Review of precast concrete sandwich panels and their innovations

Construction and Building Materials 233 (2020) 117145

Contents lists available at ScienceDirect

Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat

Review

Review of precast concrete sandwich panels and their innovations Richard O’Hegarty ⇑, Oliver Kinnane School of Architecture, Planning and Environmental Policy, Richview Research, University College Dublin, Ireland

h i g h l i g h t s
Precast concrete sandwich panels are reviewed.
There is a strong research focus on innovative thin designs.
Thin/light precast concrete sandwich panels can provide a thermally efficient solution.
Further thermal evaluation of panels is needed.
Further structural evaluation on the repeatability of results that use novel connectors is required.

a r t i c l e

i n f o

Article history: Received 16 April 2019 Received in revised form 5 September 2019 Accepted 3 October 2019

Keywords: Precast concrete Cladding Sandwich panel Thermal efficiency Composite action

a b s t r a c t Precast concrete cladding is experiencing a resurgence in popularity as a means of building cladding. Precast concrete sandwich panels are a specific type of precast cladding, characterised by high thermal resistance provided by a layer of insulation which is sandwiched between two concrete wythes and fixed together with mechanical connectors. Hence, they have the potential to offer a thermally efficient cladding solution for new buildings and as replacement cladding in the case of renovation. Due to this thermal efficiency potential, and the resurgent popularity of precast concrete generally, considerable innovation in sandwich panel design has occurred in recent years. New high-performance concrete, with novel means of reinforcement, have enabled lighter and thinner concrete wythes. Numerous new fibre reinforced polymer connectors have been developed and tested that facilitate shear load transfer across the layers while also minimising localised heat loss. State-of-the-art lowconductivity insulation technologies allow for remarkably low U-values for thin wall build-ups. All these innovations are increasing the applicability of sandwich panels for a wider range of building typologies and are augmenting the inherent benefits of precast cladding. A review of precast sandwich panels is therefore timely to evaluate the body of research undertaken and analyse proposed design-solutions and test-results to identify future research focuses. This review determines particular advancements in the development of high strength concretes with no steel but that achieve high flexural strengths. It reviews the considerable number of studies focused on the structural performance of sandwich panels but highlights a need for further thermal validation of novel sandwich panel designs and further testing of different connector types. Ó 2019 Elsevier Ltd. All rights reserved.

Contents 1. 2.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 Technology and industry overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 2.1. Manufacturing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

Abbreviations: PCSP, precast concrete sandwich panel; NC, normal concrete; FC, foamed concrete; SCC, self compacting concrete; HPC, high performance concrete; RPC, reactive powder concrete; HPFRC, high performance fibre reinforced concrete; UHPC, ultra high performance concrete; FRP, fibre reinforced polymer; AR, alkali resistant; CFRP, carbon fibre reinforced polymer; BFRP, basalt fibre reinforced polymer; GRC, glass fibre reinforced concrete; PCM, phase change material; TRC, textile reinforced concrete; EPS, expanded polystyrene; XPS, extruded polystyrene; PUR, polyurethane; PIR, polyisocyanurate foam; VIP, vacuum insulated panels; FIB, Fédération Internationale du Béton; FEA, finite element analysis. ⇑ Corresponding author. E-mail addresses: [email protected] (R. O’Hegarty), [email protected] (O. Kinnane). https://doi.org/10.1016/j.conbuildmat.2019.117145 0950-0618/Ó 2019 Elsevier Ltd. All rights reserved.

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3.

4.

5. 6.

7.

8.

2.2. Surface finishes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 2.3. Transportation and installation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 Defining sandwich panels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 3.1. Composite and non-composite panels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 3.2. Loadbearing and non-loadbearing panels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 3.3. Heat transfer in PCSPs. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 Testing methods for sandwich panel evaluation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 4.1. Flexural tests . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 4.2. Shear tests . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 4.3. Application specific testing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 Thermal performance of sandwich panels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 Components of sandwich panels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 6.1. Concrete wythes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 6.2. Insulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 6.2.1. Vacuum insulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 6.2.2. Foamed concrete. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 6.2.3. Mineral wool. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 6.3. Connectors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 6.3.1. Concrete connectors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 6.3.2. Metallic connectors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 6.3.3. Fibre reinforced polymer connectors. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 6.3.4. Concrete-insulation bond only. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 Thin sandwich panels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 7.1. Thin panel designs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 7.2. Flexural behaviour. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 Conclusions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

1. Introduction Precast Concrete Sandwich Panels (PCSPs) are a composite cladding type encompassing concrete wythes that embed a layer of thermal insulation. Hence, they constitute a rainscreen, thermal layer and internal wall in a full wall build-up. Shear connectors are used to structurally link the concrete layers and ensure structural performance (Fig. 1). They are most typically used in lowrise industrial buildings but increasingly in a wider range of building typologies including mid to high-rise residential and commercial buildings [1,2]. Precast concrete cladding offers a number of advantages over other common cladding materials, deriving from the inherent fire resistance and durability of concrete. Cladding panels can be cast at large scale, enabling extensive façade covering with fewer joints and allowing for larger vertical spaces between supports than most façade systems [1]. The PCSP wall system additionally provides a

thermally efficient cladding solution [3,4]. The full wall is prefabricated in the factory and should therefore achieve advantages of greater quality control and reduced risk of poor detailing onsite. However, traditional PCSPs are heavy and often favoured for lowrise buildings where panels are simply tilted up into place [5]. Typically constructed of steel reinforced concrete, PCSPs often have thicknesses that exceed 300 mm [1] and associated weights of ~500 kg per m2 of wall area. To expand their applicability to a wider range of buildings the weight and thickness of PCSPs needs to be reduced and hence a number of recent projects [6,7] and studies [8–10] have focused on designing for thinner sections. However, reducing the size and weight pose significant challenges as the structural and thermal requirements of the cladding cannot be compromised. The primary aim of this paper is to review the increasing body of research literature focused on the structural and thermal evaluation of precast concrete sandwich panels. Specifically, the study

Fig. 1. 3D model of a sandwich panel (left), a photograph of full height sandwich panel (middle) and an annotated section drawings of a typical precast concrete sandwich panel section (right).

R. O’Hegarty, O. Kinnane / Construction and Building Materials 233 (2020) 117145

investigates the innovations and challenges associated with this cladding technology by focusing on the individual components that make up the sandwich panel (the concrete wythes, insulation core and mechanical connectors) and that enable their structural and thermal efficiency. Innovations in each of concrete, insulation and connector technology are reviewed in the context of PCSPs. Given the current research focus on reducing the thickness of these panels, an in-depth review of thin sandwich panel designs is presented. The paper begins with an overview of the technology and industry. Following this, methods of defining performance, behaviour and typology of sandwich panel are introduced. The varied testing methods used to assess the performance of PCSPs are reviewed in Section 4, results of studies are analysed and compared. Studies that have focused on the thermal performance of PCSPs are reviewed in Section 5, and proposals are made for further investigation and thermal validation. Key components of a PCSP, presented in Fig. 1, are discussed in detail in Section 6, and recent innovations of each are reviewed. In Section 7 all thin-PCSP designs are reviewed in detail, comparing different designs and test results. 2. Technology and industry overview PCSPs have been used in the construction industry for more than 60 years [1]. According to Gleich [2] the first PCSP design comprised two wythes structurally connected with cast concrete ribs along the full length of the panel. In subsequent iterations solid square concrete zones replaced the continuous ribs with the aim of reducing the amount of concrete penetrating the insulation layer while still providing the lateral shear load transfer between the two wythes [2]. Steel trusses were later used to further reduce the amount of material bridging the insulation layer. However, these steel trusses also present significant bridging across the insulation due to the high thermal conductivity of the steel [11]. Smaller, thinner steel ties became popular in the 1980s [2]. They resulted in thermal benefits from the reduced amount of steel, although they still enabled a path for considerable heat loss [12]. Also, they reduced the overall structural capacity of the panel; as will be reviewed in subsequent sections. Fibre-Reinforced Poly-

3

mer (FRP) connectors were introduced later to replace metallic connectors and represented a disruptive technology for the industry. They are widely used today. FRP is characterised by a lower thermal conductivity (~1 W m1∙K1) than steel (~50 W m1∙K1) and hence, as connectors, they transfer less heat when crossing the insulation layer [13]. More recently there has been a notable increase in work concerned with reducing the overall thickness of the panels by, for example, using high performance insulation [14,15], or through the use of textile reinforced concrete [8,10]. These technologies are reviewed in detail in Sections 6.2 and 6.3 of this report. Today PCSPs are widely used globally in projects ranging from prisons to multi-storey apartment blocks. A selection of these buildings clad in PCSPs are presented in Fig. 2. The Proximity Hotel (Fig. 2 – building 5) is an example of where PCSPs are used to achieve high building efficiency and is described in a study of green hotels that achieved the highest possible energy rating in the US [16]. In keeping with this theme of high energy efficiency, sandwich panels have been designed as solar thermal panels by integrating piping into the outer concrete wythe to enable a thermally active façade [17,18]. As concrete has a much lower thermal conductivity than materials commonly used in solar thermal technologies, its best used for low-temperature applications such as for preheating applications in a building [19]. This review is primarily focused on the structural and thermal characteristics of PCSPs and studies that have reported on the testing of these. However, the processes of manufacturing, finishing, transportation and installation are all also undergoing innovative changes. These topics are reviewed in the following sub-sections, beginning with the manufacturing process. 2.1. Manufacturing The manufacturing of a PCSP varies depending on the specific type of panel constructed and the precast facility, but in general a common process is followed, beginning with the preparation of timber/steel formwork. Once ready, steel reinforcement is placed and the first layer of concrete is poured. If prestressed concrete is specified, the prestressing strands are laid down and can be prestressed either before or after the concrete is poured. While the

Fig. 2. Examples of projects that use PCSPs. Each caption lists Building | Location | Panel manufacturer. Pictures 1–3 & 6 [20], Picture 4 [21], Picture 5 [16].

