Reduction In Strength Of Concrete Research Articles

PERFORMANCE OF REINFORCED CONCRETE BEAM COLUMN CONNECTIONS EXPOSED TO FIRE UNDER CYCLIC LOADING

ABSTRACTFire could dramatically reduces the characteristic strength of the reinforced concrete elements [1,2]. A lot of efforts has been devoted to study the effect of fire on the reinforced concrete elements during past decades. These previous studies focused on evaluating fire resistance for concrete elements, [3] and provide some recommendations of structural fire-resistance [4] such as cover thickness, reinforcement ratio and coating materials. A lot of research focused on RC columns [5,6,7], beams [8] and slabs [9] which has been exposed to fire, where less efforts aimed to study the effect of fire on RC beam column connections [10,11,12].The main objective of the present work is to study the structural behavior of reinforced concrete beam-column connections exposed to fire under cyclic loading [13,14] and to evaluate the reduction in concrete strength during fire process. To achieve these goals, this research consists of two parts, the first part is experimental program, and the second is the theoretical analysis. The experimental program include 12 one- third scale specimens half of them casted from normal concrete and other half from SCC. The effect of some parameters on the connection's behavior like reinforcement ratio, type of concrete (normal or self-compacted concrete) [15] and exposure fire time were studied. Experimental program extended also to study heat distribution inside RC beam–column connections by measuring temperature at 65 point Distributors at 5 sections all over the RC connection. Theoretical analysis was done by using finite element program (ANSYS12.1) and a comparison between experimental program and theoretical analysis was done SCC showed good properties in hardening state when exposed to high temperature compared with normal concrete [16]. Increasing reinforcement ratio of RC connections increases its fire resistance Good agreement between analytical and experimental work was observed.

Design implications of a new load introduction mechanism into concrete-filled steel tubular columns

This paper presents the design implications of a new load introduction mechanism into concrete-filled steel tubular columns (CFST) proposed by the authors recently. According to the new load introduction mechanism, the beam-column connection load is introduced into the composite column through the column length above and within the connection. Based on the new load introduction mechanism, for slender CFST column design, a “concrete strength reduction factor” should be used to account for incomplete load introduction, when calculating the CFST column cross-section flexural stiffness. For CFST columns under combined axial compression and bending, the concrete strength reduction factor should be used when calculating the compression resistance, but should be ignored when calculating the bending resistance because composite action is always guaranteed for bending of the CFST column. The most important implication of the new load introduction mechanism is that the concrete core above the connection should be designed to resist the additional compressive force introduced to the concrete core below the connection. A further implication is that for ambient temperature design, the steel contribution ratio (steel section plastic resistance/plastic resistance of the composite cross-section) of the top floor column should be at least 0.25. For fire resistance design, the steel contribution ratio of the top floor columns, those on the floor below the top floor, and those two floors below the top floor, should not be less than 0.5, 0.33, and 0.25 respectively.

Porosity generation arising from steel fibre in concrete

The present study concerns a measurement of porosity at the steel fibre interface in mortar. Mortar cast with a centrally located steel fibre (Ø0.39×15.0mm) was segmented for microscopic observation at a backscattered electron (BSE) image. In the BSE image, the interfacial zone was defined as long as 270μm from the surface of steel fibre, where the porosity was measured by counting pixels after binarising the image for porosity. As a result, it was found that the pores generated in the vicinity of the steel fibre ranged from 5.40% to 11.45%, equivalent to 0.51–1.08% of air void in a bulk concrete. The high porosity at the interface may impose a reduction of concrete strength (i.e. up to a debondment with cement paste) and a potential risk of further supply of aggressive ions through the pores, leading to a lower durability against chemically severe environments.

Properties of rubberized concrete modified by using silane coupling agent and carboxylated SBR

Using waste rubbers as aggregates in concrete is a feasible approach to recycle the material. However, due to the poor adhesion of rubber to cement paste, the replacement of normal aggregate with rubber will lead to a reduction in the mechanical strength of concrete. This paper presents a new approach to overcome this deficiency by treating waste rubbers with silane coupling agent (SCA) and carboxylated styrene–butadiene rubber (CSBR) latex to develop chemical bonds between rubbers and cement paste. This study investigates the influence of this surface treatment on the mechanical strength and chloride penetration resistance of rubberized concrete. Experiments were conducted with six different volume fractions of rubbers (between 0% and 30%) to replace fine aggregate in the concrete. The experimental results showed that the compressive strength and flexural strength of concretes incorporating proper amount of treated rubbers could be improved by 4% and 13% respectively as compared to the control concrete with no rubbers. Moreover, the chloride penetration resistance was also improved by 35%. The good adhesion of waste rubbers to cement paste, the formation of rubber networks and the embedding effect of rubbers as shown in SEM images are explanations for the enhancement in the mechanical strength and durability for the treated rubber concrete.

