Some reflections on the application of the concept of compatibility to Heritage

Pierre SMARS

8 December 1998

Text of a paper read at the congress "Initiative for promoting the use of compatible materials in the protection of European cultural heritage from environmental risks", Athens, December 11th-12th, 1998

Let me start introducing myself briefly. I am Belgian, architectural-engineer and I am working at the Raymond Lemaire Centre for Conservation. This centre, situated in Leuven (Belgium), offer a post-graduated training program in conservation of monuments for architect, art historians, engineers, archaeologists of different origins (mainly Europeans). My personal research-field is stability of structures and I prepare a Ph.D. on the modeling of vaults. But, if I am here, it is mainly as a member of ICOMOS. In my talk, I would like to underscore aspects that are probably slightly parallels to the main stream of discussion, aspects often known but I believe worth being repeated from time to time.

1 Extension of the definition of compatibility

When speaking about compatibility, one thinks maybe firstly at the relations between adjacent materials: the chemical compatibility, the mechanical compatibility (dilatation, rigidity, resistance), the physical compatibility (diffusion, porosity). In the context of Heritage, the concept can usefully be extended, to include global aspects and so feed profitable thoughts at all levels.

The definitions of compatibility can change but three elements are always present: some differences, a changing environment and permissible damages. One speaks for instance of two materials having characteristics allowing them to deform together. Because of the differences of characteristics of the elements, differences in the behaviour are induced by variations of the external conditions and relative tensions arise. These tensions can provoke damages to some elements, which cannot be too intensive. What characterizes the cultural heritage, what makes the difficulty of its conservation but maybe eventually the interest of it, is the multiplicity of its dimensions. As the definition of heritage evolves, this multiplicity intensifies, new aspects are considered (economical, sociological, technological, etc). Numerous possibilities of incompatibility exist then.

2 Determination of compatibility

We have to make choices, compromises and absolute compatibility is more a wishful thinking than an objective reality. Incidentally, it is probably the case of most concepts used in restoration: starting by the word "conservation" itself, but what to say about reversibility, authenticity, etc. These choices are even more difficult to take when the cultural heritage under consideration stands in a risky environment, prone to earthquakes, violent winds, where the human aspects are naturally dominating. Fixing permissible damages is often the key issue.

If we are interested in the cultural heritage, it is to try to preserve its peculiarities and so its difference, its uniqueness. Now, engineers have a natural tendency to integrate their actions in known schemes. It can be for ease, by lack of knowledge or for responsibility issues. Interventions on the heritage should nevertheless be an opportunity to call into question, to discuss "obviousness". The recommendations of the ICOMOS structural committee and the organisation of events like today's are going in that way.

To call some habits into question is only possible with a thorough knowledge of the situation, of the state of the building, of the evolution of its state and of the variations of the external conditions. It is only on that condition that compatibility can be discussed, the terms of the problem being known. The complexity of the venture requires a multidisciplinary approach. To be really useful to conservation, knowledge of the ancient techniques, theoretical and laboratory researches, development of new technology or rediscovery of ancient ones should progress together. It is only possible if good contacts between teams exist, only if some compatibility is present. These remarks, maybe obvious, speak for the development of frames, of projects (international but also local and vertically oriented). In that scenario, the observation in general, from the monitoring of the deformations to the verification of the effectiveness of measures taken will have an increasing role.

3 Limits of intervention

Heritage, inevitably, is changed, transformed when interventions are carried out. Sometimes works are suggested to allow the accommodation of new functions. Sometimes, and it is rather the congress theme, reinforcement are needed to diminish the structure vulnerability. Now, one of the objectives is to try to preserve. One could at that stage speak of compatibility between object and intervention.

Sometimes, there is really incompatibility between the structure in its originality and the claim to resist to an earthquake or another solicitation. Maybe in some of these cases one should just not intervene. It seems at least useful to search for the limits of intervention. The recommendations of ICOMOS structure committee admit in some cases a lowering of the security level.

In that way, the exposure of the monument is important: is it inhabited? In the contrary, in the case of archaeological ruins for instance, that is in case of structures very often hardly weaken, can we not let some elements fall down instead of applying extreme consolidations measures. Is it not possible to consider measures just limiting the damage in case of collapse, looking for some kind of "retreatability" or even to admit the destruction of some parts?

