Macromechanical Properties and ITZ of Lightweight Aggregate Concrete from the Deck of Nanjing Yangtze River Bridge after 50 Years

Abstract

Lightweight aggregate concrete (LWAC) is a widely used material in civil engineering. This paper aims to evaluate the macro- and microcharacteristics of lightweight aggregate concrete which has served as the deck in the Nanjing Yangtze River Bridge for 50 years. The average compressive strength and the stress-strain curves were obtained from limited-size core specimens. The interfacial transition zone (ITZ) between lightweight aggregate and bulk cement paste was characterized by scanning electron microscopy (SEM), backscattered electron microscopy (BSEM), and nanoindentation to determine the morphological, mineralogical, and mechanical features. The results show that the average compressive strength, obtained from limited-size core specimens, is 76% of the initial compressive strength after 50 years of service in a real condition, including freeze/thaw and dynamic load. The ITZ of LWAC is not apparent in SEM and BSEM micrography owing to the long-term hydration of cement around the lightweight aggregate. The ITZ is around 100 μm thick according to the micromechanical properties, including indentation modulus and hardness by nanoindentation.