A screw includes a head and a shank extending from the head to terminate at a frusto-conical tip portion. A first thread section has asymmetric upper and lower thread surfaces, a plurality of V-shaped notches formed at a juncture of the surfaces, and a fillet surface formed at a juncture between the shank and the upper thread surface. Two second thread sections are disposed adjacent to each other and spaced apart from the first thread section. A third thread section extends from the first thread section toward the tip portion and has an outer surface of an arc-shaped cross-section. The screw-in movement of the screw is guided by the third thread section to prevent the screw from being slanted relative to a concrete object.