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Fig. 3. Selection of different surface finishes (Images from Techrete [20]).

concrete is still wet the connectors and insulation are fixed into place in accordance with the engineering specifications. The final layer of steel rebar is placed on top of the insulation and final layer of concrete is then poured. The exposed surface of concrete is finished to meet a desired appearance, for example with a trowel for a smooth appearance or brush for a rough appearance, while the bottom surface will have a smooth formwork finish. Once cured the panels are stripped, tilted vertically and lifted out of the formwork and stored until dispatch. Innovations in the manufacturing process over the last decades have generally been led by industry and are primarily focused around increasing the speed of production. Precast facilities typically use rapid hardening cement in their mixes to achieve high strengths at 12–24 h (or earlier), while heat curing is also sometimes applied to enable early-age strength development. But in the case of PCSPs attention must be made to limit the maximum curing temperature to avoid a differential change in volume of the insulation layer and concrete wythes [1]. An innovative casting method that casts the first layer of the PCSP with the connectors, allows it to cure and then tilts, and dips, it into the second layer of fresh concrete is one innovative method of PCSP manufacturing used by Vollert [22]. An advantage of this highly automated method is the ability to achieve two formwork surface finishes on both wythes, but a disadvantage is the requirement for specialised equipment, additional curing time and limited flexibility in terms of finishing and geometry. 2.2. Surface finishes As well as formwork finishes, a wide range of other surface finishes are available. The precision, consistency and control with which these can be applied in the precast factory, is a primary reason precast cladding is commonly specified by architects today. Similar to single skin concrete, sandwich panels can benefit from these finishes. Surface finish treatments can be applied to both fresh concrete (e.g. water washing, brushing, sand casting, brick faces) and hardened concrete (e.g. acid etching, sand blasting, polishing, power brushing). A selection of popular surface finishes are presented in Fig. 3. Honing involves the grinding away of a surface with a grit to achieve a smooth finish. Polishing uses a finer grit to achieve an even smoother finish. Surface retarders can be used on the face of the mould so that the concrete surface doesn’t set as quickly as the rest of the panel and therefore can be washed away to expose the aggregates. Acid etching involves washing of the concrete with a dilute acid solution to achieve a textured finish. A deeper texture can be achieved by blasting the panel with sand or grit. Stone, ceramic or brick fronted panels can also be achieved by placing them in the mould and pouring concrete over them to bond them in a prefabricated panel. These technologies are all well-established, however they are being applied with greater control and precision today than before, enabling extravagantly finished concrete claddings. There is little

related to surface finish technology documented in the academic literature and instead innovation is led by industry and patents. As examples of innovators in the industry the Finnish company Graphic ConcreteÒ, use what seems like surface retarders to achieve very well resolved patterns and graphics. The Tactility Factory have created INFUSED CONCRETEÒ that integrate textiles and fabrics to great effect, primarily for interior finishes. Detailed reliefs (e.g. upscaled Elgin marbles images [20]) have been applied to panels using inventive computational and mould milling technologies. Similarly 3D printing is proposed as an innovative solution for achieving textured mould finishes [6].

2.3. Transportation and installation While industry continues to develop more innovative ways to manufacture and finish PCSPs, most of the latest academic work focuses on reducing size and weight of PCSPs. One of the great benefits of lighter, thinner PCSPs is the reduced impact on transportation and installation as a consequence of the weight reduction. Typical panel sizes vary from country to country with different allowances for unescorted transportation. Special permits are typically required if height, width or weight exceed specific limits. For example, in the US there is a maximum allowable width of 3.7 m for unescorted transportation [1]. Sandwich panels are typically transported on their edge so care must be taken to avoid chipping and spalling. Without proper care and attention, damage (such as that illustrated in Fig. 4) often requires laborious patch-work finishing, or in some cases, complete reconstruction of the damaged panel. Panel handling can dictate the minimum allowable thickness of PCSP. For example, a minimum bolt diameter of 19 mm should be used for any precast handling, regardless of load requirement [15]. This means that even if structural design requirements are

Fig. 4. Damage to one of the wythes of a PCSP that occurred during transport.

R. O’Hegarty, O. Kinnane / Construction and Building Materials 233 (2020) 117145

5

achieved there are minimum allowable thickness requirements if conventional installation processes are to be used.

y1 ¼ c1 

t1 2

ð2Þ

3. Defining sandwich panels

y2 ¼ c2 

t2 2

ð3Þ

The majority of academic literature related to sandwich panels report on the structural testing of novel panel designs to investigate if design requirements can be met. These studies focus on flexural and shear testing of sandwich panels, with results of both dependent on the level to which the concrete wythes transfer loads to each other – referred to as composite or non-composite action. Panels may also be designed to carry the dead loads of building elements above – these load-bearing panels demand additional strength to resist the different load conditions and other designs and test methods are consequently investigated. PCSPs are typically introduced in studies as being thermally efficient, this study investigates the validity of these claims, with reference to the heat loss through the panel. Relevant structural and thermal descriptors of PCSPs are introduced in this section, and studies from the literature are reviewed throughout this review paper with reference to these.

where c1 (m) and c2 (m) are the distances from the top and bottom surfaces of the panel to the neutral axis of the entire panel. c1 can be calculated according to Eq. (4) where A (m2) is the area of the concrete sections (Eq. 5) and c2 is the difference between the height h (m) and c1 (m) (Eq. 6). Dimensions of a composite PCSP are shown in Fig. 5.

3.1. Composite and non-composite panels PCSPs are often categorised as either composite or noncomposite to describe the structural behaviour of the panel. A composite PCSP is one in which the two concrete wythes act together to resist bending. A non-composite PCSP is one in which the two wythes act independently to resists the applied loads. These two different structural categories are introduced here and are referred to throughout this review. In the design of non-composite panels, the wythes are treated as individual elements, and the bending moment capacity is calculated by assuming the load is distributed in proportion to the bending stiffness of each wythe. This distribution for a point load is shown in Fig. 5. For a fully composite panel the bottom and top wythe work together to resist bending. To achieve this behaviour the connectors between the two concrete wythes must provide lateral load transfer. The moment of inertia of a composite panel, Ic (m4), depends on the location of the Neutral Axis (NA) of the panel, which depends on the thickness of each wythe and is calculated according to Eq. (1).

Ic ¼

bt31 12

þ bt 1 y21 þ

bt32 12

þ bt 2 y22

ð1Þ

where b (m) is the width of the panel, t1 (m) and t2 (m) are the thickness of the two wythes and y1 (m) and y2 (m) are the distances from the NA of each individual wythe to the NA of the entire panel, and are calculated following Eqs. (2 and 3).



c1 ¼

 0:5bt21 þ bt2 ðh  0:5t 2 Þ A

ð4Þ

A ¼ bðt1 þ t2 Þ

ð5Þ

c2 ¼ h  c1

ð6Þ

In reality the behaviour of most PCSPs lies somewhere between fully composite and non-composite. The degree of composite action is typically quantified by way of experimental testing but some analytical methods have also been developed [23]. Indicative strain profiles associated with the varying degrees of composite action are presented in Fig. 6. The smaller strains, and hence stresses, show how a higher degree of composite action leads to thinner wall designs for the same loading conditions. 3.2. Loadbearing and non-loadbearing panels A PCSP may also be required to carry the load of the walls and/ or floors above – referred to as loadbearing PCSPs [15]. The load is transferred vertically to a foundation as shown in Fig. 7-a and b. Most of the academic literature reviewed in this paper is, however, focused on non-loadbearing PCSPs – panels that don’t carry the load of any other building component (Fig. 7-c). This allows nonloadbearing PCSPs to be thinner than loadbearing panels. These panels are typically supported at each floor level with a support at either the top or the bottom of the panel, used to transfer its self-weight to the primary structure and a restraint at the other end to provide overall stability – idealised as a simply supported beam from floor to floor. 3.3. Heat transfer in PCSPs There is a general trend globally to improve the energy efficiency in buildings and reduce the heat loss. PCSPs differ from many cladding options in that they can achieve high thermal resistance. The thermal performance of the panel is usually quantified by the thermal transmittance, U-value (W m2∙K1), in Europe and the thermal resistance, R-value (m2 K∙W1), in the US for

Fig. 5. Relevant dimensions and neutral axes of a PCSP for composite and non-composite cases. Load distribution is shown for the non-composite case. (Moment of inertia of Top wythe = I1 | Bottom wythe = I2).

