Mechanical Characterization of TRC Precast Façade SandwichPanel for Energy Retrofitting of Existing Buildings

ABSTRACT of STUDIES AND RESEARCHES – V.34, 2015 - Graduate School in Concrete Structures – Fratelli Pesenti - Politecnico di Milano, Italy)
Residential and commercial buildings are responsible of a large energy consumption, both for the heating in winter and the air conditioning in summer.
Standards have introduced severe limits to guarantee the energy saving in new buildings; however, there is a significant need of retrofitting existing buildings,
because of their large impact on the real estate in continents like Europe.
In the framework of a FP7 European project, a multilayer prefabricated façade panel is proposed as an outdoor solution. It is characterized by an internal EPS
layer, 100 mm thick and by two external layers made of textile reinforced concrete (TRC), 12 mm thick. The insulating material is also used to transfer the shear
between the external TRC layers. The maximum size of the panel is 1.50 x 3.30 m2; the panel height is properly chosen in order to fasten it to the frame concrete
beams. In the paper the design of the panel and the results of tests performed on full scale panels are resumed. The panels were punctually fastened on the corners
through suitable anchors and were loaded by means of a distributed load (considering wind pressure and suction as the main load acting on the panel). The
panels were also tested according to a 4 point-bending set-up up to collapse to check their ultimate limit state on full-size prototypes.

1. INTRODUCTION

Sandwich structures have been applied primarily in aircraft, missile and spacecraft industry since 1940s (Vinson, 1999); their use has been extended to other fields, such as building industry, since 1960s, when a worldwide boom in prefabricated building elements favoured the diffusion of these sandwich products (Davies, 2001).

Two main kind of sandwich panels are used in buildings: metallic face panels and R/C cladding sandwich panels.
Panels characterized by both the inner and the outer faces formed of metal sheets usually act compositely with a relatively low-strength core (with suitable insulating and stiffening properties). The bond between components can be obtained through a line forming process, by using adhesive or through mechanical fastenings. According to Davies (2001), these sandwich solutions are designed in such a way that they act as a composite load-bearing unit for the expected working life.

Pre-cast R/C cladding sandwich panels are generally made of two external reinforced concrete layers connected through the insulation layer by means of various type of shear connectors (Einea et al., 1991).

Those panels are not designed in order to behave as composite panel, exploiting the adhesive bond;

depending on the strength and stiffness of the shear connectors used, a sandwich behaves as fully-composite, partially-composite or non-composite panel (Salmon et al., 1997; Benayoune et al., 2008, Naito et al., 2011).

With respect to metallic face panels, their weight is considerably higher as the thickness of each concrete layer is never smaller than 40 mm.
Hegger and Horstmann (2009) proposed a lightweight panel with both the concrete layers made of textile reinforced concrete (TRC). The use of TRC guarantees to significantly reduce the thickness of the layers if compared to traditional panels. Furthermore, the fine-grained concrete used in TRC allows to obtain good durability and finishing, features that are very important for façade elements. Finally, exploiting adhesive bond it is possible to limit or to completely prevent the use of shear connectors, thus avoiding thermal bridges. In real applications, they introduced connecting devices in order to guarantee a proper sandwich action and a durable connection between the TRC layers. Shams et al. (2014) studied the influence of shear connectors on the behavior of these TRC panels.
Colombo et al. (2008), Dey et al. (2015), di Prisco et al. (2012), di Prisco and Zani (2012), Ferrara et al. (2008) and Mu?ller et al. (2012) also studied cement based sandwich elements in which advanced cementitious composites are used for the external layers and the connection between the layers is obtained only through the bond between the insulating material and the cementitious layer, without any connector.

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 TRC sandwich panel is proposed for the energy retrofitting of existing building (EASEE, 2012-2016). Targets of the project are multi-storey multi-owner residential buildings, dated before 1975, characterized by reinforced concrete frame structures and hollow-core brick walls.

