An experimental method for determining the shear softening constitutive relation of rock-concrete interface under compression-shear conditions

Abstract

A test method for determining the shear softening constitutive relationship of a rock-concrete interface under compression-shear conditions. The specimen consists of a rock block and a concrete block, with initial cracks reserved at the upper and lower parts of the specimen interface. Starting from the tip of the initial crack, horizontal line segments are drawn at equal intervals on the rock surface along the Y direction, and strain gauges are arranged at equal intervals on each line segment. A clamp-on extensometer is arranged at the middle position of two adjacent line segments along the interface ligament. A uniformly distributed compressive stress perpendicular to the rock-concrete interface is applied along the X direction using load control, and a compressive load is applied to the concrete along the Y direction using a displacement control mode. The Y-direction strain and the interface sliding displacement are obtained by the strain gauge and the clamp-on extensometer respectively to derive the interface shear softening constitutive relationship. By using the method of the present invention, the shear softening constitutive relationship of the rock-concrete interface under compression-shear conditions can be accurately and conveniently determined by performing a compression-shear test. The present invention is also applicable to the determination of the shear softening constitutive relationship of the rock-concrete interface under tension-shear conditions.

Description

Technical Field

[0001] The invention belongs to the field of rock-concrete fracture performance research and relates to a test method for measuring the shear softening constitutive relationship of a rock-concrete interface under compression-shear conditions. Background Art

[0002] For concrete structures built on rock foundations, such as concrete gravity dams, the interface between rock and concrete is a weak point and has become a weak link in the structural load-bearing and durability performance. If improper pouring and curing are performed during construction or if the structure is subjected to complex load and temperature changes during service, the contact surface is prone to initial defects and gradually evolves into macroscopic cracks. The propagation of interfacial cracks greatly weakens the bearing capacity of the structure and poses a potential threat to the safe operation of concrete gravity dams. Similar to single-medium materials such as concrete, the rock-concrete interface can be considered a quasi-brittle material. Crack propagation is preceded by the fracture process zone, where cohesive stress acts to characterize the material's strain-softening behavior. When applying nonlinear fracture mechanics to analyze interfacial crack propagation, accurately characterizing the softening behavior within the interfacial fracture damage zone is the basis for interfacial fracture analysis. The softening constitutive relation describes the quantitative relationship between the tensile / shear cohesive stress transmitted within the fracture process zone and the crack opening / slip displacement, and is a hot topic in interfacial nonlinear fracture mechanics research.

[0003] The applicant has long been dedicated to researching the fracture mechanism of rock-concrete interfaces and the failure modes of gravity dams. They previously proposed an experimental method for determining the shear constitutive relation of rock-concrete interfaces and were granted an invention patent (application number: 201611152877.8). In actual engineering, under the combined effects of deadweight and external water pressure, the rock-concrete interface of a concrete gravity dam is in a state of combined compression-shear stress. The softening properties and fracture mechanism of the interface under this combined compression-shear stress state differ significantly from those under pure shear conditions. The frictional resistance caused by the normal compressive stress significantly enhances the cohesive toughening effect and shear strength of the interface. Furthermore, even when shear failure occurs at the interface, residual shear stress caused by frictional resistance persists at the interface. Ignoring the influence of the normal compressive stress and directly using a pure shear softening constitutive relation to approximate the softening behavior of the interface under compression-shear conditions and predict failure modes and bearing capacity will significantly underestimate the cohesive toughening effect of the interface, leading to inaccurate calculations. According to the applicant's preliminary research, existing research has not yet proposed a method for determining the shear softening constitutive relation of the rock-concrete interface under this combined compression-shear stress state. Based on this, the applicant proposed an experimental method to determine the shear softening constitutive relationship of the rock-concrete interface under compression-shear conditions. Summary of the Invention

[0004] The present invention proposes a test method for determining the shear softening constitutive relationship of the rock-concrete interface under compression-shear conditions. The invention is as follows:

[0005] (1) Specimen size: The composite specimen consists of a rock block and a concrete block with the same geometric size, with the rock block on the left and the concrete block on the right. The elastic modulus of the rock is E. Initial cracks are set at both ends of the rock-concrete interface, such as Figure 1 The length L and height H of the composite specimen are not less than 10 times the maximum aggregate size of concrete, the thickness T is not less than 5 times the maximum aggregate size of concrete, and the initial crack length a0 is set to 0.1H~0.2H;

[0006] (2) Loading mode: The specimen loading mode is as follows: Figure 2 As shown. The horizontal and vertical directions are set as X and Y directions. First, horizontal compressive stress is applied to the rock surface along the X direction at a loading rate of 0.02MPa / s to 0.05MPa / s. Then, vertical load is applied to the upper and lower surfaces of the concrete along the Y direction at a loading rate of 0.1mm / min. Three layers of anti-friction plates are set between the loading plate and the composite specimen and lubricating oil is applied to eliminate friction between the two.

