Monument Future

Conclusion

This paper presents a new experimental approach to estimate the ice and remaining water content in porous limestone and sandstone during FT cycles, using relationships between the time necessary to crystallize all the water and the sample temperature. When resulting ice contents are compared to the theoretically expected contents, an estimation can be made of the water mass transported from 280the micropores and frozen inside macropores by cryosuction. The time-based approach relies on a contstant crystallization rate and needs to be verified on other stone systems.

References

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281

THE EVALUATION OF THE MASONRY QUALITY IN HISTORICAL LOAD-BEARING STONEWORK

Giuliana Cardani

IN: SIEGESMUND, S. & MIDDENDORF, B. (EDS.): MONUMENT FUTURE: DECAY AND CONSERVATION OF STONE.

– PROCEEDINGS OF THE 14TH INTERNATIONAL CONGRESS ON THE DETERIORATION AND CONSERVATION OF STONE –

VOLUME I AND VOLUME II. MITTELDEUTSCHER VERLAG 2020.

Dept. of Civil and Environmental Engineering – Politecnico di Milano, Italy

Abtract

Historical buildings may have different types of wall texture, often as a result of their function. In fact, depending on the function for which they were built, particular types of masonry were realized with a construction technique appropriate for each kind of purpose: defensive architecture, religious buildings, residential buildings, rural buildings, etc.

Some buildings have been designed and built to last for centuries and, therefore, the load-bearing structures have been carefully constructed with the best knowledge of that age and that particular geographical location, using traditional materials locally available.

For this reason, the stone masonry walls of historic buildings often show a great variety, not only in terms of materials but, above all, of constructive technique. The study of the morphology of a load-bearing masonry wall is fundamental for the evaluation of its mechanical behaviour under different actions. Characterization only of the individual components (stones and bedding mortar) is not suffcient.

Therefore, understanding the construction technique and assessing its quality is essential for evaluating structural safety, especially if the construction is located in a seismic-prone area.

The paper presents a complete and systematic method for the quality assessment of stone masonry. The method is based firstly on a visual inspection and then on non-destructive or slightly destructive investigation techniques. This systematic approach allows to compare different stone walls surveyed in different buildings in the same area, in order to recognise the characteristics of the local “rule of art”, according to geographical conditions and recurring risks.

Keywords: stone masonry, historical building, masonry quality, diagnostic investigation

Introduction

The stone masonry of historic buildings often show a great variety, not only in terms of materials but, above all, of construction techniques. It is necessary to study not only the architectural works left by the Roman Empire to observe different stone masonry typologies, as clearly reported in many handbooks and treaties (Barbaro 1567-Vitruvio-and Rondelet 1802, just as an example). It is enough to observe the load-bearing stone masonry walls that characterize the minor architecture of the many small historic towns in Italy, as well in other countries, to verify how they change from one area to another according 282to the different historical and territorial realities: local masonry materials, ease of retrieval and the ability of local workers (Giuffrè 1993). The historical centres are characterized by a complex texture of buildings usually “non-monumental” but not less important and that constitute a fundamental historical artistic and socio-cultural evidence.

Masonry is a non-homogenous material made of mortar and stones or bricks. As it is a composite material, its structural behaviour depends both on the characteristics of the single components and on their interaction.

The study of the structural behaviour of stone masonry is different and more complex than that of brick masonry. The standards (Eurocode 6) in fact are generally able to provide semi-empirical formulas for estimating the strength only to the newly built masonry, based on the mechanical properties of its components, and does not take into account their interaction, the different constructive techniques adopted in the past without following a standard and the widely used historical materials such as the hydrated lime for bedding mortar.

The study of the morphology of a load-bearing masonry wall is fundamental for the evaluation of its mechanical behaviour under different actions.

The experiences achieved in recent decades in Italy in the conservation and strengthening of historical buildings in seismic areas, which have evolved into the latest versions of the seismic regulations (IMIT 2018), have shown that, before designing any structural intervention, it is necessary to have an in-depth knowledge of the construction, based on accurate surveys and specific techniques to assess its construction characteristics, its structural behavior, materials and the actual state of damage of the structure.

Therefore, understanding the construction technique and assessing its quality is essential for evaluating structural safety, especially if the construction is located in a seismic-prone area (Binda et al. 2005, Binda et al. 2007).

The research, carried out by the author in more than 15 years in Italy, is addressed to develop guidelines for the characterization of masonry quality, with the elaboration of a specific survey template. The method is based firstly on a visual inspection and then on non-destructive or slightly destructive investigation techniques.

