support an engine 15 to which the inner ends of the spider frame bars 14 are attached a disadvantage in the use of the conventional machines as thus far described is illustrated in fig4 of the drawing where the concrete 12 is to be finished adjacent a vertical obstruction such as the wall 16 the guard ring 13 abuts the wall at a spacing of an inch or two from the blade tips 17 accordingly the concrete 12 between the blade tips 17 and the wall 16 is not subject to the smoothing action of the paddles 11 and a narrow furrow or band 18 of rough concrete is left adjacent the wall 16 if not removed this furrow 18 will harden and remain adjacent the wall and will constitute an imperfection in the finished concrete furrow 18 can be removed or smoothed by hand but this task involves laborious and expensive hand smoothing operations in accordance with the present invention the guard ring 13 is provided with one or more relieved segments which are or can be offset inwardly of the guard ring circle thus to permit the blades 11 to be brought into near proximity to a wall in the embodiments of fig1 3 and 5 the ring.