https://doi.org/10.4334/JKCI.2020.32.2.117

권순철(Sun-Chul Gwon) ; 김선희(Sun-Hee Kim) ; 최원창(Won-Chang Choi)

Concrete confined with both steel pipe and fiber reinforced plastic tube termed as confined concrete filled steel tube. Confining effect from steel pipe and fiber reinforced plastic tube can increase compressive strength of confined concrete filled tube and is the main element for compression design. However, design code that clearly consider the combined confining effect of steel and FRP have not been established to date. This study propose the prediction equation for compressive strength of confined concrete filled tube. This paper presents the experimental results of the axial compression tests of concrete cylinders confined with both steel tube and glass fiber reinforced plastic tube. The main parameter of the compression test is the thickness of glass fiber reinforced plastic tubes (0, 2, 4, 6 mm). The test result shows that compressive strength increases with the thickness of glass fiber reinforced plastic tube. The prediction equation for compressive strength of confined concrete filled tube were proposed by analyzing the test results. In addition, the analytical results obtained using the proposed model and previously proposed model from other studies were compared with test results.

https://doi.org/10.4334/JKCI.2020.32.2.127

최승원(Seung-Won Choi) ; 이민희(Min-Hi Lee) ; 이기열(Gi-Yeol Lee)

This study proposes the maximum reinforcing steel ratio of reinforced concrete flexural member using the material factors and the nonlinear material model of concrete. For this purpose, the concrete material model defined by parabolic-rectangle stress-strain relationship defined by the current KCI model code 2017 and the material factors of reinforcing steel and concrete were adopted. and also, the maximum reinforcing steel ratio was induced by using the force equilibrium relationship between the maximum neutral axis depth and the strain compatibility condition of the flexural member cross section. In addition, the maximum reinforcing steel ratio was presented as a ratio for the corresponding balanced reinforcing steel ratio according to the yield strength of the tensile reinforcing steel to provide convenience of design practice. The maximum reinforcing steel ratio proposed in this study can guarantee the ductile failure of the reinforced concrete flexural member and secure the constructability and economical efficiency at the same time compared to the specifications of other design codes.

https://doi.org/10.4334/JKCI.2020.32.2.137

이재욱(Jae-Uk Lee) ; 양근혁(Keun-Hyeok Yang) ; 문주현(Ju-Hyun Mun)

The present study examined the fluidity and compressive strength development of foamed concrete with partial replacement of fly ash (FA) and ground granulated blast furnace slag (GGBS). The FA contents varied from 0 to 40 % with regard to the unit binder content by weight at the fixed contents (0 or 40 %) of GGBS. Test results showed that the defoamed depth of foamed concrete tended to increase as the FA content increased from 0 to 20 %, beyond which it decreased. Thus, all specimens, except for the mixtures with FA content of 20 %, achieved the limitation for defoamed depth for 0.6 grade quality of KS F 4039. With the increase in FA contents, the flow of foamed concrete slightly increased whereas the compressive strength decreased. As FA contents increased from 0 to 40 %, 28-day compressive strength decreased as much as 48 %. Thus, foamed concrete mixtures with FA exhibited approximately 36 % lower 28-day compressive strength than ordinary portland cement-based mixtures at the same apparent density. ACI 209R equations were modified to reliably assess the compressive strength gain of foamed concrete with FA at different ages.

https://doi.org/10.4334/JKCI.2020.32.2.145

김철구(Chul-Goo Kim) ; 박홍근(Hong-Gun Park) ; 김영문(Young-Moon Kim)

Autoclaved lightweight concrete (ALC) was developed to attain lighter weight and lower thermal conductivity than normal concrete by forming macroscopic air bubbles. In this study, the material properties of ALC blocks currently used in Korea were investigated. By performing a number of material tests, the compressive strength, the modulus of elasticity, and the flexural tensile strength of ALC blocks and the compressive strength of ALC mortar were evaluated. The test results were compared with domestic and international design codes. The compressive strength, the modulus of elasticity, and the flexural tensile strength of ALC blocks increased in proportion to the specific gravity of the blocks. The ACI 523 design equations for the modulus of elasticity and the flexural tensile strength, which is defined as a function of compressive strength, agreed with the test results.

https://doi.org/10.4334/JKCI.2020.32.2.155

김태완(Taewan Kim) ; 강충현(Choonghyun Kang)

An experimental study was carried out on the effects of polyaluminum chloride (PAC) on the mechanical properties of cement mixed with ordinary Portland cement (OPC) and fly ash (FA). PAC is a flocculant for water treatment and consists of aluminum and chloride ion. FA replaced 20, 40, and 60 % of the OPC mass. PACs were mixed at 0, 2, 4, 6, 8, and 10 % of the mixing-water mass. As a result, the compressive strength increased and the pore diameter and total porosity decreased as the amount of PAC increased. PAC also formed Friedel’s salt. In the microstructure analysis, Friedel’s salt formed in the pores was observed. OPC-FA cement paste using PAC forms a dense hydration reactant. However, as the FA substitution rate increases, the overall compressive strength decreases, so an additional method is needed to improve the initial strength. In conclusion, PAC affects the hydration of OPC-FA paste, which changes the pore structure, the hydration reactant and improves the compressive strength.

