적외선 체열촬영을 이용한 당뇨병성 족부궤양의 평가

이수영,유두식,정순열,정진상 건국대학교 의과학연구소 1998 건국의과학학술지 Vol.8 No.-

To assess whether the development of plantar foot ulceration could be predicted from the mean plantar foot temperature(MFT), as assessed by digital infrared thermographic imaging(DITI), in patients with or without diabetic polyneuropathy or foot ulceration, 46 feet with no diabetic polyneuropathy(group Ⅰ), 40 feet with diabetic polyneuropathy(group Ⅱ), and 17 feet with diabetic foot ulcer(group Ⅲ) were assessed by DITI in six standard plantar sites and were correlated with laboratory findings (blood sugar, hemoglobin Alc, C-peptide). Total MFT was significantly higher in group Ⅲ(33.18±1.86℃) than group Ⅰ(29.13±1.63℃) or group Ⅱ(30.89±1.51℃). There were increasing tendency of focal MFT in the third metatarsal head and greater toe where revealed higher incidence of foot ulceration. There was a trend toward elevation of glycosylated hemoglobin(HbAlc) in groupⅢ. In conclusion, DITI is a simple, noninvasive method of idenfifying the neuropathic foot at increased risk of ulceration. Patients with high plantar foot temperature are at increased risk of neuropathic foot ulceration.

Bond performance of steel rebar embedded in 80–180 MPa ultra-high-strength concrete

Yoo, Doo-Yeol,Shin, Hyun-Oh Elsevier 2018 Cement & concrete composites Vol.93 No.-

<P><B>Abstract</B></P> <P>Ultra-high-strength concrete (UHSC) has attracted attention from engineers because of its great capability on reducing the size of reinforced concrete columns. However, the bond performance of steel rebar embedded in UHSC has not been examined enough yet, although it is a fundamental information for structural design. So, this study comprehensively investigates the bond performance of deformed steel rebar embedded in high-strength concrete (HSC), very-high-strength concrete (VHSC), and UHSC with compressive strengths ranging from 80 to 180 MPa. Different bar diameters (12.7, 15.9, and 19.1 mm), embedment lengths (0.5, 1.0, and 1.5 × bar diameter), cover depths (1, 2, and 3 × bar diameter and center), steel fiber volume fractions (0, 0.5, 1.0, and 1.5%), and yield strengths of steel rebar (normal-strength vs. high-strength) were all considered. Test results indicate that the average bond strength increased significantly with an increase in the compressive strength of the concrete and decreased slightly with an increase in the bar diameter. Average bond strengths of steel rebars in HSC, VHSC, and UHSC were all increased by increasing the embedment length. The cover depth marginally influenced the bond strength when pullout failure was generated, and it significantly increased the bond strength when splitting failure occurred. The incorporation of steel fibers most effectively increased the bond strength in the UHSC mixture, compared with the HSC and VHSC mixtures. Lastly, the widely used prediction models for average bond strength were not accurate for concrete with a compressive strength greater than 80 MPa; thus, a new, appropriate model needs to be proposed in the near future.</P>

Mitigating shrinkage cracking in posttensioning grout using shrinkage-reducing admixture

Yoo, Doo-Yeol,Ryu, Gum-Sung,Yuan, Tianfeng,Koh, Kyung-Taek Elsevier 2017 Cement & concrete composites Vol.81 No.-

<P><B>Abstract</B></P> <P>This study aims to reduce the cracking potential of posttensioning high-performance grout (HG) through use of shrinkage-reducing admixture (SRA). With this regard, an HG mixture was initially developed to possess appropriate fluidity with low bleeding and settling. Various amounts of SRA were subsequently incorporated into the developed HG mixture at 1% and 2% by weight to the cementitious components. A widely used ordinary grout (OG) mixture was also considered for comparison. Test results indicated that the HG mixture exhibited similar flowability to the OG mixture, while imparting much better performance with regard to strength, bleeding, and settling. The addition of SRA to the HG mixture led to higher compressive and tensile strength values after 28 days, lower shrinkage strain, lower maximum internal temperature due to hydration heat, and delayed shrinkage cracking. On the other hand, the degree of restraint due to an uneven surface of duct and the filling capacity of the HG were insignificantly affected by the inclusion of the SRA. Complete filling of ducts was observed for the HG samples. The OG mixture exhibited the smallest shrinkage strain and the best performance with regard to shrinkage cracking resistance; however, the OG mixture resulted in insufficiently filled ducts, leading to atmospheric exposure of prestressing strands. Consequently, the HG mixture with 2% SRA was proposed to be most appropriate for posttensioning grout with regard to the several properties denoted above.</P>

