Inspection of bridges for effects of air-entrainment on the porosity and compressive strength of concretes

Abstract

Air entrained concrete is used for outdoor structures in cold climate to enhance freeze-thaw resistance. A well dispersed air void system is incorporated in the fresh concrete, consisting of spherical air voids that provide expansion space for freezing water and thus act as pressure-relief. The amount of air entrainment (AE), which results from adding an air entrainment agent to the fresh concrete during mixing, needs to be controlled so as not to significantly reduce the strength of the hardened concrete. Excessive AE has been reported in some bridge concretes in Finland, triggering a comprehensive inspection of 96 bridges. The paper describes the testing methodologies used in this case-study of comprehensive national bridge-inventory level inspection, assess the main findings (compressive strength and porosity), and concludes on the lessons learned concerning the effectiveness of inspection methods. The large dataset of concretes, presented here, was treated statistically. The amount of AE in hardened concrete had been steadily increasing in recent years Salparanta et al. [1]. The study shows the importance of a representative number of samples when conducting inspections and found that the use of rebound hammer and core testing correlation to establish additional in situ strength measurements led to overly conservative results. It was verified that the immediate safety of the inspected bridges was not in question, and that a re-evaluation of the safety margins and special maintenance protocols may be required to assure adequate long-term service life for some of the bridges that exhibited exceptionally high AE.