In contrast, vertical movement at joints almost always is undesirable. When the two sides of a joint move up and down relative to each other by more than a few thousandths of an inch, a number of things happen, including:
- trip hazards
- ugly bumps under carpet or vinyl
- damage to vehicle tires
- structural damage to materials-handling vehicles
- damage to warehouse racks and products
- premature wear and tear on the floor
- destruction of joint fillers
- corner cracking from excessive bending stress
Rearing its ugly head
Vertical movement shows up in two ways. One way is for the slabs to fault gradually, leaving one side permanently higher than the other. The total movement can be large, sometimes exceeding 1/2 inch, but it takes place slowly. In other cases, the slab edges stay level when unloaded, but shift every time a vehicle drives across the joint.. The movement ranges from barely detectable to about 1/8 inch, rarely more, and is almost instantaneous. Although long-term faulting may look worse, both kinds of movement cause serious trouble. The exact nature of the trouble depends on traffic.
Where traffic consists of people walking around, as in houses, offices, retail stores, and institutions, the problem is long-term faulting that leaves one side high. Pedestrians don't weigh enough to cause joint movement on their own, but they will trip over an edge that sticks up. Even a minor fault, unlikely to trip anyone, can wreck the appearance of vinyl tile and some other floor coverings. On bare concrete, you can correct a faulted joint by grinding. But repairs get harder, and a lot more expensive, when the concrete lies under a floor covering.
Where traffic consists of wheeled vehicles, as in warehouses and factories, the bigger problem usually comes from instantaneous movement under loads. As a vehicle approaches the joint, it pushes the near side down. That leaves the far side sticking up for the vehicle's tires to hit. This doesn't happen just once, but every time the vehicle crosses the joint. The result is an unpleasant ride and damage to the tire or joint, or both. Materials-handling efficiency goes down, and in the worst cases, the floor can become unusable.
Though joints that move under loads have been a problem for decades, changes in the materials-handling industry have made that problem worse. In the past, the typical forklift had soft tires that could handle bad joints without much distress. Today hard plastic tires, often of small diameter, are the norm in many warehouses. Those tires fail quickly if the joints aren't in top condition.
What causes vertical movement?
Many factors can contribute, including slab curl, poor ground support, frost heave, excessive drying shrinkage, excessive joint spacing, large temperature swings, and overloading. But the root cause, always, is inadequate load transfer. If a joint contains an effective load-transfer device, it won't suffer from differential vertical movement. The two sides of the joint may move up and down, but they will move in unison.
Solving the problem
The obvious solution to the problem is to make sure joints include good load-transfer devices, but that's easier said than done. The most effective ways to limit vertical movement lock the joint so tightly it can no longer relieve stresses that might cause cracks. Tie bars and continuous reinforcement fall in that category. Dowels are designed to allow desirable horizontal movement while limiting undesirable vertical movement, but they aren't perfect and they cost money. Where low cost and ease of construction rule the day, designers still rely on aggregate interlock and keyways to transfer loads, but they often are disappointed.
If you don't prevent the problem by installing an effective load-transfer device during construction, you can sometimes fix it later. For long-term faulting, grinding is the standard remedy. For instantaneous movement under load, the list of repairs includes joint fillers, retrofit dowels, and mechanical joint stabilizers.
Brad Helminem is head of technical sales for Somero Matson Group LLC, U.S. distributors of the SD7 Joint-Saver device for stabilizing joints and cracks in concrete floors. Contact him at firstname.lastname@example.org. George Garber is a concrete floor consultant, a partner in the firm Face Consultants, and the author of Design and Construction of Concrete Floors, published by Elsevier Press. Contact him at email@example.com.