Myoung-Sung Choi

Ultimate Strength of Concrete Barrier by the Yield Line Theory

When the yield line theory is used to estimate the ultimate strength of a concrete barrier, it is of primary importance that the correct assumption is made for the failure mode of the barrier. In this study, a static test was performed on two full-scale concrete barrier specimens of Korean standard shape that simulate the actual behavior of a longitudinally continuous barrier. This was conducted in order to verify the failure mode presented in the AASHTO LRFD specification. The resulting shape of the yield lines differed from that presented in AASHTO when subjected to an equivalent crash load. Furthermore, the ultimate strengths of the specimens were lower than the theoretical prediction. The main causes of these differences can be attributed to the characteristics of the barrier shape and to a number of limitations associated with the classical yield line theory. Therefore, a revised failure mode with corresponding prediction equations of the strength were proposed based on the yield lines observed in the test. As a result, a strength that was more comparable to that of the test could be obtained. The proposed procedure can be used to establish more realistic test levels for barriers that have a similar shape.

Graphical Assessment for Span Ranges of PSC Girder Bridges

Although the prestressed concrete (PSC) girder bridge is known to be more economical than other types of bridges in short and medium spans, a longer span has also been achieved by applying several strategies. This paper presents a systematic procedure that can be used to assess the effects of these strategies on the span. The proposed scheme adopts a graphical approach that represents a relationship between the number of prestressing tendons and the span and is derived on the basis of stress assessment equations of the girder at each stage of fabrication and in service. A quantitative evaluation for the extension of the span is performed by adopting a sample bridge. A number of advantages of the proposed scheme are apparently shown for determining why and how each strategy contributes to the span extension and for suggesting further improvement for a longer span. The results imply that increasing the strength of the girder, making the girders continuous, multistage prestressing, and the decked PSC girder are very effective.

Failure Mode and Ultimate Strength of Precast Concrete Barrier

This study examines the validity of design formulas adopted from the yield line theory of American Association of State Highway and Transportation Officials load and resistance factor design (AATSHTO LRFD) specifications for precast concrete barriers with a potential longitudinal discontinuity and for barrier sections with a slope discontinuity. A full-scale static test was performed on newly-developed type of precast concrete barrier system. Protruded reinforcements and a mortar filling were used to combine the barrier and deck. Five specimens were fabricated with different test variables that simulate the cantilevered part of the deck and a series of precast barriers. Test levels and the corresponding loading patterns simulating a vehicle crash were adopted from AASHTO LRFD specifications. Findings show that the new precast concrete barrier system had an ultimate capacity equivalent to that of a conventional barrier. The longitudinal continuity of a series of precast barrier segments significantly affected the ultimate strength of the whole barrier system. The shapes of the yield liens obtained in the static test were different than those presented in the AASHTO specifications. An alternative failure mode and corresponding predictive equations of the strength based on actual yield lines were proposed. The proposed theories may be applicable to barriers with a tapered section with some points of slope discontinuity.

An Advanced Assessment Strategy of Thermal Cracks Induced by Hydration Heat and Internal Restraint

Control of the temperature difference across a section is an effective strategy to minimize the hydration-heat-induced cracks for the structures where internal restraint is dominant. The domestic code, however, overestimates probability of the crack occurrence judging from the foreign codes and construction experiences of real structures. Therefore, the background of the equation presented in the domestic code was investigated step by step to examine validity of the equation, and, as a result, it was found that the equation is established on a basis of simple elastic model where the change of elastic modulus in an early age is not considered. An advanced assessment strategy was proposed taking into account the hypoelastic model which corresponds to an incremental constitutive equation. The presented procedure resulted in an increased crack index, i.e. decreased crack risk, the value of which depends on various conditions of the mix and structures. Also, a prediction equation of the temperature difference was proposed which can readily consider the effect of the curing condition and ambient temperature in a hand calculation. For further study, the assessment equation may be more classified to strictly consider the characteristics of the mix and structures if the analytical and experimental data are accumulated.

Determination of Convection Heat Transfer Coefficient Considering Curing Condition, Ambient Temperature and Boiling Effect

The setting and hardening of concrete is accompanied with nonlinear temperature distribution caused by development of hydration heat of cement. Especially at early ages, this nonlinear distribution has a large influence on the crack evolution. As a result, in order to predict the exact temperature history in concrete structures it is required to examine thermal properties of concrete. In this study, the convection heat transfer coefficient which presents thermal transfer between surface of concrete and air, was experimentally investigated with variables such as velocity of wind, curing condition and ambient temperature. At initial stage, the convection heat transfer coefficient is overestimated by the evaporation quantity. So it is essential to modify the thermal equilibrium considered with the boiling effect. From experimental results, the convection heat transfer coefficient was calculated using equations of thermal equilibrium. Finally, the prediction model for equivalent convection heat transfer coefficient including effects of velocity of wind, curing condition, ambient temperature and boiling effects was theoretically proposed. The convection heat transfer coefficient in the proposed model increases with velocity of wind, and its dependance on wind velocity is varied with curing condition. This tendency is due to a combined heat transfer system of conduction through form and convection to air. From comparison with experimental results, the convection heat transfer coefficient by this model was well agreed with those by experimental results.