1. Introduction
3D printing technology is one of the rapidly developing areas and it has increasingly attracted academic researches and industries, as it helps with complex designs which are challenging to create with conventional manufacturing methods [
1,
2]. As time progressed, the potential of 3D printing has been clearly recognised in wide range of applications such as food, medical supplies, aeronautics and even agriculture [
2,
3,
4] At present, the building codnstruction industry has also adopted this technique with the aim of turning the complex building design into reality [
5]. 3D Printed Concrete (3DPC) techniques that are based on layered extrusion seem to be the most promising approach with respect to both its economic feasibility and to its prospective use in construction practices [
2,
6]. Adopting 3D printing can be a great asset, regardless of the type of manufacturing sector, as it offers innovative solutions, better time management, faster production, manpower and cost reduction, waste reduction, multi-material printing, and smaller environmental footprint [
6,
7,
8,
9]. The technology of producing houses and other structures using this method has dramatically improved since 2010 [
2,
10]. However, in terms of structural integrity and durability of the structure, there is more to discover.
Concrete is the most popular construction material in the world due to its numerous advantages [
6,
11] and it has also shown excellent fire performance [
12,
13,
14,
15]. Even though concrete has many advantages, one of the vital drawbacks in concrete is its high self-weight [
16]. Therefore, many research studies have been conducted to develop lightweight concrete. Lightweight concrete can be categorised as lightweight aggregate concrete and lightweight cellular concrete [
17]. The lightweight cellular concrete can be further categorised into two forms that are Autoclaved Aerated Concrete (AAC) and Foamed Concrete (FC) [
18]. Foamed concrete is produced by mixing foaming agent to the concrete mix, which makes the concrete cellular with compressed air [
19,
20]. The potential use of foamed concrete has increased in the industry due to its low self-weight, thermal insulation, acoustic absorption, earthquake resistant, long life span due to fire resistance, weatherproof, workability, and material savings [
20,
21,
22,
23,
24,
25,
26]. Hence, this study includes foamed concrete as the material used to develop the numerical models of 3DPC walls.
Structural fire damage or fire losses can be identified as a common accidental disaster throughout the world which causes thousands of deaths, injuries, and millions of property damage each year [
12,
16,
26]. Structural design code of practices has identified the fire situation as an accidental loading condition to the structure. Therefore, it is essential to design the structures to withstand a fire scenario for a prescribed period [
26,
27,
28] Fire can be identified as a time-dependent temperature variation. ISO 834 temperature time relationships [
29] given in Equation (1), where T is the fire temperature,
is the ambient temperature and t is time in minutes.
At present, Weng et al. [
30] and Cicione et al. [
31] performed the preliminary experimental studies to analyse the behaviour of 3DPC at elevated temperatures. Following the results presented by Cicione et al. [
31], preliminary numerical studies were conducted by Suntharalingam et al. [
32,
33] focusing on investigating the fire performance of 3DPC composite wall panels under standard fire condition and different fire scenarios. (i.e., hydrocarbon fire, rapid, and prolong).
Another core aims of construction industry is to reduce significant amount of energy consumption, while achieving the desired structural and thermal performance [
34,
35,
36]. 3DPC technique can be addressed as an environmentally friendly solution which offers sustainable construction. In addition, the adoption of modular building system (MBS) in high rise structure is in demand nowadays and the 3DPC technique will definitely play a major role in the future of MBS. Any complicated architecture, with complicated shapes could be made possible in modular construction by incorporating the 3DPC elements. Moreover, many researchers have discussed the need for SHS corner post section to be covered to protect against fire. Hence, the 3DPC walls with better fire performance could be incorporated with MBS to improve the fire behaviour of the whole structure.
Alkhalidi and Hatuqay [
34], investigated and developed energy efficient and low-cost residential 3DPC elements that can be accomplished through a green and sustainable method. He et al. [
37], developed 3D concrete printed modular building with integrated vertical greenery system, called 3D printed Vertical Green Wall (3D-VtGW). The energy saving potential of a small commercial building was established using 3D printed modular living wall system. Moreover, Mohammad et al. [
38] developed a high strength, lightweight concrete mixture suitable for 3DPC which showed improved thermal insulation, while reducing the energy consumption within the life cycle of the concrete structure.
However, researchers are focusing more on the structural performance of 3DPC structures. Moreover, many design guidelines and performance under elevated temperature are available for normal weight concrete, whereas very few studies have evaluated the fire performance and thermal energy of 3DPC structures. Therefore, there is an inevitable absence of literatures to investigate the performance of 3DPC structures at elevated temperatures experimentally and numerically. Hence, this study is more concerned about the fire performance and thermal comfort of the 3DPC buildings in a MBS. This study numerically investigates the fire performance of the innovative 3DPC wall configurations proposed by Alkhalidi and Hatuqay [
34]. To conclude, this article describes the fire performance of 3DPC wall panels subjected to standard fire, via a detailed parametric study of 20 numerical models covering five different 3DPC wall configurations with cavity and Rockwool infilled walls with two varying densities. The consequences of the study will potentially help to the growth in practice of safe and sustainable 3D printing technology in the building and construction industry.