칼슘 용출 열화에 의해 표면이 손상된 콘크리트 부재의 구조적 거동에 관한 해석적 연구

다양한 열화 인자에 의해 저감된 콘크리트의 내구성은 구조물의 구조적 성능과 사용 수명에 부정적인 영향을 미치게 되며 콘크리트 연구 분야 중에서도 매우 중요하고 매력적인 주제라고 할 수 있다. 이러한 이유로 콘크리트의 내구성과 관련된 많은 연구들이 발표되었으나 물리-화학적 열화에 기인하는 콘크리트의 본질적인 물성 변화에 주된 초점이 맞춰져 왔으며, 콘크리트 내구성과 구조물의 구조적 성능 사이의 관계 정립은 아직 미흡한 실정이다. 본 연구에서는 콘크리트의 강도 감소에 원인이 되는 칼슘 용출 열화를 적용하였으며, 열화 정도에 따른 구조적 거동을 평가하기 위하여 열화 손상을 입은 콘크리트 부재의 압축 및 휨 거동 실험을 수행하고 그 결과를 비선형 유한요소해석 결과와 비교 분석하였다. 연구 결과에 따르면 칼슘 용출 열화는 콘크리트의 압축 강도를 저하시키며, 열화가 진행됨에 따라 취성 거동에서 연성 거동으로 변화되는 경향을 나타냈다. 또한 열화에 의한 압축 영역의 손상 정도가 심화될수록 RC 부재의 내하력과 강성은 저하되었으며, 이러한 구조적 거동은 ABAQUS의 CDP 모델을 사용한 비선형 유한요소해석의 결과와도 비교적 잘 일치하였다. Durability is one of the most important and attractive subjects in concrete research field because not only durability of concrete is reduced by various degradation factors but also its reduction adversely influences the structural performance and service life of concrete structure. For this reason, a considerable amount of papers associated with concrete durability have been published and those researches were mainly focused on the changes of intrinsic properties of concrete due to chemicophysical degradations. However, the relationship between durability of concrete and structural behavior of concrete member has not been well established yet. In this study, calcium leaching degradation, a cause of concrete strength reduction, was dealt with. The experiments of compressive and flexural behavior of degraded concrete member were performed to evaluate the characteristics of structural behavior according to degradation level. Finally, the results from the experiments were compared with those obtained from nonlinear FEM analysis. The results from this study clearly showed that leaching degradation leads to decrease in compressive strength and compressive behavior evolves from brittle to ductile failure pattern during the degradation process. Load capacity and flexible rigidity of the degraded RC member decreased when the degradation level increased, in compressive zone. Additionally, it was found that the values from nonlinear FEM analysis, CDP model in ABAQUS, coincided well with the experimental results.

Resistance of Polymer Dispersion Additive Modified Concrete to Light Liquids Action

Numerous concrete and reinforced concrete structures (industrial floors, petrol station or manoeuvring park pavements, elements of sewage system) are subjected to danger of permanent or periodical action of petroleum liquids that often causes the considerable reduction in concrete strength as well as decrease of bond between concrete and steel, which usually results in structure damages. The subject of the paper is estimation of influence of polymer dispersion additive (styrene-acrylic co-polymer and styrene-butadiene latex) onto durability of concrete subjected to action of different types of light liquids. The influence of polymer dispersion additive was confronted with the action of silica fume usually applied for sealing the structure of concrete. In the paper there are presented tests results for different types of light liquids onto such cement-polymer concrete properties as: strength, penetration depth of light liquids into material structure.

