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CONTENTS
Volume 24, Number 1, July 2019
 

Abstract
Within the scope of this study, the foam solution was prepared by properly mixing sulfonate based foam agent with water. Furthermore, this solution was mixed with the mixture of fine sand, cement, and water to produce foamed concrete. The mixture ratios which are the percentage of foam solution used in foam concrete were chosen as 0, 20, 40 and 60% by vol. After these groups reached 28 days of strength, they were heated to 20, 100, 400 and 700oC respectively. Afterward, high-temperature effects on the foamed concrete were obtained by employing physical and mechanical properties tests. Additionally, SEM (scanning electron microscope) and EDX (energy-dispersive X-ray spectroscopy) tests were employed to analyze the microstructure, and u-CT (micro computed tomography) images were used to reconstruct 3-D models of the heat-treated specimens. Then, these models are analyzed to examine the void structures and the changes in these structures due to the high temperatures. The study has shown that the void structures reduce the high-temperature effects and the foam solution could be mixed with concrete up to 40 % by vol. where the high strength of foamed concrete is non-mandatory.

Key Words
foamed concrete; high-temperature effects; microstructure; SEM; EDX; u-CT

Address
Mehmet Canbaz, Hafid Dakman, Baris Arslan: Eskişehir Osmangazi University, Civil Engineering Department, 26480, Eskisehir, Turkey
Arda Büyüksungur: Middle East Technical University, Biotechnology Research Lab., 06800, Ankara, Turkey

Abstract
The complex phenomenon of the bond formation in geopolymer is not well understood and therefore, difficult to model. This paper present applied statistical models for evaluating the compressive strength of geopolymer. The applied statistical models studied are divided into three different categories - linear regression [least absolute shrinkage and selection operator (LASSO) and elastic net], tree regression [decision and bagging tree] and kernel methods (support vector regression (SVR), kernel ridge regression (KRR), Gaussian process regression (GPR), relevance vector machine (RVM)]. The performance of the methods is compared in terms of error indices, computational effort, convergence and residuals. Based on the present study, kernel based methods (GPR and KRR) are recommended for evaluating compressive strength of Geopolymer concrete.

Key Words
geopolymer concrete; modelling; compressive strength; kernel

Address
Prabhat Ranjan Prem, A. Thirumalaiselvi and Mohit Verma: CSIR-Structural Engineering Research Centre, Chennai, India

Abstract
In this paper, the effect of the type and amount of fibers on the physicomechanical properties of concrete containing fine recycled refractory brick (RRB) and natural aggregate subjected to elevated temperatures was investigated. For this purpose, forta-ferro (FF), polypropylene (PP), and polyvinyl alcohol (PVA) fibers with the volume fractions of 0, 0.25, and 0.5%, as well as steel fibers with the volume fractions of 0, 0.75, and 1.5% were used in the concrete containing RRB fine aggregate replacing natural sand by 0 and 100%. In total, 162 concrete specimens from 18 different mix designs were prepared and tested in the temperature groups of 23, 400, and 800oC. After experiencing heat, the concrete properties including the compressive strength, ultrasonic pulse velocity (UPV), weight loss, and surface appearance were evaluated and compared with the corresponding results of the reference (unheated) specimens. The results show that using RRB fine aggregate replacing natural fine aggregate by 100% led to an increase in the concrete compressive strength in almost all the mixes, and only in the PVA-containing mixes a decrease in strength was observed. Furthermore, UPV values at 800oC for all the concrete mixes containing RRB fine aggregate were above those of the natural aggregate concrete specimens. Finally, regarding the compressive strength and UPV results, steel fibers demonstrated a better performance relative to other fiber types.

Key Words
recycled refractory brick; fiber-reinforced concrete; elevated temperatures; ultrasonic pulse velocity; weight loss; mechanical properties

Address
Mahdi Nematzadeh and Ardalan Baradaran-Nasiri: Department of Civil Engineering, University of Mazandaran, Babolsar, Iran

Abstract
In this study, the shear behaviour of reinforced concrete (RC) beams that were retrofitted using precast panels of ultra-high performance fiber reinforced concrete (UHPFRC) is presented. The precast UHPFRC panels were glued to the side surfaces of RC beams using epoxy adhesive in two different configurations: (i) retrofitting two sides, and (ii) retrofitting three sides. Experimental tests on the adhesive bond were conducted to estimate the bond capacity between the UHPFRC and normal concrete. All the specimens were tested in shear under varying levels of shear span-to-depth ratio (a/d=1.0; 1.5). For both types of configuration, the retrofitted specimens exhibited a significant improvement in terms of stiffness, load carrying capacity and failure mode. In addition, the UHPFRC retrofitting panels glued in three-sides shifted the failure from brittle shear to a more ductile flexural failure with enhancing the shear capacity up to 70%. This was more noticeable in beams that were tested with a/d=1.5. An approach for the approximation of the failure capacity of the retrofitted RC beams was evolved using a multi-level regression of the data obtained from the experimental work. The predicted values of strength have been validated by comparing them with the available test data. In addition, a 3-D finite element model (FEM) was developed to estimate the failure load and overall behaviour of the retrofitted beams. The FEM of the retrofitted beams was conducted using the non-linear finite element software ABAQUS.

