Microstructures and interfaces in concrete after 15 years of exposure in tidal environment

A detailed investigation on the microstructures (matrix with and without pores) and interfaces in concrete (aggregate-matrix and steel-matrix) specimens 150 mm in diameter and 300 mm in height was carried out. The specimens were exposed in a tidal pool for 15 years. Cement types were ordinary portland cement (OPC), Types A, B, and C (SCA, SCB, and SCC) slag cements, and Type B fly ash cement (FACB). The water-cement ratio was 0.45. The microstructure of the concrete was evaluated by scanning electron microscope (SEM). Interfaces in concrete were evaluated by SEM and electron probe micro analyzer (EPMA). Fibrillar morphology of C-S-H is observed in the pores of the specimens made with OPC and FACB. In the case of slag cement, however, dense morphology of C-S-H is observed. The microstructure at the outer region of the specimens becomes denser compared with the inner region. It creates a barrier against the chloride ingress, especially for SCB and SCC. Relatively more chlorides are observed at the aggregate-matrix interface at the outer region of the specimens. In the case of slag and fly ash cements, very little amounts of chlorides are observed at the aggregate-matrix interface at the inner region of the specimens. The steel-matrix interface is more porous than the aggregate-matrix interface irrespective of the cements and depth of cover concrete over the steel bars.