Experimental assessment of Perfobond and T-Perfobond shear connectors’ structural response

This paper presents an evaluation of the structural behaviour of Perfobond and T-Perfobond connectors, suitable to transfer shear in composite structures. The results of an experimental programme performed at the Civil Engineering Department of the University of Coimbra, Portugal, are presented and discussed. The experimental programme consisted of twelve push-out tests configured according to the Eurocode 4 [EUROCODE 4. EN 1994. Design of composite steel and concrete structures Part 1.1 General rules and rules for buildings. Brussels: CEN European Committee for Standardisation; 2005]. The connection resistance, ductility and collapse modes are presented and discussed, namely by comparison to results from available analytical models. It was shown that the evaluated types of connectors may provide an economical and efficient solution to transfer shear in composite structures.     

Influence of location of large-scale asperity on shear strength of concrete-rock interface under eccentric load

The location and geometry of large-scale asperity present at the foundation of concrete gravity dams and buttress dams affect the shear resistance of the concrete-rock interface.However,the parameters describing the frictional resistance of the interface usually do not account for these asperities.This could result in an underestimate of the peak shear stre ngth,which leads to significantly conservative design for new dams or unnecessary stability enhancing measures for existing ones.The aim of this work was to investigate the effect of the location of first-order asperity on the peak shear strength of a concrete-rock interface under eccentric load and the model discrepancy associated with the commonly used rigid body methods for calculating the factor of safety(FS) against sliding.For this,a series of direct and eccentric shear tests under constant normal load(CNL) was carried out on concrete-rock samples.The peak shear strengths measured in the tests were compared in terms of asperity location and with the predicted values from analytical rigid body methods.The results showed that the large-scale asperity under eccentric load significantly affected the peak shear strength.Furthermore,unlike the conventional assumption of sliding or shear failure of an asperity in direct shear,under the effect of eccentric shear load,a tensile failure in the rock or in the concrete could occur,resulting in a lower shear strength compared with that of direct shear tests.These results could have important implications for assessment of the FS against sliding failure in the concrete-rock interface.     

含一阶和二阶起伏体节理剪切强度的试验研究

 利用高强石膏材料浇注含不同一阶和二阶起伏度的岩体节理模拟试件,通过节理在不同一阶和二阶起伏度及法向应力下的常法向荷载剪切试验,对其剪切强度特性进行研究,分析一阶和二阶起伏度及法向应力对剪切强度的影响规律。试验结果表明：含二阶起伏体节理试件的剪切强度–剪切位移曲线有多个峰值剪应力出现,只含一阶起伏体节理的峰值剪应力不明显;节理剪切强度随着一阶和二阶起伏度及法向应力的增大而增大;随着二阶起伏度与一阶起伏度比值的增大,剪切强度先增大后降低。对试验数据进行回归分析,提出能反映一阶和二阶起伏度及法向应力影响的剪切强度经验公式。     

考虑各向异性特征的三维岩体结构面峰值剪切强度研究

 由于岩体结构面形貌的复杂性,目前,所提出的峰值剪切强度模型不能够很好地体现其各向异性特征。鉴于此,综合考虑岩体结构面起伏角和起伏幅度,提出一个考虑各向异性特征的综合参数?表征岩体结构面粗糙度的新方法;?由某一方向的起伏角参数SRv和起伏幅度参数A表示;其中参数SRv表示结构面粗糙度的各向异性特征,用变异函数分析方法进行计算获得。然后,应用该法分析Barton十条标准轮廓线,拟合出JRC与参数SRv,A的关系表达式;结合Barton强度公式,给出考虑各向异性特征的岩体结构面峰值剪切强度模型。最后,基于ShapeMetriX3D三维形貌测量系统,在实验室开展类岩体结构面形貌的量测;并应用上述方法估算结构面4个方向上的峰值剪切强度。于此同时,开展该类岩体结构面各向异性的剪切力学试验,通过试验值与估测值的对比分析,验证了所提模型的正确性。研究结果为准确预测岩体结构面抗剪强度提供一种新方法。     

Evaluating damage during shear tests of rock joints using acoustic emissions

The acoustic emission (AE) signals generated during direct shear test were evaluated on different types of joints (rock–rock, rock–concrete and concrete–concrete). Several boreholes were cored from a dam body, rock mass and interface between dam and rock mass, and the samples were prepared and tested under direct shear test. A laser profilometer scanner was used for scanning the joint surfaces in order to assess surface roughness. By correlating the AE signals with the shear graphs one can predict the starting point of shearing during direct shear test. Count and energy parameters were analyzed in two different methods to monitor the shear behavior of the joints: a graph of the count and energy rates, and a graph of cumulative count and energy. Four separated periods were observed for bonded and non-bonded joints: linear pre-peak period, non-linear pre-peak period, post peak period and residual period. This study showed that AE has enough accuracy to monitor the shear behavior of the joints and it can be used in site confidently.     

