How do we construct expansion joints? First, there is one cardinal rule for expansion joints of all types. It is mandatory that the joint, whatever depth, be free and clear of all debris prior to the construction of the joint. The expansion joint's purpose is to isolate the tile installation from other hard surfaces. If debris is allowed to remain, it defeats the purpose.
With that said, let's look at the simpler expansion joints. Cold joints can be accomplished by simply using a flexible caulking instead of hard grout. These products are described in the section dealing with grouts. Being flexible, they allow the field of tile to move and not fracture. They are available in colors that closely match the grout color chosen. A more appropriate term for flexible caulking is an electrometric sealant compound. Electrometric denotes that the product is and will remain flexible. Electrometric also refers to a product that can stretch to two times it's length without fracture. The product also has to return to it's original shape after the stress is relieved. It is also important for the sealant to have high bonding capabilities, especially to ceramic tile or stone. For floor applications, the sealant must have a certain hardness.
Why is this important? Well, imagine a high heel piercing the sealant in an expansion joint. The wearer could fall as a result of getting their shoe caught in the joint. It is therefore recommended that the sealant have a shore-A hardness of at least 35 to compensate for this problem.
Tile Doctor Tip: When choosing a sealant for a floor expansion joint, be sure that it is Electrometric and that it has a shore-A hardness of at least 35
You will note from the diagrams, that a closed cell polyethylene filler is installed behind the caulking or sealant. This product is used as it is not affected by moisture and it will not bond to caulking compounds. You will also note that the top of the backing is rounded. This creates a sort of hourglass shape within the joint when the top of the joint is finished in a concave shape. This is important for the proper movement of the joint. A note about backup fillers. In CTIOA report 83-1-3, we note that a backup filler must be larger than the expansion joint width. This is due to the backer rod's ability to form a watertight backing on which to apply the sealant. It serves no purpose to have a joint that is not as watertight as possible.
Why do we want the sealant to bond to the tile or stone and not to the backing? Additionally, why is the hourglass shape so important? Picture the joints in use. The installation expands and contracts. If the sealant is a block shape and or is bonded at the sides and bottom, it will suffer undue stress and will fail by pulling apart from the sides of the joint. Sometimes there is not room for backup filler. In this case a bond breaking tape is used at the bottom of the joint to keep the sealant from bonding with the substrate.
Is it always necessary to use backup fillers and bond breaking tape? Not necessarily. In the case of butt or perimeter joints, it may only be necessary to leave the joint open, especially if base molding is to be applied over the perimeter joint. Also, in the case of 90-degree joints in tile it may only be necessary to use the sealant instead of hard grout.
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