SP-326—76

76.1

TRC Sandwich Panel for Energy Retrofitting Exposed to

Environmental Thermal Actions

Isabella G. Colombo, Matteo Colombo, Marco di Prisco, Graziano Salvalai,

and Marta M. Sesana

Synopsis: In the framework of the European project EASEE (Envelope Approach to improve Sustainability and

Energy efficiency in Existing multi-storey multi-owner residential buildings), a textile reinforced concrete

sandwich-prefabricated panel has been designed for the energy retrofitting of existing building built between 1925

and 1975. The feasibility of the solution has been evaluated by assessing the production process and the structural

behaviour. Thanks to the application of panels on a test façade at Politecnico di Milano and on a whole demo-

building in Cinisello Balsamo, it was possible to evaluate all the topics related to handling and mounting and to

monitor the energy performance of the system.

During summer and winter monitoring, superficial temperatures were collected: sensors were placed on panel

extrados, in the cavity between panel and existing wall and on the internal surface of existing wall. In addition,

the thermal transmittance of the retrofitted wall was measured. These data allow evaluating the overall efficiency

of the adopted system.

Using superficial temperatures as input data, the effect of temperature variation on the mechanical behaviour of a

sandwich panel has been evaluated through a finite element analysis performed in Abaqus on a 3D model.

Keywords: environmental loading; finite element analysis; sandwich panel; temperature variation; thermo-

mechanical coupling; textile reinforced concrete

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Biography: Isabella G. Colombo is a Postdoctoral Research Fellow at Politecnico di Milano (Department of

Civil and Environmental Engineering), where she received her PhD in 2015. Her research interests include the

investigation on the behavior of advanced cementitious composites (textile reinforced concrete and fiber

reinforced concrete) and their use in sandwich structures and in retrofitting of damaged structures.

Matteo Colombo is Assistant Professor at Politecnico di Milano (Department of Civil and Environmental

Engineering). He has taken part in several national and international research projects. His research interests

include the investigation on mechanical behavior and on structural applications of advanced cementitious

composites (fiber reinforced concrete and textile reinforced concrete) in standard and extreme conditions (e.g. fire

and blast).

Marco di Prisco is Professor of Structural Design at Politecnico di Milano. Honorary Editor of the European

Journal of Environmental and Civil Engineering, member of the Editorial Board of the Journal of Cement and

Concrete Composites, member of fib presidium and of Rilem DAC, he is the convenor of the Committee

TC250/SC2/Wg1/Tg2 to introduce FRC design criteria in EC2. His research interests include composite cement-

based materials, fracture mechanics, reinforcement-concrete interaction, r/c and p/c structural elements,

prefabricated structures, structural response at exceptional loads.

Graziano Salvalai is Assistant Professor at Politecnico di Milano (Department of architecture built environment

and construction engineering). His research interests include the implementation and analysis of innovative

technology solution for energy efficient buildings. He is part of the research group of RE3_Lab - REfurbishment

and Energy Efficiency Lab (Politecnico di Milano – Polo Territoriale di Lecco). He has taken part in several

national and EU founded research projects.

Marta M. Sesana is Postdoctoral Fellow at ABC Department of Politecnico di Milano, where she received a PhD

in Building Engineering whit a thesis titled "Net zero energy houses in Temperate climate. Toolbox for an

integrated design approach". She is currently involved in different national and EU founded research projects on

the topic of energy efficient buildings, in particular for dynamic simulations and georeference repository. On these

topics she is author of publications for international conference and scientific journals.

INTRODUCTION

The importance of building performance for the mitigation of climate change have been included in the EU policy

(e.g. Energy Performance of Buildings Directive and Energy Efficiency Directive). While the energy efficiency

of new building has improved over time, existing buildings need energy retrofitting interventions in order to

decrease their impact on energy consumption. The European building stock is heterogeneous across Europe;

however, in all the states, residential buildings compose the majority of floor area (varying from 60% in the north-

center of Europe to more than 85% in the southern countries) [1].

