Cơ chế giam giữ cột bê tông gia cố FRP: Luận án Tiến sĩ của Thong Minh Pham, ĐH Wollongong

Conffinemeent Meechaniism off FRP--Confiined Con ncrete Colum mns By y T Thong Min nh Pham BEn ng Th his thesis iss submitted d in fulfilmeent of the award a of thhe degree off Dooctor of Philosophy P y (Civil Engineering) School of o Civil, Miining and Environme E ntal Enginneering Universiity of Wollo ongong, Au ustralia Augustt 2014 i DECLARATION I, Thong Minh Pham, hereby declare that all materials in this thesis, submitted in fulfilment of the requirements of the award of Doctor of Philosophy, in the School of Civil, Mining and Environmental Engineering, University of Wollongong, is wholly my own work unless otherwise referenced or acknowledged. This document has not been submitted for qualification at any other academic institution. (Signed) Thong Minh Pham (Student name) ii ACKNOWLEDGEMENTS First and most importantly I would like to give my heartfelt thanks to my supervisor Associate Professor Muhammad N. Hadi for his generous support and enlightening guidance throughout my PhD study.

I am grateful to The Vietnamese Government and The University of Wollongong for providing me the full PhD scholarship. It would have been much more difficult to complete the work without the help and support of all technicians of the High Bay Lab, especially, Messrs Alan Grant, Fernando Escribano, Cameron Neilson, Ritchie McLean and Colin Devenish. I also acknowledge Mr Eric Lume for his advice on casting high strength concrete. My sincere thanks also go to my friends in the School of Civil, Mining and Environmental Engineering, especially, Drs Ida Bagus Rai Widiarsa, Pezhman Sharafi, Messrs Xu Lei, Tung Minh Tran, Le Viet Doan, and Tan Duy Le.

The support from Dr Veysel Yazici for my research proposal is appreciated. I would like express my thanks to Dr Mehmet Eren Uz for the coffees that we had together. He had shown me much useful advice at the beginning of my PhD. I am also grateful to my Vietnamese friends, who supported me at the beginning time in Australia, Dr Thanh Duc Nguyen, Dr Trong Vo Nguyen, Mrs Ha Thi Viet Vu, and Dr Huong Thu Pham.

Lastly, I would like to express my profound gratitude to my parents Pham Chinh and Do Thi Phan, to my sisters and brothers-in-law, to whom this thesis is dedicated. My heartfelt thanks also go to my wife Mrs Huong Vu Quynh Nguyen (Hana). Without her everlasting love, support and encouragement I would never have finished my PhD. iii LIST OF PUBLICATIONS Technical papers are written based on the results of this thesis Journal papers [1] Pham, T.

“Confinement model for FRP-confined normal- and high-strength concrete circular columns.” Construction and Building Materials. Accepted on 23 June 2014. “Predicting Stress and Strain of FRP Confined Rectangular/Square Columns Using Artificial Neural Networks.” Journal of Composites for Construction. "Stress Prediction Model for FRP Confined Rectangular Concrete Columns with Rounded Corners." Journal of Composites for Construction, 18(1), 04013019.

“Strain estimation of CFRP confined concrete columns using energy approach.” Journal of Composites for Construction, 17(6), 04013001. “Strengthening square reinforced concrete columns by circularization and FRP confinement.” Construction and Building Materials, 49(0), 490-499. “A new method of strengthening reinforced concrete square columns by circularizing and wrapping with FRP or steel straps.” Journal of Composites for Construction, 17(2), 229-238. "Maximum Usable Strain of FRP-Confined Concrete.

Conference papers [8] Pham, T. Strengthening square reinforced concrete columns by shape modification and CFRP. The 2013 Structures Congress. Retrofitting square RC columns using FRP and precast concrete segments.

First International Conference on Concrete Sustainability, ICCS13. Effect of eccentric load on retrofitted reinforced concrete columns confined with FRP. 22nd Australasian Conference on the Mechanics of Structures and Materials (ACMSM22) (pp. London: Taylor & Francis Group.

Behaviour of modified RC columns retrofitted with CFRP. 26th Biennial Conference of the Concrete Institute of Australia. Gold Coast, Queensland, Australia. Confinement effect of FRP and transverse steel on retrofitting square concrete columns.

The 4th Asia pacific Conference on FRP in Structures. Comparative behaviour of FRP confined square concrete columns under eccentric loading. 6th International Conference on Bridge Maintenance, Safety and Management, IABMAS 2012 (pp. The Netherlands: CRC Press/Balkema.

Behaviour of CFRP wrapped square RC columns under eccentric loading. Australasian Structural Engineering Conference (pp. Australia: Engineers Australia. Technical papers written out of the scope of this thesis Journal papers [15] Tran, T.

