Sung-Yong Choi

An Experimental Study on Mechanical Properties of Hybrid Fiber Reinforced Concrete Pavement

Cement concrete pavement offers long-term service life and excellent applicability for heavy traffic. It is easier to purchase and more durable and economical than the asphalt pavement. However, it is difficult to repair and rehabilitate compared to the asphalt pavement when it comes to the maintenance problem. Since the crack is the main reason of the damage of concrete pavement, it is necessary to control the early and long-term crack in the concrete pavement. In this experimental study, the basic performance tests have been carried out to investigate the effect of hybrid fibers which were composed of micro fibers with small diameter and high aspect ratio and macro fibers with large diameter and low aspect ratio on the concrete pavement, in which lower water ratio and larger aggregates were used compared to the general concrete mixture. The test results showed that the flexural strength and toughness of concrete pavement mixture have been increased with the use of hybrid fibers in the concrete pavement mixture, even though they were less effective compared to the normal concrete mixture. It was found that the hybrid fibers were effective to control the early shrinkage of the concrete pavement which is one of the main reasons of the damage in the concrete pavement.

Mechanical Performance of Fiber Reinforced Lean Concrete for Subbase of Newly Developed Multi-Functional Composite Pavement System

PURPOSES: This study is to investigate the mechanical performance of the fiber reinforced lean concrete with respect to different types of fibers. METHODS: Increased vehicle weight and other causes from the exposed conditions have accelerated the deteriorations of road pavement. A new multi-functional composite pavement system is being developed recently in order to extend service life and upgrade the pavement. A variety of tests were conducted before and after hardening of the concrete. RESULTS: From the test results, it was found that the use of different types of fibers did not affect the compressive strength development. This might be due to the inherent property of the lean concrete. When steel fibers were used relatively greater flexural strength and flexural fracture toughness were developed. Also addition of fly ash by replacing a part of Portland cement the fracture toughness was slightly increased. CONCLUSIONS: It has been known that the addition of fibers and use of mineral admixture can be positively considered in the development of multi-functional composite pavement system as its required mechanical performance is obtained.

A Study on the Bond Performance Improvement of Polypropylene Macro Fibers According to the Change of Surface Area

Fiber Reinforced Concrete (FRC) is a representative strengthening method adopted to improve the sudden brittle failure of concrete under tension and dynamic loadings and to control the generation and propagation of cracks occurring during the curing process. The strengthening performance provided by the fiber is sensitively influenced by the physical properties of the fiber itself as well as by its volume ratio but is also varying according to the difference in the bond performance at the interface between the fiber and concrete. Even if the fiber exhibits outstanding physical performance, if the bond performance does not reach similar level, the fiber will not be able to develop fully its performance. Currently, the methods improving the fiber bond performance are essentially focusing on the improvement of the mechanical bond performance by changing the longitudinal shape of the fiber such as the application of crimped and hooked fibers. A very few methods intend to improve the bond performance by increasing the bond area of the fiber itself. Therefore, this study increases the surface area of the fiber by changing its section while preserving its volume during the fabrication process, conducts tests on the bond performance between the cement paste and the fiber with changed surface area, and examines the changes in the bond performance according to the increase of the surface area.

Performance Analysis of Strength Development of FRC Base Depending on Maturity

PURPOSES : In this study, we analyzed the compressive strength characteristics of lean base concrete in relation to changes in the outdoor temperature after analyzing the cold and hot weather temperature standards and calculated the minimum and maximum temperatures when pouring concrete. We examined the rate of strength development of lean base concrete in relation to the temperature change and derived an appropriate analysis formula for FRC base structures by assigning the accumulated strength data and existing maturity formula. METHODS : We measured the strength changes at three curing temperatures (5, 20, and ) by curing the concrete in a temperature range that covered the lowest temperature of the cold period, , to the highest temperature of the hot period, . We assigned the general lean concrete and FRC as test variables. A strength test was planned to measure the strength after 3, 5, 7, 14, and 28 days. RESULTS : According to the results of compressive strength tests of plain concrete and FRC in relation to curing temperature, the plain concrete had a compressive strength greater than 5 MPa at all curing temperatures on day 5 and satisfied the lean concrete standard. In the case of FRC, because the initial strength was substantially reduced as a result of a 30% substitution of fly ash, it did not satisfy the strength standard of 5 MPa when it was cured at on day 7. In addition, because the fly ash in the FRC caused a Pozzolanic reaction with the progress into late age, the amount of strength development increased. In the case of a curing temperature of , the FRC strength was about 66% on day 3 compared with the plain concrete, but it is increased to about 77% on day 28. In the case of a curing temperature of , the FRC strength development rate was about 63% on day 3 compared with the plain concrete, but it increased to about 88% on day 28. CONCLUSIONS : We derived a strength analysis formula using the maturity temperatures with all the strength data and presented the point in time when it reached the base concrete standard, which was 5 MPa for each air temperature. We believe that our findings could be utilized as a reference in the construction of base concrete for a site during a cold or hot weather period.

Evaluation of the Flexural Performance of Fiber Reinforced Mortar with Low Fiber Content

This study investigates the relationship between the amount of fiber reinforcement and flexural tensile capacity by evaluating the degree of dispersion and the fiber reinforcement effect in mortar mixes with low fiber contents (lower than 0.2%) in order to derive solutions enabling to improve the tensile performance of mortar with low contents in macro fibers. The results show that a certain degree of improvement of the toughness is achieved even in the case of fiber reinforced mixes with low fiber contents and that the variation of the toughness capacity is sensitive to the degree of dispersion of the fibers.

An Experimental Study on the Creep of FRC Sub-Base for Pavement Using a New Testing Device

The hydraulic loading device commonly used for creep test necessitates continuous recharge of the hydraulic pressure with time and is accompanied by slight variation of the permanent load at each recharge. Therefore, accurate test results cannot be obtained for long-term creep tests requiring time-dependent behavioral analysis during more than 6 months. This study conducts creep test as part of the analysis of the long-term characteristics of fiber-reinforced lean concrete sub-base of pavement. The creep test is executed using the new load-amplifier device not a conventional loading device. Since the results of the preliminary verification test on the new creep test device show that constant permanent load is applied without significant variation, it can be expected that more accurate measurement of the creep will be possible in a long-term compared to the conventional hydraulic device. In addition, the creep test results of sub-base specimens reveal the occurrence of large instantaneous elastic strain, differently from the strain curve observed in ordinary concrete, as well as the occurrence of small creep strain leading to low creep coefficient.

Mechanical Properties of Concrete Pavement by Low Fraction of Macro Fiber

PURPOSES : The purpose of the study was to examine dynamic features of concrete after mixing a little macro fiber with small aspect ratio and long length utilized for bridge, tunnel and shotcrete for tensile performance and crack control in domestic/overseas countries with cement concrete pavement mix. METHODS : Coarse aggregates with small aspect ratio and macro fibers with maximum length of approximately 32 mm are introduced in small quantities in the mix proportions of concrete pavement so as to prevent loss of the workability. Then, this study intends to evaluate the applicability of macro fibers in the mix proportions of concrete pavement by examining the basic construction performance, as well as the change of toughness, the equivalent bending strength and the flexural toughness index caused by compression, bending, tension and the flexural stress-displacement curve. RESULTS : As the results, in each kind of macro fiber, polyvinyl alcohol fiber and steel fiber displayed a good performance. CONCLUSIONS : In 0.2 and 0.3% of fiber contents, it is appeared that polyvinyl alcohol fiber has a large effect on improvement of tensile performance and steel fiber on improvement of deforming performance of bending stress.