The voided slab system has been known as an effective technique to replace a heavy reinforced concrete slab system without the decrease in flexural strength. However, according to the previous studies, the flexural capacities such as flexural strength, stiffness and ductility of the voided slabs were practically lower than that of the solid slabs depending on the void shapes and details. Therefore, in this study, an analytical and experimental study were conducted to derive the optimal void shape and details focused on the flexural capacities of voided slabs. Based on a finite element (FE) analysis, a donut-type void shaper, which was a hexahedron with rounded edges and a hole penetrating the center, was suggested as the optimal shape in voided slabs, and an experimental study was conducted to verify flexural capacities of the donut-type voided slab. The flexural strength, stiffness and deflection of the donut-type voided slab were investigated by void shape and fixing method of void shaper as variables. The ductility of voided slab was also evaluated, because ductility is as important as strength for the safe design of slab member. The test results showed that the flexural strength of the donut-type voided slabs was equivalent to 98% and 105% that of the solid RC specimen, and the donut-type voided slab specimens had enough ductility for the flexural member. The stiffness of the donut-type voided slab was decreased about 8~9% compared with the solid slab, but it was improved up to 7% compared to the non-donut-type voided slab. Based on test results, the flexural design method of the donut-type voided slab associated with the void shape and fixing device of void shaper was suggested, and it was confirmed that the donut-type voided slab is one of the efficient alternatives to replace heavy flat plate slabs.

This study experimentally investigates the strengthening performance and failure behavior of reinforced concrete slabs coated with high elastic modulus polyurea(HEPU). To address the limited strengthening effect of conventional polyurea due to its low elastic modulus and strength, HEPU was applied to slab specimens with varying coating thicknesses (1, 2, and 3 mm). To evaluate the effectiveness of the HEPU reinforcing method, experiments were conducted with varying coating thicknesses, and the results were compared with those of unstrengthened specimens. The results showed that HEPU strengthening significantly enhanced flexural strength and stiffness, with improvements increasing proportionally with coating thickness. Additionally, the polyurea coating enhanced crack resistance and adhered well to the concrete, preventing delamination or detachment up to final failure. The slabs maintained ductile behavior even after HEPU rupture, although load resistance decreased post-rupture. However, the strengthening effect diminished due to the rupture of the HEPU, indicating limited effectiveness in improving ductility. These findings provide insights into the design and application of polyurea-based strengthening techniques for reinforced concrete structures.

In reinforced concrete (RC) structures, bond behavior between concrete and deformed steel bar (rebar) has a great effect on the structural behavior of members including voided slabs. According to previous research, bond behavior of a rebar in voided slabs is different from that in general RC slabs by the concrete inner cover formed between rebar and void. However, the bond characteristics of rebar in voided slabs have not been clearly verified yet. This study investigated the bond characteristics of rebar in the donut-type voided slab. Like other voided slabs, the donut-type void-shaper make concrete inner cover formed between rebar and void, and some part of the donut-type voided slab has smaller inner cover thickness than 2.5 d_b. Furthermore, inner cover thickness changes along the longitudinal rebar due to the shape of void. These were expected to affect the bond behavior of rebar in the donut-type voided slab. In this study, pull-out tests were performed to find out the effect of bond condition by the region due to voids. To this end, the donut-type voided slab was divided into three regions according to the shape of void, such as ‘Insufficient region’, ‘Transition region’, and ‘Sufficient region’. Main variables were inner cover thickness, embedded length, and bond location, which is affected by void shape. Bond characteristics of rebar in the donut-type voided slab were evaluated through comparison of bond stress–slip relationship, maximum bond strength, and bond stress distribution of each region. Test results showed that the average bond stress was found to be 25~50% lower than that of the general RC slab in the region of the inner cover thickness of 2.5 d_b, which was not satisfied due to void, and the bond stress–slip relationship and the bond stress distribution of the donut-type voided slab was also different from that of the general RC slab according to the type of bond region and inner cover thickness. In addition, the calculation method of bond strength of rebar in donut-type voided slab was proposed, which indicated that the error range between the estimated value and the experimental value was within 5%.

The purpose of this study was to evaluate the structural performance of composite deck slabs containing macro-synthetic fibers. after a fire by proposing a deflection estimation method for non-fireproof structural decks. Therefore, this study evaluated the fire resistance performance and deflection of deck slabs mixed with macro-synthetic fibers. Afterward, the deflection estimation method considering the thermal characteristics of concrete and deck plates was proposed. A material test was first conducted to evaluate the mechanical properties of concrete mixed with macro-synthetic fibers. This test found that the compressive strength and elasticity modulus of concrete mixed with macro-synthetic fibers was greater than that of general concrete. A flexural tensile test confirmed that residual strength was maintained after the maximum strength was achieved. The fire resistance of the deck slab was adequate even when a fire-resistant coating was not applied. The internal temperature was lowest for the specimen with macro-synthetic fibers. Deflection was evaluated using previously published equations and standards. The deflection evaluation confirmed that the temperature distribution should be applied differently in the estimation method that uses the thermal load of the deck slab.

An underground electric power utility tunnel is an important piece of infrastructure. Securing the durability of such structures is essential to ensuring their long lifecycles. However, the concrete of an electric power utility tunnel is likely to be carbonized, owing to long-term exposure to high carbon dioxide. Moreover, the carbonation of concrete may be intensified by damage, such as cracks. In this study, the characteristics of carbonation in electric power utility concrete were examined based on crack width and depth. The results of the experiment showed that the crack width was an important variable in the carbonation of concrete in an electric power utility tunnel. Carbonization proceeded at the tip of the crack, even if the crack was deep and the width was small.