Historic Concrete: From Concrete Repair to Concrete Conservation

Concrete like materials were already applied during the Roman Empire. After the decline of the Roman Empire, a wide scale application of concrete only reappeared in the 19th century. Here lies also the origin of modern (reinforced) concrete. Since then, both concrete application and composition have significantly changed. Today, concrete from the 19th and early 20th century is considered as historic concrete. In this thesis, we study historic concrete in the Netherlands, that is to say the concrete dating back from the middle of the 19th century up to the 1960s. Within this period, the application and properties of concrete varied, due to increasing technological insight and social acceptance of concrete as a building material. Although the application of concrete in the Netherlands was late compared with neighbouring countries, there are quite specific and interesting developments and innovations in Dutch concrete, especially with regard to surface finishes. Historic concrete in the Netherlands is thus specific enough to require a dedicated study. Inaddition to this, the commonality of the history of concrete in different countries makes the relevance of this study broader than the Dutch case. Knowledge on historic concrete is increasingly relevant, as conservation of historic buildings dating from the 19th and 20th century internationally emerges as a new field within heritage care. This period is characterised by both architectural and structural innovations and by novel construction materials, of which (reinforced) concrete is probably the most important. Although a growing number of concrete buildings is listed as monuments, conservation specialists have still to become acquainted with the history and properties of historic concrete. Even though the structural history is frequently addressed, only little is known about the composition and surface finish of historic concrete. For the conservation of historic concrete buildings, generally ordinary repair techniques, which have been developed for modern concrete, are applied. From a technical point of view, their performance is not always satisfactory, as the properties of historic concrete can deviate from modern concrete. From a conservation point of view, additional threats exist, because requirements such as respecting the historic material and heritage values are commonly not considered. An uncritical application of repair approaches and a lack of knowledge on historic concrete induce the risk of loss of historic evidence and historic material. Therefore, the aim of this thesis is to develop a real concrete conservation approach, which takes into account the specific properties and the heritage values of historic concrete. The main research questions are therefore the following: - How can historic concrete be preserved together with its ascribed heritage values? - How can a proposed conservation strategy be evaluated for its impact on both the heritage values and the technical performance? - How can we balance the technical demands and the preservation of heritage values, when heritage values and durability issues originate in the same material properties? The structure of the thesis reflects its aim to achieve a dedicated conservation strategy for historic concrete: Part I - Understanding and characterisation of historic concrete The first part offers a compendium of historic concrete addressing the development of design rules and standards in the Netherlands, and in particular the constituents used (binders, aggregates, mixing water), mix design, reinforcement, and surface finishes applied. This part is illustrated with Dutch examples, supporting both technical and historical surveys in the field, and offers historical background information for further (material) investigation. Part II - Interpretation and evaluation of historic concrete The second part gives guidance to identify risks for both material and heritage values. This includes methods to correlate heritage values with the historic concrete and the state of conservation. Additionally, the risks and benefits of different repair techniques are evaluated from a conservation point of view. The new approach is applied to three case studies: Fort Bezuiden Spaarndam (1897-1901), the Hofplein railway viaduct (1900-1908, A.C.C.G. van Hemert), and the earth retaining walls for the unfinished Groot Museum (1921, H. van der Velde). Part III - Development and evaluation of dedicated conservation strategy The concluding part addresses the decisions to be made when developing a dedicated conservation strategy. The proposed approach considers balancing the preservation of heritage values and technical demands. Case specific criteria are formulated, which allow the characterisation of possible conservation strategies. This thesis presents new knowledge on historic concrete, combining historical developments with material properties. Different periods within the life in service of concrete are linked: the past by explaining its possible significance and durability, the present by evaluating its in practice performance and state of conservation, and the future by supporting decisions on an appropriate conservation strategy. This thesis aims to strongly support the necessary transition from concrete repair to concrete conservation.