THE JOINT DURANET/CEN WORKSHOP ON DESIGN FOR DURABILITY OF CONCRETE: BERLIN 1999

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1 THE JOINT DURANET/CEN WORKSHOP ON DESIGN FOR DURABILITY OF CONCRETE: BERLIN 1999 Hywel Davies Hywel Davies Consultancy, UK Abstract Successful understanding and prediction of concrete durability is an urgent task facing construction. This urgency has increased in recent years because of the importance of serviceability of concrete or reinforced concrete structures. Failure to achieve the required or anticipated serviceability has economic impacts well beyond construction, causing immediate problems for users, and unexpected costs for owners. A social burden may fall on those who depend on the facility, either directly because of the failure, or because resources are diverted to address it. Concrete durability is therefore studied intensively world-wide, and in particular in Europe. It is now important to clarify how and to what extent standardised procedures can be developed for durable, serviceable design of concrete and reinforced concrete, building on these research and development activities and knowledge and experience in the field of concrete durability. To review progress in these issues, members of the EC funded DuraNet project, a network to promote performance based durability design of concrete structures, met with the European Standards Committee responsible for concrete in June The aim was to clarify and summarise the state of the art in durability design of concrete structures and to consider future research and standardisation activities. 1. Introduction The present approach to durability of concrete structures is largly empirical, based on deemed-to-satisfy rules for minimum cover, maximum water/cement ratio, and so on. There is an implied assumption that if these rules are met, structures will achieve «acceptably long» life. The length of service life likely to be achieved by this approach by a particular structure is unknown, and there is no mechanism for predicting likely future maintenance requirements (or costs). 187

2 Several recent European infrastructure projects such as the Great Belt and Oresund Links have demanded a more rigorous approach to the durability of the structure throughout the design. Project Sponsors have insisted that the service life design of the structure is able to deliver the life on which the financial case for the project is based. A traditional empirical approach does not allow assessment of the future durability. The response to this has been the development of a probabilistic service life design approach. Probabilistic service life design treats service life as a limit state. To develop a probabilistic design a time-, material- and environmental-dependent deterioration model based on known physical and chemical deterioration processes is developed for the particular structure being considered. A performance limit is defined and the model is used to demonstrate that the probability of performance falling below that limit is acceptably low. The modelling process identifies key factors of the design and environment which will influence durability and then assesses the likely rate of deterioration. This is an iterative process during which the design is modified to address identified weaknesses until the design and service life are comparable. The approach offers the possibility of 'tailor made' design by applying accurate design models and material properties to specific projects. It can also be applied to repair of existing structures, and gives guidelines for optimal preventive maintenance. 2. Development of Probabilistic Design Probabilistic design has developed over the last decade. From large Scandinavian infrastructure projects, interest has spread across Europe. In 1995 a European research project, «DuraCrete», was funded to support development of the approach as a more widely useable tool. Following the success of that project «DuraNet», a European funded dissemination project has now begun. «DuraNet» is a network of engineering professionals which aims to promote adoption and wider use of probabilistic service life design for reinforced concrete structures. It brings together 19 partners from across Europe, led by TNO Bouw, committed to improving the durability design, assessment and repair of concrete structures in Europe. The network exists to promote the use of the «DuraCrete» model for service life design of concrete structures based on probabilistic design. The project began in November 1998 and runs for three years. The main formal activities of the network are a series of 3 annual workshops. The first workshop was held in Berlin in June 1999, in conjunction with CEN TC104, Concrete, to review the treatment of durability in European Concrete standards. With speakers from both TC104 and the «DuraCrete» research project the workshop focused on measures needed to incorporate service life design approaches into standards. 188

