OLD AND NEW CONCRETE ACTING TOGETHER AT BRIDGE REPAIRS

If the water-jet technique is used to remove the deteriorated top layer of a concrete bridge deck the deteriorated concrete can be replaced by a new concrete topping without providing the surface with stud bolts which traditionally have been used to ensure the shear capacity. The water-jet technique makes it possible to remove concrete selectively and to create a clean and rough surface without inducing any microcracks into the remaining concrete. A good bond between existing and new concrete also demands a careful compacting of the new concrete. Composite concrete slabs fulfilling these demands have developed ultimate loads comparable to those developed by slabs of similar thickness cast in one lift. For the replacement of deteriorated concrete on vertical and overhead surfaces shotcrete is more convenient than convential concrete casting with appurtenant complicated formwork. According to tests on composite beams shotcrete seems to be a good repair method if removing, cleaning and shotcreting operations all are carried out carefully. Furthermore, the interface between old and new concrete ought not to coincide with the reinforcement plane. (TRRL)