3. Development of Finite Element Model
This section explains the development of the three-dimensional Finite Element (FE) model for analysing the heat transfer thermal behaviour of the 3DPC wall panels with different cross-sectional arrangements. The ABAQUS [
40] software is used in this study which allows uncoupled and coupled thermal analysis to examine the thermal behaviour of structures. The overall fire performance of a structure has to be analysed under three primary criteria such as insulation, integrity, and structural load bearing capacity [
16,
26]. This is known as coupled analysis which investigates the combined mechanical-thermal behaviour. As this study is focused only on non-load bearing walls, uncoupled heat transfer analysis has been performed. The selected wall panels were exposed to normal fire under ISO 834 standard fire scenario [
41] and the insulation failure analysis was conducted by measuring the unexpected surface temperature variation. Rockwool material is used as the fire insulation material here in this study.
In order to perform the detailed heat transfer analyses of 3DPC non-load bearing wall configurations with and without cavity insulation, precise temperature dependent thermal properties such as thermal conductivity, specific heat, and relative density have to be specified for concrete and the insulation material Rockwool. Alkhalidi and Hatuqay [
34] presented the ambient temperature thermal properties of three concrete mixtures used in their study, which were derived from the equation by Craveiro et al. [
42]. Since the presented properties are similar to that of the foam concrete at ambient temperature, thermal properties of foam concrete at elevated temperatures have been used in this study. The thermal properties of lightweight foamed concrete (LFC) at high temperatures with densities 650 kg/m
3 and 1000 kg/m
3 were obtained from the experimental and analytical study by Othuman and Wang [
19].
Figure 3a–c shows the thermal properties of foamed concrete at elevated temperatures and
Figure 4 illustrates the thermal conductivity variation of Rockwool insulation material. The density and specific heat values of Rockwool at elevated temperatures are 100 kg/m
3 and 840 J/kg.°C, respectively [
43].
7. Results and Discussion
The standard fire curve, ISO 834 [
41] was applied on firesides of the heat transfer FE model and the time-dependent fire unexposed side temperature was measured from ABAQUS CAE tools. The fire behaviour of different wall configurations and densities in terms of insulation fire rating are discussed herein extensively.
Figure 8a,b illustrate the unexposed surface temperature variation for all five considered wall configurations with densities 650 kg/m
3 and 1000 kg/m
3, respectively. Increment in insulation fire rating with the increase in density is clearly identified for all the wall panels. Both the cavity and Rockwool infilled walls showed superior fire resistance such that insulation fire rating is not exceeded the limiting insulation fire rating temperature of 160 °C (140 °C + 20 °C) for all the wall configurations within four hours. Moreover, it is obvious that the temperature increment is very low for the wall configurations with double row and triple row.
Figure 9a–e illustrate the comparison of unexposed surface temperature variation up to 4 h for all the wall configurations under standard fire condition for varying material densities. The temperature rise of all the different wall configurations in four hours is presented in
Table 3. The temperature distribution of the cavity wall panels, and Rockwool insulated wall panels at 0 min, 30 min, 1 h, 2 h, and 4 h of exposure to the standard fire are represented in the
Table 4,
Table 5 and
Table 6.
Considering the results, both the cavity and Rockwool infilled wall configurations have displayed superior fire resistance in terms of insulation failure criteria within the initial four hours. Moreover, increasing fire behaviour is obviously identified with the increase in density for all wall configurations. However, this heat transfer analysis only considers the insulation failure criteria and does not include the combined mechanical–thermal behaviour. Hence, Rockwool insulated single row 10 cm wall configuration with 1000 kg/m
3 density could be proposed to have better performance against fire with less material requirement. However, Alkhalidi and Hatuqay [
34] proposed E-PLA filled double row 15 cm configuration wall configuration with 1522 kg/m
3 density to have higher energy performance. Since, the fire performance will be amplified with increasing wall thickness and density, double row 15 cm configuration wall configuration with density 1522 kg/m
3 could be proposed to have energy efficient, thermally comfortable 3D printed built environment with enhanced fire performance.
Moreover, with the aim of developing cost effective sustainable buildings with enhanced structural performance and with increased construction speed the following MBS with 3DPC walls with improved thermal energy and fire performance could be proposed.
Figure 10 shows one of the proposed 3DPC walls in MBS and
Figure 11a,b illustrate the recommendations on how to integrate 3DPC walls around SHS corner posts in steel construction. The configuration shown in
Figure 11a illustrates the arrangement of steel corner posts with 3DPC walls. This could be used to protect the steel elements from fire source inside the building which is also easy to install onsite. The in-situ installation of the configuration given in
Figure 11b would be a challenge since the corner post t integration with printing process. However, it could be used to protect the steel elements from fire inside the building, as well as external environmental and chemical factors.