Tensile Stresses In Concrete Research Articles

Determination of Tensile Creep and Stress Relaxation of Concrete by Ring Test

The understanding of stress relaxation and tensile creep behavior is extremely important in accurate stress analysis and crack prediction of early-age concrete. The free shrinkage deformations of concrete with different strength grade were examined. The early-age tensile elastic modulus of concrete was investigated through temperature-stress testing machine. The tensile creep and shrinkage stress were obtained through the modified restrained ring test. The results indicate that the development of free shrinkage coordinates well with the inner strain of steel ring. Tensile creep decreases as water-binder ratio increases. Creep counteracts tensile stress of concrete by 28%~40% , decreases the possibility of cracking of concrete at early ages.

THE INFLUENCE OF THE PRESTRESS OF REINFORCED STEEL ON THE BEHAVIOUR OF THE FLEXURAL ELEMENTS OF REINFORCED CONCRETE / ARMATŪROS IŠANKSTINIŲ ĮTEMPIŲ ĮTAKA GELŽBETONINIŲ LENKIAMŲJŲ ELEMENTŲ ELGSENAI

This article briefly discusses the essence of prestressed concrete,its advantages and disadvantages. The analysis of prestress lossesis done according to different standards. The paper explainspretensioning force and selection of its eccentricity as well asanalyzes the influence of beam cracking according to limitationson concrete tensile and compressive stresses. Santrauka Publikacijoje glaustai aptarta įtemptojo gelžbetonio esmė, pranašumai ir trūkumai. Atlikta nuostolių vertinimo skirtingų normų metodikomis skaičiavimo analizė. Analizuojamas racionalios armatūros įtempimo jėgos ir jos pridėties ekscentriciteto parinkimas. Nagrinėjama leistinųjų betono tempimo ir gniuždymo įtempių įtaka sijų pleišėjimui.

비개착공법으로 건설된 지하구조물 상부 콘크리트 도상 안정성에 관한 연구

최근 철도주변의 개발이 확대되고 있는 가운데 비개착공법을 이용하여 철도노반하부를 횡단하는 구조물을 시공하는 경우가 불가피하게 증가하고 있다. 현재, 기존선 하부를 횡단하는 구조물을 시공하는 경우 구조물 및 열차주행에 대한 안정성만을 고려하고 있으며, 구조물 주변에 대한 안정성 검토 규정은 미비한 실정이다. 따라서 본 연구에서는 실내모형실험을 통하여 비개착공법시 주변지반의 특성에 대해서 파악하였고, 이를 토대로 3D 이동하중을 고려한 수치해석을 통해 콘크리트도상의 동적 거동특성에 대하여 파악하였다. 변위의 경우 토피고가 낮아짐에 따라 접속부와 구조물과의 변위차이는 크게 나타났다. 응력은 토피고가 3 m일 때는 안전한 것으로 나타났으나 1m, 2m일 경우 허용인장응력을 초과하는 것으로 나타났다. As the development of around railway area, the subsurface construction using non-opencut method under the railway has been recently increased. When the structure under railway is constructed, the track support stiffness of the ground is not considered as an important design factor. In this study, laboratory tests were performed to investigate the ground movement characteristics nearby the subsurface structure. Based on the laboratory test results, 3D numerical analyses were performed to find out the dynamic characteristics of the concrete track. The numerical analyses results showed that displacements above the structure were varied with the soil thickness. When soil thickness is 3m, tensile stress of the results was less than allowable tensile stress. However, when the soil thickness are 1m and 2m, concrete tensile stress on the concrete track exceeds the allowable tensile stress.

Cracking of the concrete cover due to reinforcement corrosion: A two-dimensional lattice model study

Corrosion of steel reinforcement is a serious problem for durability and serviceability of reinforced concrete. As the reinforcing steel corrodes, it expands and exerts pressure on the surrounding concrete cover, causing tensile stresses in concrete. This ultimately leads to cracking and spalling of the concrete cover, further exacerbating the durability problems of a structure and increasing the rate of its deterioration. In order to study cracking mechanisms due to reinforcement corrosion, mechanics of the problem was implemented in a two-dimensional lattice model. Heterogeneous nature of concrete was taken into account in the mechanical analysis. Firstly, the case of uniform corrosion was tested, and successfully verified using experimental data from the literature. Also, it was found that cracking pressure is not a deterministic value, and depends on local mechanical parameters. Secondly, two pitting scenarios were tested and compared to the uniform corrosion case. Pitting results in significant reduction of cracking pressure, compared to the uniform corrosion case. Based on the proposed model, some conclusions with implications for engineering practice were drawn.