Reliability of core test – Critical assessment and proposed new approach

Core test is commonly required in the area of concrete industry to evaluate the concrete strength and sometimes it becomes the unique tool for safety assessment of existing concrete structures. Core test is therefore introduced in most codes. An extensive literature survey on different international codes’ provisions; including the Egyptian, British, European and ACI Codes, for core analysis is presented. All studied codes’ provisions seem to be unreliable for predicting the in-situ concrete cube strength from the results of core tests. A comprehensive experimental study was undertaken to examine the factors affecting the interpretation of core test results. The program involves four concrete mixes, three concrete grades (18, 30 and 48MPa), five core diameters (1.5, 2, 3, 4 and 6in.), five core aspect ratios (between 1 and 2), two types of coarse aggregates (pink lime stone and gravel), two coring directions, three moisture conditions and 18 different steel arrangements. Prototypes for concrete slabs and columns were constructed. More than 500 cores were prepared and tested in addition to tremendous number of concrete cubes and cylinders. Results indicate that the core strength reduces with the increase in aspect ratio, the reduction in core diameter, the presence of reinforcing steel, the incorporation of gravel in concrete, the increase in core moisture content, the drilling perpendicular to casting direction, and the reduction in concrete strength. The Egyptian code provision for core interpretation is critically examined. Based on the experimental evidences throughout this study, statistical analysis has been performed to determine reliable strength correction factors that account for the studied variables. A simple weighted regression analysis of a model without an intercept was carried out using the “SAS Software” package as well as “Data Fit” software. A new model for interpretation of core test results is proposed considering all factors affecting core strength. The model when calibrated against large number of test data shows good agreement. The proposed model can effectively estimate the in-situ concrete cube strength from core test results.

Experimental Study on the Corrosion Law of Chloride in Concrete under Artificial Climate

Corrosion experimental were conducted in order to study concrete samples of two strength grades during different erosion period under the condition of designed artificial chloride erosion.Through drilling powder of concrete surface at various depths,chloride ions’concentration at different depths of corroded samples were determinated,and the diffusion coefficients of chloride ion after 90,180,240 days of corrosion were calculated on the basis of Fick’s second law.It turns out that obvious convection zone exitsin corroded concrete,and the thickness of convection zone increases with the erosing time and the reduction of concrete strength grade.In addtion,fitted datas of chloride ion diffusion coefficient increases with the reduction of concrete strength grade.

Experimental Research on Tensile Strength Reduction of Concrete Caused by Microwave Irradiation

In order to understand the law on concrete degradation induced by microwave irradiation, the experimental research is conducted in which the following works are completed. At first, the different concrete specimens are exposed to microwave of various power levels for various time periods, and then some specimen irradiated by microwave are cooled in air, the other cooled by spraying water. After that, the tensile strengths of the heated and unheated specimens are measured with Brazilian test method. According to the test results, the reduction of tensile strength of plain concrete and steel-fiber reinforced concrete induced by microwave irradiation is analyzed, and the relations between the tensile strength reduction and the microwave exposure time and the effect of microwave power level on the residual tensile strength of concrete are obtained. The experimental research shows that the tensile strength reduction of concrete induced by microwave irradiation is dependent on the concrete component, microwave exposure time, microwave power level, and cooling method after irradiation. It is expected that the findings in the paper will become the basis of the future research of concrete and rock degradation induced by microwave irradiation, and will aim in searching the new method of breaking concrete and rock more efficiently.

The Study on Bearing Capacity of Concrete-Filled Double Skin Steel Tubular Columns of Octagon Section under Axial Compression

Based on unified strength theory solution of thickness cylinder, according to the introducing concrete strength reduction factor and equivalent restriction factor, this paper deduced the axial compressive bearing capacity formula of concrete-filled double skin steel tubular columns of octagon section. Compared the results of the paper with the experiment results，both coincide in good condition. The results show that the theoretical formula are correct and viable. The theoretical formula is expected to provide a theoretical foundation to the research of concrete-filled double skin steel tubular columns of octagon section.And it provides references for the engineering design.

