What are "cold joints" in the context of concrete?

Cold joints occur when two separate batches of concrete are poured at different times, leading to an incomplete bond between the cured concrete and the fresh concrete. This situation arises when the first batch begins to harden before the second batch is placed, which can happen if there is a significant delay between pours. The interface where the two batches meet does not achieve the same strength as a monolithic pour, which can adversely affect the integrity and durability of the concrete structure.

The phenomenon of cold joints is critical for concrete flatwork finishing because it can lead to issues such as water infiltration, reduced load-bearing capacity, and the potential for cracking. Understanding cold joints can help finishers to manage their pour schedules diligently to minimize their occurrence and ensure a continuous, strong, and cohesive concrete surface.