After placing repair material on concrete substrate,the moisture exchange takes place between the repair material and the concrete substrate,driven by the capillary pressure gradient.The resultant change of water content in the repair material affects the cement hydration process,the microstructure development and thus the bond strength.In this paper,the influence of moisture exchange on the degree of hydration and porosity of the repair material and on the bond strength between two materials was investigated.The experimental results show that the saturated concrete substrate can provide the additional water for the hydration of the repair material and lead to a higher degree of hydration.When placing a repair material on an unsaturated concrete substrate,the unsaturated concrete substrate can absorbs a large amount of water from the repair material.The water loss in the repair material results in a de-crease in water cement ratio.Consequently,the repair material cast on the unsaturated substrate shows a lower degree of hydration and a lower porosity compared to those cast on the saturated substrate.Also the decrease of water cement ratio leads to a high bond strength between the repair material and the concrete substrate.