A cold joint is a discontinuity between layers of concrete caused by curing before the rest is poured. It can cause visual defects, moisture infiltration, and compromised strength. Avoidance or mitigation is possible through waterproofing, surface preparation, rebar insertion, or strategic placement.
A cold joint is an unwanted discontinuity between layers of concrete that occurs when one layer of concrete is allowed to cure before the rest of the concrete is poured into what should be a single, solid mass. Discontinuity occurs between layers due to the inability of freshly poured, wet concrete to adequately mix and bond with the cured concrete. Such discontinuity is often the result of logistical issues such as a contractor’s work schedule or an unexpected material shortage.
Problems associated with cold joints range from relatively minor to very serious. At the less severe end of the spectrum, a cold joint can result in a visually unappealing discontinuity that is visible on the surface once the concrete has hardened. This type of cosmetic defect can simply be hidden rather than repaired.
A more serious problem associated with a cold joint is the possibility of moisture infiltration into the concrete section. If water settles in the joint, it can lead to degradation of the concrete under certain environmental conditions. For example, because water expands when it freezes and then contracts when it melts, water trapped in a cold joint can cause material to crack or erode. Moisture can also damage other things besides the concrete mass if it is able to seep all the way through.
Also, a cold junction is an area of compromised strength. Concrete is distinguished by its high compressive strength, but is much weaker under tension. A cold junction is even weaker under tension and is susceptible to shear at discontinuity.
Whenever possible, these joints should be avoided in concrete construction by completing the entire pour for a given section in one session. This allows the entire section to harden into a solid, continuous mass. If this is not possible, there are several steps you can take to mitigate more serious problems.
Special waterproofing joint sealant can be applied to the joint to make it waterproof, thus protecting it from potentially harmful moisture infiltration. A special surface preparation can be applied to the hardened layer before applying a new layer of concrete. This will strengthen the bond between the two layers. Another way to increase cold joint strength is to insert rebar, or rebar, into the first layer before pouring the next layer. This will help bind them together better and increase the tensile strength of the joint. It is also sometimes possible to locate the weakened joint in an area that is not critical to supporting a heavy load.
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