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Showing 78 results for Concrete

Mazloom M., Ramezanian Pour A.a.,
Volume 2, Issue 1 (3-2004)

This paper presents the long-term deformations of reinforced high-strength concrete columns subjected to constant sustained axial forces. The objective of the study was to investigate the effects of binder systems containing different levels of silica fume on time-dependent behaviour of high-strength concrete columns. The experimental part of the work focused on concrete mixes having a fixed water/binder ratio of 0.35 and a constant total binder content of 500 kg/m3. The percentages of silica fume that replaced cement in this research were: 0%, 6%, 8%, 10% and 15%. The mechanical properties evaluated in the laboratory were: compressive strength secant modulus of elasticity strain due to creep and shrinkage. The theoretical part of the work is about stress redistribution between concrete and steel reinforcement as a result of time-dependent behaviour of concrete. The technique used for including creep in the analysis of reinforced concrete columns was age-adjusted effective modulus method. The results of this research indicate that as the proportion of silica fume increased, the short-term mechanical properties of concrete such as 28-day compressive strength and secant modulus improved. Also the percentages of silica fume replacement did not have a significant influence on total shrinkage however, the autogenous shrinkage of concrete increased as the amount of silica fume increased. Moreover, the basic creep of concrete decreased at higher silica fume replacement levels. Drying creep (total creep - basic creep) was negligible in this investigation. The results of the theoretical part of this researchindicate that as the proportion of silica fume increased, the gradual transfer of load from the concrete to the reinforcement decreased and also the effect of steel bars in lowering the concrete deformation reduced. Moreover, the total strain of concrete columns decreased at higher silicafume replacement levels.
Kheyr Aldin A., Mortezaei A.r.,
Volume 2, Issue 1 (3-2004)

Structural walls are used extensively in moderate- and high-rise buildings to resist lateral loads induced by earthquakes. The seismic performance of many buildings is, therefore, closely linked to the behavior of the reinforced concrete walls. The analytical models used in this paper are developed to study the push-over response of T-shaped reinforced concrete walls andinvestigate the influence of the flange walls on laterally loaded walls and nonlinear behavior of shear walls, namely strength, ductility and failure mechanisms. A layered nonlinear finite element method is used to study the behavior of T-shaped and rectangular (barbell) shear walls. This paper introduces a computer program to practically study three-dimensional characteristics of reinforced concrete wall response by utilizing layered modeling. The program is first verified bysimulated and reported experimental response of 3-D reinforced concrete shear walls. Subsequently, a study considering eighteen analytical test specimens of T-shaped and barbell shear walls is carried out. Finally, based on analytical results, a new equation for minimum ratio of shear wall area to floor-plan area is proposed.
M.r Esfahani , M.r Kianoush, M. Lachemi ,
Volume 2, Issue 3 (9-2004)

This paper compares the results of two experimental studies on bond strength of steel and GFRP bars in the case of self-consolidating concrete (SCC). Each study included pull-out tests of thirty six reinforcing bars embedded in concrete specimens. Two types of concretes, normal concrete and self-consolidating concrete were used in different studies. Different parameters such as bar location and cover thickness were considered as variables in different specimens. The comparison between the results of GFRP reinforcing bars with those of steel deformed bars showed that the splitting bond strength of GFRP reinforcing bars was comparable to that of steel bars in both normal strength and self-consolidating concrete (SCC). The bond strength of bottom reinforcing bars was almost the same for both normal concrete and self-consolidating concrete. However, for the top bars, the bond strength of self-consolidating concrete was less than that of normal concrete.
Ghodrati Amiri G., Sedighi S.,
Volume 2, Issue 4 (12-2004)

