ISSN: 1402-1544 ISBN 978-91-86233-XX-X Se i listan och fyll i siffror där kryssen är DOCTORAL T H E SIS Markus Bergström Department of Civil, Mining and Environmental Engineering Division of Structural Engineering Assessment of Existing ISSN: 1402-1544 ISBN 978-91-86233-11-2 Concrete Bridges Luleå University of Technology 2009 Assessment Bending Stiffness as a Performance Indicator of Existing Concrete Bridges Bending stiffness stiffness Bending Bending Stiffness as a Performance Indicator I II III IV Load/Time Markus Bergström Luleå University of Technology Doctoral Thesis Assessment of Existing Concrete Bridges Bending Stiffness as a Performance Indicator Markus Bergström Division of Structural Engineering Department of Civil, Mining and Environmental Engineering Luleå University of Technology SE-971 87 Luleå Sweden http://www.ltu.se/ Sweet are the memories sweet is the goal, heading for new challenges to inspire the soul Assessment of Existing Concrete Bridges – Bending Stiffness as a Performance Indicator MARKUS BERGSTRÖM Avdelningen för byggkonstruktion Institutionen för samhällsbyggnad Luleå tekniska universitet Akademisk avhandling som med vederbörligt tillstånd av Tekniska fakultetsnämnden vid Luleå tekniska universitet för avläggande av teknologie doktorsexamen, kommer att offentligt försvaras i universitetssal F1031, fredagen den 6 mars 2009, kl. 10.00. Fakultetsopponent: Professor Paulo Cruz, University of Minho, Portugal Betygsnämnd: Professor Björn Engström, Chalmers tekniska högskola, Sverige Professor Sven Thelandersson, Lunds tekniska högskola, Sverige Professor Uday Kumar, Luleå tekniska universitet, Sverige Universitetstryckeriet, Luleå ISBN 978-91-86233-11-2 ISSN 1402-1544 Luleå 2009 WWW.LTU.SE Front page: The illustration shows a reinforced concrete (RC) member which passes through four phases; Un-cracked (I), Crack forming (II), Crack stabilised (III) and Failure (IV). The propagation is governed by increased loads or degradation. Both of these could be time dependent during certain conditions, which imply that the stiffness of an RC members in time could also develop in this manner. The present research has focused on the bending stiffness used as a performance indicator, and how it can be used from a bridge management perspective. Preface The present doctoral thesis reflects my work between July 2004 and March 2009 at the Division of Structural Engineering at Luleå University of Technology (LTU). Without funding from “the Swedish Construction Industry’s Organisation for Research and Development (SBUF)”, and “the Swedish Research Council for Environment, Agricultural Sciences and Spatial Planning (FORMAS)” this would not have been possible. A sincere thanks is addressed to these organisations for the interest in my research. The scholarships from Elsa and Sven Thysell’s Foundation, J Gust Richerts Memorial Fund and the Wallenberg Foundation have made it possible to represent LTU and the present research at international conferences, and are of course also greatly acknowledged. All the staff at the Department of Civil, Mining and Environmental Engineering, especially the staff at the Division of Structural Engineering, are apprechiated for the discussions in the coffee room. I can only remember good things about working close to my colleagues and friends within the research group “Innovative Materials and Structures”. I would like to thank my PhD-student companions for valuable times signed both work and leisure, and ofcourse my supervisor Prof. Björn Täljsten and assistant supervisor Dr. Anders Carolin for supporting me during these years. It has been valuable and pleasant to have you as supervisors, and not only at work. I hope we can keep in contact in the future. I would also like to thank the staff at Testlab for all their help with my tests and measurements. Special attention should be given to Tech. Dr. Claes Fahleson, Civ. Eng. Georg Danielsson and Mr. Lars Åström for excellent cooperation. All my colleagues at Norut Narvik AS, located in Narvik (Norway), are appreciated both for valuable collaboration and kind hospitality during my visits. Prof. Lennart Elfgren and Tech. Dr. Martin Nilsson should be acknowledged for giving valuable comments during the final work with the thesis, and also Prof. Mats Emborg for leading the division during this time. Special thanks go to my family, friends and especially to my fiancé Sofi, who all bring joy into my life. Markus Bergström March 2009 I Summary Optimizing the use of existing civil reinforced concrete (RC) structures could be interpreted in such a way as to say that the capacity should be used and taken care of in an effective manner, both from a technical and economical point of view, keeping the safety in mind. Achieving this requires