The effects of different sandstone aggregates on concrete strength

Sandstones vary in composition and consequently when used in concrete as aggregate may cause different concrete strengths. However, there are few data about correlating the effects of different sandstone aggregates. In this work we have highlighted some mechanical aspects concerning the use of different sandstones as concrete aggregate. The sandstone samples were first tested to determine their petrographic characteristics and aggregate properties. Then, concretes were prepared by using these aggregates, and fresh and hardened concrete properties were determined. The influence of different sandstone aggregates on the strength of the concrete was evaluated. According to the results obtained, subarkose–arkose, sublitharenite–litharenite and arkose aggregates which have clay cement caused approximately a 40–50% reduction in concrete strength when compared to subarkose, quartz sandstone and arkose aggregates which have carbonate cement, because these aggregates result in weaker bonding between aggregate and cement than others.

Compressive Strength Reduction of Concrete Subjected to Hydrostatic Compression

An experimental study of the damage behavior of two kinds of concrete with different strength grades has been performed using 100mm cubes subjected to increasing hydrostatic loading history, namely, the isotropic compression at high pressure. The compressive strength and ultrasonic velocity are measured before and after loading history, respectively. The damage degree of these cubes is defined as the reduction of compressive strength based on the continuum damage mechanics theory. Linear and exponential curve fit of experimental data is performed respectively to describe the evolution of damage as well as the descent of ultrasonic velocity with respect to the loading history. It can be seen that, the influence of hydrostatic loading history upon strength and ultrasonic velocity could really reflect that upon the degree of damage development. In general, ultrasonic inspection is convenient and applicable to estimation of damage of concrete due to loading history in engineering practice.

Biaxial Stress due to Shrinkage in Concrete Jackets of Strengthened Columns

The effect of restrained concrete shrinkage on the strength of concrete used to strengthen existing structures is investigated in this study. A series of compressive tests are performed on square prism concrete specimens where steel plates are placed in order to restrain free shrinkage. Results indicate that there is a significant reduction of concrete strength (20 to 30%) due to a biaxial stress state induced by the restrained concrete shrinkage. The experimental results are used to validate a numerical procedure proposed to simulate concrete shrinkage. A numerical investigation is then conducted on models that represent the concrete of a reinforced concrete jacket applied to an original column for strengthening purposes where the degree of restraint is different to that of the examined square prism specimens. It was found that that there is a further reduction of concrete strength (30 to 40%).

Test Results and Model for the Residual Compressive Strength of Concrete After a Fire

An investigation into temperature induced degradation of the compressive strength of concrete including that under cooling phase is carried out. The paper gathers and reviews a considerable amount of test data, considering the influence of different test parameters such as initial compressive strength, aggregate type, cooling regime and specimen shape. It is found that the compressive strength of concrete at high temperature is in accordance with the model proposed in the Eurocodes for calcareous concrete. However, during cooling phase, an additional reduction of compressive strength in concrete is observed, which can be as high as 20% of the initial strength for elevated temperatures around 500°C. Finally, a generic concrete model for temperature dependent compressive strength, accounting for both growth and cooling phase of temperature is proposed. The model can be used for simulating fire response of concrete structures subjected to natural fires or for the evaluation of residual load capacity of concrete structures after fire.

Ultimate Bearing Capacity Research on the Steel Tube Composite Column Filled with Steel Reinforced Concrete

The ultimate load-bearing capacities of axially-loaded steel tube composite column filled with steel reinforced concrete under three-dimensional stress based on the unified strength theory are analyzed in this paper. The influence of thickness-length ratio and scale effect are considered by introducing the reduction factor of equivalent constraints and concrete strength reduction factor, respectively. The nonlinear three-dimensional finite element analysis of the steel tube composite column filled with steel reinforced concrete is performed by the finite element software ANSYS. The numerical and the analytical results are compared with experimental results and good agreement can be observed. A series of numerical simulation technologies is studied and described in detail, such as selecting element type, defining material model of steel and concrete, establishing global finite element model with discrete reinforced bars elements, applying loads to the specimens, and setting solution controls option. The results indicate that ANSYS finite element software may well simulate the behavior of the steel tube composite column filled with steel reinforced concrete under axial compression through reasonably selecting parameters.

Seismic analysis of reinforced concrete frame-wall systems considering ductility effects in accordance to Eurocode

This paper presents the results of experimental and numerical analysis of the seismic resistance of a reinforced concrete coupling system considering the plastic capacity in accordance with the standard requirements STN ENV 1998 (2005), ENV 1998 (2003), Önorm B4015 (2002) and FEMA 368 (2001). The plastic capacity of the structure can be established by parameter q in the case of the spectral analysis to determine the seismic response. The experience from dynamic analysis of a hospital structure in accordance with standard requirements is presented in this paper. Dynamic parameters of the building structure are checked by experiment and the calculation model is modified on the basis of the experiment. The nonlinear analysis of the coupling system was realized in the program CRACK under system ANSYS for Kupfer’s failure condition and Červenka’s model of the concrete strength reduction.

