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. HeinemannChair of Building Conservation, Faculty of Architecture, Technical University of Delft, The Netherlands

R.P.J. van HeesChair of Building Conservation, Faculty of Architecture, Technical University of Delft, The NetherlandsTNO Built Environment & Geosciences, Delft, The Netherlands

T.G. NijlandTNO Built Environment & Geosciences, Delft, The Netherlands

ABSTRACT: The 20th century built heritage is one of the new conservation challenges, due to its architecturaldifferences from the traditional heritage and new materials. One major new material is concrete; its quantity andimportance for the new heritage requires a tailored conservation approach. Until now, there is a dependence on arepair approach. The dangers of such an approach for the cultural-historical values of the original concrete willbe shown by the means of a case study, Fort Bezuiden Spaarndam (1901), part of the UNESCO World HeritageSite 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 conservationapproaches and techniques is therefore needed.

1 INTRODUCTION

With the aging of the 20th century building stock,we are now facing the challenge to evaluate themand make selections for objects which should be keptas part of our built cultural heritage. It is consistentthat the values which justified the selection shouldbe preserved and respected during any intervention.Interventions cannot be avoided as aging often goesalong with some degree of deterioration. The demandsfor interventions according to (international) conser-vation guidelines are applicable for the entire builtheritage, independent from age or construction type.

However, for the implementation of the demandsa tailored conservation approach for the 20th centurybuilt heritage is needed.The new heritage does not onlydiffer in architecture, structure, quantity, and buildingtypology from the traditional heritage, but also in theused materials such as concrete. The different materialproperties make a transfer of knowledge and experi-ence gained with traditional materials such as brickand mortar less feasible.

For concrete, the current approach is often a con-ventional repair approach as no tailored conservationapproach exists. As in practice a loss of values cor-related to the original concrete is often encountered,the question arises if this is a pure technical problem

such as incompatible repair materials or a more funda-mental problem such as the way concrete conservationprojects are approached.

During this research, studies of completed interven-tions of historic concrete buildings indicated a relationbetween the following of repair approaches and theloss of monumental values. In this paper, this rela-tion will be illustrated by means of the case study FortBezuiden Spaarndam, the Netherlands. It shows thatthere are general, underlying problems in the field ofconcrete conservation which endanger values, inde-pendent from the technical performance of a repair.

2 METHODOLOGY

The current research aims at understanding the rela-tion between the loss of value and following a repairapproach. To evaluate the impact of a repair approach,case studies of former concrete conservation projectsin the Netherlands have been carried out. In this paperthe results of the case study Fort Bezuiden Spaarndam,part of the Defence Line of Amsterdam (Stelling vanAmsterdam), will be presented to explain the underly-ing problems of the preservation of values of historicconcrete.

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During the research, the most important involvedparties were interviewed (architect, surveyor, and con-crete repair company) to understand their background,knowledge, and experience and to see where thedependence on concrete repair experts starts. Liter-ature, documents, and surveys related to the fort, itsconstruction and conservation were consulted to inves-tigate the historical background, conservation aims,and conservation process. During a visual inspectionin November 2006, the performance and impact of theconservation was evaluated.

The study focussed on concrete which was usedfor exterior elements, stressing the complexity to bal-ance technical, aesthetical and conservation demands,meaning a minimal loss of existing fabric and a maxi-mal preservation of values. Additionally, petrographicanalyses of samples were made to determine the con-crete composition and possible damage. To evaluatethe suitability of standard concrete surveys for con-servation tasks, literature and guidelines related toconcrete assessment were studied.

3 BACKGROUNDS OF REPAIR ANDCONSERVATION

To understand why following a repair approach in aconservational context can endanger values, it is nec-essary to understand why and how the main aimsof repair and conservation diverge radically. Repairmethods are usually developed to preserve a structuralfunction for a limited lifetime, lately also influenced bysustainability aspects. Surveys focus on physical andmechanical properties and the criteria for the choiceof a repair approach are, beside the structural perfor-mance, the service life, economical issues, and thepossibilities of maintenance (ENV 1504-09 1997).

Conservation, however, is much more than ful-filling technical and financial requirements. Besidesconsidering structural performance and safety, thepreservation of the authenticity of and values attachedto the site, building and material must be considered.The value and authenticity of the cultural heritageis understood in ‘artistic, historic, social, and sci-entific dimensions’ and the conservation principles‘respect. . . the existing fabric’ and the cultural heri-tage should be preserved for ‘present and futuregenerations’ (ICOMOS 1999).

