Stress–strain behaviour and acoustic emission characteristic of gangue concrete under axial compression in frost environment
Introduction
Coal gangue is an industrial solid waste that is discharged during coal mining. In China, the cumulative stock of coal gangue has reached more than 5 billion tons. A large amount of coal gangue accumulation has occupied land resources and seriously polluted the environment. Therefore, effective utilisation of coal gangue has become an urgent social problem. In the 1960s and 1970s, many countries have focused on the comprehensive utilisation of coal gangue [1], [2]. Since then, this material has been widely used in various fields, such as building material preparation, roadbed filling, mineral extraction, electricity generation and chemical raw material preparation [3], [4], [5]. One of the main utilisation methods is the preparation of building materials that largely consume coal gangue and yield low production cost.
Coal gangue concrete is an environment-friendly building material prepared by replacing natural stone or sand of concrete with coal gangue at a certain proportion. Using coal gangue coarse aggregate in concrete offers economic and environmental benefits. The advantages of gangue concrete include low environmental pollution, saving of natural aggregate resources and reduction of energy consumption. The amount of coal gangue is huge in areas with abundant coal resources, especially in the northern regions at high latitudes of China. Coal gangue concrete materials are increasingly used in engineering construction, such as paving roads, roadway support and secondary structures. However, the collective effect of the penetration of external water and the alternation of positive and negative temperatures in the coal gangue concrete structure at high-latitude areas results in frost-swell stress and osmotic pressure in the pores of the concrete, seriously jeopardizing the durability, load-bearing performance and service life of structure. Thus, the research on mechanical and durability of coal gangue concrete is crucial.
At present, the frost-resistance durability, mechanical property and capillary absorption of normal concretes, such as forming concrete [6], [7], [8], shotcrete [9], alkali-activated concrete [10] and recycled concrete [11], [12], have been thoroughly studied. However, the antifreeze durability and mechanical properties of coal gangue concrete have been seldom studied, and the performance of axial compression and damage law after freezing and thawing have not been reported. The existing research is mainly focused on the microstructure and performance of coal-gangue-mixed materials and the preparation and compressive strength of coal gangue concrete. For instance, Dongxu Li [13] studied the mechanical property of coal gangue in the activation process using the method of orthogonal experiment, and then the microstructure of coal gangue was determined by SEM and NMR. Jinman Wang [14] studied on the compressive strength of coal gangue concrete and found that when the coal gangue substitution rate is between 15% and 25%, the compressive strength of coal gangue concrete can meet the test requirements. Cong [15] studied the preparation technology and the microstructure of self-ignition coal gangue autoclaved aerated concrete for the optimum mixtures. Yi Cheng [16] reported that the coal gangue geopolymer has a higher initial strength and a lower 28-day compressive strength than P.O 42.5 cement sample when the liquid–solid ratio is more than 0.32.
Accumulated energy is released in the form of elastic wave during the internal crack development of concrete brittle materials; this phenomenon is called acoustic emission. Therefore, the acoustic emission signal can be used to understand the evolution law of damage during concrete material loading [17], [18]. At present, the applications of acoustic emission technology are mainly on the relationship between basic physical and mechanical properties and acoustic emission phenomena in various types of concrete materials. Acoustic-emission characteristic parameters can be used in studying the failure mechanism and the damage characteristics of concrete under different conditions. Mainili [19] revealed the failure mechanism of the frost damaged shotcrete using AE technology. Abdelrahman [20] established a damage assessment method for prestressed concrete structures based on acoustic emission technology. Aggelis [21] analysed the acoustic emission characterisation of fracture process in fibre-reinforced concrete. Geng [22] studied the dynamic damage failure of concrete based on acoustic emission. Tam [23] studied the characterisation of damage in concrete beams under bending using acoustic emission and found that a strong correlation exists between damage evolution and acoustic emission signals.
In this study, the frost-resistance durability was investigated through accelerated freeze–thaw experiment, and the stress–strain relationship was measured to determine the degradation law of mechanical property of coal gangue concrete. Capillary absorption and acoustic emission characteristics of coal gangue concrete under freeze–thaw cycles were examined to explain the damage evolution and the mechanisms for failed durability.
Section snippets
Materials
The raw materials information used to prepare coal gangue concrete specimens was listed in Table 1. Fig. 1 illustrates the mineral of coal gangue and the particle diameter distribution of cement.
Gangue concrete mixture
Four mixtures, namely, C1, C1M2, C1M4 and C1M6, were designed with the same water–binder ratio of 0.53 in this experiment, but the volume ratio of replacement stone with coal gangue was 0%, 20%, 40% and 60%, respectively. Then, the influence law of coal gangue content on frost-resistance durability of
Sorptivity of gangue concrete
Under freeze–thaw cycles, the variation of cumulative water absorption of both ordinary and coal gangue concrete specimens with different substitution rates and square root of time is shown in Fig. 12. Clearly, the cumulative water absorption progress of ordinary and coal gangue concretes increased nonlinearly with the square root of time but exhibited a relatively rapid increase in the early stage (T1/2 = 0–147 s), a slower increase in the middle stage (T1/2 = 147–518 s) and a stable state
Stress–strain relationship
Fig. 17 showed the complete stress–strain curve of gangue concrete specimens with different replacement rates before the freeze–thaw cycles under axial compression. As shown in the figure, the slope of rise phase and decline phase and the overall shape of curve were similar, the peak stress and the peak strain appeared in 30–35 Mpa and 0.0015–0.002, respectively. The mechanical properties of coal gangue concrete did not significantly decline, compared with that of ordinary concrete.
On one hand,
Conclusion
The comprehensive utilisation of coal gangue, which is the most important industrial solid waste, is crucial. Using coal gangue to prepare concrete can effectively utilise coal gangue and reduce the exploitation of natural stone, which has significant environmental and economic benefits. In a cold environment with high latitude and altitude, concrete structures of coal gangue suffer from freeze–thaw cycles, physical and mechanical properties and bearing capacity of coal gangue concrete
Declaration of Competing Interest
The authors declare that they have no conflict of interest to this work.
Acknowledgements
This project is financially supported by the National Natural Science Foundation of China (No. 51808443), Postdoctoral Science Foundation of China (No. 2017M613166), The Open Project of State Key Laboratory of Green Building in Western China (No. LSKF201808).
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