[0007] (3) Measurement scheme: The strain in the Y direction of the rock surface is monitored by pasting strain gauges, and then the constitutive relationship of interface shear softening is derived. The strain gauge arrangement scheme is as follows: Figure 3 As shown in the figure, starting from the initial crack tip, k+1 horizontal line segments are drawn on the rock surface along the Y direction at equal intervals, with a line segment spacing Δh of 15mm to 25mm; n strain gauges are arranged at equal intervals on each line segment to monitor the rock surface strains ε1, ε2…ε at that location. n-1 , ε n , strain gauge spacing Δl = L / 2n; along the rock-concrete interface, clip-on extensometers are arranged at the midpoint between two adjacent line segments to monitor the interface slip displacement, with a total of k measuring points; based on the strain gauge data on the two adjacent line segments and the clip-on extensometer data at the midpoint between the two adjacent line segments, a constitutive relationship curve for interface shear softening can be derived; considering the discreteness and uncertainty of the shear test, the number of clip-on extensometers k is not less than 3, and the number of strain gauges n is not less than 5;

[0008] (4) Data processing: Take the rock block between two adjacent line segments as a calculation unit, such as Figure 4 As shown in the figure, the stress distribution curve in the Y direction is calculated based on the strain distribution curve. The integral of the stress distribution curve in the Y direction on the rock cross section is the resultant force in the Y direction. To simplify the calculation, the area of ​​the rectangle on the stress distribution curve in the Y direction is used instead of the actual area of ​​the curved trapezoid. The resultant force F in the Y direction on the rock cross section is calculated by formula (1). This method is used to obtain the resultant forces F1 and F2 on the upper and lower surfaces of the calculation unit respectively. The average shear stress τ at the interface is calculated by formula (2).

[0009] F=ETΔl(ε1+ε2+ε3+...+εn ) (1)

[0010]

[0011] The monitoring results of the clip-on extensometer at the middle position of the two line segments of the ligament are regarded as the average sliding displacement s at the interface, and the average shear stress-average sliding displacement (τ-s) curve of the whole loading process is drawn, as shown in Figure 5 As shown in Figure 1, the descending section of the curve is the shear softening constitutive relation of the rock-concrete interface under compression-shear conditions. The results obtained from k calculation units are averaged to obtain the final shear softening constitutive relation of the rock-concrete interface under compression-shear conditions.

[0012] The present invention has the following beneficial effects: Using the test method proposed in the present invention, a simple compression-shear test can be performed to accurately and conveniently determine the shear softening constitutive relationship of the rock-concrete interface under compression-shear conditions. Furthermore, the present invention is also applicable to determining the shear softening constitutive relationship of the rock-concrete interface under tension-shear conditions. BRIEF DESCRIPTION OF THE DRAWINGS

[0013] Figure 1 Schematic diagram of the specimen dimensions, (a) is the front view, (b) is the cross-sectional view;

[0014] Figure 2 It is a schematic diagram of loading form;

[0015] Figure 3 Schematic diagram of the measurement scheme;

[0016] Figure 4 Schematic diagram of data processing method;

[0017] Figure 5 is the average shear stress-average sliding displacement (τ-s) curve. DETAILED DESCRIPTION

[0018] To further clarify the objectives, technical solutions, and beneficial effects of the present invention, the following describes a method for determining the shear softening constitutive relationship of the rock-concrete interface under compression-shear conditions, using the rock-concrete natural bonding interface under 1 MPa compressive stress as an example. Obviously, the described embodiments are only a portion of the embodiments of the present invention, not all of them. Based on the embodiments of the present invention, all other embodiments obtained by persons of ordinary skill in the art without inventive effort are within the scope of protection of the present invention.

[0019] (1) Specimen preparation

[0020] The specimen size used in this embodiment is H×L×T=150mm×150mm×75mm, and the initial crack length a0 at both ends of the interface is 15mm. Granite is used as the rock, and C30 concrete is used as the concrete. The elastic modulus of the rock is 40.88GPa, and the Poisson's ratio is 0.117; the elastic modulus of the concrete is 28.79GPa, and the Poisson's ratio is 0.268. In order to more realistically reflect the surface characteristics of the bedrock in actual engineering, the natural cross-section of the rock is obtained as the bonding surface through a three-point bending fracture test. Two pieces of PVC sheets with a size of 15mm×75mm are pasted on both ends of the natural cross-section of the rock to form an initial crack of 15mm in length. The rock block is fixed on one side of the mold, and concrete is poured on the other side of the mold. The rock cross-section is fully bonded to the concrete by sufficient vibration.