Key Words
ultra-high performance fiber reinforced concrete; shear behaviour; retrofitting; epoxy agglutinant; bond capacity; failure mode; finite element model

Address
Mohammed A. Al-Osta: Department of Civil & Environmental Engineering, King Fahd University of Petroleum & Minerals (KFUPM), Dhahran 31261, Saudi Arabia

Abstract
Compressive ability is one of the most important mechanical properties of concrete material .The compressive failure process of concrete is pretty complex with internal tension, shear damage and friction between cracks. To simulate the complex fracture process of concrete at meso level, methodology for meso-structural analysis of concrete specimens is developed; the zero thickness cohesive elements are pre-inserted to simulate the crack initiation and propagation; the constitutive applied in cohesive element is established to describe the mechanism of crack separation, closure and friction behavior between the fracture surfaces. Aseries of simulations were carried out based on the model proposed in this paper. The results reproduced the main fracture and mechanical feature of concrete under compression condition. The effect of key material parameters, structure size, and aggregate content on the concrete fracture pattern and loading carrying capacities was investigated. It is found that the inner friction coefficient has a significant influence on the compression character of concrete, the compression strength raises linearly with the increase of the inner friction coefficient, and the fracture pattern is sensitive to the mesostructure of concrete.

Key Words
concrete; mesostructure; fracture pattern; constitutive; cohesive element; friction

Address
Yi-qun Huang: College of Mechanics and Materials, Hohai University, 1 Xikang Road, Nanjing, China
Shao-wei Hu: School of Civil Engineering, Chongqing University, 174 Shazheng Road, Chongqing, China

Abstract
Thermal energy from high temperatures can cause concrete damage, including mechanical and chemical degradation. In view of this, the residual mechanical properties of high-strength fiber reinforced concrete with a design strength of 75 MPa exposed to 400-800oC were investigated in this study. The test results show that the average residual compressive strength of high-strength fiber reinforced concrete after being exposed to 400-800oC was 88%, 69%, and 23% of roomtemperature strength, respectively. In addition, the benefit of steel fibers on the residual compressive strength of concrete was limited, but polypropylene fibers can help to maintain the residual compressive strength and flexural strength of concrete after exposure to 400-600oC. Further, the load-deflection curve of specimen containing steel fibers exposed to 400-800oC had a better fracture toughness.

Key Words
fiber reinforced concrete; residual mechanical properties

Address
Chao-Wei Tang: Department of Civil Engineering & Geomatics, Cheng Shiu University, No. 840, Chengching Rd., Niaosong District, Kaohsiung City, Taiwan R.O.C.; Center for Environmental Toxin and Emerging-Contaminant Research, Cheng Shiu University, No. 840, Chengching Rd., Niaosong District, Kaohsiung 83347, Taiwan; Super Micro Mass Research & Technology Center, Cheng Shiu University,
No. 840, Chengching Rd., Niaosong District, Kaohsiung 83347, Taiwan

Abstract
Concrete reinforced with fiber reinforced polymer (FRP) bars (FRP-RC) has attracted a significant amount of research attention in the last three decades. Alimited number of studies, however, have investigated the effect of bond slip on the performance of FRP-RC columns under eccentric loading. Based on previous experimental study, a finite-element model of eccentrically loaded FRP-RC columns was established in this study. The bondslip behavior was modeled by inserting spring elements between FRP bars and concrete. The improved Bertero-Popov-Eligehausen (BPE) bond slip model with the results of existing FRP-RC pullout tests was introduced. The effect of bond slip on the entire compression-bending process of FRP-RC columns was investigated parametrically. The results show that the initial stiffness of bond slip is the most sensitive parameter affecting the compression-bending performance of columns. The peak bond stress and the corresponding peak slip produce a small effect on the maximum loading capacity of columns. The bondslip softening has little effect on the compression-bending performance of columns. The sectional analysis revealed that, as the load eccentricity and the FRP bar diameter increase, the reducing effect of bond slip on the flexural capacity becomes more obvious. With regard to bond slip, the axial-force-bendingmoment (P-M) interaction diagrams of columns with different FRP bar diameters show consistent trends. It can be concluded from this study that for columns reinforced with large diameter FRP bars, the flexural capacity of columns at low axial load levels will be seriously overestimated if the bond slip is not considered.

Key Words
FRP reinforced concrete; bond slip; compression-bending performance; P-M interaction diagram

Address
Chunyang Zhu, Li Sun: Civil Engineering School, Shenyang Jianzhu University, Hunnan Road 9#, Shenyang,110168, China
Ke Wang: China Communications Construction Company First Highway Consultants Co. Ltd., Keji 2nd Road 63#, Xi\'an 710075, China
Yue Yuan: Highway Administration Bureau of Liaoning Communications Department, 13Wei Road 19#, Shenyang 100168, China
Minghai Wei: Civil Engineering School, Shenyang Jianzhu University, Hunnan Road 9#, Shenyang,110168, China

Abstract
The ACI building code is allowing for higher strength reinforcement and concrete compressive strengths. The nominal strength of high-strength concrete columns is over predicted by the current ACI 318 rectangular stress block and is increasingly unconservative as higher strength materials are used. Calibration of a rectangular stress block to address this condition leads to increased computational complexity. A triangular stress block, derived from the general shape of the stressstrain curve for high-strength concrete, provides a superior solution. The nominal flexural and axial strengths of 150 highstrength concrete columns tests are calculated using the proposed stress distribution and compared with the predicted strength using various design codes and proposals of other researchers. The proposed triangular stress model provides similar level of accuracy and conservativeness and is easily incorporated into current codes.

Key Words
high-strength concrete; flexural and axial strengths; triangular stress block; interaction curve; column; beam

Address
Mustafa Kamal Al-Kamal: Department of Civil Engineering, Al-Nahrain University, Baghdad, Iraq

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