Development of Temperature and Stress during Foundation Slab Concreting of National Supercomputer Centre IT4

In the paper the method of experimental temperature and stress measurement of reinforced concrete foundation slab during construction of the National Supercomputing Centre in Ostrava, Czech Republic is described. Measured results are compared with numerical analysis and time dependent development of concrete strength. Decreasing the tensile stress in concrete by use of the sliding joint was verified.

Thermal Stress Numerical Simulation on Concrete Hydration Heat of Giant Floor in Deep Foundation Pit

The finite element analysis software, during the pouring of concrete for the temperature field and stress field in a numerical simulation analysis. Analysis showed that the concrete core temperature changes greater maximum thermal stress in mass concrete bottom and around the site and soil transfer, far less than the concrete tensile stress, the temperature will not crack.

The Discussion on Deflected Pre-Stressed Tendons and Diagonal Cracks of Webs in PSC Box Girders

Abstract In order to find the relationship between deflected pre-stressed tendons and diagonal cracks of webs and provide basis to design and construction of bridges, a characteristics section model of a long span pre-stressed concrete continuous box girder, which is now under construction in Chongqing is analyzed by using nonlinear finite element program Midas-FEA. A numerical simulation analysis to the tensile stress in the webs was down based on the actual construction situation. As the research shows: 1, Stress concentration exists at the point where pre-stressed tendons are anchored, and the stress value was higher in anchorage area, which accord with the Saint-Venant Principle.2, The main tensile stress of webs in the anchorage area is lower than 1.04MPa, which did not surpass the standard 2.74MPa of the concrete tensile stress of C55, as a result, webs would not craze from finite element analysis. 3, Cracks brought by other normal or abnormal reasons existed in the process of casting the cantilever...

Control Crack Width by Nominal Tensile Stress for RC Beams

In order to convenient application in optimum design of large and complex structures, theoretical formula of nominal tensile stress of concrete is deduced relating to the maximum crack width of RC beam in serviceability. By further statistical analysis of test data in certain reliability, simplified and practical formulas are proposed for calculating the nominal tensile stress of concrete. Therefore, the control of maximum crack width is replaced by control of nominal tensile stress of concrete, which shall be easier to realize in optimum design of concrete structures.

Determination of the Main Parameters of Alkali Silica Reaction Using System Identification Method

Although concrete is widely considered a very durable material, if conditions are such, it can be vulnerable to deterioration and early distress development. Alkali-silica reaction (ASR) is a major durability problem in concrete structures. The product of this reaction is a gel that is hygroscopic in nature. When the gel absorbs moisture, it swells leading to tensile stresses in concrete. When those stresses coming from the gel swelling exceed the tensile strength of concrete, cracks occur. This paper is part of a broader research study to assist engineers to effectively mitigate ASR in concrete, leading to an increase in the life span of concrete structures. To achieve this objective, a comprehensive study on different types of aggregates of different reactivity was conducted to formulate a robust approach that takes into account the factors affecting ASR, such as temperature, moisture, calcium concentration, and alkalinity. A kinetic model was proposed to determine aggregate ASR characteristics which we...

Extended cracked membrane model for the analysis of RC panels

A new model considering fixed and interlocked cracks for the analysis of reinforced concrete (RC) cracked membrane elements is presented. The model can be regarded as an extension of the cracked membrane model proposed by Kaufmann and Marti. Its value lies in eliminating the need to resort to empirical averaged stress–strain relations for the reinforced concrete material, while being sufficiently simple for implementation in a robust finite element formulation. Equilibrium is formulated in terms of stresses at the cracks, enabling the use of fundamental theoretical models for the individual mechanical phenomena governing shear behaviour, such as concrete compression softening, aggregate interlock (including crack dilatancy effects) and tensile bridging stresses. Reinforcement stresses at the cracks are obtained from the average strains assuming a stepped rigid plastic bond shear stress–slip law according to the tension chord model. This allows for a consistent treatment of the tension stiffening effects and a proper evaluation of steel deformation capacity in the post-yielding stage. A rational and quantitative description of the stress/strain fields both at the cracks and in between the cracks is derived, which allows a deeper understanding of the complex mechanics involved in shear behaviour of cracked membrane elements. A validation campaign was undertaken using a database with the experimental results of 54 RC panels subjected to in-plane shear and axial stresses. The failure modes, failure loads and stress–strain curves are, in general, accurately predicted.

