Eu-Artech Seminar on Small Samples - Big objects , Edited by Bayerisches Landesamt für Denkmalpflege, Munchen 2007, pp. 15-26.

SAMPLING AND CHARACTERISATION ISSUES IN THE STUDY OF A STONE PORTAL WITH MICRODRILLING

José Delgado Rodrigues* & A. P. Ferreira Pinto** * Geologist, Principal Research Officer (ret.), National Laboratory of Civil Engineering, Lisbon, [email protected] ** Civil Engineer, Assistant Professor, Department of Civil Engineering and Architecture, IST, Technical University of Lisbon, Portugal [email protected]

ABSTRACT

The portal in the Old of is built of limestones, some of them containing relevant amounts of clay minerals. The degradation state was very severe and the abandonment it suffered for decades has left it virtually crumbling in pieces. The study here presented is part of a broader study carried out in this portal aiming at providing assistance for the definition of the conservation concept and for the delineation of the specific conservation actions to be carried out. The DRMS (drilling resistance measurement system) was used for identifying the degradation condition and it showed to be of major importance for the study carried out. Through a progressive sampling strategy, it was possible to get information on the conservation state that showed to be decisive to build up a precise model of the decay processes responsible for the major degradation features. A deep fragmentation process and a superficial softening were identified and a profuse presence of past treatments was also detected. The implications in terms of the conservation concept are also briefly addressed in the paper.

1. INTRODUCTION Porta Especiosa is a Renaissance portal applied on the north façade of the Romanesque Old Cathedral of Coimbra (Fig. 1). It is organised in three main architectonic stages, the lower portico and entrance, the balcony in the intermediate zone and the upper stage. For several decades it was abandoned and a critical state of degradation was reached. Most surfaces exhibited heavy mass losses and severe lacunae were quite frequent. These conditions have lead the authorities to act on the portal and a preparatory condition survey was carried out. LNEC was asked to provide assistance in this process and was charged of the identification of the degradation profiles and of the study of conservation treatments of potential interest for application in the portal. A subsequent conservation intervention was carried out in the entire portal and a conservation concept was defined for supporting the intervention. The onsite tests carried out by LNEC were instrumental for the definition of the consolidation alternatives and were considered decisive for selecting the options to be followed. This paper addresses the characterisation of the degradation profiles, especially in the areas where more severe damage features were visible, as well as the identification of past consolidation treatments. The scope of the paper is to highlight the perspective of Eu-Artech Seminar on Small Samples - Big objects , Edited by Bayerisches Landesamt für Denkmalpflege, Munchen 2007, pp. 15-26. characterising a large “object” by departing from some microdestructive tests that for this specific case could be considered virtually innocuous in terms of their intrusive character. In complementary studies, several consolidation treatments were tested, but this subject is outside the theme of this seminar and out of the scope of the present paper.

Figure 1 – North portal (“Porta Especiosa”)of the Old Cathedral of Coimbra

2. THE BUILDING MATERIALS AND THEIR PROBLEMS The north portal of the Old Cathedral of Coimbra is made of limestones quarried in the region of Ançã-Portunhos 1, in the surroundings of that city. There is no specific information on the exact provenance of the stone varieties of the portal, and the large number of quarries still existing in that area precludes the attribution of a correct origin to these materials. Two main varieties are present in the portal. One is a whitish, compact and fine grained micritic limestone with some conspicuous calcite veinlets crossing the blocks. The second variety is an impure clay limestone, grey to brownish coloured and apparently less compact. A number of small samples were collected in the portal, mainly consisting of fragments that were detaching and about to fall down. These samples were studied in the lab for determining the insoluble residue and porosity and gave the following results 2: • Porosity range = 10.0 to 14.4% • Insoluble residue = 8.9 to 13.1% It is worth stressing the fairly high value of the insoluble residue, the more so that its main constituents are clay minerals. Since only small fragments were available for testing, the swelling behaviour could not be determined, but using the correlation functions obtained in a previous work 3, it is possible to estimate that the swelling strain would reach values as high as 10 -3 (=1mm/m). The extensive surface scaling and deep fragmentation are a direct consequence of the high swelling capacity derived from the high insoluble residue. Eu-Artech Seminar on Small Samples - Big objects , Edited by Bayerisches Landesamt für Denkmalpflege, Munchen 2007, pp. 15-26.