Yet, due to the youth of the field of concrete con-servation, concrete repair experts are entrusted withconcrete conservation projects.Although these expertshave considerable knowledge of the technical aspects,they are not trained in specific conservation aspects,as the complexity and variety of monumental valuesand authenticity or the properties of historic con-crete. Because conservation experts often do not havesufficient knowledge on the material, an evaluation ofthe suitability of such a process is difficult.

Figure 1. Site plan of Fort Bezuiden Spaarndam.

4 FORT BEZUIDEN SPAARNDAM

4.1 Historical background of the Defence Line ofAmsterdam

The Defence Line of Amsterdam, the Netherlands(1880–1920), is a ring of fortifications (45 forts)around Amsterdam, intended to defend the city by sys-tematic inundation in case of an attack. This ingeniousdefence technique was one justification for the listingas a UNESCO World Heritage Site in 1996. In con-nection with the listing, attention was drawn to theneed for conservation and re-use, as the condition ofmany forts declined after the military function of theforts ceased in the 1960s. Fort Bezuiden Spaarndam(1901), a provincial monument in the province NorthHolland, was constructed as part of the Defence Lineof Amsterdam.

4.2 Historical background of Fort BezuidenSpaarndam

Fort Bezuiden Spaarndam is a non-reinforced concretefort built in 1901. It was planned as a standard modelof the Defence Line ofAmsterdam on a fort island withtwo cupolas with disappearing turrets (Fig. 1). The fortitself did not suffer from major damages during theSecond World War, though the cupolas were blown upby the German occupation army. The remains of thecupolas still lay spread around the grounds. Since thecease of the military function in 1960, the fort wasused for agricultural storage.

4.3 Historical background of the concrete ofFort Bezuiden Spaarndam

To recognise the historical value of the original con-crete, it is necessary to understand the construction andtechnology history of the late 19th century. Knowl-edge of concrete and its construction was mainlybased on empiric studies; standard design codes only

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appeared at the beginning of the 20th century. Inthis context, the knowledge gained from the con-struction of the Defence Line of Amsterdam wasimportant for the Netherlands, where early concretetechnology was dominated by the army corps ofengineers (e.g. Scharroo 1910). The first Dutch rein-forced concrete guidelines Eerste Gewapend-Beton-Voorschriften from 1912 were influenced by the expe-rience gained during the construction of the DefenceLine of Amsterdam (Noord-Holland 1996).

Therefore, the forts of the Defence Line of Ams-terdam are the tangible artefacts of this pioneer phaseof concrete technology. Initially, the forts were builtas traditional brick constructions; yet the inventionof brisance grenades demanded new and strongerconstruction methods. Hence from 1897 onwards,the forts were built as non-reinforced concrete con-structions, later as reinforced concrete constructions(ICOMOS 1996). This transition from brick to rein-forced concrete and the still preserved high degreeof authenticity was part of the justification for theNetherlands to nominate the Defence Line of Ams-terdam as a UNESCO World Heritage Site (ICOMOS1996). The value of the concrete as a preservablecultural-historical artefact was stated prior in thedescription of the Fort Bezuiden Spaarndam in con-nection with the listing as a provincial monument inthe early 1990s (Provincie Noord-Holland 1998).

5 CONSERVATION 1996–1999

Fort Bezuiden Spaarndam was one of the first fortsto be restored after the listing of the Defence Lineof Amsterdam as a World Heritage Site in 1996. Itwas supposed to be a role-model conservation projectwith extra subsidies and attention, as further fortsneeded restoration in the near future. The first aimwas to find a new locally orientated function for thefort, which should be compatible with the surroundingnature recreation area. In 1996, it was decided to usethe northern wing as a day nursery, the postern as anart gallery, and the southern wing as storage.

The task of conservation was handed over to arestoration architect, who had experience with theconservation of a brick fort, but no experience withthe material concrete. This is not unusual for thelate 1990s, as little knowledge and experience withthe conservation of concrete existed. The heritagecare authorities of the province North Holland had nospecialist for the material concrete either, and an exter-nal concrete expert was entrusted with the survey ofthe fort.