The age of existing buildings is a good indicator concerning the energy efficiency of the building stock: the higher

the percentage of new dwellings (built with higher efficient standards) the higher the overall energy performance.

In the EU countries, about 50% of residential buildings were built before 1970, when the first thermal regulations

have been introduced [1].

In Italy, there are about 12.2 million residential buildings, 56.7% of them built before 1970 and 17.4% built

between 1971 and 1980 [2]. In Lombardy region (where the two demo-buildings described in this paper are

placed), 1.5 million residential buildings have been detected in the census, 56.3% of them built before 1970 and

16.7% built between 1971 and 1980 [2]. Considering that the first national Standard on energy saving in buildings

was released in 1976 [3], the amount of final energy due to heating and cooling of the existing building stock is

remarkable and close to 40% of the overall energy consumption [4].

In this framework, the consortium of the European project EASEE (Envelope Approach to improve Sustainability

and Energy efficiency in Existing multi-storey multi-owner residential buildings) developed innovative solutions

for the inner envelope, the cavity wall and the external façade aimed at enhancing the energy performance of

buildings, thus reducing their energy consumption [5]. The paper concerns the solution proposed for the outer

façade: a precast sandwich panel (Fig. 1) characterized by external Textile Reinforced Concrete layers (TRC) 12

mm thick (0.47 in) and an internal expanded polystyrene layer 100 mm thick (3.9 in). The maximum dimensions

of the panel are 3.03 x 1.50 m (119 x 59 in).

EASEE panels were firstly applied on a test façade at Politecnico di Milano (Fig. 2a,b) in order to carefully assess

the handling and mounting process and the structural and energy behavior of the retrofitted envelope. The building

chosen for the installation of the panels was built in the early 70’s and is characterized by a precast concrete

column-beam frame and concrete walls. It hosts classrooms, research laboratories and departmental offices. The

lower two floors of the West façade of the building has been retrofitted through the installation of 13 panels with

dimensions of 3.03 x 1.50 m (119 x 59 in -10 panels), 3.03 x 1.43 m (119 x 56 in - 1 panel) and 3.03 x 0.55 m

TRC Sandwich Panel for Energy Retrofitting Exposed to Environmental Thermal Actions

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(119 x 22 in - 2 panels). Panels are characterized by different colors (light gray, gray and charcoal) and textures

(smooth and knurled).

A building and environmental monitoring system was installed on the test façade, as shown in Fig. 2b, to evaluate

the thermal transmittance of the retrofitted envelope and the energy efficiency of the intervention.

This experience allowed the EASEE consortium to evaluate the installation procedure and the performance of the

proposed solution under real environmental conditions. In addition, thanks to this experience, some improvements

to the manufacturing process have been developed in order to obtain higher aesthetic features.

Once the EASEE solution has been validated on the test façade, a whole demo building has been retrofitted in

Cinisello Balsamo, a medium municipality located in the Metropolitan area of Milan (Fig. 2c,d) [6,7]. The test

façade constituted an opportunity for the demo-building owner to choose colors and textures suitable for his

building. The building is constituted by three floors built over a basement that hosts garages; it was built in 1971

and it is a property of ALER (the Social Housing Agency of Lombardy Region). As it emerged from the thermal

survey, an energy retrofitting of the building was needed in order to solve energy losses through the envelope.

The intervention covered a surface of more than 580 m2 (6,243 ft2). The demo-building activity allowed to:

- apply the methodology of building assessment (from a geometrical and an energy point of view);

- apply a BIM approach to the EASEE retrofitting approach;

- verify the installation procedure;

- test the realization of joints and technical details in correspondence of balconies, corners, doors and

windows, etc.;

- evaluate the impact of the intervention on occupant’s life (reduced impact with respect to Exterior

Insulation and Finishing System as scaffoldings are not required and times of intervention are shorter);

- evaluate the energy performance of the retrofitted building.