The grey to brownish varieties were used in the lower part, while the more compact and pure whitish limestone was used in the upper zones of the portal. Fig. 2 illustrates some aspects of the degradation state of the greyish varieties. The extensive fissuring and the widespread powdering and scaling have induced severe mass losses and many decorative features are already lost. Some blocks showed an intensive network of fractures associated with active detachment of particles, situation that raised serious doubts on the integrity of those blocks. To deepen the investigation of these peculiar degradation features was one of the main aims of the work carried out whose results will be presented here. The more compact stone variety is crossed by frequent calcite veinlets that correspond to a family of weakness planes that pervade the stone (Fig. 3). These planes trigger the rupture of the most exposed pieces and are responsible for many lacunae found in the upper zones of the portal.

Figure 2 – Degradation features of the greyish stone varieties. Notice the multiple fractures, the heavy mass losses and the generalised precarious condition of the stone surfaces

In some areas of the intermediate zone, some dark areas showing a decay pattern similar to black crusts were present. The external layer of these zones was very dark and some times very hard, while in some areas a white powdering layer could be identified after the Eu-Artech Seminar on Small Samples - Big objects , Edited by Bayerisches Landesamt für Denkmalpflege, Munchen 2007, pp. 15-26. indurated layer was lost. Later in the preparatory studies, a cleaning test carried out in these zones has shown that the black colour was deeply rooted in the stone and virtually impossible to be removed. The results presented below will demonstrate that these decay features are largely due to the presence of consolidating products extensively applied in the portal in undated previous interventions. The object of this presentation is to highlight the use of the DRMS – Drilling Resistance Measurement System - for the characterisation of the main aspects of the degradation features occurring in this portal and to illustrate how these results were integrated in the definition of the conservation concept for supporting the conservation intervention.

Figure 3 – Degradation features in the whitish and more compact stone variety. The calcite veinlets (left) materialise a network of weakness planes that trigger the detachment of large stone fragments and induce important lacunae, as seen on the cornice (right)

4. DRILLING RESISTANCE AND DECAY PROFILES DRMS is a drilling instrument capable of making a hole till 5cm depth under precise drilling conditions and measuring accurately the resistance offered by the stone during the drilling operation 4. For the present study, a common hard steel drill bit suitable for hard stone and concrete (Fisher type) with 5mm diameter was used under drilling conditions of rotation speed of 600rpm and advancing rate of 20mm/min. From the observation of the surface condition in the portal serious doubts were raised on what could be done to fight against the decay processes under active progress. Some blocks showed intense powdering and scaling meaning that it would be impossible to conserve these surfaces as they are. Some material loss had to be accepted, even if a treatment was to be considered. Other areas showed conspicuous cracks and multiple fissures, leaving the impression that they were not superficial occurrences but that would rather be massive events that possibly could develop inwards till great depth. To know the type of degradation profiles that were prevailing in those blocks was considered essential for defining an adequate approach to tackle all these complicated conservation problems. The DRMS was elected as the main tool for elucidating these intriguing questions. The study started with the execution of a few trial drill holes in the more decayed blocks. The results were immediately considered of a very relevant interest and the testing campaign was then extended to the entire portal. To characterise the different types of Eu-Artech Seminar on Small Samples - Big objects , Edited by Bayerisches Landesamt für Denkmalpflege, Munchen 2007, pp. 15-26. degradation features and identify the presence of past treatments were the two main aims of this onsite testing campaign. Fig. 4 and 5 illustrate the results obtained in the areas where the surface showed an intensive powdering and scaling and where some fractures were also visible. The drilling resistance profiles show a very thin superficial layer where the stone shows an outwards decrease of strength, meaning that the superficial powdering and scaling is mostly a superficial phenomenon. In depth, the graphs show the occurrence of multiple zones where strength reaches very low values, thus meaning that fractures were present very deep inside those blocks.