https://doi.org/10.4334/JKCI.2020.32.2.165

신태용(Tae Yong Shin) ; 한성호(Seong Ho Han) ; 김재홍(Jae Hong Kim)

The yield stress and plastic viscosity represent the rheological behavior of freshly-mixed concrete. Their measurements need a rheometer using a sample more than 20 L volume. A delicate control and analysis on the measurements, together with the high-volume sample requirement, makes it hard to apply the use of a rheometer in field. This paper proposes an analytical model to estimate the yield stress and plastic viscosity using the results of the slump flow test. The model is linearly approximated with its volume-of-fluid simulation results. A general tendency on the rheological properties is clearly confirmed with the proposed model: A high yield-stress concrete shows a higher slump flow, and the time to get 500 mm flow (T50) is dominantly proportional to its plastic viscosity. In addition, the model reflects a small but non-negligible effect of the plastic viscosity on the slump flow, also the effect of yield stress on T50, for the estimation. Finally, the model estimations are compared with the measurements using a commercialized rheometer such as ICAR and BML. Their biased measurements are discussed with the estimates using the proposed model.

https://doi.org/10.4334/JKCI.2020.32.2.173

김규익(Kyu-Ik Kim) ; 정주성(Ju-Seong Jung) ; 이복기(Bok-Gi Lee) ; 이강석(Kang-Seok Lee)

A wire-woven bulk Kagome truss was recently developed. The Kagome truss, a periodic cellular metal type, is composed of evenly distributed helical wires with a constant pitch and helical radius in six directions. In this study, a new seismic strengthening method by attaching a concrete wingwall panel with embedded Kagome truss (Kagome Concrete Panel) was proposed for strengthening medium-to-low-rise reinforced concrete buildings. The method is one of the strength design approach by retrofit which can easily increase the ultimate lateral-load capacity of R/C buildings controlled by shear. Two full-size two-story reinforced concrete frames based on the existing school building without seismic details in Korea were designed and fabricated for the structural test. The pseudo-dynamic test was carried out in order to verify the seismic strengthening effects of the proposed method in terms of the maximum load carrying capacity and ductility. Test results revealed that the proposed Kagome Concrete Panel strengthening method installed in reinforced concrete frame enhanced conspicuously the strength and displacement capacities, and the method can resist markedly under the large scaled earthquake intensity level.

https://doi.org/10.4334/JKCI.2020.32.2.185

김도겸(Do-Gyeum Kim) ; 양근혁(Keun-Hyeok Yang) ; 서은아(Eun-A Seo) ; 윤현섭(Hyun-Sub Yoon)

The present study estimated an effective dry treatment process to remove cement pastes attached on recycled coarse aggregates and investigated the physical properties of such aggregates. The dry treatment processes conducted in these tests included conventional trituration, trituration for 24 hours, trituration and light-calcination treatments, trituration and microwave treatments, and trituration and ultrasonic treatments. The physical properties (density, water absorption, and particle distribution) of aggregates treated through different processes were compared with minimum requirements specified in KS F 2527 for natural coarse aggregates. The paste removement was estimated from a immersion tests using 5 % sulfuric acid solution and pore size analysis on the surface of aggregates. Test results revealed that the recycled aggregates treated by trituration process for 24 hours possessed a similar level to the density and water absorption of natural aggregates, indicating lower variations in the mass and porosity decreases due to the detachment of cement pastes than those treated by other processes. Thus, trituration process for 24 hours can be practically useful for reducing cement pastes on the recycled aggregates without additional water treatment.

https://doi.org/10.4334/JKCI.2020.32.2.193

이용준(Yong-Jun Lee) ; 김형국(Hyeong-Gook Kim) ; 김민준(Min-Jun Kim) ; 김동환(Dong-Hwan Kim) ; 김길희(Kil-Hee Kim)

In this study, flexural tests were performed to evaluate the structural performance of prestressed concrete hollow core slabs (HCS). Four-point loading simple beam tests were performed for eighteen HCS specimens with the height of section and topping concrete as variables. As a result of the flexural tests, HCS showed the average flexural strength of about 14 % higher than the factored flexural strength according to KCI Model Code 2017, regardless of the topping concrete. In addition, the horizontal shear strength acting on the joint surface at failure obtained from the horizontal shear force of composite HCS was 1.01 to 1.22 times higher than KCI Model Code 2017 regardless of the height of section. It was confirmed that composite HCS satisfies the required horizontal shear strength even if only roughness is applied to the joint surface between topping concrete and HCS.