Abstract: A method and apparatus for forming a longitudinal joint in a concrete surface. A vertical knife is mounted on a concrete slip-forming machine and extends to a predetermined depth in the concrete surface. As the knife is pulled through the formed concrete by the machine, aggregate or reinforcing fibers are deflected to either side of the knife. An aggregate- or fiber-free strip of concrete paste fills in the gap behind the knife creating a weakened zone which acts to constrain the formation of cracks in the concrete surface to the formed joint. The longitudinal joint in the roadway is formed without sawing, cleaning, or sealing and does not require maintenance.