In the past decade design procedure changed to �performance-based design� from�force-based design�, by this mean many researchers focused on nonlinear static analysis (NSA)and the procedure named �PUSHOVER�. Advantages of this method are defining the inelasticbehavior of structure without nonlinear dynamic analysis (NDA) effort and also defining plastichinges formation in critical elements, and the order of formed plastic hinges. In spite of these goodadvantages NSA is limited to short and planar structures and application of that in tall andtorsionaly asymmetric structures may yield unreliable results.In this study reliability of NSA is investigated by performing both nonlinear static and dynamicanalysis on six 2D moment resisting concrete frames. Non linear dynamic analysis has been doneby the suggested method in FEMA356 guideline called �Target Displacement Method�. A groupof 4 different lateral increasing loads were used in pushover analysis and 3 different groundmotions were applied in NDA. Results indicate that same responses can be obtained by performingNSA, but errors will be increased by frames height increment.
Khalou A.r., Ghara Chour Lou A.,
Volume 3, Issue 1 (3-2005)

This paper presents the results of analytical studies concerning the flexuralstrengthening of reinforced concrete beams by external bonding of high-strength lightweightcarbon fiber reinforced plastic (CFRP) plates to tension face of the beam. Three groups of beamswere tested analytically and compared with existing experimental results. Results of the numericalanalyses showed that, although addition of CFRP plates to the tension face of the beam increasesthe strength, it decreases the beam ductility. Finite element modeling of fifteen different beams in aparametric study indicates that steel area ratio, CFRP thickness, CFRP ultimate strength andelastic modulus considerably influence the level of strengthening and ductility.
M. Naderi,
Volume 3, Issue 1 (3-2005)

Having observed the costly failures of different cutoff walls, that had been constructed according to the mix design specified by reputable consultants in Iran, a research programme was conducted to study the effects of constituent materials on the properties of plastic concrete. The main properties, such as compressive strength, biaxial and triaxial strains, permeability, and modulus of elasticity have been investigated using different mixes, obtained from prototype production line plant, situated on site, because it was realized that the site production line and the systems employed have major effects on the properties of plastic concrete. Statistical analysis of the results, revealed the coefficients of influence of main constituent materials of plastic concrete namely cement, bentonite, aggregate and water on its compressive strength and modulus of elasticity. Having realized the cancelling effects of bentonite and aggregates on the measured properties, some equations relating the quantities of cement and water to the compressive strength and modulus of elasticity are introduced. Effects of clay and hydrated lime powder, as fillers were also investigated leading to the proposal of limits for their safe and economic use. Since most of the cutoff walls are buried structures, failure strains under both uniaxial and triaxial tests, with values of cohesion and internal friction, are also presented in this paper.
Sabouri Ghomi S., Kharazi M.h.k., Asghari A., Javidan P.,
Volume 3, Issue 1 (3-2005)

Design and construction of efficient and economic Reinforced Concrete (R.C.) Hyperbolic Cooling Towers have driven the engineers toward the design of tall and thin-shell towers which have considerable high slenderness aspect ratio. Consequently, the shell of R.C. Cooling Towers with relative high slenderness aspect ratio is extremely prone to buckling instability due to wind loading. To increase the structural stability or buckling safety factor, one economic approach is to design and construct stiffening rings for the R.C. Hyperbolic Cooling Towers. Despite the research previously performed to determine the effect of stiffening rings on the buckling behavior of the R.C. Hyperbolic Cooling Towers, information resulting in maximum buckling stability is absent considering the optimized utilization of the quantity and dimension as well as the location of this type of stiffeners. In this paper, not only the effect of the stiffening rings on the buckling stability of the R.C. Cooling Tower is studied but also the optimized location,quantity and dimension of the stiffening rings are carried out for a sample RC Cooling Tower. The dimensions of the selected sample cooling tower are in average typical dimensions which are used in the current practice. In this study, finite element (F. E.) analyses has been carried out to define the buckling modes and resistance of this tower due to wind loading for different number of stiffening ring configurations. Based on the conducted buckling analysis, the optimized number, location and dimension of the stiffening rings that maximizes the tower.s buckling stability are defined and the methodology to achieve this information is discussed in this paper.
Khaloo R., Sharifian M.,
Volume 3, Issue 3 (9-2005)

