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CONTENTS
Volume 5, Number 2, April 2008
 

Abstract
This research examines the engineering properties and color intensities of mortar containing different amounts of fly ash (0, 5, 10 and 20%) mixed at different water-to-binder ratios (w/b = 0.23, 0.47 and 0.59) and exposed at different temperatures (T = 25, 100, 200, 400, 600 and 800oC). Results show that there is greater mass loss on ignition with high w/b and higher temperatures. In addition, the color channel image analyzer (Windows software written in Delphi) is utilized to study the relationship between the curing temperature and intensity of three primary colors, red, green and blue (RGB), of the fly ash mortar specimens. The results show that the RGB intensities on the specimen surface increases from that at 25oC. The mortar specimen becomes white with increase in w/b but without the addition of fly ash. Moreover, for mortar specimens with greater content of fly ash, red on the specimen surface has the greatest increase in intensity at elevated temperature. Observation the variations in color on the specimen surface may help estimate the highest elevated temperatures that concrete structures can withstand.

Key Words
elevated temperature; fly ash; mortar; image analyzer.

Address
Department of Civil Engineering, National Kaohsiung University of Applied Sciences, Kaohsiung, 807, Taiwan

Abstract
The microscopic determination of air void characteristics in hardened concrete, defined in EN 480-11 as the linear-traverse method, is an extremely time-consuming and tedious task. Over past decades, several researchers have proposed relatively expensive mechanical automated systems which could replace the human operator in this procedure. Recently, the appearance of new high-resolution flatbed scanners has made it possible for the procedure to be automated in a fully-computerized and thus cost-effective way. The results of our work indicate the high sensitivity of such image analysis automated systems firstly to the quality of sample surface preparation, secondly to the selection of the air void threshold value, and finally to the selection of the probe system. However, it can be concluded that in case of careful validation and the use of the approach which is proposed in the paper, such automated systems can give very good estimate of the air void system parameters, defined in EN 480-11. The amount of time saved by using such a procedure is immense, and there is also the possibility of using alternative stereological methods to assess other, perhaps also important, characteristics of air void system in hardened concrete.

Key Words
concrete; air void system; linear-traverse method; image analysis; high-resolution flatbed scanner; air void threshold value.

Address
University of Ljubljana, Faculty of Civil and Geodetic Engineering, Jamova 2, 1000 Ljubljana, Slovenia

Abstract
An experimental and numerical research was conducted to gain a deeper insight on the structural behaviour of deep-beams with indirect supports and to assess the size effects in the ultimate state behaviour. The experimental campaign focused on the influence of the reinforcement tie distribution height on the compression check of the support region and on the benefits of using unbonded prestressing steel. Three reduced scale specimens were tested and used to validate the results obtained with a nonlinear finite element model. As a good agreement could be found between the numerical and the experimental results, the numerical model was then further used to perform simulations in large scale deep-beams, with dimensions similar to the ones to be adopted in a practical case. Two sources of size effects were identified from the simulation results. Both sources are related to the concrete quasi-brittle behaviour and are responsible for increasing failure brittleness with increasing structural size. While in the laboratory models failure occurred both in the experimental tests as well as in the numerical simulations after reinforcement yielding, the numerically analysed large scale models exhibited shear failures with reinforcement still operating in the elastic range.

Key Words
non-linear analysis; non-linear fracture mechanics; deep-beams; shear; strut-and-tie models; size effects; unbonded post-tensioning.

Address
Mario Pimentel; LABEST, University of Porto, Faculty of Engineering, Rua Dr. Roberto Frias s/n 4200-465 Porto, Portugal
Paulo Cachim; University of Aveiro, 3810-193 Aveiro, Portugal
Joaquim Figueiras; LABEST, University of Porto, Faculty of Engineering, Rua Dr. Roberto Frias s/n 4200-465 Porto, Portugal

Abstract
This paper presents a nonlinear finite element analysis procedure for the seismic performance assessment of reinforced concrete bridge piers with unbonded reinforcing or prestressing bars. A computer program named RCAHEST (Reinforced Concrete Analysis in Higher Evaluation System Technology) is used to analyze reinforced concrete structures; this program was also used in our study. Tensile, compressive and shear models of cracked concrete and models of reinforcing and prestressing steel were used account for material nonlinearity of reinforced concrete. The smeared crack approach was incorporated. To represent the interaction between unbonded reinforcing or prestressing bar and concrete, an unbonded reinforcing or prestressing bar element based on the finite element method was developed in this study. The proposed numerical method for the seismic performance assessment of reinforced concrete bridge piers with unbonded reinforcing or prestressing bars is verified by comparison of its results with reliable experimental results.

Key Words
seismic performance; reinforced concrete bridge piers; unbonded reinforcing or prestressing bars; material nonlinearity; finite element method.

Address
T.-H. Kim1, J.-G. Park2, Y.-J. Kim1 and H. M. Shin*2
1Civil Engineering Research Team, Daewoo Institute of Construction Technology, 60 Songjuk-dong, Jangan-gu, Suwon, Kyonggi-do, 440-210, Korea
2Department of Civil and Environmental Engineering, Sungkyunkwan University, 300 Chunchun-dong, Jangan-gu, Suwon, Kyonggi-do, 440-746, Korea

Abstract
A new approach for nonlinear finite element analysis of corroded reinforcements in RC structures is elaborated in the article. An algorithmic procedure for producing the tension-stiffening curve of RC elements taking into consideration most of effective parameters, e.g.: the rate of steel bar corrosion, bond-slip behavior, concrete cover and amount of reinforcement, is illustrated. This has been established on both experimental and analytical bases. This algorithm is implemented into a nonlinear finite element analysis program. The abilities of the resulted program have been studied by modeling some experimental specimens showing a reasonable agreement between the analytical and experimental findings.

Key Words
nonlinear finite element method; reinforced concrete; tension-stiffening; bond-slip; corrosion.

Address
Mohsen A. Shayanfar; Civil Engineering Department, Iran University of Science and Technology,
Narmak 16846, Tehran, Iran
Amir Safiey; Moshanir Consultants Engineering Inc., Park Prince Buildings, Vanak, Tehran, Iran


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