The Underground Committee of the American Shotcrete Association (ASA) has published two new position statements covering two critical topics of underground shotcrete application – the application of shotcrete overhead and shotcreting on synthetic sheet waterproofing membranes.
ASA is a non-profit organization of contractors, suppliers, manufacturers, designers, engineers and owners with a common interest in advancing the use of shotcrete. The vision of ASA entails that “Structures built or repaired with the shotcrete process are accepted as equal or superior to cast concrete.” The mission of the ASA supporting the vision is to provide knowledge resources, qualification, certification, education and leadership to increase the acceptance, quality and safe practices of the shotcrete process.
The two new position papers titled “Spraying Shotcrete Overhead in Underground Applications” and “Spraying Shotcrete on Synthetic Sheet Waterproofing Membranes” were developed by ASA’s Underground Committee, that is chaired by the author. The purpose of the position statements is to provide the underground and shotcrete industry guidance and a common technical understanding of the topics covered, following the mission of the Underground Committee to educate and promote the use and proper application of shotcrete in the underground construction and mining industry. The position statements were developed by two task groups, consisting of Frank E. Townsend, Axel G. Nitschke and Lihe “John” Zhang for the overhead paper, and Frank E. Townsend, Axel G. Nitschke and William T. Drakeley for the waterproofing statement.
Spraying Shotcrete Overhead in Underground Applications
In underground applications, spraying shotcrete overhead is a common and often daily operation but the successful application is challenging. Shotcrete placement propels concrete at high velocity against the overhead surface and the dynamic energy compacts the concrete in place. However, plastic shotcrete has no immediate strength. Depending on the applied thickness, the weight of the shotcrete layer can be significant. The weight of a single layer or multiple layers of shotcrete can be enough to pull the shotcrete down and cause local or largescale fallouts, which may pose a significant safety hazard and lost productivity. In addition, improper application of overhead shotcrete and other circumstances can lead to delamination and voids in the installed shotcrete.
The position statement introduces the basic elements of the adhesion and cohesion of overhead shotcrete, proper application techniques, and a discussion about socalled “reentry criteria” for working under freshly installed shotcrete. The position paper also provides recommendations for contractors and owners from ASA’s perspective on not only how to properly apply shotcrete, but also on how to specify and inspect overhead shotcrete in underground projects and summarizes the topic in a conclusion.
Understanding the adhesion and cohesion effects is a key element of overhead shotcrete:
“During the plastic shotcrete phase, the adhesion of the plastic material to the ground surface in combination with the cohesion of the plastic shotcrete material to itself are the forces acting against the selfweight of the plastic shotcrete. Because the chemical reaction of the cement in the mixture is still in progress and has not yet created significant strength, the plastic shotcrete sticks overhead only if the conditions for adhesion and cohesion are right.”Further key elements of preparation and execution to achieve proper conditions are discussed in more detail and typical mistakes are highlighted.
The position statement also provides guidance for the proper application of shotcrete overhead by covering mixture proportioning, surface preparation, the thickness of each pass, utilization of anchors, lattice girders, and reinforcement. In addition to discussing shotcreting procedures and techniques, it also highlights the importance of qualification of the key personnel.
One of the key questions for overhead shotcrete operations regarding safety is the definition of a re-entry criteria, that allows crew movements underneath the newly installed overhead shotcrete. Although the position statement provides some general guidance, it also emphasizes the need for a project-specific definition and evaluation of the re-entry criteria.
Spraying Shotcrete on Synthetic Sheet Waterproofing Membranes
Applying shotcrete for final linings against synthetic sheet waterproofing membranes has become more common in underground projects. The sprayed shotcrete is propelled at high velocity against the membrane and the dynamic energy consolidates the plastic shotcrete in place. However, the smooth surface and potential pillowing or vibrating of the waterproofing membrane poses a challenge compared to a shotcrete application against a stiff and rough surface.
The use of shotcrete final linings in underground applications is a standard placement technique. However, improper application of shotcrete against waterproofing membrane in a double shell system, with a separate temporary lining at the extrados and the final lining at the intrados of the membrane, can lead to poor-quality shotcrete due to delamination and voids, and produce local or large-scale fallouts, and can be the subject of technical disputes about cause and effect responsibilities.
ASA’s position paper about spraying shotcrete on sheet waterproofing membranes discusses the basic elements of the adhesion of overhead and vertical shotcrete on a waterproofing membrane, the proper application techniques, and emphasizes the use of mockups. This position paper also provides recommendations for contractors and owners from ASA’s perspective, on how to properly apply, specify and inspect shotcrete applied against waterproofing membranes.
The key technical challenge is the weight of the plastic shotcrete before it gains sufficient strength to support itself:
“Adhesive forces of the plastic shotcrete can only be transferred into the initial lining at the fixation points of the membrane or where the membrane is supported by a self-supporting reinforcing bar cage.
Reinforcement and lattice girders provide an important element during the installation of shotcrete. It is important to understand this concept and ensure that either the self-supporting rebar and/or the anchoring system is strong enough. However, the weight to be carried strongly depends on the shotcreting procedures and patterns as well as the strength development of the shotcrete over time.
When using fiber-reinforced shotcrete on top of the waterproofing membrane, the potential for damaging the membrane by fibers is a frequent question. There are two different scenarios. The first scenario deals with fiber-reinforced shotcrete in the substrate. To avoid damages to the membrane typically a non-fiber-reinforced smoothing layer is installed. The second scenario is applying fiber-reinforced shotcrete as a final lining on top of the membrane.
“Spraying steel fiber-reinforced shotcrete does not damage or puncture the membrane because the forces acting on the fiber are not strong enough to push the fiber into the membrane and typically the fibers tend to orient parallel to the membrane on impact.”
Coordination between the membrane installer, the reinforcement installer and the shotcrete contractor is another key element. An improperly installed membrane can lead to performance issues of the shotcrete lining.
The recently published ASA position papers covering applying shotcrete overhead and on synthetic sheet waterproofing membranes address two technical applications of shotcrete, which are frequently subjects of discussion on underground projects. The position papers provide guidance in areas that are currently not well covered by other guidelines or generally accepted literature. ASA’s goal is to support the industry by filling these gaps. Both position papers are available for download, free of charge, on ASA’s website www.shortcrete.org under the Products/Services & Information tab.
ASA welcomes feedback from the industry and is available as a resource. If you are interested, feel free to join ASA and the ASA Underground Committee.
Axel G. Nitschke is Tunnel Practice Leader (NATM) for WSP USA. He is an ASA Executive Committee Member and Treasurer, and Chair of the ASA Underground Committee.