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Fig. 6. Degree of composite action displaying approximate strain plots for a) composite b) non-composite and c) partially composite.

Fig. 7. Simple section examples of loadbearing (a and b) and non-loadbearing (c) PCSP fixing system.

example. The U-value (W∙m2∙K1) is used in this study for comparison of different designs. It is calculated according to Eq. (7) where tc and ti is the thickness of the concrete and insulation layers (ISO 6946 [24]). External and internal surface resistance values (Rse = 0.04 m2∙K∙W1 ; Rsi = 0.125 m2∙K∙W1) are taken from the CIBSE Guide A [25].

1 tc ti ¼ Rse þ þ þ Rsi U kc ki

ð7Þ

The higher the U-value the greater the heat loss for a given temperature difference. In colder climates, building regulations typically specify lower U-values then in warmer climates. For example, in Finland the maximum allowable U-value is 0.17 W∙m2∙K1 while in Spain a U-value of 0.74 is deemed acceptable [15]. Low U-values are equally important for keeping airconditioned spaces cool in hot climates. As well as considering U-values, thermal bridging also needs to be considered. A thermal bridge is a discontinuity in the insulation layer which results in an increase in the heat flow through that particular section. In the context of PCSPs thermal bridges occur at the location of the connectors between the panels. The impact these thermal bridges have on the thermal performance of PCSPs is dependent on the size and the material of the connector. With larger, more-conductive, connectors (e.g. steel trusses) creating a

greater thermal bridge and hence more heat loss than a smaller, less-conductive, connector (e.g. FRP pin). 4. Testing methods for sandwich panel evaluation PCSPs have been introduced, a brief summary of the history of PCSPs has been outlined and the terminology needed to describe them has been defined. Different experimental and theoretical test methods, used throughout the literature, to evaluate the performance of PCSPs are described in this section. And the studies that use the various methods are listed. The results from the listed studies are not presented here but are documented instead in the proceeding sections that evaluate individual components (Section 6) as well as innovative designs (Section 7). This section therefore provides a review of the methodologies – it does not aim to provide a review of the results as the results are not directly comparable. 4.1. Flexural tests Wind induced flexural loading is common to all cladding elements. As a result, the majority of PCSP research has focused on testing these panels in flexure. 3-point [26,27], 4-point [9] and also 6-point [28] bending tests are most common as these tests can be done with standard laboratory equipment (displacement sensors, load cell, hydraulic jack, test frame). Wind loads are, however, bet-

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ter represented by applying a uniformly distributed load to simulate either pressure or suction on the panel – this can be achieved by using an air bag between one face of the PCSP and another external rigid surface (e.g. the ground) and blowing air into the bag to create a uniform load on one face of the panel [29]. While this method is more representative of the actual loads which the panel will sustain in operation it is also significantly more complicated to set up. In any test set-up the bending moment capacity of the panel can be determined both at first cracking and at ultimate failure loads. Schematics of different flexural test set-ups are presented in Fig. 8. Flexural test results for different thin-PCSP designs are presented and compared in detail in Section 7. Different methods have been introduced that define a degree of composite action based on flexural test results. Bush and Stine [30] calculated the degree of composite action using strain measurements across the top and bottom of both wythes of concrete and compared the internal moment to the applied moment. Pessiki and Mlynarczyk [29] defined the degree of composite action based on measured deflection to determine the flexural stiffness of the tested panel and compare it to the theoretical fully composite and non-composite panels. This stiffness method is particular useful and is commonly applied as only load and deflection measurements are required. Kim and You [31] calculated the composite action by comparing the measured ultimate load of a partially composite panel with the measured ultimate load of both composite and non-composite panels. Fully composite action was ensured by joining the wythes with thick concrete ribs; the non-composite panels contained no connectors. Tomlinson and Fam [32] also used a strength based method to determine the degree of composite action at ultimate, yield and serviceability loads. They found that the strength based methods gave very different degrees of composite action (51%90%) when comparing the composite action of the same panels using the stiffness based method proposed by Pessiki and Mlynarczyk [29] (3% to 6.9%). All these methods require fullscale experimental test results. The flexural behaviour of PCSPs have also been tested theoretically using Finite Element Analysis (FEA). Complex non-linear FE models that predict the flexural behaviour of specific PCSP designs have been developed and validated with experimental results [9,33,34]. A simple linear-elastic FE model has also been presented which is used to understand the pre-cracking composite behaviour of PCSPs [35]. Analytical models used for the analyses of PCSPs are reviewed by Bai and Davidson [36] who themselves present a discrete model that captures the complex shear deformation and independent flexural resistance of PCSPs. The model is validated with experimental results and provides accurate results for precracking analysis. Tomlinson and Fam [37] present an analytical model which accounts for cracking and connector failures. However, as noted by Gombeda et al. [23], the shear tie properties are limited to the elastic range. Recently Gombeda et al. [23] proposed a simplified analytical method which does not require

7

experimental results of the full sized panels. The method is based on determining the total shear force contribution of the connectors by way of an iterative process and super-imposing this moment contribution onto the non-composite scenario. To use this method, however, the mechanical properties of the connectors are required, and so small-scale testing is still neccessary.

4.2. Shear tests The structural efficiency of PCSPs is defined by a degree of composite action. A panel with a higher degree of composite action will have a greater global bending moment capacity and stiffness than a panel of equal material and geometry but with a connector that does not achieve the same level of shear transfer and hence composite action. Therefore, the importance of the shear flow between the two wythes is also specifically investigated in a number of studies. Naito et al. [38] conducted a comprehensive experimental investigation of the behaviour of 12 different connector configurations and showed average strength variations of between 5.5 and 18.4 kN for discrete connectors (e.g. pins) and between 17.2 and 57.8 kN∙m1 for distributed connectors (truss or gird type). The continuous type connector (such as the CFRP C-GridÒ from Table 5e) provided greater initial stiffness than discrete ties (such as the ThermomassÒ GFRP pin from Table 5-a). They used results from their experimental work to develop a simplified trilinear model for each of the shear connectors. A shear test is typically carried out on a 5 layered test panel (3 wythe and 2 layers of insulation) with the two outer wythes fixed to a test frame and the middle wythe pushed away from these two fixed outer wythes by way of a hydraulic jack or equivalent while load and deformations are measured (Fig. 9-b). Shear tests can also be conducted on typical 3 layered panels, but this test may be subject to rotation of the outer wythe [39]. Shear testing specifically investigates the behaviour of the connectors but it is well documented that the insulation can have also have an impact on the shear behaviour [10,40,41]. Bunn [40] specifically investigated the impact of insulation type and thickness using a grid type connector. Kazem et al. [42] looked at the effect of short term weathering on the shear capacity of PCSPs and their connectors, concluding the 7-months of exposure had negligible impact on the performance.

4.3. Application specific testing Flexural and shear tests are by far the most common tests conducted on PCSPs but other test methods have also been carried out, such as those shown in Fig. 10. Some PCSPs are designed as a structural load bearing wall (Section 3.2) and have therefore also been tested under an axial loading regime [43–48] (Fig. 10-a). These load bearing panels require additional steel and/or concrete to meet

Fig. 8. Flexural test set-ups and associated bending moment diagrams for a) 3-point loading b) 4-point loading and c) pressure loading.

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mal loading and bowing of the panels need to be accounted and subsequently tested for [56–58].