[0021] (2) Arrangement of strain gauges and clamp-on extensometers

[0022] Starting from the initial crack tip, four horizontal line segments were drawn equally spaced along the Y direction on the rock surface, with a line segment spacing of Δh = 20 mm. Five strain gauges were placed equally spaced on each line segment to monitor the rock surface strain at that location. The strain gauge spacing was Δl = 15 mm. A clip-on extensometer was placed midway between two adjacent line segments along the interfacial ligament to monitor interfacial slip displacement, for a total of three measurement points.

[0023] (3) Loading and data collection

[0024] To ensure uniform stress distribution, two loading plates were placed between the testing machine and the specimen. Three layers of polytetrafluoroethylene (PTFE) sheeting were added between the specimen and the loading plates to eliminate friction. The relative positions of the specimen, loading plates, and loading head were adjusted to avoid eccentric loading. First, the X-direction was loaded at 0.02 MPa / s to 1 MPa in load control mode, and the X-direction load value remained unchanged during the test. Then, the Y-direction was loaded at 0.1 mm / min in displacement control mode. To prevent damage to the testing machine, loading was stopped after the load value reached its peak. The strain gauge data and clip-on extensometer data were recorded throughout the Y-direction loading process.

[0025] (4) Derivation of the shear softening constitutive relation

[0026] Obtain the rock surface strain data and sliding displacement data at each measuring point during the entire loading process. According to the above data processing method, three average shear stress-average sliding displacement (τ-s) curves are obtained. The three curves are averaged to obtain the final shear softening constitutive relationship of the rock-concrete interface under 1MPa compression-shear conditions, as shown in the figure. Figure 5 shown.

Claims

1. A test method for determining the shear softening constitutive relation of rock-concrete interface under compression-shear conditions, characterized in that: Here are the steps: (1) Specimen size: The composite specimen consists of a rock block and a concrete block of the same geometric size, with the rock block on the left and the concrete block on the right. The elastic modulus of the rock is E. Initial cracks are set at both ends of the rock-concrete interface. The length L and height H of the composite specimen are not less than 10 times the maximum aggregate size of the concrete, and the thickness T is not less than 5 times the maximum aggregate size of the concrete. The initial crack length a0 is set to 0.1H~0.2H. (2) Loading mode: The horizontal and vertical directions are set as X and Y directions respectively; first, horizontal compressive stress is applied to the rock surface along the X direction at a loading rate of 0.02 MPa / s to 0.05 MPa / s. The load value in the X direction remains unchanged during the test; then, vertical load is applied to the upper and lower surfaces of the concrete along the Y direction at a loading rate of 0.1 mm / min; to prevent damage to the testing machine, loading is stopped after the load value reaches the peak; three layers of anti-friction plates are set between the loading plate and the composite specimen and lubricated with lubricating oil to eliminate friction between the two; (3) Measurement scheme: The strain on the rock surface in the Y direction is monitored by pasting strain gauges, and then the constitutive relationship of interface shear softening is derived. Starting from the initial crack tip, k+1 horizontal line segments are drawn on the rock surface along the Y direction at equal intervals, with the line segment spacing Δh being 15 mm to 25 mm. On each line segment, n strain gauges are arranged at equal intervals to monitor the rock surface strains ε1, ε2, …, ε at that location. n-1 , ε n , strain gauge spacing Δl = L / 2n; along the rock-concrete interface, clip-on extensometers are arranged at the midpoint between two adjacent line segments to monitor the interface slip displacement, with a total of k measuring points; based on the strain gauge data on the two adjacent line segments and the clip-on extensometer data at the midpoint between the two adjacent line segments, a constitutive relationship curve for interface shear softening is derived; considering the discreteness and uncertainty of the shear test, the number of clip-on extensometers k is not less than 3, and the number of strain gauges n is not less than 5; (4) Data processing: The rock block between two adjacent line segments is taken as a calculation unit. The stress distribution curve in the Y direction is calculated based on the strain distribution curve. The integral of the stress distribution curve in the Y direction on the rock cross section is the resultant force in the Y direction. To simplify the calculation, the area of ​​the rectangle on the stress distribution curve in the Y direction is used instead of the actual area of ​​the curved trapezoid. The resultant force F in the Y direction on the rock cross section is calculated by formula (1). This method is used to obtain the resultant forces F1 and F2 on the upper and lower surfaces of the calculation unit respectively. The average shear stress at the interface is: Calculated by formula (2); F=ETΔl(ε1+ε2+ε3+...+ε n ) (1) The monitoring results of the clip-on extensometer at the middle position of the two line segments of the ligament are regarded as the average sliding displacement s at the interface, and the average shear stress-average sliding displacement ( -s) curve, the descending section of the curve is the shear softening constitutive relation of the rock-concrete interface under compression-shear conditions; the results obtained by k calculation units are averaged to obtain the final shear softening constitutive relation of the rock-concrete interface under compression-shear conditions.

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