thorough understanding of the structure and also of the tools used for assessing current and future capacity and needs. Monitoring together with finite element modelling could give relevant and important information about a structure’s capacity. In a case where monitoring alone is used, it is beneficial if a quantity is monitored which is interpretable on material and geometrical level. It is further important that the measure is practically possible to capture, and that it reflects the behaviour in a theoretically well-known mode. One example of a quantity which fulfils these requirements is the bending stiffness. In the Serviceability Limit State (SLS), in particular, a high bending stiffness is beneficial as this reduces deflections, vibration amplitudes and crack widths. It is shown within the thesis that four phases are distinguished during loading of an RC member; Un-cracked phase (I), Crack forming phase (II), Crack stabilised phase (III) and Failure phase (IV). It is also shown that corrosion and flexural strengthening are possible to capture through the bending stiffness by monitoring the curvature. Linear elastic theory has in addition been concluded to give satisfactory results in terms of good agreement between measured and theoretical results. It is shown that it is possible to determine the highest load which the structure has been previously exposed to, presuming that the structural element has not reached phase (III). The stiffness is almost constant in phase (III) which implies that it is the same for a certain load interval. One limitation coupled to the stiffness plateau formed in phase (III) is that it is difficult to predict a possible failure by monitoring the bending stiffness, caused by the limited forewarning prior to the beginning of phase (IV). Other tools, such as reliability-based assessment, become especially important here since active degradation, for example, is difficul to verify by curvature measurements in phase (III). Estimating the safety, and also finding the probable failure mode is important since curvature measurement is not effective in the Ultimate Limit States (ULS) and only captures the behaviour in bending. In the reliability-based assessment, the agreement between analytical results and actual capacity of the particular failure mode must be treated with special attention, since it has been shown that the model uncertainty can affect both the safety level and also probable failure mode. If it can be shown from monitoring that the structure is located within phase (I) the load effect during the past time has not affected the integrity of the structure in terms of bending cracks. III It is preferrable to use the global curvature when evaluating the bending stiffness, since this property gives a more robust average curvature and also additional information about the structural member. Especially changing bond properties, during e.g. corrosion, is more likely to be detected if the global curvature is monitored. Another important conclusion is that the local and global stiffness development is very similar. This implies that a crack at a certain location is not allowed to increase without redistribution of stresses, which affects the global stiffness in an comparable extent. Two criteria are suggested for the least distance over which the global curvature should be measured, LG. The first one concerns the fact that at least four macro cracks is suggested to be covered and is based on the maximum crack spacing recommended by Eurocode (2004). The other requirement is that the distance should not be that small that the estimated deflection become less than one hundred times the in-built measurement error in the displacement gauge. A measurement error above one percent is hence not allowed. Curvature assessment could be useful from three different aspects Condition assessment. The monitored quantity is back-calculated to input data, such as material property or geometry. That is, solving the inverse bridge management problem. Decisions about the use of the structure are then based on the outcome of this assessment. Refined calculations in serviceability and ultimate limit states. Use the results to refine the models used for SLS and ULS performance. For example, it might be possible to treat the structure in its actual condition. Optimized LCC. Time until a major repair and/or strengthening procedure is estimated using the bending stiffness development captured by curvature measurement. The approach using bending stiffness as a performance indicator is applied in two case studies in Sweden, the Panken road bridge located east of Karlstad and the railway bridge located in Örnsköldsvik. The Panken Bridge was located within phase (III) (crack stabilised phase), while the Örnsköldsviks