Influence of substrate moisture conditions on microstructure of repair mortar and water imbibition in repair-old mortar composites

Appling fresh mortar onto the surface of old concrete has been a common repair and reinforcement method. In this work, the influence of initial moisture conditions of old mortar on the microstructure of repair mortar was checked. Two types of composites were prepared by bonding fresh repair mortar with a pre-dried or a watersaturated old cement mortar. By analyzing X-ray CT images, severe shrinkage cracking was found in the repair mortar bonded with a pre-dried old mortar. A large discontinuity was revealed between repair mortar and pre-saturated old mortar by X-ray CT images. Various porosities in the repair mortar were identified by Mercury Intrusion Porosimetry tests. Dynamic water imbibition in the two types of composites was followed by neutron radiography. Shrinkage cracks in repair mortar can significantly promote water infiltration in both repair mortar and substrate, which poses a threat to the durability of the repair system.