The air permeability, carbonation and chloride content along a concrete highway underpass

Abstract

In reinforced concrete structures carbonation and chloride ingress are two main phenomena that induce rebar corrosion. Agents such as CO2 gas or water containing chloride enter the cementitious material mainly throughout the pore system. The air permeability was measured with a non-destructive rapid test method. The aim of the present work was to identify a relation between the carbonation, chloride ingress and the permeability to air of a reinforce concrete underpass. Concrete cores were drilled from a 40 years old in service highway underpass located in the South alpine region. The carbonation values were between 11-25 mm after 40 years exposure to the atmosphere. They corresponded to the conventional carbonation rate, in spite of the cyclic exposure condition of the artefact. In general, the wing wall east and west exhibited lower values as compared to the shoulders. This also indicated the importance of the geometry, orientation and exposition of the structure. The chloride content were generally well beyond 0.025 % referred to the concrete mass. This was due to the location of the structure close to the Alps at 1000 meters above sea and the spreading of the deicing salts during winters. The chlorides were leached from the concrete surface, so that higher contents were sometimes found with concrete depth. A correlation existed between the carbonation (CO2 entrance) and the permeability to air. The higher the permeability, the deeper the carbonation. At concrete depths 0-20 mm chloride ingress and carbonation seemed to be antagonist phenomena. They could not occur simultaneously to a great extent. When humidity is present, gas penetration is lowered. While from 30 to 40 mm chloride ingress, transport and carbonation appeared to be both present.