Textile Reinforced Concrete
Contemporary research and applications
TU1303 short-term scientific mission
November 2016
Will Hawkins
University of Bath
Overview
This report details a short-term scientific mission (STSM) undertaken during November 2016 as part of the
research framework of COST Action TU1303. This involved visiting four universities across Europe aiming to
gather knowledge relating to textile reinforced concrete (TRC). An itinerary is shown in Table 1.
Table 1 – STSM itinerary
Dates Institution Department Principle contact/host
7 -9
th th
Vrije Universiteit Brussel Department of Mechanics of Materials and Prof. Tine Tysmans
November Constructions (MeMC)
10th - 11th RWTH Aachen University Institut für Massivbau (IMB) Dr. Rostislav Chudoba
November
14th - 15th TU Dresden Institute of Concrete Structures Dipl.-Ing. Sebastian May
November
17th – 18th ETH Zurich Institute of Technology and Architecture Dr. Andrew Liew
November
TRC is the subject of considerable research activity at present and has a wide range of developing structural
applications. It combines woven textile reinforcement (typically of glass or carbon fibres) with a cementitious
matrix to create a durable material with a high tensile strength, and is particularly well suited to creating thin
sections with complex geometries due to the flexible nature of the reinforcement.
Research background
In my own PhD research, I am developing a flooring system for buildings using thin TRC shells as a low
embodied energy alternative to traditional reinforced concrete flat slabs. In the proposed system, a thin,
curved TRC shell spans between column supports to create a vaulted ceiling (Figure 1). A self-levelling fill of
lightweight, low-strength, foamed concrete is then applied using the shell as formwork. As well as creating a
level floor surface, this restrains the shell against buckling and provides acoustic and thermal insulation. Steel
ties span between adjacent columns in order to counter the thrust imbalance on the columns at the building’s
edge. By integrating the services within the structural zone, the total depth is similar to an equivalent flat slab
arrangement.
Figure 1 – Proposed TRC shell flooring system
The project is now entering its second year. A finite element analysis framework has been developed,
accounting for non-uniform loadings, differential column settlement and non-rigid support conditions. This was
used to explore a range of shell geometries and corresponding formwork options. A design for a typical UK
office floor structure (with spans of 7.5 m) showed a 53% self-weight saving compared to an equivalent flat
slab, based on a 60 mm thick shell section. Future developments in the design and analysis process will aim to
further increase efficiencies through improvements in modelling accuracy and optimisation of geometry and
materials.
Construction methods, particularly formwork systems, were considered from the outset during the
development of this proposal. Figure 2 shows options for simple formwork systems of straight timber
elements or fabric stressed around a rigid frame. Future work will explore these systems and their effect on
the geometry and performance of the shells. A programme of prototyping and physical testing has also been
developed in order to examine the construction process, verify the computational analysis and investigate the
behaviour of the system at the point of structural failure.
Figure 2 – Potential formwork solutions
A vault such as that proposed derives its efficiency from acting primarily in compression. If the geometry is
designed correctly, the minimisation of bending forces keeps stresses low and allows a thin shell section to be
used. Concrete is an ideal material for vaults since it is relatively cheap and abundant, strong in compression
and can be readily formed into complex forms due to its fluidity. In practice however, reinforcement must be
provided to overcome concrete’s low tensile strength and tendency fail in a brittle manner. The textile
reinforcement provides ductility and robustness in the event of failure as well as increasing bending and tensile
strength where required. Using textiles also allows simple customisation of reinforcement distribution by
varying the number of layers reinforcement throughout the shell.
Although there is a considerable body of published research relating to TRC, it was felt that formulating an
appropriate design approach and constructing a prototype without the assistance of an experienced
researcher would be a difficult and time consuming task. Examples of TRC research or applications in the UK
are very limited. A number of key topics to be investigated regarding the application of TRC to this project
were therefore identified prior to the commencement of the STSM:
– Fire performance for exposed use in buildings
– Computational modelling of stiffness and strength
– Behaviour at ultimate failure
– Durability
– Construction methods and formwork systems
– Reinforcement material options and effect on cost, embodied energy and performance
– Interface with concrete cast onto TRC surface
– Design of shell geometry
The following sections detail the key findings of the STSM, including existing applications, current research and
experiments and points of discussion.
Classification of TRC
Whilst the term ‘textile reinforced concrete’ has been adopted throughout this report, an issue which came
up in multiple discussions was that of nomenclature, with several terms in active use often with overlapping
definitions. Below are some examples of the terms used in publications:
– Textile reinforced concrete (TRC): The most commonly used term for the applications discussed in
this report (Brameshuber, 2006).
– Textile reinforced mortar (TRM): This definition acknowledges that, since aggregate sizes are
typically small, the matrix is more accurately described as a mortar than concrete (Triantafillou and
Papanicolaou, 2006).