Results of an experimental investigation performed to evaluate the effect of various concrete strength levels on behavior of lightweight concrete (LWC) under pure torsion are reported.The principle variable of the testing program was compressive strength of concrete (�'c) which ranged between 6.9 and 81.4 MPa. Ten mixture proportions were utilized for LWC of 1500 to 2050 kg/m3 unit weight. In total, sixty four (thirty two pairs) rectangular specimens with 100x 200 mm cross-section were tested. Ultimate torsion strength of LWC increases as uniaxial compressive strength increases however the increase rate reduces for high levels of concrete strengths. The test results are compared with predictions of elastic and plastic theories for torsion and the ACI Code. The Code underestimates the cracking torque of LWC under pure torsion. A regression equation incorporating test results is higher than the ACI equation prediction by a factor of 1.12.
H. Oucief, M.f. Habita, B. Redjel,
Volume 4, Issue 2 (6-2006)

In most cases, fiber reinforced self-compacting concrete (FRSCC) contains only one type of fiber. The use of two or more types of fibers in a suitable combination may potentially not only improve the overal properties of self-compacting concrete, but may also result in performance synergie. The combining of fibers, often called hybridization, is investigated in this paper for a cimentetious matrix. Control, single, two fibers hybrid composites were cast using different fiber type steel and polypropylene with different sizes. Flexural toughness tests were performed and results were extensively analysed to identify synergy, if any, associated with various fiber combinations. Based on various analysis schemes, the paper identifies fiber combinations that demonstrate maximum synergy in terms of flexural toughness.
M. Naderi,
Volume 4, Issue 2 (6-2006)

This paper introduces an innovative partially destructive method, called “Twist-off”, for the assessment of in situ concrete strength. In this method a 40mm diameter metal probe is bonded to a concrete surface by means of a high strength epoxy resin adhesive. To measure the concrete compressive strength, a torque is applied using an ordinary torque-meter and the maximum shear stress at failure is used to estimate the cube compressive strength by means of a calibration graph. The relationship between the results of this new method and compressive strengths of concrete cores is also presented in this paper. The average coefficient of variation of the results of this method was seen to be of the order of 8 percent and the correlation coefficients of its comparative results with concrete cube and core compressive strengths were found to be 0.97 and 0.90 respectively. In order to assess the performance of this method on site, tests were undertaken on a number of buildings. Although the method was found to perform well but with some of the structures tested, the differences between the strengths of sample cubes and estimated in situ compressive strength of concrete were seen to be significant.
M.h. Baziar, Sh. Salemi, T. Heidari,
Volume 4, Issue 3 (9-2006)

Seismic behavior of a rockfill dam with asphalt-concrete core has been studied utilizing numerical models with material parameters determined by laboratory tests. The case study selected for these analyses, is the Meyjaran asphalt core dam, recently constructed in Northern Iran, with 60 m height and 180 m crest length. The numerical analyses have been performed using a nonlinear three dimensional finite difference software and various hazard levels of earthquakes. This study shows that due to the elasto-plastic characteristics of the asphalt concrete, rockfill dams with asphalt concrete core behave satisfactorily during earthquake loading. The induced shear strains in the asphalt core, for the case presented in this research, are less than 1% during an earthquake with amax=0.25g and the asphalt core remains watertight. Due to large shear deformations caused by a more severe earthquake with amax=0.60g, some cracking may occur towards the top of the core (down to 5-6 m), and the core permeability may increase in the top part, but the dam is safe.
F.m. Wegian, M.t. Alkhamis, S.r. Sabbagh Yazdi,
Volume 4, Issue 4 (12-2006)

This study evaluates two different types of techniques for concrete hollow-block sections reinforced with traditional steel rebars and wire meshes, and compares their structural behaviour to that of an ordinary reinforced concrete beam section. The comparisons are based on the responses both before and after they were repaired with glass fibre reinforced polymers (GFRP). The specimens were subjected to concentrated loading up to initial failure. After failure, the specimens were repaired and loaded once again until ultimate failure. It was shown that the success of the repair by GFRP depended on the mode of failure of the hollow-block concrete beams.
Ali Kheyroddin, Hosein Naderpour,
Volume 5, Issue 1 (3-2007)

