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CONTENTS
Volume 14, Number 3, September 2014
 

Abstract
Prestressed hollow-core slabs (HCS) are widely used for modern lightweight precast floor structures because they are cost-efficient by reducing materials, and have excellent flexural strength and stiffness by using prestressing tendons, compared to reinforced concrete (RC) floor system. According to the recently revised ACI318-08, the web-shear capacity of HCS members exceeding 315 mm in depth without the minimum shear reinforcement should be reduced by half. It is, however, difficult to provide shear reinforcement in HCS members produced by the extrusion method due to their unique concrete casting methods, and thus, their shear design is significantly affected by the minimum shear reinforcement provision in ACI318-08. In this study, a large number of shear test data on HCS members has been collected and analyzed to examine their web-shear capacity with consideration on the minimum shear reinforcement requirement in ACI318-08. The analysis results indicates that the minimum shear reinforcement requirement for deep HCS members are too severe, and that the web-shear strength equation in ACI318-08 does not provide good estimation of shear strengths for HCS members. Thus, in this paper, a rational web-shear strength equation for HCS members was derived in a simple manner, which provides a consistent margin of safety on shear strength for the HCS members up to 500 mm deep. More shear test data would be required to apply the proposed shear strength equation for the HCS members over 500 mm in depth though.

Key Words
prestressed; hollow-core; slab; web shear; cracking; shear stress distribution

Address
Deuck Hang Lee1a, Min-Kook Park, Jae-Yuel Oh and Kang Su Kim: Department of Architectural Engineering, University of Seoul, 5 Seoulsiripdae-ro, Dongdaemun-gu, Seoul 130-804, Republic of Korea

Ju-Hyeuk Im and Soo-Yeon Seo: Department of Architectural Engineering, Korea National University of Transportation, 50 Daehak-ro, Chungju-si, Chungbuk 380-702, Republic of Korea


Abstract
In this paper, a rule based Mamdani type fuzzy logic model for prediction of slippage at maximum tensile strength and slippage at rupture of structural lightweight concretes were discussed. In the model steel rebar diameters and development lengths were used as inputs. The FL model and experimental results, the coefficient of determination R2, the Root Mean Square Error were used as evaluation criteria for comparison. It was concluded that FL was practical method for predicting slippage at maximum tensile strength and slippage at rupture of structural lightweight concretes.

Key Words
bond strength; bending; structural lightweight concrete; fuzzy logic approach

Address
Mehmet E. Arslan: Department of Civil Engineering, Düzce University, 81620, Düzce, Turkey

Mehmet E. Arslan: Department of Civil Engineering, Karadeniz Technical University, 61080, Trabzon, Turkey

Abstract
Partial replacement of Portland cement by slag can reduce the energy consumption and CO2 emission therefore is beneficial to circular economy and sustainable development. Compressive strength is the most important engineering property of concrete. This paper presents a numerical procedure to predict the development of compressive strength of slag blended concrete. This numerical procedure starts with a kinetic hydration model for cement–slag blends by considering the production of calcium hydroxide in cement hydration and its consumption in slag reactions. Reaction degrees of cement slag are obtained as accompanied results from the hydration model. Gel-space ratio of hardening slag blended concrete is determined using reaction degrees of cement and slag, mixing proportions of concrete, and volume stoichiometries of cement hydration and slag reaction. Furthermore, the development of compressive strength is evaluated through Powers\'gel-space ratio theory considering the contributions of cement hydration and slag reaction. The proposed model is verified through experimental data on concrete with different water-to-binder ratios and slag substitution ratios.

Key Words
slag; hydration model; compressive strength; microstructure; concrete

Address
Wang Xiao-Yong: Department of Architectural Engineering, Kangwon National University, Chuncheon, Korea

Lee Han-Seung: School of Architecture & Architectural Engineering, Hanyang University, Ansan, Korea

Abstract
This study investigated four electric field configurations of two-dimensional accelerate lithium migration technique (ALMT), including line-to-line, plane-to-line, contour-to-line and plane-to-plane, and analyzed the ion migration behavior and efficiency. It was found that the free ion distribution diagram and voltage distribution diagram were similar, and ions migrated in the power line direction. The electrode modules were used for the mortar specimen with w/c ratio of 0.5. The effectively processed areas accounted for 14.1%, 39.0%, 49.4% and 51.4% of total area respectively on Day 28. Larger electrode area was more advantageous to ion migration. In addition, it was proved that the two-dimensional electric field could be divided into different equifield line active regions, and regarded as affected by one-dimensional electric field, and the ion migration results in various equifield line active regions were predicted by using the duration analysis method based on the theoretical model of ion migration obtained from one-dimensional test.

