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The adhered cement mortar in coarse aggregates and fines from demolished concrete has the potential to induce a residual cementing property upon recycling. A procedure for quantifying the contribution of recycled fines to the strength gain within the new mortar matrix is proposed. The strength gain is found to be more significant in recycled aggregates and fines from brick aggregate concrete than in those from stone aggregate concrete. Isothermal calorimetry measurements indicate the existence of large heat flows immediately after wetting and a prolonged initial reaction period during the hydration of recycled fines due to the presence of unreacted cement compound fractions and depleted amounts of gypsum. This phenomenon has been further confirmed, particularly in recycled fines from brick aggregate concrete, through electron microscopy observations of the formation of new gel structures due to rehydration. The chemical compositions determined using two independent methods indicated possible interactions between CH (the hydration product) and pozzolan (from brick) to induce strength gain in new mortars and new concrete obtained by recycling brick aggregate concrete. This explanation is consistent with the strength test results and the evolution of the heat events observed calorimetrically.

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An experimental investigation was carried out on chloride induced corrosion of steel bars in concrete under marine environment with the variation of micro-structure of steel-concrete interface. To create the variation of micro-structure of steel-concrete interface, steel bars coated with cement paste of different W/Cs and cement types were used. For investigation, cylindrical concrete specimens were made with steel bars (with and without cement paste coating). W/C ratios of cement paste coat were 0.3, 0.5, 0.7 and 1.0. The specimens were exposed to an accelerated seawater exposure controlled with automatic wetting and drying. The specimens were tested till 45 cycles in the exposure. Test items include compressive strength of concrete, chloride ingress into concrete, electrochemical evaluation of corrosion, microscopic investigations of steel-concrete interface by optical microscope, scanning electron microscope (SEM), and linear traverse. It is revealed that the initiation of corrosion is significantly influenced by the nature of micro-structure of the steelconcrete interface, such as size of voids at the steel-concrete interface.

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