L. Calleja

Characterization by computed X-ray tomography of the evolution of the pore structure of a dolomite rock during freeze-thaw cyclic tests

Abstract The study of the pore structure of rocks is very important for the interpretation of their behaviour when subjected to weathering. In the case of rocks used in historic buildings or in objects of art where other techniques can not be applied, the Non-Destructive Testing method (NDT) for determining porosity, such as X-ray computed tomography (X-ray CT), becomes a very interesting alternative. In this paper this technique has been used for imaging textural features and the internal pore structure of a dolomitic rock used as building material in Oviedo Cathedral, Spain. The reconstructed 3D image shows a non uniform distribution of porosity. The rock has been subjected to freeze-thaw cycles to evaluate its durability. The evolution of the void structure has also been studied by X-ray CT and the images obtained allow one to follow the progressive decay and to assess the internal depth of the damage. Because of the low resistance to weathering of the rock, damage, mainly in the form of fissures, was observed even in the initial tests (third cycle in some of the specimens). Breakdown of the rock appeared as early as cycle 12 in some specimens.

Characterization by X-ray computed tomography of water absorption in a limestone used as building stone in the Oviedo Cathedral (Spain)

Abstract Water plays a fundamental role in rock weathering processes. Its penetration and movement inside rocks greatly influences the nature and intensity of damage affecting building stones. X-ray computed tomography (CT) is a useful technique for non-destructive mapping of water penetration. For the reported study, CT was used to investigate the internal structure and water penetration patterns for the Piedramuelle Stone, a Cretaceous limestone used as building material in the Oviedo Cathedral (North of Spain). X-ray CT provides good images of the internal structure of the samples: the sedimentary layering due to differences in composition and porosity is clearly seen, as well as other textural features. The movement and penetration rate of water was monitored during standard free absorption water tests. The images that were obtained show a clear difference between dry and wet zones in the interior of the sample. Water movement is related to the petrographic characteristics of the rock, mainly to layering which controls the direction of water penetration. Hounsfield Unit numbers provide a quantitative approach for assessing the penetration rate of water.

NON-DESTRUCTIVE ULTRASONIC PROCEDURE TO EVALUATE IN SITU THE RELATIVE DETERIORATION OF MONUMENTAL STONES: PRELIMINARY RESULTS

To evaluate in-situ the relative deterioration of monumental stones, a non-destructive ultrasonic procedure is under development: ultrasonic waves are introduced into the rock and after travelling through it are processed for evaluation of their Vp, amplitude, duration, energy and count number. The procedure was applied to two Spanish Monasteries, one built with a granodiorite (El Escorial, Madrid, which results are included here) and the other built with sandstone (Sta. Maria de Ripoll, Gerona). Laboratory ageing tests, involving monitoring of the above mentioned ultrasonic parameters, were performed as a basis for interpreting the in-situ measurements. The energy seems to be the most sensitive parameter as an indicator of the rock deterioration, so, the value obtained for a masonry block in the monument can be used to classify the rock into one of preset deterioration groups.

Acoustic emission during swelling and contraction tests

An expansive clayey rock belonging to the Asturias Carboniferous of NW Spain has been petrophysically characterized before, during and after swelling tests. The evolution of the microfissuring and the induced deterioration have been studied by acoustic emission during tests and by ultrasonic techniques before and after swelling/contraction cycles. A good correlation has been found between the AE curves and swelling strain. Damage due to the generation and growth of microfissures is most important during the first moments of each swelling/contraction cycle. Microfissures mainly develop in the direction of the foliation planes, as can be shown by linear crack density and measurements of the propagation velocity of longitudinal waves.

Acoustic Emission Monitoring of the Cathedral of Palma de Mallorca (Spain)

The presence of fractures and fissures in a building is the consequence of an inadequate response of its structure and/or materials to the stresses affecting them. Frequently, primarily in large buildings, the soil-structure interaction modifies the distribution of stress on the structure: differential settlement, unforeseen strains, underground instability, etc. are some of the causes that can induce fractures in the structure or in building materials. Generally speaking, the fractures and fissures caused by these factors cease to grow after a short time and do not represent a problem for the buildings stability.

Durabilidad en ambientes húmedos de la Arenisca de La Marina (Jurásico superior de Asturias)

Se ha estudiado la alterabilidad de dos variedades (gris y amarilla) de una roca ornamental asturiana, denominada Arenisca de la Marina, perteneciente a la Formacion Lastres (Jurasico Superior). Ademas de las clasicas tecnicas de microscopia, para el estudio textural se han utilizado la velocidad de propagacion de ondas elasticas y la tomografia axial computerizada de rayos-X. Se ha determinado la variacion de algunas propiedades fisicas (densidad, porosidad abierta y velocidad de propagacion de ondas elasticas) antes y despues de los ensayos de alterabilidad (humedad-sequedad, hielo-deshielo y cristalizacion de sales), comprobandose que los ciclos de cristalizacion de sales son los mas agresivos y que la variedad amarilla es mas susceptible al deterioro que la variedad gris. La textura ejerce una gran influencia en la durabilidad de ambas muestras, de modo que las bandas con mas matriz y mayor porosidad presentan una mayor desagregacion superficial, determinandose la existencia de un comportamiento anisotropo, mas marcado en el caso de la variedad amarilla que en la gris.