Understanding the behavior of recycled aggregate concrete by using thermogravimetric analysis

The physio-chemical changes in concrete mixes due to different coarse aggregate (natural coarse aggregate and recycled coarse aggregate (RCA)) and mix design methods (conventional method and Particle Packing Method (PPM)) are studied using thermogravimetric analysis of the hydrated cement paste. A method is proposed to estimate the degree of hydration (\alpha) from chemically bound water (W_{\mathrm{B}}). The PPM mix designed concrete mixes exhibit lower \alpha. Recycled aggregate concrete (RAC) mixes exhibit higher and \alpha after 7 d of curing, contrary to that after 28 and 90 d. The chemically bound water at infinite time (W_{\mathrm{B}\infty }) of RAC mixes are lower than the respective conventional concrete mixes. The lower W_{\mathrm{B}\infty }, Ca(OH)₂ bound water, free Ca(OH)₂ content and FT-IR analysis substantiate the use of pozzolanic cement in the parent concrete of RCA. The compressive strength of concrete and \alpha cannot be correlated for concrete mixes with different aggregate type and mix design method as the present study confirms that the degree of hydration is not the only parameter which governs the macro-mechanical properties of concrete. In this regard, further study on the influence of interfacial transition zone, voids content and aggregate quality on macro-mechanical properties of concrete is needed.     

Determination of shear strength of steel fiber RC beams: application of data-intelligence models

Accurate prediction of shear strength of structural engineering components can yield a magnificent information modeling and predesign process. This paper aims to determine the shear strength of steel fiber reinforced concrete beams using the application of data-intelligence models namely hybrid artificial neural network integrated with particle swarm optimization. For the considered data-intelligence models, the input matrix attribute is one of the central element in attaining accurate predictive model. Hence, various input attributes are constructed to model the shear strength “as a targeted variable”. The modeling is initiated using historical published researches steel fiber reinforced concrete beams information. Seven variables are used as input attribute combination including reinforcement ratio (\rho \mathrm{\%}), concrete compressive strength ({{\displaystyle f}}_c^{\text{'}}), fiber factor ({{\displaystyle F}}_{\mathrm{1}}), volume percentage of fiber ({{\displaystyle V}}_f), fiber length to diameter ratio ({\displaystyle {{\displaystyle l}}_f{{\displaystyle l}}_d}) effective depth (d), and shear span-to-strength ratio ({\displaystyle ad}), while the shear strength ({{\displaystyle S}}_s) is the output of the matrix. The best network structure obtained using the network having ten nodes and one hidden layer. The final results obtained indicated that the hybrid predictive model of ANN-PSO can be used efficiently in the prediction of the shear strength of fiber reinforced concrete beams. In more representable details, the hybrid model attained the values of root mean square error and correlation coefficient 0.567 and 0.82, respectively.     

Shear design of high strength concrete prestressed girders

Normal strength prestressed concrete I-girders are commonly used as the primary superstructure components in highway bridges. However, shear design guidelines for high strength PC girders are not available in the current structural codes. Recently, ten 7.62 m (25 feet) long girders made with high strength concrete were designed, cast, and tested at the University of Houston (UH) to study the ultimate shear strength and the shear concrete contribution (V_c) as a function of concrete strength ({f^{\prime }}_{\mathrm{c}}). A simple semi-empirical set of equations was developed based on the test results to predict the ultimate shear strength of prestressed concrete I-girders. The UH-developed set of equations is a function of concrete strength (\sqrt{{f^{\prime }}_{\mathrm{c}}}), web area (b_wd), shear span to effective depth ratio (a/d), and percentage of transverse steel (ρ_t). The proposed UH-Method was found to accurately predict the ultimate shear strength of PC girders with concrete strength up to 117 MPa (17000 psi) ensuring satisfactory ductility. The UH-Method was found to be not as overly conservative as the ACI-318 (2011) code provisions, and also not to overestimate the ultimate shear strength of high strength PC girders as the AASHTO LRFD (2010) code provisions. Moreover, the proposed UH-Method was found fairly accurate and not exceedingly conservative in predicting the concrete contribution to shear for concrete strength up to 117 MPa (17000 psi).     

