https://doi.org/10.14311/APP.2022.33.0098
Acta Polytechnica CTU Proceedings 33:98–104, 2022 © 2022 The Author(s). Licensed under a CC-BY 4.0 licence

Published by the Czech Technical University in Prague

TRC SANDWICH SOLUTION FOR ENERGY RETROFITTING

Isabella Giorgia Colombo, Matteo Colombo, Marco di Prisco∗

Politecnico di Milano, Department of Civil and Environmental Engineering, P.za L. da Vinci, 32 - 20133
Milano, Italy

∗ corresponding author: marco.diprisco@polimi.it

Abstract.
Concerning energy improvement of existing façades, a favourable system involves prefabricated

multilayer panels, made of internal insulation core and outer textile reinforced concrete layers. It is
a convincing alternative to external thermal insulation composite systems (ETICS) and ventilated
façades, and it meets all the requirements for façade systems. The main advantage is the possibility to
apply the panel using a crane, without any scaffolding. The paper considers two solutions: the former
uses expanded polystyrene (EPS) as insulating material; the latter substitutes EPS with an innovative
green insulation material made of inorganic diatomite. The paper aims at comparing the solutions
in terms of mechanical properties of the components and behaviour of the composite sandwich at
lab-scale level. Numerical models, previously calibrated, will be instrumental for the discussion.

Keywords: Energy performance, sandwich panels, textile reinforced concrete.

1. Introduction
In Europe, building sector uses a large amount of en-
ergy (40% of the total) and produces relevant CO2
emissions (36%). For this reason, the focus of many
European policies is to improve energy efficiency [1].
Each year, only approximately 1% of buildings are en-
ergy renovated, in the framework of a building stock
that is old and energy inefficient, with the half of res-
idential buildings dated before 1970, year in which
the first European thermal Standard was emanated
[2]. Hence, improvement in energy efficiency of build-
ings is a crucial topic.

In multi-storey buildings, façades cover a large
amount of the total outer envelope surface. Hence,
solutions devoted to the energy retrofitting of existing
façade result particularly effective in terms of build-
ing energy saving. A convenient solution is repre-
sented by multilayer precast sandwich panels made
of an inner insulation core and thin external textile
reinforced concrete (TRC) layers.

TRC sandwich panels were proposed about 15
years ago by German researchers [3] and constituted a
new idea in the framework of cement-based sandwich
elements: TRC thickness (10-15 mm) is smaller than
that of traditional R/C external claddings (>40 mm)
[4], leading an important decrease in the weight of
the panel. Other conveniences are durability and the
good appearance, obtained using fine-grained mor-
tar. Although connectors were introduced between
the layers for a durable connection [5], advantage is
taken from adhesive bond between TRC and the in-
sulation layer to transfer the shear stresses.

2. TRC sandwich solutions
The research concerns prefabricated sandwich panels
constituted by thin external TRC layers and an in-

ternal insulation core. Large size panels (3 × 1.5 m2)
have been designed to reduce the manufacturing
and installation costs. The installation is performed
through a crane, without scaffolding, thus minimiz-
ing installation timing and inconveniences for inhab-
itants. Other advantages are: the integration of
the insulation and finishing systems; the durability
and possibility to obtain different colour and tex-
ture as finishing; the protection of the insulation core
through the external, waterproofed layer of TRC; the
good resistance to concentrated loads. Only the inner
TRC layer is connected to the existing R/C frame;
the shear transfer between the two TRC layers is en-
trusted to the insulation material (for fire safety, some
supplementary connectors could be inserted).

2.1. Panel with expanded polystyrene
core (EASEE project)

The Consortium of the European project EASEE [6]
developed a prefabricated TRC sandwich panel with
expanded polystyrene (EPS) as insulation material.
A thickness of 100 mm was considered for the core.
All the details concerning the panel design, testing
at different scale levels, durability, and modelling can
be found in [7–12]. EASEE main output is repre-
sented by a demonstration building [11] in Cinisello
Balsamo, Italy, where a 70% decrease in the thermal
transmittance of the wall was achieved [13]. One of
the main criticisms of this panel is that EPS may melt
during fire, leading to the loss of mechanical perfor-
mances of the composite solution.

2.2. Panel with diatomite core (Smart
P.I.QU.E.R.)

Smart P.I.QU.E.R. project [14] was aimed at solving
the criticisms of the previous project. Among the

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vol. 33/2022 TRC Sandwich Solution for Energy Retrofitting

Figure 1. Test results: TRC tensile behaviour (a) and core material compressive behaviour (b).

goals, particular relevance is assumed by the devel-
opment of a green insulation material, with suitable
mechanical properties and good fire and insulating
capabilities. At the same time, TRC layers need to
be optimised, reducing the shrinkage and weight of
the mortar, and enhancing the fabric fire behaviour
and sustainability.

