“We must enhance the sustainability of our infrastructure,” argues Claudia Ostertag, T.Y. and Margaret Lin Professor of Engineering. “Concrete structures are deteriorating at a much faster rate than expected, resulting in a massive need for repairs and premature replacement that costs billions of dollars annually.”
Deterioration is caused by mechanical loading conditions and expansive deterioration processes (corrosion, frost action, alkali-silica reaction, and sulfate attack) that create tensile stresses and cause the concrete to crack.
Cracks in concrete begin as small microcracks. Once microcracks become interconnected and form macrocracks, ingress of water and aggressive ions is enhanced and the deterioration process accelerated.
In order to enhance the service life of concrete structures, concrete must be designed so that it complies with performance criteria such as crack resistance, high durability, and deflection or tension hardening behavior.
Ostertag and her research group, Gabriel Jen, William Nguyen, Rotana Hay, and Jake Duncan, have developed such a material.
“We have developed a performance-based materials approach called Deterioration Reduction through Multi-scale crack Control (DRMC),” says Ostertag. “DRMC enhances the durability and service life of concrete structures.”
Multi-scale crack control is achieved by combining the use of micro and macrofibers. The microfibers control microcracks, and macrofibers resist and delay subsequent macrocrack formation.
“DRMC is a holistic approach,” explains Ostertag, "It provides multiple lines of defense against damage due to both mechanical and environmental loading conditions by reducing the rate of damage initiation and damage progression. Contrary to the more traditional approach which treats each deterioration process in isolation and proposes different remedies for each expansive deterioration mechanism, her approach concentrates on the common signature to all expansive deterioration processes, which is cracking."
DRMC limits the ingress of aggressive agents and water to the reaction sites, since crack formation is delayed up to strain levels exceeding the yield strain of conventional steel reinforcing bars. It also limits the egress away from the reaction sites due to the presence of microfibers which bridge microcracks at onset in close vicinity to the reaction sites.
“The control of these microcracks is essential for enhancing the durability of reinforced concrete structures,” says Ostertag. The microfibers pin the crack surfaces, thereby mechanically confining the reaction product (i.e. alkali silica reaction gel or corrosion product), and preventing the reaction product from leaving the reaction site.
The lack of egress modifies the density, the composition, and the amount of the alkali silica gel and corrosion products. It also reduces their reaction rates. In case of alkali silica reaction (ASR) it was the change in the ASR gel composition that starved the reaction.
“DRMC, based on materials science and fracture mechanics principles, has the potential to provide us with materials that enhance the service life and the sustainability of concrete structures,” says Ostertag. “It also has the potential to spawn new investigations of the deterioration mechanisms.”
Hybrid fiber reinforced concrete (HyFRC) composites developed according to the DRMC approach and tested under severe environmental and seismic loading conditions exhibit superior performance in regards to durability, crack resistance, and ductility.
When exposed to seismic loading conditions, the HyFRC delayed damage initiation and spalling in bridge columns. Because of its superior crack resistance, when it was exposed to severe environmental loading conditions, HyFRC provided excellent corrosion resistance to the embedded steel reinforcing bars.
Conventional concrete not only revealed severe cracking due to the expansive nature of the corrosion products, but it also completely lost its load-carrying capacity due to splitting crack formation and loss of bond to the rebar.
Damage of reinforced concrete versus reinforced HyFRC after 2 years of exposure to chloride induced corrosion
The superior crack control in HyFRC prevented the splitting crack formation associated with corrosion. “Even after two years of exposure to a chloride environment,” says Ostertag, “the reinforced HyFRC remains nearly free of corrosion damage.”
T.Y. and Margaret Lin Professor of Engineering
CEE Vice Chair for Research & Technical Services