Key Words
electrochemical; alkali-silica reaction; ALMT; lithium

Address
Chih-Chien Liu: Department of Civil Engineering, ROC Military Academy, No.1, Wei-Wu Rd., Fengshan District, Kaohsiung 83059, Taiwan, R.O.C

Wen-Ten Kuo: Department of Civil Engineering, National Kaohsiung University of Applied Sciences, No. 415, Chien-Kung Rd., Sanmin District, Kaohsiung 80778, Taiwan, R.O.C

Abstract
This paper investigates the effects of confinement reinforcement and concrete strength on nonlinear behaviour of reinforced concrete buildings (RC). For numerical application, an eleven-storey and four bays reinforced concrete frame building is selected. Nonlinear incremental static (pushover) analyses of the building are performed according to various concrete strengths and whether appropriate confinement reinforcement, which defined in Turkish seismic code, exists or not at structural elements. In nonlinear analysis, distributed plastic hinge model is used. As a result of analyses, capacity curves of the frame building and moment-rotation curves at lower end sections of ground floor columns are determined. These results are compared with each other according to concrete strength and whether appropriate confinement reinforcement exists or not, respectively. According to results, it is seen that confinement reinforcement is important factor for increasing of building capacity and decreasing of rotations at structural elements.

Key Words
confinement reinforcement; concrete strength; distributed plastic hinge; nonlinear pushover analysis

Address
Burak Yön and Yusuf Calay

Abstract
Effects of polypropylene (PP) fibers, steel fibers (SF) and hybrid on the properties of high-strength fiber reinforced self-consolidating concrete (HSFR-SCC) under different volume contents are investigated in this study. Comprehensive laboratory tests were conducted in order to evaluate both fresh and hardened properties of HSFR-SCC. Test results indicated that the fiber types and fiber contents greatly influenced concrete workability but it is possible to achieve self consolidating properties while adding the fiber types in concrete mixtures. Compressive strength, dynamic modulus of elasticity, and rigidity of concrete were affected by the addition as well as volume fraction of PP fibers. However, the properties of concrete were improved by the incorporation of SF. Splitting tensile and flexural strengths of concrete became increasingly less influenced by the inclusion of PP fibers and increasingly more influenced by the addition of SF. Besides, the inclusion of PP fibers resulted in the better efficiency in the improvement of toughness than SF. Furthermore, the inclusion of fibers did not have significant effect on the durability of the concrete. Results of electrical resistivity, chloride ion penetration and ultrasonic pulse velocity tests confirmed that HSFR-SCC had enough endurance against deterioration, lower chloride ion penetrability and minimum reinforcement corrosion rate.

Key Words
polypropylene fibers; steel fibers; high-strength self-consolidating concrete; mechanical properties; durability

Address
Bui Le Anh Tuan: Department of Civil Engineering, Can Tho University, Campus II, 3/2 street, Ninh Kieu district, Can Tho city, Vietnam

Mewael Gebregirogis Tesfamariam,Chao-Lung Hwang and Chun-Tsun Chen: Department of Construction Engineering, National Taiwan University of Science and Technology, 43, Keelung Rd., Sec. 4, Taipei 106, Taiwan, R.O.C.



Abstract
Concrete is an important material in most of civil constructions. Many properties of concrete can be determined through analysis of concrete images. Image segmentation is the first step for the most of these analyses. An automated system for segmentation of concrete images into microstructures using texture analysis is proposed. The performance of five different classifiers has been evaluated and the results show that using an Artificial Neural Network classifier is the best choice for an automatic image segmentation of concrete.

Key Words
microstructural analysis; image segmentation; FLD; KNN; artificial neural networks; SVM; bayesian classification; co-occurrence matrix; texture analysis

Address
Mehran Yazdi: Department of Electronics and Computer Engineering,Shiraz University, Shiraz, Iran

Katayoon Sarafrazi: Department of Electronics and Computer Engineering, Shiraz University, Shiraz, Iran

Abstract
This paper presents the application of multi-gene genetic programming (MGP) technique for modeling the bond strength of ribbed steel bars in concrete. In this regard, the experimental data of 264 splice beam tests from different technical papers were used for training, validating and testing the model. Seven basic parameters affecting on the bond strength of steel bars were selected as input parameters. These parameters are diameter, relative rib area and yield strength of steel bar, minimum concrete cover to bar diameter ratio, splice length to bar diameter ratio, concrete compressive strength and transverse reinforcement index. The results show that the proposed MGP model can be alternative approach for predicting the bond strength of ribbed steel bars in concrete. Moreover, the performance of the developed model was compared with the building codes\' empirical equations for a complete comparison. The study concludes that the proposed MGP model predicts the bond strength of ribbed steel bars better than the existing building codes\' equations. Using the proposed MGP model and building codes\' equations, a parametric study was also conducted to investigate the trend of the input variables on the bond strength of ribbed steel bars in concrete.

Key Words
genetic programming; bond strength; ribbed steel bars; concrete

Address
Department of Civil and Environmental Engineering, Amirkabir University of Technology,
424 Hafez Ave, Tehran, Iran

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