https://doi.org/10.4334/JKCI.2020.32.3.213

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

Autoclaved lightweight concrete (ALC) can improve the thermal insulation performance due to formation of macroscopic air bubbles, attaining lower density and thermal conductivity. However, since the compressive and tensile strengths of ALC are significantly lower than those of ordinary concrete, strengthening methods are additionally required when ALC blocks are used, particularly for lateral load resistance systems. In this study, the flexural tensile performance of ALC blocks and prisms strengthened with textile reinforced mortar (TRM) method was investigated. In this method, glass fiber reinforced polymer (GFRP) meshes were attached to ALC blocks by means of ALC mortar and staples. Flexural tests on ALC blocks and prisms were performed. The results showed that GFRP meshes improved the flexural tensile strength and deformation capacity of ALC blocks and prisms as the meshes resisted tensile force after flexural cracking. GFRP meshes between ALC blocks had no significant effect on the bond strength, while meshes on the face of ALC blocks increased the tensile strength.

https://doi.org/10.4334/JKCI.2020.32.3.225

황승현(Seung-Hyeon Hwang) ; 양근혁(Keun-Hyeok Yang) ; 심재일(Jae-Il Sim) ; 최용수(Yong-Soo Choi)

The present study examined the structural efficiency of prefabricated V-ties that were devised to improve the constructability of supplementary ties by comparing the compressive and seismic performances of columns with different arrangements of supplementary ties. Test results revealed that the compressive and flexural strengths of columns with the prefabricated V-ties can be conservatively estimated by the ACI 318-14 procedure. Columns with the prefabricated V-ties possessed ductility values 1.58 times in compression and 1.13 times in flexure than the counterpart columns with conventional crossties. The corresponding increase values were 1.25 times and 1.02 times, respectively, when compared with ductility values measured in columns with individual V-ties. Overall, the prefabricated V-ties had better potentials than the crossties and individual V-ties in confining core concrete and delaying the buckling of longitudinal reinforcement.

https://doi.org/10.4334/JKCI.2020.32.3.233

김문길(Mun-Gil Kim) ; 천성철(Sung-Chul Chun) ; 김영호(Young Ho Kim)

A design procedure for truss encased slab was developed that doesn’t require slab formwork or shoring system and can be applied to parking lots. Truss encased slab is composed of half PC, tension rebar, truss rebar, U-shaped channels, filling concrete, and topping concrete. In the construction stage, the half PC and tension bars resist the tensile force of the lower part, and the U-shaped channels and filling concrete resist the compression force of the upper part. In a service load state, topping concrete and Half PC are combined to act like reinforced concrete beams. Among the flexural, shear and horizontal shear strengths, the smallest strength is determined as the design strength against the factored design load. The flexural tests were conducted to evaluate the performance of the truss encased slab in the service state. The test variables were the presence or absence of voids. Test results show that both specimens showed sufficient ductility behavior and both specimens developed higher strength than expected flexural strength. The average of [actual flexural strength]/[expected flexural strength] ratio is 1.26, and it can be evaluated safely even if the current flexural strength design equations is used. The deflection under the service load is 79 % of the expected deflection, indicating that the serviceability of truss encased slab is excellent. The moment-curvature relations of tests and prediction were compared to investigate the composite behavior of the truss encased slab. Fully composite behavior of the truss encased slab was confirmed by the moment-curvature relations and the location of the neutral axis with varying the load. The developed truss encased slab is believed to secure proper flexural performance for the design load.

https://doi.org/10.4334/JKCI.2020.32.3.241

강현욱(Hyunuk Kang) ; 강성훈(Sung-Hoon Kang) ; 정연웅(Yeonung Jeong) ; 문주혁(Juhyuk Moon)

This study investigated the hydration reaction of GGBFS with a substitute using X-ray diffraction based quantitative analysis methods. To overcome the limitation of X-ray diffraction technique of quantifying the amount of amorphous material, internal standard and PONKCS methods were adapted and compared. There was negligible hydration reaction on the used GGBFS reacted with water so strength development was not noticeable. However, when calcium hydroxide was used to activate the GGBFS, the compressive strength of 50 MPa at 28 days was achieved owing to significant hydration reaction. Several reaction phenomena of the consumption of calcium hydroxide and productions of chemically bound water and calcium carbonate, were successfully calculated as chemical reaction progresses. However, due to the existence of different types of amorphous materials within the raw material and hydration product (i.e., C-S-H), the degree of GGBFS consumption could not be directly calculated.

https://doi.org/10.4334/JKCI.2020.32.3.251

배영환(Young Hwan Bae) ; 김성도(Seongdo Kim) ; 임홍재(Hong Jae Yim)

Self-consolidating concrete has been widely used for complex structures that require high performance. Several chemical and mineral admixtures are used to enhance the fluidity of fresh concrete. However, excessive use of superplasticizer and failure of construction can induce unexpected segregation in static or dynamic condition, and segregation affects concrete durability due to non-homogeneous and potential damage to components. This study used electrical resistivity measurement method to evaluate the segregation of various types of concrete mixtures including superplasticizer or fly ash. In addition, conventional segregation measurement method, the wet-sieving method for measurement of settled coarse aggregate, was implemented to verify the electrical method.