Investigation of Generalized SIFs of cracks in 3D piezoelectric media under various crack-face conditions

This paper investigates the influence of crack geometry, crack-face and loading conditions, and the permittivity of a medium inside the crack gap on intensity factors of planar and non-planar cracks in linear piezoelectric media. A weakly singular boundary integral equation method together with the near-front approximation is adopted to accurately determine the intensity factors. Obtained results indicate that the non-flat crack surface, the electric field, and the permittivity of a medium inside the crack gap play a crucial role on the behavior of intensity factors. The mode-I stress intensity factors (K_{\mathrm{I}}) for two representative non-planar cracks under different crack-face conditions are found significantly different and they possess both upper and lower bounds. In addition, K_{\mathrm{I}} for impermeable and semi-permeable non-planar cracks treated depends strongly on the electric field whereas those of impermeable, permeable, and semi-permeable penny-shaped cracks are identical and independent of the electric field. The stress/electric intensity factors predicted by permeable and energetically consistent models are, respectively, independent of and dependent on the electric field for the penny-shaped crack and the two representative non-planar cracks. Also, the permittivity of a medium inside the crack gap strongly affects the intensity factors for all crack configurations considered except for K_{\mathrm{I}} of the semi-permeable penny-shaped crack.     

A novel finite element formulation for static bending analysis of functionally graded porous sandwich plates

This article aims to propose a finite element formulation based on Quasi-3D theory for the static bending analysis of functionally graded porous (FGP) sandwich plates. The FGP sandwich plates consist of three layers including the bottom skin of homogeneous metal, the top skin of fully ceramic and the FGP core layer with uneven porosity distribution. A quadrilateral (Q4) element with nine degrees of freedom (DOFs) per node is derived and employed in analyzing the static bending response of the plate under uniform and/or sinusoidally distributed loads. The accuracy of the present finite element formulation is verified by comparing the obtained numerical results with the published results in the literature. Then, some numerical examples are performed to examine the effects of the parameters including power-law index k and porosity coefficient \xi on the static bending response of rectangular FGP sandwich plates. In addition, a problem with a complicated L-shape model is conducted to illustrate the superiority of the proposed finite element method.     

基于超大型三轴仪的堆石料缩尺效应研究

对某一爆破筑坝堆石料开展了试样直径1m(最大粒径为200mm)的超大型和试样直径0.3m(最大粒径为60 mm)的常规大型三轴固结排水剪切试验,研究了平行相似级配条件下最大粒径对应力–应变关系、颗粒破碎、特征应力状态、割线模量及邓肯张E-B模型参数的影响。试验结果表明:(1)大型三轴试验的峰值内摩擦角大于超大型三轴试验,并且峰值应力处的轴向应变小于超大型三轴试验;(2)大型三轴试验的相变应力比大于超大型三轴试验,并且相变处的体变及轴向应变均小于超大型三轴试验;(3)大型三轴试验的颗粒破碎率小于超大型三轴试验,但二者平行粒组的变化规律相同;(4)大型三轴试验在应力水平50%处的割线弹性模量及割线体积模量均大于超大型三轴试验。大型三轴试验的邓肯张E-B模型参数k,kb分别是超大型三轴试验的1.22倍、1.38倍。研究成果可为建立考虑缩尺效应的堆石料本构模型和分析土工构筑物变形提供重要的试验依据。     

加卸载条件下吸力对黄土变形特性影响的试验研究

为了探讨基质吸力和净围压对黄土的变形–强度及卸载–再加载模量的影响,用非饱和土三轴仪对原状Q_3黄土及其重塑土共做了3组27个试验,即9个控制吸力和净围压等于常数的原状Q_3黄土的三轴固结排水剪切试验、9个原状Q_3黄土和9个重塑Q_3黄土的控制吸力和净围压等于常数的三轴固结排水卸载–再加载剪切试验。研究结果表明:基质吸力和净围压均对试样的强度及变形特性有显著影响;原状Q_3黄土的偏应力–应变曲线随净围压的增大从理想弹塑性型演变为硬化型,因其结构性导致偏应力–应变曲线的初始阶段存在交叉现象,试样一直处于剪缩状态;重塑试样在净围压较小时由剪缩逐渐变为较强的剪胀,在净围压较大时则只发生剪缩;表观凝聚力、切线杨氏模量和卸载–再加载模量均随着吸力的增大而增大;而吸力对有效内摩擦角及参数n(原状Q_3黄土的初始杨氏模量随净围压增大而变化的指数)、n_(ur)(原状Q_3黄土的卸载—再加载模量随净围压增大而变化的指数)和n_(ur)'(重塑Q_3黄土的卸载—再加载模量随净围压增大而变化的指数)的影响很小,皆可视为常数。基于试验研究结果,提出了考虑吸力和净围压影响的表观凝聚力、切线杨氏模量和卸载—再加载模量的修正计算公式,完善了非饱和土的增量非线性模型。     

