Sustainable Masonry: Stability and Behavior of Structures

Preface

This book is the result of the meeting of two complementary approaches to the same subject: masonry. The first is the approach developed by Thierry Ciblac at the Ecole Nationale Supérieure d’Architecture de Paris la Villette (National School of Architecture of Paris La Villette), which revisits historical design methods using digital tools. The second consists of studies led by Jean-Claude Morel at the Ecole Nationale des Travaux Publics de l’Etat (National Civil Engineering School), which are based on experiments with masonry structures of earth materials. The convergence of these two approaches occurs through the common use of the theory of yield design.

This book was written to promote understanding of the mechanical stability of masonry structures in a contemporary context and to introduce it to the readers. This approach will allow contractors to carry out diagnostics on existing heritage and to design new structures.

The challenges presented by sustainability criteria have provided – or restored – respectability to masonry constructions using earth materials. The latest research in this area has been formalized by putting it into perspective with historical approaches. This is done with the dual purpose of making design methods used for old structures (mostly from the eighteenth century) more accessible and providing simple tools for understanding their behavior. In particular, developments relative to graphic statics, take on new educational and demonstrative values with the use of digital tools.

We wish to thank Noël Challamel for proposing the idea for this book, for his detailed proof-reading of the manuscript and for his valuable advice.

Such a book is the fruit of a collective effort. Experimental work, in particular, is the result of a team effort where students, contractors, technicians, engineers and researchers cooperate.

Work on dry stone began in 1998 at ENTPE, instigated by Patrick Cohen in the Luberon Regional Natural Park.

Work on earth began in 1981 at ENTPE, instigated by Myriam Olivier and later, Ali Mesbah. These two researchers were eager to share their knowledge, and J.C. Morel benefitted from their expertise upon arrival at ENTPE. Note that Claude Boutin encouraged J.C. Morel to study the theory of yield design in order to apply it to earth materials.

The authors are particularly thankful to a number of PhD students, whose work has enriched this book. Laboratories and funding involved in these PhDs are specified as following:

– Abalo P’kla (DGCB, Ecole Nationale des Travaux Publics de l’Etat – National Civil Engineering School);

– Boris Villemus (DGCB, Ministère de l’Ecologie, du Développement Durable et de l’Energie-MEDDE – Ministry of Ecology, Sustainable Development and Energy);

– Givanildo Azeredo (DGCB, Conselho Nacional de Desenvolvimento Científico e Tecnológico CNPQ-Brésil – National Council for Scientific and Technological Development CNPQ-Brazil);

– Anne-Sophie Colas (DGCB, Ministère de l’Ecologie, du Développement Durable et de l’Energie-MEDDE – Ministry of Ecology, Sustainable Development and Energy);

– Quoc Bao Bui (Centre National de la Recherche Scientifique-CNRS - National Centre for Scientific Research);

– Apostolia Th. Oikonomopoulou, (ARIAM-LAREA, Ministère de la Culture et de la Communication – Ministry of Culture and Communication);

– Hong Hanh Le (LGCB, Ecole Nationale des Travaux Publics de l’Etat – National Civil Engineering School).

These PhDs were carried out with the assistance of technical staff:

– Odile Roque (MEDDE technician);

– Jean-François Halouze (MEDDE technician);

– Sébastien Courrier (MEDDE technician);

– Erwan Hamard (MEDDE technician);

– Stéphane Cointet (MEDDE technician);

– Joachim Blanc-Gonnet (CNRS research engineer).

The experimental work which forms the basis of the first part of this book was carried out in close cooperation with builders, notably via the Ecobâtir network, which includes Nicolas Meunier, Vincent Rigassi and Alain Marcom, experts in earthen construction. In the field of dry stone, the series of experiments were conducted by Paul Arnaud (OPUS) and Philippe Alexandre (Lithos-APARE) at Le Beaucet. The second series was conducted in Saint-Germain-de-Calberte by the Artisans Bâtisseurs en Pierres sèches (Dry Stone Builders’ Association) led by Marc Dombre and Christian Emery. The third series was conducted at Pont de Montvert by the Artisans Bâtisseurs en Pierres sèches, led by Bruno Durand and Thomas Brasseur.

Denis Garnier co-supervised Anne-Sophie Colas’ thesis, the second thesis on dry stone retaining walls, bringing her valuable skills to the optimum implementation of yield design.

Rabia Charef-Morel carried out careful proof-reading of the manuscript and created Figures 4.1, 4.2, 4.3, 11.1 and 11.3.

Paul McCombie, Nicolas Meunier, Bruno Durand provided the photographs in Figures 1.1 and 1.3.

