This shotcrete overlay failed as a result of several factors—specification of improper materials and finishing methods, poor workmanship, and lax inspection and quality assurance programs. Had cracks in the underlying concrete been sealed properly to stop leaks, and had wet-mix shotcrete been applied by a well-trained, conscientious crew, city officials likely would have gotten the results they wanted.
Shotcrete is defined as “mortar or concrete pneumatically projected at high velocity onto a surface.” Application of shotcrete can be an effective, efficient construction and repair method, but it relies heavily on proper technique and careful workmanship. Two publications from the American Concrete Institute—ACI 506.2-95 Specification for Shotcrete and ACI 506R-90 Guide to Shotcrete—provide detailed information on the materials, properties, and use of shotcrete, including application procedures, equipment requirements, and crew responsibilities. ACI 506R-90 also discusses preconstruction, prequalifying, and acceptance testing of workers, materials, and finished shotcrete. In addition, ASTM has numerous standard test methods and specifications that pertain to shotcrete.
A key quality issue is obtaining sound shotcrete behind reinforcing bars. In the Dix Reservoir project in Barre, Vt., the gridlike pattern of cracks and calcite deposits on the training walls clearly indicated voids behind the steel reinforcement. A qualified nozzle operator knows how to position and move the nozzle so that shotcrete will encase the bars and not produce voids behind the rebar.
Preconstruction test panels are the best way to qualify a shotcrete crew and ensure the mix design and application can provide acceptable results. A panel is fabricated by shooting shotcrete onto a form of heavy plywood or steel plate. The test panel should match the thickness of the structure, and at least part of it should contain the same reinforcement configuration as the structure.
Cores or cubes obtained from the sample panels should be tested for compressive strength and other properties considered important for the project. As prescribed in ACI 506R-90, cut surfaces of the specimens should be carefully examined, and additional surfaces should be exposed by sawing or breaking to check soundness and uniformity. All cut and broken surfaces should be dense and free from laminations and sand pockets. ACI 506.2 contains a core grading system used to determine whether or not the work is of acceptable quality.
Public works officials can use the ACI shotcrete guide and specification as educational tools that will help them to recognize and insist upon quality shotcrete work. One way to increase your chances of getting a qualified shotcreting crew is if they have at least one member who has been certified as an ACI Certified Shotcrete Nozzleman.
— Steven H. Gebler is a senior principal engineer with CTLGroup, Skokie, Ill.