First Advisor

Franz Rad

Term of Graduation

Fall 2020

Date of Publication

11-23-2020

Document Type

Dissertation

Degree Name

Doctor of Philosophy (Ph.D.) in Civil & Environmental Engineering

Department

Civil and Environmental Engineering

Language

English

Subjects

Carbon fiber-reinforced plastics, Concrete columns -- Maintenance and repair, Strength of materials, Flexure -- Testing, Reinforced concrete

DOI

10.15760/etd.7472

Physical Description

1 online resource (xviii, 174 pages)

Abstract

The use of FRP materials in strengthening and repairing of reinforced concrete (RC) structures has increased in the past two decades. Recently, FRP materials have become one of the most used materials in rehabilitation engineering. For seismic retrofitting of RC structures, usually the process involves strengthening or repairing the vertical support elements of the buildings or bridges. Several studies focused on the use of FRP materials in strengthening and repairing RC columns. Externally bonded (EB) FRP sheets or laminates and near-surface-mounted (NSM) FRP rods have been used for enhancing the strength and ductility of RC columns. Although glass FRP (GFRP) and basalt FRP (BFRP) rods have been used in flexural strengthening of RC columns, NSM-CFRP rods have not been used yet. In addition, studies in the use of CFRP ropes in flexural strengthening or repairing of RC columns are not available.

To address these gaps, an experimental investigation was conducted on the use of CFRP materials in strengthening and repairing RC columns. The investigation consisted of three main parts. The three parts focused on strengthening and repairing slender RC columns using CFRP materials. The first part of the investigation focused on the use of CFRP sheets, rods, and ropes in flexural strengthening of RC columns. Half scale square (150 x 150 mm) RC columns were fabricated and tested. Each specimen consisted of two 1.065-meter long columns connected in the middle by a stiff element (concrete stub). All columns were designed based on older codes (pre 1970s). The specimens were (1) as built specimen, (2) strengthened with CFRP confinement only, (3) strengthened with CFRP confinement and NSM-CFRP rods, and (4) strengthened with CFRP confinement and NSM-CFRP ropes (two specimens). Another specimen was strengthened with EB-CFRP sheets. The results showed that both EB and NSM techniques can effectively be used in strengthening slender RC columns. In addition, CFRP ropes are very effective in strengthening RC columns. The strength enhancement ranged from 35% to 60%. Finally, a theoretical model was created to predict the load-displacement response of RC columns strengthened with EB-CFRP sheets and NSM-CFRP rods and ropes. The theoretical results showed good agreement with the experimental results. The second part of the investigation focused on the use of CFRP sheets and ropes in repairing damaged RC columns. Recently, RC columns have been upgraded and strengthened with FRP confinement all around the globe. Future cases of repair will likely encounter RC columns that were strengthened previously with FRP confinement. However, studies on repairing damaged columns that were enhanced by confinement before damage are not currently available. Moreover, there is a vital need for an emergency repair technique that can be used to rapidly repair damaged columns of essential structures after a seismic event. In this study, the time required to complete the repair was a key factor to propose a rapid CFRP-based repair technique. A total of four specimens, each one representing two columns, were fabricated and tested. The technique was applied to square RC columns (150 mm x 150 mm x 1,065 mm) subjected to combined axial and cyclic lateral loads. The process of applying the repair technique was completed in three days. Test results showed that the proposed repair technique was effective not only by restoring the original strength, but also by improving the strength significantly. Moreover, the measured and idealized load-displacement response showed that the ductility of the repaired column was reasonably sufficient. The proposed technique appears promising and may be considered as a permanent repair technique.

The third part of the investigation focused on the tensile strength of CFRP ropes anchored to concrete using chemical epoxy. One of the major problems with using fiber reinforced polymer (FRP) in strengthening reinforced concrete (RC) structures is premature debonding of FRP. Anchoring FRP materials to concrete has become associated with most of the strengthening techniques. One of the anchoring techniques is using handmade anchors made from FRP materials. In previous studies, most FRP anchors were made from rolling pre-cut FRP sheets with short embedment (mm) as they were used for flexural or shear strengthening of RC beams. In the present study, FRP anchors were made from carbon fiber reinforced polymer (CFRP) ropes and had long embedment to be used for flexural strengthening of RC columns. A total of twenty-one pullout tests were conducted on CFRP rope anchors bonded to concrete using chemical epoxy. The test parameters included embedment lengths of 45 mm, 90 mm, 135 mm, 180 mm, 270 mm, and 315 mm; anchor hole diameters of 12.7 mm, 19.1 mm, and 25.4 mm; and two epoxy types, Hilti 500 and MasterBrace SAT 4500. Test results showed that the pullout strength of CFRP anchors increased with the increase in embedment length, and no significant effect of the hole diameter on the pullout strength was observed. However, the bond strength increased with decreasing embedment length and hole diameter. For shorter embedment lengths, the distribution of the bond stress along the length of the anchor is expected to be more uniform than that of anchors with longer embedment lengths. The average bond stress (strength) is calculated by dividing the maximum pullout force by the embedment length. Therefore, increasing the embedment length without significant improvement in the pullout load results in lower bond strength. The observed pullout results and failure modes were compared to the predictions using available models. Finally, a modified model was proposed to predict the pullout strength of CFRP rope anchors.

Rights

© 2020 Yasir Matloob Saeed

In Copyright. URI: http://rightsstatements.org/vocab/InC/1.0/ This Item is protected by copyright and/or related rights. You are free to use this Item in any way that is permitted by the copyright and related rights legislation that applies to your use. For other uses you need to obtain permission from the rights-holder(s).

Persistent Identifier

https://archives.pdx.edu/ds/psu/34448

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