Many contractors use long-strip construction as an efficient concrete placement method for slabs on grade. It involves placing concrete in alternate long strips, then filling the adjacent strips using the existing slabs as edge forms. Contractors must understand, however, that long-strip construction affects levelness. ACI 117 specifies a level alignment of + or - 3/4 inch of the top elevation of a slab on grade. The two alternate strips can satisfy the ACI tolerance for top-of-slab elevation and levelness, but the in-fill strip may not satisfy the levelness criteria. Alternate concrete strips provide the edge forms for the infill strips and these edges can change elevation. Infill strips are always placed to match existing elevations of the two adjacent strips. If an adjacent concrete strip curls or settles before an infill strip is place, the elevation difference, and therefore the levelness, may be worse. Slabs exposed to different temperature or moisture conditions create moving edge forms. Especially susceptible are slabs half-protected and half-exposed to the environment, such as a slab that extends beyond the roof line.