Innovative laboratory testing

Three laboratory test techniques were developed to study and quantify damage development in loaded rock specimens and to provide data for use in the calibration and solution of numerical models. Damage-controlled tests involved incremental loading–unloading cycles with the stress at volumetric strain reversal (crack damage stress σ_cd) and elastic properties calculated for each successive cycle. Damage was quantified by measuring the irreversible volumetric strain upon unloading and the degradation of rock strength and elastic properties. In long-term loading tests, a creep stress greater than σ_cd was applied to the specimen until failure. The time to failure was dependent upon the ratio of the creep stress to σ_cd, and to confining pressure. It was shown that σ_cd is a reasonable estimate of the long-term rock strength. Damage was quantified by measuring the degradation of elastic properties through periodic unloading–reloading cycles. The test data were used for the calibration of Itasca's PFC stress-corrosion model. Five types of thermoporoelasticity tests were designed to determine seven of the nine thermoporoelastic parameters required for the modelling of the thermoporoelastic response of rock. Tests were performed by applying increments of confining pressure, temperature or pore pressure on the rock specimen, and measuring the changes in confining pressure, specimen volume and pore pressure. The measured values of five parameters were found to agree reasonably well with the estimated values.     

A study of the mechanical behaviour of coal for pillar design

A series of triaxial compression tests was carried out on 61, 101, 146 and 300 mm diameter samples to investigate the effects of scale on the mechanical behaviour of coal. The “inherent variability” of laboratory coal strength data was found to be related to the degree of cleating or brightness of the coal samples. Using this observed relationship, a method was developed for estimating in situ coal seam strength based upon the “intact” properties of dull coal samples and seam brightness profiles. The peak strength criterion for in situ coal uses the parameters σ_c, m and s of the Hoek–Brown empirical strength criterion for rock masses. Unloading cycles were used to investigate the yield behaviour of the coal samples. The results show that, depending on the magnitude of the confining stress, the failure mechanism of coal alters significantly with a change from an axial splitting to a shearing mechanism. Measurements of the volumetric strain response also indicate an absence of scale dependence in the deformation behaviour of the three larger sample sizes. Unloading cycles were used as a means of investigating the development of recoverable (elastic) and irrecoverable (plastic) components of strain. The irrecoverable strain loci, for the larger sizes, were taken to be representative of the in situdeformation response of coal.     

Particle loss: An initial investigation into size effects and stress-dilatancy

Drained triaxial tests have been performed to explore the effect of particle loss on shearing behaviour and critical states in granular mixtures. The mixtures comprise Leighton Buzzard sand (d₅₀ = 0.8 mm), to which was added 15% by mass of salt particles of different nominal sizes: 0.063 mm, 0.25 mm and 0.5 mm. Shearing behaviours before and after particle loss (by dissolution) were compared. A good fit is observed between the test data and a stress-dilatancy relationship for the post-dissolution tests, highlighting the ability of the stress-dilatancy analysis as a means to interpret the effects of particle loss on shearing. It was noted that critical state strength parameter M is determined by the post-dissolution grading regardless of size of removed particle. However, the duration of contractant volumetric strain increased with the larger removed particles (0.25 mm & 0.5 mm) even when initial specific volumes were virtually identical. It is suggested that a loose volumetric state is reached if the sand particle network is initially disrupted by the amount and/or size of salt particles, which following dissolution results in structural or fabric phenomena that are not reflected in scalar volumetric measures such as specific volume.     

控制吸力的非饱和土真三轴试验研究

基于西安理工大学的新型真三轴仪,在压力室底座与试样顶盖上分别安装高进气值陶土板与多孔板,从而增加孔隙气压力与孔隙水压力独立控制和量测装置,进而利用轴平移技术对基质吸力进行控制,并且新增水气控制面板和气动二联件对可控制吸力的真三轴仪进行了进一步完善。在此基础上初次尝试对重塑黄土进行了同时控制吸力(s=100 kPa)与净平均应力(p=300 kPa)为常数的真三轴固结排水剪切试验,分析了不同中主应力参数(b=0,0.25,0.5,0.75,1)条件下,各主应变(e_1,e_2,e_3)随偏应力(q)的变化情况;给出了重塑黄土在同时控制吸力与净平均应力路径下p平面上的破坏轨迹,并与Lade–Duncan强度破坏准则和Mohr–Coulomb强度破坏准则的预测破坏线进行了比较。     