Stone Masonry survey (1st level of analysis)

Historic masonry buildings present different masonry textures according to their typology and consequently their functional role. Thus, according to the importance of the building, a peculiar masonry typology can be observed in each historical construction, built with a different and suitable constructive technique. In addition, historical masonry buildings may have experienced a constructive evolution over time, with the addition of volumes or with partial reconstructions, that led to an additional number of masonry typologies in the same building.

A clear classification of the different masonry typologies is so necessary. In figure 1 different common textures of stone masonry are represented. The worst defect for a masonry wall, as made by different components, is to show no monolithicity under loads, mainly when loads produce an out-of-plane thrust; this can happen for instance when the wall is made by small pebbles or by two external leaves even well-ordered but not mutually connected or containing rubble infill (Fig. 2). This causes the wall to become more brittle especially when external horizontal forces act. The same problem can happen under vertical loads if they act eccentrically (Binda & Saisi 2010).

A research developed by the author within the frame of the Italian RELUIS 2009 project (Reluis-DPC 2009), had the aim of giving guidelines to characterize the masonry quality at different levels of investigation with the elaboration of a special proposed template for the on-site survey.

The masonry has to be surveyed by pictures, taken as parallel as possible to the masonry surface, and by placing close to the cross section or to the texture a ruler or a graduated stick in order to know the wall dimension, when redrawing it. The 2D graphic plotting is realised with a special care in the representation of stones shape, mortar 283joints, wedges, voids and the arrangement in horizontal courses (Fig. 1), in order to be clearer than a picture and to make easier the comparison among different masonry texture.

Figure 1: Masonry textures: a) with cut stones in regular horizontal courses; b) with partially cut stones in rather regular courses; c) with roughly cut stones in sub-horizontal courses; d) with pebbles or irregular stones of different dimension in irregular courses.

It is worth to remark that regular textures on façade often do not correspond to regular morphology in the cross section. Therefore, a correct analysis of the mechanical behaviour of existing masonry structures, especially when multi-leaf walls are present, cannot disregard the proper investigation of the arrangement of materials in all its thickness. The presence, dimension and distributions of voids in the cross section are useful for the design of possible strengthening intervention (e. g. grout injections).

Figure 2: Examples of masonry cross sections: a) one single leaf (stones well interlocked); b) two leaves well interlocked (with one transversal connecting stone and one partial connecting stone); c) two leaves partially interlocked; d) two leaves not interlocked; e) three leaves or multiple leaf not interlocked.

Data can be collected in a dedicated survey form (Binda & Cardani 2011, Cardani & Binda 2015) following, in general, a procedure developed for the definition of the masonry quality that should start from the choice of the most representative areas of the load-bearing masonry walls. The stratigraphic method allows subdivision of the building into homogeneous parts characterised by relative chronological relationships. Each part corresponds to a building phase, recognized by the observation of constructive details. This identification will help to choose the most representative masonry walls, which constitute the structure of a historic building, mainly in the basement, which is the most stressed part.

After the selection of the different areas (it could be necessary to remove a portion of the plaster), the survey of the masonry texture has to report in the survey form the following descriptions:

— the type of masonry units: stonework, brickwork, mix of stone and brick masonry;

— the shape of the stone elements: a) regular or irregular, b) the average stones dimension, c) the type of manufacturing: cut sides and sharp edges, split sides, non-manufactured sides, round pebbles, and so on;

— the thickness of the bedding mortar joint, with a general description of the mortar consistence; presence of new re-pointing mortar;

— the horizontality of the courses (masonry can show horizontal, sub-horizontal or irregular courses), 284the stagger of the vertical joints (respected, partially respected or non-respected), the presence of wedges and levelling of other materials;

— the type of cross section of the masonry wall (Cardani & Binda 2015): one or multiple leaf, well interlocked or not (Fig. 2), the presence of transversal connecting elements (“diatoni”).

The correct survey of the masonry texture should refer to an area at least of 1 × 1 m² wide.

If no large cracks or collapsed portions are visible, and so when it is impossible to observe directly masonry sections, a small masonry disassembling can be carried out, not larger than 40 × 40/50 cm (depending on the stones dimension) and ¾ of the section deep, so not to reach the opposite side. (Fig. 3) This destructive operation should be realised in the same wall portion, after the Non-Destructive or Minor Destructive diagnostic tests, when planned, such as after a sonic pulse velocity test or a double flat jack test, so to better interpret the test results (Binda & Cardani 2015).

