Research on the influence of contact surface constraint on mechanical properties of rock-concrete composite specimens under compressive loads

Abstract

The contact form of rock-concrete has a crucial influence on the failure characteristics of the stability of rock-concrete engineering. To study the influence of contact surface on the mechanical properties of rock-concrete composite specimens under compressive loads, the two different contact forms of rock-concrete composite specimens are designed, the mechanical properties of these two different specimens are analyzed under triaxial compressive condition, and analysis comparison on the stress-strain curves and failure forms of the two specimens is carried out. The influence of contact surface constraint on the mechanical properties of rock-concrete composite specimens is obtained. Results show that the stress and strain of rock-concrete composite specimens with contact surface constraint are obviously higher than those without. Averagely, compared with composite specimens without the contact surface, the existence of contact surface constraint can increase the axial peak stress of composite specimens by 24% and the axial peak strain by 16%. According to the characteristics of the fracture surface, the theory of microcrack development is used to explain the contact surface constraint of rock-concrete composite specimens, which explains the difference of mechanical properties between the two rock-concrete composite specimens in the experiment. Research results cannot only enrich the research content of the mechanics of rock contact, but also can serve as a valuable reference for the understanding of the corresponding mechanics mechanism of other similar composite specimens.

Keywords

rock-concrete / composite specimen / contact surface / mechanical properties / failure mechanism

Introduction

The study of the mechanical properties of composite materials has always been a hot and difficult topic in mechanics and materials science. Because the basic composition, structure, and mechanical properties of each part of the materials are different when two or more different materials are combined together and subjected to mechanical deformation, the difference of mechanical properties of composite materials is huge [¹]. In common projects, concrete and rock are the two most commonly used materials. Even though the basic composition and mechanical properties of the two materials differ greatly, but they are often combined to form composite materials in use, such as the combination of tunnel surrounding rock and primary lining concrete, the combination of dam body and mountain rock of the reservoir dam, and the contact between foundations of the deep foundation pit and foundation rock [²]. However, due to construction and other reasons, the two materials could not be closely bonded together and could not form a stable contact surface during forming of the composite material, thus affecting the overall mechanical properties of the composite material [³].

Currently, the mechanical response mechanism of rock-concrete composite materials has been studied by many experts and scholars. Based on the deformation analysis of the deep soft rock tunnel, Wang et al. [⁴] have carried out an on-site experimental study on the rock-concrete lining structure with the Chinese LiangJiaBa coal mine as the engineering background. Do and Dias [⁵] analyzed the coordinated deformation between tunnel lining and soft rock. Zhao et al. [⁶] discussed the strengthening function of various composite materials on tunnel surrounding rock and tunnel lining. Schänzlin and Fragiacomo [⁷] discussed the elastic deformation characteristics of wood-concrete composite structures and derived the effective creep coefficient of a wood-concrete composite structure. Using digital image correlation (DIC) technology, Dong et al. [⁸,⁹] studied the fracture process of rock-concrete composite beams with different pre-crack locations under the action of three-point bending (TPB) and four-point shearing (FPS). Based on the direct shear test of the indoor foamed concrete-rock composite, Zhao et al. [¹⁰] analyzed the basic mechanical properties of foamed concrete-rock composite and studied the bond-slip behavior of foamed concrete-rock contact surface. Bloodworth and Su [¹¹] simulated the rock-concrete composite lining structure by analyzing the relationship between the coordinated deformation of tunnel surrounding rock and concrete lining. Based on the Brazilian shear test, Chang et al. [¹²] discussed the failure mode of the rock-concrete composite and the fracture length of the failure contact surface. Hong et al. [¹³] mainly analyzed the basic relationship between the fracture behavior of rock-concrete contact surface and contact surface constraint by numerical simulation. Eleni et al. [¹⁴] discussed the delayed mechanical behavior of rock-concrete interface under shear stress. Farinha et al. [¹⁵] taking the actual dam as an example, analyzed the coordinated deformation mechanism between the dam rock and concrete. Hussein et al. [¹⁶] through the research that the shear strength of concrete and rock interface is a key factor determined based on the stability of hydraulic structures. Simanjuntak et al. [¹⁷] discussed the co-deformation of rock and concrete in tunnel lining structure. Based on the analysis of the stress level and characteristics of rock-concrete interface of dam, de Granrut et al. [¹⁸] analyzed the mechanical characteristics of rock-concrete interface Andjelkovic et al. [¹⁹] analyzed the shear properties of the concrete and the surface of the rock, and presents a new explanation for the mechanism of shear and destruction of the rock. Used the limit equilibrium method (LEM), Krounis et al. [²⁰] analyzed the rock and concrete interface damage, and think that belongs to brittle failure of rock and concrete interface. Kang et al. [²¹] analyzed the deformation behavior of rock-concrete interface by single and cyclic shear tests.

