Study on mechanical properties and constitutive model of rock-like materials under acid corrosion
Yongsheng Liu1,Wang Liu1,Cui Wang2
1 School of civil engineering and architecture,East China Jiaotong University, Nanchang 330013, China2 Department of Civil Engineering, Jiangxi institute of construction, Nanchang 330200, China
Correspondence should be addressed to Cui Wang; 2019019082300008@ecjtu.edu.cn

Abstract

Mechanical specimens of rock-like materials were prepared from cement, gypsum, quartz sand and water. After the specimens were formed, they were immersed in acid solutions of pH=2 and pH=4 for 60d and 90d respectively, and uniaxial compression tests were carried out before and after immersion to obtain mechanical parameters.Quantification of the extent of degradation of mechanical parameters and introduction of acid corrosion damage variables.Based on the Lemaitre strain equivalence principle, the Weibull distribution statistical damage model for rock-like micro-fractures was combined with a correction for damage variables that takes into account the effect of residual strength to derive a damage constitutive model under acid corrosion.The theoretical damage constitutive curves were compared with the experimental curves to analyse the applicability of the constitutive equation.The results show that after corrosion by the acid solution, the initial compression-density portion of the stress-strain curve of the specimen becomes longer and the elastic deformation phase becomes shorter,the elastic modulus, peak stress and residual strength of the specimen all decrease, and the final strain of the compression-density portion, final elastic strain, peak strain and residual strength strain values all increase,the longer the maintenance time and the more acidic the solution, the more significant the above characteristics are.Comparing the theoretical and experimental curves, the damage constitutive model, which includes acid damage and residual strength corrections, agrees well in the elastic, plastic and post-peak phases.

Introduction

With the rapid development of underground engineering, the design and construction of tunnels, mines and galleries are closely related to the mechanical properties of the surrounding rock[1].The deterioration of the mechanical properties of rock is an important factor in the occurrence of engineering hazards, and the study of the deterioration of the mechanical properties of rock is important for the prevention and control of underground engineering hazards[2].The rock-like material test is easy to operate, safe, reliable and cost-effective, and is widely used to study the mechanical properties of rock through rock-like material model tests.
Tian used white cement, quartz sand and water as raw materials and obtained the relevant ratios for rock-like materials through extensive trial and error testing. The rock-like materials were subjected to uniaxial compression tests to obtain the compressive strength and observe the damage characteristics, while numerical simulations of the uniaxial compression of the specimens were carried out, and the simulation results were in good agreement with the experimental results[3].Haeri prepared rock-like specimens using silicate volcanic ash cement, mica flakes and water. Uniaxial compression tests were performed to observe micro-crack propagation in the specimens, while the accuracy of the rock-like material batching was verified based on numerical testing principles[4].Cheng used sand, gypsum, limestone and water in a certain mass ratio as the raw material for the rock-like specimens. The specimens were also subjected to uniaxial compression tests to observe and analyse the crack expansion pattern of the specimens[5].Based on the fracture pattern and energy release mechanism of rocks, Zhang investigated the crack extension pattern and secondary crack propagation pattern in single and double fracture rock materials under compression[6].Bi investigates the crack initiation and propagation patterns of rock-like materials under impact loading. Based on the experimental results and numerical tests, the crack initiation and propagation are verified to be reasonable[7].Using cement and mortar as raw materials, Wang prepared and grouted multi-fractured rock-like materials and subjected these specimens to fracture uniaxial compression tests. The effect of fracture on the ultimate strength, residual strength and deformation resistance of the rock-like material was investigated at the microscopic and macroscopic levels[8].Zhao investigated the crack propagation pattern and plastic deformation mechanism of a single-crack preexisting rock-like material with different dip angles under static loading through laboratory tests using the Digital Image Correlation Method (DICM)[9].
Based on the Lemaitre strain equivalence assumption, combined with the Weibull statistical damage model for micro-cracks and the fracture mechanics model for macroscopic joints, Liu derived the coupled damage tensor for intermittently jointed rocks and established a cyclic uniaxial compression damage intrinsic structure model for intermittently jointed rocks[10].Based on Lemaitre’s equivalent effect variation hypothesis and energy dissipation theory, Xu established a constitutive model of rock thermal damage. The present constitutive model can better reflect the phenomenon of post-peak stress decrease and better describe the whole stress-strain curve of rock damage, which verifies the adequacy of the model[11].Based on the statistical distribution of micro-fractures and the theory of fracture mechanics, Niu analysed the mechanical properties and damage mechanisms of brittle rocks under stress and established an intrinsic model of micro-damaged brittle rock materials[12].JiM combined the Mohr-Coulomb strength criterion and the normal distribution to develop a principal structure model for rock damage under uniaxial stress conditions, and numerically fitted the uniaxial compression test data and experimental data for sandstone to verify the rationality of the principal structure equations[13].
The water environment surrounding the rock has a major influence on its stability, especially in an acidic environment.The damage and fracture mechanisms of water-rock interactions and acid-corroded sandstone were studied by immersion corrosion of sandstone specimens using different acid solutions[14].Li investigated the dynamic mechanical properties of limestone in acidic environments, testing the dynamic mechanical properties at five different strain rates[15].Studies have shown that acidic groundwater affects the mechanical properties of the surrounding rock and that the physical and mechanical properties of the rock material change to varying degrees after acid corrosion[16].Based on the concept of damage mechanics, the relationship between the degradation of the mechanical properties of granite and the acid concentration was established, and uniaxial compression tests were carried out on granite specimens to investigate the changes in mechanical properties and micro-structure under acid corrosion[17,18].
The above studies provide a basis for the use of rock-like materials. In this paper, based on the Lemaitre strain equivalence assumption, the Weibull distribution statistical damage model for rock-like micro-fractures is combined with a correction of the damage variables by considering the residual strength to derive an intrinsic damage model under the influence of acid corrosion.

Uniaxial compression test on rock-like materials under acid corrosion

The rock-like material was made from cement, gypsum, quartz sand and water as raw materials, with a sand content of 65%, a gypsum-cement ratio of 2:1 and a water content of 8%. An appropriate amount of boric acid was added as a retarding agent to ensure the quality of the sample.
In accordance with the relevant requirements of the Engineering Rock Test Method standard, the specimen is formed into a cylindrical specimen with a diameter of 60 mm and a height of 125 mm. After the specimens were formed, they were polished sufficiently to control the parallelism of the specimen ends to within 0.05 mm, the height and diameter of the specimen to within 0.3 mm, and the end face perpendicular to the axial direction of the specimen to within 0.25°. Twelve specimens were prepared under natural and acid conditions.
The specimens were air dried under natural conditions for 7d and then kept in hydrochloric acid solutions at pH=2 and pH=4 for 60d and 90d respectively.To ensure that the acidic environment is at a preset value and to reduce the effect of solution volatilisation on the pH, the pH of the solution is calibrated weekly, as shown in Figure.1 and Figure.2.