3 3. Purpose and Structure of the Berlin Workshop on Durability of Concrete CEN TC104 SC1 drafted European Standard EN206: Concrete-performance, production and conformity, incorporating rules for durability of concrete. SC1 was unhappy that resistance of concrete to environmental actions could not be assessed using performance tests or design methods. The standard therefore addresses durability of concrete by provisions for the composition of the concrete, based on experience at national level with the relevant climatic conditions and locally available constituent materials. TC104 decided that the situation could not be changed in the first edition of the Standard (which has just received a positive formal vote earlier in 2000, and is due to be published shortly). Design for durability needs to be addressed in a future revision of the Standard. TC104 and the «DuraNet» project arranged a joint workshop, the Berlin Workshop on durability of concrete, for experts from the Standards Organisations and active researchers in the field to come together.. Its purpose was to discuss the treatment of durability in concrete in order to identify the future direction in which standardisation should go and which research is still necessary to support this development. The workshop began with a series of presentations setting out the latest thinking within the standards committee and amongst the experts in service life design. This was followed by sessions in four parallel working groups, looking at specific issues relevant to the durability of concrete, as follows: WG 1 for steel corrosion (carbonation; chlorides) WG 2 for frost attack WG 3 for chemical attack WG 4 for in situ assessment of existing structures In particular the following items were addressed: - a critical review of the performance-testing concept (TC51/TC104 approach) - a critical review of the design concept of the Duracrete approach. in each case the following factors influencing durability were considered: - quality of the deterioration models - the materials parameters and characteristics needed for the models - test methods available and their sensitivity - the classification of environmental actions eg. EN 206 Table. - the possibility of developing simplified rules for durability design. 4. Action Plan for the future development of durability design of concrete. 4.1 WG 1: Steel Corrosion The term "performance-testing" was considered misleading. A distinction should be made between laboratory performance and in-situ performance testing. An attempt was made to distinguish between laboratory performance testing and material testing, namely 189

4 that material testing gives a material parameter, while performance testing simulates the actual environmental load. The relation between these tests and conditions is: "Performance-testing" for carbonation The method derived by CEN TC51&TC104 was discussed and it was concluded that this test procedure was in the border between a laboratory performance test and a traditional material test. Results are of limited value for exposure conditions other than those of the test itself, and in such a dry environment, corrosion would not occur anyway. To benefit from a carbonation test, a mathematical model was needed to bridge the gap between lab values and real in-field exposure conditions. Such models would need other in-put parameters than that from the carbonation test, e.g. characteristic design values for the environment, moisture transport in the concrete, threshold values for ph and moisture to support corrosion. The prime need is therefore a mathematical model transforming the information from laboratory tests to in-situ behaviour of the structure. A number of models are presently available, and it should be possible to reach international consensus on a unified model within 5 years. Performance testing for chlorides CEN currently has neither laboratory performance nor material tests available or under development in this field. Empirical based mathematical models exists to predict the long term behaviour for marine structures based on laboratory tests or short term in-field assessments. Examples of tests that are applied to obtain data for such models are the resistivity test, bulk diffusion test, the "Chalmers test" and the "Coulomb" test. 190

