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
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
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.
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.
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.