Hyunjin Ju

Estimation of Concrete Carbonation Depth Considering Multiple Influencing Factors on the Deterioration of Durability for Reinforced Concrete Structures

While the durability of concrete structures is greatly influenced by many factors, previous studies typically considered only a single durability deterioration factor. In addition, these studies mostly conducted their experiments inside the laboratory, and it is extremely hard to find any case in which data were obtained from field inspection. Accordingly, this study proposed an Adaptive Neurofuzzy Inference System (ANFIS) algorithm that can estimate the carbonation depth of a reinforced concrete member, in which combined deterioration has been reflected based on the data obtained from field inspections of 9 buildings. The proposed ANFIS algorithm closely estimated the carbonation depths, and it is considered that, with further inspection data, a higher accuracy would be achieved. Thus, it is expected to be used very effectively for durability estimation of a building of which the inspection is performed periodically.

Application of Hydrophilic Silanol-Based Chemical Grout for Strengthening Damaged Reinforced Concrete Flexural Members

In this study, hydrophilic chemical grout using silanol (HCGS) was adopted to overcome the performance limitations of epoxy materials used for strengthening existing buildings and civil engineering structures. The enhanced material performances of HCGS were introduced, and applied to the section enlargement method, which is one of the typical structural strengthening methods used in practice. To evaluate the excellent structural strengthening performance of the HCGS, structural tests were conducted on reinforced concrete beams, and analyses on the flexural behaviors of test specimens were performed by modified partial interaction theory (PIT). In particular, to improve the constructability of the section enlargement method, an advanced strengthening method was proposed, in which the precast panel was directly attached to the bottom of the damaged structural member by HCGS, and the degree of connection of the test specimens, strengthened by the section enlargement method, were quantitatively evaluated by PIT-based analysis.

Shear Behavior Models of Steel Fiber Reinforced Concrete Beams Modifying Softened Truss Model Approaches

Recognizing that steel fibers can supplement the brittle tensile characteristics of concrete, many studies have been conducted on the shear performance of steel fiber reinforced concrete (SFRC) members. However, previous studies were mostly focused on the shear strength and proposed empirical shear strength equations based on their experimental results. Thus, this study attempts to estimate the strains and stresses in steel fibers by considering the detailed characteristics of steel fibers in SFRC members, from which more accurate estimation on the shear behavior and strength of SFRC members is possible, and the failure mode of steel fibers can be also identified. Four shear behavior models for SFRC members have been proposed, which have been modified from the softened truss models for reinforced concrete members, and they can estimate the contribution of steel fibers to the total shear strength of the SFRC member. The performances of all the models proposed in this study were also evaluated by a large number of test results. The contribution of steel fibers to the shear strength varied from 5% to 50% according to their amount, and the most optimized volume fraction of steel fibers was estimated as 1%–1.5%, in terms of shear performance.

Shear Strength Reduction Factor of Prestressed Hollow-Core Slab Units Based on the Reliability Approach

This study investigated the shear design equations for prestressed hollow-core (PHC) slabs and examined the suitability of strength reduction factors based on the structural reliability theory. The reliability indexes were calculated for the shear strength equations of PHC slabs specified in several national design codes and those proposed in previous studies. In addition, the appropriate strength reduction factors for the shear strength equations to ensure the target reliability index were calculated. The results of the reliability index analysis on the ACI318-08 equation showed that the shear strengths of the members with the heights of more than 315 mm were evaluated to be excessively safe, whereas some members with low depths did not satisfy the target reliability index.

Properties and applications of non-shrinkage chemical grout for repairing and strengthening of concrete structures

Epoxy resins are widely used in structural repairing and strengthening work, but most of them typically require primer works and are vulnerable to water. To overcome such limits, a non-shrinkage chemical grout (NCG) has been developed in this research, which provides a strong chemical bond strength utilizing Silanol/Si-OH. The material characteristics of the NCG were investigated from experiments and the applications of non-shrinkage chemical grout were also provided. The experimental investigation indicates that NCG has much improved performance compared to a typical epoxy product, which implies that the NCG can be widely used for repairing and strengthening work.

Consideration on punching shear strength of steel-fiber-reinforced concrete slabs

The flat plate slab system is widely used in construction fields due to its excellent constructability and savings in story height compared to the conventional beam-column moment-resisting system. Many researchers are, however, concerned about the punching shear failure that can happen in a two-way flat plate slab system, for which many shear-strength-enhancement techniques have been suggested. One of the effective alternatives is the application of steel-fiber-reinforced concrete. However, most previous studies on the punching shear strength of steel-fiber-reinforced concrete flat slabs had presented empirical formulas based on experimental results. On the other hand, theoretical models proposed in previous studies are difficult to be applied to practical situations. Therefore, in this study, a punching shear strength model of the steel-fiber-reinforced concrete two-way flat slab is proposed. In this model, the total shear resistance of the steel-fiber-reinforced concrete flat slab is expressed by sum of the shear contribution of steel fibers in the cracked tension region and that of intact concrete in the compression zone. A total of 91 shear test data on steel-fiber-reinforced concrete slab–column connection were compared to the analysis results, and the proposed model provided a good accuracy on estimating the punching shear strength of the steel-fiber-reinforced concrete flat slabs.

Minimum Torsional Reinforcement Ratio of Reinforced Concrete Members for Safe Design

Dept. of Architectural Engineering, Sungkyunkwan University, Suwon 440-746, KoreaABSTRACT Current design codes regulate the minimum torsional reinforcement requirement for reinforced concrete members to prevent their brittle failure. The minimum torsional reinforcement ratio specified in the current national code and ACI318-11, however, have problems in the minimum longitudinal reinforcement ratio for torsion, the equilibrium condition in space truss mo del, and a marginal strength, etc. Thus, in order to overcome such shortcomings, this study presents a rational equation for minimum torsional reinforcement ratio that can provide a sufficient margin of safety in design. The minimum torsional reinforcement ratio proposed in this study was compared to the test results available in literature, and it was confirmed that it gave a proper margin of safety for all specimens studied in this paper.Keywords : minimum torsional reinforcement, reserved strength, reinforced concrete, torsional cracking strength