5 Conclusion For marine structures, the technology concerning test methods and modelling is sufficiently mature to reach consensus on a harmonised approach within 5 years. The key properties to be included will be diffusion coefficient, environmental load and threshold values for supporting corrosion (e.g. chloride concentration). For structures subject to de-icing salts, it is not realistic to reach a consensus within 5 years, but work to develop such technology should be initiated. For modelling the propagation phase, it is not realistic to develop a reliable model within 5 years. Critical review of the "DuraCrete" design concept The DuraCrete concept as presented at the workshop was endorsed in principle. It was agreed that a probabilistic approach (simple or sophisticated) is needed when dealing with deterioration mechanisms. It was noted that such an approach also forms the basis for the present prescriptive approach mainly based on w/c ratios, execution rules and cover for the structural resistance and environmental classes for the load. Conclusion Further development and consensus on the philosophy and models for service life design is needed. The "DuraCrete" approach forms a good basis for such a process, which could develop a new edition of the FIP/CEB "Model Code" for structural design. It would provide guidance when working outside the scope of standards (eg. length of working life and trade offs between curing, concrete quality, cover etc). It would also assist code writers developing simplified prescriptive rules based on w/c, cover, etc. Classification of environmental actions in EN 206 Concern was expressed that the great number of classes would confuse users. On the other hand, the detailed description of each class raised a question of whether the number of classes was adequate to take into account the different environmental loads in Europe. In particular the absence of temperature as a parameter was noted. Due to this absence, jetties in both Le Havre and Helsinki would be classified in "XF-4", in spite of major differences in number of frost cycles and in minimum temperatures. There were no recommendations to improve the present system of classification. However, one way might be to give qualitative descriptions of the classes on an European level, and leave to the different nations to exemplify the interpretation. The need for simplified rules The WG concluded that the present prescriptive system primarily based on environmental classes, w/c, execution procedures and cover, would serve the purpose of securing the working life for the majority of structures in the future. However, it was agreed that a more advanced approach using modelling is needed for code writers to improve the present simple requirements. This is also needed for deriving requirements for binder types other than CEN CEM

6 Recommended Actions The international bodies listed in Table 1 were identified as possible hosts for groups to take forward the actions recommended by the Working Group. Table 1. Organisations to take forward Working Group 1 Recommendations Organisation Main Field of Work FIB Pre-normative work, design philosophy, modelling RILEM Pre-normative work, test procedures CEN Implementation of technology in European Standards EC Funding It was noted that the same experts are active in these bodies, and that future activities should be well co-ordinated. Action plan for carbonation related problems To undertake this work, the WG recommend joint working groups formed by experts drawn from the bodies listed, as follows: Table 2: Recommended groups to take forward recommendations on carbonation Organisation Main Area of responsibility FIB Commission V Modelling CEN TC51 &TC104 WG12 TG5 Test procedures Table 3: Contribution to the development of models for chloride related problems. Organisation Main Contribution FIB Commission V Modelling RILEM TC178 TMC Test procedures CEN TC104 Representation as an end user of the ouputs 4.2 WG2: Freeze-thaw attack Critical synthesis of the rules in pr EN 206 The prescriptive design rules in pr EN 206 for adequate resistance against freeze-thaw attack with and without de-lcing salt are empirical and descriptive, based on field and laboratory experience. Following a concept of performance classes they consist of a classification of exposure classes related to environmental actions and corresponding limiting values for concrete composition. In general the design rules for durability of concrete as specified in pr EN 206 were considered adequate and in line with the commonly accepted state of the art. However, it was noted that alternative design approaches should be developed, using performance based design and verification procedures, although such procedures are not yet available. 192

7 Classification of exposure classes The systematic engineering classification of exposure classes was considered a valuable step toward a rational design approach. The detail of the normative classification of the exposure classes for freeze-thaw attack was considered adequate for the majority of exposure conditions in Europe. The informative list of examples should be used to cope with different regional climatic situations in Europe and may be different from country to country. In particular it was noted that freeze-thaw attack combined with sea-water exposure should be more specifically addressed in these examples if relevant. Limiting values for concrete composition Qualitative parameters selected for limiting values (maximurn w/c, minimum strength class and cement content and if relevant minimum air content additional requirements on aggregates and types of cement) were generally considered to be adequate. Specific limiting values for concrete composition should be normative in each place of validity of EN 206. The lack of European harmonisation of such specific limiting values was not considered a serious disadvantage. There was a suggestion that standard concrete composition classes could be used in specification. Critical review of existing test methods Existing test methods assess the integral behaviour of concrete test specimens under freeze-thaw attack. The test conditions are based on standardised freeze-thaw cycles, controlled within the test specimen, the freezing medium or the cooling liquid. Performance is assessed based on surface scaling (weight loss expressed in g/m') and on internal deterioration (decrease of dynamic modulus of elasticity in %). Verification of freeze-thaw performance is based on allowable rates of frost damage expressed in terms of weight loss and the decrease of dynamic modules of elasticity. Conventional limits for these allowable values are set empirically based on laboratory and field experience with conventional concretes. Extrapolation of test results to assess novel concretes (composition, type of constituent materials) requires expert evaluation. European standard tests have only been proposed for measuring scaling resistance under freeze-thaw attack with and without de-icing salt. Existing test methods are appropriate for comparative-testing, initial type testing, conformity and control testing. They do not allow monitoring of real deterioration processes (initiation and propagation of frost damage) and identification of controlling materials parameters. The test exposure conditions do not differentiate between different environmental conditions (numbers and duration of cycles, minimum freezing temperature, moisture conditions etc.). Systematic knowledge of the correlation between real and laboratory performance is not available. Knowledge and modelling of deterioration mechanisms under freeze-thaw attack Extensive research results are available on the behaviour of concrete under freeze-thaw attack in laboratory tests. Only very limited systematic information is available on the modelling of real exposure conditions (freezing and moisture conditions). 193