2.3. Materials
2.3.1. Textile reinforced concrete
TRC developed in the EASEE project is made by
high strength fine grain mortar reinforced with one
alkali-resistant glass fabric. The warp is aligned in
longitudinal direction. This fabric [7] (leno-weave
production; SBR coating: water resin based on
styrene butadiene rubber) is characterised by a nom-
inal tensile strength in warp direction of 820 MPa
(computed on the equivalent cross-sectional area of
the glass). The cementitious matrix has a water to
binder ratio equal to 0.225, a superplasticiser to ce-
ment ratio of 9.3% and 1 mm maximum aggregate
size. The average cubic compressive strength (fcc) of
mortar, measured as specified in [15], is equal to 71.89
MPa on 12 specimens, with a coefficient of variation
of 9.13%.

Concerning Smart P.I.QU.E.R. project, several
mortar mix-designs and alkali-resistant glass fabric
reinforcements were investigated [14]. In this paper,
a high-performance mortar that includes shrinkage
reducing admixtures and is reinforced with one alkali-
resistant glass fabric coated with a thermosetting
epoxy coating is considered (see "HPC+SRA_AP1"
in [14]). The epoxy coating is particularly suitable in
case of fire exposure. The fabric is characterised by a
nominal strength in the warp direction of 1041 MPa.
The cementitious matrix used is characterised by a
maximum aggregate size of 1 mm, a water to binder
ratio of 0.20 and a super-plasticiser to cement ratio of
5.1%. The average cubic compressive strength (fcc)
of mortar measured according to [15] is equal to 97.53

MPa on 32 specimens, with a coefficient of variation
of 8.33%.

In both cases, three 400×70×10mm3 TRC samples
were tested in uniaxial tension using an electrome-
chanical press INSTRON 5867 with a maximum load
capacity of 30 kN, imposing a constant crosshead dis-
placement rate of 0.02 mm/s. The same test appa-
ratus and modalities described in [7] are used. The
test results are shown in Figure 1a in terms of nominal
stress (σ = P/A, with P=load and A=specimen cross
section) vs. normalised displacement (δ/l, with δ =
imposed displacement and l = free length) curves.

2.3.2. Core material
In the framework of the EASEE project, expanded
polystyrene was used as insulation material in sand-
wich panels. The product EPS250, characterised by a
nominal density of 35 kg/m3, was selected. As result-
ing from experimental tests [10], it is characterised by
a uniaxial compressive yield stress of 0.188 MPa, an
estimated Young’s modulus in compression of 13.7
MPa, a uniaxial tensile yield stress of 0.392 MPa,
a shear yield stress of 0.160 MPa and an estimated
shear modulus of 5.04 MPa.

As already anticipated, one of the main aims of
the Smart P.I.QU.E.R. project was to develop a core
material that could be more sustainable and more
suitable in case of fire with respect to EPS. Hence, a
novel lightweight sustainable insulation core (DHCF)
was developed by Verdolotti and other authors [16]
using diatomite (natural source) as matrix. Short fi-
bres were added to guarantee a ductile behaviour of
the insulation material both in compression and in
bending, in order to guarantee a proper global be-
haviour of sandwich panels. These fibres also con-
tribute to the enhancement of thermal performances
of the material, by reducing the thermal conductivity.
Polypropylene (PP) fibres were selected among dif-
ferent kind of reinforcement tested (jute, waste syn-
thetic, mixed and hemp fibres, as such and chopped).

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I. G. Colombo, M. Colombo, M. di Prisco Acta Polytechnica CTU Proceedings

Figure 2. Four-point bending test set-up (Smart P.I.QU.E.R.).

This choice was related not only to the proper ther-
mal and mechanical performances of the foam in
which PP fibres are included, but also because they
are the only ones that allow the production at indus-
trial level by means of an industrial plant. Consid-
ering that the level of the plastic plateau is related
to specimen density, a target density of 300 kg/m3
was fixed to guarantee comparable performances of
the core material with those of expanded polystyrene.
The core mix design is collected in Table 1.

Diatomite (wt%) 19.57
Polysilicate (wt%) 65.24
Si (wt%) 0.046
Catalyst (wt%) 8.39
Vegetable (wt%) 5.82
PP fibres (wt%) 1

Table 1. DHCF_PP mix design.

