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REFERENCE LINKING PLATFORM OF KOREA S&T JOURNALS
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Journal of the Korea Concrete Institute
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Korea Concrete Institute
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Volume & Issues
Volume 28, Issue 4 - Aug 2016
Volume 28, Issue 3 - Jun 2016
Volume 28, Issue 2 - Apr 2016
Volume 28, Issue 1 - Feb 2016
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Strut-Tie Models and Load Distribution Ratios for Reinforced Concrete Beams with Shear Span-to-Effective Depth Ratio of Less than 3 (I) Models and Load Distribution Ratios
Chae, Hyun-Soo ; Yun, Young Mook ;
Journal of the Korea Concrete Institute, volume 28, issue 3, 2016, Pages 257~265
DOI : 10.4334/JKCI.2016.28.3.257
The failure behavior of reinforced concrete beams is governed by the mechanical relationships between the shear span-to-effective depth ratio, flexural reinforcement ratio, load and support conditions, and material properties. In this study, two simple indeterminate strut-tie models which can reflect all characteristics of the failure behavior of reinforced concrete beams were proposed. The proposed models are effective for the beams with shear span-to-effective depth ratio of less than 3. For each model, a load distribution ratio, defined as the fraction of load transferred by a truss mechanism, is also proposed to help structural designers perform the rational design of the beams by using the strut-tie model approaches of current design codes. In the determination of the load distribution ratios, the effect of the primary design variables including shear span-to-effective depth ratio, flexural reinforcement ratio, and compressive strength of concrete was reflected through numerous material nonlinear analysis of the proposed indeterminate strut-tie models. In the companion paper, the validity of the proposed models and load distribution ratios was examined by applying them to the evaluation of the failure strength of 335 reinforced concrete beams tested to failure by others.
Strut-Tie Models and Load Distribution Ratios for Reinforced Concrete Beams with Shear Span-to-Effective Depth Ratio of Less than 3 (II) Validity Evaluation
Chae, Hyun-Soo ; Yun, Young Mook ;
Journal of the Korea Concrete Institute, volume 28, issue 3, 2016, Pages 267~278
DOI : 10.4334/JKCI.2016.28.3.267
In this study, the ultimate strength of 335 simply supported reinforced concrete beams with shear span-to-effective depth ratio of less than 3 was evaluated by the ACI 318-14's strut-tie model approach implemented with the indeterminate strut-tie models and load distribution ratios of the companion paper. The ultimate strength of the beams was also estimated by using the experimental shear strength models, the theoretical shear strength models, and the current strut-tie model design codes. The validity of the proposed strut-tie models and load distribution ratios was examined by comparing the strength analysis results classified according to the prime design variables of the shear span-to-effective depth ratio, flexural reinforcement ratio, and compressive strength of concrete.
Simplified Moment-Curvature Relationship Model of Reinforced Concrete Columns Considering Confinement Effect
Kwak, Min-Kyoung ; Yang, Keun-Hyeok ;
Journal of the Korea Concrete Institute, volume 28, issue 3, 2016, Pages 279~288
DOI : 10.4334/JKCI.2016.28.3.279
The present study simplified the moment-curvature relationship to straightforwardly determine the flexural behavior of reinforced concrete (RC) columns. For the idealized column section, moments and neutral axis depths at different stages(first flexural crack, yielding of tensile reinforcing bar, maximum strength, and 80% of the maximum strength at the descending branch) were derived on the basis of the equilibrium condition of forces and compatibility condition. Concrete strains at the extreme compression fiber beyond the maximum strength were determined using the stress-strain relationship of confined concrete, proposed by Kim et al. The lateral load-displacement curves converted from the simplified moment-curvature relationship of columns are well consistent with test results obtained from column specimens under various parameters. The moments and the corresponding neutral axis depth at different stages were formulated as a function of longitudinal reinforcement and transverse reinforcement indices and/or applied axial load index. Overall, curvature ductility of columns was significantly affected by the axial load level as well as concrete compressive strength and the amount of longitudinal and transverse reinforcing bars.