The North façade was chosen for monitoring in the winter season. The external sensor applied on a panel is

highlighted in Fig. 2d.

On the basis of the results of the monitoring activity, the panel is subjected to high thermal gradients in its

thickness, thus causing thermal stresses on the TRC layers. Hence, it is important to verify if these stresses cause

cracks on the TRC layers, compromising the panel durability.

In the study, the superficial temperatures measured during the summer and winter monitoring campaigns were

imposed on a panel 3D model built in Abaqus FE environment, and a coupled thermal-mechanical analysis was

performed in order to investigate the consequent strain state of the panel cross-section.

PANEL DESCRIPTION

As already said in the introduction, the panel is characterized by two external layers 12 mm thick made of TRC

and an internal insulation layer 100 mm thick in expanded polystyrene (EPS). The maximum length of the panel

is equal to 3030 mm (119 in) and its maximum width is equal to 1500 mm (59 in).

Shear forces are mainly transferred through the insulation layer to the external TRC layers; however, in order to

prevent the detachment of the layer connection in extreme conditions (e.g. fire), four stainless steel AISI 310S

bent bars (ϕ5) were embedded in both the longitudinal panel edges at the upper and lower ends.

The fastening of the panel to the existing structure is made through four high-performance fiber reinforced

concrete (HPFRC) thickenings placed on the lower and upper edges of the panel (see Fig. 1b). The upper

connectors (see point F1, Fig. 1a) are aimed at resisting only the wind pressure acting on the panel, while the

lower connectors (see point F2, Fig. 1a) are loaded by both wind pressure and the self-weight of the panel.

TRC layers were obtained by reinforcing a high strength fine grain mortar with one alkali-resistant glass fabric.

The matrix mix design has the following characteristics: water to binder ratio equal to 0.19; superplasticizer to

cement ratio equal to 0.055; maximum aggregate size equal to 2 mm. As a result, the mortar is characterized by

a high flowing capability, thus guaranteeing good matrix-fabric and matrix-EPS bond. The average cubic

compressive strength, determined according to EN 196 Standard for mortar [8], is equal to 87.7 MPa (12,720 psi)

at 28 days (STD=15.6% on 10 specimens). The characteristics of the fabric selected as reinforcement are in Table

1. Fabrics are placed with warp yarns along the panel longitudinal direction.

The expanded polystyrene - commercially known as “EPS250” - has a density of 35 kg/m3 (2.18 pcf) and a thermal

conductivity of 0.034 W/mK (0.236 BTU in/ft2hr.°F) [9]. According to the test results [10], it is characterized by

an elastic modulus in compression of 13.7 MPa (1,987 psi), a uniaxial compressive yield stress of 0.19 MPa

(28 psi), a uniaxial tensile yield stress of 0.39 MPa (57 psi), a shear yield stress of 0.16 MPa (23 psi) and a shear

modulus of 5.04 MPa (731 psi).

The mechanical characterization of the panel has been performed at material, lab-scale and full-scale levels; in

particular, load-controlled tests [11] and displacement-controlled tests [12] were performed on full-scale panels.

Considering loads that are typical for the wind pressure (Serviceability Limit State), panels were cracked, but,

thanks to TRC multi-cracking, cracks were not visible to the naked eye (crack width lower than 50 μm (0.002 in)),

thus satisfying an important requirement for a façade panel.

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MONITORING CAMPAIGN

A thermal monitoring campaign was conducted after retrofitting on the test façade at Politecnico di Milano (west

exposure) during summer and on the demo building in Cinisello (north façade) during winter. North exposure was

chosen for winter monitoring in order to exclude the effect of the solar radiation, thus maximizing the heat loss.

All the details concerning the methodology and applied sensors can be found in [6,7], together with the main

results obtained in the demo-building case study. A heat flux sensor LSI LASTEM BRS-ESR240, characterized

by an operative temperature ranging between -30°C (-22°F) and +70°C (158°F) and an accuracy of 5% F.S. in 12

hours measurement, was placed on the inner surface of the existing envelope. Surface temperature sensors were

placed on the existing envelope (inner and outer surfaces) and on the external surface of the applied panel (Fig.