Influence of Early Temperature Rise on Movements and Stress Development in Concrete Decks

The primary focus of this paper is to develop an understanding of temperature changes introduced by hydration heat release in the first few hours after casting in the thermal movements and stresses of the concrete deck and girders. Temperature and strain measurements from a simply supported, single-span, steel girder bridge with a composite concrete deck are presented. It is shown that setting occurs during the temperature rise and partial strain compatibility between steel girder and concrete deck is initiated at the end of the concrete temperature rise. Full strain compatibility between concrete deck and steel girder is achieved at the end of the cooling period following the initial temperature rise. The stresses in the steel girder associated with temperature changes are interpreted using an analytical model. It is shown that the concrete deck gains sufficient stiffness at the end of the temperature rise to restrain the movement of the top flange. Concrete deck movement in the period associated with cooling following the initial temperature rise is restrained, which could potentially produce tensile stress in concrete. The magnitude of tensile stress at the end of the cooling period depends upon the difference in the temperatures of the concrete deck and top flange and on the temperature gradient in the steel girder at the end of the heating period.

Time-dependent early-age behaviors of concrete under restrained condition

To investigate the early-age behaviors of concrete under a restrained condition, a set of apparatus was developed. In this way, the tensile creep and other early-age properties can be investigated in depth. By measuring the modulus of elasticity of concrete, synchronous shrinkage of concrete and steel rings and free shrinkage of concrete, the deformations of concrete ring can be quantified respectively. The experimental results show the tensile stress in concrete is time-dependent, and the stress at cracking is much lower than the tensile strength at that age; the tensile creep plays an important role in relaxing the tensile stress and postponing the cracking of concrete.

Influence of Shrinkage-Reducing Admixtures on Development of Plastic Shrinkage Cracks

Cracking that forms between concrete placement and concrete setting is generally described as plastic shrinkage cracking. The authors discuss how concave menisci may form on fresh concrete surfaces due to water evaporation. Potential development of plastic shrinkage cracks may occur when concrete surface tensile stress and concrete settlement develop because of the menisci. The authors focus on how mortars containing a shrinkage-reducing admixture (SRA), which is commercially available, develop plastic shrinkage cracks. When exposed to the same environmental conditions, fewer and narrower plastic shrinkage cracks are shown in SRA-containing mortar than plain mortar. The authors propose that pore fluid of SRA-containing mortar has lower surface tension, which can result in lower crack-inducing stresses at the mortar's topmost layer, reduced capillary tension, reduced settlement, and less evaporation.

Crack Resistance Analyses on RC Composite Floor Slab

Cracking behavior of reinforced concrete composite floor slab, which has a significant effect on durability of concrete structures, is studied. Crack resistance of one-way composite floor slab is analyzed during transfer stage, handling stage, construction stage and serviceability stage, and calculation formulas are presented. Precast prestressed plank and topping cast-in-situ concrete layer can be designed as composite floor slab. It is assumed that prestressed unit is in elastic stage under service loads and hence every stress subentry can be calculated on the base of linear elastic theory and can be added linearly. For two-way composite floor slab, principal tensile stress in concrete is deduced on the base of theory of three-dimensional stress state in material mechanics. In addition, two-way effect on stress and practical distribution pattern of moment and shear force are considered. A number of structure experiment and experiences from engineering practice show that cracking behavior of reinforced concrete composite floor slab is a necessary consideration in design and analysis method shown in this paper is convenient and effective.

Nonlinear analysis of beam–column joints using softened truss model

The aim of this study is to expand the application of the nonlinear softened truss model for membrane elements on beam–column joints. The softened truss model employs three equations for equilibrium, three for compatibility and four equations for the constitutive laws of materials. The constitutive equations for both the concrete and steel are based on the actually observed stress–strain relationships. The model has three important attributes. The first is the nonlinear association of stress and strain. The second, and conceivably more noteworthy, is the softening of concrete in compression due to tensile strains in the perpendicular direction. The third is that the influence of the concrete tensile stresses between cracks on the average stress–strain relationship for reinforcing steel and the influence of orthogonal tensile stresses on the compression stress–strain relationship for concrete can be considered in the model. For beam–column joints, one of the most important factors influencing the behaviour is certainly the bond conditions of the beam bars. In this study, the softened truss model is expanded to take into account the influence of this important factor into account. In the revised version of the model, full strain compatibility does not exist between the steel reinforcement and the surrounding concrete and thus the factors influencing the bond-slip between concrete and reinforcement is adequately considered. The improved softened truss model is applied on 51 exterior beam–column joint tests. It is apparent from the results that the revised model gives very accurate predictions of the shear strength of joints and is an improvement on the existing version of the model proposed by Hsu.