Effect of soil–infilled joints on the stability of rock wedges formed in a tunnel roof

The use of an equivalent continuum for a rock mass is not always suitable for situations, where the failure is structurally controlled by discontinuities as in the case of wedges in the tunnel roof. In these instances, discontinuum approaches are usually preferred. Rock joints that are filled with soft infill are likely to be the weakest planes in a rock mass, having a dominant influence on its overall shear behaviour. In this case, the joint material model adopted for the discontinuities should be able to describe important mechanisms, such as asperity sliding and shearing, post-peak behaviour, asperity deformation, and the effect of the soft infilling. The latest version of a soil–infilled joint model is discussed here. It describes more comprehensively than previous models the occurrence of dilation and compression with lateral displacements, and also represents the hardening mechanism related to asperity interference as observed in the laboratory that cannot be readily captured by the existing joint models. An analytical approach for the analysis of rock wedges structurally controlled by soil–infilled joints and a numerical simulation based on a metro station collapse which occurred in Brazil in 2007 are presented.     

An approach to measure infill matric suction of irregular infilled rock joints under constant normal stiffness shearing

Rock joints infilled with sediments can strongly influence the strength of rock mass. As infilled joints often exist under unsaturated condition, this study investigated the influence of matric suction of infill on the overall joint shear strength. A novel technique that allows direct measurement of matric suction of infill using high capacity tensiometers (HCTs) during direct shear of infilled joints under constant normal stiffness (CNS) is described. The CNS apparatus was modified to accommodate the HCT and the procedure is explained in detail. Joint specimens were simulated by gypsum plaster using three-dimensional (3D) printed surface moulds, and filled with kaolin and sand mixture prepared at different water contents. Shear behaviours of both planar infilled joints and rough joints having joint roughness coefficients (JRCs) of 8–10 and 18–20 with the ratios of infill thickness to asperity height (t/a) equal to 0.5 were investigated. Matric suction shows predominantly unimodal behaviour during shearing of both planar and rough joints, which is closely associated with the variation of unloading rate and volumetric changes of the infill material. As expected, two-peak behaviour was observed for the rough joints and both peaks increased with the increase of infill matric suction. The results suggest that the contribution of matric suction of infill on the joint peak normalised shear stress is relatively independent of the joint roughness.     

Joints in unsaturated rocks: Thermo-hydro-mechanical formulation and constitutive behaviour

A formulation for the coupled analysis of thermo-hydro-mechanical (THM) problems in joints is first presented. The work involves the establishment of equilibrium and mass and energy balance equations. Balance equations were formulated taking into account two phases: water and air. The joint element developed was implemented in a general purpose finite element computer code for THM analysis of porous media (Code_Bright). The program was then used to study a number of cases ranging from laboratory tests to large scale in situ tests. A numerical simulation of coupled hydraulic shear tests of rough granite joints is first presented. The tests as well as the model show the coupling between permeability and the deformation of the joints. The experimental investigation was focused on the effects of suction on the mechanical behaviour of rock joints. Laboratory tests were performed in a direct shear cell equipped with suction control. Suction was imposed using a vapour forced convection circuit connected to the cell and controlled by an air pump. Artificial joints of Lilla claystone were prepared. Joint roughness of varying intensity was created by carving the surfaces in contact in such a manner that rock ridges of different tip angles were formed. These angles ranged from 0° (smooth joint) to 45° (very rough joint profile). The geometric profiles of the two surfaces in contact were initially positioned in a “matching” situation. Several tests were performed for different values of suctions (200, 100, and 20 MPa) and for different values of vertical stresses (30, 60, and 150 kPa). A constitutive model including the effects of suction and joint roughness is proposed to simulate the unsaturated behaviour of rock joints. The new constitutive law was incorporated in the code and experimental results were numerically simulated.     

The influence of the thickness of the walls and their properties on the treatment of rising damp in historic buildings

Intervention in older buildings increasingly requires extensive and objective knowledge of what one will be working with. In old buildings, rising damp in walls that are in direct contact with the ground leads to the migration of soluble salts that are responsible for many of the pathologies observed. Our research allows us to conclude that the most efficient way of treating rising damp is by ventilating the wall base [2], [4]. This technique was experimentally validated to limestone walls 20 cm thick. As it experimental validation of different thicknesses and different compositions has not been possible, numerical investigation has been carried out in order to analyse their influence.In this paper we will present the results of the work developed in the Department of Civil Engineering of the University of Coimbra in collaboration with the Department of Civil Engineering in the Faculty of Engineering of the University of Porto. The main purpose is to analyse the influence of the thickness and composition of walls on the new treatment for rising damp in historic walls: the ventilation of the wall base.     

