Both pour lines and cold joints are common in concrete construction, but cold joints have more serious consequences. Using case histories, the authors discuss several testing methods that can be used to evaluate whether a visible pour line found on an exposed concrete surface is just a noncritical surface imperfection or a sign of a significant structural defect.
Evaluating Core Samples
The first step in evaluating a surface line is realizing that the important information does not lie at the surface. Core samples must be removed from representative locations within the structural element under consideration. Core samples should coincide with the surface line and be located such that the surface line is at the center of each core. Then each core must be visually examined (both by the unaided eye and by microscope) and tested. Visual examination can determine how far the line extends beyond the surface. Microscopic examination can identify evidence of carbonation, drying, or changes in paste microstructure--all conditions that indicate a possible cold joint.
Another step in core evaluation consists of performing split-cylinder tests in accordance with ASTM C 496-96 (Ref. 3). Cores must be tested with the pour line or cold-joint line oriented vertically at the center of the test machine. A statistical evaluation of the tensile test values is then compared with the expected strength of monolithic concrete. If the value falls below the expected strength, structural implications should be investigated.