– Textile reinforced cement composites (TRCC): Here the nature of the matrix is described more
generally (Mobasher, 2011; Tysmans et al., 2011).
– Fibre reinforced cement composites (FRCC): A more general term in which the reinforcement is
not necessary woven into a textile (Balaguru and Shah, 1992).
– Fibre reinforced cementitious matrix (FRCM): More commonly used in strengthening applications
with high fibre volumes (Trapko, 2013).
– Strain-hardening cement composites (SHCC): This term is again more general, and focuses on the
resulting (tensile) behaviour of the material rather than the nature of the reinforcement (Soranakom
and Mobasher, 2008).
This is potentially confusing to the uninitiated reader and perhaps likely to inhibit the dispersal of knowledge
between relevant researchers. The variety of terms used reflects the wide range of material combinations and
applications which are being researched in this field.
Research context
The term ‘textile reinforced concrete’ appears in literature towards the end of the 1990s, and early research
findings were summarised in a RILEM state of the art report in 2006 (Brameshuber, 2006). Since then, steady
progress towards greater understanding of the material and commercial adoption has been made.
In 2014, the German Centre for Competence in Construction (DIBt) approved the use of TRC for
strengthening applications, confined to specific combinations of permitted materials. Use of TRC outside of
these restrictions is still possible however specific approvals must be granted, increasing project costs. A
significant research effort is underway aiming to add further knowledge and bring wider applications of the
technology.
A major recent source of research funding in TRC is from the Carbon Concrete Composite (C3) project,
founded in 2014. With funding from the German government and industrial partners totaling 67.5 million
euros, the aim of the project is for carbon reinforced concrete to enter the market by 2020 and to become a
competitive alternative to steel reinforcement by 2025. Today this involves 155 partners across academia and
industry, and is being coordinated from Dresden.
Reinforcement materials
The majority of research and application in TRC has used two fibre materials: carbon and glass. Each of the
projects discussed on the STSM features one of these materials. The key differences between these materials
are summarised below:
– Composition: Carbon fibres are produced from polymeric raw materials, typically derived from
fossil fuels. Glass fibres are inorganic and use silica sand as well as a variety of other natural minerals
as a raw material.
– Stiffness: Carbon fibres have a significantly higher stiffness (200-500 GPa) than glass fibres (70-80
MPa).
– Strength: The ultimate tensile strength of carbon fibre (3000-5000 MPa) is higher than glass (around
1400 MPa). In all cases, the tensile strength of yarns or woven textiles are lower than that of
individual filaments.
– Durability: The alkaline environment which comes from using Portland cement causes degradation
of glass fibres and significant loss of strength. ‘Alkali resistant’, or AR-glass fibres are now standard for
use in concrete, however the durability issue is only reduced, not eliminated. Reducing the alkalinity
of the matrix is another effective approach to improve the durability of glass fibres. Carbon fibres
show good durability characteristics by comparison.
– Fire resistance: Carbon fibres can withstand higher temperatures than glass fibres before losing
strength (Sim et al., 2005). In both cases, the use of resin impregnation of the fibres (which
improves structural performance) can lead to fire resistance issues where the melting temperature
of the resin is low.
– Cost: Carbon fibres currently cost significantly more to produce than glass fibres, by a factor of 20 as
estimated by Ashby and Jones (2012). Around half of the cost of carbon fibres is associated with the
raw material PAN (Polyacrylicnitrile), which is used in 90% of carbon fibre production and accounts
for 50% of its cost (Mainka et al., 2015). Significant research is being conducted into the use of
alternative precursors with a lower cost, such as lignin (a waste product from paper production).
– Embodied energy: The combination of high temperatures and raw materials currently required to
produce carbon fibres make their embodied energy around ten times higher than glass fibres (Song
et al., 2009).
The individual yarns are woven into textiles, providing many possible variations in fibre orientation, spacing,
and weight. This allows customisation of the textile for specific applications Often the textile is then
impregnated with a polymeric matrix material in order to improve load transfer between fibres as well as
bond characteristics and durability. The ‘scale’ of the TRC can also vary considerably, from fine reinforcement
meshes with thin layers and very small aggregate sizes to large textiles similar in construction to steel
reinforcement (Figure 3).
Figure 3 – Fine reinforcement meshes and chopped mats (left) and a heavy 50 mm textile grid (right)
A number of manufacturers in Germany produce textiles designed to be used in composite with concrete,
including V.Fraas, Tudatex and solidian. At TU Dresden, the full manufacturing process including fibre
production, weaving and coating of carbon fibre textiles can be done in-house, allowing full control and
investigation of the material production (Figure 4).
