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What To Do With Control and Expansion Joints

Updated: Mar 19



Filling joints in a concrete slab before installing a resinous floor can be a little more complicated than it may seem. There are many ways to fill a joint and some contractors do not even fill them at all, which leads to a lot of problems and greatly reduces the longevity of the floor.


Concrete floor slabs have several different kind of joints, which require different treatment with respect to the installation of resinous flooring.


There are two main types of joints: expansion joints and control joints. Control joints are different from expansion joints in that they are essentially non-moving once the concrete has reached full cure, while expansion joints need to be able to expand and contract.


Because of this, sealant used in expansion joints should be able to move with the joint while, sealants used to seal construction and control joints are typically rigid.


If the joints are improperly sealed than the resinous material on top of the joint can crack and disbond from the walls of the joint.


Prior to applying a resinous coating on concrete, the joints need to be filled. To do this first the joints need to be sawcut and vacuumed 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 joint needs 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.




Control joints can mirror through epoxy floor toppings, causing a crack to form in the topping over these joints. Using a combination of epoxy based joint filler and epoxy-saturated, crack-bridging reinforcing fabrics helps prevent reflective cracking.


This type of treatment is often also suggested to address concrete floor cracks however, use of a rigid filler system does not always yield long-term success.


Concrete cracks develop because the slab is moving so future movement causing a stress riser at the same location can cause the concrete to crack again. Often a new crack will develop immediately adjacent to the original, filled crack. In such cases, a flexible sealant is usually a more practical solution.


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.


Likewise, when the joints converge, the material in the joints needs to be able to squeeze together, instead of just crack and crumble. It is important to note that use of backer rod can be necessary when the joints are larger than ½”.


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 backer rod, when the joint squeezes it pushes the material upward creating a lump in the floor where it will probably crack however, if there is backer rod the path of least resistance is downward so it is pushed down instead.


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.

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