Testing and comparison of concrete barriers containing shredded waste tire chips

In this study, concrete specimens containing shredded waste tire chips were evaluated using laboratory and field tests. A total of 24 cylindrical specimens, 15 cm × 30 cm in size, were tested to determine the compressive strength of concrete at 7 and 28 days. For the dynamic tests, on the other hand, 6 New Jersey shaped concrete barriers were constructed using the identical mix designs used in the static tests. Results of the study showed that tire addition reduced the compression strength and modulus of elasticity of specimens. Dynamic impact tests show that increase in tire percentage has significant effect on the reduction in vehicle peak deceleration forces and thus impact severity. Based on the static and dynamic test results, it can be concluded that specimens with 20 to 40% aggregate replacement gives the best impact performances without significant reduction in concrete strength. Concrete barriers containing larger amounts of tire can be used at highway applications where concrete fracture is desired for energy dissipation, such as crash cushion and end treatment applications.

Correlation of strength, rubber content, and water to cement ratio in rubberized concrete

Waste rubber tires that cannot be processed for useful applications are numbered in the millions around the world. The build up of old rubber tires in landfills is commonly considered a major threat to the environment, and it is unquestionably a burden on landfill space. This research project was an investigation into the possibility of using fine rubber particles in concrete mixtures. The experimental testing program was designed to study the effect of the addition of crumb rubber, as replacement of a portion of fine aggregates (sand), on the strength of concrete. Rubber was added to concrete in quantities of 5%, 10%, and 15% by volume of the mixture. Three different water/cement ratios were used: 0.47, 0.54, and 0.61. A total of 180 concrete cubes were made. The cubes were tested in compression at 1, 7, 14, 21, and 28 d with the load continuously and automatically measured until failure. The load values were used to calculate compressive stress as related to different rubber contents and water/cement ratios. Compression test results were used to develop several plots relating rubber content and water/cement ratio to compressive stress of concrete. Test results gathered in this research project indicated that the addition of crumb rubber to concrete results in a reduced strength as compared with that of conventional concrete. Based on the experimental results, correlations have been developed to estimate the reduction in concrete strength as a function of the rubber content in the mix.Key words: compressive strength, concrete, crumb rubber, rubberized concrete.

Artificial Neural Networks in Prediction of Concrete Strength Reduction Due to High Temperature

This paper demonstrates how artificial neural networks (ANN) can be used to predict the residual strength of heated concrete. The authors report on ongoing research that is trying to account for the effect of exposure time (to high temperatures) on residual strength of concrete. Results reveal that loss of strength becomes significant as exposure times are prolonged at exposure temperatures of 400 deg C. and higher. Analyses of residual pulse velocity and residual strength shows that a certain linear relationship exists for exposure times between 30 and 120 min. Due to the absence of a theoretical relationship, neural network analysis is applied to identify a possible general relationship between residual strength and variables including exposure temperature, exposure time, water-cement ratio (w/c), and residual pulse velocity. The authors note that ANN bears better tolerance than regression analysis to an imprecise data set. The authors conclude that good linear relationships are found when comparing the residual strength predicted by networks with correspondent target values. This type of testing may prove valuable as part of the assessment for fire-damaged concrete prior to the repair of structures.

Performances of concrete under elevated curing temperature

The behaviors of concrete at elevated curing temperature were studied. The test results show that when concrete is cured at elevated temperature, a harmful consequence occurs. The later strength decreases significantly compared to that under normal curing condition. Incorporating silica fume, fly ash and slag or lowering w/c ratio can effectively alleviate this harmful consequence. Comparatively, incorporation of silica flume is the most efficient means to decrease the later strength reduction. The harmful consequence is not caused by the difference in degree of hydration since the degree of hydration is similar between elevated curing temperature and normal curing condition. The SEM analysis shows that it is the uneven distribution of hydration products caused by elevated curing temperature that leads to the later strength reduction of concrete.