Thermal properties of carbon nanofibers enhanced lightweight cementitious composite under high temperature
Introduction
Lightweight concrete (LWC) has become an attractive construction material in Prefabricated Prefinished Volumetric Construction (PPVC) as Building Construction Authority in Singapore is pushing for greater productivity on site. PPVC is a construction method which 3-dimensional modular units are constructed with internal finishes and fixture in a precast yard, before it is assembled on-site. LWC has significant advantages to its use as structural members due to the reduced dead load. Due to its porous structure, LWC has excellent thermal and acoustic insulation properties that improve energy efficiency and sustainability of construction. Lightweight aggregate concrete (LWAC) is one type of LWC used for structural applications. The lightweight aggregates are either mined naturally or manufactured by calcining raw material. The former causes excessive exploitation of resources while the latter consumes great energy and is not eco-friendly. Cellular concrete is another type of LWC and it can be classified as autoclaved aerated concrete (AAC) and foam concrete. AAC has uncontrollable air pore structures and the autoclaved process is not only environmentally unfriendly but also limits the dimensions of ACC components [1], [2]. In this regard, foam concrete is a type of green material due to its composition and energy saving during production. However, traditional foam concrete is mainly used for non- or semi-structural applications because of its low mechanical and durability properties [2], [3], [4].
A new type of foam concrete named carbon nanofibers enhanced lightweight cementitious composite (CNF-LCC) for structural applications was developed and reported by Wang et al. [5]. CNF-LCC is produced by blending micro-foam bubbles with carbon nanofibers enhanced ultra-high performance concrete (ceUHPC). Despite the density of 1500 ± 50 kg/m3, CNF-LCC possesses superior mechanical properties, long-term properties and bond performance with steel bars compared to conventional foam concrete. While the previous work [5] looks at the abovementioned engineering properties, this manuscript studied the thermal properties of CNF-LCC under ambient and fire conditions.
The cellular microstructure gives foam concrete excellent thermal insulation property because the introduced air bubbles have almost negligible thermal conductivity (i.e. 0.026 W/mK). In literature, although thermal conductivity of foam concrete at room temperature is reported, there is no published experimental data at high temperature. Table 1 summarises the studies on the ambient-temperature thermal conductivity of foam concrete with different densities and mix designs. It can be found that thermal conductivity of foam concrete ranged from 0.06 to 0.7 W/mK for density varying from 150 to 1600 kg/m3, while NWC had a thermal conductivity of around 1.6 W/mK [6]. The thermal conductivity of foam concrete decreased with reducing density due to the increased volume of air [3]. Addition of mineral admixtures such as fly ash or pulverized fuel ash effectively reduced thermal conductivity because the lower density and cenospheric particle morphology of particles delay heat transfer through the material [7], [8], [9]. Meanwhile, adding fly ash or nanomaterials promoted uniform pore size distribution and reduced pore size by preventing air bubbles from merging, which subsequently reduced the thermal conductivity of foam concrete [7], [10].
However, there is a lack of experimental work on the high-temperature thermal properties of foam concrete, which is critical for evaluating its fire resistance [29]. Available research works showed acceptable or even superior fire resistance performance of foam concrete when compared to NWC [2], [18], [30], [31], [32], [33], but they were largely qualitative. Othuman and Wang [34] proposed an analytical model to predict thermal conductivity and specific heat of foam concrete from 20 to 1000 ℃. They conducted transient heating tests on foam concrete slabs and compared the experimental results with the analytical results from a one-dimensional heat transfer program to indirectly validate the predicted thermal properties. However, there was no direct testing results for the thermal properties of foam concrete under elevated temperatue.
Compared to traditional LWC, CNF-LCC uses eco-friendly ingredients and production process but also possesses superior engineering properties for structural applications as reported in the authors’ previous work [5]. As a new type of foam concrete, CNF-LCC possesses excellent sustainability due to its intrinsic thermal insulation properties and fire resistance. Due to the lack of study on temperature-dependent thermal properties of foam concrete, it is necessary to investigate the thermal properties of CNF-LCC under high temperature so that this material can be designed for a fire situation. These thermal properties include thermal diffusivity, specific heat, thermal conductivity and thermal expansion of ceUHPC base mix, CNF-LCC and LCC (without carbon nanofibers but of the same mix design as CNF-LCC) and they were determined experimentally from ambient to 800 ℃. In addition, one-dimensional heat transfer tests using the ISO834 standard fire curve were carried out with CNF-LCC and LCC samples. The measured thermal properties were validated using recorded temperatures in one-dimensional heat transfer tests with ABAQUS software. Thermogravimetric analysis and X-ray diffraction were used to characterise the dehydration reaction and analyse the measured thermal properties of samples under high temperature. All the experimental results of CNF-LCC and LCC were compared with traditional foam concrete, NWC and LWAC from literature to evaluate its fire resistance performance. The effect of foam bubbles and carbon nanofibers on the microstructures and thermal properties was also investigated.
Section snippets
Mix design
The details of mix design and preparation of CNF-LCC and LCC have been reported in [5], [35]. CNF-LCC was produced based on the pre-foaming method by blending ceUHPC base mix with micro-foam bubbles. Table 2 shows the mix designs of CNF-LCC and LCC. The ceUHPC base mix was optimised according to the investigation of Chen et al. [36] where optimum workability and minimised entrapped air were achieved to improve the mechanical properties of resulting UHPC. The cement shown in Table 2 is CEM 1
Mechanical properties
The mechanical properties of CNF-LCC and LCC were exhibited in [5] and they are briefly summarised in Section 4.1 for completeness of this paper.
Laser Flash
Thermal diffusivity is an inherent thermal property of a material and is related to thermal conductivity. The thermal diffusivity of a sample was obtained using TA Discovery Laser Flash DLF1200. The samples were cast as 2 mm thick discs with 12.5 mm diameter. A thin layer of graphite coating was applied over the samples to ensure that the energy pulse
Mechanical properties
The 28-day mechanical properties of CNF-LCC and LCC were reported and discussed in [5]. The results are briefly summarised in Table 3, along with those of NWC and LWAC with same compressive strengths based on EC2 [41]. CNF-LCC showed higher compressive strength at 28 days than traditional foam concrete of densities between 240 and 1900 kg/m3 [3]. This is because CNF-LCC was formulated based on ceUHPC base mix that created stronger and denser microstructure of pore borders. In addition,
Conclusion
Based on the excellent engineering properties and potential structural applications reported in previous work [5], this paper investigated the thermal properties of CNF-LCC under elevated temperature. As a structural lightweight concrete, CNF-LCC also showed the potential for fire resistance due to its good thermal insulation properties and low thermal strain under high temperature. The conclusions of the study are as follows:
- 1.
Adding a low dosage of CNFs had no effect on thermal diffusivity,
CRediT authorship contribution statement
Su Wang: Conceptualization, Data curation, Formal analysis, Investigation, Methodology, Software, Validation, Writing – original draft. Yan Hao Ng: Methodology, Data curation, Formal analysis, Software, Validation, Writing – original draft. Kang Hai Tan: Conceptualization, Funding acquisition, Methodology, Resources, Supervision, Writing – review & editing. Aravind Dasari: Conceptualization, Methodology, Resources, Supervision, Writing – review & editing.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgment
The authors would like to acknowledge financial and materials support from ceEntek Pte Ltd.
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