The lack of durability in concrete repairs induces repair failures in field and endless “repair of repairs”. Most often drying shrinkage of “new” repair material restrained by “old” concrete substrate causes cracking in the repair material, combined with interface delamination between the repair and the concrete substrate. The surface cracking and interface delamination may also introduce chlorides, oxygen, moisture, alkali or sulphate into the repaired concrete structure and accelerate further deterioration.
This research proposed a material solution to the described drying shrinkage induced concrete repair failures. Engineered Cementitious Composites (ECC) is a material micromechanically designed with high ductility and toughness indicated by multiple micro-cracking behavior under uniaxial tension. Experimental study was conducted on layered repair systems based on concrete, SFRC and ECC repair materials respectively. Under environment with the same relative humidity and temperature, the ECC repaired system exhibited the most desirable performance despite the fact that the ECC’s drying shrinkage strain was higher than the other two repair materials. The crack width of the ECC repair and the interface delamination were both very small (<60µm), which was ideal for achieving durability. Conversely, the concrete repaired system had several localized fractures with much bigger crack width (220-270µm). The delaminated length of both ECC and concrete repaired system was around 50mm. Surprisingly, although SFRC repair had the smallest shrinkage strain, the SFRC repaired system exhibited both large crack width (120-140µm) and large interface delamination height (275µm) and length (350mm), which could be severe enough for introducing undesirable agents into the repaired system, resulting in a loss of durability.
The experimental study verified that the high ductility of ECC can relieve shrinkage induced stresses in the ECC repair layer and at the ECC/concrete interface, thereby suppressing large surface cracks and interface delamination. The concept of translating ECC repair material ductility to the whole repair system durability can be widely applied to many concrete structures repair applications for developing cost-effective and durable concrete repairs.
Fig. 1 Layered repair system configuration and potential failure modes