Figure 4 – Weaving of carbon fibres into an orthogonal reinforcing grid at TU Dresden
Typically the reinforcement consists of a 2D layer of woven fibres which is then built up in successive layers
with concrete to create the composite. Each layer therefore comprises a separate manufacturing process. An
alternative is to use a 3D textile, where stiff links are woven in between textile layers creating a separation.
This enables a full section to be created in only a single application of concrete. This method is being used at
Vrije Universiteit Brussel to create TRC strengthened sandwich panels (with a foam insulating core), as can be
seen in Figure 5.
Figure 5 –TRC sandwich panel with 3D textile reinforcement (Vrije Universiteit Brussel)
Matrix materials
There are a wide variety of matrix materials currently being used in TRC research, demonstrating
considerable variations in both binders and aggregates. The choice of matrix is dictated primarily by the nature
of the reinforcement. For example, the size of the openings in the textile dictate the maximum aggregate size
which can be used in order to ensure sufficient penetration of the matrix. A typical matrix material, which is
certified for commercial use with carbon textile reinforcement in Germany, is Pagel TF10 (Figure 6, right).
This is a ready–mixed concrete with a maximum aggregate size of 1mm, allowing good penetration for most
textile grids.
Inorganic Phosphate Cement (IPC) is a cementitious matrix developed specifically for use with uncoated glass
fibres which has been developed at Vrije Universiteit Brussel (Figure 6, left). Since the matrix is non-alkaline,
problems of degradation of glass fibre reinforcement are eliminated. The very fine particle size allows the
material to be used with closely spaced filaments of reinforcement, typically the randomly orientated chopped
mats as shown in Figure 3 (left). This enables very thin sections to be created (around 1 mm per layer) and
with very high volume fractions of reinforcement of up to 25% (Cuypers and Wastiels, 2011). The resulting
material has a high tensile strength, good durability and fire resistance characteristics and takes advantage of
the availability and cost efficiency of glass fibres.
Figure 6 - Inorganic Phosphate Cement (IPC) (left) and Pagel TF10 mortar (right)
Structural behaviour, testing and modelling
As with any composite material, the behaviour of TRC is dictated by the properties of the constituent
materials, the interface between them and their geometrical arrangement. A significant proportion of past and
current research is focused on determining accurate and reliable testing methods as part of efforts to
understand, model and ultimately commercialise the material.
The majority of past research has been focused on the tensile behaviour of TRC. This is particularly relevant
for strengthening and retrofit applications where the high tensile strength, strain-hardening behaviour and
adaptable application process of TRC are taken advantage of. A number of tensile testing arrangements have
been investigated in the past (Hartig et al., 2012). The TRC specimen is typically clamped between steel
plates, and this clamping force must be small enough to avoid damage. There must also be sufficient bond
length to avoid pull-out failure of the reinforcement, as can be seen in Figure 7 (right). Standard tensile test
methods are being developed at RTWH Aachen and TU Dresden (Brameshuber et al., 2016; Holz, 2015).
The strength and volume fraction of reinforcement affects the nature of the structural response in tension. If
the reinforcement is insufficient, single cracks grow in size as the fibres fail. The volume fraction must therefore
be sufficiently large to allow distributed cracking, as can be seen in Figure 7 (left). A recent paper from RWTH
Aachen shows how TRC behaviour can be replicated in a finite element simulation by calibrating the material
model directly from tensile test results (Chudoba et al., 2016).
Figure 7 – Tensile test specimens showing distributed cracking (left) and textile pull-out failure (right)
Further testing to characterise the tensile behaviour of the material is being carried out at TU Dresden,
including long term and fatigue testing, as well as pull-out tests using a variety of reinforcement materials and
coatings (Figure 8).
Figure 8 – Pull-out (left) and anchorage (right) tests being conducted on TRC samples at TU Dresden
In contrast, investigations into the compressive behaviour of TRC are much more limited, since for many
applications it is the tensile strength which is critical for design of the section. Ongoing research at TU Dresden
has indicated that the layering of the concrete as well as the presence of the reinforcement causes a reduction
in the compressive capacity of the section (Bochmann, 2015). There is a strong dependence on the
orientation of the compressive force with respect to the reinforcement, with the greatest reduction in capacity
observed at a reinforcement inclination of around 45°, due to shearing between the layers (Figure 9).
Figure 9 – Compressive tests on TRC specimens at TU Dresden (left image courtesy Jakob Bochmann)
Applications
One of the earliest practical applications of TRC was as cladding panels at RWTH Aachen in 2001 (Hegger et
al., 2006). Reinforcement is required for strength and robustness despite these not being a load bearing
structure, and non-corroding reinforcement is particularly important for the material when exposed to all
weathers (Figure 10).
