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, Portugal [email protected] ** Civil Engineer, Assistant Professor, Department of Civil Engineering and Architecture, IST, Technical University of Lisbon, Portugal [email protected] ABSTRACT The Renaissance portal in the Old Cathedral of Coimbra 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.