A parametric study is performed to assess the influence of the tension reinforcement index, ( ω = ρ fy /f Bc), and the bending moment distribution (loading type) on the ultimate deformation characteristics of reinforced concrete (RC) beams. The analytical results for 15 simply supported beams with different amounts of tension reinforcement ratio under three different loading conditions are presented and compared with the predictions of the various formulations and the experimental data, where available. The plastic hinge rotation capacity increases as the loading is changed from the concentrated load at the middle to the third-point loading, and it is a maximum for the case of the uniformly distributed load. The effect of the loading type on the plastic rotation capacity of the heavily reinforced beams is not as significant as that for the lightly reinforced beams. Based on the analytical results obtained using the nonlinear finite element method, new simple equations as a function of the tension reinforcement index, ω, and the loading type are proposed. The analytical results indicate that the proposed equations can be used for analysis of ultimate capacity and the associated deformations of RC beams with sufficient accuracy.
P. Ghoddousi, A.m. Raiss Ghasemi, T. Parhizkar,
Volume 5, Issue 4 (12-2007)

Plastic shrinkage is one of the most important parameter which must be considered in hot weather concreting. If plastic shrinkage is not prevented, cracking will be significant, especialy if silica fume is used in the mix. In this paper, the effect of silica fume in bleeding and evaporation was investigated in laboratory. The results showed that in restrained shrinkage, beside relative humidity, temperature and wind velocity, sun rediation also is very important factor in evaporation rate. It is found that under solar radition condition, the evaporation was much larger than the estimated value in ACI 305 Nomogram. The rate of evaporaion under solar radiation was about two folds of evaporation rate under shade condition. The results showed that in terms of crack initiation time, crack width and total cracking area, concrete containing silica fume is more severe than concrete with no silica fume. Reduction of water cement ratio in concrete with silica fume makes the concrete more sensitive in cracking. The results of this project also showed that the severity of the cracking is not related only to rate of bleeding but all environmental factors including like sun radiation or shading and also mix compositions have important roles.
A. Foroughi-Asl, S. Dilmaghani, H. Famili,
Volume 6, Issue 1 (3-2008)

Self-Compacting Concrete (SCC) is a highly fluid yet stable concrete that can flow consistently under its own weight, pass between bars, and fill in formwork without the need of compaction. The application of SCC effectively resolves the difficulties of concreting in situations with complicated formwork and congested reinforcements. In this paper, the bond between SCC and steel reinforcement was investigated. The bonding strengths of reinforcing bars were measured using cubic specimens of SCC and of normal concrete. The SCC specimens were cast without applying compaction, whereas the specimens of normal concrete were cast by conventional practice with substantial compaction and vibration. The results showed that SCC specimens generated higher bond to reinforcing bars than normal concrete specimens and the correlation between bond strength and compressive strength of NC is more consistent.
Alireza Mortezaei, Ali Kheyroddin,
Volume 7, Issue 1 (3-2009)

The work presented in this paper investigates the causes of size effects in structural-concrete members. It is

based on the use of a finite-element model found to yield realistic predictions of structural-concrete behavior in all

cases investigated to date. In fact, the previous use of this model in investigations of size effects in reinforced-concrete

beams indicated that such effects reflect the dependence of load-carrying capacity on small unintended eccentricities

of the applied load and/or load-induced anisotropy, rather than, as widely considered, on fracture-mechanics

characteristics. The present work extends the scope of the above investigation so as to include the case of reinforced

concrete flanged shear walls, the behavior of which is already established experimentally. It is found that, unlike the

flanged shear walls with a height-to-length ratio larger than 2, the shear walls investigated in the present work, in

contrast with the interpretation given to recently published experimental findings, are size-effect independent.