Breakage and shear behaviour of intermittent rock joints

The breakage and shear behaviour of intermittent rock joints have been investigated in a series of direct shear tests with a new shear device, specifically designed for this purpose. The tests have been performed on specimens of rock-like material or hard rock, respectively, incorporating idealized non-persistent joints, made up of a number of short cracks in an en-échelon arrangement along the central shear axis.The shear behaviour of such a joint constellation has been found to be composed of different phases. The first phase of shearing is that of the actual rupture, initiated by the formation of wing cracks, starting from the existing cracks and growing into the material bridges, and concluded by the generation of additional new fractures connecting the initial cracks in the zone between the wing cracks. The second phase of shearing is characterized by friction processes and volume increase in the then continuous shear zone. Finally, the third phase of shearing, reached after large shear displacements, is determined by sliding processes inside the strongly fractured shear zone.In a large number of shear tests the geometrical parameters of the discontinuous joints as well as the loading conditions have been found to influence the activated shear resistance in each phase of shearing to a noticeably different extent. The orientation of the initial cracks and the normal stress, however, have been identified as the most influential parameters. Depending on the test conditions, an initially discontinuous rock joint can activate the largest shear resistance not just before rupture but in one of the two subsequent phases of shearing as well.The mechanisms which govern the different shear phases could be identified as (1) tensile rupturing, (2) rolling and sliding friction of dilatant joint zones and (3) sliding within the joint filling composed of brecciated material.     

Influence of asperity degradation on the mechanical behavior of rough rock joints under cyclic shear loading

A cyclic shear testing system was established to investigate the mechanical behavior of rough rock joints under cyclic loading conditions. Laboratory cyclic shear tests were conducted for two joint types of Hwangdeung granite and Yeosan marble: saw-cut and split tensile joints. Prior to test, the roughness of each specimen was characterized by measuring the surface topography using a laser profilometer. Several important aspects of cyclic joint behavior, such as high peak shear strength and non-linear dilation in the first loading cycle, different frictional resistance for the reversed shear loading direction, and anisotropic shear behavior and its dependence on the normal stress level were identified from the cyclic shear test results. These features and their variations in the subsequent loading cycles are mainly due to the effect of second order asperities and strength of rock material. It was also observed from experimental results that degradation of asperities under cyclic shear loading also followed the exponential degradation laws for asperity angle and the mechanism for asperity degradation would be different depending upon the shearing direction and the type of asperities. Based on the experimental results an elasto-plastic constitutive model, which can consider the degradation of second order asperities, was proposed. Numerical simulations for the monotonic and cyclic shear loading indicated agreement with the laboratory test results.     

基于数字图像处理技术的岩石节理分形描述

岩石节理的粗糙度对结构面的剪切强度起着重要的作用.分形维数对节理的次级细微粗糙结构能够进行很好的描述,而对节理的一级起伏结构体现并不明显.考虑了节理形态的层次性,用分形维数代表节理的次级精细结构,即节理的粗糙度;用起伏度表示节理的一级波状形态,据此提出用分形维数D和起伏度W_d两个指标来描述JRC(节理粗糙度系数),并给出相应的计算公式.另外,针对工程实践中分形维数准确快速量测存在的困难,本文以摄影测量为基础,借助数字图像处理技术,在VC++平台下开发了节理轮廓线分形维数计算程序,为分维较准确方便地应用于工程实践做了有益的探索.     

A review of shear strength models for rock joints subjected to constant normal stiffness

The typical shear behaviour of rough joints has been studied under constant normal load/stress(CNL)boundary conditions,but recent studies have shown that this boundary condition may not replicate true practical situations.Constant normal stiffness(CNS) is more appropriate to describe the stressestrain response of field joints since the CNS boundary condition is more realistic than CNL.The practical implications of CNS are movements of unstable blocks in the roof or walls of an underground excavation,reinforced rock wedges sliding in a rock slope or foundation,and the vertical movement of rock-socketed concrete piles.In this paper,the highlights and limitations of the existing models used to predict the shear strength/behaviour of joints under CNS conditions are discussed in depth.     

Development of an automated servo-controlled direct shear apparatus applying a constant normal stiffness condition

Correct assessment of the shear strength of rock joints is essential for several rock-engineering projects. The shear behavior of rock joints is usually investigated in the laboratory using a direct shear apparatus, wherein the forces or stresses acting normal to the direction of shear displacement are maintained constant during the shear process. However, this apparatus may be quite inappropriate for situations in which the normal stress on the joint surfaces changes considerably during the shear process. In the present study, an automated servo-controlled direct shear apparatus and computer control system have been developed to automatically accommodate the change in normal stress with dilation under the constant normal stiffness (CNS) boundary condition. A new LabVIEW-based system is used to described the performance of instrument control, data acquisition and storage operations of the new apparatus. The results of the shear tests conducted using the artificially prepared joint specimens show that the normal stiffness has a significant influence on the mechanical behavior of joints during the shear process.     

