Anti-Fracture Membranes: Making Concrete Floor Joints Work for You
But that need not be the case. With a good understanding of what each type of joint is designed to do, the architect/designer and tile contractor can work together to reconcile their differences and create a floor surface that fully meets the standards of the industry.
Understanding the types of floor joints and their functions is critical to designing and executing proper ceramic tile installations. What are commonly called “expansion joints” are, in reality, four different functional joints in a concrete slab. Let’s discuss each of the four types.
Control Joints. Control joints function exactly as their name implies — they control shrinkage cracks in a concrete slab. Before a slab cures, pre-determined control joints are placed anywhere from 24 to 36 feet in each direction for interior applications, and 12 to 16 feet in each direction for exterior applications. This latter spacing standard also applies to certain interior applications, such as those that are close to windows and skylights.
Control joints permit horizontal movement in the plane of the slab. Because concrete cures from the top down, it tends to curl or lift. As a result, uncontrolled shrinkage cracks can develop in a concrete slab. When control joints are used, however, the cracks develop at these planned points. The resulting breaks, in turn, cause the cement to snap at these pre-determined points.
Cold or Construction Joints. These joints are located at the point where concrete work is concluded for the day, or at the end of the pour. Unlike control joints, the cold/construction joint breaks go all the way through the slab. Sometimes, they are treated with expansion joint material. Other times, they’re simply handled as stopping points. Cold/construction joints also can be treated in the same fashion as control joints.
Isolation Joints. Isolation joints separate columns and walls from the concrete slab. In doing so, they permit horizontal and vertical movement between the two. These joints extend through the full depth of the slab and normally make use of backer rod and joint filler. Isolation joints are the most active in a concrete slab.
Expansion Joints. A true expansion joint, which is made of metal or plastic, works freely horizontally and vertically. Because these joints are designed to expand and contract with the movement of the building, they should never be covered with any type of floor covering.
Working with jointsThe placement of control joints is decided by the structural engineer. The engineer designs the floor, determines how it will move and where joints should be placed to control lateral shrinkage. Shrinkage between two new concrete slabs can total as much as up to 1/8 inch (1/16 inch of shrinkage for each slab).
In general, control joints are formed while the concrete is still “plastic” (before it cures). Saw cuts are made in the surface of the concrete to a depth of approximately one-quarter of the slab’s thickness. They also can be made with grooved forms, by scoring, or by inserting molded non-metallic strips. According to literature published by the American Concrete Institute (ACI), “the spacing, in feet of control joints, should be equal to two to three times the thickness, in inches, of the concrete slab.”
While certainly important to the success of the finished flooring, these joints create both challenges and dismay for the architect and contractor. The ceramic tile industry, through the auspices of the Tile Council of America (TCA) Handbook, has long recommended that all “joints in the structure should continue through the tilework.” This means that, if the tile joints don’t line up with the control joint, the tile must be cut to meet the joint. If any portion of the tile is installed over this joint, it is likely to crack.
When the joint is filled with a silicone or urethane sealant, which usually is called a “soft joint,” problems can result. Wheeled traffic that runs over the joint tends to push the tile down into the soft compound, which often causes breakage at the tile edges. So, not only is the break in the tile pattern aesthetically unpleasing, but the owner is likely to sustain damage to his tiles. I estimate that approximately 20% of all cut soft joints in airports and shopping malls are damaged by heavy-wheeled traffic.
The problem in planning for tile jointsWith these challenges in mind, let’s follow the procedure after the structural engineer has decided on the location of the control joints. Even before he receives these specifications, however, the architect may have already considered various ceramic and marble types and sizes for the floor.
Say, for instance, the architect selects a tile pattern that does not run parallel with the joints in the slab. Must the tile contractor cut tiles to meet the joints in the slab? Must the tile contractor abandon the desired flooring pattern because the joints in the slab will destroy the pattern’s desired effect? In the real world, tile selections and patterns change for any number of reasons. In such instances, no amount of planning will prevent tile design problems when joints in the slab must be accommodated.
Products that accommodate joint relocation. Anti-fracture membranes can be used to transfer joint movement (lateral shrinkage) to accommodate the intended tile pattern. The membranes literally transfer the lateral movement of the joint to a point in the tile pattern chosen by the contractor.
(Insert joint relocation drawing)
There are some joints, however, that the anti-fracture membrane or any other product should not cover. One example is mechanical exposed expansion joints, which are designed to move freely to accommodate structural movement between slabs and buildings.
In most cases, however, anti-fracture membranes remain a cost-effective solution for joint relocation, strip applications over cracks, renovation of floors, and complete coverage of structurally damaged floors. Certainly, these product innovations have given architects, designers and tile contractors greater flexibility in floor design.