Fire resistance of a prefabricated bushfire bunker using aerated concrete panels

Highlights

• Full-scale fire testings of 3 m × 3 m PLAC panels are conducted.

• A 125,000 elements model is developed to simulate fire performance of PLAC panels.

• A dual-skin PLAC fire bunker is concluded to ensure fire safety compliance.

• Super light PLAC panels of 500 kg/m³ is required in severe bush fire conditions.

Abstract

Prefabricated lightweight aerated concrete (PLAC) panels provide low thermal conductivity, potentially high stiffness-to-weight ratios, cost-effective material and structural systems and rapid modular construction. These panels can be utilised as floor slabs or external walls for various applications in building construction. The fire performance of the PLAC panel is examined in this work for a particular case, namely a prefabricated emergency bushfire shelter, which is one of the key applications of PLAC panels. Since, bushfires have unique heating curves, standardised tests are not useful and the system needs to be tested in a manner such that the heat flux of an actual bush fire can be reproduced. In this study, the fire performance enhancement of dual-skin bushfire bunkers, which are comprised of lightweight concrete and base metal thickness (BMT) steel, are examined experimentally and validated numerically. The Speedpanel PLAC modular panel explored in this work is a lightweight wall system primarily used for acoustic and thermal insulation purposes. Burning experimental studies of a single panel and dual-skin bunkers are carried out on a full scale. The experimental results are compared with fire safety codes for building materials to identify the key areas for improvements. A fire dynamic numerical model has been developed in this work using the Fire Dynamics Simulator (FDS) to simulate the burning process of PLAC structures. Numerical results of heat production are presented in comparison with experimental observations for validating the computational model. The proposed numerical model is used to predict the fire performance of a dual-skin bushfire bunker, demonstrating the need to have at least two PLAC layers to ensure fire safety compliance.

Introduction

Developments in structural mechanics and materials technology have created a trend towards using hybrid composite panels based on lightweight and low-emission materials in the construction industry. Lightweight composite panels provide efficient systems, offering high stiffness-to-weight ratios and high strength-to-weight ratios [1], [2]. These panels could be used effectively in many different applications in modular construction, namely multifunctional walls, slabs and roof panels.

Fire safety continues to be a critical issue around the world, especially with severe bushfires in Australia. Every summer, Australia is facing the danger of bushfires, which cost lives, destroy forests and rage into the suburban fringes of major cities. The worst bushfires that ever occurred in Australia include Black Saturday in Victoria 2009, Ash Wednesday in Victoria 1983, Black Friday in Victoria 1939, Black Tuesday in Tasmania 1967, and the Gippsland fires and Black Sunday in Victoria in 1926 [3]. Bushfires are unpredictable and they can happen anywhere in Australia. While fire prevention is not sufficient [4], a plan for fire management is encouraged in the event of bushfires so that the loss of human lives can be avoided. Fire bunkers, which are mainly made of composite materials, including steel and concrete, can save human lives in the event of a fire.

Composite materials have played an important role in the construction industry over the past decade [5], [6]. They have been proven to exhibit numerous benefits in comparison to conventional construction materials. Chen et al. revealed that the use of combined material systems improved the fire resistant performance of internal and external walls [7]. A steel hollow section filled with concrete has higher strength and larger stiffness than a conventional structural steel section, as well as reinforced concrete. The purpose of using composite materials in construction is to reduce the amount of formwork and site labor. Concrete filled steel hollow sections (CFSHS) have been used widely in numerous applications such as columns, beams and flooring systems, as they offer significant advantages in strength, which are attributed to the high tensile strength of steel and high compressive strength of the concrete [8], [9]. This composite material also facilitates a high fire rating for structures [10], [11]. In concrete filled steel construction, concrete and steel work together to transfer external loads [12] whilst preventing premature buckling of the structural steel section. The concrete infill provides thermal insulation to the steel due to the concrete’s ability to withstand high temperatures. Extensive research has been conducted on the behavior of CFSHS with numerous shapes, such as circular [13], elliptical [14], octagonal [15], square [16] and rectangular [17]. Han et al. [18] investigated the fire resistance of concrete-filled square hollow section (SHS) and rectangular hollow section (RHS) columns, where a high strength concrete with a density of 2400 kg/m³ was used in the experiment. This paper also points out that at that time, the most economical solution to the design of a fire resistant concrete-filled SHS or RHS is to design for the maximum structural efficiency, and obtain the required fire resistance by applying a conventional insulating system.

Research from Go et al. [19] showed that a lightweight aggregate concrete wall performs better in terms of fire resistance than a conventional concrete wall. The lightweight aggregate walls maintain superior post-fire structural behavior in comparison to normal-weight aggregate walls [19]. Other studies also showed that lightweight concrete has a higher strength to weight ratio, a higher tensile strength, a lower coefficient of thermal expansion, and superior heat and sound insulation characteristics due to air voids in the lightweight aggregate [20], [21], compared to conventional concrete. Lightweight aerated concrete (LAC) has also been chosen over normal concrete due to its light weight, cost effectiveness and environmental sustainability [22]. LAC can have a density as low as 400 kg/m³, offering a potential weight saving of 83% over normal concrete [22]. On the other hand, the disadvantages of lightweight aerated concrete have also been investigated. Studies have found that lightweight concrete has not been used in structural members due to problems with its engineering properties [23]. Furthermore, the brittleness of lightweight concrete is higher than that of conventional concrete for the same mix proportion and compressive strength [24], [25]. In addition, the mechanical properties of lightweight concrete are lower than normal weight concrete [23]. It is possible to produce lightweight concrete using lightweight aggregates or air entraining agents with a density as low as 250 kg/m³ [26].

There are three main methods of production of lightweight concrete, which include: the use of lightweight aggregates, the incorporation of voids by aeration, and little or no fine aggregate addition [27], [28]. In this study, foaming agents such as sodium lauryl sulfate (SLS) and aluminum powder have been utilised to generate voids and produce lightweight concrete.

While most researchers have investigated the fire resistant performance of normal density CFSHS, there has been a lack of research on low density CFSHS. In order to have a comprehensive understanding on light CFSHS, this research investigates the performance of the panel under standard fire conditions. A full scale lightweight CFSHS panel is specifically designed for bushfire prevention and tested under standard fire conditions in accordance with AS1530.4-2014 [29]. A computational fluid dynamics (CFD) model is also developed and validated. The validated model is then applied to predict the fire performance of a dual-skin fire bunker.