Unified strength model based on the Hoek-Brown failure criterion for fibre-reinforced polymer-confined pre-damaged concrete columns with circular and square cross sections

Fibre-reinforced polymer(FRP)has the advantages of high strength,light weight,corrosion resistance and convenient construction and is widely used in repairing and strengthening damaged concrete columns.Most of the existing strength models were built by regression analysis of experimental data;however,in this article,a new unified strength model is proposed using the Hoek-Brown failure criterion.To study the strength of FRP-confined damaged and undamaged concrete columns,900 test data were collected from the published literature and a large database that contains the cross-sectional shape of each specimen,the damage type,the damage level and the FRP-confined stiffness was established.A new strength model using the Hoek-Brown failure criterion was established and is suitable for both circular and square columns that are undamaged,load-damaged and fire-damaged.Based on the database,most of the existing strength models from the published literature and the model proposed in this paper were evaluated.The evaluation shows that the proposed model can predict the compressive strength for FRP-confined pre-damaged and undamaged concrete columns with good accuracy.

Brittleness Generation Mechanism and Failure Model of High Strength Lightweight Aggregate Concrete

The brittleness generation mechanism of high strength lightweight aggregate con-crete(HSLWAC) was presented, and it was indicated that lightweight aggregate was the vulnerable spot, initiating brittleness. Based on the analysis of the brittleness failure by the load-deflection curve, the brittleness presented by HSLWAC was more prominent compared with ordinary lightweight aggregate concrete of the same strength grade. The model of brittleness failure was also established.

Refined mathematical model for the breaching of concrete-face sand-gravel dams due to overtopping failure

Overtopping is one of the main reasons for the breaching of concrete-face sand-gravel dams(CFSGDs).In this study,a refined mathematical model was established based on the characteristics of the overtopping breaching of CFSGDs.The model characteristics were as follows:(1)Based on the Renormailzation Group(RNG)k-εturbulence theory and volume of fluid(VOF)method,the turbulent characteristics of the dam-break flow were simulated,and the erosion surface of the water and soil was tracked;(2)In consideration of the influence of the change in the sediment content on the dam-break flow,the dam material transport equation,which could reflect the characteristics of particle settlement and entrainment motion,was used to simulate the erosion process of the sand gravels;(3)Based on the bending moment balance method,a failure equation of the concrete face slab under dead weight and water load was established.The proposed model was verified through a case study on the failure of the Gouhou CFSGD.The results showed that the proposed model could well simulate the erosion mode of the special vortex flow of the CFSGD scouring the support body of the concrete face slab inward and reflect the mutual coupling relationship between the dam-break flow,sand gravels,and concrete face slabs.Compared with the measured values,the relative errors of the peak discharge,final breach average width,dam breaching duration,and maximum failure length of the face slab calculated using the proposed model were all less than 12%,thus verifying the rationality of the model.The proposed model was demonstrated to perform better and provide more detailed results than three selected parametric models and three simplified mathematical models.The study results can aid in establishing the risk level and devising early warning strategies for CFSGDs.

Deformation and failure modes of composite foundation with sub-embankment plain concrete piles

With the development of high-speed railway in China, composite foundation with rigid piles has become a stamdard solution of meeting the high requirements of stability and post-construction settlement of embankment on soft subgrade. Among several im- provement pattems, plain concrete piles have been extensively used to treat soft ground supported embankment. To investigate the deformation and failure modes of unimproved soft ground and soft ground reinforced by sub-embankment plain concrete piles, and to learn the influences of track and vehicle load, the effect of pile spacing, as well as the compression moduli of soil layers and upper load condition on the failure modes, a series of centrifuge model tests were performed. Test results indicate that the dis- placement of unimproved soft ground under the embankment increases continuously as embankment, track and train loading, and slip circle failure takes place. The deformation law of soft ground reinforced by sub-embankment plain concrete piles depends on pile spacing, compression modulus of the soft ground, and loading conditions. It was also found that plain concrete piles show displacement and failure patterns depending on its location, compression modulus of soft soil around the pile, and loading condi- tions. Furthermore, the evaluation of improved ground stability as well as the model test procedure is also presented.

