materials used for tie bars and joint fillers should be durable and capable of withstanding environmental conditions such as moisture and temperature fluctuations.
• Thickness of Slabs: The thickness of the concrete slab influences joint placement; thicker slabs may require different spacing or additional joint types to effectively manage stresses.