5. Thermal performance of sandwich panels

Fig. 9. Shear test set-ups. a) Two wythe shear test b) Typical three wythe shear test used to eliminate rotational movement.

the additional structural requirements. Goh et al. [45] developed a non-linear FE model to simulate the axial compression of a PCSP. Impact loading is a concern for most cladding panels and its effects on a specific PCSP have been documented [49]. Traditionally impact loading is conducted by using a pendulum to swing an impactor (both soft and hard objects are used) against a full wall build up [50] (Fig. 10-b). This test is costly and, although important, has not attracted much academic attention. PCSPs can also provide a solution for quick and temporary military buildings. These specific panels need to be resistant to blast loading and therefore have been tested accordingly [51–53] (Fig. 10-c). PCSPs have gained attention as a solution for flooring and roofing systems [26,27]. Such structural elements are designed to withstand sustained dead loads and as a result creep behaviour is a concern. This has been investigated by Hopkins et al. [54] (Fig. 10-d) who found that confining the PCSP in FRP plates reduced the rate of creep. It is also important for PCSPs installed on buildings in seismic zones to resist lateral or seismic loading conditions [55] (Fig. 10e). And importantly in countries with extreme temperatures ther-

PCSPs first and foremost need to resist any load imposed on them, but this would be achieved by using a single wythe reinforced concrete element. PCSP differ from a standard reinforced concrete wall in that they contain the insulation layer within to provide the thermal resistance for the building. It is therefore also important that they are thermally investigated. However, to date, the majority of research is focused on the structural testing of PCSPs with only a small number of experimental thermal testing studies [3,59,60] and number of simulation based studies [3,12,14,59–63]. Those PCSPs, which have been structurally tested, are thermally assessed in this section. A summary of the effective U-values, U (W m2 K1), of different tested panels are calculated and presented in Table 1. The effective U-values neglect the impacts of thermal bridging as to do so would be a significant study in itself [64] and is outside the scope of this paper. By neglecting thermal bridging in the calculation, the best-case scenario is documented. The thermal conductivity values of insulation materials are taken from Table 3 unless specified. The thermal conductivity of the concrete, kc (W m1 K1), is not reported in almost all the reported studies and a value of 1.65 W m1 K1 is therefore assumed [65]. The thermal resistance of the panels is dominated by the resistance of the insulation, because the thermal resistance from the concrete layer is significantly lower than that of the insulation. For example, the U-value obtained for the panel designs of Colombo et al. [66] and Frankl et al. [13] is the same but the overall thickness differs by 80 mm. This is because the insulation type and thicknesses are the same. Interestingly, most panels would not abide by current Irish building regulations which stipulate a required U-value of 0.18 W m2 K1 for new domestic buildings [67] and 0.21 W m2 K1 for new non-domestic buildings [68] while only two of the listed PCSPs meet passive house standards (of 0.15 W m2 K1 or better [69]). And this is importantly before thermal bridging is accounted for, which can account for up to 70% of the heat loss in highly insulated panels [4]. Evidently there is a requirement for greater focus on the thermal performance of this cladding typology.

Fig. 10. Illustrations of other testing methods, including a) Axial, b) impact with pendulum, c) Blast from explosions, d) Sustained creep load and e) Seismic test of wall with window.

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R. O’Hegarty, O. Kinnane / Construction and Building Materials 233 (2020) 117145 Table 1 Review of insulation used in panels and their associated U-values. Reference

Hodicky et al. O’Hegarty et al. Hegger et al. Shams et al. Shams et al. Teixeira et al. Tomlinson & Fam Frankl et al. Chen et al. Portal et al. Choi et al Flansbjer Choi et al Kim and You Hopkins et al. Frankl et al. Joseph et al. Colombo et al. Enfedaque et al. Al-Rubaye et al. Bush and Stine Pessiki and Mlynakrzyk Benayoune et al. Amran et al. Amran et al. Lee et al.

[70] [4] [71] [72] [10] [73] [32] [74] [26] [9] [75] [76] [77] [31] [54] [78] [79] [66] [80] [28] [30] [29] [43] [34] [46] [81]

U (W∙m2∙K1)

Total panel thickness (mm)

0.08 0.09 0.16 0.20 0.20 0.22 0.22 0.31 0.31 0.31 0.31 0.31 0.31 0.31 0.31 0.32 0.32 0.32 0.35 0.41 0.57 0.57 0.70 0.89 1.04 3.55

330 150 180 280 260 270 270 255 254 230 260 200 220 220 254 200 150 120 110 280 203 203 121 150 125 65

Insulation Type

Thickness (mm)

PF PF + VIP PUR EPS EPS EPS EPS EPS EPS FC EPS FC EPS EPS EPS EPS EPS EPS EPS XPS EPS EPS EPS EPS EPS EPS-concrete

290 90 150 160 160 150 150 102 102 150 100 150 100 100 102 100 100 100 90 76 51 51 41 30 25 55

*Conductivity of Foamed Concrete (FC) = 0.05 W m1 K1 – Average value from [76]. **Where Polystyrene (PS) is used it is assumed that it is an EPS. ***In studies where a number of panels were tested the best thermally performing panel is referenced – i.e. the panel that specific study with the best insulation. ****Thermal bridging is not accounted for in any of the calculations.

6. Components of sandwich panels This section provides a thorough review of the innovations of the individual key components (numbered in Fig. 1) as documented in the academic literature. Results from the tests outlined in Section 4 are also outlined in relation to these specific components. 6.1. Concrete wythes The concrete wythes are the structural part of the wall system and form the inner and outer surfaces. Although some three wythe PCSP designs have been proposed [60,63] most of the reviewed

panels are constructed of two concrete wythes. The two wythes typically use the same type of concrete for both wythes. Comparison between different PCSPs in the literature are generally made with interest to the different insulation and connector types. Few studies directly compare the performance of a sandwich panel with various concrete mixes. The compressive and tensile strengths of the concrete mixtures used in PCSPs are compared here in Table 2. Normal Concrete (NC), Foamed Concrete (FC), Self-Compacting Concrete (SCC), High Performance Concrete (HPC), High Performance Fibre Reinforced Concrete (HPFRC) Glass Reinforced Concrete (GRC), Reactive Powder Concrete (RPC) and Ultra High Performance Concrete (UHPC) have all been used in PCSPs. Different concrete compressive strengths are reported

Table 2 Material properties of different concrete wythes in increasing order of compressive strength. Material

Reference

Reinforcement

Compressive strength* (MPa)

Tensile strength* (MPa)

FC FC NC FC NC NC NC SCC SCC HSC GRC FRC HPC HPFRC RPC UHPC UHPC

[45] [46] [33 43,44] [34] [51 26] [29] [7731] [79] [9] [8366] [57] [71] [56] [35] [76] [72] [81]

Steel Steel Steel Steel Steel Prestressing steel Steel Steel Carbon textile AR glass textile AR Glass fibres AR glass textile Polypropylene fibres Coated glass fibres Carbon textile Carbon textile Steel fibres

8 25 24 26 28 41 45 46 72 73 78 82 90 96 147 175 193

0.9 1.9 2.5 2.1 NA NA NA 4.3 3.2 4.5 3.9 2.1 6.0 15.3 5.1 18.8 32

*The type of compressive test (e.g. cube, core, cylinder) and tensile test methods (e.g. flexural, direct, indirect) are not explicitly stated in all references and the values are therefore a representation of the approximate compressive and tensile/flexural strengths at 28 days.

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throughout the PCSP literature, ranging from 8 MPa for lightweight foamed concrete [45] to 193 MPa for UHPC [81]. In relation to reinforcement, both reinforced and prestressed concrete require concrete cover to protect the embedded steel from corrosion. Prestressed concrete is particularly common in PCSPs that serve as a load bearing wall [74,78,84–86]. In many non-loadbearing PCSPs the wythe thickness is determined by the required cover to the reinforcement, rather than the structural requirements. To overcome this, a number of studies have proposed the use of non-corrosive Textile Reinforced Concrete (TRC) as the wythes of their PCSP designs [8,87,88]. Different textile materials have been used and tested throughout the PCSP literature, each offering different advantages. For example, carbon fibre is stronger than Alkali Resistance (AR) glass fibre but it is also more expensive. A HPC, combined with an epoxy coated carbon textile grid, was successfully implemented by Portal et al. [9] as part of a PCSP make-up. Hortsmann and Hegger [89] also used a Carbon Fiber Reinforced Polymer (CFRP) mesh as the reinforcement of their TRC wythes while Shams et al. [72] used CFRP pretensioned tendons to achieve a high performance PCSP. AR Glass Fibre Reinforced Polymer (GFRP) [8,71,90] and Basalt Fibre Reinforced Polymer (BFRP) meshes [70] have also been used in PCSP construction. Other types of concrete used include Glass-fibre Reinforced Concrete (GRC) [8,57,80], geopolymer concrete [91,92] and Phase Change Material (PCM) concrete [93,94]. Recently UHPC with a compressive strength of up to 175 MPa and a flexural strength of almost 19 MPa has been used in a curved PCSP design by Shams et al. [72]. Compressive strengths of 193 MPa and flexural strengths of 32 MPa were achieved in the UHPC used by Lee et al. [81] in a PCSP. 6.2. Insulation While the concrete provides the structural and durability characteristics of the PCSP, the insulation provides the thermal resistance. Different insulation types have been reported throughout the literature and are reviewed here in the context of PCSPs. A number of studies have already carried out extensive reviews on insulation materials used in buildings in general [95–98], showing that considerable variance exists with regard to the thermal properties of these materials. The conductivity values, ki [W m1 K1], of different insulation types are presented in Table 3 from just one manufacturer (KingspanÒ) and therefore are unlikely to contain any bias towards one over another. Typical rigid foam insulation includes Expanded Polystyrene (EPS), Extruded Polystyrene (XPS), Polyurethane (PUR), Polyisocyanurate foam (PIR) (a slight upgrade of PUR) and Phenolic Foam (PF). The European standard EN 13501–1 [99] devised a rating system based on the parameters where insulation materials are rated from A1 (highest quality of fire resistance) to E (lowest quality of fire resistance). The fire ratings of the listed insulation materials are also documented in Table 3. Other more novel insulation materials such as Vacuum Insulation Panels (VIPs) and Foamed Concrete (FC) have also been used

