Effect of elevated temperatures on the mechanical behavior of basalt textile reinforced refractory concrete
Introduction
Textile reinforced concretes (TRC) are a new generation of cementitious materials with enhanced tensile strength and ductility [1], [2]. With its excellent mechanical properties, the TRC’s are used, currently, in a wide range of applications that include: strengthening and repair in structural elements, protective linings, thin-walled elements, façade elements, bridges and also freeform and lightweight structures. In the last two decades significant improvements in the development of TRC have been achieved resulting in high performance composites that can present uniaxial tensile strength up to 50 MPa and tensile strain up to 5% [3], [4]. It is important to consider, however, that in many of these applications the concrete and the textile undergo thermal effects, becoming the study of thermo-mechanical performance of TRC absolutely indispensable [5], [6], [7].
Although there is a growing interest in the use of TRC elements, little is known about their thermo-mechanical performance and even less about applications using refractory concrete as a matrix. Conventional fiber refractory concrete present increased ductility and toughness compared to the plain refractory matrix [8], [9]. In most cases, the use of a fiber reinforcement in refractory concretes is related to the control of cracking during the heating or drying process (bridging the crack and limiting the crack propagation) [8], [10] and as an attempt to reduce the fragmentation process of the concrete when subjected to high temperatures (spalling) [10], [11], [12].
It is known that high alumina cement has alkali content typically less than 0.5% of its weight [13]. Therefore, the risk of alkali attack, common to basalt, glass and natural fibers, decreases substantially in refractory concretes. Taking advantage of these properties, several researches used high alumina cements in the past to produce fiber reinforced composites (specially using glass fibers) [14].
Basalt is a low cost material that brings interesting opportunities to the construction industry. Recent researches developed by Larrinaga et al. [15] contributed to explain the mechanical behavior of basalt fabric as a reinforcement in a TRM (textile reinforced mortar). Several basalt layers were used as reinforcement in a non-commercial cement-based mortar containing a redispersible resin to achieve fire-proof properties. The tensile strength showed to be strongly affected by the reinforcement ratio. There were visible differences in the cracking spacing as well as in the crack width and the strain capacity surpassed 2% for the most of the studied TRM specimens. Sim et al. [16] investigated the applicability of the basalt fiber as a strengthening material for structural concrete members through durability, mechanical, thermo-mechanical and structural tests. The results obtained indicated that, compared to FRP (fiber reinforced polymer) strengthening systems, the basalt fiber strengthening may be a good alternative when characteristics such as moderate structural strengthening and high resistance for fire are simultaneously sought (such as for building structures). When tested under high-temperature (over 600 °C) and compared to carbon and S-glass fibers, only the basalt maintained its volumetric integrity and 90% of the strength.
When a coating is used in the textile reinforcement, the bond performance between fibers and matrix may change with temperature. Recent researches developed by Silva et al. [6] using TRC reinforced with carbon fibers showed that when heating polymer coated carbon fibers TRC under temperatures up to 150 °C a polymer interlocking mechanism between filaments and matrix is observed. This mechanism results in significant increases in the maximum pullout load. Krüger and Reinhardt [5] performed fire tests on four different I-shaped mortar beams reinforced with AR-glass and carbon textiles. The investigation was focused on the load bearing capacity of the composite during a fire test under constant load. In one of the cases a SBR thermoplastic resin was used as coating in the carbon fabric. The results showed to be very dependent on the fire behavior of the used fibers. Due to the softening of the SBR coating (at about 90 °C) the fiber–matrix interface was rapidly impaired, resulting in fiber pullout and, subsequent, failure.
The aim of this article is to investigate the effect of elevated temperatures on the mechanical properties of a textile refractory composite reinforced with basalt fabric submitted to tensile loading. At first, the refractory composites were produced with a cementitious matrix, made of calcium aluminate aggregates and high alumina cement (HAC), reinforced with basalt fabrics. The composites were tested under tensile load after being submitted to different temperatures regime ranging from 25 to 1000 °C. The influence of the number of textile layers on the tensile behavior of TRC was also addressed. The influence of the exposure time on the tensile behavior of basalt TRC was investigated for the temperature of 200 °C. The crack formation was investigated using a high resolution imaging procedure. Crack spacing was measured using image analysis and correlated with the applied strain under tensile loading. Tensile tests on basalt fabric specimens were carried out at room temperature and at 400 °C. The identification of the dehydration reactions caused by the heating regime was addressed using thermo-gravimetric analysis (TGA) and X-ray diffraction (XRD). The present study adds an important knowledge to the existing literature on the thermo-mechanical behavior of textile reinforced concrete.
Section snippets
Refractory concrete matrix
The matrix used in this research (compressive strength of about 45 MPa) was designed following the compressible packing model (CPM) routine [17], [18] and then adapted to the rheology necessary to produce laminated TRC’s. As a result of the small diameter of the continuous filaments and the small distance between the reinforcement textile layers, the maximum aggregate diameter had to be less than 1.18 mm. The materials used in the TRC composition were a calcium aluminate cement (Secar 51) with
The heating regime
The TRC specimens were heated up to 75, 150, 200, 300, 400, 600 and 1000 °C and subsequently cooled by a natural process inside the furnace. Experiments on specimens stored at room temperature were also carried out. In order to better understand the mechanical behavior of TRC under elevated temperatures, additional tensile tests using different exposure times (at the target temperature of 200 °C) were performed. The thermo-mechanical behavior of basalt fabric was also investigated, but using only
Mechanical testing
The direct tensile tests on the TRC plates and on the fabric samples were performed in a Shimadzu universal testing machine model AGX – 100 kN and controlled by the actuator displacement at a rate of 0.4 mm/min. For the composites, the force was transferred to the specimens via rotatable steel plates screwed to the TRC plates. Four rectangular shaped specimens measuring 400 mm × 60 mm × 13 mm (length × width × thickness) were tested using a gage length of 200 mm with fixed-hinged boundary conditions.
Materials characterization
Before and after the heating process, the fiber–matrix interfaces were investigated using a scanning electron microscope (SEM) FEI Quanta 400. The TRC samples were sectioned in specimens with dimensions of 20 × 20 mm (length × width). The samples were coated with 20 nm of gold to become conductive and suitable for conventional SEM analysis. The SEM was operated using 25 kV of acceleration tension and about 30 mm of working distance. The TGA analyses were carried out using fragments of the samples
Influence of the number of layers on the TRC tensile behavior (at room temperature)
Fig. 3(a) shows representative stress–strain curves obtained from the tensile tests (at room temperature) performed with the plain matrix and with TRC reinforced with 1, 3 and 5 bi-directional basalt fabric layers. The typical cracking patterns are shown in Fig. 5(b)–(e). The results obtained from the specimens tested at 7 days of age are given in Table 3. The bend-over point (BOP) of each curve corresponds to the end of the linear elastic region and to the formation of a first matrix crack
Conclusions
The following conclusions can be drawn from the present work:
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The behavior of the TRC is strongly influenced by the reinforcement ratio. The TRC’s produced with 3 and 5 fabric layers showed an effective cracking control and, therefore, significant increases on the tensile response and on the overall ductility of the composite system.
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The heating–cooling regimes used in the present work showed to affect the tensile response of the TRC. Preheating up to 150 °C had a significant impact on the maximum
Acknowledgement
The authors gratefully acknowledge the Brazilian Agencies CNPq and FAPERJ for their partial financial support and Kerneos (France) for supplying the cement, superplasticizer and aggregate.
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