https://doi.org/10.4334/JKCI.2021.33.6.551

고성현(Seong-Hyun Ko) ; 이재훈(Jae-Hoon Lee)

This investigation deals with monotonic behavior for longitudinal reinforcement of reinforced concrete bridge piers. The purposes of this study are to verify currently used monotonic models and to propose a rational monotonic tension?compression model for longitudinal reinforcement of manufactured reinforcing steel in Korea. One hundred thirty-eight specimens of longitudinal reinforcement were tested under axial strain control. The tests were performed for test variables of yield strength and reinforcement bar size. Modified models of monotonic tension stress-strain and monotonic compression stress-strain were proposed on the basis of the monotonic tension and compression test results of longitudinal reinforcement.

https://doi.org/10.4334/JKCI.2021.33.6.561

이혜진(Hye-Jin Lee) ; 김학영(Hak-Young Kim) ; 양근혁(Keun-Hyeok Yang) ; 전용수(Yong-Soo Jeon)

The present study aimed to examine the compressive strength development of precast bottom ash aggregate (BAA) concrete cured under high temperatures at early age. The main parameters investigated were the natural aggregate content for replacing bottom ash fine and coarse particles, and curing temperature histories up to the age of 1 day. The empirical equations to estimate the compressive strength development of BAA concrete were formulated by using the maturity function specified in the fib model considering the curing temperature histories. Test results showed that higher compressive strengths were commonly obtained at early ages and at 28 days for concrete cured under 60 °C for 12 hrs; the values were higher by 2~10 % only than those measured for concrete under 60 °C for 8 hrs. The means of the ratios between the predictions using the proposed equations and the experimental compressive strengths were 1.05, 0.96, and 1.03 at ages of 1, 3, and 91 days, respectively, indicating that the equations can be useful for estimating the compressive strength development of precast BAA concrete.

https://doi.org/10.4334/JKCI.2021.33.6.569

김종원(Jong-Won Kim) ; 양근혁(Keun-Hyeok Yang) ; 문주현(Ju-Hyun Mun)

The objective of this study is to develop the lightweight insulating concrete using bottom ash (BA) aggregates and expanded polystyrene (EPS) bead. The main parameters investigated in the BA aggregate concrete were the volume fraction of EPS (0~15 %), addition of the polymer and polyvinyl alcohol fiber, and curing temperatures (20~60 °C). The polymer and polyvinyl alcohol fiber were used to maintain the workability and reduce the crack development in the tested concrete due to the addition of EPS bead. Empirical design models were proposed to reliably assess the compressive strength gain, splitting tensile strength, rupture modulus, and thermal conductivity of lightweight insulating concrete. From the test results and proposed models, it was recommended to add 5~10 % volume fraction of EPS bead for BA aggregate concrete to achieve the requirements specified in KS F 4736 for the extrusion lightweight concrete panels.

https://doi.org/10.4334/JKCI.2021.33.6.579

민근형(Geunhyeong Min) ; 이일근(Ilkeun Lee) ; 정유석(Yoseok Jeong) ; 김우석(WooSeok Kim)

Repair cases of deteriorated concrete bridges are increasing due to aging and the use of de-icing chemicals. In addition, damage is repeatedly inspected on repaired locations of deteriorated bridges. Types of damage fall into types of shrinkage-induced cracks and delaminations caused by lower bond strength. This study aims at improving performance of repair of deteriorated concrete structures. Therefore, bond strength of repaired concrete bridges in service was evaluated and a comparative study was also conducted on guidelines of concrete repairs among countries. Based on results of field tests and the comparative study, the following conclusions for enhancing performance of repaired concrete structures are drawn: ① Compressive strength of the repair material should be raised to 25 MPa, and the bond strength should be equal to or larger than 1.5 MPa ② The removal area of the deteriorated concrete should expand 150 mm from the delaminations or cracks ③ Concrete substrates should be in a saturated surface-dry (SSD) condition immediately before casting ④ The repair material should be placed in one layer; otherwise, second layer should be placed before setting of former layer or casting second layer on a SSD conditioned former layer ⑤ Repair materials should be wet-cured.

https://doi.org/10.4334/JKCI.2021.33.6.589

이정윤(Jung-Yoon Lee) ; 김길희(Kil-Hee Kim) ; 신동익(DongIk Shin)

The shear design and torsion design of reinforced concrete members are interrelated. The maximum yield strength of shear and torsional reinforcement in the KCI-21 code are the same at 500 MPa. Members subjected to both shear and torsion are designed taking into account the effects of both forces. In the KCI-21 design code, the provisions for maximum shear strength have been modified, but there are no modifications for maximum torsional strength. As a result, the provisions of the KCI-21 code for the maximum shear strength are contradictory. In this paper, the maximum torsional strength derived from the space truss model was evaluated. The maximum torsional strength derived from the space truss model over-estimated the strengths of 143 RC beams. For the flexural compression region and three-dimensional node region, and because of the effect of warping on the maximum torsional strength, the maximum torsional strength reduction factor of 0.5 should be applied. In addition, to apply the maximum yield strength of shear reinforcement of 500 MPa to the torsional reinforcement, the maximum torsional strength reduction factor of 0.5 should be applied.

https://doi.org/10.4334/JKCI.2021.33.6.601

김동현(Dong-Hyun Kim) ; 김형남(Hyoung-Nam Kim) ; 궁빙청(Bing Cheng Gong)