Methodology for estimating probability of dynamical system's failure for concrete gravity dam

Methodology for the reliability analysis of hydraulic gravity dam is the key technology in current hydropower construction.Reliability analysis for the dynamical dam safety should be divided into two phases:failure mode identification and the calculation of the failure probability.Both of them are studied based on the mathematical statistics and structure reliability theory considering two kinds of uncertainty characters(earthquake variability and material randomness).Firstly,failure mode identification method is established based on the dynamical limit state system and verified through example of Koyna Dam so that the statistical law of progressive failure process in dam body are revealed; Secondly,for the calculation of the failure probability,mathematical model and formula are established according to the characteristics of gravity dam,which include three levels,that is element failure,path failure and system failure.A case study is presented to show the practical application of theoretical method and results of these methods.

Evaluation of Blast-Resistant Performance Predicted by Damaged Plasticity Model for Concrete

in order to evaluate the capacity of reinforced concrete （RC） structures subjected to blast Ioadings, the damaged plasticity model for concrete was used in the analysis of the dynamic responses of blast-loaded RC structures, and all three failure modes were numerically simulated by the finite element software ABAQUS. Simulation results agree with the experimental observations. It is demonstrated that the damaged plasticity model for concrete in the finite element software ABAQUS can predict dynamic responses and typical flexure, flexure-shear and direct shear failure modes of the blast-loaded RC structures.

Mesoscale Modeling of Hooked-End Steel Fiber Reinforced Concrete under Uniaxial Compression Using Cohesive Elements

Based on the cohesive zone model, the 2D mesostructures were developed for numerical studies of multi-phase hooked-end steel fiber reinforced concrete under uniaxial compression. The zero-thickness cohesive interface elements were inserted within the mortar, on interfaces of mortar and aggregates and interfaces of mortar and fibers to simulate the failure process of fiber reinforced concrete. The results showed that the numerical results matched well the experimental results in both failure modes and stress-strain behavior. Hooked-end steel fiber reinforced concrete exhibited ductile failure and maintained integrity during a whole failure process. Compared with normal concrete, HES fiber reinforced concrete was greater stiffness and compressive strength;the descending branch of the stress-strain curve was significantly flatter;the residual stress was higher.

Effect of curing, capillary action, and groundwater level increment on geotechnical properties of lime concrete: Experimental and prediction studies

Lime concrete and lime treatment are two attractive techniques for geotechnical engineers.However,researches have rarely been carried out to study the effects of moisture and capillary action due to increasing groundwater level on geotechnical properties of lime concrete.The aim of this study is to investigate the effects of curing time and degree of saturation on some of geotechnical properties of lime concrete such as unconfined compressive strength(UCS),secant modulus(ES),failure strain,brittleness index(IB),and deformability index(ID) using unconfined compression tests.First of all,geotechnical and chemical properties of used materials were determined.After curing times of 14 d,28 d,45 d,and 60 d in laboratory condition,the specimens were exposed to saturation levels ranging from 0 to 100%.The results showed that the moisture and curing time have significant effects on the properties of lime concrete.Based on the results of scanning electron micrograph(SEM) test,it was observed that the specimen was characterized by a rather well-structured matrix since both the filling of a large proportion of the coarse-grained soil voids by clay and the pozzolanic activity of lime led to retaining less pore water in the specimen,increasing the UCS and ES,and consequently resisting against swelling and shrinkage of the clay soil.Moreover,due to the pozzolanic reactions and reduction of water,by increasing the curing time and decreasing the degrees of saturation,UCS,ES,and IBincreased,and IDdecreased.Based on the experimental results,a phenomenological model was used to develop equations for predicting the properties in relation to the ratio of degree of saturation/curing time.The results showed that there was a good correlation(almost R2> 90%) between the measured parameters and the estimated ones given by the predicted equations.