5.1 The survey of the concrete

The survey did not differ from standard, non-conservation concrete surveys. The assignment was to

assess the technical state of the concrete and to deter-mine the causes of the encountered leaching, cracking,and leakages. Several cores were taken, not only todetermine the composition and damage causes, butalso to gain information on the wall construction.

It was stated that the walls were massive concreteand the concrete was composed of cement with groundgranulated blast furnace slag (GGBS) and crushedrocks (granite, porphyry) as aggregates. The followingdamages were assessed in the survey:

– shrinkage cracks– cracks along casting segments caused by missing

expansion joints– water bearing cracks with leaching– map cracking caused by the formation of thaumasite

or ettringite.

The advice was to reconstruct the missing earthshelter of the roof and to seal the roof to preventfurther infiltrations. Cracks that influenced the struc-tural performance should be injected; the injection ofother cracks was optional. In general, the conditionof the concrete, considering the construction time andmethod, was stated as good and with adequate repairs,the building could last another 100 years.

5.2 Repair of the concrete shell

The main goal of the intervention was to repair theouter shell of the fort in a way that would solve theproblems ‘for once and for all’, repairing the roofand treating all cracks. As a detailed advice for repairtechniques was not commissioned, the choice of anadequate repair technique was left to the executingconcrete repair company. In 1997, the repair campaignof the fort started and included for the exterior concretewalls following intervention steps:

– high water pressure cleaning of the façade– filling of cracks with standard repair mortars– injection of cracks with micro-cement– creating expansion joints along the cracked vertical

segment joints.

No specific requirements concerning durability andcompatibility were prescribed. The concrete repairworks were not supervised by external experts. Main-tenance and regular inspections were not planned,since it was assumed that the repairs would be longlasting.

6 EVALUATION OF THE CONSERVATIONPROCESS

Although intended to be a role-model conservationproject, the approach of Fort Bezuiden Spaarndamis an example of a concrete repair approach. This

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case shows the problems resulting from an unsuit-able approach and reflects an underlying problemof concrete conservation: unawareness of the valuesof historic concrete. Examples from the survey andexecution illustrate the consequences and show howvaluable information could have been gained or valuespreserved.

First, the crucial phase of assessment and decision-making was not guided by adequate experts whounderstood both the material, including its history,and conservation principles. Consequently, valuesattached to the concrete were neither determined northe impact of the repair techniques on the valuesattached to the concrete evaluated.

However, it is possible to gain information onpossible values during a survey (Heinemann 2007).A review of the conservation process show how valuescould have been detected and thus incorporated in theconservation process; it also reinforces that knowl-edge on the material, its history, conservation aims andrepair techniques is needed, and that trained experts forthese tasks are still missing.

6.1 Present condition 2006

A visual inspection of the performance and impactof the interventions was made during a site visit ofFort Bezuiden Spaarndam in 2006. From far distance,the impact of the repaired cracks was already visible,changing the appearance from a monolithic structureto a patched structure (Fig. 2). On closer examination,it appeared that the repaired cracks next to the newlycreated expansion joints re-cracked and the concreteof the casemates showed map-cracking with leachingand local disintegration. On several places on the mainfaçade, traces of camouflage paint were found thatwere not documented in former surveys.The interior ofthe fort showed no signs of infiltrations or re-cracking,and according to the tenants, who appreciated the

Figure 2. Gorge façade after repair.

interior of the fort (day nursery and art gallery), theclimatic conditions were satisfactory.

6.2 Awareness of values

The visual impact of the conservation on the façaderaised the question why the aesthetical values of themonument were not respected. A possible aestheticalvalue of a building is usually widely accepted and thevisual impact of a repair technique can be evaluatedwithout large-scale research. However, standard repairtechniques do not consider aesthetical compatibility.

Therefore, the question arose how the conservationof concrete was realised and if values were consid-ered in the process. During our investigation, it becameapparent that the concrete was only perceived as thestructural material of the shell. This reflects the under-lying problem of concrete conservation, that concreteis often not seen as a valuable historical material.

In the case of Fort Bezuiden Spaarndam, it was offi-cially stated that the original concrete was valuable inthe justification for the listing as a UNESCO WorldHeritage Site (ICOMOS 1996) and as a provincialmonument in 1992 (Noord-Holland 1998). The origi-nal concrete is important because of its uniqueness andremains of this phase of the technical development ofconcrete in Europe are rare. However, an implemen-tation of the preservation of and awareness for thesevalues was missing in the conservation process.