One of the main project objectives is the design of a technological solution representing a valid and more durable alternative to the exterior insulation and finishing system (EIFS), which is usually used for the energy retrofitting of existing buildings. The main advantages of the solution, if compared with the thermal coating (EIFS system), are: the lower impact on occupant life (no scaffolding required), the possibility to obtain the desired finishing in terms of surface roughness, color, pattern (including the reproduction of the original façade), the increase in impact resistance, the higher quality of finishing and an higher expected durability. The latter aspect is particularly important, especially considering a residual expected building life of at least 30 years. Aesthetic and durability aspects are directly related to the use of a high strength fine-grained concrete in TRC.
Durability problems could occur mainly in the EPS/TRC interface and in both the materials (TRC layers and EPS), because of temperature conditions correlated to freezing and thawing cycles in winter and sun radiation in summer. Moreover, alkali-resistant glass fabric could be affected in the time by some degradation, resulting in a loss in strength of the TRC composite: in previous tests performed on cementitious materials reinforced with AR-glass fibers, a loss in strength of about 20% was detected after 10 years (Purnell et al., 2006). The use of modified matrices and coatings can determine substantial improvements in TRC durability (Purnell et al., 2006).

The proposed panel is also characterized by all the advantages related to precasting in terms of quality control and fast mounting. The use of TRC allows the producer to keep the weight of the panel under 80 kg/m2; that means building site safety during panel handling and low building mass increment, requirement that plays a key role in seismic areas.
This paper is focused on the mechanical characterization of full-scale panels.

2. TRC PRECAST FAÇADE SANDWICH PANEL
The final solution proposed by the EASEE consortium consisted in a prefabricated façade sandwich panel characterized by an internal insulation layer 96 mm thick, made of expanded polystyrene (EPS 250) and by two external layers 14 mm thick in textile reinforced concrete (Figure 1).
The maximum size of the panel was 1.50 x 3.30 m2. The panel height was properly chosen in order to fasten it to the frame concrete beams by means of four connectors placed near to the corners on the short edges: the two upper connectors are aimed at resisting only the wind pressure acting on the panel, while the two connectors placed at the bottom are loaded both by wind pressure and the selfweight of the panel.

When the panel is applied to a façade, the anchoring system connects the internal TRC layer to the bearing structure of the building. When the wind pressure acts on the external surface of the panel, the load is transferred from the outer to the inner TRC layer through the insulation layer and from the inner TRC layer to the bearing structure through the anchors. A proper anchoring system, which is not discussed in this paper, has been developed by the consortium of the European EASEE project in order to properly transfer the stresses from the panel to the bearing structure and to resist the wind load and the panel self-weight.

The thickness of the polystyrene layer is chosen in order to guarantee a proper thermal insulation, significantly reducing heat losses of the building. In order to correct the out of plumb of the existing façade, an air cavity is left between the panel and the wall (Figure 1a).

Thermal bridges caused by the connectors are prevented by using the insulating material to transfer the shear between the two external TRC layers.
However, in order to prevent the outer layer detachment in extreme conditions (e.g. fire), the panel is equipped with four stainless steel AISI 310S bent bars (Diam. 5) embedded in both the longitudinal panel edges at the upper and lower ends (Figure 1b).
The fabric warp is aligned with the longitudinal direction of the panel. The detail of the four edges is visible in Figure 1c; the choice of this geometry is related to the requirement of a staff bead on all the corners in order to prevent any damage during handling and to guarantee an adequate aesthetic finishing of the joints. Furthermore, the mortar corner, together with a proper elastomeric joint, protects the insulation layer from the atmospheric agent attack.

A vertical formwork is used to cast the panels in order to guarantee a proper thickness of the concrete layers, minimizing the presence of voids in the mortar and ensuring an adequate level of finishing.

The panels designed in the project have been applied on a test façade at Politecnico di Milano and on three demo-buildings respectively in Gdynia (Poland), Madrid (Spain) and Cinisello Balsamo (Italy). The last one in Cinisello Balsamo is a full demo-building, which was completely retrofitted by applying the EASEE panels. It is a four storey building (ground floor for the basement plus three floor of apartments) characterized by a reinforced concrete load-bearing structure with central concrete columns and external reinforced concrete walls. Pictures of the test façade and of the Italian demo building are shown in Figure 2.

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