The present invention provides a hammer drill attachment for contacting external walls of formwork for imparting external vibration to concrete poured into the formwork, the attachment comprising: an input shaft configured to receive motion from an output of a hammer drill; an output shaft configured to contact the external walls of the formwork for imparting the external vibration, the output shaft being arranged transversely relative to the input shaft to output pulsed percussions in a transverse direction relative to a longitudinal axis of the output shaft, a connecting portion for connecting the output shaft with the input shaft to transmit motion from the input shaft to the output shaft when the input shaft is receiving motion from the output of the hammer drill. In another aspect of the present invention, there is provided a method for smoothing concrete contained within external walls of formwork.