Figure 10 – TRC cladding panels at the Institute Für Massivbau (RWTH Aachen)
Some of the clearest demonstrations of the potential of TRC shell structures are also to be found at RWTH
Aachen, and include barrel-vaulted canopies for a bicycle shelter and a pavilion roof structure shown in Figure
11, and detailed in two recent papers from the department (Scholzen et al., 2015a; Scholzen et al., 2015b).
Both of these are reinforced with layers of uncoated carbon fibre mesh which are woven with flat, wide
rovings to maximise contact area thus improving bond with the concrete matrix. Long term testing of the shell
sections is planned in order to investigate changes in behaviour over several years.
Figure 11 – TRC pavilion at RWTH Aachen using a 60 mm deep section reinforced with 12 layers of carbon fibre textile
Another TRC shell roof canopy is currently under development for the Nest HiLo project, an experimental
research building being developed in collaboration with ETH Zurich, architects Supermanoeuvre and
structural engineers Bollinger + Grohmann (ETH Zurich, 2016). A roof canopy as seen in Figure 12 (left) is to
be constructed using a hybrid fabric and cable net formwork system as proposed by Veenendaal and Block
(2014) and reinforced with a carbon textile. The geometry of the shell can be controlled and structurally
optimised by controlling the prestress on individual cables within the formwork system. The project also
features a precast flooring system, part of which is shown in Figure 12 (right), which is a rib-stiffened concrete
shell. The steel fibre reinforcement simplifies construction and creates ductile behaviour at failure.
Investigations into the behaviour and modelling of TRC shells has also been undertaken at Vrije Universiteit
Brussels, particularly concerning very thin IPC shells. For these structures, the buckling behaviour and nature
of initial imperfections is of particular importance (Verwimp et al., 2016).
Figure 12 – Form-found shells using a flexible formwork (left, image: hilo.arch.ethz.ch) and a CNC cut mould (right)
The geometrical flexibility of TRC continues to lead to the development of new creative applications. Two
novel TRC flooring systems are shown in Figure 13. The left hand image shows 3 m long barrel shells
designed for use in existing buildings (Senckpiel, 2015). The use of a thin section is made possible by
optimisation of the form, and allows manual transportation and handling. A different approach to the same
problem, also using TRC, has been developed at Vrije Universiteit Brussel (De Sutter et al., 2015). In this case
the TRC is formed into rectangular hollow beams and insulated ceiling elements upon which in-situ concrete
is poured (Figure 13, right). Special attention has also been given to the interface between these two materials
in the PhD work of Maciej Wozniak, which aims to characterize the bond-slip relationship.
Figure 13 – Thin lightweight TRC floors with in-situ concrete at TU Dresden (left) and Vrije Universiteit Brussel (right, image
courtesy Sven De Sutter)
TRC has also been used in combustion with prestressed concrete to create bridges. Figure 14 shows two
examples of this, where in both cases the textile is used to reinforce the deck perpendicular to the span. As
well as corrosion resistance, the flexibility of the concrete enables curved or complex geometric details to be
manufactured without the additional process of bending steel reinforcement.
Figure 14 – Examples of TRC used in bridge decks at TU Dresden (left) and RWTH Aachen (right)
A final example, demonstrating an original application of the material, is a system developed at RWTH Aachen
known as origami concrete or ‘oricrete’ (Chudoba et al., 2014). By leaving strips of bare reinforcement during
casting the structure can be folded into a variety of forms, before being fixed in position by the application of
mortar along these hinges (Figure 15).
Figure 15 - ‘Oricrete’ experimental TRC shell structure (RWTH Aachen)
Outlook & conclusions
The wide variation in materials, their different behaviours, composite interaction, the multitude of possible
construction processes and flexibility of structural form makes TRC undoubtedly a complex material to
understand and characterise. However, it is also this diversity which leads to such a vast array of potential
applications for the material. The future of TRC is closely linked to parallel developments in neighboring fields
of research, manufacturing and construction. For example, the sophisticated computational modelling
techniques required to analyse geometrically complex composite structures are becoming more mainstream
in industry. In addition, the development of new fibre production techniques and materials has the potential to
reduce costs and improve performance. The growing adoption of prefabricated building elements also favours
the use of lightweight TRC sections. As changes such as these take place, even more potential research topics
and applications emerge.
By undertaking this STSM I have been able to vastly improve my understanding of this material and how it can
be applied in my own research. It has been a clear demonstration of how observation, demonstration and
conversation can accelerate both the understanding of concepts and the generation of ideas.
Acknowledgements
I would very much like to thank Professors Lars De Laet and Marijke Mollaert of TU1303 for accepting this
STSM proposal. I am also very grateful to each of my very welcoming hosts for introducing me to their
research teams and taking me on tours of their facilities. Finally, I would like to thank all the researchers who
gave up their time to discuss their research, listen to my proposal and show me around their cities. I hope that
they found this as interesting, useful and enjoyable as I did.
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