PSI - Issue 64

1136 Mahdi M.K. Zanjani et al. / Procedia Structural Integrity 64 (2024) 1134–1141 Zanjani et al. / Structural Integrity Procedia 00 (2019) 000 – 000 3 where , , and represent the specific heat capacity of foam, air, paste and mPCM, respectively. In air and paste, the heat capacity is purely sensible in the range of outdoor and indoor air temperatures, relevant to the study of building energy efficiency. On the contrary, the mPCM must be chosen such that it solidifies and melts (i.e., it releases and absorbs latent heat) in the range of interest, making the heat capacity ( ) a nonlinear function of the temperature with a peak in the solidification range. Fig. 1 shows the ( ) curve for the microencapsulated PureTemp23 PCM (Mues, 2022). These PureTemp23 are characterized by a melting point around 23°C which is ideal for its use as a passive thermal regulator within the 19°-27° thermal comfort zone of the building under investigation.

Fig. 1. Specific heat of the microencapsulated PureTemp23 PCM.

The material properties of the constituents of an NRG&STRUCT-foam are listed in Table 1.

Table 1. Thermal properties of the constituents of an NRG&STRUCT-foam. Material Density

Specific heat Thermal conductivity

Cement paste

1624 kg/m 3 1.225 kg/m 3

790 J/kg/°C 1000 J/kg/°C

0.588 W/m/°C 0.026 W/m/°C

Air

995 kg/m 3 (solid) 920 kg/m 3 (liquid)

0.255 W/m/°C (solid) 0.155 W/m/°C (liquid)

mPCM

See Fig. 1

2.2. Thermal conductivity The thermal conductivity of a foamed concretes, including the NRG&STRUCT ones, is highly influenced by the pore structure of the system, which is characterized not only by the porosity itself but also by shape and size distribution of them. Fachinotti et al. (2023b) employed a Finite Element-based homogenization, utilizing a detailed (computationally generated) representative volume element, to accurately determinate foams heat transfer properties. However, the approach turned to be too computationally expensive, for our current application, where the porosity is treated as a design variable. Luckily, as an incidental finding of our work, we also found that the analytical formula proposed by Nielsen (1974) provided satisfactory results. This formula defines the effective thermal conductivity of a composite with dispersed inclusions as: = 1 + ⁄ −1 ⁄ + 1− ⁄ −1 ⁄ + ( + 1− 2 2 ) , (3) being and the thermal conductivities of the matrix and the inclusions, respectively, the volume fraction of inclusions, a constant associated to the inclusions shape, and the maximum attainable , which depends on the type of packing. To obtain the effective thermal conductivity ≡ of the NRG&STRUCT-foam, Eq. (3) is called with ≡ & (effective thermal conductivity of the base mix, made of cement paste and mPCM), ≡ ,