https://doi.org/10.4334/JKCI.2020.32.3.259

이성철(Seong-Cheol Lee)

For nonlinear analysis of reinforced concrete members under multi-directional stress state, the Disturbed Stress Field Model (DSFM) has been developed from the Modified Compression Field Theory (MCFT) so that crack slip behavior can be rigorously considered in the analysis. In the DSFM, however, shear stress-slip behavior along a crack is considered only for monotonic loading condition; crack surface behavior under cyclic load cannot be properly considered. In this paper, based on the Basic Contact Density Model (BCDM), a simple model has been developed to consider shear stress-slip behavior along a crack under cyclic load. In spite of the simplification, it has been found that the proposed model reasonably predicted the shear behavior along a crack under cyclic load. In addition, using the proposed model, it was shown that crack slip behavior can be more rationally described for a reinforced concrete panel under cyclic load. This paper is expected to be useful for relevant studies on structural behavior of reinforced concrete under cyclic load.

https://doi.org/10.4334/JKCI.2020.32.3.267

최종권(Jongkwon Choi) ; 김창혁(Changhyuk Kim)

Due to the drastic increase of aging infrastructures, the demands for the repair and strengthening of existing structures have become of extreme importance. One of the most economical and efficient repair and strengthening materials, which has been extensively researched and widely used to extend the service life and improve the structural capacity of existing structures, is carbon fiber reinforced polymer (CFRP). To accurately evaluate the structural performance of repaired and strengthened structures with CFRP, basic mechanical properties of CFRP has to be qualitatively measured. Though a standard test method (ASTM D3039) is currently available, no specific details for specimen dimensions are provided. This may lead to confusion and inconsistency among researchers. In this paper, to investigate the influence of specimen dimensions on the tensile behavior, three representative dimensions - length, thickness, and width - were selected as test variables and a series of CFRP strips were fabricated and tested. CFRP strips were subjected to a monotonically increasing uniaxial tensile load in accordance with ASTM D3039. A total of nine strain gauges were installed on each specimen and continuously monitored until a tensile failure was observed. To comprehensively evaluate the test results, a shape parameter was introduced and used to propose an optimal shape of the CFRP strip test specimen.

https://doi.org/10.4334/JKCI.2020.32.3.275

윤현광(Hyun-Kwang Yoon) ; 김선우(Sun-Woo Kim) ; 장영일(Young-Il Jang) ; 김선웅(Sun-Woong Kim) ; 이종원(Jong-Won Lee)

In this study, the flexural performance of steel tubes filled with PVA (Polyvinyl Alcohol) fiber-reinforced cement composites (FRCCs) was experimentally evaluated. For the experimental program, a total of four specimens were fabricated to evaluate the effects of recycled materials such as fly ash and recycled fine aggregate: STC specimen is plain mortar filled steel pipe; STF specimen is a steel pipe filled with PVA FRCCs; STFR1 specimen is a steel pipe filled with PVA FRCCs replaced with 25 % fly ash and 25 % recycled fine aggregate; STFR2 specimen is a steel pipe filled with PVA FRCCs replaced with 25 % fly ash and 50 % recycled fine aggregate. As a result, STFR1 and STFR2 specimens incorporating fly ash and recycled fine aggregate showed a tendency to increase the denting depth due to the lower elastic modulus and the higher Poisson’s ratio of the filling material. In view of energy dissipation capacity, the STFR1 and STFR2 specimens showed a greater contribution of denting depth to energy dissipation capacity due to the damage of the stressor than the STC and STF specimens, indicating that the redistribution capacity of the local damage was improved. In addition, when the PVA fiber was incorporated, the steel strain at the bottom of the mid-span was smaller than that at the top of the end due to the crack dispersion in the tension part, and the stress distribution was stable when the recycled materials were used.

https://doi.org/10.4334/JKCI.2020.32.3.285

윤다애(Da-Ae Youn) ; 김지현(Ji-Hyeon Kim) ; 이건철(Gun-Cheol Lee) ; 서수연(Soo-Yeon Seo) ; 윤현도(Hyun-Do Yun)

Smart hybrid steel and polyethylene (PE) fiber reinforced strain-hardening cement composite (smart Hy-SHCC) sensors were manufactured by incorporating carbon nanotube (CNT) into SHCC material. The influence of CNT dosages on the tensile and self-sensing properties of smart Hy-SHCC was investigated. Also, the effect of measurement technique (two-probe and four-probe) on the piezoresistive response of smart Hy-SHCC material. Smart Hy-SHCC was reinforced with 0.5 % steel fiber and 1.0 % PE fiber to give strain hardening and multiple cracking performance to brittle cement composite and included 0.5 % and 1.0 % CNT to enhance self-sensing capacity. Test results show that within 1 % volume fraction of CNT, the addition of CNT improved compressive strength, direct tensile behavior and multiple cracking behavior of smart Hy-SHCC material. The gauge factor of smart Hy-SHCC material from four-probe measurements was higher than that of two-probe method. The best performance as strain sensor was obtained for the Hy-SHCC with 0.5 % CNT, reaching values of gauge factor up to 33 with R2 Pearson’s coefficient above 0.99.