Concrete: Too Young for Conservation?

Structural Analysis of Historic Construction – D’Ayala & Fodde (eds) © 2008 Taylor & Francis Group, London, ISBN 978-0-415-46872-5 Concrete: Too young for conservation? H.A. Heinemann Chair of Building Conservation, Faculty of Architecture, Technical University of Delft, The Netherlands R.P.J. van Hees Chair of Building Conservation, Faculty of Architecture, Technical University of Delft, The Netherlands TNO Built Environment & Geosciences, Delft, The Netherlands T.G. Nijland TNO Built Environment & Geosciences, Delft, The Netherlands ABSTRACT: The 20th century built heritage is one of the new conservation challenges, due to its architectural differences from the traditional heritage and new materials. One major new material is concrete; its quantity and importance for the new heritage requires a tailored conservation approach. Until now, there is a dependence on a repair approach. The dangers of such an approach for the cultural-historical values of the original concrete will be shown by the means of a case study, Fort Bezuiden Spaarndam (1901), part of the UNESCO World Heritage Site Defence Line of Amsterdam. An underlying problem is that concrete is too young for conservation experts, causing a dependence on concrete repair expertises. The used repair techniques are developed for structural aims, not integrating conservation aims. Fundamental research on the history of concrete and suitable conservation approaches and techniques is therefore needed. 1 INTRODUCTION such as incompatible repair materials or a more fundamental problem such as the way concrete conservation With the aging of the 20th century building stock, projects are approached. we are now facing the challenge to evaluate them During this research, studies of completed interven- and make selections for objects which should be kept tions of historic concrete buildings indicated a relation as part of our built cultural heritage. It is consistent between the following of repair approaches and the that the values which justified the selection should loss of monumental values. In this paper, this rela- be preserved and respected during any intervention. tion will be illustrated by means of the case study Fort Interventions cannot be avoided as aging often goes Bezuiden Spaarndam, the Netherlands. It shows that along with some degree of deterioration. The demands there are general, underlying problems in the field of for interventions according to (international) conser- concrete conservation which endanger values, inde- vation guidelines are applicable for the entire built pendent from the technical performance of a repair. heritage, independent from age or construction type. However, for the implementation of the demands a tailored conservation approach for the 20th century built heritage is needed.The new heritage does not only 2 METHODOLOGY differ in architecture, structure, quantity, and building typology from the traditional heritage, but also in the The current research aims at understanding the rela- used materials such as concrete. The different material tion between the loss of value and following a repair properties make a transfer of knowledge and experi- approach. To evaluate the impact of a repair approach, ence gained with traditional materials such as brick case studies of former concrete conservation projects and mortar less feasible. in the Netherlands have been carried out. In this paper For concrete, the current approach is often a con- the results of the case study Fort Bezuiden Spaarndam, ventional repair approach as no tailored conservation part of the Defence Line of Amsterdam (Stelling van approach exists. As in practice a loss of values cor- Amsterdam), will be presented to explain the underly- related to the original concrete is often encountered, ing problems of the preservation of values of historic the question arises if this is a pure technical problem concrete. 151 During the research, the most important involved parties were interviewed (architect, surveyor, and concrete repair company) to understand their background, knowledge, and experience and to see where the dependence on concrete repair experts starts. Liter- ature, documents, and surveys related to the fort, its construction and conservation were consulted to inves- tigate the historical background, conservation aims, and conservation process. During a visual inspection in November 2006, the performance and impact of the conservation was evaluated. The study focussed on concrete which was used for exterior elements, stressing the complexity to bal- ance technical, aesthetical and conservation demands, meaning a minimal loss of existing fabric and a maxi- Figure 1. Site plan of Fort Bezuiden Spaarndam. mal preservation of values. Additionally, petrographic analyses of samples were made to determine the concrete composition and possible damage. To evaluate 4 FORT BEZUIDEN SPAARNDAM the suitability of standard concrete surveys for conservation tasks, literature and guidelines related to 4.1 Historical background of the Defence Line of concrete assessment were studied. Amsterdam The Defence Line of Amsterdam, the Netherlands 3 BACKGROUNDS OF REPAIR AND (1880–1920), is a ring of fortifications (45 forts) CONSERVATION aroundAmsterdam, intended to defend the city by sys- tematic inundation in case of an attack. This ingenious To understand why following a repair approach in a defence technique was one justification for the listing conservational context can endanger values, it is nec- as a UNESCO World Heritage Site in 1996. In con- essary to understand why and how the main aims nection with the listing, attention was drawn to the of repair and conservation diverge radically. Repair need for conservation and re-use, as the condition of methods are usually developed to preserve a structural many forts declined after the military function of the function for a limited lifetime, lately also influenced by forts ceased in the 1960s. Fort Bezuiden Spaarndam sustainability aspects. Surveys focus on physical and (1901), a provincial monument in the province North mechanical properties and the criteria for the choice Holland, was constructed as part of the Defence Line of a repair approach are, beside the structural perfor- of Amsterdam. mance, the service life, economical issues, and the possibilities of maintenance (ENV 1504-09 1997). 4.2 Historical background of Fort Bezuiden Conservation, however, is much more than ful- Spaarndam filling technical and financial requirements. Besides considering structural performance and safety, the Fort Bezuiden Spaarndam is a non-reinforced concrete preservation of the authenticity of and values attached fort built in 1901. It was planned as a standard model to the site, building and material must be considered. of the Defence Line ofAmsterdam on a fort island with The value and authenticity of the cultural heritage two cupolas with disappearing turrets (Fig. 1).The fort is understood in ‘artistic, historic, social, and sci- itself did not suffer from major damages during the entific dimensions’ and the conservation principles Second World War, though the cupolas were blown up ‘respect... the existing fabric’ and the cultural heri- by the German occupation army. The remains of the tage should be preserved for ‘present and future cupolas still lay spread around the grounds. Since the generations’ (ICOMOS 1999). cease of the military function in 1960, the fort was Yet, due to the youth of the field of concrete con- used for agricultural storage. servation, concrete repair experts are entrusted with concrete conservation projects.Although these experts 4.3 Historical background of the concrete of have considerable knowledge of the technical aspects, Fort Bezuiden Spaarndam they are not trained in specific conservation aspects, as the complexity and variety of monumental values To recognise the historical value of the original con- and authenticity or the properties of historic concrete, it is necessary to understand the construction and crete. Because conservation experts often do not have technology history of the late 19th century. Knowl- sufficient knowledge on the material, an evaluation of edge of concrete and its construction was mainly the suitability of such a process is difficult. based on empiric studies; standard design codes only 152 appeared at the beginning of the 20th century. In assess the technical state of the concrete and to deter- this context, the knowledge gained from the con- mine the causes of the encountered leaching, cracking, struction of the Defence Line of Amsterdam was and leakages. Several cores were taken, not only to important for the Netherlands, where early concrete determine the composition and damage causes, but technology was dominated by the army corps of also to gain information on the wall construction. engineers (e.g. Scharroo 1910). The first Dutch rein- It was stated that the walls were massive concrete forced concrete guidelines Eerste Gewapend-Beton- and the concrete was composed of cement with ground Voorschriften from 1912 were influenced by the expe- granulated blast furnace slag (GGBS) and crushed rience gained during the construction of the Defence rocks (granite, porphyry) as aggregates.The following Line of Amsterdam (Noord-Holland 1996). damages were assessed in the survey: Therefore, the forts of the Defence Line of Ams- – shrinkage cracks terdam are the tangible artefacts of this pioneer phase – cracks along casting segments caused by missing of concrete technology. Initially, the

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