This unawareness led to the commission of a stan-dard (i.e. purely technical) survey of the concretewhich was not adapted to the needs of conservation.Without sufficient knowledge of historic concrete andpossible values, the weaknesses of a purely technicalsurvey in this context were not seen. Consequently, thecorrelation between values, the material, and the stateof conservation was not determined. Therefore, thevalues were not further integrated in the conservationprocess.

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6.3 Survey of the surface

Aesthetics are criteria for the value assessment ofbuildings and for the compatibility of conservationtechniques. Therefore, it is important to describe thesurface of a monument during a survey. For techni-cal surveys, as it was commissioned at Fort BezuidenSpaarndam, a description of the surface is not includedas it usually does not influence the mechanical orphysical properties of the concrete.

A description of the properties of the surface suchas colour, texture, deposits, weathering, or traces offormer events can be used to evaluate the impact of anintervention (e.g. material loss due to cleaning) or helpto adapt the structure and colour of a repair mortar tothe surrounding concrete.

Other essential information which should havebeen documented is the presence of camouflage painttraces and the original surface finish. Further colour-historical research should have been carried out toevaluate the options for reconstruction of the paint andto broaden the historical knowledge of the forts.As thiscase was supposed to be a role-model, opportunitiesto study the forts in connection with the survey shouldhave been taken.

How far the cleaning of the façade with high-waterpressure caused losses of the original surface, andwith it historical information, cannot be determinednow, since there is no detailed documentation of thefaçade to refer to. Given experience with façade clean-ing, however, such a loss is highly likely. It also showshow easily standard approaches are followed, and con-sequences for the monument and its values are notconsidered.

6.4 Survey of the composition and construction

An analysis of the composition, structure, and causeof damage of concrete, as for example by petrographicexamination, is not standard, as it might not always benecessary to diagnose the cause of damage. However,if samples are taken, they can be used to gain insightinto the composition and construction of historic con-crete. Besides giving relevant technical information,it can reveal important historical information, andtherefore indicate value.

Historic concrete differs from modern concrete. Inthe case of Fort Spaarndam Bezuiden, it is known thatit was constructed in a time when concrete construc-tions were not standardised.Also the statement that theconcrete is a rare remainder of concrete developmentshould have led to an investigation of the samples froma historical point of view as well.

Our petrographic analysis of samples of the south-ern cupola and of the main building show that theconcrete is composed of a coarse-grained Portlandcement and not ground granulated blast furnace slag(GGBS) cement as described in the survey. The

coarse aggregate was crushed porphyry from Quenast,Belgium, and the fine aggregate river sand (Fig. 3).

From the study of historical documents, we foundout that crushed rock was assumed to be superior togravel because of its rough and angular surface whichwas supposed to improve the bond to the cement paste.The use of crushed rock and in particular porphyry ofQuenast as an aggregate was advised in contempo-rary books (Kloes 1908). In the construction plans ofthe fort from 1900, it can be seen that a differentia-tion between the used aggregates and function of theconcrete element was made. For important elements,the use of porphyry was designed, while secondaryelements where designed with cheaper gravel or evenwith rubble.

The high amount of voids and the average to highcapillary porosity of the concrete mentioned in thesurvey can be explained with knowledge of the com-position and construction method. They are in thiscase typical, because the rough shape of the aggre-gate complicated the manual compaction. To obtainthe workability, the use of a higher water-cement ratiowas common. Both the compaction and the high water-cement ratio influence the porosity and durability ofconcrete.

Another result from the composition is the occur-rence of alkali-silica reaction (ASR) (Fig. 4). The usedaggregate, porphyry of Quenast, is now known tobe alkali-silica reactive (Nijland et al. 2003). Whenused in combination with Ordinary Portland Cement,which, in contrast to the GGBS cement that waswrongly considered to be present in the original survey,ASR is not inhibited. With knowledge of the construc-tion history of concrete certain damage types can beexplained, here the assumed superior combination ofporphyry and Ordinary Portland Cement causingASR.

Figure 3. Microphotograph showing detail of the cementpaste, with relics of unhydrated Portland clinker (indicatedas calcium silicates C2S and C3S), and no blast furnace slagat all (view 0.7 × 0.45 mm, plane polarized light).

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Figure 4. ASR-gel (A) in voids (V) and porphyry (P)and sand (S) as aggregates in the cement paste (C) (view1.8 × 1.12 mm, plane polarized light).