As can be seen from the above research fields and directions, now, many types of research at home and abroad are still focused on the actual engineering analysis, numerical software simulation and overall deformation and failure analysis of rock-concrete structures, and few studies have been reported on the contact surface constraint and mechanical response of rock-concrete composite specimens under compressive loads. In this paper, two different type of rock-concrete composite specimens are designed. Based on the indoor triaxial test results, the stress, strain, and failure modes of the two composite specimens are analyzed and compared, and the reasons for the influence of contact surface on the mechanical properties of the composite specimens are explained from the perspective of the development of microcracks. Research results can provide theoretical support and guiding ideology for the study of the overall mechanical properties of rock-concrete composite specimens, and can also promote the study and analysis of similar composite mechanical properties in the future.

Experimental preparation and experimental plan

Preparation of specimen

To better analyze the influence of contact surface constraint on the mechanical properties of rock-concrete composite materials, two different type of composite specimens with contact constraint and without contact constraint are designed. The rock-concrete composite specimen with contact surface constraint is made by pouring the designed mixed concrete on the rock mass, and after the concrete curing is completed, according to the International Society of Rock Mechanics (ISRM) test code [²²], and making a standard cylinder specimen with a height of 100 mm and a diameter of 50 mm through drilling, cutting, and polishing in the laboratory, wherein the specimen of concrete and rock parts are about 50 mm in height, and the error of specimen diameter is less than 0.3 mm and that of end face non-parallelism is not more than 0.05 mm. Composite specimens without contact surface constraint are made according to the same requirements, and rock and concrete specimens are processed into standard cylinder specimens, with a height of 50 mm and a diameter of 50 mm, respectively, and then stack them together.

The rock used in the experiment is white low-strength sandstone with compact structure and fine particles, among which the mineral materials mainly include quartz, feldspar, calcite and a small amount of iron, and chemical compositions mainly include, SiO₂, Al₂O₃, CaO, and Fe₂O₃; the concrete specimen used in the experiment is designed according to C40 strength, and its 28-day compressive strength is slightly lower than the rock strength. Prepared specimens are shown in Fig. 1.

Experimental equipment

In the experiment, TFD-2000 microcomputer servo-controlled rock triaxial rheological test machine used in key experiments of Chongqing University of Science and Technology in energy engineering mechanics and disaster prevention and mitigation is mainly adopted. The test machine has a maximum axial test force of 2000 kN, a maximum confining pressure of 100 MPa and a temperature control range (room temperature to 200°C) and can complete rock and concrete mechanical tests under multiple coupling conditions such as uniaxial and triaxial compression tests, uniaxial and triaxial rheological tests, triaxial compression tests under different temperatures, triaxial rheological tests under different temperature, rock permeability tests at normal temperature and high temperature. The experimental equipment is shown in Fig. 2.

Experimental process and plan

First, the wave velocities of two rock-concrete composite specimens are measured by a non-metal wave velocity tester, and those with similar wave velocities are selected as same groups; the geometric size and mass of specimen are measured with the vernier caliper and electronic balance to calculate the density of the specimen. The measured rock-concrete composite specimen is fixed on the base of the triaxial apparatus, and lateral and axial extensometers are installed to measure the lateral and axial deformation of the rock-concrete composite specimen during the experiment; then, the confining pressure cylinder is sealed and filled with silicone oil to apply confining pressure to the rock-concrete composite specimen. Finally, the triaxial experimental software of the experimental equipment is opened and the experiment shall be carried out according to the following experimental plan:

1) Referring to the Code for Rock Tests of Hydroelectric and Water Conservancy Engineering (sl26-2001) [²³], the axial stress {\sigma }_{\mathrm{1}} and confining pressure {\sigma }_{\mathrm{3}} are applied at a speed of 0.5 MPa/s to make sure the rock specimen under hydrostatic pressure (i.e., {\sigma }_{\mathrm{1}}={\sigma }_{\mathrm{2}}={\sigma }_{\mathrm{3}}), and the value of confining pressure shall be 0 (uniaxial compression), 7, 15, and 22 MPa, respectively, according to the design value.

