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Fibre-Reinforced Concrete for Industrial Construction - a Fracture Mechanics Approach to Material Testing and Structural Analysis

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Fibre-Reinforced Concrete for Industrial Construction - a Fracture Mechanics Approach to Material Testing and Structural Analysis Fibre-reinforced Concrete for Industrial Construction - a fracture mechanics approach to material testing and structural analysis INGEMAR LÖFGREN Department of Civil and Environmental Engineering Structural Engineering CHALMERS UNIVERSITY OF TECHNOLOGY Göteborg, Sweden, 2005 THESIS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY Fibre-reinforced Concrete for Industrial Construction - a fracture mechanics approach to material testing and structural analysis INGEMAR LÖFGREN Department of Civil and Environmental Engineering Structural Engineering CHALMERS UNIVERSITY OF TECHNOLOGY Göteborg, Sweden, 2005 Fibre-reinforced Concrete for Industrial Construction - a fracture mechanics approach to material testing and structural analysis INGEMAR LÖFGREN Göteborg, 2005 ISBN 91-7291-696-6 © INGEMAR LÖFGREN, 2005 Doktorsavhandlingar vid Chalmers tekniska högskola Ny serie nr. 2378 ISSN 0346-718X Archive no. 35 Department of Civil and Environmental Engineering Structural Engineering Chalmers University of Technology SE-412 96 Göteborg Sweden Telephone: + 46 (0)31-772 1000 Cover: A schematic picture illustrating the suggested and applied approach for material testing and structural analysis of FRC. Printed by Chalmers Reproservice Göteborg, Sweden, 2005 Fibre-reinforced Concrete for Industrial Construction - a fracture mechanics approach to material testing and structural analysis INGEMAR LÖFGREN Department of Civil and Environmental Engineering Structural Engineering Chalmers University of Technology ABSTRACT More efficient and industrialised construction methods are both necessary for the competitiveness of in-situ concrete and essential if the construction industry is to move forward. At present, the expenditure on labour (preparation and dismantling of formwork, reinforcing, and casting and finishing of concrete) almost equals the cost of material. Fibre-reinforced concrete (FRC) extends the versatility of concrete as a construction material, offers a potential to simplify the construction process and, when combined with self-compacting concrete, signifies an important step towards industrial construction. However, a barrier to more widespread use of FRC has been the lack of general design guidelines which take into account the material properties characteristic of FRC, i.e. the stress-crack opening (-w) relationship. The presented work has been focused on FRC, showing a strain-softening response, and the interrelationship between material properties and structural behaviour. This has been examined by investigating and developing test methods and structural analysis models. A systematic approach for material testing and structural analysis, based on fracture mechanics, has been presented which covers: (1) material testing; (2) inverse analysis; (3) adjustment of the -w relationship for fibre efficiency; and (4) cross-sectional and structural analysis. Furthermore, recommendations for using the wedge-splitting test (WST) method for FRC have been provided. The relative small scale of the WST specimens makes it ideal for use in laboratory studies, e.g. for development and optimisation of new mixes. By conducting experiments, the approach was demonstrated and it was shown that it is possible to adjust the -w relationship for any difference in fibre efficiency between the material test specimen and the structural application considered. Full-scale experiments were conducted on beams, made of self-compacting fibre-reinforced concrete, with a small amount of conventional reinforcement. The results indicate that FRC can be used in combination with low reinforcement ratios; the amount of conventional reinforcement could be reduced to half that of conventional reinforced concrete (for the same load-carrying resistance) but still lead to improved structural performance (reduced crack width and increased flexural stiffness). The results also suggest that the approach used for the material testing provides the necessary properties to perform analyses based on non-linear fracture mechanics. Finally, when comparing the peak loads obtained in the experiments with the results from the analyses, the agreement was good, with a high correlation (>0.9). Hence, this demonstrates the strength of the fracture-mechanical approach for material testing and structural analysis. Key words: concrete, in-situ cast, fibre-reinforced, self-compacting, non-linear fracture mechanics, stress-crack opening relationship, inverse analysis. I Fiberarmerad betong för ett industriellt platsgjutet byggande - materialprovning och strukturanalys baserad på brottmekanik INGEMAR LÖFGREN Institutionen för bygg- och