PSI - Issue 64

Mahdi M.K. Zanjani et al. / Procedia Structural Integrity 64 (2024) 1134–1141 Zanjani et al. / Structural Integrity Procedia 00 (2019) 000 – 000

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1. Introduction The construction sector is called to significantly change its current practices to address environmental challenges, improve energy efficiency, and mitigate the global warming. Within this framework, replacing energy-inefficient building materials with more sustainable (CO 2 cutting) alternatives (Caggiano et al., 2018 and 2023; Mankel et al. 2019a-b) has garnered huge interest from the civil engineering community, including academician, researchers, industry professionals, and building end-users (Fachinotti et al., 2023b; Ramón-Álvarez et al., 2023). In this context, improving the thermal performance of buildings and their envelope is a key goal in making them sustainable, energy-efficient and CO 2 neutral (Jayalath et al., 2024). Achieving this requires shaping the synergistic interplay between insulation and thermal energy storage (TES) (Mankel et al., 2023), while also considering structural integrity of the material. To achieve this goal, a novel material for building envelopes has been engineered: this is called NRG&STRUCT-foam: it integrates tuneable insulation, thermal energy storage (TES), and structural properties. NRG&STRUCT-foam is a foamed concrete (Gilka-Bötzow, 2021) blended with microencapsulated phase-change materials (mPCM), which are embedded within the cement-based mixture. Controlling the porosity and mPCM content emerges as critical factor governing insulation efficiency, TES capability, and structural integrity in NRG&STRUCT foam (Frahat et al., 2023). The NRG&STRUCT-foam can be designed as a thermal metamaterial (i.e., a customizable material that offers a unique set of properties, such as thermal conductivity, heat capacity, density, etc.), which are not commonly found in natural nor commercially available (Wang et al. ,2020). Hence, the study aims at optimizing the balance between insulation, TES, and mechanical response within building envelopes. We achieve this by formulating an optimization problem where the objective function is the energy consumption for indoor thermal comfort, and the design variables mainly influence the functional composition of NRG&STRUCT-foam within the envelope. The focus lies on multilayered external walls, being each layer made of a specific NRG&STRUCT-foam determined by two design variables: porosity and mPCM content (Fachinotti et al., 2020). In a previous study (Fachinotti et al., 2023a), we had focused only on TES performance to determine the optimal envelope. With this investigation, we extend our analysis to include the load bearing (strength) capacity, imposing constraints on the density of NRG&STRUCT-foams. The resulting envelope, which balances thermal optimization with structural feasibility, is compared with the original envelope outlined in the benchmark BESTEST 900 ANSI/ASHRAE Standard 140-2017. 2. The NRG&STRUCT Foam The NRG&STRUCT-foam is a composite material made of cement-based mix (possible enhanced with microencapsulated phase-change materials, mPCM) and a preformed foam. In its hardened state, it becomes a two phase composite, with a matrix consisting of hardened cement paste with mPCM and an air-bubble dispersed phase generated by the foamed structure. The matrix itself also represents a two-phase composite, comprising cement paste with dispersed mPCM capsules. To achieve an NRG&STRUCT-foam that optimally balances insulation, TES, and structural performance, we manipulate in our optimization approach two key parameters: the porosity and the volume fraction of mPCM within the base mix. Our approach involves determining the functional dependency of thermal properties of NRG&STRUCT-foam along these parameters. where , and are the air, cement paste and mPCM densities, respectively. The volumetric heat capacity of the NRG&STRUCT-foam also follows the mixture rule (Maxwell, 1873) as: = +(1− )[(1− ) + ], (2) 2.1. Density, heat capacity and enthalpy The density of an NRG&STRUCT-foam is defined by the following mixture rule: = +(1− )[(1− ) + ], (1)