2) Keeping the confining pressure constant, changing the axial loading mode to displacement control, and applying axial stress to the rock specimen at a rate of 0.005 mm/min to make it lose its bearing capacity and fail. The installation of the specimen and extensometer is shown in Fig. 3, and the loading diagram is shown in Fig. 4. Basic mechanical parameters of the two rock-concrete composite specimens are shown in Table 1.

Results and discussions

Figure 5 shows the stress-strain curves obtained from the different type of two rock-concrete composite specimens and the experimental plan made through the above design, wherein the triaxial compression stress-strain curve of the composite specimen with rock-concrete contact surface constraint is shown in Fig. 5(a), and the triaxial compression stress-strain curve of the composite specimen without rock-concrete contact surface constraint is shown in Fig. 5(b). As can be seen from Fig. 5: 1) Under the same confining pressure, the axial peak strain, lateral peak strain and peak stress intensity of the composite specimen with contact surface constraint are significantly larger than those of the composite specimen without contact surface constraint; 2) Along with the increase of confining pressure, the peak strain and peak stress of the two specimens increase, and such increments of the specimens with contact surface constraint are greater than that of those without.

Comparative analysis of strain of two rock-concrete composite specimens

Figure 6 shows the relationship among axial peak strain, lateral strain and confining pressure of two rock-concrete composite specimens. Combined with Fig. 5, it can be concluded that the axial peak strain of composite specimens with contact surface constraint is 0.6%, 0.83%, and 1.06%, respectively; when confining pressure increases step by step, the axial peak strain increases linearly by about 0.2%, and when confining pressure further increases to 22 MPa, the rate of increase of axial peak strain also increases. For rock-concrete composite specimens without contact surface constraint, the axial peak strain is 0.56%, 0.80%, 0.96%, and 1.01% in the process of confining pressure from 0 to 22 MPa, and the incremental increase of axial peak strain is 0.24%, 0.16%, and 0.05% in the sequence. It can be seen from this that along with the increase of confining pressure, the axial peak strain of rock-concrete composite specimen without contact surface constraint is gradually decreased, which is just the opposite of the axial peak strain of rock-concrete composite specimen with contact surface constraint. In addition, there are not only obvious differences in axial peak strain, but also very obvious differences in lateral peak strain of two different rock-concrete composite specimens. When the specimen is restrained by contact surface, its lateral peak strain gradually decreases from 1.18% in the uniaxial state to 1.00%, 0.94%, and 0.84%. Especially when the confining pressure is from 7 to 22 MPa, the rate of change between each stage is about 1%, showing a very obvious linear decreasing trend. However, the rock-concrete composite specimen without contact surface constraint is just the opposite, its lateral peak strain gradually increases from 0.36% to 0.45%, 0.68%, and 1.32% in the uniaxial state with the increase of confining pressure, and there is no obvious rule to follow.

From the above analysis, the following conclusions can be made: after the concrete is poured on the rock, a contact surface is formed, and the overall mechanical properties of the contact surface composite specimen are obviously improved. From further detailed observation, there are a large number of micropores and cracks inside the rock and concrete. After pouring the concrete on the rock, due to the axial load and the constraint of the contact surface, the deformation and compaction of the micro-pores and cracks inside the rock are synchronous, which makes the rock-concrete two different materials show a strong coordination and integrity when stressed at the same time; however, as for the composite specimens without contact surface constraint, even they also subjected to axial loads when they are stacked together, but the two materials are always in a relatively independent state of stress deformation. For example, when they are subjected to the same axial stress, the closing of micro-pores in concrete and the degree of compaction will be much greater than those in rock. Therefore, if either of the two materials is subjected to stress failure in advance, the overall failure of the composite specimens will be caused.

Through the axial and lateral peak strain curves of the rock-concrete composite specimens under different confining pressures shown in Figs. 6(a) and 6(b), the change of peak strain of the two different rock-concrete composite specimens with confining pressure can be clearly seen. To further understand and better predict the change of peak strain of the two rock-concrete composite specimens, the change law is expressed by a formula through curve fitting, wherein the fitting formulas of axial peak strain and lateral peak strain of the rock-concrete composite specimens with contact surface constraint are shown in Eqs. (1) and (2):

(1)y=\mathrm{0.001}{x}^{\mathrm{2}}+\mathrm{0.0183}x+\mathrm{0.61},R^{\mathrm{2}}=\mathrm{0.9416},

(2)y=\mathrm{0.0004}{x}^{\mathrm{2}}-\mathrm{0.0226}x+\mathrm{1.1721},R^{\mathrm{2}}=\mathrm{0.9703.}

