Chang-Il Jang

High Ssrength polymer-modified repair cementitious composite for fire protection

The purpose of this study was to evaluate the mechanical performance and fire resistance of a high strength polymer-modified cementitious composite, to test its ability to repair concrete tunnel structures that are in danger of collapse due to cracks or deterioration. In particular, because existing repair materials are not fire-resistant and commercial fire-resistant materials have low strength, this study was aimed at increasing the water tightness and strength of a repair material and also making it resistant to fire. In addition, this study evaluated changes in internal temperature depending on the cover thickness of repair materials to determine the optimal cover thickness at which a high strength polymer-modified cementitious composite could protect existing concrete tunnel structures from fire. Results indicated that the high strength polymer-modified cementitious composite had superior strength and water tightness than commercial fire-resistant materials and it also provided good fire resistance. The high strength polymer-modified cementitious composite required to be applied in an optimal layer thickness of at least 40 mm to protect existing structures from fire.

Tensile fracture and bond properties of ductile hybrid FRP reinforcing bars

Fibre reinforced plastic (FRP) rebar has only linear elastic behaviour, whereas steel rebar has linear elastic behaviour up to its yield point, followed by a large amount of plastic deformation. With no plastic deformation, a small increase in the load acting on concrete structures containing FRP rebar can cause a catastrophic collapse without warning. A ductile hybrid FRP rebar was manufactured and evaluated in this study. The tensile and bond strengths of the hybrid FRP rebar were tested to determine its fracture properties and bond behaviour. It showed elastic behaviour up to the point of early fracture, but had very irregular behaviour thereafter. The fibres in the sleeves were broken at very irregular lengths, while the cores appeared to fracture in a regular pattern. The stress–strain curves were linearly elastic, with a definite yield point followed by pseudo-plastic deformation. In addition, the hybrid rebar specimens exhibited a large amount of slip when the peak load was reached.

Fire Resistance Performance for Hybrid Fiber Reinforced High Strength Concrete Column Member

Dam Engineering Research Center, Korea Institute of Water and Environment, Ansan 305-730, KoreaABSTRACT This study evaluated fire resistance performance for hybrid (polypropylene + steel) fiber reinforced high strengthconcrete column. Full-size columns were constructed and tested with or without fibers using ISO-834 fire curve. As the result oftest, Control specimen occurred serious spalling and indicated rapidly internal temperature increasing. Specimen with polypro-pylene fiber occurred not spalling but steady internal temperature increasing. Specimen with hybrid fiber occurred not spalling aswell as does not propagated temperature distribution. Therefore, hybrid fiber reinforced column specimen indicated a good fireresistance performance than other cases.Keywords : fire resistance, high strength column, polypropylene fiber, steel fiber

Durability Characteristics of High Performance Shotcrete for Permanent Support of Large Size Underground Space

This study evaluated the durability of high-performance shotcrete mixed in the proper proportions using alkali-free and cement mineral accelerators as a permanent support that maintains its strength for the long term. Durability tests were performed the chloride permeability, repeated freezing and thawing, accelerated carbonation, and the effects of salt environments. Test results showed that all the shotcrete mixes included silica fume had low permeability. In addition, after 300 freeze/thaw cycles, the shotcrete mix had excellent freeze/thaw resistance more than the 85% relative dynamic modulus of elasticity. The accelerated carbonation test results were no effect of accelerator type but, the depth of carbonation was greater in the shotcrete mix containing silica fume. No damage was seen in a salt environments. Therefore, the high performance shotcrete mix proportions used in this study showed excellent durability.

Mechanical Properties and Microstructures of Gfrp Rebar after Long-Term Exposure to Chemical Environments

This study describes accelerated degradation tests on glass fibre-reinforced polymer (GFRP) rebar to evaluate its long-term durability. The tests used simulated building acid, sulphate and de-icing environments. We measured the resulting reduction in the mechanical properties of the rebar and performed a microstructural degradation analysis of the rebar. The acidic environment consisted of a 0.6% acetic acid solution at a pH of 2.92, whereas the sulphate environment contained 3% sodium sulphate solution, and the de-icing environment was composed of 4% calcium chloride solution. Tensile strength tests were performed to measure the mechanical properties of the rebar after it was immersion in one of the environments for 50 to 100 days. We also measured the change in the internal pore distribution and in the shape of the rebar surface morphology using mercury intrusion porosimetry (MIP) and scanning electron microscopy (SEM) technology to observe the microstructure degradation. The tests demonstrated that GFRP rebar exposed to degradation environments exhibited only slight reductions in mechanical properties with time. Areas of surface degradation were difficult to observe in the SEM analysis, and the MIP pore values were very small, indicating a dense structure. Since the durability of GFRP rebar was not adversely affected to an appreciable degree by exposure to the degradation environments, it was deemed suitable for use in concrete reinforcements.

Durability Performance of Roller Compacted Concrete Using Fly Ash

This study investigated the durability performance of roller-compacted concrete (RCC) with fly ash. The effects of long-term durability of RCC adding fly ash was studied. In this study, fly ash replaced 20, 30, 40 and 50% by mass of the cement. Laboratory tests of the chloride ion permeability, abrasion, drying shrinkage were conducted. The test results demonstrated that 30% fly ash replacement has excellent durability properties.

Bond Properties of Glass Fibre-Reinforced Polymer Dowel Bars in Jointed Concrete

Dowel bars installed at the transverse joints of concrete slabs reduce the deflection and stress at the joint edges while transferring the traffic load from one slab to the next. We used draw-out and push-back tests to simulate the movement of a glass fibre-reinforced polymer (GFRP) dowel bar installed in a horizontal linear section of a concrete pavement undergoing repeated shrinkage and expansion movements. The tests were carried out on three different sizes and shapes of dowel bar. The pull-out strength was lower in GFRP dowel bars with an elliptical cross section, which would be a better fit as a load transfer device in concrete pavement. Since they do not restrict shrinkage and expansion of concrete pavement and allows free movement, they would prevent destruction of a concrete slab by curling.

Service life prediction of GFRP rebars in an alkaline environment

This study proposes a model that predicts the degradation of a GFRP rebar exposed to an alkaline environment, and based on this estimate, predicts the service life of the GFRP rebar. The alkaline environment was a pH 12.6 solution of 0.16% calcium hydroxide, 1% sodium hydroxide, and 1.4% potassium hydroxide, and the GFRP rebar was immersed for 300 days. We performed a tensile test to measure the degradation. The measured tensile strength for each time period was the remaining residual tensile strength (as a percentage) of the reinforcing GFRP rebar under a controlled state. The time shift factor was taken as the time at which the residual tensile strength was 80% of the control strength. A rebar immersed in an alkaline solution for 300 days at 80 °C was calculated to have a TSF of 13,125 days, or about 36 years, under normal South Korean regional conditions, based on the statistics of Koreas mean annual temperature. This study provides a more appropriate model for the environment of Korea than the one suggested by Dejke.

Bond Properties of High Strength Steel Rebar in High Strength Steel Fiber Reinforced Concrete

This study was to evaluate bond properties between high-strength steel fiber reinforced concrete and high strength steel rebar. An direct bond test were performed to evaluate the bond performance of high strength steel rebar in two types of high-strength concrete with steel fiber volume fraction (0, 20, ). Also, relative bond strength was defined to determine the effect of steel fiber volume fraction on bond strength. The bond test results showed that the bond performance of high strength steel rebar and high strength concrete tended to increase with higher compressive strength and steel fiber volume fraction. Relative bond strength which performed to analyze effect of steel fiber volume fraction showed increased relative bond strength with increased steel fiber volume fraction.