8 An agreed understanding of freeze thaw deterioration and models of the processes are not yet available. Further research must be directed at modelling of exposure conditions coupled with a classification of the initiation and propagation phenomena and the identification of the governing material parameters. Recommended Actions Principles Design for freeze-thaw resistance requires controlling performance of concrete under freeze-thaw attack. (Structural measures to avoid freeze-thaw attack are not considered). Traditional and novel design rules should be based on state of the art knowledge of the behaviour of concrete under freeze-thaw attack. Both traditional and novel design rules should result in the same probability of limiting frost damage to an acceptable degree. Design rules for freeze-thaw resistance must address material and structural requirements and labour costs. Competitiveness of concrete as a structural material requires simple design rules, since these are only one aspect of the measures needed to ensure sufficient freeze-thaw resistance; planning and execution also being important. Tasks 1. Further exchange and analysis of experience-gained with the traditional design rules under various climatic conditions. A WG of TC104 SC1 could address this. 2. Further development of freeze-thaw resistance tests with the following aims: - Assessment of internal deterioration; - Classification of the preconditioning of test specimens and of test exposure conditions to differentiate climatic regimes in freeze-thaw resistance tests. This task may be given to TC51 WG12 or to RILEM. 3. Correlation of laboratory test results with field test results and with the performance of real structures under natural exposure conditions. This task could be given to the existing Network within the EU Measurement and Testing Program. 4. Further research to reach a better synthesis of existing scientific findings in view of their implementation in traditional and novel design rules. 5. Modelling of the real deterioration mechanism under freeze-thaw attack. This task will be taken up in a research programme at the University of Stuttgart. 4.3 WG3: Chemical Attack Introduction Four attack mechanisms were considered: External attack due to sulphates and acids Internal attack due to alkali aggregate reaction (AAR), delayed ettringite formation (DEF) (lime leaching was briefly considered as a form of internal attack). 194

9 Only single actions were considered. This issue of combined actions was raised but considered to be too complex to be dealt with in the limited time available. Issues of performance testing, the Duracrete design approach, material parameters, test methods, environmental actions and simplified rules were considered in relation to the general approach to durability design, ie. whether the approach is by AVOIDANCE or by (DESIGN) CONTROL. Sulphate Attack Background The current national prescriptive limits are reasonably consistent and have generally been derived from long term exposure trials (20 years or more). There is agreement that, two types of binder may be considered sulphate resisting: Portland, Cement with C3A < 3% Binders with slag contents > 60 %. National experience with other binder types differs. For example, the UK prescribes fly ash for some exposure classes. France has had bad experience with the use of fly ash. The current prescriptive approach is used as it is accepted that avoidance (within the lifetime of structures) can be achieved using one or other of the two prescribed binders. It was noted that not all slags (or other mineral additions) perform in the same way and some limits on the constituents of the material should be defined. However, there is a range of binders that may be acceptable under some conditions. The current test is of the "pass" or "fail" type and while this may be used to minimise the risk of sulphate attack, the results from the test cannot be used to predict performance in the structure of different cement types under the range of exposure condition across Europe. It was agreed that a performance based design method should be developed to deal with sulphate attack. This is needed, for long life structures eg. tunnels, where neither of the prescribed sulphate resisting cements may be acceptable for other technical reasons such as chloride resistance or high early strength. This requires both a predictive model and a test method (or methods) which provide data from the critical parameters. It was felt that an empirical model may be developed by reviewing available European data. 4.4 WG4 Assessment of existing structures Objectives for standards for existing structures : consist of basic concepts and principles, primarily concerning the assessment methodologies and «updating» of material properties. (Due to their unique nature, structures must be investigated individually, they can not be standardised.) 195