Figure 5. Microphotograph of the section through thesurface, showing three lime-based layers (plane polarizedlight).

It has to be kept in mind that the perception ofcement types differed from our current perception. Atthe time, the use of Ordinary Portland Cement was pre-ferred to the use of blast furnace slag cement. Latterwas assumed to be of poor quality, mainly because itwas a cheap waste product. It became, due to economicreasons, only widely used in the Netherlands from the1920s onwards.

A petrographic analysis of the concrete can alsotell stories about the history of a building. A sectionthrough the surface (Fig. 5) shows three layers of alime based layers on the surface. The core was takenbefore the surface was cleaned with high-water pres-sure. Therefore it cannot be determined if they areremains of a finish (Noord-Holland 1996) or deposits.If the indications from the thin section would have beenused prior to the intervention, research could have beencarried out in situ and decisions made if the surface is

preservable.Yet it is likely that the high pressure clean-ing of the façade affected this layer of the surface. Asno references were found on the building anymore norcould be found in the documentation of the project,the amount of loss of original fabric cannot be said.

With historical and technical knowledge, a petro-graphic analysis can help to determine the authenticityof the original concrete, which is important when deci-sions have to be made of how much material loss isacceptable. The results from our petrographic analy-sis show that the composition of the concrete of FortBezuiden Spaarndam is of high quality relative to thestate of the art of 1900. The given historical informa-tion confirms the value of the authentic concrete andany intervention should respect that value.

6.5 Advices and decision-making

As the assignment of the survey was pure technical,additional historical aspects of the concrete were notconsidered.As a result, no special treatment respectingthe historical values (significance of original concreteand the visual appearance) was advised. The choice ofthe repair techniques was done by the restoration archi-tect and the concrete repair company. Neither of themwas familiar with the properties of historic concreteand the complexity and consequences of repair choiceswere not seen. One example of the consequences isthe repair approach of the cracks caused by thermalmovements.

A common problem of the forts of the Defence Lineof Amsterdam is the lack of expansion joints. Expan-sion joints were not common at that time, in the 1920sthere was still discussion if expansion joints are needed(Kleinlogel 1927). Also here the missing insight intothe properties of concrete caused damage, cracking,which becomes a historical evidence of the trials anderrors of the technological development.

The concrete forts of the Defence Line of Amster-dam are approximately 150 m long, most of them notreinforced. The stresses caused by thermal movementcaused regular linear cracks along the weakest points,usually the casting segments. Cases of severe struc-tural damage are not known, as each casting segmentis equivalent to one vault and the cracks appeared atthe points of support in the middle of the cross walls.

Yet, the ingress of aggressive agents can be facil-itated by the cracks. As there was no reinforcement,corrosion was not possible but the composition of theconcrete was ASR sensitive, a reaction which needswater and can lead to severe damage. However, nosigns for increased deterioration were visible next tothe cracks.

During the restoration campaign of Fort BezuidenSpaarndam is was decided to repair the outer shell‘properly’, which included a repair of the crackscaused by thermal stresses. The idea was to compen-sate the thermal stresses with expansion joints, which

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Figure 6. New expansion joint (arrow) next to patchedcracks (CR) caused by thermal expansion.

would be intersected. The façade was cleaned withhigh water pressure and the cracks injected. The injec-tion was originally carried out with a polyurethaneinjection resin. The high porosity, the numerous voidsalong the segment joints and the thickness of the wall(1.25 m) complicated the work. As a complete fillingof the cracks was unlikely, the cracks were injectedwith non-elastic micro cement instead. As the thermalstresses would be compensated by the new joints, theelasticity of the mortar was not considered important.

After patching and injecting, the exterior façade wasvertically intersected (0.05 cm–0.10 cm depth) alongthe casting segments with the idea to create expansionjoints which would prevent further cracking of the con-crete. The dimensions (width, depth) of the cut werenot based on calculations but on the working experi-ence of the contractor with non-reinforced industrialfloors.A possible effectiveness of the intervention wasnot studied prior to application, neither tryouts on alimited scale were done nor by simulations.

As the concrete is not reinforced, the structure issensitive to stresses; instead of releasing the struc-ture from stresses, stresses concentrated at the endof the intersection. The reappearance of cracks in thepatched area within less than a decade indicates thatthe intervention does not work (Figs 6–7).