A.a. Ramezanianpour, M. Mahdi Khani, Gh. Ahmadibeni,
Volume 7, Issue 2 (6-2009)

Rice Husk Ash (RHA) is a by-product of the agricultural industry which contains high amount of silicon dioxide (SiO2). In this research, for the first time in the Middle East, in order to supply typical RHA, a special furnace was designed and constructed in Amirkabir University of Technology. Afterwards, XRD and XRF techniques were used to determine the amorphous silica content of the burnt rice husk. Attempts were made to determine the optimum temperature and duration of burning. Results show that temperature of 650 degrees centigrade and 60 minutes burning time are the best combination. Then various experiments were carried out to determine properties of concretes incorporating optimum RHA. Tests include compressive strength, splitting tensile strength, modules of elasticity, water permeability and rapid chloride permeability test. Results show that concrete incorporating RHA had higher compressive strength, splitting tensile strength and modulus of elasticity at various ages compared with that of the control concrete. In addition, results show that RHA as an artificial pozzolanic material has enhanced the durability of RHA concretes and reduced the chloride diffusion.
I. Rasoolan, S.a. Sadrnejad, A.r. Bagheri,
Volume 7, Issue 2 (6-2009)

Concrete is a heterogeneous material with a highly non linear behavior, which is mainly caused by the

initiation and propagation of micro cracks within the several components of the material. The damage behavior of

concrete is usually simulated on the macro scale using complex constitutive models. The direct determination of the

homogenized material parameters is often difficult and sometimes impossible. Furthermore these materials models do

not explicitly represent effects and bond behaviors of interfaces between the several components. So in order to predict

of concrete behaviors and characteristics, it should be modeled as a three phase composite material consisting of

aggregate, interfacial transition zone (ITZ) and cement paste. The size and distribution of aggregate affects concrete

characteristics. Because of the random distribution and size variation of aggregate in concrete, the modeling of

concrete behavior based on component in meso structure is difficult and so we must use simple assumption. In this

paper with mixing design and grading curve we developed a simple method to replace real aggregate with equivalent

sphere aggregate with effective diameter. So we can use simple methods instead of complex numeral and randomness

or x ray methods to find effective diameter and use it to determine two arrangements with maximum and minimum

aggregate volume as a repeatable basical element .As a result we can use this element to modeling the behavior of

sample concrete in meso scale and three phases.

M. Saiidi, C. Cruz, D. Hillis,
Volume 8, Issue 1 (3-2010)

Three unconventional details for plastic hinges of bridge columns subjected to seismic loads were developed,

designed, and implemented in a large-scale, four-span reinforced concrete bridge. Shape memory alloys (SMA),

special engineered cementitious composites (ECC), elastomeric pads embedded into columns, and post-tensioning

were used in three different piers. The bridge model was subjected to two-horizontal components of simulated

earthquake records of the 1994 Northridge earthquake in California. The multiple shake table system at the University

of Nevada, Reno was used for testing. Over 300 channels of data were collected. Test results showed the effectiveness

of post-tensioning and the innovative materials in reducing damage and permanent displacements. The damage was

minimal in plastic hinges with SMA/ECC and those with built in elastomeric pads. Conventional reinforced concrete

plastic hinges were severely damaged due to spalling of concrete and rupture of the longitudinal and transverse

reinforcement. Analytical studies showed close correlation between the results from the OpenSEES model and the

measured data for moderate and strong earthquakes.

A.r. Khaloo, I. Eshghi, P. Piran Aghl,
Volume 8, Issue 3 (9-2010)

In this paper the response of cantilevered reinforced concrete (RC) beams with smart rebars under static lateral loading has been numerically studied, using Finite Element Method. The material used in this study is SuperelasticShape Memory Alloys (SE SMAs) which contains nickel and titanium elements. The SE SMA is a unique alloy that has the ability to undergo large deformations and return to their undeformed shape by removal of stresses. In this study, different quantities of steel and smart rebars have been used for reinforcement andthe behavior of these models under lateral loading, including their load-displacement curves, residual displacements, and stiffness, were discussed. During lateral loading, rebars yield or concrete crushes in compression zone in some parts of the beams and also residual deflections are created in the structure. It is found that by using SMA rebars in RC beams, these materials tend to return to the previous state (zero strain), so they reduce the permanent deformations and also in turn create forces known as recovery forces in the structure which lead into closing of concrete cracks in tensile zone. This ability makes special structures to maintain their serviceability even after a strong earthquake

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