Shotcrete, concrete applied pneumatically at high velocity, has the potential to save costs and construction time when used to replace cast-in-place concrete for retaining walls and slope stabilization. It is becoming popular for vertical and overhead applications where conventional formwork and repairs are difficult and costly.
However, the life expectancy of structures that incorporate shotcrete can be compromised if the bond between the shotcrete and the substrate is not well developed. In addition, long-term freeze-thaw weathering in northern states can degrade the bond strength of the interface and result in both debonding from existing substrate concrete structures and corrosion of the rebar. Therefore, proper shotcrete placement and accurate characterization of its bond properties are paramount to successful shotcrete performance and durability.
This study evaluated the shotcrete-concrete interface bond by testing four representative substrate surface preparation methods—chipping, pressure-washing, sandblasting, and no preparation—under three different loading conditions: tensile, shear, and Mode-II fracture. The study also investigated the long-term freeze-thaw durability of these bonds and introduced a probabilistic damage model to predict their service lives. The estimated surface texture depth and bond behavior were also correlated by using laser texture scans of the substrate.
The results indicated that chipped substrates produced the highest interface bond strength and mode-II fracture energy. Of the other surface preparation methods, pressure-washing produced an interface bond that was more tensile-resistant, and sandblasting led to an interface bond that was more shear-resistant. Therefore, the researchers recommended that agencies consider the actual loading on the shotcrete-concrete interface bond when they choose appropriate surface preparation and testing techniques.
The researchers proposed recommendations to be included in shotcrete specifications. Specifically, they recommended a tensile bond minimum strength of 145 psi for overhead shotcrete applications and a shear bond minimum strength of 300 psi for slope stabilization with shotcrete. They also recommended that a chipped surface preparation be used whenever possible. Otherwise, pressure washing can be used for the application of shotcrete to overhead structures and sandblasting for slope stabilization. Because the predicted service life of shotcrete-concrete interface bonds is significantly shorter in regions prone to higher numbers of freeze-thaw cycles, the researchers recommended that methods to improve the long-term durability of bond strength be further investigated.
With the increasing desire of highway agencies to use shotcrete to save time and money, the results of this project will be helpful to them in achieving the best structural quality and durability.
Authors:
Haifang Wen
Pizhong Qiao
Ayumi Manawadu
WSU Department of Civil and Environmental Engineering
Sponsor: WSDOT
WSDOT Technical Monitor: Scott Sargent
WSDOT Project Manager: Mustafa Mohamedali