Mechanical and structural behaviors of ultra-high-performance fiber-reinforced concrete subjected to impact and blast

Yoo, Doo-Yeol,Banthia, Nemkumar Elsevier 2017 Construction & building materials Vol.149 No.-

<P><B>Abstract</B></P> <P>This study comprehensively investigates impact and blast resistances of ultra-high-performance fiber-reinforced concrete (UHPFRC) by considering various influential factors. At a material level, rate-dependent fiber pullout behavior, dynamic compressive behavior, and impact tensile and flexural behaviors were examined in detail, and the benefits of using UHPFRC to improve the impact resistance of ordinary concrete were discussed. It was obvious that (1) UHPFRC is able to dissipate much higher energy by impact than ordinary concrete with and without fibers, (2) the use of long straight steel fiber is effective in improving the impact resistance of UHPFRC compared to that of deformed steel fibers at high volume fractions, (3) fiber orientation significantly influences the impact resistance of UHPFRC: when more fibers are aligned in the tensile load direction, better impact resistance is achieved, and (4) size effect on the dynamic increase factor versus strain-rate relationship is insignificant. Impact and blast resistances of UHPFRC beams, slabs, columns, and composite structures were also examined at structural level, and several useful conclusions were drawn. (1) UHPFRC is favored for impact- or blast-resistant structures as compared with ordinary concrete due to its much better impact and blast resistance at identical dimensions, reinforcement configuration, and load magnitude, (2) the use of high-strength steel rebar provides the better blast resistance of UHPFRC beams or slabs as compared with that of normal-strength steel rebar, and (3) seismic detailing applied in UHPFRC columns leads to better blast resistance than is seen for columns without seismic detailing. Further research is suggested to address the remaining complicated problems or conflicts and to inspire proper design of structural UHPFRC members in an attempt to increase the use of UHPFRC.</P> <P><B>Highlights</B></P> <P> <UL> <LI> UHPFRC dissipates much higher energy by impact than ordinary FRC. </LI> <LI> Fiber orientation significantly influences the impact resistance of UHPFRC. </LI> <LI> Long straight steel fiber is effective in improving the impact resistance of UHPFRC. </LI> <LI> Using high-strength steel in UHPFRC is efficient in enhancing the blast resistance. </LI> <LI> Adopting seismic detailing in UHPFRC columns improves the blast resistance. </LI> </UL> </P>

Development of cost effective ultra-high-performance fiber-reinforced concrete using single and hybrid steel fibers

Yoo, Doo-Yeol,Kim, Min Jae,Kim, Sung-Wook,Park, Jung-Jun Elsevier 2017 Construction and Building Materials Vol.150 No.-

<P><B>Abstract</B></P> <P>This study investigates the flexural behavior of ultra-high-performance fiber-reinforced concrete (UHPFRC) with single and hybrid steel fibers. To do this, three different types of steel fibers, i.e., hooked, twisted, and straight fibers, were considered, and a UHPFRC commercially available in North America was used as a comparison. To suggest a low-cost UHPFRC exhibiting the best flexural performance, test data and cost of fibers were analyzed based on a literature review. Test results indicate that straight steel fibers provide the best flexural performance, including strength, deflection capacity, energy absorption capacity, and cracking behavior, compared with hooked and twisted fibers, especially when many fibers (2% by volume) were incorporated. Hybrid reinforcement (hooked+straight fibers) efficiently improved the flexural performance of the UHPFRC with single hooked fibers, but the twisted+straight fibers were less effective than the UHPFRC with single twisted fibers. The optimum UHPFRCs contained 2vol% single straight steel fibers (l<SUB>f</SUB>/d<SUB>f</SUB> of 19.5/0.2) or hybrid 0.5vol% long (l<SUB>f</SUB>/d<SUB>f</SUB> of 30/0.3) and 1.5vol% medium-length (l<SUB>f</SUB>/d<SUB>f</SUB> of 19.5/0.2) straight steel fibers; they showed better flexural strength and cost effectiveness than other types of UHPFRCs.</P> <P><B>Highlights</B></P> <P> <UL> <LI> The use of S fibers is most efficient in improving flexural performance of UHPC at <I>v<SUB>f</SUB> </I> of 2%. </LI> <LI> Hybrid H and S fibers are effective in improving flexural performance of UHPFRC with single H fibers. </LI> <LI> Hybrid T and S fibers are less efficient in terms of flexural performance than single T fibers. </LI> <LI> The optimum UHPFRC mixture is suggested in terms of flexural strength and cost effectiveness. </LI> </UL> </P>

Impact Resistance of Reinforced Ultra-High-Performance Concrete Beams with Different Steel Fibers