and are reviewed here. Examples of the required thickness of the three insulation materials are presented in Fig. 11 illustrating how high-performance insulation materials enable slimmer designs while maintaining thermal performance. 6.2.1. Vacuum insulation Of the foam insulation materials listed in Table 3, phenolic foam insulation has the lowest thermal conductivity of 0.021 W m1 K1. Vacuum Insulated Panels (VIPs) have a thermal conductivity three times lower (0.007 W m1 K1 [100]), and provide a particularly attractive solution if the overall wall thickness is limited. A detailed review of different VIPs is presented by Kalnaes and Jelle [101]. In general, they comprise a rigid core (typically fumed silica) encapsulated in an air tight enclosure (typically aluminium foil) which has been evacuated. A thorough review of VIPs and their uses in buildings can be found in [102]. The use of VIPs in PCSP construction has gained attention in recent years with current trends to reduce both thickness and U-value. The International federation of structural concrete, or Fédération Internationale du Béton (FIB), discuss VIPs and their uses in PCSPs in their latest state of the art report bulletin on PCSPs [15] and present notched VIP geometries to facilitate the incorporation of pin connectors [103]. While VIPs have significantly lower thermal conductivity than other insulation products they are relatively fragile and upon being damaged lose approximately one third of their thermal resistance [104]. Voellinger et al. [14] propose the application of a composite insulation layer that encapsulates the VIPs between rigid foam insulation [105] to protect the VIPs from damage. KingspanÒ [106] have recently released a VIP product encapsulated in PIR foam insulation for roofing and flooring applications. This product, presented in Fig. 12, could be also used in PCSPs. 6.2.2. Foamed concrete As well as standard rigid foam and state of the art VIPs other insulation products have been used in PCSPs. Foamed Concrete (FC) has been used as the insulation layer in a number of PCSP designs [9,76]. FC, as listed in Table 2, has also been used as the material for the concrete wythes [34,45,46,82]. There is considerable variance in the mechanical and thermal properties of FC and it is designed according to the application of the material. There is a strong correlation between the compressive strength of the FC and the density which can be controlled by the amount of air

Table 3 List of typical commercially available insulation types suitable for PCSPs. Density and conductivity values taken from example KingspanÒ products. Fire ratings taken from [98]. Insulation type

Conductivity (W∙m1∙K1)

Density (kg∙m3)

Fire Rating

EPS XPS PF PIR VIP

0.034 0.035 0.021 0.025 0.007

15 38 35 30 195

E E B-C B A1

Fig. 11. Required insulation thickness for a PCSP to meet a U-value of 0.18 W∙m2∙K1 using different materials.

R. O’Hegarty, O. Kinnane / Construction and Building Materials 233 (2020) 117145

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forces between the two concrete layers to achieve composite action. The degree of composite action is dependent on the type of connector used [110]. Early PCSPs used cast-concrete connectors, but in modern panels metal and plastic ties are more common. All ties bridge the insulation layer creating a thermal bridge – the extent of which depends on the size and material of the connector. The challenge is to provide structural shear transfer while minimizing the thermal bridge. Given the difficulty in developing a connector solution to this challenge there is a strong academic focus on the testing of these connectors – which is reviewed here. Fig. 12. KingspanÒ Optim R E – Vacuum insulation encapsulated in PIR foam.

in the concrete [76]. FC used for the structural wythes is designed to have a higher density and hence compressive strength. Conversely, if used for the insulation layer, it needs to have a low conductivity and hence low density. Silva et al. [107] propose the use of a foamed concrete aerogel in sandwich panel construction. FC has a thermal conductivity ranging from 0.04 to 0.06 W m1 K1 [76] while Silva et al. [107] note that the addition of QuartzeneÒ can decrease the thermal conductivity to between 0.03 and 0.035 W m1 K1. Lee et al. [81] developed a concrete with EPS foamed beads and used it as the core material in a PCSP. A relatively high thermal conductivity of 0.49 W m1 K1 was reported. 6.2.3. Mineral wool The thermal conductivity of mineral wool can range from approximately 0.03 to 0.04 W m1 K1 depending on the type – so in terms of thermal performance it is similar to the standard foam-based insulation. The problem with mineral wools is that typical manufacturing methods which pour the second layer directly onto the insulation might not be suitable and because of their lack of stiffness the load transfer between the two concrete wythes is entirely reliant on the connectors. This means that larger and stronger connectors are required to connect the two concrete wythes, resulting in bigger thermal bridges. Correia [57] use mineral wool as well as EPS in their PCSP design but 10 mm thick concrete ribs are used to connect the two wythes together. Wool is however cheaper and has superior fire resistance [98] and in some countries there are restrictions on the use of combustible materials. In the UK for example, in the wake of the Grenfell disaster [108], an amendment to the building regulations has been enforced since December 2018 which prohibits the use of any combustible materials (Rated less than A2) in domestic buildings above 18 m [109]. As a result, wool based materials are likely to be a large player in the future PCSP market in the UK.

6.3.1. Concrete connectors Historically the two concrete wythes of a PCSP were connected with continuous ribs (Fig. 13-a) or discrete zones (Fig. 13-b) of concrete crossing through the insulation layer [2]. Near fully composite behaviour has been achieved with the use of these concrete ribs [31,77] or solid concrete zones [29]. Recent studies have also included continuous concrete sections in their PCSP designs at the top and bottom edge of the panels [26] as well as around all edges of the panel [79] (Fig. 13-c). All these concrete connectors result in significant thermal bridging that compromises the thermal resistance of the PCSP. Three wythe PCSPs with staggered layers of insulation and concrete zones have also been tested structurally [86] and thermally [60] (Fig. 14), but these three wythe over-complicate the manufacturing process and do not significantly improve the thermal performance. 6.3.2. Metallic connectors To reduce the area of insulation bridged by the connector, metallic connectors were introduced to improve upon the concrete connectors. They are now available in the form of a truss, tube or plate if composite action is desired [30,33,46,79,111], or in the form of discrete pin-type connectors if no composite action is designed for [29,55,112]. These metallic connectors are supplied by a number of different manufacturers (e.g. Halfen, PeikkoÒ and Fixinox). Example geometries are displayed in Fig. 15. A fully composite PCSP was achieved by Hou et al. [113] using evenly spaced diagonal steel connectors. Hegger et al. [71] attempt to minimise the use of metallic connectors in their design and use them only around the edges of the panel. Metallic connectors cre-

6.3. Connectors The connectors join the insulation (thermal layer) and concrete (structural layers) together. They can also transfer lateral shear

Fig. 14. Cross section of two different three-wythe PCSPs [86]. WWF = Welded Wire Fabric.

Fig. 13. Examples of concrete connectors.

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Fig. 15. Examples of typical metallic connectors.

ate substantial thermal bridges across the insulation layer as a result of their high thermal conductivities [3,60]. This is evident by the focus of most recent studies on the study of PCSPs with non-metallic connectors with lower thermal conductivities. 6.3.3. Fibre reinforced polymer connectors Of the 12 connector types assessed by Naito et al. [38] 6 of them were made from non-metallic Fibre Reinforced Polymer (FRP) materials. FRP materials are a composite material made up of a polymer matrix embedded with fibres. These composite materials are inherently variable, and their properties vary as a consequence. A table summarising the various different FRPs that could be used as connectors and their relevant parameters are presented in Table 4. Carbon Fibre Reinforced Polymers (CFRP) are made up of carbon fibres which are produced using different chemical processes with the aim of achieving a minimum of 90% carbon [114]. Glass Fibre Reinforced Polymer (GFRP) is made up of fibres produced from silica sand with the addition of zircon to give alkali resistance characteristics [115]. Basalt Fibre Reinforced Polymer (BFRP), is made from mineral fibres extracted from basalt rock. The fibres do not require additive materials; this results in a simpler production process and cheaper material [47]. A qualitative comparison of the various materials is presented by Portal [114] who concluded that an obvious choice does not exist but that GFRP appears to be the most cost effective option. CFRP presents a number of structural advantages but is expensive and BFRP is a promising material but requires further research as a material to be used with concrete. Different geometrical configurations of these FRP have been investigated throughout the literature. They may be broadly categorised by their geometry and load transfer capabilities as described in Table 5. 6.3.3.1. Glass fibre reinforced polymer. GFRP connectors have been used throughout the PCSP literature in various configurations. GFRP was introduced as a truss configuration (Table 5 – type D)

Table 5 Description of FRP based connectors categorised by their geometric configuration. Geometric type

Description

A. Discrete pin connectors

Pin connector typologies are designed to transfer in-plane loads during lifting. They can also carry minimal vertical loads such as the self-weight of the panel by way of dowel action. They are not designed to transfer lateral shear between the two concrete wythes during bending. They are similar to the metallic connector in Fig. 15 – d. This connector can transfer all the loads that the pin connector can – but because of its greater thickness in one orthogonal plane a lower quantity are required. Some of these tie types are advertised as being able to achieve composite action and transfer lateral shear forces. They are similar to the metallic connector in Fig. 15 – a. This connector is similar in concept to the discrete plates. The lateral shear loads in such a connector are, however, transferred by way of axial forces in the connector rather than shear forces. They are similar to the metallic connector in Fig. 15 – c. Essentially the same as a steel truss joining the two concrete layers – but instead of steel a lower conductivity FRP based material is used. The lateral shear loads are transferred by way of axial tensile and compression forces in the diagonal members – similar to the discrete X-shaped connectors. This connector is a mesh or grid made up of a series of FRP tendons. They act as a membrane and have no out of plane stiffness and transfer loads by in plane tension. They rely also on the compressive strength of the insulation layer to transfer lateral loads between the two wythes.

