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Thermal strain behavior and strength degradation of ultra-high-strength-concrete
Lee, Y. W.,Kim, G. Y.,Gucunski, N.,Choe, G. C.,Yoon, M. H. Rilem Publications 2016 Materials and Structures Vol.49 No.8
<P>With the increasing application of high-strength-concrete (HSC) in high-rise-buildings for which structural safety performance is needed. However, the thermal mechanical behavior of HSC exposed to fire differs from that of normal-strength concrete (NSC). HSC is known to show different thermal strain behavior and strength degradation from NSC. It is needed to consider the thermal strain of HSC at elevated temperature under loading condition. In this study, the thermal strain behavior and strength degradation of HSC when exposed to elevated temperatures under loading conditions were examined experimentally. The compressive strength, thermal expansion, total strain, and hightemperature-creep at elevated temperatures were evaluated. To evaluate the thermal expansion of HSC at elevated temperatures, HSC with compressive strengths of 80,130, and 180 MPa concrete were heated to 700 degrees C at a rate of 1 degrees C/min. The total strain and high-temperature-creep were measured under the loading condition of 25 % of the compressive strength at room temperature. The experimental results clearly showed that the strength degradation of HSC increased with the higher compressive strength at elevated temperature. Thermal expansion occurred consistently regardless of the strength level without loading. However, 180 MPa concrete failed while being heated to around 300 degrees C. The transient creep had a large influence on the high temperature-creep as the HSC was heated at elevated temperature while under a load. It is considered that the reduced amount of aggregate and increased binder content make extremely density matrix in HSC, and it is particularly evident as the compressive strength increased.</P>
You, Y. J.,Kim, J. H.,Park, Y. H.,Cho, C. M. Rilem Publications 2016 Materials and Structures Vol.49 No.11
<P>Since FRP rebars developed up to present are produced without specific guidelines on their content ratio, fiber type, and surface shape, the performance capacity of FRP reinforced concrete (RC) members cannot be fully guaranteed even though the concrete members were designed according to the existing design guidelines. Moreover, since investigations of FRP RC members have focused mainly on their short-term behavior, knowledge of their long-term behavior (i.e., creep and shrinkage effects) of FRP RC members is greatly needed for a proper design of the members. In order to verify material and structural creep behavior of a deformed glass FRP (GFRP) rebar with a guaranteed tensile strength of minimum 900 MPa manufactured from modified braidtrusion process, long-term material and structural creep tests of the GFRP rebar and GFRP RC beam, respectively, were conducted. The material creep test result showed that a service life of 100 years can be guaranteed when the sustained load magnitude is less than 50 % of the maximum tensile load capacity of the rebar. The structural creep test results showed that current design guidelines either overestimate or underestimate the long-term deflection of GFRP RC members. Therefore, a coefficient accounting for the time-dependent characteristics must be properly selected.</P>
Effect of aggregate on residual mechanical properties of heated ultra-high-strength concrete
Kim, Y. S.,Ohmiya, Y.,Kanematsu, M.,Kim, G. Y. Rilem Publications 2016 Materials and Structures Vol.49 No.9
<P>When concrete structures are exposed to fire temperatures, their internal structure will change, and then the service life of the structure will decrease due to the deterioration of its strength and deformation capacity. The deterioration level will depend on the temperature reached, exposure time, mix proportions of the concrete, aggregate property and the material's own characteristics. This study was carried out to evaluate the influence of water to cement ratio, fine-grained aggregate to aggregate ratio and maximum size of aggregate on the thermal behavior of ultra-high-strength concrete. At room temperature and 500 degrees C the tests for compressive strength, static modulus of elasticity, ultrasonic pulse velocity, dynamic modulus of elasticity and weight loss were measured using the cylindrical specimens of empty set100 9 200 mm. The test data indicated that the fine-grained aggregate to aggregate ratio and the maximum size of aggregate have a significant influence on the residual mechanical properties of ultra-high strength concrete. And by measuring the ultrasonic pulse velocity of the ultra-high-strength concrete specimens, it was confirmed in this study that predicting the residual compressive strength is a distinct possibility.</P>