Yoo, Doo-Yeol,Banthia, Nemkumar,Yoon, Young-Soo American Concrete Institute 2017 ACI structural journal Vol.114 No.1

<P>Ten large reinforced ultra-high-performance concrete (UHPC) beams were fabricated and tested under drop-weight impacts. The test parameters included the potential energy, fiber volume content, and steel fiber type and length. The important parameters obtained from an experimental program were summarized to provide a fundamental data set belonging to a research area that is limited within the literature. The test results showed that the addition of 2% (by volume) steel fibers was effective in decreasing the maximum and residual deflections by impact, improving residual capacities after impact damage, redistributing the tensile stress associated with microcracking, and preventing local failure at the contact surface. The use of long smooth steel fibers also resulted in the improvement of both the impact and residual capacities-that is, a decrease in the maximum and residual deflections by impact and an increase in the residual moment capacity and deflection capacity at the ultimate state. In contrast, the fiber content and type had negligible influences on the ratios of the moment capacities under impact and quasi-static loadings. Finally, a step-by-step procedure to assess the residual capacities after impact damage was proposed based on the quasi-static flexural response and the maximum deflection by impact.</P>

Experimental and numerical study on flexural behavior of ultra-high-performance fiber-reinforced concrete beams with low reinforcement ratios

Yoo, Doo-Yeol,Banthia, Nemkumar,Yoon, Young-Soo National Research Council of Canada 2017 Canadian journal of civil engineering Vol. No.

<P> Flexural behaviors of reinforced ultra-high-performance fiber-reinforced concrete (UHPFRC) beams were experimentally and numerically investigated in terms of reinforcement ratio. To do this, four UHPFRC beams with different reinforcement ratios (0%-1.71%) were fabricated and tested. Since we focused on the placement technique of the steel reinforcing bars, only a small number of reinforced UHPFRC beams were deliberately considered. Test results indicated that with an increase in the reinforcement ratio, post-cracking stiffness and load carrying capacity were increased, whereas first cracking load was decreased. The cracking behavior was characterized by numerous vertical micro-cracks up to near the peak, followed by crack localization with a gradual decrease in load carrying capacity. The number of cracks and average crack spacing were marginally influenced by the reinforcement ratio. Sectional analysis incorporating a linear compressive model and tension-softening curves obtained from inverse analyses and direct tensile test were performed and verified through comparison with the experimental moment-curvature responses. </P>

Comparative Biaxial Flexural Behavior of Ultra-High-Performance Fiber-Reinforced Concrete Panels Using Two Different Test and Placement Methods

Yoo, Doo-Yeol,Banthia, Nemkumar,Zi, Goangseup,Yoon, Young-Soo ASTM AMERICAN SOCIETY FOR TESTING AND 2017 Journal of testing and evaluation Vol.45 No.2

Effects of stirrup, steel fiber, and beam size on shear behavior of high-strength concrete beams

Yoo, Doo-Yeol,Yang, Jun-Mo Elsevier 2018 Cement & concrete composites Vol.87 No.-

<P><B>Abstract</B></P> <P>This study investigates the effectiveness of steel fibers and minimum amount of stirrups on the shear response of various sized reinforced high-strength concrete (HSC) beams. For this, six large reinforced HSC beams with a shear span-to-depth ratio (a/d) of 3.2 were manufactured. Three of them contained 0.75% (by volume) steel fibers without stirrups as per ACI Committee 318, while the rest were reinforced with the minimum amount of stirrups without fibers. Test results indicate that, with increasing beam size, significantly lower shear strength was obtained for steel fiber-reinforced high-strength concrete (SFR-HSC) beams without stirrups, than for the plain HSC beams with stirrups. The inclusion of steel fibers effectively limited crack propagation, produced more diffused initial flexural cracks, and led to higher post-cracking stiffness, compared to plain HSC. On the other hand, the use of minimum stirrups gave better shear cracking behaviors than that of steel fibers, and effectively mitigated the size effect on shear strength. Therefore, a large decrease in shear strength, with an increase in the beam size, was only obtained for SFR-HSC beams without stirrups. A shear strength decrease of 129% was obtained by increasing the effective depth from 181 mm to 887 mm. The shear strengths of reinforced steel fiber-reinforced concrete beams were not accurately predicted by most previous prediction models. Therefore, a new shear strength formula, based on a larger dataset, that considers the size effect, is required.</P>

Influence of ring size on the restrained shrinkage behavior of ultra high performance fiber reinforced concrete

Yoo, Doo-Yeol,Park, Jung-Jun,Kim, Sung-Wook,Yoon, Young-Soo Springer - RILEM Publishing 2014 Materials and Structures Vol.47 No.7