Seismic Performance of Circular RC Columns Retrofitted Using Ductile PET Fibers
Vachirapanyakun, Sorrasak ; Lim, Myung-Kwan ; Choi, Dong-Uk ;
Journal of the Korea Concrete Institute, volume 28, issue 3, 2016, Pages 289~298
DOI : 10.4334/JKCI.2016.28.3.289
An experimental research was performed using fibers for the purpose of retrofitting existing reinforced concrete circular columns. Glass fiber (GF) and polyethylene terephthalate (PET) were used as well as combined GF+PET (HF). PET has high tensile strength (over 600 MPa) and high ductility (about 15%), but has very low elastic modulus (about 1/6 of GF). A total of four columns was tested against laterally applied reverse cyclic load: control column, GF-, PET-, and HF-strengthened columns. All columns retrofitted using fibers demonstrated improved moment capacity and ductility. Moment capacity of GF-, PET-, and HF-strengthened columns was 120%, 107%, and 120% of the control column, respectively. Drift ratio of all retrofitted columns also increased by 63 ~ 83% over the control column. The final failure mode of the control column was main bar buckling. The final failure mode of the GF- and HF-strengthened columns was GF rupture while that of the PET-strengthened column was main bar rupture in tension. No damage was observed for PET at the ultimate stage due to excellent strain capacity intrinsic to PET. Current test results indicate that PET can be effectively used for seismic retrofit of RC columns. It is noted that the durability characteristics of PET needs to be investigated in the future.
Microstructure and Strength Properties of Alkali-activated Binder mixed with Sea Water
Jun, Yubin ; Oh, Jae-Eun ;
Journal of the Korea Concrete Institute, volume 28, issue 3, 2016, Pages 299~308
DOI : 10.4334/JKCI.2016.28.3.299
This paper presents an investigation of the mechanical and microstructural properties on hardened samples that were synthesized using blended binder(fly ash(FA) and blast furnace slag cement(BFSC)), alkali activator and sea water or distilled water. Binders were prepared by mixing the FA and BFSC in different blend weight ratios of 6:4, 7:3 and 8:2. Sodium hydroxide and sodium silicate were used 5 wt% of binder, respectively, as an alkaline activator. The compressive strength and absorption were measured at the age of 3, 7 and 28 days, and the XRD, TGA and MIP tests were performed at the age of 28 days. An increase in the content of BFSC leads to an increase in the quantities of ettringite and C-S-H formed, regardless of the type of mixing water. And it also shows higher strength due to the reduction of pores larger than ~50 nm. All hardened samples in this study have common hydration products of C-S-H,
and calcite. Hydrocalumite of all reaction products formed was only present in hardened sample mixed with sea water. For each FA/BFSC mixing ratio, the compressive strength of hardened sample mixed with sea water was similar to that mixed with distilled water. It is proposed that the slight increase of strength of samples mixed with sea water is mainly due to the presence of hydrocalumite phase containing chlorine ion, contributing to the change of total porosity and pore size distribution in samples.
Investigation of Early-Age Concrete Strength Development Using Hardening Accelerator
Kim, Gyu-Yong ; Kim, Yong-Ro ; Park, Jong-Ho ;
Journal of the Korea Concrete Institute, volume 28, issue 3, 2016, Pages 309~316
DOI : 10.4334/JKCI.2016.28.3.309
In this study, performance of hardening accelerator types which promote setting and hardening of cement has been reviewed in order to develop early age strength of concrete with compressive strength of 21~27 MPa after examination of strength development of the concrete at early age according to curing temperature and unit cement(binder) content. As results, soluble mineral salt showed better hardening acceleration effect than organic salt in the scope of this study. Also, hydration reaction accelerating effect of
by Soluble mineral salt is effective on development of early age compressive strength and it was shown that the Pt's hydration reaction accelerating effect was the best. Construction duration reduction can be expected by securing compressive strength for prevention of early aged freezing damage in 25hour-curing time under curing temperature at
. Also, it was shown that compressive strength of specimen cured at
was similar with plain specimen cured at
. Therefore, it is expected that fuel costs and carbon dioxide can be reduced when the same construction duration is considered.