“A new empirical model for shear strength of reinforced concrete beam-column connections.” Magazine of Concrete Research, 66(10), 514-530. “Behaviour of CFRP wrapped square RC columns under eccentric loading.” Concrete in Australia, 38 (3), 45- 50. v ABSTRACT Strengthening concrete columns by externally wrapping fibre reinforced polymer (FRP) around the perimeter of column sections is rapidly growing. This strengthening technique confines the column cores thus increases their carrying loads and ductility.

This thesis is concerned with the confinement mechanism of FRP- confined concrete. Particular attention is given to a new technique for strengthening existing concrete columns. The confinement mechanism of FRP-confined concrete is comprehensively investigated and analysed, which resulted in confinement models for FRP-confined concrete columns. The confinement model for FRP-confined circular concrete columns covers a wide range of unconfined concrete strengths with higher accuracy than other existing models.

The confinement model for FRP-confined rectangular concrete columns takes the stress concentration at the corners of sections into account, which has not been done by previous studies. In addition, this study introduces the use of artificial neural network (ANN) to generate analytical equations for calculating the compressive strength and strain of FRP-confined rectangular concrete columns. These equations significantly increase the accuracy compared to existing models. Additionally, the progressive failure mechanism of FRP-confined concrete that has not been previously investigated is experimentally studied.

Experimental results show that the maximum usable strain of 1% recommended by ACI 440.2R (2008) and Concrete Society (2012) is un-conservative for FRP-confined concrete. A new model is then proposed to calculate the residual strength of a concrete core at a given axial strain. Finally, a new practical method called circularisation technique is proposed to strengthen existing square reinforced concrete columns. The new technique significantly increases the axial capacity of the existing square columns.

Through experimental studies, the proposed technique was verified for not only normal strength concrete but also high strength concrete. Two sets of experimental testing proved the viability of the proposed circularization technique. vi TABLE OF CONTENTS DECLARATION. ii LIST OF PUBLICATIONS.

v TABLE OF CONTENTS. vi LIST OF FIGURES. viii LIST OF TABLES .2 Motivation and Objectives. 5 2 MECHANISM OF FRP-CONFINED CIRCULAR CONCRETE COLUMNS.

7 3 MECHANISM OF FRP-CONFINED SQUARE CONCRETE COLUMNS. 27 4 PREDICTING STRESS/STRAIN OF FRP-CONFINED SQUARE CONCRETE COLUMNS BY ARTIFICIAL NEURAL NETWORKs. 39 5 MAXIMUM USABLE STRAIN OF FRP-CONFINED CONCRETE. 49 6 CIRCULARIZING SQUARE COLUMNS TO CIRCULAR COLUMNS .1 Circularizing by Normal Strength Concrete .2 Circularizing by High Strength Concrete.2 FRP-confined Circular Columns .3 FRP-confined Rectangular/Square Columns .4 Application of ANN .5 Maximum Usable Strain of FRP-Confined Concrete.

111 viii LIST OF FIGURES Chapter 2 Figure 1. Stress-strain relationship of FRP-confined concrete…………………. Performance of the proposed model for both NSC and HSC specimens. Comparison of the selected strength models…………………………….

Accuracy of the selected strength models……………………………. Performance of the modified proposed model………………………. Energy relationship of FRP-confined circular columns………………. Performance of the proposed strain model…………………………….

Comparison of the selected strain models……………………. Accuracy of the selected strain models………….14 Chapter 3 Compressive Strength Model Figure 1. Confinement behaviour at the corner of the section: (a) mechanism of the tension force; (b) distribution of confining stress……………………………. Relationship between factor A and FRP strain efficiency factor (k)…….

Relationship between normalized confining stress and normalized confined strength: strength equation…………………………………………………………. Relationship between normalized confining stress and normalized confined strength: minimum amount of FRP for sufficient confinement……………………. Performance of the selected models (ascending type specimens)……. Accuracy of the selected models………………………………………….

Performance of the proposed models (ascending and descending types specimens)………………………………………………………………………….25 Compressive Strain Model Figure 1. (a) Load - displacement diagram; (b) A typical stress-strain curve of FRP- confined concrete……………………………………………………………………30 Figure 2. Energy relationship of circular sections…………………………………. (a) Effective confinement area; (b) Confining pressure of square sections; (c) Round corners of square sections………………………………………….

Energy relationship of square sections……………………………………34 ix Figure 5. Performance of models on circular specimens…………………………. Accuracy comparisons for strain prediction of circular specimens among the models…………………………………………………………………………. Performance of models on circular specimens (insufficient confinement).

