As granite is one of the most widely encountered lithologies in studies of high-temperature rock mechanics, the effects of high temperatures on the physical and mechanical properties of granite have attracted considerable attention. In this study, to identify the physical and mechanical properties of granite observed before and after high-temperature treatment from room temperature to 1000 °C and to determine the influence of high temperatures on granite and on its intrinsic connections, appearance, and mineral morphology, longitudinal wave testing and uniaxial and triaxial tests were performed. Our primary conclusions are as follows: (1) effects of high temperatures on microscopic minerals of granite primarily include the oxidation of dark minerals, the recrystallization of feldspar minerals, and the generation and extension of mineral cracks. (2) High temperatures decrease the speed of longitudinal waves in granite and cause longitudinal wave energy to be absorbed. Cracks created by high temperatures cause longitudinal waves to not be detected. (3) Relationships between stress and strain processes at 400 °C in granite first soften and subsequently harden, and mechanical properties change only slightly overall. After a temperature of 400 °C is reached, with increasing temperatures, softening characteristics emerge. At 800 °C, plastic deformation occurs. At 1000 °C, the strength and deformability of rock specimens are largely diminished. We found 600 °C to be the threshold of mechanical properties and the transition temperature of the failure mode. (4) Finally, 400 °C, 600 °C, and 800 °C were found to be characteristic temperatures of three different stages of physical and mechanical properties for granite.

Physical and Mechanical Properties of Granite After High-Temperature Treatment

This study experimentally investigates the influence of heating/cooling cycles on the mechanical properties and cracking features of Rucheng granite. The specimens, which were previously subjected to various numbers of heating (350 °C) and cooling (in 21 °C tap water) cycles, were tested under unconfined compression. Integrated acoustic emission (AE) and digital image correlation (DIC) techniques were used to study the micro/macrocracking characteristics of the specimens upon different heating/cooling cycles. The changes in the amount of surface cracks and microcracks, P-wave velocity, unconfined compressive strength (UCS), elastic modulus, and Poisson’s ratio of the specimens prior to and after various heating/cooling cycles were comprehensively analyzed. The results showed that as the number of heating/cooling cycles increased, more surface cracks and internal microcracks (including intergranular and intragranular) were observed, leading to more remarkable decreases in P-wave velocity and substantial loss of the strength and stiffness. The change in the P-wave velocity is more sensitive than the UCS upon the cyclic heating/cooling treatments. An increase in the cycle number yielded an increase in the brittleness and a decrease in the ductility with respect to the stress-strain response. Furthermore, after exposure to a greater number of heating/cooling treatment cycles, the onset of unstable crack propagation will initiate at a lower stress threshold, while the postpeak softening response becomes more prominent. In addition, the failure mode changes from a typical shear failure to an axial splitting fashion as the number of heating/cooling cycles increases.

Influence of heating/cooling cycles on the micro/macrocracking characteristics of Rucheng granite under unconfined compression

Isophorone diisocyanate-loaded microcapsules were synthesized with controllable size for self-healing of structural polymer materials. The obtained microcapsules showed good compatibility with polymer resins, e.g., one-component epoxy and multi-component polyurethane/water glass structural materials. During the processing of composite self-healing materials, the highly cross-linked composite capsule wall ensured the integrity of microcapsules and prevented the leakage and degradation of reactive liquid isocyanates effectively. A relatively larger amount of self-healing agent could be released to the local cracks with the increasing size and content of microcapsules in polymer matrix which improved the self-healing efficiency of the composites. Based on the microcapsules with a diameter of ~96 μm, the healing efficiency of epoxy composites containing 15 wt% of capsules and polyurethane/water glass composites containing 20 wt% of capsules could reach to 105 and 101% recovery of fracture toughness, respectively. The crack-healing effect was further verified by scanning electron microscopy. Meanwhile, a possible self-healing mechanism of microcapsules to the cracks of composite polymer materials was proposed.

Self-healing isocyanate microcapsules for efficient restoration of fracture damage of polyurethane and epoxy resins

External confinement of concrete using fiber reinforced polymer FFRP) composites has emerged as a popular method for retrofitting of existing reinforced concrete columns for increased strength and ductility. The paper examines the axial stress-strain behavior of FRP confined concrete, understanding of which is necessary for the design of FRP retrofitting measures for columns, particularly when the column is subject to combined bending and axial load and when seismic ductility is of concern. Presented is a new, accurate stress-strain model for concrete with a sufficient amount of confinement from FRP which leads to significant compression strength enhancement. The model is based on a careful interpretation of large test databases gathered from extensive surveys of the existing literature.