10 focus on aspects of (structural) safety and durability through thorough consideration of time dependent phenomena (corrosion, change in loads, fatigue) include economic and social aspects suggest procedures, in stages, from simple to advanced methods should be formulated in a probabilistic environment Probabilistic treatment includes : Introduction of probabilistic thinking and analysis (instead of deterministic thinking) Systematic subdivision, classification and analysis of existing structures according to a segmental approach (considering both structural and durability aspects). Elaboration of acceptable reliability levels for both structural safety and durability These acceptable levels should be expressed as a range of t values that depend on hazard scenario, economic constraints, immaterial values, etc. (New) Structures with durability design A «birth certificate» should accompany new structures consisting of: - documentation (utilisation plan, safety plan, construction drawings, ) - instructions for the users - a monitoring and maintenance manual - statement of acceptance and liability issues - a schedule for service life updating (based on in-situ durability properties) (Existing) Structures without durability design A methodology based on the service record, in-situ testing and structural evaluation is required. Acceptable conditions of structural segments should be determined and desired levels should be assigned to the initiation phase (requiring preventive interventions) and the propagation phase (calling for curative interventions). It is necessary to think in terms of hazard scenarios (or failure scenarios); local and global hazards should be determined; limit states and target values should be re-evaluated. Design for durability of concrete structures If required, re-evaluation of the service life based on updated (durability) properties, use of the structure (updated loads) and limit states may be performed after construction, based on an assessment of the performance. Design for durability of concrete structures should also take account of whole life cost issues and requirements for sustainability. Design for durability has implications for education and communication to engineers, students, owners and society, in particular relating to the use of probabilistic thinking and a whole life approach to performance and cost, as well as sustainability. 196

11 Recommended Actions Guidance Documents (to be established by fib and partly by RILEM): - service life and residual service life design approach, service life definition - assessment and maintenance methodology on a probabilistic basis - identification of predominant parameters - in-situ test methods It is important to ensure active participation of end users in development of guides Standards (to be established by CEN, ISO, RILEM, JCSS) - basic principles for structural and durability assessment on a probabilistic basis - acceptable levels of reliability - «birth certificate» - test methods respecting the probabilistic basis standards should cover only what is needed, not all that could be standardised. Research and development - update service life & residual service life design approach - develop assessment and maintenance methodology on a probabilistic basis - develop test methods with defined levels of reliability there is a need to lobby sponsors to persuade them to fund and undertake R&D on difficult issues. 5. Summary Since the Berlin Workshop various activities with CEN, FIB and RILEM have sought to implement the recommendations. The DuraNet project continues until November No project in «millenium year» is complete without a web presence, and «DuraNet» is no exception. The website has details of the project, the partners, the workshops and the newsletters. It can be found at To subscribe to the newsletter or for further information please use the contact details on the wesite, or contact Hywel Davies Consultancy at hywel@gec.org.uk. 6. Acknowledgement DuraNet is funded by the European Community Fourth Framework programme. The full title of the «DuraNet» project is: "Network for supporting the development and application of performance based durability design and assessment of concrete structures". Its official reference number is: Brite-EuRam project BET