In recent years, some states have replaced cast-in-place (CIP) concrete with shotcrete for structural earth retaining components such as fascia walls. Shotcrete also has the potential to be a solution for traditional reinforced concrete retaining walls—if it shows economic benefits and good long-term performance. This study’s aim was to evaluate the freeze-thaw durability of shotcrete in cold regions and to develop better ways to predict its long-term performance.
Although shotcrete is promising, certain of its characteristics could potentially reduce the 75-year life expectancy of retaining walls, particularly in cold regions. Furthermore, shotcrete is prone to early drying shrinkage cracking and debonding from reinforcing bars or existing structures, compounding long-term durability concerns, especially in cold regions.
This research, in combination with related research work for the Washington State Department of Transportation (WSDOT) (see WA-RD 870.1), applied a novel testing protocol to characterize long-term shotcrete performance. The researchers also sought to evaluate the damage accumulation and durability of shotcrete in cold temperatures with damage mechanics models and to recommend test methods for long-term performance characterization of shotcrete in cold regions.
One benchmark mix design from WSDOT was chosen to prepare samples for evaluation. Long-term freeze-thaw performance after certain numbers of cycles was evaluated by using the dynamic modulus of elasticity test, fracture energy test, and X-ray CT microstructure imaging analysis.
Because of the smaller degradation ratios obtained with the fracture energy test, it was found to be a more sensitive test method than the dynamic modulus of elasticity for screening material deterioration over time and for capturing accumulative material damage caused by rapid freeze-thaw action.
X-ray CT imaging analysis was found to be capable of detecting microcracks that formed and pore evolution under rapid freeze-thaw attacks in the aggregate, cement matrix, and interface transition zone of conditioned samples.
The relationship between the degradation of the dynamic modulus/fracture energy of shotcrete and the number of freeze-thaw cycles was established at different damage levels. As expected, the failure rate increased as the number of freeze-thaw cycles increased, indicating that shotcrete structures exhibit more potential risk of failure and less reliability as service life goes on and freeze-thaw attacks continue.
The findings of this study will benefit the construction and maintenance of transportation infrastructure in cold climates by improving the testing of accelerated aging in shotcrete, the evaluation and quantification of its damage and durability, and the understanding and prediction of its long-term performance and failure mechanisms.
Authors:
Pizhong Qiao
Zhidong Zhou
WSU Department of Civil and Environmental Engineering
Sponsors:
Center for Environmentally Sustainable Transportation in Cold Climates
WSDOT