An Approach for Quantifying the Influence of Seepage Dissolution on Seismic Performance of Concrete Dams

Many concrete dams seriously suffer from long-term seepage dissolution,and the induced mechanical property deterioration of concrete may significantly affect the structural performance,especially the seismic safety.An approach is presented in this paper to quantify the influence of seepage dissolution on seismic performance of concrete dams.To connect laboratory test with numerical simulation,dissolution tests are conducted for concrete specimens and using the cumulative relative leached calcium as an aging index,a deterioration model is established to predict the mechanical property of leached concrete in the first step.A coupled seepage-calcium dissolutionmigrationmodel containing two calculation modes is proposed to simulate the spatially non-uniformdeterioration of concrete dams.Based on the simulated state of a roller compacted concrete dam subjected to 100 years of seepage dissolution,seismic responses of the damare subsequently analyzed.During which the nonlinear cracking of concrete,the radiation damping of the far-field foundation is considered.Research results show that seepage dissolution will seriously weaken the seismic safety of concrete dams because of the dissolution-induced decrease of effective thickness of the dam body.The upstream surface,dam toe and gallery wall suffer from a large degree of dissolution,whereas it is minimal and basically the same inside the dam body,at a degree of 0.19%within 100 years.The horizontal displacements of dam crest under the design static load and fortification against earthquake increase by 6.9%and 21.9%,respectively,and the dissolution-induced seismic cracking leads to the failure of dam anti-seepage system.This study can provide engineers with a reference basis for reinforcement decision of old concrete dams.

Damage response of conventionally reinforced two-way spanning concrete slab under eccentric impacting drop weight loading

Reinforced concrete(RC)structures are generally designed to carry quasi-static gravity loads through almost indispensable components namely slab,however,it may be subjected to high intense loads induced from the impact of projectiles generated by the tornado,falling construction equipment,and also from accidental explosions during their construction and service lifespan.Impacts due to rock/boulder falls do occur on the structures located especially in hilly areas.Such loadings are not predictable but may cause severe damage to the slab/structure.It stimulates structural engineers and researchers to investigate and understand the dynamic response of RC structures under such impulsive loading.This research work first investigates the performance of 1000×1000×75 mm^(3)conventionally reinforced two-way spanning normal strength concrete slab with only tension reinforcement(0.88%)under the concentric impact load(1035 N)using the finite element method based computer code,ABAQUS/Explicit-v.6.15.The impact load is delivered to the centroid of the slab using a solid-steel cylindroconical impactor(drop weight)with a flat nose of diameter 40 mm,having a total mass of 105 kg released from a fixed height of 2500 mm.Two popular concrete constitutive models in ABAQUS namely;Holmquist-Johnson-Cook(HJC)and Concrete Damage Plasticity(CDP),with strain rate effects as per fib MODEL CODE 2010,are used to model the concrete material behavior to impact loading and to simulate the damage to the slab.The slab response using these two models is analyzed and compared with the impact test results.The strain rate effect on the reinforcing steel bars has been incorporated in the analysis using the Malvar and Crawford(1998)approach.A classical elastoplastic kinematic idealization is considered to model the steel impactor and support system.Results reveal that the HJC model gives a little overestimation of peak displacement,maximum acceleration,and damage of the slab while the predictions given by the CDP model are in reasonable agreement with the experimental test results/observations available in the open literature.Following the validation of the numerical model,analyses have been extended to further investigate the damage response of the slab under eccentric impact loadings.In addition to the concentric location(P1)of the impacting device,five locations on a quarter of the slab i.e.,two along the diagonal(P2&P3),the other two along the mid-span(P4&P5),and the last one(P6)between P3 and P5,covering the entire slab,are considered.Computational results have been discussed and compared,and the evaluation of the most damaging location(s)of the impact is investigated.It has been found that the most critical location of the impact is not the centroid of the slab but the eccentric one with the eccentricity of 1/6th of the span from the centroid along the mid-span section.