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Figure 4 – Block of greyish clay limestone with severe mass loss and heavy scaling and several drilling profiles made in it. The fractures seen at the surface have a clear impact in the drilling profiles at depth, as seen in the very low strength regions found at several depths

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Figure 5 – From the observation of the extensive superficial degradation it was not possible to deduct that multiple zones of extreme weakness were present until large depth, as the drilling profiles clearly demonstrate

These two main types of results extracted from the graphs have raised two major doubts in terms of the conservation actions that could and should be done. Firstly, the decayed superficial layer is very thin and stays in contact with the sound substrate, meaning that there is no transition zone between the decayed and the sound stone. Acting to preserve Eu-Artech Seminar on Small Samples - Big objects , Edited by Bayerisches Landesamt für Denkmalpflege, Munchen 2007, pp. 15-26. this thin decayed layer would mean to consolidate a “powder” thick of 1mm staying on top of a hard stone substrate. In practical terms this is a virtually impossible endeavour. On the other hand, the weak zones existing deep inside the stone signify that the stone block may collapse at depth, loosing several centimetres of their external most parts. Consequently, addressing the preservation of these blocks means that a solution had to be found to consolidate these fractures at great depth. In practical terms, it became clear that these fractures could not be consolidated from the stone surface and therefore a direct access to the fractures had to be created to provide additional strength to these areas. Both conclusions were taken as critical issues and were integrated in the conservation concept, as indicated below. The next sampling target was the area with “black crusts”. The morphological details fit the model of black crusts, but the dark and homogeneous colour occurring in large areas raised doubts on the origin of this decay form. Fig. 6 illustrates the results taken in one of those areas. The drilling profiles have systematically shown that a superficial harder layer was present in these dark areas and this was interpreted as an indication that a past consolidation action might have been applied here. In terms of the conservation implications, it became clear that the model of black crust had an additional complication here with the introduction of a hardened layer that presumably would also be less permeable and that could act as a barrier to any possible introduction of treatments, if required. A cleaning test trial had subsequently shown that the black colour was at least partially due to the ageing of the consolidating product, fact that turned the cleaning operation extremely difficult and slow.

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Figure 6 – Area with “black crusts” and the corresponding drilling profiles. The presence of a very hard superficial layer demonstrates that a past consolidation treatment was applied in these areas. The two small peaks resulting from the splitting of the original high strength peak illustrate the progress of the degradation process

When closely observing the portal, several signs of past treatments were identified in other parts of the portal, even in areas that visibly did not show signs of degradation that would justify their consolidation. The campaign headed then to a few spots where signs Eu-Artech Seminar on Small Samples - Big objects , Edited by Bayerisches Landesamt für Denkmalpflege, Munchen 2007, pp. 15-26. of past treatments were visible and to others that were undamaged and with no visible signs of them. Some of the results obtained in these areas are illustrated in Fig. 7 and 8.

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Figure 7 – Drilling profiles in a “sound” area. Notice the high strength peak in the first 3mm. The graph with squares (hole 2) was obtained in a zone where a broken fragment had recently detached

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Figure 8 – Drilling profiles in an original surface. The very hard superficial layer demonstrates that an alien agent had been applied. The detachment of the thin plaquette in the photo illustrates the consequence of the presence of a very hard layer of high strength contrast with the lower substrate. The incipient peak splitting of the resistance graph (arrow) is the imprint of that degradation form

The results have identified the widespread presence of a thin hardened layer on top of the sound stone, even in areas with no visible need to be consolidated. The hard layer has a significant contrast with the untreated stone in terms of mechanical strength, and very likely also in terms of thermal properties and drying behaviour. With such highly contrasting properties, it is not surprising to find well defined plaquettes being formed and detaching from the blocks. A potential consequence of the consolidation treatments in this stone type found dramatic illustrations here and raised a serious warning on the risks that a possible consolidation action would bring about. Eu-Artech Seminar on Small Samples - Big objects , Edited by Bayerisches Landesamt für Denkmalpflege, Munchen 2007, pp. 15-26.

When analysed in detail, the graphs show that the consolidation action was not similar in all the areas, since the peaks have not the same shape and strength increase. We may argument that the application conditions might have been inhomogeneous, but it is also admissible that several consolidation campaigns might have taken place in the past. The severe degradation condition of the portal has certainly raised serious concern to authorities and it is not surprising that several consolidation actions might have been carried out to save this important artistic and architectonic piece.