The fitting formulas for axial peak strain and lateral peak strain of rock-concrete composite specimens without contact surface constraint are shown in Eqs. (3) and (4):

(3)y=-\mathrm{0.0009}{x}^{\mathrm{2}}+\mathrm{0.0405}x+\mathrm{0.562},R^{\mathrm{2}}=\mathrm{0.998},

(4)y=\mathrm{0.0027}{x}^{\mathrm{2}}-\mathrm{0.0169}x+\mathrm{0.3865},R^{\mathrm{2}}=\mathrm{0.988.}

Comparative analysis of strength of two rock-concrete composite specimens

Analyses on the stress-strain curves of two kinds of rock-concrete composite specimens and the influence of contact surface constraint on the deformation characteristics of rock-concrete composite specimens have been carried out. To further explain the influence of contact surface on the mechanical characteristics of rock-concrete composite specimens, comparative analysis of strength of two different rock-concrete composite specimens is conducted. Figure 7 shows the relationship between peak stress and confining pressure of two different rock-concrete composite specimens. As can be seen from Fig. 7, the existence of the contact surface constraint greatly increases the peak stress of the composite specimen, and this situation becomes more and more obvious with the increase of the confining pressure.

The study of rock strength criterion has always been the focus of rock mechanics. Among the many strength criteria at present, such as Coulomb criterion, Hawk-Brown criterion, generalized Hawk-Brown criterion and exponential strength criterion, Coulomb criterion is widely used in engineering because as for parameters in calculation, only cohesion and internal friction angle are needed, which is very convenient and concise [²⁴]. When using the maximum principal stress{\sigma }_{\mathrm{1}}, {\sigma }_{\mathrm{3}} to express the Coulomb criterion, the criterion can be written as: {\sigma }_{\mathrm{1}}=M+N{\sigma }_{\mathrm{3}}, briefly Q(M,N). The formula expresses the linear relationship between the axial peak stress {\sigma }_{\mathrm{1}} and the confining pressure {\sigma }_{\mathrm{3}} of the test specimen. Moreover, the values of M and N depend on the internal friction angle and cohesion of the specimens, and their relations are shown in Eqs. (5) and (6):

(5)M=\mathrm{2}c\cos \varphi /(\mathrm{1}-\sin \varphi ),

(6)N=(\mathrm{1}+\sin \mathrm{phi;})/(\mathrm{1}-\sin \varphi )={\tan }^{\mathrm{2}}({\mathrm{45}}^{\circ }+\varphi /\mathrm{2})\mathrm{.}

Therefore, based on Coulomb criterion, the fitting of relationship between peak stress and confining pressure of two different rock-concrete composite specimens is carried out, and the rationality of the fitting is further judged by comparing the values of the correlation coefficient R^{\mathrm{2}}. The peak stress and confining pressure obtained from the experiment are used in the formula, and the Coulomb expressions of rock-concrete composite specimens with and without contact surface constraint are shown in Eqs. (7) and (8):

(7){\sigma }_{\mathrm{1}}=\mathrm{34.50}+\mathrm{3.82}{\sigma }_{\mathrm{3}},R^{\mathrm{2}}=\mathrm{0.987},

(8){\sigma }_{\mathrm{1}}=\mathrm{26.52}+\mathrm{3.35}{\sigma }_{\mathrm{3}},R^{\mathrm{2}}=\mathrm{0.966},

where {\sigma }_{\mathrm{1}} is the axial peak stress, and {\sigma }_{\mathrm{3}} are different confining pressure values.

Comparative analysis of failure characteristics of two kinds of rock-concrete composite specimens

Except the obvious difference in stress and strain, the fact that whether the rock-concrete composite specimen has or has not the contact surface constraint also has a great influence on the failure mode of the composite specimen. It can be seen from Figs. 8(a) and 8(b) that under the same confining pressure, the failure modes of the rock-concrete composite specimen with contact surface constraint and that without contact surface constraint are completely different. After the rock-concrete composite specimen with contact surface constraint has reached its ultimate bearing capacity, the specimen will undergo oblique shear failure from top to bottom, except for crushing failure at some areas, the rest of the failure surface is complete. And the contact surface will always be in good condition during the whole compression failure process, without any cracking or transverse cracking. Under the constraint of the contact surface, the composite specimen shows a high degree of coordinated deformation capability. However, the failure mode of rock-concrete composite specimens without contact surface constraint is quite different. As mentioned in the previous article, the uniaxial compressive strength of rock specimens is slightly higher than that of concrete specimens. Therefore, when the rock-concrete composite specimens are free from contact surface constraint, both materials are in independent deformation process, which causes that the lower strength concrete specimens reach the bearing limit ahead of time and break in advance, while the rock specimens with slightly higher strength remain intact except for crushing failure of some end surfaces.

