the depth limiting action of the sleeve 36 with respect to the sleeper the fastener 34 cannot be driven into the concrete so far as to precompress the resilient pads 26 further assurance that the fasteners are driven into the concrete the correct amount is provided by the fact that the stroke of the hammer 86 is limited the limited stroke of the hammer also substantially eliminates any possibility that the hammer will crush the sleeve with the foregoing apparatus anchoring of floor systems of the type described in u s pat no re37 615 is greatly facilitated the automatic mode using a magazine type driver is preferred over the manual mode not only because of time saving and convenience but because the manual mode requires a higher air pressure supply for the driver tool than does the magazine type that is because in the magazine type the mass of the fastener pre positioned in the hammer bore by action of the magazine is effectively added to the mass of the hammer during the driving action providing a more forceful impact for a given air pressure supply referring to fig1 it will be seen that the fastener may be provided with a smaller head 40 and a washer 58 placed inside the countersink 66 of the plastic sleeve 36 and in engagement with a bottom end wall 59 the washer 58 is engaged by the fastener head 40 when the fastener 34 is driven into the base 28 the.