Explaining the Riddle of Tension Stiffening Models for Shear Panel Experiments

Analyses of reinforced and prestressed concrete structures based on a smeared crack approach generally include a tension stiffening relationship to estimate the average concrete tensile stress after cracking. Many such equations have been developed over the years and show significant differences between them. In this study, three of these equations are compared and it is shown that the variation between the equations can be explained by the different bond conditions of the specimens tested to calibrate the equations. An expression is proposed to quantify this effect and sample results are shown. Use of this relationship should allow more realistic estimates of crack width and stiffness at service loads to be obtained.

Determination of crack spacing and crack width in reinforced concrete beams

In this paper spacing and width of flexural cracks in reinforced concrete beams are determined using two-dimensional finite element analysis. At early loading stages on the beam the primary crack spacing is based on the slip length, which is the development length required to resist the steel stress increment that occurs at a cracked section on the formation of the first flexural crack. A semi-empirical formula is presented in this paper for the determination of the slip length for a given beam. At higher load levels, the crack spacing is based on critical crack spacing, which is defined as the particular crack spacing that would produce a concrete tensile stress equal to the flexural strength of concrete. The resulting crack width is calculated as the relative difference in extensions of steel reinforcement and adjacent concrete evaluated at the cracked section. Finally a comparative study is undertaken, which indicates that the spacing and width of cracks calculated by this method agree well with values measured by other investigators.

鋼板コンクリート(SC)構造スラブの保有火災耐力

To examine properties and load-carrying capacity of composites slabs of steel plates with steel studs and concrete with top reinforcements in ISO fire, fire tests and analyses of heat transfer and elasto-plastic stress-deformation were carried out. And a simple calculation method to evaluate load-carrying capacity of composites slabs was proposed. This calculation method based on simple plastic theory on the viewpoint of considering in strength degradation zone of concrete by heat transfer during fire. After this study, the following conclusion could be obtained. 1) The composites slabs designed within the limit of ordinary live load have flexural failure mode in fire and the load-carrying capacity of composite slabs can be calculated by using the formula proposed. 2) Composite slabs with steel studs having shear stress more than 0.5N/mm^2 collapse by shear stress on the initial stage of ISO fire. This shear failure is caused by decreasing of shear strength of concrete due to giving rise to tensile stress in concrete. Tensile stress in concrete increase on the initial stage of fire, because of the restraint of elongation of steel plates through steel studs. 3) However, generally, composite slabs have axial restraint more than 30% of full axial restraint in structure, the decreasing of shear strength of concrete doesn't occurred during fire, so shear failure can be avoided.

Development of Rapid Solutions for Prediction of Critical Continuously Reinforced Concrete Pavement Stresses

A rapid solution is presented for predicting critical tensile stresses on the top surface of continuously reinforced concrete pavements (CRCP). These tensile stresses are responsible for the development of CRCP punchouts and have to be considered in a mechanistic-based design procedure. The solution is based on a combination of the neural network (rapid solution) and finite element (numerical analysis) techniques. This approach combines the convenience and computational efficiency of neural network solutions with the flexibility and power of the finite element analysis. Such a combination is quite efficient for analyzing damage accumulation in CRCP, which requires predicting portland cement concrete tensile stresses for a large number of loading and site condition combinations. The procedure for stress prediction is based on the finite element model developed with ISLAB2000. The neural network has been trained with the results from ISLAB2000. A concept developed specifically for this study—the equivalent CRCP structure—was used extensively to reduce the number of independent parameters of the neural net work and speed up its training. The proposed rapid solution provides a good match of the ISLAB2000 stress values for a small fraction of the computation cost. This makes the rapid solution CRCP a natural choice for analyzing CRCP stresses and for inclusion in a mechanistic-empirical CRCP design procedure.

Analytical Prediction of Flexural Behavior of Concrete Beams Reinforced by FRP Bars

An analytical model to predict the flexural strength and deformations in concrete beams reinforced with fiber reinforced plastic (FRP) bars is presented. It is based on the equilibrium of forces and compatibility of deformation. The model uses a more rational representation of the behavior of all composite constituents. It also accounts for the concrete tensile stresses, the variation of stresses, strains and curvature over the depth of the cross section and the length of the beam. The model predicts the moment-curvature, load-deflection, and load-strain of any of the composite constituents. The model predictions were checked against the experimental results of nine test beams reinforced with steel or FRP bars. The behavior of the concrete tested beams were characterized by their moment-curvature, load-strain in the reinforcement, and load-deflection response up to failure. The results indicate that the analytical model predicts the behavior of GFRP reinforced concrete beams with good accuracy. Furthermore, the accuracy of the model was checked by comparing its capacity prediction with that of a parallel model available in the literature based on measured data and reported by other researchers. Results of the comparison showed that the suggested model provided better prediction.