Sealing construction and expansion joints in concrete pavement, drives, and walks is a moisture resistant measure that can benefit the pavement in many ways. Expansion and contraction joints are permeable zones which can receive water and eventually erode the aggregate and soil sub base under concrete flatwork. This eventual subgrade failure can cause the slab to crack and fail far earlier than necessary. Also when water can easily make its way through these unprotected cracks into the sub-base and subgrade below, contributing to freezing and thawing in colder climate regions.
Expansion joints (often housing an expansion fiber, foam, or other material) are the most common joints to receive caulk. Contraction joints may also be setup to receive caulk sealant though this is more common on interior slabs if at all, as contraction joints are smaller and less permeable than expansion.
Lastly, if the joint exists between a slab and a building foundation, it can seep water through to the foundation wall which is always something to be avoided.
To provide an effective long term seal against the passage of water, a concrete joint sealant should be durable, resilient, and have cohesive and adhesive strength. For concrete, the appropriate type of caulk will typically be polyurethane due to its ability to withstand prolonged foot traffic and it’s long lasting, durable, and flexible features which perform well in an exterior application.
A joint in concrete will require preparation to assure the substrate is ready to accept the caulk and form a good bond. This might consist of light pressure washing and or air blasting. Once the surface is dry, if the joint cannot accept the flowable self-leveling material without waste falling down the crack, a backer rod (circular foam) is pressed into the joint as a caulk dam.
Also zip strip expansion joint material may be used which has a removable top cap which uncovers a preformed channel meant to receive caulk sealant.
Then the caulk material is applied with a caulk gun. Self leveling caulk is available and is popular with exterior concrete as it shapes to larger irregular joints well. Care must be taken not to track the material as it is quite sticky immediately following placement, especially on hot days.
For exterior concrete pavement, it is common for hot asphalt cement be applied to the joints with a heated tar kettle and wand. This approach is typically more productive and cost effective for large amounts of wide joints.