Which type of joint separates pavement slabs on grade filled with compressible material?

The type of joint that separates pavement slabs on grade filled with compressible material is the expansion joint. This joint is specifically designed to accommodate the expansion of materials due to temperature changes and other environmental factors. By incorporating compressible material, the expansion joint allows for horizontal and vertical movements while maintaining the integrity of the pavement structure.

In the context of pavement design, expansion joints are particularly important because they prevent stress buildup that can lead to cracking or structural failures. The compressible material within the joint absorbs any expansion, thereby safeguarding adjoining slabs.

Other types of joints, such as contraction joints and control joints, serve different purposes related to managing cracking and ensuring structural stability under loading but do not specifically address the need for movement caused by expansion. An isolation joint, meanwhile, provides a similar function by allowing adjacent structures to move independently, but again, it is not primarily focused on accommodating expansion. This is why the expansion joint is the most appropriate choice in this context.