This is part one of the New Concept in Concrete Control Joints. Part II, which covers the treatment of control joints, will be featured online in June 2013.
Control joints are used to accommodate concrete shrinkage during the curing and aging process in an attempt to “control” where a crack will locate. However, they can potentially create issues relative to surface integrity, sanitation, or aesthetics. Traditionally, these joints have been filled with a semi-flexible epoxy to support traffic, to maintain a monolithic surface, and to move with the contraction and expansion of the concrete. But are there other options?
The control, or contraction, joint is designed to control the crack location caused by the shrinking, or contraction, of the concrete slab. The change in volume of the concrete slab is primarily due to the amount of paste in the concrete and the corresponding loss of the excess water used to facilitate placement. It has to do with volume reduction caused by the hydration process on a molecular level. Think of what happens when a wet sponge dries. It shrinks, as does the concrete slab when it cures.
Shrinking can occur throughout the drying process, and usually occurs in three stages. Plastic shrinkage occurs prior to initial set when the concrete is in a “plastic” state. Then, short-term drying shrinkage can occur. This generally occurs within the first 30 days after placement and typically accounts for 30 to 40 percent of the total shrinkage for standard concrete. Finally, long-term drying shrinkage can occur. This refers to the first year after placement. After one year, standard concrete will have achieved 90 percent of the total potential contraction. Generally, concrete will shrink 1/8 of an inch (0.32 cm) for every 20-foot (6.10 m) span over the entire process.
Several variables, including the concrete mix and the external environment, will impact the degree of shrinkage:
• Cement type (listed here in order of decreasing shrinkage):
1. ASTM C 150 Type III
2. ASTM C 150 Type I
3. ASTM C 150 Type II
4. ASTM C 845 Type K
• Water-to-cement (w/c) ratio. Increasing the amount of water in cement paste increases shrinkage
• Temperature of plastic and hardened concrete. As temperatures increase, hydration- and moisture-loss
increase, which encourages shrinkage
• Lack of chemical or mineral admixtures. By reducing the amount of water needed for a given level of
workability, admixtures can be used to reduce drying shrinkage
• Concrete finishing techniques
• Curing procedures
Control Joint Design
Concrete is made up of a collection of materials — such as several aggregate types, cement, pozzolans, water, and air — that are glued together with a cement paste. The aggregates provide the compressive strength. The cement, on the other hand, is relatively weak; its job is to hold the aggregates together. Additionally, the tensile strength of typical concrete is about one-tenth the compressive strength, or about 350 to 500 psi (2,413.17?3,447.38 kPa). With such low tensile strength, then, it shouldn’t be surprising that cracking occurs.
As material-handling vehicles or even shopping carts cross over these joints, their wheels can wear or break off the concrete edges. This is called “spalling,” which will continue to erode the concrete under these conditions. It can result in safety issues and potential damage to vehicles, just like when a small defect in a heavily trafficked road can grow to a pot hole that can cause accidents and/or vehicle damage.
To prevent the joint edge from experiencing repeated impact from traffic, a control joint filler is used. The filler is designed to support the anticipated traffic, to maintain edge-to-edge integrity, and to provide a smooth, contiguous surface that can be easily cleaned, will not harbor dirt, and will not support an environment for microbial growth.
To prevent uncontrolled cracking in concrete slab, control joints must be placed properly. For standard concrete, the maximum joint spacing depends on the thickness of the slab. The joints should be spaced (in feet) 2.5 times the slab thickness (in inches). For example, in a 6-inch-thick (0.15 m) slab, the joints should be no further apart than 2.5 times 6 between joints, which is 15 feet (4.57 m). The panel length should not exceed 1.5 times the width. The depth of the joint groove (saw-cut or tooled) should be one quarter the thickness of the slab, and not less than one inch. For example, our 6-inch slab should have a control joint depth of 1.5 inches (3.81 cm). The width of the joint will vary, although a quarter inch (0.64 cm) is the most common. The width of the joint will affect the degree that a joint filler can accommodate the joint expansion as a result of concrete shrinkage.