Figure 3: Study of the morphology of the cross masonry section by a small dry dismantling and redrawing of the section in lateral view.

In the case of stone masonry, the load-bearing capacity is linked to the efficiency of construction details, which can also generate local mechanisms, such as bulging or detachment between the layers. The surveyed masonry quality influences also the choice of strengthening intervention: in fact, for some types of stone masonry there is an objective difficulty in applying certain intervention techniques (Binda et al. 2003). For example, the results obtained from various direct experiences have confirmed that masonry cannot be injected if the voids do not exceed a certain percentage and are not communicating (Fig. 2); injections are very effective where there are cracks, but it is not possible to inject the mortar itself, even if it is poor and very porous, nor to inject very fine material such as silt or clay or, in any case, with poor composition as often found inside masonry.

It is therefore essential to carry out accurate assessments and recognise the morphological-constitutive characteristics of the types of masonry and especially of the masonry sections. The only observation of the external elevation often does not reveal how the masonry was built.

The survey form, therefore (Fig. 4), has the advantage of systematically reporting all the data necessary to attribute a qualitative judgment based on visual observation alone and to allow a comparison between different masonry. It certainly does not claim to give quantitative judgements (Borri et al. 2015) which results would be altered by the failure to fill in some items. The form is splitted in 4 sections (Fig. 4): definition of the building and its function, analysis of the façade, analysis of the cross-section and definition of the masonry typology with reference to the masonry typological classes in Italian standard (IMIT 2018).

In Italy, the assessment of the masonry quality and the recognition of its typology, in the absence of diagnostic investigations, is even more important because it allows to use with greater confidence the well-defined mechanical parameters for the structural evaluation of existing masonry buildings, reported in the (IMIT 2018) national standards. These suggested structural parameters vary according to the different typological classes of masonry that can be recognized.

Stone Masonry assessment (2nd level of analysis)

The judgement on the masonry quality can assume quantitative values if the analysis is accompanied by the execution of some in situ diagnostic investigations (Binda et al. 2000), aimed at defining its physical-mechanical properties.

The suggested tests for the masonry quality evaluation have to be carried out on a unique selected area. Before stripping a portion of plaster (if present) to clear the masonry, it is advisable to carry 285out a thermographic survey in order to identify the most significant portion of masonry to be analysed and without any punctual alterations.

Figure 4: Part of the survey form for the evaluation of the masonry quality according to the level of knowledge 1.

If there is no plaster, the following diagnostic non-destructive investigations can be carried out: (a) sonic pulse velocity test by direct transmission on a grid of about 1x1 m with a graphical elaboration of the results represented on the drawn area through the calculation of the velocity distribution (Fig. 5.); (b) single flat jack test to define the masonry local vertical state of compressive stress, (c) double flat jack test and elaboration of the stress-strain behaviour indicating also the measured local state of compressive stress, (d) local disassembling of a small masonry area for the identification of the cross section typology and for the materials sampling for laboratory analyses.

Figure 5: Comparison between the results of double flat jack and sonic pulse velocity tests on different buildings typologies in the same historic center, realized with the same masonry materials but with different constructive techniques.

In case the masonry disassembling is not possible, coring can be carried out horizontally with a minimum diameter compatible with the stones dimension. If the diameter is too small, it is necessary however to carry out more than one drill, in order to overcome the small and local analysis and to have representative data describing the masonry.

The NDT sonic pulse velocity test is based on the generation of elastic waves in the frequency range of sound (20 Hz–20 kHz), by means of mechanical impulses at a point of the structure. In the case of masonry, due to its heterogeneity, the pulse velocity represents a qualitative characteristic of the masonry. The velocity is influenced by the composition of the masonry as well as by the presence of inhomogeneities, voids and deteriorated areas, as well as the number of intersected mortar joints. A velocity reduction corresponds to an increase of mortar joints or voids or to cracks presence. Higher velocity peaks states higher density of the materials and in stone masonry could represent the presence of single stones with the role of transversal connection of the layers. Double flat jack test when coupled with sonic tests is useful to classify different types of masonry (solid, multiple leaf, stone, brick masonry, etc) (Fig. 5). It is useful also to calibrate the sonic test results on the other masonry walls of the same building, so to compare efficiently the graphical maps using only non-destructive tests for evaluating the state of conservation of the whole construction. This is the case of architectural heritage, where no diffused destructive tests are allowed.