2b, 2d). These sensors (LSI LASTEM Pt100) were characterized by an operative temperature ranging between -

40°C (-40°F) and +80°C (176°F), a resolution of 0.01°C (0.018°F) and an uncertainty of 0.1°C (0.18°F). Data

were collected every 6 minutes by an Environmental data logger LSI LASTEM ELO515.

The thermal transmittance values measured on the retrofitted wall were equal to 0.31 and 0.27 W/m2k (0.055 and

0.048 BTU/ft2hr.°F) respectively for the test façade at Politecnico and the demo building in Cinisello. Considering

that the thermal transmittance values before retrofitting were respectively 0.99 and 0.83 W/m2K (0.174 and 0.146

BTU/ft2hr.°F), a reduction of this parameter of 69% and 67.5% has been obtained.

In Fig. 3, the superficial temperatures collected in the summer and winter campaign are shown: “TS_ext” is the

temperature measured on the external surface of the applied panel, and “TS_int” is the temperature measured in

the cavity on the outer surface of existing wall.

NUMERICAL MODEL

Description of the model

A thermo-mechanical model of the panel was implemented in Abaqus /CAE 6.13-1 (Fig. 4). The maximum

dimensions of the panel are considered. In order to reduce the numerical effort, half of the panel has been

reproduced exploiting yz symmetry.

As visible in Fig. 4a, the panel layers (TRC and EPS) and the mortar thickenings for panel fixing are modelled as

solid and homogeneous; perfect bond is assumed at EPS/TRC interfaces, as no debonding was observed during

the experimental campaign. Bent bars were not modelled as their influence on the panel global behavior is

negligible (see numerical results in [12]).

Concerning constraints (Fig. 4a), displacements orthogonal to the yz symmetry plane are prevented, displacement

in y-direction is prevented at points F1 and F2, and displacement in z-direction is prevented at point F2. The

gravity load is accounted in z-direction and superficial temperatures are imposed on internal and external surfaces

of the panel according to the data collected in the monitoring campaign. In particular, two scenarios are

considered: a summer week (July 2015, 20th – 28th) and a winter week (November 2015, 21st – 29th). In Fig. 5 (a,

b) the imposed superficial temperatures are shown.

The mesh - shown in Fig. 4b - is constituted by 158,846 four-node linear tetrahedral coupled temperature-

displacement elements (C3D4T). There are one element over the thickness of each TRC layer, four elements over

the EPS thickness.

Definition of material properties

In the following, the constitutive laws adopted for each materials are described.

Concerning TRC, the elastic behavior is defined introducing a Young's modulus of 30 GPa (4,351,132 psi) [13]

and a Poisson's ratio of 0.2. Abaqus Concrete Damage Plasticity model is used for the definition of the plastic

behavior. As no damage variable is introduced, the model simply behaves as a plasticity model. In compression,

an elastic-perfectly plastic behavior is assumed considering a yield strength of 87.7 MPa (12,720 psi) [section

“Panel description”]. In tension, the stress-irreversible strain relationship based on experimental results already

adopted in [12] is used. It is worth to note that TRC tensile behavior is assumed homogeneous over the layer

thickness; the reliability of this assumption was discussed and proved in [10].

Expanded polystyrene is modelled as elastic, considering a Young's modulus of 13.7 MPa (1,987 psi) [10] and a

Poisson's ratio of 0.1. Also HPFRC is considered elastic, assuming a Young’s modulus equal to 47,500 MPa

(6,889,293 psi) and a Poisson’s ratio equal to 0.2 [14,15].

Physical and thermal properties adopted in the model for materials are collected in Table 2.

NUMERICAL RESULTS

In this section, the results of the finite element thermo-mechanical analysis are shown and discussed. In particular,

the out-of-plane displacement U2 (y-direction) of point C1 (central point of the panel – Fig. 4a) is plotted in Fig.