miljöteknik Konstruktionsteknik Chalmers tekniska högskola SAMMANFATTNING Ökade krav på produktivitet och kvalitet i byggbranschen har aktualiserat behovet av att utveckla ett resurssnålt byggande. Fiberarmerad betong i kombination med självkompakterande betong innebär en möjlighet att förenkla byggandet och är ett stort steg mot ett industriellt platsgjutet byggande. Ett hinder för denna utveckling är avsaknaden av generella dimensioneringsregler som beaktar de materialegenskaper som är karakteristiska för fiberarmerad betong, det vill säga sambandet mellan spänning- spricköppning (-w). Arbetet i avhandling har fokuserats på fiberarmerad betong och sambandet mellan materialegenskaper och strukturrespons vilket har analyserats genom att undersöka och utveckla metoder för materialprovning och modeller för strukturanalys, båda baserade på brottmekanik. I avhandlingen presenteras en metodik som omfattar: (1) materialprovning; (2) parameteridentifikation (för att bestämma -w sambandet); (3) korrigering av -w sambandet avseende skillnad i fibereffektivitetsfaktor; samt (4) tvärsnitts- och strukturanalys. Genomförda experiment har påvisat att det är möjligt att ta hänsyn till skillnader i fibereffektivitetsfaktor och att det därför går att korrigera -w sambandet, vilket även behövs om strukturresponsen skall beskrivas realistiskt. I avhandlingen presenteras även förslag på hur ”kil-spräck” metoden (wedge-splitting test method) kan använda för fiberbetong. Kil-spräck metoden är väl lämpad för laboratoriestudier, t ex vid utveckling och optimering av nya fiberbetonger, tack vare att relativt små provkroppar används. En slutsats av arbetet är att fiberarmerad betong i kombination med konventionell armering medför att denna kan halveras (för samma bärförmåga), men trots detta erhålls en bättre prestanda (mindre sprickvidd och ökad böjstyvhet). Detta påvisades i utförda fullskaleförsök som genomfördes på balkar, gjutna med självkompakterande fiberarmerad betong, med en liten mängd konventionell armering. Slutligen, genom de försök som har utförts (både materialprovning och fullskaleförsök) har den föreslagna metodiken demonstrerats och när resultaten från fullskaleförsöken jämfördes med beräknade var överensstämmelsen god, med en hög korrelation (>0.9). Detta belyser således styrkan i en brottmekanisk approach för materialprovning och strukturanalys. Nyckelord: betong, platsgjuten, fiberarmerad, självkompakterande, ickelinjär brottmekanik, samband spänning-spricköppning, parameter- identifikation. II LIST OF PUBLICATIONS This thesis is based on the work contained in the following papers, referred to by Roman numerals in the text. I. Löfgren, I. and Gylltoft, K.: In-situ cast concrete building: Important aspects of industrialised construction, Nordic Concrete Research, 1/2001, pp. 61-80. II. Löfgren, I.: Lattice-girder elements – Investigation of structural behaviour and performance enhancements, Nordic Concrete Research, 1/2003, pp. 85-104. III. Löfgren, I., Stang, H., and Olesen, J.F.: The WST method, a fracture mechanics test method for FRC. Paper submitted for publication in Materials and Structures (2005-04-03), 11 pp. IV. Löfgren, I., Olesen, J.F., and Flansbjer, M.: The WST method for fracture testing of fibre-reinforced concrete. Paper accepted for publication in Nordic Concrete Research, 2/2005, 19 pp. V. Löfgren, I., Stang, H., and Olesen, J.F.: Fracture properties of FRC determined through inverse analysis of wedge splitting and three-point bending tests, Journal of Advanced Concrete Technology, Vol. 3, No. 3, pp. 425-436, October 2005, Japan Concrete Institute. VI. Löfgren, I.: Fracture behaviour of reinforced FRC beams. Paper submitted for publication in Structural Concrete, Journal of the fib, October 2005. III OTHER PUBLICATIONS BY THE AUTHOR During the course of this work, subsequent results and supplementary work have been presented on several occasions. Moreover, some of the work has been presented in national engineering magazines for a wider audience. This work has been presented in the following publications: LICENTIATE THESIS Löfgren, I.: In-situ concrete building systems – Developments for industrial construction. Licentiate Thesis. Publication 02:2, Department of Structural Engineering, Chalmers University of Technology, Feb. 2002, 138 pp. CONFERENCE PAPERS Esping, O. and Löfgren, I.: Investigation of early age deformation in self-compacting concrete. Presented at the Knud Højgaard conference on Advanced Cement-Based Materials - Research and Teaching, at Technical University of Denmark, Lyngby, 12- 15 June 2005. Löfgren, I., Stang, H., and Olesen, J.F.: Wedge splitting test – a test to determine fracture properties of FRC. In Fibre-Reinforced Concretes - BEFIB 2004 – Proceedings of the Sixth RILEM symposium. Eds. M.di Prisco, R. Felicetti, and G.A. Plizzari, pp. 379-388, Varenna, Italy, 20-22 September 2004. PRO 39, RILEM Publications S.A.R.L, Bagneaux. Löfgren,
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