B. Discrete plate/ thick pin connectors

C. Discrete ‘X’ shaped connectors

D. Continuous rigid truss connectors

E. Grid/mesh connectors

Commercial example ThermomassÒ | MC Series

ThermomassÒ | CC Series

ThermomassÒ | X Series

Aslan FRPÒ | Nu-TieTM

Atlus GroupÒ| C-GridÒ

Table 4 Properties of steel compared with different FRP materials that have been used in concrete sandwich panels.

a b c d e f

Material

Tensile strength (MPa)

Young’s modulus (GPa)

Thermal conductivity (W m1 K1)

Steel CFRP GFRP BFRP

483-690a 600-3690a 483-1600a 1100b

200a 120-580a 35-51a 70b

50c 5f-8e 0.3d–1.0e 1.0b

[116]. [47]. [65]. [31]. [26]. [117].

in early research in the 1990s [11,111] to replace the thermal bridging steel truss. In the four full scale (9.14 m high) flexural tests conducted by Salmon et al. [111], the two panels with steel trusses behaved non-compositely while the panels with GFRP trusses behaved compositely. The authors were not able to conclusively explain the reason for this but suggested that it might be because of the compressive members of the steel truss buckling. Different GFRP truss geometries have also been tested using inplane shear loading conditions by Choi et al. [75]. An increase in maximum shear flow was found for thicker connectors with greater embedment while no significant difference in the shear

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behaviour was noted between different insulation types (XPS and EPS). Chen et al. [26] carried out a series of 3-point and 4-point flexural tests on PCSPs with different configurations of a 2.2 mm thick GFRP plate provided by CraneÒ Composites (Table 5 – type B). They achieve high bending moment capacities (up to 32 kNm), but the insulation in this tested panel was not continuous through the length of the panel and the top and bottom 300 mm of panel was a solid concrete zone. This concrete zone would have aided in the lateral shear transfer and therefore the performance cannot be attributed to the connector alone. The 300 mm of concrete also forms a clear thermal bridge across the insulation layer. Similar plate-shaped GFRP connectors are also used in PCSP studies to assess the impact of blast [51] and creep [54] loading conditions for military buildings and floor slabs respectively. Portal et al. [9] use a GFRP plate, similar to the typical steel plate connector (Fig. 15-a), and test a series of 6 PCSPs with a system of this GFRP plate in conjunction with GFRP pin connectors. Composite action of less than 10% were calculated for these tested panels when using the deflection based method [29] (described in Section 4.1). A GFRP grid (Table 5 - type E) have also been used in an experimentally tested PCSP for both pressure [31] and suction [77] flexural loading conditions. In this study the PCSPs with EPS insulation performed better and ultimately failed by shear failure of the core whereas the panel with XPS failed by debonding and then rupture of the connector. A similar but different GFRP shear grid was used by Choi et al. [118] who found that panels with XPS had a higher maximum shear flow than those with EPS. This is in opposition to the findings of Kim and You [31] and Choi et al. [77] who found that XPS insulation resulted in superior performance. It is likely true that insulation has an impact on the performance, but it is not clear whether an extruded or expanded polystyrene insulation provides better results and it is more likely that the manufacturer’s specific insulation or the PCSP construction method is affecting performance. Some studies have compared different GFRP connectors. A GFRP flat plate, corrugated plate and hexagonal tube have been developed and tested for their shear capacity [119]. While the three different connector geometries had similar shear flow capacities the hexagonal tube offers potential as a two-way connector, performing similarly along two orthogonal directions (similar to the metallic sleeve in Fig. 15-b). Al-Rubaye et al. [28] investigate the flexural behaviour of PCSPs using three different commercially available GFRP based connector systems, namely:

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to corrosion, their high fatigue characteristics and their relatively low thermal conductivity. PCSPs that use C-GridÒ (The commercially available CFRP grid from Table 5 - E) have undergone rigorous structural testing [40,41,85,120]. Specific research on investigating the performance of this CFRP grid under shear loading has been conducted [121,122] while a method to measure the shear flow capacity of this CFRP grid is presented by Bunn [40] who tested over 60 PCSPs. The calculation method calculates the shear flow by modifying a base capacity of 17.5 kN m1 (for a panel with the grid alone) with factors that account for the insulation thickness, insulation type, number of grids and orientation of grid. Individual strands of the CFRP grid (~4–7 mm2) have been tested having an average tensile capacity of 3.7 kN [121]. Pure tensile and compressive strengths were conducted on panel segments with CFRP connectors with varying thicknesses of insulation. The CFRP grid (Table 5-E) behaved similarly regardless of insulation thickness showing tensile strengths greater than 450 MPa but compressive strengths of approximately 1 MPa [123], the results from this study are shown in Fig. 16. The compressive members buckled while the tensile members failed by rupture [122,123]. The low compressive strength of the grid type connectors shows that the insulation is relied on to provide the compressive resistance as part of a hybrid insulation-connector truss system which ultimately provides the composite action for the panel. This reliance on insulation when using a grid based FRP connector has also been referred to by Shams et al. [10] and O’Hegarty et al. [35]. A PCSP with prestressed concrete wythes and CFRP grid for the connector tested under flexure has shown signs of almost 100% composite action [74,78,85]. 6.3.3.3. Basalt fibre reinforced polymer. Basalt Fibre Reinforced Polymer (BFRP), when compared with CFRP and GFRP, is a relatively new material for use in connectors of PCSPs. Naito et al. [38] tested a discrete ‘X’ shaped BFRP connector (Table 5-C) under a shear loading condition and found that it provided greater shear resistance than standard pin-type GFRP connectors (Table 5-A). A new inverted ‘V’ shaped BFRP connector is introduced by Tomlinson et al. [124]. Tomlinson and Fam [32] carried out a series of flexural tests on PCSPs with these discrete BFRP connectors and found that the connectors provided less composite action than similar shaped steel connectors. The same ‘V’ shaped BFRP connectors were used in the PCSPs tested by Teixeira et al. [73] who again found that the steel connectors outperformed the BFRP connectors. A contin-

(i) the AslanTM Nu-Tie for THiN-WallTM (Table 5 – type D), (ii) the HKTM Composites tie (Table 5 – type B) and (iii) the ThemromassÒ CC and X tie series. (Table 5 – type B and C) Two panels of each type were made and tested. The panels with HK (ii) and Thermomass (iii) ties outperformed the panels the NuTie (i). A variance in the maximum loads of less than 8% between the two duplicates of Panels (ii) and (iii) was also reported whereas a variance of 39% between the two duplicates of Panel (i) was noted. The post cracking behaviour of the two duplicates of Panel (i) also significantly differed with one panel showing signs of increased load carrying capacity after a major crack (strain hardening) and the other showing signs of reduced load carrying capacity after a major crack (strain softening). This highlights the importance of using a connector that provides repeatable results. 6.3.3.2. Carbon fibre reinforced polymer. Carbon Fibre Reinforced Polymer (CFRP) has become more common in the precast concrete sandwich panel industry in recent years because of their resistance

Fig. 16. Behaviour of CFRP grid within rigid foam subjected to pure tension and compression tests [122].