球应力和偏应力对粗粒土变形影响的真三轴试验研究

为了研究p,q,θσ单独变化对粗粒土变形的影响,使用河海大学TSW-40型真三轴仪,对粗粒土进行了等q等b(b=(σ2-σ3)/(σ1-σ3))试验,等p等b试验和等p等q试验,试验结果表明:q,b保持不变,p单独减小时,初期几乎不产生偏应变,但会产生膨胀的体积应变,随着p的减小,体积膨胀增大,偏应变也逐渐增大,但偏应变数值上比体积应变小,到后期体积应变和偏应变都加速发展,直至破坏;p的减小直接引起体积膨胀,体积膨胀后颗粒结构松动,进而导致偏应变的产生;p,b保持不变,q单独增大时,初期几乎不产生体积膨胀,但会产生偏应变,随着q的增大,偏应变增大,体积膨胀也逐渐增大,但体积应变数值上比偏应变小,到后期体积应变和偏应变都加速发展,直至破坏;q的增大直接产生偏应变,偏应变使得颗粒之间产生错动,进而导致体积膨胀的产生;p,q保持不变,应力罗德角θσ单独变化会产生不可恢复的体积应变和偏应变,但数值上很小。引入参数sp(sp=(p/q-p0/q0)/(1/Mf-p0/q0))和sq(sp=(q/p-q0/p0)/(Mf-q0/p0)),p0,q0分别为初始球应力和偏应力,Mf为破坏应力比,发现q,b保持不变,p单独减小时,dεv/dp与1/(1-sp)1/2-1成正比例关系,dεs/dp与-sp[1/(1-sp)1/2-1]成正比例关系,应力–剪胀方程为dεv/dεs=-1/sp;p,b保持不变,q单独增大时,dεs/dq与1/(1-sq)1/2-1成正比例关系,dεv/dq与-sq[1/(1-sq)1/2-1]成正比例关系,应力–剪胀方程为dεv/dεs=-sq。最后根据本文试验结果对粗粒土柔度矩阵元素的特性进行了分析。     

Higher-order shear and normal deformable theory for functionally graded incompressible linear elastic plates

We use the principle of virtual work to derive a higher-order shear and normal deformable theory for a plate comprised of a linear elastic incompressible anisotropic material. The theory does not use a shear correction factor and employs three components of displacement and the hydrostatic pressure as independent variables. For a Kth order plate theory, a set of 4(K+1) coupled equations need to be solved for the (K+1) pressures and the 3(K+1) displacements defined on the reference surface of the plate.

Equations for free vibrations of a plate are derived, and equations for the determination of frequencies and the corresponding mode shapes of a simply supported rectangular plate are given.     

A new developed expression to determine more realistically the shear buckling stress in steel plate structures

This paper summarises and presents main results of an in-depth numerical analysis dealing with the shear buckling resistance of stainless steel plate girders. The studies conducted have permitted the development of a simple design expression to determine the critical shear buckling stress in steel web panels. This expression takes into account the effects of the material nonlinearity together with the actual boundary conditions of the web panel given by the geometry of the plate girder by two defined parameters: η and , which is the shear buckling coefficient. Expressions given to obtain η will quantify mainly the effect of the material nonlinearity whereas the proposed analytical expression of will reproduce principally the influence of the real boundary conditions in a web panel.     

Parametric finite element study of slotted end connections to circular hollow sections

The influence that the shear lag phenomenon may have on the strength of tubular connections has been addressed in design provisions with variable efficiency factors that affect the tube net area. However, an experimental program carried out on gusset plate connections to the ends of circular hollow section (CHS) members has indicated that these provisions may be overly conservative. As part of the experimental program, a total of eight specimens was tested under quasi-static tension and compression loading considering three connection types frequently used in practice. Moreover, results of a parametric analysis undertaken based on finite element models of these connections (where the responses were verified with the test results) showed considerable differences between the calculated connection strength and the predicted capacity by design provisions. In the finite element analysis, a nonlinear time step analysis was performed considering nonlinear material properties and 8-noded solid elements were used throughout the modeling. The gradual propagation of cracks in the material was emulated by definition of a maximum equivalent strain as the failure criterion with the activation of a “death feature” of the elements. The influence of parameters such as: the weld length (L_w), a proposal to use the eccentricity reduced by half the flange-plate thickness and the tube diameter-to-thickness (D/t) ratio have been studied. The analysis results have shown that a gradual transition between several failure modes takes place as the weld length increases. Also, the likelihood of developing the full efficiency of the tube net cross-sectional area, if a minimum ratio of L_w/w=1.0 is used, is illustrated.