Effect of Internal Curing by Super-Absorbent Polymer (SAP) on Hydration, Autogenous Shrinkage, Durability and Mechanical Characteristics of Ultra-High Performance Concrete (UHPC)
Kang, Sung-Hoon ; Moon, Juhyuk ; Hong, Sung-Gul ;
Journal of the Korea Concrete Institute, volume 28, issue 3, 2016, Pages 317~328
DOI : 10.4334/JKCI.2016.28.3.317
This research intends to understand the impact of super-absorbent polymer (SAP) as an internal curing agent in Ultra-High Performance Concrete (UHPC). Two different types of SAPs of acrylic acid (SAP_AA) and acrylic acid-co-acrylamide (SAP_AM) were examined with UHPC formulation. Isothermal calorimetry and x-ray diffraction experiments revealed the impact of polymers with the different chemical bonds on cement hydration. To test its feasibility as a shrinkage reducing admixture for UHPC, a series of experiments including flowability, compressive strength, rapid chloride permeability and autogenous shrinkage profile was performed. While both SAPs showed a reduction in autogenous shrinkage, it has been concluded that the SAP size and chemical form significantly affect the performance as an internal curing agent in UHPC by controlling cement hydration and porosity modification. Between the tested SAPs, SAP_AM which absorbs more water in UHPC than SAP_AA, shows better mechanical and durability performance.
Optimum Carbonation Reforming Period of Recycled Aggregate Based on the Microscopic Carbonation Conduct
Shin, Jin-Hak ; Kim, Han-Sic ; Ha, Jung-Soo ; Chung, Lan ;
Journal of the Korea Concrete Institute, volume 28, issue 3, 2016, Pages 329~340
DOI : 10.4334/JKCI.2016.28.3.329
Increase in demotion and repair works on buildings in the construction market generates a large amount of construction waste. Recycling of construction waste is important for saving of resources, preservation of environment and constant advance of the construction industry. Accordingly, the environmental and economic value of recycled aggregate, which is produced after waste concrete is crushed, is increasingly highlighted. It is generally known that compared to concrete made of ordinary aggregate, concrete made of recycled aggregate has low quality, and the low quality is dependent on the amount of the bonding heterogeneous (cement paste and mortar) as well as the amount of the pores within the bonding heterogeneous. Reports on carbonation mechanism shows that the pores of cement-based materials are filled up by the progress of carbonation. Therefore, this study aims at an estimation of the period for optimum carbonation reforming appropriate for the thickness of the bonding heterogeneous of recycled aggregate, based on carbonation mechanism, with a view to improving the product quality by means of filling up the pores of the bonding heterogeneous of recycled aggregate. This study drew the carbonation depth according to the passage of age by calculating the bonding ratio and bonding thickness of the bonding heterogeneous as against the particle size distribution of recycled aggregate as well as by chemical quantitative analysis according to the age of accelerated carbonation of mock-up samples imitating bonding heterogeneous. Based on the correlation between the age of accelerated carbonation and carbonation depth, this study also proposed the estimated period of carbonation reforming of recycled aggregate appropriate for the thickness of the bonding heterogeneous.
Pore Structure and Physical Properties of Heterogeneous Bonding Materials of Recycled Aggregate according to Carbonation Reforming
Shin, Jin-Hak ; Kim, Han-Sic ; Chung, Lan ; Ha, Jung-Soo ;
Journal of the Korea Concrete Institute, volume 28, issue 3, 2016, Pages 341~348
DOI : 10.4334/JKCI.2016.28.3.341
At present, about 40 million tons of concrete is dismantled each year, which accounts for the largest portion of the total amount of construction waste with 60.8%. It is known about 97.5% of it is recycled. However, most of the usage of waste concrete is limited to lower value-added business areas, and considering the increasing amount of waste concrete generated due to the deterioration of structures, the need for converting waste concrete to structural concrete is urgent. Therefore, this study aims at estimating the period for the optimum carbonation reforming to improve the quality of recycled aggregate, by making use of the method of accelerated carbonation reforming of the bonding heterogeneous (cement paste and mortar) for the purpose of converting recycled aggregate to structural concrete. Based on the period appropriate for the heterogeneous thickness and each bonding thickness of recycled aggregate which was drawn from previous studies, the changes in the characteristics and physical properties of pore structure according to progress of accelerated carbonation were analyzed. The result shows that with the progress of carbonation, the pore volume and the percentage of water absorption of the bonding heterogeneous decreased and the density increased, which indicates improvement of the product quality. But after certain age, the tendency was reversed and the product quality deteriorated. Synthesizing the results of previous studies and those of the present study, this study proposed 4 days and 14 days respectively for the period for the optimum carbonation reforming of recycled fine aggregate and recycled coarse aggregate.