Compressive tests were performed on EPS speci-
mens with a size of 100 × 100 × 150 mm3, and DHCF
specimens with a dimension of 40 × 40 × 40 mm3.
The tests were displacement controlled by imposing a
constant displacement rate of the machine crosshead
equal to 1e-3 mm/s and 2e-3 mm/s respectively. The
compressive behaviour of both expanded polystyrene
and diatomite-based insulation material is shown in
Figure 1b where the nominal stress σ vs. nominal
strain ε curves are plotted (σ = P/A, P = load, A
= unloaded specimen cross-section; ε = δ/h, δ = top
displacement, h = specimen height).

3. Experimental behaviour of
lab-scale sandwich beams with
diatomite-based core

A wide experimental campaign on lab-scale sand-
wich specimens were developed within the EASEE
project. A four-point bending set-up was adopted
for both deep (550 × 150 × 120 mm3) and slender
(1200 × 300 × 120 mm3) sandwich beams, with EPS
100 mm thick. Detailed description of experimental
procedures and results can be found in [8]. Hence,

this Section is focused on the experimental tests per-
formed at lab-scale level within Smart P.I.QU.E.R.
on beams in bending.

3.1. Test set-up
Four-point bending tests were performed on sand-
wich beams using the same electromechanical press
INSTRON 5867 already mentioned. Two nominally
identical specimens with size of 600 × 150 × 80 mm3
and the warp of the glass fabric aligned in the beam
longitudinal direction were tested. The specimen ge-
ometry and the test set-up are shown in Figure 2. The
insulation panel densities are collected in Table 2.

Specimen ρ1st_measure ρ2nd_measure(kg/m3) (kg/m3)
PIQUER1 330 (31 days) 303 (125 days)
PIQUER2 348 (28 days) 297 (122 days)

Table 2. Density of the insulation core.

The insulation layer of full-scale panels designed
in the Smart P.I.QU.E.R. project is obtained plac-
ing side by side different core panels with size of
600×600×100mm3. For this reason, some cuts are in-
troduced in the insulation layer also in the lab-scale
beams (Figure 2). With the aim of reducing stress
concentration, 50 mm wide steel plates are glued on
the specimen over the cylindrical supports and un-
der the loading-knives. The loading and the sup-
porting cylinders are free to rotate on their axis and
in the plane perpendicularly to the longitudinal axis
of the specimen. During the initial phase of each
test, specimens were instrumented with displacement
transducers in order to measure: the specimen deflec-
tion under the loading knives, the deflection of the
upper TRC layer under the loading knives, the su-
perior longitudinal displacement on the compressed
side inside the constant bending moment region, and
the crack opening displacement on the tense side.
However, these aspects are not treated in this pa-
per. Tests were displacement-controlled, considering
the machine crosshead displacement (stroke) as feed-
back parameter. An initial stroke rate of 2E-3 mm/s

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vol. 33/2022 TRC Sandwich Solution for Energy Retrofitting

Figure 3. Experimental results (Smart P.I.QU.E.R.): load vs. stroke curves (a) and pictures of specimen PIQUER
2 during the test (b) and at failure (c).

was imposed; then, after crack formation, the rate is
increased up to 5E-3 mm/s.

3.2. Experimental results
The test results are shown in Figure 3a in terms of
load (P) vs. stroke curves. Looking at the results,
it is interesting to note that specimen PIQUER1 ex-
perienced a premature shear failure of the core, thus
leading to a localised failure and preventing the multi-
cracking of TRC layers that guarantees the hard-
ening behaviour of the sandwich solution. In fact,
while performing a check before testing, core panel
used to cast specimen PIQUER1 presented - to the
touch - some softer regions at the edges. On the
contrary, specimen PIQUER2 showed a ductile hard-
ening global response, experiencing multi-cracking of
TRC faces. A picture of this specimen during testing
is shown in Figure 3b. The final failure is related to
the debonding between the beam core and the lower
TRC layer (Figure 3c).

4. Numerical modelling of the
tests and comparison between
the two sandwich solutions

4.1. Geometry and constraints
Abaqus /CAE 6.14-5 was adopted for the modelling
of the panel. Yz symmetry was exploited, and half
of the beam was reproduced, thus minimising the nu-
merical effort (Figure 4a).

As shown in the figure, the panel layers (TRC and
core), and the steel plates are modelled as solid and
homogeneous. Perfect bond is assumed at core/TRC
interfaces and between the panel and steel plates.
The interaction between adjacent core panels is char-
acterized by a hard contact in the normal direction
and by a frictionless contact in the tangential direc-
tion.

Concerning constraints, displacement orthogonal
to the symmetry plane are prevented, the lower steel

plate is constrained in the vertical direction on the
bottom face and a vertical displacement δ is im-
posed on the loading line. The mesh is shown in the
same figure: eight-node linear hexahedral elements
(C3D8R - Continuum, 3-D, 8-node - reduced integra-
tion) are used.