5. CONSERVATION ISSUES AND CONCLUSIONS The use of DRMS (drilling resistance measurement system) requires the execution of a 5mm hole and since it is slightly micro-destructive some restrictions in its applicability are unavoidable. In the study carried out in “Porta Especiosa” portal the use of this tool was feasible because of the extreme degradation condition and of the unique information that it can provide for carbonate stones. The sampling approach of this study followed a strategy of progressive advances, taking advantage of the previous results and progressing towards new objectives, when the first were considered reasonably clarified. The study started by searching about the degradation condition of the more decayed blocks where any damage caused by drilling the investigating holes could be considered as irrelevant. The results made it possible to characterise the uppermost decayed zone of the block as well as to identify the presence of fractures at multiple depths. The upper most decayed zone was very soft, but very thin too, with virtually no transition to the underlying sound substrate. Then, it was conclude that to preserve it would be an almost impossible endeavour. The deep rooted fractures constituted an important structural problem to be solved, but the known extreme difficulty to consolidate this type of stone excluded the possibility that these deep zones could be reached directly from the surface. With these two basic constraints in mind, the conservation concept therefore adopted for the portal stipulated that no conservation treatments were to be applied in the decayed areas. Any stone fragment in precarious condition was reattached to the substrate by means of a soft inorganic fine mortar and soft brushing was the only accepted cleaning action. Water leakage points were carefully treated and horizontal architectonic features were protected with a zinc cover to divert water from reaching the vertical surfaces (see detail on the right photo of Fig. 3). The deep fracture zones were treated with an injection mortar applied through syringes and peristaltic pumps in holes distributed over the fragmented blocks. This injection mortar, especially designed for this portal by Techno Edile Toscana, is made of a hydraulic binder and highly deformable “aggregates” in order to reach a very low defor- mability modulus 5. The results obtained in the areas where “black crusts” occurred have shown that a past treatment was a prominent feature of these areas and raised the hypothesis that the model of black crust formation could simply play a minor role. These findings were used to prepare the conservation actions, namely on what concerns cleaning operations. In fact, the dark coloured surface was mainly caused by the ageing of the consolidation product used in the past and its removal was obviously impossible. The work carried out consisted of a lowering of the dark colour to an acceptable greyish patina that was considered acceptable to be left. Eu-Artech Seminar on Small Samples - Big objects , Edited by Bayerisches Landesamt für Denkmalpflege, Munchen 2007, pp. 15-26.

The demonstration that extensive consolidation actions had been made in the past without any significant and definitive result support the decision to not consolidate again this problematic type of stone. The portal was carefully treated in every detail, by eliminating every water leakage entrance and diverting the runoff water outwards from the portal aiming at avoiding that water might enter in contact with this sensitive stone and promote additional damage. Finally, it is worth stressing once again that the use of this micro-destructive investigation tool, following a step by step sampling procedure, fully demonstrates its potential interest as an investigating tool. The results that it has made available served as decisive arguments in the definition of the conservation strategy.

REFERENCES

1 Aires-Barros, L. and Dionísio, A. – Monument stone damage diagnosis and mapping: the case of Porta Especiosa at Old Cathedral of Coimbra-Portugal. Proc. 5th Int. Symp. on the Conservation of Monuments in the Mediterranean Basin. , 2000, pp.53-55. 2 Bakshandegi, N. 2002 – Preparação de intervenções de conservação. O caso da Porta Especiosa”. MSc thesis, Univ. Évora, 3 Delgado Rodrigues, J. 1988 – “Proposed geotechnical classification based on Portuguese and Algerian examples”. Engineering Geology, vol.25, pp.33-43. 4 Tiano, P.; Delgado Rodrigues, J.; De Witte, E.; Vergès-Belmin, V.; Massey, S.; Snethlage, R.; Costa, D.; Cadot-Leroux, L.; Garrod, E.; Singer, B., 2000 – “The conservation of monuments: a new method to evaluate consolidating treatments”. International Journal for Restoration of Buildings and Monuments, Vol. 6, N.º 2. AEDIFICATIO Verlag. Fraunhofer IRB Verlag, pp. 133-150 5 Delgado Rodrigues, J.; Ferreira Pinto, A. P.; Proença, N., 2005 – “ Porta Especiosa from Old Cathedral of Coimbra. Methodologies and notes on a conservation intervention”. Património Estudos, Vol.8, pp.67-72 (in Portuguese).