Evaluation of the ultimate strength for the UHPFRC (ultra-high-performance fiber-reinforced concrete) flexural members was conducted. In this study, an experimental program about UHPFRC beams was conducted with the effect of fiber volume fraction, shear span to depth ratio, and compressive strength of matrix as the main variables. Among them, it was found that fiber volume fraction was the variable that had the greatest influence on the ultimate strength. The inclusion of 2% volume fraction steel fiber increases the shear and flexural strength of UHPFRC beams significantly. In particular, steel fiber inclusion changed the mode of failure of beams from diagonal shear failure into flexural failure. For the classification of failure patterns, the ultimate flexural strength and shear strength of UHPFRC members were evaluated using the current design code and UHPC guidelines. Flexural ultimate strength was affected by the size and shape of the stress block and consideration of the matrix’s tensile strength. For the accurate shear strength prediction of UHPFRC beams, the tensile strength of the high strength matrix and the effect of steel fiber should be considered.

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.

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%.

Design guidance for headed-bar development remains uncertain for large-diameter bars at cutoff points, where bar termination increases anchorage demand and confinement is often limited. This study quantified the anchorage behavior of 43 and 57 mm headed bars and established a regression-based strength model grounded in a splitting-controlled bond–bearing mechanism. Nineteen reinforced concrete beam specimens were tested under four-point loading configured to place the bending-moment inflection point at the head location. The primary variables were the development length (l_dt = 12–28d_b), concrete compressive strength (f_c′ = 42 and 70 MPa), clear side cover, clear spacing, and transverse reinforcement index (K_tr/d_b = 0–2.0). All the specimens failed by splitting prior to bar yielding, characterized by longitudinal cracking along the development region and cover spalling near the head. The anchorage strength increased with concrete compressive strength and development length and was most strongly enhanced by transverse reinforcement (up to ~60%). At failure, the bond contributed 70–86% of the developed stress, while the head-bearing contribution increased with confinement. Existing ACI 318-19 and KDS-2021 provisions were generally unconservative, particularly for unconfined specimens. The proposed bond–bearing model showed a close agreement with the test database (mean test/prediction = 0.99; COV = 4.72%) within stated parameter limits.

Recently, the service lives of electric power utility tunnels have been increasing rapidly; however, inspection and maintenance tasks are difficult, and the economic loss owing to the end of tunnels’ lifespans is substantial. Moreover, electric power utility tunnels are exposed to high carbon dioxide. Because their interiors are sealed environments, the tunnels are significantly affected by carbon dioxide. Carbon dioxide can cause concrete carbonation, which can lead to a decline in the performance of electric power utility tunnels. In this study, the carbonation characteristics of concrete were analyzed, and the effect of carbonation on the mechanical performance was evaluated.

Recently, the service life of electric power utility tunnels has been rapidly increasing; however, inspection and maintenance are difficult because of the characteristics of electric power utility tunnels. In addition, there are no detailed guidelines for electric power utility tunnels regarding the safety and maintenance of existing facilities. Therefore, it is necessary to establish a comprehensive performance evaluation system that reflects the characteristics of electric power utility tunnels. In this study, we determine and evaluate the factors influencing durability for the development of a comprehensive performance evaluation system for electric power utility tunnels via a field investigation.