4.2. Constitutive laws
In this Section, a description of the constitutive laws
adopted for each material is proposed.

The elastic phase of TRC is defined introducing a
Poisson’s ratio of 0.2 and a Young’s modulus of 30
GPa (according to literature results [17] on mortar
characterized by equal maximum aggregate size and
compressive strength).

The plastic behaviour of TRC is modelled through
Abaqus Concrete Damage Plasticity model. No dam-
age curve is inserted; hence, the model behaves
as a plasticity model. In compression, an elastic-
perfectly plastic behaviour is assumed, imposing a
yield strength of 97 MPa according to the values
shown in Section 2.3.1. The stress-irreversible strain
relationship (Table 3) adopted in tension is extracted
from the experimental results of PIQUER specimens
shown in Figure 1a, considering strains computed
basing on the measurements of displacement trans-
ducers applied on TRC. TRC tensile behaviour is as-
sumed homogeneous over the layer thickness; this as-
sumption is reliable according to [10].

Point Yield stress Cracking strain
T1 2.5 0
T2 5 0.00275
T3 20.81 0.017

Table 3. TRC plastic tensile constitutive law.

Concerning the core material, the elastic phase is
defined introducing a Young’s modulus of 20 MPa

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Figure 4. FE model: geometry and constraints (a) and validation of core material model considering three-point
bending (b).

Figure 5. FE model results: numerical response compared with experimental results (a) and with EASEE bending
behaviour (b); failure modes for specimen PIQUER2 showing core strut yielding at point C1 (c) and TRC multi-
cracking at point T3_sup (d).

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vol. 33/2022 TRC Sandwich Solution for Energy Retrofitting

(according to experimental results in compression
shown in Figure 1b) and a Poisson’s ratio of 0.2.

The plastic behaviour of the core material is in-
troduced using Abaqus Concrete Damage Plasticity
model. Also in this case, no damage curves are in-
troduced. An elastic-perfectly plastic behaviour is
assumed both in tension and in compression: a com-
pressive yield strength of 0.30 MPa is used according
to experimental results shown in Figure 1b (average
value for specimens of the same batch of core sample
PIQUER2); a tensile yield strength of 0.053 MPa is
adopted according to average experimental results of
tensile tests performed on core specimens.

With the aim of checking the validity of the mate-
rial model adopted for the core, a three-point bending
test performed on core prismatic specimen with size
of 160×40×40mm3 was modelled. Experimental and
numerical curves are compared in Figure 4b, demon-
strating that the adopted material model is reliable.

Steel is assumed elastic with a Young’s modulus of
210 GPa and a Poisson’s ratio of 0.1.

4.3. Numerical results
The numerical response is shown in Figure 5a in terms
of load (P) vs. stroke. A good correlation is found
with respect to the behaviour of specimen PIQUER2
that showed a hardening behaviour. A perfect over-
lapping is obtained in the initial phase of the test; the
divergency of the two curves while the test is progress-
ing could be related to a bond weakness at TRC-core
interfaces, leading to the final failure due to debond-
ing already underlined in Figure 3c.

In order to investigate the failure mechanisms oc-
curred for a specimen characterised by a good TRC-
core bond, critical points of material constitutive laws
are underlined on the numerical curve. In particu-
lar: T points refer to the TRC layers (Table 3), with
subscript "inf" related to lower TRC layer and sub-
script "sup" related to the upper TRC layer; point C1
indicates the yielding of the strut in the core mate-
rial. It is worth noting that the change in the slope
of the global response corresponds to point C1 (Fig-
ure 5c), when both TRC layers are already cracked
(Figure 5d).

In Figure 5b the numerical response of PIQUER
specimens, in which the core thickness was enhanced
to 100 mm, is compared with the experimental be-
haviour of EASEE sandwich beams characterised by
the same cross-section and size.

5. Conclusions
The bending behaviour of sandwich solutions de-
veloped within the EASEE and Smart P.I.QU.E.R.
projects was compared at lab-scale level. Even
though the response of the PIQUER sandwich beam
showed lower maximum load and final displacement,
a ductile hardening behaviour and a reasonable sus-
tained load are guaranteed by the solution. It is
worth noting that a good bond at TRC-core interfaces

must be guaranteed to obtain a proper behaviour of
the composite.

Acknowledgements
The research was developed within the Smart P.I.QU.E.R.
project, funded by Regione Lombardia (Smart Living; de-
cree n. 14782 - 24/11/2017; ID SiAge: 379284). Authors
would like to thank the partners; in particular, iPCB-
CNR and Isoltech involved in the development of the core
material.

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