Failure mechanism of rock-concrete composite specimens

There are many microcracks and micro-pores inside the rock and concrete [²⁵], so when the rock and concrete are suffered external stress, the microcracks and micro-pores inside the rock and concrete will quickly close tightly to cope with external stress changes. The number of microcracks and micro-pores and the development mode after stress all will have a significant impact on the macro-mechanical properties of materials, such as rock and concrete [²⁶]. Therefore, the analysis failure mechanism of rock and concrete from the perspective of the development of microcracks and micro-pores has become very important.

According to known theoretical and experimental studies, there are many microcracks and micro-pores inside rock and concrete, and these cracks and pores generally meet the Weibull distribution rule. In addition, under different external stress conditions, these cracks and pores can show different mechanical properties. According to Ref.[²⁷] and other studies, it is found that the microcrack development angle of rock specimens basically conforms to in Eq. (9), and the development mode of cracks is shown in Fig. 9.

(9){\theta }_i+{\phi }_i={\mathrm{90}}^{\circ }

where {\theta }_i is the dip angle of microcrack i, {\phi }_i is the development angle of the microcrack i.

Therefore, according to the development and failure theory of microcracks, the influence of the surface constraint on the rock-concrete composite specimen is discussed, wherein the microcrack development diagram of the rock-concrete composite specimen with surface constraint is shown in Fig. 10, and the microcrack development diagram of the rock-concrete composite specimen without is shown in Fig. 11. Through these diagrams, it can be clearly seen that the microcracks and pores of the rock and concrete parts are different. Without contact surface constraint, the two parts of materials of the specimen are under their respective stress deformation processes under the action of axial stress. In the process, the stress and strain change rates of the two parts of material of the specimen are different.

From the theoretical point of microcrack development, the paper analyzes the influence of contact surface constraints on the overall mechanical properties of rock-concrete composite specimens, which is helpful to better understand the differences in mechanical properties between the two rock-concrete composite specimens and the reasons for the differences. As can be clearly seen from Figs. 10 and 11, when there is contact surface constraint, the two different materials can be connected through the contact surface, microcrack and pore development can be conducted to each other, so that the deformation, strength and failure modes of the two materials are almost the same in this case. Even though the strength of the concrete specimen is lower than that of the rock specimen, such difference has been well adjusted by the effect of the contact surface constraint. On the other hand, when there is no contact surface constraint, the two specimen materials cannot conduct microcracks through the contact surface, and the microcracks of the respective specimen materials can only develop inside respectively. Therefore, under the same stress conditions, concrete specimens with lower strength will fail first, which is consistent with the experimental results.

Conclusions

1) Contact surface constraint has a significant impact on the mechanical properties of rock-concrete composite specimens. The mechanical properties of rock-concrete composite specimens with contact surface constraint are better than those without, especially in axial peak stress and overall strain. The maximum axial stress and axial strain of composite specimens with contact surface constraint can reach 42% and 46%, respectively, compared with those without contact surface constraint.

2) Contact surface constraint significantly affects the lateral deformation of the composite specimen. When there is contact surface constraint, the lateral peak strain of the rock-concrete composite specimen decreases along with the increase of confining pressure. However, the lateral peak strain of rock-concrete composite specimens without contact surface constraints increases along with the increase of confining pressure.

3) From the perspective of microcrack development, it is concluded that the contact surface constraint can effectively transfer the microcrack development path between two different materials and significantly improve the coordinated stress and deformation of the specimen; however, for composite specimens without contact constraint, the microcrack development path can only expand inside their respective materials and cannot be coordinated with other materials.

4) The study on the influence of contact surface constraint on mechanical properties can provide a valuable reference for the understanding of the mechanics mechanism of rock and concrete contact engineering. In practical engineering, for example, in the engineering of the contact between rock and concrete such as tunnel surrounding rock-lining concrete and dam rock-mass concrete, our research results, namely ‘the influence of contact surface constraints on the overall mechanical properties of composite samples’, can guide the pouring of concrete so as to improve the overall quality of the project. In addition, for the completed project, we can also analyze the contact between rock and concrete to judge the local or overall stability of the project.

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