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uous BFRP truss, similar in geometry to that of Table 5-D, has been patented [125]. A BFRP grid geometry has also been used in a panel that has been tested under a flexural loading regime [70]; it showed no obvious difference with a similar panel containing CFRP grids. Further research of the use of BFRP connectors in PCSPs is required given the lower volume of research on this material – the results to date however are promising. 6.3.4. Concrete-insulation bond only Some PCSPs have been designed to rely solely on the bond between insulation and concrete, removing the connector completely [81]. Such a design is typical of aluminium composite panels with thin aluminium face-sheets which are bonded onto either side of an insulation core using adhesives [126]. Regarding concrete panels, a thin layered TRC sandwich panel with no connectors and EPS foam insulation has been experimentally [66] and numerically [83] investigated. These studies found that the shear deformability of the insulation layer plays an important role in the global deformation of the panel. Lateral shear transfer was also relied upon by the concreteinsulation bond in panels developed by Hegger et al. [8]. Successful improvements of the bond in these panels were then made by using notches in the concrete and insulation interface. The positive influence of a superior insulation-concrete bond is widely reported [29,31,38,78,84,85,118,127]. However, durability of the bond has not been established, and it is unclear whether it can be relied upon with confidence in design [5,55]. 7. Thin sandwich panels It is evident from the literature that current research is focused on reducing overall thickness of PCSPs. This is a pertinent aim as it results in space and weight saving, with onsite efficiencies and material savings. Reducing thickness is also a challenge as the same thermal and structural requirements need to be maintained so the panel can function as a safe and efficient cladding element. Thin-PCSP designs are therefore reviewed here in detail. Thinner PCSPs can be achieved by using non-corrosive reinforcement, using low conductivity insulation or by achieving composite action. A number of studies have focused on one or all of these methods to reduce panel thickness and weight, while ensuring high structural and thermal performance. With standard reinforced concrete wythes it has been noted that a minimum thickness of approximately 80 mm (depending on exposure class) is required to provide sufficient cover to the

embedded steel reinforcement. This paper considers any panel designs with concrete wythe thicknesses lower than this standard thickness to be advancements towards thin-PCSP systems. PCSPs that incorporate high performing (low conductivity) insulation products to reduce overall panel thickness are also included in this section, such as those PCSPs that incorporate VIPs. As discussed in Section 3.1, if the two wythes of concrete can work together in bending then an overall more efficient and slimmer design can resist the same load. The aim therefore is to use a connector that can provide the required lateral shear transfer while reducing the impact of thermal bridging. Additional to the matter of cover, other factors limit the minimum possible thickness of PCSPs. These include the method by which the panel is lifted and transported as well as the design requirements for protection against fire. Commonly, lifting eyes or bolts are cast-in for lifting purposes. Minimum diameters of these bolts are specified in the standards, and hence define the thickness. The matter of fire resistance is particularly pertinent in the case of load bearing elements. In that case, measures are taken to ensure levels of structural performance are maintained for specified time periods that vary across international standards (e.g. EN 1992-1-2 [128]). These are achieved by ensuring adequate concrete cover. For non-load bearing PCSPs the standards are less stringent but adequate compartmenting needs to be ensured with fire stops between the floors and the PCSPs to prevent the spread of flames. Most of the thin designs referenced in this section are designed as non-load bearing elements. In the UK these are affected by new regulations currently being introduced to limit the use of combustible materials in cladding for high-rises above 18 m [109]. However, fire regulations vary from country to country and are under review in many locations due to recent fire disasters. 7.1. Thin panel designs Components and dimensions of different thin-PCSP designs are documented in Table 6. As discussed in Section 6.1, the minimum thickness of concrete in PCSPs is often determined by the requirement to avoid corrosion problems with steel reinforcement. For example Brameshuber [129] proposes the use of non-corrosive textile reinforcement to significantly reduce the thickness of the concrete wythes. Examples of thin, textile-reinforced PCSPs include those developed by Hegger et al. [8], who propose thin panels for use in a modular building. This design uses concrete reinforced with short glass-fibres (GRC), in conjunction with larger mats of textile reinforcement. It uses polyurethane foam and steel

Table 6 List of thin-PCSP designs in order of overall thickness.

No.

Authors

Reference

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

Lee et al. Gopinath et al. Correia et al. Hülsmeier Enfedaque et al. Colombo et al. Hyde et al. O’Hegarty et al. Hegger et al. Hegger et al. Flansbjer et al. Shams et al. Portal et al. Shams et al. Hodicky et al.

[81] [130] [57] [104] [80] [66] [91,92] [35] [71] [8] [76] [10] [9] [72] [70]

Overall thickness (mm)

Wythe thickness (mm) Outer

Inner

Concrete

Insulation

Connector

Reinforcement

65 70 100 110 110 120 120 155 180 180 200 220 230 280 350

5 10 10 15 10 10 20 25 15 15 25 30 30 40 30

5 10 10 30 10 10 20 40 15 15 25 30 50 80 30

UHPC TRC GRC TRC GRC TRC Geopolymer UHPC GRC GRC RPC TRC TRC UHPC TRC

EPS-concrete None EPS + Wool VIP EPS EPS PF + VIP EPS PUR PUR FC XPS, EPS, PUR FC PUR EPS, PF

None Profiled steel sheet Concrete ribs GFRP I sections None Insulation bond only CFRP grid CFRP grid Pins and steel ties None GFRP truss GFRP pins and CFRP grid GFRP plates GFRP pins and CFRP grid BFRP grid

Steel fibres AR- Glass Mesh Glass fibre GFRP or CFRP Glass fibre CFRP CFRP Glass fibres AR- Glass Mesh AR-Glass Mesh CFRP GFRP or CFRP CFRP GFRP + CFRP BFRP

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pins and relies heavily on the bond between the concrete and insulation to achieve peak structural performance. Other options for the wythes that have been studied include GRC without any textile or steel reinforcement [57], and textile reinforced geopolymer [66,67]. To enhance the connection between concrete and connectors where exceptionally thin wythes are used, local thickening of wythes at joint locations have been attempted [10,113]. Correia et al. [57] incorporated mineral wool into their panel (in addition to EPS). Other insulating materials in thin panels include foamed concrete, sandwiched between two layers of textile reinforced concrete by Portal et al. [9]. However, the most effective thermal insulation, receiving increasing research attention, relies on vacuum insulation panels (VIPs) which are reviewed in Section 6.2.1. VIPs have been included in panels with textile reinforced wythes from 15 to 30 mm thick [104]. VIPs present a number of unique challenges, such as the need to produce them in fixed sizes that cannot be cut. These difficulties have not however prevented their use in practice [103]; while the challenges are described in more detail by the FIB [15]. 7.2. Flexural behaviour Table 7, an extension of Table 6, outlines the testing conducted on these thin PCSPs and also documents the flexural capacity of the panels tested. All current testing is predomently structural. A common performance indicator and a standard way of presenting results has not established throughout the reviewed studies, and so, a methodology to normalise the results from the reviewed data is first outlined. The bending moment capacities per meter width of wall (kNm∙m1) of the individual panels are estimated from the

figures/tables provided in the different studies. The method for estimating this varies depending on the available data. An example is presented in Fig. 17 using the load vs displacement data from Gopinath et al. [130]. If duplicate panels are tested the average is taken. Some studies test more than one type of panel – and are discussed in the following paragraphs. The majority of testing applies 4-point bending. Reported peak load bending moment capacities range from less than 5 kNm∙m1 to more than 80 kNm∙m1. For context a 1.6 kPa design wind pressure (as specified in [15]), for a 3.4 m high panel, would result in a bending moment of 2.31 kNm per meter width of wall. Designs with first crack bending moment capacities of less than this are therefore unsuitable as unsupported structural elements. Almost all reviewed panels which have been tested are structurally sufficient to meet this requirement. The impressively thin PCSP design (5 mm inner and outer wythes) presented by Lee et al. [81] reached bending moment capacities of as high as 7.4 kNm∙m1. However, a very high U-value of 3.55 W m2 K1 is calculated in Section 5 – the highest of all panels compared in Table 1 by more than a factor of 3. An ultimate bending moment capacity of 6.3 kNm∙m1 was measured for the thin panel with 10 mm TRC wythes and profiled steel core tested by Gopinath et al. [130]. First cracking occurred at a bending moment of approximately 2.6 kNm m1 m (Table 7 – Panel No. 1). Again, the thermal performance of this panel is poor as it contains no insulation. A similar first crack moment of 2.8 kNm∙m1 is estimated for the 120 mm thick PCSP designed by Colombo et al. [66] (Table 7 – Panel No. 5). This panel which does not contain any connectors and relies solely on the bond between insulation and concrete has an ultimate moment of approximately 4.3 kNm∙m1. The ther-

Table 7 Test methods and flexural test results of the thin-PCSP designs presented in Table 6. No.

Authors

Reference

Test method

Panel label

Panel comments

1

Lee et al.

[81]

4-point bending

M-U1 U-U1

2 3 4 5 6 7 8

Gopinath et al. Correia et al. Hülsmeier Enfedaque et al. Colombo et al. Hyde et al. O’Hegarty et al.

[130] [57] [104] [80] [66] [91,92] [35]

4-point bending – – Impact test 4-point bending – 3-point bending

9 10

Hegger et al. Hegger et al.

[71] [8]

4-point bending 4-point bending

11

Flansbjer et al.

[76]

4-point bending

– – – – Average* – Average* Average* Average* P1 P2 P3 P4 S – Average* D – Average*

12

Shams et al.

[10]

4-point bending and shear testing

Panel with EPS mortar core Panel with a high-performance EPS concrete core – – – – 0.55 m long panel – 155 mm total thickness 170 mm total thickness – notched - qi = 32 kg∙m3 glued - qi = 32 kg∙m3 notched - qi = 200 kg∙m3 notched - qi = 40 kg∙m3 Single connector Double connector EPS  low bond

13 14

Portal et al. Shams et al.