Mechanical Properties of Self-Compacting Rubberized Concrete with Different Rubber Types under Triaxial Compression

Different rubber aggregates lead to changes in the effect of stress conditions on the mechanical behavior of concrete,and studies on the triaxial properties of self-compacting rubber concrete(SCRC)are rare.In this study,35 cylindrical specimens taking lateral stress and rubber type as variables were prepared to study the fresh properties and mechanical behaviors of SCRC under triaxial compression,where the rubber contains two types,i.e.,380μm rubber powder and 1–4 mm rubber particles,and four contents,i.e.,10%,20%and 30%.The test results demonstrated that SCRC exhibited a typical oblique shear failure mode under triaxial compression and had a more moderate descending branch compared with self-compacting concrete(SCC).The presence of lateral stress can significantly improve the compression properties,including initial elastic modulus,peak stress and peak strain,with an improvement range of 3%–73%for peak stress.While rubber aggregates mainly targeted the deformation abilities and toughness for improvement,and the peak strain improvement ranges were 0.1–3.1 times and 0.1–1.0 times for SCRC containing rubber powder and SCRC containing rubber particles,respectively,relative to SCC.At a high lateral stress of at least 12 MPa,the loss of strength due to the addition of rubber can be controlled within 10%,in which case the content of rubber powder and rubber particles was recommended to be at most 20%and 30%,respectively.Based on the Mohr-Coulomb theory,the failure criteria of SCRC with different rubber types were established.For analysis and design purposes,an empirical model was proposed to predict the stressstrain behavior under triaxial compression,considering the influence of different rubber content and lateral stress.The results obtained in this study can provide a valuable reference for the design and application of self-compacting rubberized concrete in practical projects,especially those involving three-way compression states and requiring high-quality deformation and energy dissipation.

Damage and progressive failure of concrete structures using non-local peridynamic modeling

Peridynamics (PD), a recently developed theory of solid mechanics, which employs a non-local model of force interaction and makes use of integral formulation rather than the spatial partial differential equations used in the classical continuum mechanics theory, has shown effectiveness and promise in solving discontinuous problems at both macro and micro scales. In this paper, the peridynamics theory is used to analyze damage and progressive failure of concrete structures. A non-local peridynamic model for a rectangular concrete plate is developed, and a central pairwise force function is introduced to describe the interior interactions between particles within some definite distance. Damage initiation, evolution and crack propagation in the concrete model subject to in-plane uni-axial tension, in-plane uni-axial compression and out-of-plane impact load are investigated respectively. The numerical results show that discontinuities appear and grow spontaneously as part of the solution to the peridynamic equations of motion, and no special failure criteria or re-meshing techniques are required, which proves the potential of peridynamic modeling as a promising technique for analyzing the progressive failure of concrete materials and structures.

New Unified Failure Model on Evenly Distributed Reinforced Concrete Members with Box Section

It is urgently needed to describe the structural collapse process under extreme conditions to survive people.For reinforced concrete structures it is still a difficulty to describe the failure of reinforced concrete members under complex internal force combination,such as under axial forces,bending moment,shear forces, and torsion working together.In this paper,based on the traditional Nielsen model,a new unified failure model on reinforcement evenly distributed concrete members with box section under combined forces is introduced.The advantages of the proposed new model are to consider the dowel actions of reinforcements and reasonably to consider of the shear carrying capacity of concrete,especially when compression stress of concrete is in a high value.Finally,the theoretical results of the new model are compared with a series of experimental results of box section members.The comparison has verified that the new model is more accurate and feasible for the design and calculation of box section members.