The Effects of Na
on Early Strength of High Volume Slag Cement
Kim, Tae-Wan ; Hahm, Hyung-Gil ;
Journal of the Korea Concrete Institute, volume 28, issue 3, 2016, Pages 349~356
DOI : 10.4334/JKCI.2016.28.3.349
This report presents the results of an investigation on the early strength development of pastes high volume slag cement (HVSC) activated with different concentration of sodium carbonate (
). The ordinary Portland cement (OPC) was replaced by ground granulated blast furnace slag (GGBFS) from 50% to 90% by mass, the dry powders were blended before the paste mixing. The
was added at 0, 2, 4, 6, 8 and 10% by total binder (OPC+GGBFS) weight. A constant water-to-binder ratio (w/b)=0.45 was used for all mixtures. The research carried out the compressive strength, ultrasonic pulse velocity (UPV), water absorption and X-ray diffraction (XRD) analysis at early ages(1 and 3 days). The incase of mixtures, V5 (50% OPC + 50% GGBFS), V6 (40% OPC + 60% GGBFS) and V7 (30% OPC + 70% GGBFS) specimens with 6%
, V8 (20% OPC + 80% GGBFS) and V9 (10% OPC + 90% GGBFS) specimens with 10%
showed the maximum strength development. The results of UPV and water absorption showed a similar tendency to the strength properties. The XRD analysis of specimens indicated that the hydration products formed in samples were CSH and calcite phases.
Experimental Study on Mechanical Properties and Deformation Behavior of Concrete with Recycled Aggregates and Steel Fiber
Lee, Hyun-Ho ; Lee, Tae-Wang ;
Journal of the Korea Concrete Institute, volume 28, issue 3, 2016, Pages 357~363
DOI : 10.4334/JKCI.2016.28.3.357
To solve the exhaustion problem of natural aggregate which were create the high value in construction and environmental industry, recycled aggregates have considerable benefits than other materials. However, even though many researches have been conducted with recycled aggregates, building structures with recycled aggregated are rarely constructed because it has lower quality than natural aggregates have. In this study, mechanical and strain properties of recycled aggregates concrete containing steel fibers have been reviewed in order to complement performance of recycled aggregates concrete. As results, recycled aggregates concrete showed lower compressive strength and elastic modulus than plain concrete. But, recycled aggregates concrete containing steel fibers showed equivalent performance with plain concrete. In review of drying shrinkage and creep coefficient, recycled aggregates concrete containing steel fibers showed similar behavior with plain concrete in the range of 0.5 Vol.% fiber content rate by internal restraint effect, moisture transport restraint effect and strength enhancement effect of steel fiber. Therefore, it is considered that mixing steel fibers with concrete is the effective method as a active application plan for recycled aggregates.
Anchorage Strength of Headed Bars in Steel Fiber-Reinforced UHPC of 120 and 180 MPa
Sim, Hye-Jung ; Chun, Sung-Chul ; Choi, Sokhwan ;
Journal of the Korea Concrete Institute, volume 28, issue 3, 2016, Pages 365~373
DOI : 10.4334/JKCI.2016.28.3.365
Ultra-High-Performance Steel Fiber-Reinforced Concrete (SUPER Concrete) exhibits improved compressive and tensile strengths far superior to those of conventional concrete. These characteristics can significantly reduce the cross sectional area of the member and the anchorage strength of a headed bar is expected to be improved. In this study, the anchorage strengths of headed bars with
embedment length were evaluated by simulated exterior beam-column joint tests where the headed bars were used as beam bars and the joints were cast of 120 or 180 MPa SUPER Concrete. In all specimens, the actual yield strengths of the headed bars over 600 MPa were developed. Some headed bars were fractured due to the high anchorage capacity in SUPER Concrete. Therefore, the headed bar with only
embedment length in 120 MPa SUPER Concrete can develop a yield strength of 600 MPa which is the highest design yield strength permitted by the KCI design code. The previous model derived from tests with normal concrete and the current design code underestimate the anchorage capacity of the headed bar anchored in SUPER Concrete. Because the previous model and the current design code do not consider the effects of the high tensile strength of SUPER Concrete. From a regression analysis assuming that the anchorage strength is proportional to
, the model for predicting anchorage strength of headed bars in SUPER Concrete is developed. The average and coefficient of variation of the test-to-prediction values are 1.01 and 5%, respectively.