5 with reference to the summer (c) and the winter (d) scenario. In Fig. 5 (e, f), the elongation of the panel along

the longitudinal direction is shown in terms of displacement U3 of point F1 (see Fig. 4a) for both summer and

winter week.

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Looking at the results, it is worth to note that the maximum deflection registered is limited to 1/100 of the total

panel thickness. The maximum in-plane displacement is equal to 0.1 mm (0.004 in), which is smaller than the

space between adjacent panels (10 mm [0.40 in]), thus avoiding any interaction between them.

For some significant steps (t1, t2, t3 and t4) characterized by important or minimal thermal gaps, the temperature

profile and corresponding strain profile along the thickness of the panel are plotted (Fig. 6 - 7). In the strain profile,

the total strain - highlighted by a thicker continuous line and by a gray shadow – is due to different contribution:

the thermal strain, the elastic mechanical strain and the plastic mechanical strain. It is worth noting that, during

all the time path, any inelastic mechanical strain occurs, thus meaning that textile reinforced concrete remains un-

cracked.

CONCLUSIONS

Basing on the results of the numerical analysis it is possible to state that:

- even if the panel is subjected to high thermal gradients over its thickness due to environmental thermal

actions, thus causing stresses on the TRC layers, textile reinforced concrete remains un-cracked and the

durability of the panel is preserved;

- small in-plane and out-of-plain displacement are registered (in particular, a longitudinal elongation of

0.1 mm [0.004 in] and a maximum deflection of 1/100);

- both TRC and EPS do not reach the yielding strength (respectively fct and fy), hence no plastic strain

occurs;

- as the yielding strength of EPS is not reached, the elastic assumption for this material can be considered

reliable.

It is worth to note that, in the model, the superficial temperature imposed on the inner surface of the panel is the

one measured on the external face of the existing wall (cavity temperature) during monitoring. However, this

assumption is in favor of safety as the total temperature variation imposed on the panel thickness is higher than

the real one.

ACKNOWLEDGMENTS

The authors gratefully acknowledge Magnetti Building for the production and the installation of the panels, Stam

for the design of the formwork and Halfen for supplying the anchors. The research was financially supported by

the European “EASEE” project, Grant Agreement No.: 285540, Thematic Priority: EeB.NMP.2011-3 - Energy

saving technologies for buildings envelope retrofitting, Starting date of project: 1st of March, 2012, Duration:

48 months.

REFERENCES

[1] EU Buildings Database - European Commission (https://ec.europa.eu/energy/en/eu-buildings-database).

[2] ISTAT Italian national census (http://dati-censimentopopolazione.istat.it).

[3] Italian national Standard n° 373/76 “Norme per il contenimento del consumo energetico per usi termici negli

edifici”.

[4] Rapporto annuale efficienza energetica 2017 (www.efficienzaenergetica.enea.it)

[5] European Project EASEE: Envelope Approach to improve Sustainability and Energy efficiency in Existing

multi-storey multi-owner residential buildings, Grant Agreement No.: 285540, starting date: March 1st, 2012,

duration: 48 months (www.easee-project.eu).

[6] Salvalai, G., Sesana, M.M., and Iannaccone, G., “Deep renovation of multi-storey multi-owner existing

residential buildings: A pilot case study in Italy”, Energy and Building, V. 148, 2017, pp. 23-36.

[7] Salvalai, G., Iannaccone, G. Sesana, M.M., and Pizzi, E., “Outer facade retrofitting trough precast insulation

panels: method and planning tool applied to an Italian residential building”, TEMA, 3 (2) 2017, pp.12-23.

[8] Standard for mortar EN 196-1, “Methods of testing cement - Part 1: Determination of strength”, 2005.

[9] UNI EN 13163, “Thermal insulation products for buildings. Factory made products of expanded polystyrene

(EPS) – Specification”, 2009.

