Chongku Yi

Compressive Response of Concrete Confined with Steel Spirals and FRP Composites

This article presents the results of an experimental and analytical study on the behavior of concrete cylinders externally wrapped with fiber-reinforced polymer (FRP) composites and internally reinforced with steel spirals. The experimental work was carried out by testing twenty-four 150 × 300 mm 2 concrete cylinders subjected to pure compression with various confinement ratios and types of confining material. The test results show that the compressive response of concrete confined with both FRP and steel spirals cannot be predicted by summing the individual confinement effects obtained from FRP and steel spirals. This is largely attributable to differences in the inherent material properties of FRP and steel. A new empirical model to predict the axial stress-strain behavior of concrete confined with FRP and steel spirals is proposed. Comparisons between experimental results and theoretic predictions show agreement.

Strengthening and toughening mechanisms in microfiber reinforced cementitious composites

Materials with quasi-brittle stress strain curves exhibit desirable properties such as enhanced durability, flaw tolerance and toughness. This study reveals that steel microfiber reinforced cement based composites exhibit such quasi-brittle behavior. Mechanical properties of steel microfiber reinforced cement based composites are obtained through flexure and splitting tension tests. The cracking process and crack fiber interactions that lead to the quasi-brittle behavior in these composites were investigated. The strength and toughness enhancement is associated with crack wake mechanisms. Aggregate bridging and pullout and secondary crack formations associated with microfiber bridging sites are predominant during the strain hardening regime. Multiple secondary microcracks perpendicular to the fiber/matrix interface is the dominant failure mode beyond peak load in the strain softening regime.

Tensile strength enhancement in interground fiber cement composites

Abstract Interground fiber cement (IFC) is a new process where fibers are ground in with the cement clinker during the dry cement manufacturing process. With IFC considerable strength enhancement can be achieved compared to ordinary cement even at a fiber volume as low as 0.2% due to homogeneous fiber distribution and fiber surface modifications associated with the milling process. The cracking mechanisms associated with the strength enhancement were observed in real time during load application using a custom designed loading device. The homogeneous fiber distribution stabilizes crack growth. Formation of multiple, stable secondary microcracks was observed during the strain hardening regime, enhancing the strain capacity at ultimate strength. Fiber pullout was the dominant toughening mechanism in the strain softening regime. For fibers inclined to the propagating crack, fiber pullout was preceded by secondary microcrack formations along the fiber/matrix interface.

Effect of Confinement on Properties and Characteristics of Alkali-Silica Reaction Gel

This paper studies how steel microfibers can generate confinement of the resulting alkali-silica reaction (ASR) gel. ASR gel, which develops as a byproduct between certain types of aggregates and the cement paste, can lead to crack formation in concrete. As the amount of local confinement found within the material determines the degree of damage, the research also investigates how confinement affects the formation and characteristics of the ASR gel. Both solid and liquid forms of ASR gel are studied, via microprobe analysis (to determine chemical composition) and inductive coupled plasma spectroscopy. The research also measures the viscosity of the liquid alkali-silicate solution. Results show that the use of steel microfibers in controlling cracks leads to a chemo-mechanical confinement of the ASR gel. The authors note that this type of confinement reduces volumetric expansion, ASR gel formation, and the reactivity of the reactive aggregate.

Quasi-brittle behavior of cementitious matrix composites

Observations on crack interactions with bridging sites during in situ crack propagation in steel microfiber reinforced cementitious composites are presented. The loading fixture used for identifying the toughening mechanisms is briefly described. The loading fixture can be placed under an optical microscope, inside a Scanning Electron Microscope (SEM) or inside the Environmental Electron Microscope (ESEM). Controlled displacements are applied to compact tension specimens through a piezoelectric translator which is driven by a high voltage amplifier. The in situ crack propagation measurements reveal energy absorbing mechanisms in the vicinity of fiber and aggregate bridging sites which are contributing to the strength and toughness enhancement observed in these composites.

Strengthening of Concrete with Mixed Confinement Materials - Steel Hoops and FRP Composites

This paper presents the results of an experimental and analytical study on the behavior of rectangular concrete specimens externally wrapped with fiber reinforced polymer (FRP) composites and internally reinforced with steel hoops, subjected to pure compression. Axial load and strain were recorded to evaluate stress-strain behavior, ultimate strength and corresponding strain of the tested specimens with varying conf inement type (one material versus two materials) and confining pressure. The test results showed that the compressive response of concrete confined with both FRP-wrap and steel hoops was quite different f rom that of concrete confined by either FRP or steel hoops separately, and the need of experimental as well as analytical study to account for the effect of the mixed reinforcements was recognized based on the comparisons between the test results and predictions by the four existing analytical models considered in this study. K e y w o r d s : Fiber reinforced polymer; Lateral confining pressure; Confined concrete; Mechanical properties

Bond Failure Surface of Glass Fiber Reinforced Polymer Bars

The effects of concrete strength on bond-slip behavior and the failure mechanisms of glass fiber reinforced polymer (GFRP) bar embedded in concrete under direct pullout were investigated in this study. Total of twenty seven specimens were prepared by placing two different types of GFRP bars and conventional steel rebar in 25 MPa, 55 MPa, and 75 MPa concrete and tested according to CSA S806-02. The test results showed that the bond strength of the GFRP rebars as well as the steel increased with the concrete strength. However, the increase in the bond strength with respect to the concrete strength was not as significant in the GFRP series as the steel, and it was attributed to the interlaminar failure mechanism observed in the GFRP test specimens.

Mechanical Properties of Epoxy Resin Mortar with Sand Washing Waste as Filler

The objective of this study was to investigate the potential use of sand washing waste as filler for epoxy resin mortar. The mechanical properties of four series of mortars containing epoxy binder at 10, 15, 20, and 25 wt. % mixed with sand blended with sand washing waste filler in the range of 0–20 wt. % were examined. The compressive and flexural strength increased with the increase in epoxy and filler content; however, above epoxy 20 wt. %, slight change was seen in strength due to increase in epoxy and filler content. Modulus of elasticity also linearly increased with the increase in filler content, but the use of epoxy content beyond 20 wt. % decreased the modulus of elasticity of the mortar. For epoxy content at 10 wt. %, poor bond strength lower than 0.8 MPa was observed, and adding filler at 20 wt. % adversely affected the bond strength, in contrast to the mortars containing epoxy at 15, 20, 25 wt. %. The results indicate that the sand washing waste can be used as potential filler for epoxy resin mortar to obtain better mechanical properties by adding the optimum level of sand washing waste filler.

Bond degradation of glass fibre reinforced plastic bars in concrete subjected to tensile cyclic loads

An adequate bond between reinforcement and concrete is critical in ensuring the composite behaviour of a reinforced concrete structure. Although fibre reinforced plastic rebar–concrete bond behaviour under monotonic load has been studied by many researchers, little is known about its behaviour under repeated loads. In this study, a series of pull-out tests were performed to investigate the bond stress-slip behaviours under tensile cyclic loads at local slips greater than elastic range. Conventional reinforcing steel bars and two common types of glass fibre reinforced plastic bars, embedded in concrete with strength of 42 MPa, were tested under four different loading patterns (monotonic + 3 tensile cyclic load patterns), and the bond failure of each specimen was examined. The results indicate that the bond strengths of fibre reinforced plastic rebar, unlike steel rebar, can be affected by as much as 24% by tensile loading history. In addition, the analysis of the failure surfaces of the rebar after the tests showed that bond strength degradation of the glass fibre reinforced plastic rebar types considered in this study can be attributed to the degree of interlaminar failure.