Introduction:
In cement concrete pavements, tie bars are provided to enhance the structural integrity and performance of the pavement. These tie bars are reinforcement bars that are placed across joints to prevent the occurrence of cracks and to ensure the transfer of loads between adjacent sections of the pavement.

Explanation:
Tie bars are typically provided at the mid depth of the slab across longitudinal joints. This placement is chosen for several reasons:

1. Load Transfer: The main purpose of tie bars is to ensure load transfer between adjacent sections of the pavement. Placing the tie bars at the mid depth of the slab allows for effective load transfer because it is closer to the neutral axis of the pavement. This helps in minimizing differential deflections and stresses at the joint, thereby reducing the risk of cracking and failure.

2. Protection against Spalling: Placing the tie bars at the mid depth of the slab also helps in protecting the edges of the joint from spalling. When vehicles pass over the joint, the load is transferred through the tie bars, which helps in reducing the stress concentration at the edges of the joint. This prevents the development of cracks and spalling at the joint, ensuring the longevity and durability of the pavement.

3. Optimum Reinforcement: Placing the tie bars at the mid depth of the slab ensures that they are adequately embedded within the concrete. This provides optimum reinforcement and enhances the overall strength and stability of the pavement. Placing the tie bars too close to the top or bottom of the slab may result in inadequate reinforcement, reducing the effectiveness of load transfer and increasing the risk of joint failure.

It is important to note that tie bars are not provided across expansion joints in cement concrete pavements. Expansion joints allow for the natural expansion and contraction of the pavement due to temperature variations. Placing tie bars across expansion joints would hinder the movement of the pavement and may lead to cracking and distress.

Conclusion:
In cement concrete pavements, tie bars are provided at the mid depth of the slab across longitudinal joints. This placement ensures effective load transfer, protects against spalling, and provides optimum reinforcement. Tie bars are not provided across expansion joints as they would restrict the natural movement of the pavement.