Advanced Unified Failure Model on Uniformly Reinforced Concrete Box Section Members

Based on the traditional Nielsen model,a unified failure model on the uniformly reinforced concrete box section members under combined forces was introduced by Luo and Liu.One of their contributions is adjustment of the shear carrying capacity of concrete at the member failure surface.In the unified failure model,the comparison with the experimental results verified this adjustment.Nevertheless,it should be pointed out that the adjustment factor of shear carrying capacity at member failure surface for the reinforced concrete members in the unified failure model is a fixed adjustment constant for all experiment data,which is basically determined by curve fitting.However,the adjustment factor should vary with the normal stress at the member failure surface.In this paper,an advanced theoretical model is introduced,in which the adjustment factor of shear carrying capacity at failure surface is a variable related to the normal stress at failure surface.Furthermore,the advanced unified failure model on the uniformly reinforced concrete box section member can still be expressed in a simple form.Finally,the comparison with several groups of test data has verified that this advanced model is more accurate and feasible to be used in design.

Experimental and theoretical investigation of the failure behavior of a reinforced concrete target under high-energy penetration

The dynamic mechanical properties of concrete and reinforced concrete targets subjected to high-speed projectile impact loading have a significant influence on the impact resistance of protective structures.In this study,high-speed projectile penetration and perforation of concrete and reinforced concrete structures was carried out to determine the high-energy impact loading.The failure behaviors of projectile penetration and perforation of the concrete and reinforced concrete targets were investigated,and the destruction characteristics of the targets were measured.An analytical model was established using the principle of minimum potential energy.The results show that the theoretical predictions are consistent with the experimental data,indicating that the energy method is effective for predicting the dynamic mechanical properties of concrete and reinforced concrete targets under high-speed projectile penetration.

基于统一相场理论和内聚力模型的钢纤维混凝土损伤破坏细观研究

钢纤维已成为土木工程中应用需求不断增长的增强材料,因而钢纤维混凝土(steel fiber reinforced concrete,SFRC)的损伤、断裂也是工程中亟需解决的关键问题。基于最新提出的统一相场理论和内聚力模型,建立由钢纤维、骨料、砂浆及界面过渡区构成的SFRC细观有限元模型并结合试验予以验证。针对单轴拉伸、三点弯曲等工况,考察钢纤维掺量和长度、骨料形状、初始切口缺陷等因素对SFRC抗拉和抗折性能的影响,对SFRC的力学性能及损伤破坏机理进行研究。结果表明:基于相场损伤模型和内聚力模型建立的SFRC细观有限元模型具有较好的准确性和可靠性;SFRC抗拉强度受钢纤维掺量的影响较大,在掺量取2%时性价比较高,比0.5%掺量时约提高34.2%,骨料形状和钢纤维长度的影响较小;SFRC抗折强度受钢纤维掺量及切口深度的影响较大,掺入0.2%、0.35%和0.5%的钢纤维使SFRC切口梁承载力分别提高了20.7%、38.9%和63.7%,相比切口深度为25 mm的SFRC切口梁,切口深度为40 mm和55 mm的SFRC切口梁承载力分别降低了19.9%和39.8%。供钢纤维混凝土工程分析和设计参考。

正交异性钢-钢纤维混凝土组合桥面板疲劳极限状态研究

为了解正交异性钢-钢纤维混凝土(Steel Fiber Reinforced Concrete,SFRC)组合桥面板的疲劳性能及失效机理,以川南城际铁路临港长江大桥为背景,设计、制作正交异性钢-SFRC组合桥面板足尺模型进行疲劳试验,采用ANSYS软件建立试件有限元模型,研究关键细节的疲劳应力和开裂等情况。基于有限元模型,分析不同设计参数(钢顶板厚度、栓钉布置、SFRC抗拉强度及层厚)下组合桥面板疲劳极限状态的失效模式,并提出了主要控制参数取值建议。结果表明:200万次疲劳加载后足尺模型的实测最大裂缝宽度为0.136 mm,出现在SFRC层上缘,钢结构未开裂,组合桥面板疲劳性能良好;组合桥面板疲劳极限状态主要由SFRC层开裂控制,钢顶板厚度、栓钉布置对组合桥面板疲劳性能的影响较小,SFRC抗拉强度和层厚对组合桥面板疲劳性能影响较大,为主要控制参数;在常规正交异性钢桥面板上铺设薄层(厚度不超过50 mm)SFRC时,SFRC抗拉强度不应小于5 MPa,在常规正交异性钢桥面板上铺设普通SFRC(钢纤维体积含量不高于1%,抗拉强度不高于3 MPa)时,SFRC层厚不宜低于100 mm。

