Brief
Learn about the latest developments in contraction joints for slabs-on-ground that help minimize random cracks. Discover the new ACI code requirements and the technical discussion behind the effectiveness of early contraction joints.
Insight
This article focuses on the new developments for contraction joints in slabs-on-ground. The ACI Committee on Residential Concrete now recognizes the effectiveness of joints that are installed soon after concrete placement as early contraction joints.
This category includes tooled joints, inserts, and early-entry saw-cuts. The depth requirement for these joints is 1/5 the thickness of the slab or a minimum of 1 inch. A new insert type of contraction joint called a “Trak Joint” is available on the market, which is installed at the time of bull floating, making it an effective method to minimize random cracks.
Historically, the joint depth of 1/4 the thickness of the slab applied to all types of contraction joints for slabs-on-ground. However, with the introduction of early-entry sawcuts, a new criterion evolved where joints could be 1-in. deep for slab thicknesses of up to 9-in.
These joints were installed shortly after the slab was strong enough to support foot traffic. Many times, the conventional sawcuts were installed the day following concrete placement, allowing cracks to develop overnight. Field experience showed a significant improvement in reducing random cracks by using early-entry sawcuts.
The technical discussion of this article focuses on the three mechanisms that cause concrete to decrease in volume, resulting in random cracks in slabs. These mechanisms include drying shrinkage, thermal contraction, and chemical shrinkage. The first mechanism occurs slowly over days to years as water evaporates from the pores of the concrete.
The second mechanism occurs due to the high temperature of the internal concrete that decreases as the concrete hardens. The coefficient of thermal expansion permits calculation of the amount of contraction expected due to temperature change.
The third mechanism, chemical shrinkage, occurs when portland cement is exposed to water, and the cement particles partially dissolve, forming various types of crystals. Volume decrease occurs in the concrete by chemical shrinkage as cement hydrates.
Random cracks can occur in concrete on the first day due to chemical shrinkage.The article concludes by highlighting that the contraction joints installed earlier are more effective in activating than joints installed after the concrete is hard. The recognition of the effectiveness of joints that are installed soon after concrete placement as early contraction joints and the introduction of the “Trak Joint” can be effective methods to minimize random cracks in slabs-on-ground
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