[10] Colombo, I.G., Colombo, M., and di Prisco, M., “Analytical and numerical prediction of the bending

behaviour of textile reinforced concrete sandwich beams”. Journal of Building Engineering, 17C, 2018, pp. 183-

195.

[11] Colombo, I.G., Colombo, M., and di Prisco, M., “TRC precast façade sandwich panel for energy retrofitting

of existing buildings”, proc of DSCS 2015 - International Workshop on Durability and Sustainability of Concrete

Structures, ACI SP-305, Bologna, October 1st – 3rd, 2015, pp. 30.1-30.10.

[12] Colombo, I.G., Colombo, M., and di Prisco, M., “Numerical modelling of textile reinforced concrete

sandwich panels”, proc of Computational Modelling of Concrete and Concrete Structures – EURO-C 2018, Bad

Hofgastein, February 26th – March 1st, 2018, 8 pages.

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[13] Brameshuber, W., Brockmann, T., Curbach, M., Meyer, C., Vilkner, G., Mobasher, B., . . . Wastiels, J.,

“Textile Reinforced Concrete - State-of-the-art”, in W. Brameshuber (ed) Report of RILEM Technical Committee

201-TRC, 2006, RILEM Publications.

[14] Zani, G., Colombo, M., and di Prisco, M., “High performance cementitious composites for sustainable

roofing panels”, in J. Bastien, N. Rouleau, M. Fiset & M. Thomassin (eds), Proceedings of the 10th fib

International PhD Symposium in Civil Engineering 2014: 333-338.

[15] MC2010 (2013), “fib Model Code for Concrete Structures”, 2010.

[16] NTC2008, “Norme tecniche per le costruzioni - D.M. 14 Gennaio 2008”.

[17] UNI EN ISO 10456, “Building materials and products - Hygrothermal properties - Tabulated design values

and procedures for determining declared and design thermal values”, 2008.

TABLES AND FIGURES

Table 1 –Characteristics of the fabric

Fabrication technique Leno weave

Material AR-glass

Coating Water resin based on Styrene-Butadiene Rubber

Warp yarn spacing, mm (in) 10.0 (0.39)

Weft yarn spacing, mm (in) 14.3 (0.56)

Warp, Tex (Yield) 2 x 2400 (2 x 207)

Weft, Tex (Yield) 2 x 1200 (2 x 413)

Coating weight, g/m2fabric (psf) 100 (0.02)

Maximum tensile loada on 70 mm [2.76 in], kN (lb) 9.15 (2057.10)

a average values of 10 tests

Table 2 – Physical and thermal properties of materials

TRC and HPFRC EPS

Density, kg/m3 (pcf) 2,100 (131.1) [16] 35 (2.2) [producer]

Expansion, - 1e-5 [16] 5e-5 [producer]

Specific heat, J/kgK (BTU/lb°F) 1,000 (239) [17] 1,450 (346) [17]

Conductivity, W/mK (BTU in/ft2hr.°F) 1.5 (10.4) [17] 0.034 (0.236) [9]

Fig. 1 –Sketch (a) and picture (b) of the EASEE panel.

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Fig. 2 –EASEE demo buildings: test façade at Politecnico di Milano (a), location of the summer sensors on the

test façade (b), building completely retrofitted in Cinisello Balsamo (c) and north façade on which the winter

sensors are applied (d).

Fig. 3 –Surface temperatures collected in summer (a) and winter (b) monitoring campaign.

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Fig. 4 –Model of the panel: geometry with constraints (a) and mesh (b).

Fig. 5 –Imposed temperatures on external and internal surfaces (a: summer; b: winter), mid-point out-of-plane

displacement (c: summer; d: winter) and panel elongation along the longitudinal direction (e: summer;

f: winter).

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Fig. 6 –Summer scenario: temperature profile and corresponding strain profile along the thickness of the panel

at step time t1 (a, b) and t2 (c,d).

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Fig. 7 –Winter scenario: temperature profile and corresponding strain profile along the thickness of the panel at

step time t3 (a, b) and t4 (c,d).