This intervention required an unnecessary materialloss and as it failed, a ‘repair of the repair’ may follow,causing further material loss. A simpler approach by

Figure 7. Close up of an expansion joint with a reappearanceof the former cracks; the colour of the used patching mortarwas not adapted.

not filling the cracks and preventing water ingress byfixing the roof, as it was done anyway, should havebeen tried instead. Together with a long-time moni-toring of the moisture ingress through the cracks andof the crack movement, a major intervention couldhave been prevented or delayed and insight would havebeen gained on the damage mechanisms, allowing thechoice of an appropriate conservation approach.

7 CONCLUSIONS

The primary problem of the concrete conservationof Fort Bezuiden Spaarndam is not technical suchas the performance of a repair technique. The prob-lem lies in an approach, which does not respect theconcrete as a historical valuable material: a repairapproach instead of a conservation approach.Yet thereis another underlying problem in the field of conser-vation: concrete seems to be simply too young forheritage stakeholders.

7.1 Conservation Fort Bezuiden Spaarndam

Monumental values were lost during the conservationof Fort Bezuiden Spaarndam, because the concrete wasprimarily understood as a structural and not a historicalmaterial.Together with a dependence on repair expertswhich are trained to perceive concrete from a structuralpoint of view, little attention was given to the valuescorrelated to the original concrete. The conservationexperts could not evaluate if the approach was suitablefor historic concrete as they had no experience with thematerial.

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Similar to a repair process, the survey focusedon physical properties of the concrete and did notintegrate historical relevant aspects as the uniquecomposition of the concrete or the traces of camou-flage paint. Consequently, the amount of lost originalconcrete and related values was not considered.

Support and guidance for the involved partiesby concrete conservation experts was not possible,as the field of concrete conservation barely existedand the complexity of concrete conservation was notunderstood.Therefore, possible conservation aims, thecultural-historical values, the significance of the stateof the building and their relationship could not bediscussed openly before starting the conservation cam-paign. Such discussion should not be limited to pos-sible re-use or economical issues, but should includetangible and intangible values of the building/materialas well.

A critical review of the repair approaches was nottaken into account, as it was assumed that repairexperts have sufficient knowledge of the material.There was no awareness for the fundamental differ-ences between a repair and conservation, therefore theinfluence of a repair approach on the values was notconsidered.

However, this case is not isolated and it is likely thatthe results would have been similar if the conservationwould have been carried out by other people. The fun-damental problem of concrete conservation is that theconservation process is often not guided by conserva-tion experts which understand both the material andconservation principles.

7.2 Concrete conservation

Values and their preservation are key issues of heritagecare. The value of age, that, as Riegl (Riegl 1903) says,even ‘appeals the masses’ is often not seen since tothe ‘youth’ of concrete buildings and the more sophis-ticated historical values need art-historical knowledgeto be recognised. Until now, this art-historical knowl-edge hardly exists and this reflects itself in the (value)assessment of historic concrete buildings. For themajority of restoration stakeholders, concrete build-ings are too young, as their working field is mainlybuildings prior to 1900.

It is necessary to overcome the knowledge gap con-cerning 20th century buildings in the working fieldand education of conservation experts. Parallel, it isnecessary to develop a tailored conservation approachto overcome the dependence on a pure repair approach.When applying repair techniques, one should not relyon the fact that concrete repair has been carried outfor decades, but assessed whether they are applicable,since they were developed for different aims.

As long as there is a dependence on a repairapproach, the background of repair experts has to be

kept in mind. It is the task of the conservation expertto explain the diverging requirements for conserva-tion towards repair experts. This requires sufficientknowledge of the material concrete from the conser-vation expert; otherwise the deficiency of a repairapproach cannot be seen and may bring to risk thevalues which were the initial reason for listing andconservation.

As long as concrete conservation projects are stillisolated cases and no clear guidelines or references forconcrete conservation exist, general guidelines suchas the Venice Charter (ICOMOS 1964) should beconsulted to evaluate possible approaches. Finally, dis-cussions of possible values of concrete are needed tosee that it can be more than a greyish bearing mate-rial. As concrete was initially mainly understood as astructural material and only later the architectural val-ues were discovered, we have to make now a similarevolution: from structural repairs to conservation.

ACKNOWLEDGEMENTS

Figures 1 (based on the construction drawings from1900), 2, 6, and 7 by H. A. Heinemann, figures 3, 4and 5 by T.G. Nijland.

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