Universities Research Sustainable Shotcrete
Currently, the life of service tunnels are designed to last for at least 100 years or more, however, according to a team of researchers collaborating across a variety of universities, there are issues with how the expected service lives are being determined.
Specifically, Florian Mittermayr, researcher at the Institute of Technology and Testing of Construction Materials at Graz University of Technology says that, "The service life is currently calculated on the basis of theoretical key figures and empirical values. Environmental conditions such as chemically aggressive groundwater, for example, can possibly lead to cost-intensive maintenance measures earlier than expected.”
In tackling a more precise life expectancy, in a project jointly initiated by the Austrian Society for Construction Technology (ÖBV), TU Graz and OTH Regensburg, researchers worked to define a more systematic understanding of shotcrete applications, leading them to form a new basis for more durable concrete mixes in tunnels.
Looking at Shotcrete
To observe how different shotcrete formulations interacted within different environments and developing best practices, Wolfgang Kusterle from the Concrete Laboratory of the OTH Regensburg and the Austrian Society for Construction Technology, alongside Mittermayr initiated the "Advanced and Sustainable Sprayed Concrete (ASSpC) " research project.
"For four years, we investigated the process in its entirety in many laboratory and large-scale tests and provided scientific support at various tunnel construction sites," said Mittermayr. In their latest research efforts, the team examined how shotcrete should be ideally be processed and how to constitute binders for high durability.
According to the research findings, shotcrete can be designed more durable when cements, supplementary cementitious materials, admixtures and aggregates are better matched to the requirements. In the collaborated investigations, the team found that granulated blast furnace slag—in combination with other supplementary cementitious materials—is an effective way of increasing resistance against sulphate attack. These sulphate ions are traditionally observed as a result of gypsum dissolution and can be witnessed in soil or groundwater and lead to deformations and even cracks in the concrete.
Additionally, supplementary cementitious materials such as metakaolin or siderite can help to reduce the contribution of shotcrete to sinter formations (process of calcium carbonate precipitation) in the drainage system. These issues can further lead to the clogging of tunnel drainage systems, which can lead to tunnel closure for maintenance and repair.
The team also found that even the smallest addition of ultrafine limestone powder can significantly increase the early strength of shotcrete, making it possible to use the additives for more durable and sustainable shotcrete materials.
"Another milestone in terms of sustainability; we are particularly pleased about that,” said Sebastian Spaun, managing director of the Association of the Austrian Cement Industry. “The extended service life means that tunnels can now be maintained at longer intervals, the maintenance effort is reduced for the operator and for motorists this means less congestion. Also impressive is the research zeal of the universities, which is helping to position our building material more and more as an environmentally friendly material with precise and tailor-made formulations.”
The scientific consortium included researchers from the TU Graz Institutes of Applied Geosciences and of Technology and Testing of Construction Materials, the Concrete Laboratory of OTH Regensburg and the materials technology working group of the University of Innsbruck. The scientists were supported by "Austria's concentrated industrial shotcrete competence" (quoted by Kusterle).
This industry project submitted by the Austrian Society for Construction Technology (ÖBV) is being carried out in the framework of the Collective Research programme funded by Austrian Research Promotion Agency. The project results are available to the entire Austrian construction and concrete industry in line with the funding scheme.
Michael Pauser, managing director of the Austrian Society for Construction Technology concludes that, “The ÖBV-FFG research project is one more proof that these newly investigated and in practice already tested concrete formulations make a further contribution to the climate protection goals. The cooperation between universities, clients and the construction and building materials industry ensures that the research is practice-oriented and its results will be incorporated into the shotcrete guideline of the Austrian Society for Construction Technology, which is known beyond Austria's borders."