When human aspects are present they are often preponderant. One needs caution but do we have to transform traditional architecture to preserve it? Requiring strong interventions, the risk of denaturing the buildings is high, and so to eliminate it today under the pretext to preserve it for tomorrow. It is a matter of determining the values important to protect. If concessions cannot be made on the lives, it is still possible to make some on the buildings; damages cannot be totally avoided.

This problem is particularly complex in the case of badly constructed or badly kept structures. Some months ago I had the opportunity to go to Umbria, on the spot of the September/October 1997 earthquake. The quality of the masonry of the destroyed house was very often poor: three-leaves walls with a pulverulent rubble infill and not very well ordered facing. The structure of the houses and villages not corresponding with today-live requirements, numerous buildings were abandoned. They were naturally more damaged... as we know the fundamental importance of maintenance on the resistance of structures.

In some places, para-seismic interventions had been done. Some were effective other not. For instance, some masonry injections did not work properly like shown by ripped open walls, very unevenly reinforced. It was clear that closer contacts between researchers, people stating the characteristics and the application modalities of the grout, the contractors and their workmen should have been necessary and that, at different levels, tests like the ones recommended for instance by Sasse and Snethlage in a Dahlem workshop report of 1997 should have been planed.

The efficiency of methods is limited. In some circumstances the use of a technique can be impossible. Some stones were bonded with an earthy mortar, clayey, not injectable.

4 The intervention

When an intervention is decided the objective is most often to improve the global resistance. It is natural to imagine doing it increasing the local resistance. It is not always so easy. Firstly, a local reinforcement does not always bring a global reinforcement. Secondly, introducing too resistant reinforcements can, in case of heavy solicitations, damage original elements more fragile and that goes against the conservation objectives. If one has to introduce new elements, the precepts of good conservation encourage rather the use of weaker elements. Talking about repointing, one would certainly not want a too resistant new mortar to damage an especially valuable stone.

I spoke before of the strong resistance reduction due to the lack of maintenance. The reformation of the structure original quality is sometimes sufficient to reach a satisfactory safety level. Of course, it is always a question of looking for equilibrium between the risks of damages and the respect of the values. The para-seismic interventions recommended by the standards ask often the transformation of the structure into rigid entities. To increase the global rigidity of an ancient building is not obvious; the interventions needed are often incompatible with the respect of heritage values. The rigidification of some parts implies sometimes the necessity of a complete intervention, extensive and therefore very heavy.

The dynamical requirements can also conflict with the statical ones. The first taking to the formation of a rigid organism, hyperstatic and the second to the formation of a flexible one, isostatical, able to accommodate to the external conditions variations. Solutions looking to maximise the capacity of dissipation instead of trying to increase the rigidity may be more suitable.

Small-scale rigidity and resistance should not always be rejected a priori. It can be excessive to sentence a material merely on the basis of its characteristics. It is only knowing also the neighbouring materials, the external conditions fluctuation and the hierarchy of values that it can be proved to be appropriate or not. Talking about rigidity, it can for instance be permissible to use a rigid material to fix broken stone pieces back together if some more general movements are still possible.

5 Conclusion

Compatibility is always relative and good interventions should be decided and worked out on the base of the actual stakes, the values. Local considerations should always fit in a global framework.


[1] I. MAROEVI, "Material structure and authenticity of the architectural heritage in view of possible interventions for seismic protection", Proceedings of the 1st int. seminar on modern principles in conservation and restoration of urban and rural cultural heritage in seismic-prone regions, Skopje, 1988, pp. 9-15

[2] G. TORRACA, "The application of science and technology to conservation practice", in BAER, SABBIONI, SORS (eds), Science, technology and European cultural heritage, Bologna, 13-16/6/1989, Brussels/Luxembourg, 1991, pp. 221-232

[3] H.R. SASSE, R. SNETHLAGE, "Methods for the evaluation of stone conservation treatments", in N.S. BAER, R. SNETHLAGE (eds), Saving our architectural heritage, the conservation of historic stone structures, Chichester, 1997, pp. 223-243

[4] K. VAN BALEN, S. ERCAN, T. PATRÍCIO, "Reversibility versus ductility : a case study at the late hellenistic nymphaeum of Sagalassos (Turkey)", Use of and need for preservation standards in architectural conservation, ASTM STP, to be published

[5] P. SMARS, J.-J. DERWAEL, V. PEETERS, K. VAN BALEN, "Displacement monitoring in the church of Sint-Jacob in Leuven", First European congress on restoration of Gothic cathedrals, Vitoria, 20-23/5/1998, to be published