A new stress-strain model for frp-confined concrete

With the increased development of geothermal resources and nuclear waste storage technologies, studies of high-temperature rock mechanic characteristics have become a hot research topic (Stesky 1978; Tapponnier and Brace 1976; Wai et al. 1982; Simpson 1985; Kou 1987; Homand-Etienne and Houpert 1989; Somerton and Boozer 2010; Smithies et al. 2011; Just and Kontny 2012; Singh et al. 2015; Zhao et al. 2017). Many researchers have examined the mechanical properties of rock after exposure to high temperatures, including deformation processes, failure criteria of rocks under pressure, and thermal cracking. They have also provided constitutive equations and carried out rock-damage characterization in response (Brede 1993; Hueckel et al. 1994; Araújo et al. 1997; Emirov and Ramazanova 2007; Sun et al. 2015; Hartlieb et al. 2016; Su et al. 2016; Ding et al. 2016; Xu and Zhang 2018). A study on the mechanical characteristics of high-temperature rocks after cooling is at the center of scientific research interests. The effects of different cooling rates and methods on mechanical parameters, strength cracking, and failure modes of high-temperature rocks are critical knowledge (Wang et al. 1988; Kim et al. 2014; Shao et al. 2014; Kumari et al. 2017; Isaka et al. 2018; Xu and Sun 2018). Extant literature has focused on the influences of cooling by air, real-time high temperatures, and single instances of water quenching on rock’s mechanical parameters. However, there is little research on the influence of cycles of heating and cooling with cold water on the mechanical properties of rock which are often found in underground engineering projects. For example, geothermal development has become the focus of research at home and abroad in recent years, especially for enhanced geothermal system (EGS) (Barla 2016; Breede et al. 2013). Granite has become an important reservoir material for EGSs. The principle of EGS is to inject cold water into hot rock by drilling into the ground and then drawing the heated water out of the surface to achieve the utility of underground heat energy. Reservoir rocks are subjected to different heating and cooling conditions. Therefore, they exhibit very different mechanical characteristics compared to conventional hot rock. Additionally, high temperature and cold water repeatedly act on surrounding rock during tunnel fires. To ensure the stability of a tunnel, it is necessary to understand the effects of the heating and cooling cycles on the rock’s structure. Recently, related scholars have carried out research on mechanical properties of rocks after cyclic heating and cooling. Ge and Sun (2018) studied the AE and b value in the process of rock fracture with the variation trend of different stress levels and temperatures; in this study, AE is used only to evaluate the intensity characteristics of samples. Rong et al. (2018) studied the physical and mechanical properties of two bedrocks (i.e., marble and granite) in geothermal fields with the increase of the number of thermal cycle in the treatment if a high temperature is 600 °C; Jin et al. (2019) compared and analyzed the responses of physical, mechanical, and transport properties of the granite after various thermal cycling treatments (slow heating followed by slow cooling or rapid quenching). However, Rong et al. used natural cooling method to cool the samples, and Jin et al. conducted * Hongwen Jing hwjing@cumt.edu.cn

Mechanical Characteristics of Granite After Heating and Water-Cooling Cycles

Basalt fiber is cheap and has excellent mechanical performance. In addition, in combination with the environmentally friendly coconut fiber, it can contribute to sustainable construction materials. In this study, a composite material consists of basalt fiber-reinforced polymer (BFRP) tube-encased coconut fiber-reinforced concrete (CFRC) is developed. The 28-day compression strength of the plain concrete is about 15 MPa, which represents the low strength and poor-quality concrete widely existing in a large number of old buildings. The concrete was poured and cured into the steel moulds, BFRP tubes with the inner diameter of 100 mm and a height of 200 mm, and BFRP tubes with the inner diameter of 160 mm and a height of 320 mm, respectively. In total, 36 cylindrical specimens were constructed and tested. The axial compression tests were carried out to examine the strength and ductility enhancement due to the confinement of the BFRP tubes and coconut fibers. Also, the existing models were used to predict the ultimate axial compression strength and strain. The results show that the size of the specimen significantly influences the predicted compression strength.

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Compression Behavior of Basalt Fiber-Reinforced Polymer Tube-Confined Coconut Fiber-Reinforced Concrete

The empirical formulas of dynamic amplification factor (DAF) specified in current bridge codes only consider the span or fundamental frequency of reinforced concrete (RC) girders in highway. Althou...

Xue-Qian Wu

Papers

Compression Behavior of Basalt Fiber-Reinforced Polymer Tube-Confined Coconut Fiber-Reinforced Concrete

Experimental Study on Dynamic Amplification Factor of Simple-Supported Reinforced Concrete Beams Under Impact Loading Generated by an Impulse Hammer