Durable Overlay Systems with Engineered Cementitious Composites (ECC)

This paper reviews recent research on the application of an engineered cementitious composite (ECC) as overlay in the repair of deteriorated concrete structures. ECC is an ultra-ductile fiber reinforced cement based composite, that has metal-like features when loaded in tension. The uniaxial tensile stress-strain curve shows a ‘yield’ point, followed by strain-hardening up to several percent strain, resulting in a material ductility of at least two orders of magnitude higher in comparison to normal concrete or standard fiber reinforced concrete. ECC also has unique cracking behavior. When loaded to beyond the elastic range, ECC maintains crack width to below 100 μm, even when deformed to several percent tensile strain. Experimental testing of ECC overlay reveals significant improvements in load carrying capacity and in system ductility over conventional concrete or steel fiber reinforced concrete overlays. The commonly observed overlay system failures by delamination or by spalling are eliminated when ECC is applied. Numerical simulation of differential drying shrinkage also confirms the superior performance of ECC. In the presence of a vertical joint, large crack formation and/or delamination are observed in mortar and fiber reinforced concrete overlays. These failure modes are suppressed when ECC is used as the overlay material. The unique material behavior of ECC makes it an ideal candidate material for durable overlay applications.