预制混凝土夹心保温墙体GFRP连接件抗剪性能研究

纤维增强材料连接件因导热系数低、耐久性好等特点,在建筑外墙中具有广阔的工程应用前景。然而,纤维增强材料连接件抗剪性能较差,有必要开展采用纤维增强材料连接件的墙板抗剪性能研究。利用Abaqus有限元软件,选择合理的混凝土和连接件本构模型,建立4种不同类型连接件的数值模型。通过与试验结果进行对比,证明所建模型的有效性。随后探讨混凝土等级、连接件数量、连接件布置方式对夹心保温墙板抗剪承载力的影响。研究结果表明,提高混凝土等级能够提高部分夹心保温墙板在破坏阶段的承载力,但提升能力十分有限;连接件水平布置比垂直布置的墙板承载力略高;对于使用棒状及类似形状连接件的夹心保温墙板,增加连接件数量是最直接提高抗剪能力的方式。最后,分析了夹心保温墙板不同的失效模式及其计算方法。

煤矸石混凝土静态抗压破坏机理及强度预测

为厘清煤矸石混凝土静态抗压强度变化规律和破坏机理,采用试验研究与模型预测相结合的方式,揭示煤矸石混凝土破坏过程、破坏形态、尺寸效应及水灰比对抗压强度的影响规律,剖析水灰比与抗压强度之间的数理关系,利用灰色理论建立GM(1,1)模型预测煤矸石混凝土抗压强度。结果表明:煤矸石具有大多数黏土岩的特性,即干燥时强度良好,浸水后易发生崩解;煤矸石混凝土的破坏可分为3个阶段(裂缝生成阶段、裂缝发展扩大阶段、混凝土破坏阶段)、3种形态(粗骨料破坏、黏结破坏、水泥石破坏,且以粗骨料破坏为主);当水灰比由0.60降至0.25时,煤矸石混凝土抗压强度提高了2.67%~18.50%,满足C15、C20、C25、C304个强度等级;由于煤矸石物理性质差,混凝土界面过渡区充斥大量微裂缝与孔洞,导致煤矸石混凝土尺寸效应强于普通混凝土;GM(1,1)模型预测值与实测值基本吻合,预测模型精度等级为1级。灰色理论可以有效预测煤矸石混凝土强度增长规律,煤矸石混凝土在实际应用前可采用灰色模型预测抗压强度。

石灰石粉-矿渣混凝土的碳化性能及可靠性分析

为探究石灰石粉及矿渣对混凝土碳化性能以及可靠性的影响,以掺合料比例、水胶比、石粉比表面积以及龄期为参数进行快速碳化试验,得出了试验环境和自然环境下的碳化预测模型,由此展开自然碳化下的可靠性预测分析,并以分析得到的可靠度指标为因变量,选取混凝土的水胶比、保护层厚度、石粉的掺量以及龄期4个自变量设计正交试验,对4个因素的重要性进行判定。结果表明:掺加石粉或矿渣后混凝土抗碳化能力均有所降低,其中以单掺20%石粉的混凝土抗碳化能力下降最为明显。石灰石粉-矿渣混凝土抗碳化能力随着水胶比的减小、石灰石粉比表面积的增大而变强。掺加不同比例的矿物掺合料对碳化结构可靠性均有一定减弱影响,对碳化结构可靠性的影响程度为水胶比>碳化龄期>混凝土保护层厚度>石灰石粉掺量,其中水胶比大小、石灰石粉掺量大小、碳化龄期长短与可靠度指标大小负相关,保护层厚度大小与可靠度指标大小呈正相关。