Performance of models on circular specimens (heavy confinement)……. Performance of models on square specimens……………………………. Accuracy comparisons for strain prediction of square specimens among the models…………………………………………………………………………. Architecture of the proposed ANN strength model……………………….

Comparison of the selected strength models………………………. Accuracy of the selected strength models………………. Comparison of the selected strain models………………. Accuracy of the selected strain models …………………….

Architecture of the proposed ANN strength equation……………………44 Figure 7. Accuracy of the selected strength models………………………. Accuracy of the selected strain models…………………………………. Performance of the proposed strain model with or without the input r….

Stress-strain relation of concrete…………………………………………. Position of strain gauges…………………………………………………. Damage of tested specimens with high axial strain………………………. Stress-strain relation of Group C2…………………………………….

Stress-strain relation of Group C3…………………………………. Damage of tested specimens with low axial strain…………. Residual strength of tested specimens……………………………. Generation of a stress-strain curve of FRP-confined concrete (based on Jiang and Teng 2007)……………………………………………………………….

Definition of the unloading stiffness (based on Lam and Teng 2009)…. Theoretical verification of the tested specimens……………………. Determination of the maximum usable strain………………………….82 Chapter 6 Circularization by Normal Strength Concrete Figure 1. Effective Core for Steel Straps Confined Columns………………………85 Figure 2.

Centroid of Compression Zone of the Column…………………. Stress - Strain Analysis for Computing P-M Diagram…………. Plan View of Specimens………………………………………. Eccentric Loading System: (a) Loading Head, (b) “Loading” Roller, (c) A Pair of loading head and (d) The whole loading system…………………….

Formworks: (a) Core Columns and (b) Concrete Covers………. Segmental Circular Concrete Covers and Modified Section Columns: (a) Concrete Covers and (b) Bonded Specimens……………. Load - Deflection Diagram for Concentric Loading Tests………………. Failure Modes: (a) Specimen CF-0 and (b) Specimen CS-15………….

Load - Deflection Diagram for Eccentric Loading Tests (e=15 mm)…. Load - Deflection Diagram for Eccentric Loading Tests (e=25 mm). Load - Deflection Diagram for Specimens under Flexural Tests………. Axial Stress – Strain Diagrams for Concentric Loading Tests………….

Comparison of Theoretical & Experimental P-M Diagrams (Groups CF & CS)…………………………………………………………………………………. Comparison of Theoretical & Experimental P-M Diagrams (Groups N & RF)…………………………….……………………………………………………92 Circularization by High Strength Concrete Figure 1. Cross section of specimens (units in mm)………………………………. Circularization process (a) Removal of the segments from the formworks; (b) Removal of foams attached on the covers; (c) Bonding of segments; and (d) Bonded specimens………………………………………………………………….

Details of strain gauge locations…………………………………………. Failure modes of the tested columns…………………………………. Failure patterns of the tested beams…………………………………. Axial load – deflection diagrams of the concentrically loaded columns.

Axial load – deflection diagrams of the eccentrically loaded columns (e = 25 mm)………. Axial load – deflection diagrams of the eccentrically loaded columns (e = 50 mm)…………………………………………………………………………. Load – deflection diagrams of the beams……………………………. Axial load – FRP strain diagrams of the concentrically loaded columns101 Figure 11.

Axial load – FRP strain at the extreme compression fibre diagrams…. Strain distribution around the circumference of Column C80-25……. Experimental interaction diagrams for the tested columns……. Experimental and theoretical interaction diagrams for the tested columns…………………………………………………………………………….102 xii LIST OF TABLES Chapter 2 Table 1.

Pseudo-identical specimens in the database………………………………11 Table 2. Statistics of the column parameters for the proposed strength model….…11 Chapter 3 Compressive Strength Model Table 1. Summary of published models………………………………………. Test results of FRP-confined rectangular specimens……………….19 Compressive Strain Model Table 1.

Database of CFRP–confined circular concrete cylinders…………………. Database of CFRP–confined square concrete columns……………………33 Chapter 4 Table 1. Statistics of the input parameters for the proposed models…………. Test matrix…………………………………………………………………68 Table 2.

Residual strength of the tested specimens…………………………………69 Chapter 6 Circularization by Normal Strength Concrete Table 1. Test Matrix of the Experiment……………………………………………. Results of Specimens Tested under Concentric Loading………………. Results of Specimens Tested under Eccentric Loading (e = 15 mm)…….

Results of Specimens Tested under Eccentric Loading (e = 25 mm)……. Results of Specimens in Flexural Tests……………………………. Summary of Confinement Efficiency……………………….91 Circularization by High Strength Concrete Table 1. Test matrix…………………………………………………………………97 Table 2.

Summary of the tested columns under concentric loads………………….