How To Fill And Maintain Control and Expansion Joints
Updated: Jul 1
Filling joints in a concrete slab before installing a resinous floor can be complicated. There are many ways to fill a joint, and some contractors do not fill them at all, which leads to many problems and dramatically reduces the longevity of the floor.
Additionally, concrete floor slabs contain several different types of joints, which require different treatment with respect to the installation of resinous flooring.
Expansion and Control Joints
There are two main types of joints: expansion joints and control joints. The main difference between them is that control joints are essentially non-moving once the concrete has reached full cure, while expansion joints need to be able to expand and contract.
Due to this, sealants used in expansion joints should be able to move with the joint, while sealants used to seal construction and control joints are typically rigid.
Expansion joints are usually at least ½” thick and are used to separate slabs and concrete from other parts of the structure such as areas, rooms, or slabs poured at different times.
To fill expansion joints, a flexible epoxy or polyurea with high elongation is an ideal material to use because when the joints diverge, the material in the joint needs to be able to expand.
If the joints are improperly sealed, the resinous material on top of the joint can crack and disbond from the walls of the joint.
Filling The Joints
Prior to applying a resinous coating on concrete, the joints need to be filled. However, before doing this the joints need to be sawcut and vacuumed in order to remove debris.
Next, the joints should be filled with an epoxy mortar mixture, typically consisting of epoxy primer and sil-co-sil for control joints. After that, the joints need to be primed and a chopped strand fiberglass mat should be placed on it to bridge the gap between the two floors.
The fiberglass mat is then saturated with primer and rolled out to remove air bubbles. Once all of the control joints have been treated this way, the floor is ready to be primed and coated with resinous material.
Using A Backer Rod For Material Support
When the joints converge, the material in the joints needs to be able to squeeze together, and cracking and crumbling should be avoided. It is important to note that use of backer rod can be necessary when the joints are larger than ½”.
A backer rod is a flexible, soft rod that is compacted into the base of the joint. When the joint contracts, the joint filler has two directions to squeeze, up or down.
Without a backer rod, when the joint squeezes it pushes the material upward, creating a lump in the floor where a crack will likely form. However, the use of a backer rod moves the path of least resistance downward, so the material is pushed down instead.
How To Prevent Cracking Joints
Control joints can mirror through epoxy floor toppings, causing cracks to form in the topping over the joints. Using a combination of epoxy-based joint filler and crack-bridging reinforcing fabrics can help to prevent reflective cracking.
Proper Filling Of The Joints
An illustration of this can be seen below. If done right, filling the joints of a floor can be very aesthetically pleasing and leave no areas for dirt to be trapped.
The most important thing you can do to prevent your floor from failing due to poorly filled joints is to contact a trusted expert resinous flooring expert.