[9] [72]

15

Hodicky et al.

[70]

4-point bending 4-point bending and shear testing Shear

Average*

Bending moment (kNm per m width)** At first crack

At peak load

– –

3.3 7.4

2.6 – – – 2.8 – 2.7 3.7 1.7 5.0 5.0 14.0 10.0 2.5 3.1 7.0

6.3 – – – 4.3 – 7.5 14.6 12.1 8.0 6.0 46.0 28.1 6.7 11.8 10.6

EPS  high bond XPS  high bond XPS + pins XPS + C-Grid XPS + 2C-Grids XPS + CG + thicker bottom – –

17.6 26.4 26.4 28.2 28.2 28.2 6.0 38.0

30.8 48.4 48.4 88.0 70.4 70.4 13.3 50.0

–

–

–

*A number of panels have been tested and the average is taken. **Most panels are tested with different breadths so the bending moment capacity per meter width is used as per Shams et al. [11]. q insulation density.

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R. O’Hegarty, O. Kinnane / Construction and Building Materials 233 (2020) 117145

Fig. 17. Example of first crack and ultimate load reading from the load deflection plot by Gopinath et al. [130].

mal performance of this panel is a significant improvement on the panels presented by Lee et al. [81] and Gopinath et al. [130]. A 155 mm thick panel, presented by O’Hegarty et al. [35] with a Uvalue three times lower than that Colombo et al. [66] had an ultimate bending moment capacity of 7.5 kNm∙m1. The authors found that increasing the thickness of one of the wythes by an additional 25 mm increased the ultimate bending moment capacity further to 14.6 kNm∙m1. Average bending moment capacities of 6 kNm∙m-1at first crack and 13.3 at peak load are estimated for the 230 mm panel with GFRP plate connectors tested by Portal et al. [9] (Table 7 – Panel No. 11). A slightly slimmer panel (200 mm) tested by Flansbjer et al. [76] (Table 7 – Panel No. 9) measured a first crack bending moment of 2.5 kNm∙m1 and an ultimate bending moment capacity of 6.7 kNm∙m1. This panel used a GFRP truss instead of the GFRP plate. They also found that doubling up on the truss configuration increased both first crack and ultimate load bending moments to 3.1 kNm∙m1 and 11.8 kNm∙m1 respectively. Of the tested panels, those tested by Heggar et al. [8] (Table 7 – Panel No. 8) and Shams et al. [10] (Table 7 - Panel No. 10) were by far the strongest. Heggar et al. [8] tested their 180 mm thick PCSP with 15 mm TRC wythes and found that incorporating notches in the joint between the wythes and the insulation improved the performance. They also found that increasing the density of the PU insulation resulted in superior shear transfer and the panels subsequently failed by rupture of the textile reinforcement in the lower wythe. But it is also likely that the insulation with greater density also has a greater thermal conductivity and hence performs worse thermally. Shams et al. [10] tested a number of PCSPs with TRC wythes, different connectors and different insulation types. They found that a strong adhesive bond between the insulation and concrete increased the proportion of load taken by the insulation core and that insulation with higher shear moduli increased the overall load bearing capacity of the panel. Shams et al. [10] also found that FRP pin connectors (Table 5 – A) did not increase stiffness or strength but did provide additional post-cracking failure strength. On the other hand, they found that using a CFRP shear grid (Table 5 – E) increased the stiffness, ductility and load bearing capacity of the panels with the CFRP strands providing the tensile members of a truss system. They note how the compression of the hybrid truss system was provided by the stiffness of the insulation and not by the CFRP strands. As a result, more grids did not improve the stiffness, but did improve post cracking ductility. Finally, they found that the effectiveness of the CFRP and insulation core truss system

was decreased with increasing wythe thickness. They concluded that this was because of the thicker wythes compressing the insulation and caused a relaxation of the shear ties. In conclusion it would appear that the CFRP grid connectors are dependent on a number of factors and require parametric analysis to ensure a robust design solution. They do, however, appear to significantly improve post cracking ductility, when compared with pin type connectors. 8. Conclusions Since their first introduction as a cladding typology, over half a century ago, PCSPs designs have undergone significant advancements. Today, innovations in concrete and insulation materials, as well as the structural methods of concrete wythe connection, are driving new designs; in particular, thinner/lighter designs. Novel concretes such as ultra-high performance fibre-reinforced and textile-reinforced concretes have enabled thinner sections by replacing solid steel reinforcement bars with non-corrosive alternatives. High performance insulation materials, such as vacuum insulation, have also enabled thinner designs – achieving low Uvalues for considerably lower insulation thicknesses. A number of PCSP designs, with thicknesses lower than 200 mm have been validated structurally, achieving flexural capacities well in excess of what is required to meet typical design wind loads. These novel panel designs, have reduced the overall panel weight by up to a factor of 4. Many exhibit comparable strength when compared to more standard and common market available precast concrete sandwich panels. While many thin PCSP designs have been tested structurally, their thermal performance has not been rigorously evaluated. Often simple evaluation of even standard panels shows them to be thermally substandard to contemporary building envelope standards. This review compared the theoretical U-values of a range of PCSP designs and found that only two of the panels would actually meet a high thermal performance standard, such as that of the passive house standard (U = 0.15 W m2 K1) and only three would meet current Irish building regulations (0.21 W m2 K1). And this is before thermal bridging is accounted for which has been shown to be a significant source of heat loss. Future panels need to be designed to better thermal standards if they are to be used as an energy efficient solution. Many recent studies have focused on developing and evaluating connectors that can achieve composite action while minimising their thermal bridging effect. To achieve the parallel aims of ther-

R. O’Hegarty, O. Kinnane / Construction and Building Materials 233 (2020) 117145

mal and structural efficiency considerable innovations in wythe connectors have been achieved. There is considerable focus within the sandwich panel literature on the testing of bespoke, as well as commercially available, fibre reinforced polymer connectors. These integrate materials such as glass, carbon and basalt in varying geometries (pin, plate, tube). However, this range leads to a wide variability in the documented test-methods and results. A wide disparity in results is also evident for even the same connector type across different studies. Further variability in results has also been found even within the same studies. One such study tested duplicates of an identical panel that used commercially available FRP connectors and found that both panels behaved significantly differently after cracking. This variability of results, found throughout the literature, means that an individual result, or results, from a specific study showing high degrees of composite action should be taken with caution; not because the result are not to be believed but because the complex behaviour between individual components of these panels (e.g. the connector-concrete integration and the insulation-concrete interaction) under bending means that full composite action may not be achieved repeatedly. This is of particular concern where composite action between the two concrete wythes is relied upon in design. This matter is even more pertinent in thin-designs where structural boundaries are continuously being pushed and redundancy is being designed out and hence opening the risk of catastrophic failure. A full-scale validation study that would assess the repeatability and reproducibility of panel system designs would be costly but would allow for the calculation of safety factors that could be relied upon for future applications. In conclusion, the inherent durability and achievable highthermal-efficiency, advantages of PCSPs are likely to result in increased usage of this cladding typology into the future. Thin, lightweight panels, particularly, have the potential to be a disruptive technology, offering material savings, transport and onsite efficiencies. Most importantly, perhaps, they allow for a new typology of energy efficient facade cladding, that retains the benefits of precast but without the weight and depth profiles that so affect structure and fixing, and limit the applicability of current PCSPs. However, further research on the structural and thermal performance of PCSPs, and the range and methods of their innovation, is essential. Acknowledgments This work was funded by the European Union as a Horizon 2020 project (IMPRESS) under Grant No. 636717, and the authors would like to express their thanks to the funding bodies and partners for making this possible. References [1] P.C.I. Committee, State-of-the-art of precast/prestressed sandwich wall panels, PCI J. (2011). [2] H. Gleich, New Carbon Fiber Reinforcement Advances Sandwich Wall Panels, Structure Magazine (April). (2007) 61–63. [3] Y.J. Kim, A. Allard, Thermal response of precast concrete sandwich walls with various steel connectors for architectural buildings in cold regions, Energy Build. 80 (2014) 137–148, https://doi.org/10.1016/j.enbuild.2014.05.022. [4] R. O’Hegarty, A. Reilly, R. West, O. Kinnane, Thermal investigation of thin precast concrete sandwich panels, J. Build. Eng. 100937 (2019), https://doi. org/10.1016/j.jobe.2019.100937. [5] H. Wilden, PCI Design Handbook: Precast and Prestressed Concrete 7th ed., Prestressed Concrete Inst, 2014. [6] IMPRESS H2020, New Easy to Install and Manufacture PRE-Fabricated Modules Supported by a BIM based Integrated Design ProceSS, Impress. 2019. http://www.project-impress.eu (accessed April 1, 2019). [7] SESBE, SESBE – Smart Elements for Sustainable building Envelopes, (2019). http://www.sesbe.eu/ (accessed April 15, 2019).

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