不同接触状态下粗糙节理剪切强度性质的颗粒流数值模拟和试验验证

节理表面形貌和接触状态对节理剪切力学性质有重要的影响。用砂浆材料的单轴压缩试验和光滑节理直剪试验得到材料和光滑节理的宏观力学性质参数,对颗粒流数值模拟的节理细观力学性质参数进行标定。用颗粒流离散元数值软件(PFC2D)构建人工粗糙节理表面形貌,对不同表面形貌的节理在不同接触状态下的剪切强度性质进行颗粒流直剪数值模拟试验,获得其峰值剪切强度。同时进行人工材料节理直剪试验,与颗粒流直剪数值模拟试验结果进行对比分析,数值试验与直剪试验结果吻合较好,验证了颗粒流直剪数值模拟试验与直剪试验具有同等的精度,可以作为各种表面形貌的节理在不同法向应力水平下抗剪强度研究的一种补充方法,以解决节理直剪试验中表面形貌损失对其剪切强度的影响,也可以在少量节理直剪试验的基础上,预估相同形貌的节理在不同接触状态下的剪切强度性质,同时还可以在现场测定不同粗糙度的节理表面形貌,预估其在不同接触状态、不同法向应力下的剪切力学性质。从而解决节理直剪试验中在相同形貌节理试件取样和制备困难的问题,且具有经济、方便、快捷、可重复性强等特点。     

Time-dependent behaviour of the Callovo-Oxfordian claystone-concrete interface

In the context of the Cigeo project,the French National Radioactive Waste Management Agency(Andra)is studying the behaviour of a deep geological facility for radioactive waste deposit in the CallovoOxfordian(COx) claystone.The assessment of durability of this project requires the prediction of irreversible strain over a large time scale.The mechanical interaction of the host rock and the concrete support of tunnels must be investigated to ensure the long-term sustainability of the structure.The instantaneous and time-dependent behaviour of the claystone-concrete interface is experimentally investigated with direct shear tests and long-duration shear tests of a few months.The mechanical and structural state of the claystone which is affected after interaction with concrete reflects to the response of the claystone-concrete interface,and thus different types of COx claystone-concrete interfaces are tested.The delayed deformation of the interface is found to be linked to the level of the normal loading and the loading history while a different response of the interface was observed from the short-and long-duration tests,indicating a possible progressive modification of interface under long-duration loadings.     

断续节理直剪试验与PFC2D数值模拟分析

 在以往有关断续节理模型试验和数值模拟的研究基础上,设计不同连通情况和法向应力的断续节理模型材料直剪试验,并采用颗粒流离散元软件PFC2D对模型试验进行全真数值模拟。以贯通节理试样、完整试样的剪应力–应变数值模拟曲线和模型试验曲线吻合作为PFC细观力学参数选取准则,并利用获得的细观力学参数对共面断续节理试样直剪试验进行数值重现。对比分析数值模拟曲线和模型试验曲线,对断续节理受剪贯通的力学机制进行研究。根据模型试验和数值试验的成果,分析断续节理预剪面上应力随剪应变的演化过程,发现剪切过程中的剪胀效应使得岩桥承担更多的压应力,从而提高了岩桥的抗剪强度。对断续节理岩体在直剪加载条件下的破坏机制进行讨论,将整个剪切过程分为线弹性阶段、初裂阶段、峰值阶段、峰后阶段及残余阶段5个阶段。     

创建一流大学 培养一流人才——澳大利亚蒙纳士大学的教学思想与实践

蒙纳士大学（Monash University）是澳大利亚规模最大、最为国际化的一所大学。“创建一流大学,培养一流人才”是蒙纳士大学的建校目标。围绕建校目标,蒙纳士大学把教学思想和教学实践作为影响学生最直接的环节给予高度重视,并具体体现在教学过程的各个方面,由此取得了骄人的成绩。     

Shear strengthening of old concrete beams

In our March‐April issue last year, the author described work at the Czech Technical University in Prague on the use of epoxy resins to apply new flat reinforcement to old concrete beams in order to enhance or prolong their service life. Here, some further test results and general conclusions are presented on the application of this technique to shear strengthening. The author is Associate Professor of Civil Engineering at the University.     

Shear strengthening of old concrete beams

In our March-April issue last year, the author described work at the Czech Technical University in Prague on the use of epoxy resins to apply new flat reinforcement to old concrete beams in order to enhance or prolong their service life. Here, some further test results and general conclusions are presented on the application of this technique to shear strengthening. The author is Associate Professor of Civil Engineering at the University.