This study was conducted to evaluate the sulfate resistance of mortar and paste specimens to which barium carbonate was applied. A mortar and paste containing 5, 10, and 15 % barium carbonate, including the reference specimen, were prepared, subjected to standard curing for 28 days, immersed in 5 % sodium sulfate aqueous solution, and measured from 26 weeks to 38 weeks to measure compressive strength, length change, and weight change. The change was measured and verified by performing SEM, EDAX, XRD analysis and mapping analysis. As a result of the compressive strength ratio test, it was found that the specimen to which barium carbonate was added had high sulfate resistance, and from the measurement results of length change, the addition of barium carbonate also increased the sulfate resistance. As a result of measuring the mass change of the sulphate semi-immersion, the mass of all specimens increased, and the increase was found to increase in the order of P150>BC550>BC1550>BC1050. Through SEM and EDAX analysis, it was confirmed that Ettringite and gypsum formed after immersion, and it was confirmed that barium carbonate was converted to barium sulfate. As a result of the XRD analysis, it was confirmed that less gypsum and ettringite appeared in the barium- applied paste specimen. Mapping analysis revealed that sulfate did not penetrate into the specimen to which barium was applied.

https://doi.org/10.4334/JKCI.2021.33.6.609

진주현(Joo-Hyun Jin) ; 박홍근(Hong-Gun Park) ; 김철구(Chul-Goo Kim)

The use of composite members using precast concrete (PC) and cast-in-place (CIP) concrete with different compressive strengths has increased. However, current design codes do not clearly define the shear strengths of composite members with different concrete strengths. In this study, the shear strength of PC-CIP composite beams with dual concrete strength (24 MPa and 60 MPa) was experimentally evaluated. The test variables included the area ratio of two different concretes, the use of stirrups and support conditions. The test results showed that flexural cracks and splitting cracks along longitudinal top rebars affected the strength of composite beams. Although the current design code predicts the shear strength of fixed composite beams on the safe side, the shear strength predictions of composite beams without stirrups were largely conservative.

https://doi.org/10.4334/JKCI.2021.33.6.619

천성철(Sung-Chul Chun) ; 성민규(Min-Kyu Seong)

To verify a new design equation of a compression development length proposed by previous studies, two exterior beam-column connections, C40-BCJ and C70-BCJ, were tested with a variable of concrete compressive strength of 40 and 70 MPa. Both top and bottom bars of the beams had a standard hook in tension, conforming to KDS 14 20 52 except for the development length. The development length of bars in compression is longer than that of hooked bars in tension in KDS 14 20 52; however, the proposed equation was used in this study instead of KDS 14 20 52 provisions of development length of bars in compression. The two specimens showed joint shear failures after beam flexure strengths were fully developed with yielding of beam bars. High joint aspect ratios of 1.14 and 1.41 for C40-BCJ and C70-BCJ, respectively, may cause joint shear failure. Especially, C70-BCJ, having a higher joint aspect ratio, showed more significant damage of the joint. However, two specimens satisfy the criteria of moment frames required in ACI 374 and are thought to have sufficient resistance against reversed cyclic loading. At 3.0 % drift ratio, the beam bars yielded and a residual deformation was measured in the tensile side after the applied load was removed. Under reversed loading, crack gaps in the tensile side at the previous loading did not close and only the beam bars resisted the applied compression. Therefore, compressive stresses of the bars may be higher than calculated stresses according to the method used determine the nominal flexural strength. Bars with a compression development length according to the equation proposed by previous studies have sufficient anchorage capacities.

https://doi.org/10.4334/JKCI.2021.33.6.629

김춘호(Chun-Ho Kim) ; 김남욱(Nam-Wook Kim)

In this study, with the aim of investigating improvements of the strength and durability of precast concrete according to the addition of mixed industrial byproducts, a specimen was produced in which fly ash or blast furnace slag powder was mixed with cement; the compressive strength of the specimen was set to two types, 30 MPa and 45 MPa, and the characteristics of the manufactured specimen were compared using water curing method and steam curing method, which are used for precast concrete manufacturing. As a result of dry shrinkage deformation characteristics and chloride ion penetration test, concrete specimens manufactured using fly ash or blast furnace slag powder tended to decrease in dry shrinkage deformation and an effect of inhibiting chloride ion penetration was confirmed. Therefore, in the scope of this study, it is judged that the use of blast furnace slag powder and fly ash is valid when manufacturing precast concrete.

https://doi.org/10.4334/JKCI.2021.33.6.637

염현수(Hyun-Soo Youm) ; 홍성걸(Sung-Gul Hong)

This study investigated the effects of a transverse web opening on the shear strength of ultra-high-performance concrete (UHPC) beams. Test results showed that due to both the geometrical discontinuity and sectional loss, the introduction of a web opening significantly deteriorated the shear strength of the UHPC beams. It has been revealed that the reduced shear strengths, however, were effectively restored by applying various reinforcements such as diagonal shear links and textile fabrics. The test results were evaluated based on the currently available design formula for the shear strength of UHPC beams. Moreover, a strut-and-tie model was constructed that can be used to explain the ultimate strengths and failure mechanisms of the UHPC beams with web openings. However, the reinforcing mechanisms of textile fabrics are not yet clearly understood, resulting in serious overestimations. Thus, more research is required for better understanding of the complex bond behavior between fabrics and UHPC.