Su-Tae Kang

The Effect of Steel-Fiber Contents on the Compressive Stress-Strain Relation of Ultra High Performance Cementitious Composites (UHPCC)

The effect of steel-fiber contents on the compressive behavior of ultra high performance cementitious composites (UHPCC) was studied to propose a compressive behavior model for UHPCC. The experiments considered fiber contents of 0~5 vol.% and the results indicated that compressive strength and corresponding strain as well as elastic modulus were improved as the fiber contents increased. Compared to the previous study results obtained from concrete with compressive strength of 100MPa or less, the reinforcement effect on strength showed similar tendency, while the effect on the strain and elastic modulus were much less. Strength, strain, and elastic modulus according to the fiber contents were presented as a linear function of fiber reinforcement index (RI). Fiber reinforcement in UHPCC had no influence on the shape of compressive behavioral curve. Considering its effect on compressive strength, strain, and elastic modulus, a compressive stress-strain relation for UHPCC was proposed.

Improvement in Predicting the Post-Cracking Tensile Behavior of Ultra-High Performance Cementitious Composites Based on Fiber Orientation Distribution

In this paper, the post-cracking tensile behavior of Ultra-High Performance Cementitious Composites (UHPCC) was studied and an improved analytical model to predict the behavior depending on the fiber orientation distribution was proposed. Two different casting methods were adopted to estimate the influence of the casting method on the tensile behavior. The direct tensile test results showed that the post-cracking tensile behavior was considerably dependent on the casting method. The influence of the casting method was quantified by image analysis of the fiber distribution. The fiber orientation distribution obtained by image analysis may sometimes include considerable error according to the image resolution, which may cause inaccuracy when predicting the post-cracking tensile behavior based on the fiber orientation distribution. To overcome this dependency, the tensile bridging behavior by the fibers in UHPCC was simulated considering the obtained fiber orientation distribution as well as the number of fibers detected. The post-cracking behavior was then simulated by combining the bridging behavior and tension softening behavior of the matrix. The approach adopted in this study to simulate the post-cracking behavior of UHPCC showed good agreement with the experimental results.

Optimum Mix Design of Alkali-Activated Cement Mortar Using Bottom Ash as Binder

In this research, the possibility of using bottom ash as a binder for the alkali-activated cement mortar is studied. Several experiments were performed to investigate the variation of the material properties according to the mix proportion. In the experimental program, the flowability and compressive strength were evaluated for various values of water/ash ratio, activator/ash ratio, sodium silicate to sodium hydroxide ratio, curing temperature, and the fineness of bottom ash as the main variables. The experimental results showed that high strength of 40 MPa or greater could be achieved in 60oC high temperature curing condition with proper flowability. For 20℃ ambient temperature curing, the 28 days compressive strength of approximately 30MPa could be obtained although the early-age strength development was very slow. Based on the results, the range of optimized mix design of bottom-ash based alkali-activated cement mortar was suggested. In addition, using the artificial neural network analysis, the flowability and compressive strength were predicted with the difference in the mix proportion of the bottom-ash based alkali-activated cement mortar.

Performance Evaluation of Sprayed Ductile Fiber-Reinforced Mortar as a Repairing Material

Most of existing repair materials have some shortcomings such as brittle fracture, imperfect interface bonding and marked difference in modulus of elasticity compared with the structures. These problems make their repair inefficient. Some researches on using a fiber-reinforced mortar as an alternative to enhance the efficiency have been carried out recently. This paper presents the results of an experimental study on the performance of sprayed PVA fiber-reinforced mortar as a repair material. We evaluated its mechanical properties, durability and strengthening effect. This study shows that the sprayed PVA fiber-reinforced mortar is remarkably effective as a repair material.