Properties and material models for common construction materials at elevated temperatures
Introduction
Civil constructions are to be designed to satisfy codal requirements. One such requirement is to withstand extreme events (i.e. fire/thermal loading). The ability of a structure to withstand fire and associated fire-induced forces is highly reliant on, 1) the type of material the main structural members/components are composed of, and 2) how properties of such materials are influenced by elevated temperatures [1]. This ability is often measured through experimental fire testing or, for the most part, through fire resistance evaluation. In such evaluation, thermal and mechanical characteristics of construction materials are of interest as fire resistance assessment requires carrying out a two-step analysis; thermal and structural. In the first step, rise in temperature and associated temperature propagation in a load bearing member are obtained by examining how density, thermal conductivity, and specific heat properties fluctuate with increasing temperatures. Once sectional temperatures are obtained, these are then loaded into the second step of analysis. In this step, the adverse effect of increasing temperature upon mechanical properties of construction materials; primarily comprising of strength, and modulus, is considered in evaluating the assessing behavior of a fire-exposed member [1], [2].
Thus, carrying out a proper fire resistance analysis requires thorough knowledge of thermal and mechanical properties at ambient and fire conditions. While evaluating aforementioned material properties at ambient conditions can be achieved with ease mainly due to the availability of testing standards and instrumentations, assessing properties of building materials at high temperatures is shown to be a tedious task [3]. This can be primarily ascribed to the current lack of expertise and/or standardized (i.e. agreed upon) guidance, shortage and limited access of testing equipment, and most of all complexities arising from simulating fire conditions (i.e. survivability of sensors as well as ensuring proper measurements under elevated temperatures) [4]. As a result, fewer studies managed to successfully conduct material property tests under high temperatures as compared to that at ambient conditions [5].
The outcome of these studies presented the adverse effects of fire on material properties through temperature-dependent material models which were either prepared into simplified expressions, or informative charts [6], [7]. Despite the fact that most of these research studies were carried out in 1960-90’s, the outcome of such studies continues to form the basis for currently adopted temperature-dependent material models [6], [7]. A close examination of these models exposes discrepancies arising from different testing methods and equipment, specimen configuration etc. used in high temperature tests [5], [8]. Another factor that adds further complexities is the existence of distinct variations in the make-up (composition) of construction materials tested in the 1960–90s and those available today as a result of the natural progression in materials science, differences in origin/amount/type of additives, as well as from modern production/milling procedures.
A review of literature also shows that this community has accepted two material models to be used in fire resistance assessment. For the most part, these models are adopted in North America (i.e. American Society of Civil Engineers (ASCE) design guide [6]), and Europe (Eurocodes [7], [9], [10], [11]). Despite the fact that these two models have been widely used, recent works have shown that variation in fire resistance predictions can be in the range of 25% when using temperature-dependent models adopted by ASCE or Eurocodes [5], [12]. This often complicates structural evaluation at elevated temperatures; particularly in the analysis/design for compound load effects (viz. torsion, or buckling etc.), selecting appropriate fire protection material/type/thickness, or even in carrying out design/engineering (consulting) services.
Such variation arises due to a number of reasons. For a start, these codal-adopted models imply that the micro-structure of construction materials is independent of its fabrication process, or composition/origin. Further, these models were developed using vintage devices, which are inferior to the modern and state-of-the-art equipment, and thus provided scientists with restricted testing set-ups, and possibly mediocre measurements [5], [12]. Furthermore, these material models continue to be not updated since their development; dating back to 20–30 years. Finally, while Eurocode 3 suggests the use of certain models to represent behavior of contemporary construction materials (i.e. stainless steel and cold-formed steel), ASCE design guide does not provide direction nor insights into how to account for temperature-dependent effects in materials; leaving designers with limited guidance, hence complicating the process of fire resistance evaluation.
From this work’s perspective, it is infeasible to regularly conduct temperature-dependent tests on materials – given the variety in compositions, origins and mixes. Thus, a dilemma arises highlighting the need for a uniform and modern model for construction materials at elevated temperatures. With the hope of overcoming this challenge, and in support of current inertia aimed at promoting standardization for fire resistance assessment, this study aims at utilizing Artificial Intelligence (AI) to develop modern and updated temperature-dependent models for commonly used construction materials – and this could be the first step towards realizing uniform (universal/standardized) constitutive material models. As such, this work presents a novel approach to develop temperature-dependent thermal and mechanical material models for some of the commonly used building materials, namely: normal strength concrete, masonry, structural steel, stainless steel, cold-formed steel and wood. This study starts by presenting high temperature properties of common construction materials and then showcases a proper procedure to developing an AI model capable of deriving temperature-dependent material models. To ensure precision and wide acceptance, the developed AI model integrates material models adopted by notable fire codes, standards and design guides, together with models collected from past and recent published studies/reports.
Section snippets
Temperature-dependent properties of common construction materials – an overview
Response of structures once exposed to elevated temperatures is largely a function of properties of building materials. Under such effects, thermal and mechanical1 properties fluctuate with temperature mirroring the series of phase changes that occur. As such, this section delivers a brief overview on the
A Note on high temperature material tests
A comparison between various material models plotted in Fig. 1, Fig. 2, Fig. 3, Fig. 4 shows the large variations in measured properties of common building materials. This variation is not only apparent when comparing test data points carried out in 1950s-70s with those carried out in late 1990s-2000s, but also in between data points obtained from tests within the same era. For example, the early works of Powell [21], Yawata [22], and Touloukain [23], which were carried out in 1960s-70s, did
Development of artificial intelligent model
In lieu of traditional deterministic methods, artificial intelligence (AI) simulates the reasoning process through cognitive layers arranged in a specific layout (e.g. Artificial Neural Network (ANN)). As such, ANNs are made of constitutive layers and accompanying neurons (processing units) as shown in Fig. 5. Each layer has a number of neurons that is largely contingent on the complexity level of arriving at a relation between inputs and expected outcome. On the far left side, the input layer
Deriving AI-based temperature-dependent material models
As discussed above, all data points (in terms of measured properties or codal-adopted models i.e. ASCE, Eurocodes, AS 4100, AISC, BS5950 and so on), as well as those measured through researchers as plotted in Fig. 1, Fig. 2, Fig. 3, Fig. 4.) for commonly used construction materials were input into the ANN. More specifically, temperature-dependent reduction factors of modulus property of concrete were first collected at specific temperatures i.e. 25, 100 °C etc. (as shown in Fig. 4a) [95]. It is
Practical implications and future research plans
The discussion outlined in Secs. 1–3 illustrates how the result of a given fire resistance evaluation is highly sensitive to the selected material model. As the fire response of a structural member is not known beforehand, then fire resistance predictions are often viewed with caution. In such a scenario, fire designers often face a challenge as to 1) what material model to select for fire assessment, 2) justification to select such model over others. As a result, it is common for
Conclusions
This study showcases a framework to develop temperature-dependent thermal and mechanical material properties of common building materials including normal strength concrete, masonry, structural steel, stainless steel, cold-formed steel and wood. This approach applies artificial intelligence in two forms, i.e. Artificial Neural Networks (ANNs) and Genetic Programming (GP) to achieve modern temperature-dependent material models.
These following conclusions could also be gathered from the outcome
Acknowledgment
The author would also like to thank Prof. Venkatesh K. Kodur for his continuous support.
Data availability
The raw data required to reproduce these findings are available to download from [www.mznaser.com/fireassessmenttoolsanddatabases].
Conflict of interests
The author declares no conflict of interest.
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