Sprayed concrete has been widely used by the
European construction industry in stabilising rock tunnels. It has
also been used as a primary liner for soft ground tunnels, but has
never been seriously considered as part of the permanent works.
This is because of a lack of confidence in the long-term structural
and durability performance of the material and the absence of a
suitably detailed modelling and design process that takes into account
the complex interactions between ground and lining. The process
does, however, have the potential to provide tunnel designers much
greater flexibility in terms of complex tunnel junctions and passenger
access points in underground transportation networks. This potential
created an impetus to examine the possible problems with a view
to finding solutions and, as a result, a consortium of European
construction companies and researchers came together under a Brite-Euram
project. After four years of work, new equipment and formulations
have now been developed that allow sprayed concrete to be used to
its full potential.
Finding the right recipe
One of the first aims of the project was to
characterise concrete mixes with respect to strength, durability
and application properties. The project co-ordinator, Mott MacDonald,
a UK construction and civil engineering company, worked closely
with British SME Sprayed Concrete on this aspect. Sprayed Concrete
specialises in wet application techniques. This involves mixing
the concrete in a conventional manner and then adding air and an
accelerator at the spraying nozzle. The technique produces far less
dust than the alternative 'dry' application whereby water is added
at the nozzle. Consequently, it is better for both the health of
workers and in terms of material wastage.
A large number of concrete mixes were characterised and several
important guidelines for mix design were established. The mixes
were further refined with help from two other project partners,
Taywood Engineering Ltd, another UK civil engineering and construction
company, and Imperial College of Science and Technology in London.
As a result, the project identified a shortlist of mixes with good
durability properties, extremely good flexural toughness, and tensile
strengths that were improved with steel fibre reinforcement.
Project partner Dragados, a Spanish construction company, then put
the mixes to the test in Spain. Favourable results there proved
their potential for the pan-European market, an important factor
given the regional variations in aggregate types and quality, and
in cements. Dragados has subsequently benefited from its involvement
in the project by being able to market improved pre-bagged cement
products to its national sprayed concrete market.
The age question
One of the key aspects of the research was the
need to develop a time dependent behavioural model which can be
used for design purposes. Consequently, the research partners, Imperial
College and the Institute of Mechanics, Materials and Geostructures
(IMMG) in Athens developed mathematical models to describe and predict
the mature behaviour of sprayed concrete based on extensive testing
of conventional cast concrete. A series of large-scale tests which
involved spraying circular fibre reinforced concrete linings on
to a simulated clay surround, were implemented by Taywood to generate
data to calibrate and update the models. Further work on ageing
is now ongoing at Mott MacDonald.
IMMG also looked at the characteristics of the concrete in the first
few hours after application and discovered that the concrete has
considerable strength and stiffness at only two hours after spraying.
As a by-product of this work an instrument capable of providing
reliable in situ strength predictions based on the bearing strength
of the concrete surface was developed.
The research has provided good grounds
for users to be confident in the use of sprayed concrete, either
individually or in combination with other techniques, as an important
element in tunnel construction.
Since the completion of the project, Mott MacDonald has been involved
in a number of construction projects that have specified sprayed
concrete, including an access shaft and non-circular tunnels designed
particularly for passengers with limited mobility at Paddington
Underground Station in London. Outside of the UK, the company has
designed both primary and secondary fibre-reinforced spray concrete
linings in the eastward extension of the San Diego LRT, and a primary
liner in the Kastanousa Rail Tunnel in Greece.