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Economical and ecological concrete

   
 
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Traditional sprayed concrete, or 'shotcrete', is used world-wide as a preliminary tunnel lining. However, it has logistical, environmental, health and cost disadvantages, and the resulting concrete is not strong enough to serve as a permanent load-bearing lining.
In this project, three cement manufacturers, in partnership with a specialist producer of concrete additives and a materials research institute, developed new shotcrete formulations and application processes which have overcome all of the problems associated with the traditional product.
The new, patented shotcrete system has already been used successfully on major tunnelling projects in Germany and Austria, and will be marketed world-wide over the coming years.

In 1991, the German water authorities were asked to approve plans to build two massive tunnels as part of a high-speed rail link between Stuttgart and Ulm. They shocked the railways by rejecting the plans. They objected to the use of sprayed concrete, and specified stainless steel cladding throughout the 60 kilometres of the proposed tunnels, which would have added about 1,000 per cent to the cost of construction.
This local problem was the starting point for the project improvement of the shotcrete construction method. For a solution, the rail companies turned to Heidelberger Zement, the world's fifth largest cement producer. In partnership with two other cement manufacturers, an additives producer, and a scientific institute, Heidelberger Zement embarked on an urgent search for alternative shotcrete formulations and application techniques.

Shotcrete's well-known disadvantages

Sprayed concrete, or shotcrete, is an integral component of the New Austrian Tunnelling Method (NATM) now used around the world. Quick-setting concrete is sprayed onto the bare rock surface immediately after excavation, and rapidly hardens to form a preliminary support, protecting the tunnelling crew from rock falls until the final lining of conventional poured concrete can be installed.
Traditionally, shotcrete's quick-setting properties have been achieved by the injection of high-alkaline additives at the spraying nozzle. However, this method has always had its drawbacks. The resulting concrete is highly porous, and lacks strength. Caustic dust from the additives can cause skin and lung problems, and represents a real health hazard to construction workers.
Even from an economic standpoint, traditional shotcrete is less than ideal. Very rapid setting means that each succeeding layer is sprayed onto a hard surface. This means that as much as 30 per cent fails to stick and falls to the ground. It must then be shovelled up, transported out of the tunnel and disposed of, adding unnecessary costs and logistical overheads to the project.
The German water authorities were concerned about the environmental problems associated with conventional shotcrete. Due to its porous nature, large quantities of groundwater seep through causing caustic alkalines to be leached out of the concrete. These are washed into aquifers and rivers, where they constitute a serious polluant. Leaching causes problems for tunnel owners as well, because hardened leachate rapidly blocks the tunnel's drainage systems.

Meeting an urgent need

Finding a solution for the Ulm and Stuttgart tunnels was Heidelberger Zement's immediate task, but BRITE-EURAM funding gave the company the opportunity to undertake a much broader re-examination of shotcrete composition, application methods, costs and specifications.
Environmental and health problems were not confined to German tunnelling projects. Not far away, Austria and Italy were both engaged in large infrastructure projects involving extensive tunnelling, and the demand for 'clean', less expensive shotcrete was growing there too.
Cementi Buzzi from Italy, and Wietersdorfer und Peggauer from Austria, were happy to collaborate with Heidelberger Zement. They were joined by shotcrete additives manufacturer Heidelberger Baustofftechnik, and by the materials research institute of the University of Innsbruck.
The partners' 30-month work plan was ambitious. They were looking for a solution to meet the immediate environmental concerns of the German water authorities, but went further, aiming to improve the cost-effectiveness of shotcrete application as well as the strength, density and durability of the hardened concrete. They turned the problem into an opportunity, and worked towards developing a sprayed concrete capable of serving as a permanent load-bearing tunnel lining.

Vaulting ambition

There were two possible approaches. The one adopted by Heidelberger Zement's competitors was immediately available, but it had negative long-term cost and performance implications.
In normal cement, gypsum acts as a setting regulator, keeping the concrete workable for several hours. If gypsum is taken out of the shotcrete mix, then alkaline additives are unnecessary, but the cement sets immediately on contact with water. Expensive, oven-dried aggregates have to be used to prevent premature setting, and even then application is extremely difficult.
The partners took a different and technically more demanding approach. It involved the development of a new, non-alkaline additive which would retard setting for the two to three minutes necessary for easy spraying and improved adhesion, but which would then allow very rapid hardening.
By the end of the project, the partners knew that they would achieve and even surpass their objectives. By minimising the gypsum content, their formulation included no alkaline additives at all. The new additive could be premixed with the cement instead of being added at the spraying nozzle.
Perhaps most important of all, wet aggregate did not trigger the setting reaction, which only began when water was added at the nozzle. Gravel could be used straight from the quarry, avoiding the enormous cost and energy-consumption involved in oven-drying.

Multiple advantages

The products developed by the partners offers multiple advantages. Hardening is delayed for just long enough to ensure that the shotcrete remains workable until the next layer is sprayed, improving adhesion as well as strength, while at the same time greatly reducing dangerous, messy and wasteful 'rebound'.
Completely free of alkaline accelerator, the new formulations produces a far denser and less porous lining. Its early strength easily meets sprayed concrete certification standards, while final strength is up to 50 per cent greater than that of traditional shotcrete.
Additional logistical and cost benefits are offered by a new application technique which was also developed during the project. Where appropriate, the conventional system of pumping the shotcrete mix down a long pipe from the tunnel head can be completely avoided. Instead, hoppers on a specially equipped truck can be loaded with gravel and cement outside the tunnel and driven directly to the spraying site. Electronic controls regulate the mix, which is fed to traditional shotcrete mixing and spraying equipment.

Fifteen per cent of the German market

Even before the project ended, the partners had started to promote the benefits of their system to civil engineers and public authorities. The Stuttgart and Ulm tunnels had by this time been cancelled, but the technical and economical advances of the new system very quickly enabled them to win contracts to supply 30,000 tonnes of the new cement for the construction of tunnels in Austria and southern Germany. This gave the new shotcrete a 15% share of the German market in its first year.
These contracts allowed the partners to refine the application process, and to demonstrate in a very visible way the flexibility and cost-effectiveness of their system. The concrete's high strength and low leachability have now been thoroughly tested, and the new ecological shotcrete will be used for most of the tunnels in a new high-speed rail link between Frankfurt and Köln, to be started in 1996.
Cementi Buzzi is also producing the new cement, and Wietersdorfer und Peggauer will start to do so as soon as new equipment has been installed. Heidelberger Zement, with subsidiaries throughout Europe, as well as in China and the United States, is currently waiting for approval of its patent for the formulation and the application technique, and will exploit the project's outcomes world-wide.

 

Project Title:  
Improvement of the shotcrete construction method concerning quality, safety, health, environmental compatibility and economic efficiency.

Programmes:
Industrial and Materials Technologies (BRITE-EURAM/CRAFT/SMT)

Contract Reference: BE-5189

Cordis DatabaseFor more information on this project,
go to the CORDIS Database Record

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