Research was also done in the context of two national projects: PEDRA and RESTOR:

– RGCU PEDRA project No. 10 MGC S 017, studies on dry stone or weak mortar masonry of the Civil and Urban Engineering Network, coordinated by Eric Vincens of the Ecole Centrale de Lyon (Lyon Central School).

– RESTOR project, restoration of dry stone retaining structures, of the PNRCC program of the Ministry of Culture and Communication, coordinated by Eric Vincens of the Ecole Centrale de Lyon (Lyon Central School).

Studies on rockfill dams and dry stone masonry revetment were initiated at the instigation of EDF.

Finally, J.C. Morel was supported by the Rhône-Alpes region in furthering his studies in England, with a 5 month placement at the University of Bath.

The sections on graphic statics, principles of yield design and stability of curvilinear masonry, written by Thierry Ciblac, are directly related to his teaching and research activities at the MAP Maacc/CNRS-MCC UMR 3495 (ex. ARIAM-LAREA) laboratory at the Ecole Nationale Supérieure d’Architecture de Paris La Villette. The author wishes to thank Louis-Paul Untersteller and François Guéna, founders and successive directors of the laboratory, and the initiators of the research focus on digital tools to help preserve heritage masonry. The quality of their welcome, their support and their experiences as educators and researchers has been an invaluable aid. The development of graphic statics in dynamic geometry has been the subject of collaboration with the Department of Architecture of the Massachusetts Institute of Technology, under the MIT-France program, with Professor John Ochsendorf and Philippe Block, then a student, whom we also wish to thank.

Thierry CIBLAC and Jean-Claude MOREL

June 2014

Part 1

Technologies and Construction Process

1

Introduction to Sustainable Masonry

1.1. Definitions of sustainable masonry

This book is particularly focused on masonry structures made of local materials, stone and earth. They are among the first materials to have been used by humans to build shelters thousands years ago and they are called, in this book, Earth Materials. In this context, earth masonry deals with adobe or compressed earth block, or rammed earth if the material is manufactured in successive layers.

This book does not particularly apply to baked clay brick and cement sand blocks, which are within the scope of Eurocode 6.

1.1.1. Sustainable constructions

Here, we consider sustainable development as defined in [BRU 87] as a development mode that meets the needs of present generations without compromising the ability of future generations to meet their own needs.

In this book, we will only consider the mechanical stability of masonry. However, in this introductory chapter, we consider some elements of thermal and hydric behavior, socio-economic aspects, and sustainability and environmental impacts of these structures. These elements will include references so that the reader, if he wishes, can further his knowledge of all these key aspects of sustainability.

Local materials are acquired from or near the construction site. Here, ‘near’ means a distance of about 20 km. Local materials used in construction have an impact due to their transportation. When a material is taken from on-site, as was often the case for earth, rubble stone masonry (rubble stone blocks and earth or sand lime mortar) and dry stone constructions, the impact is obviously reduced [HAB 12].

This precision concerning the implied proximity of the word local is important since it implies that the production of these materials cannot be completely industrialized. Materials therefore maintain a very variable composition, depending on the soil that is available locally and the geology. Therefore, it is not possible to give a standard composition of these materials. Local materials of interest here are: cut or uncut stone assembled with (rubble stone masonry) or without (dry stone) mortar, adobe and earth mortar and finally rammed earth. These materials are earth materials.

1.1.2. Masonry structures

By masonry structures, we refer to an arrangement of blocks hand-stacked by a mason, regularly or irregularly, with or without a mortar, over successive layers. We exclude Cuzco rampart type masonry or blocks exceeding one ton, quarry cut and assembled with a crane and Opus Incertum in general.

This masonry is done manually, thus is greatly dependent on the mason’s skill, but following specific rules, which form part of the mason’s skill set. We will also consider rammed earth as a part of masonry as they depend on a mason’s art, even if they do not constitute a stack of small elements but rather layers of compacted earth.

In an industrial context, sustainable development can be interpreted as requiring structures to be designed based on criteria, taking all environmental consequences (in the broad sense of the word) induced by these constructions into account. However, the objects discussed here are the result of a systemic approach whereby an optimum is obtained according to environmental and sustainability criteria, amongst others, which revert to being current. Vernacular architecture was erected in a context of limited resources and energy shortages, thus respecting the criteria of minimum impact on the planet. Their age provides an obviously tangible guarantee of durability. However, we must also consider the innovation that characterizes sustainable masonry, as the use of old materials in a modern context can only be achieved through an adjustment that includes innovation.

1.2. Challenges of sustainable development in construction

The use of earth materials in construction allows a technological leap in terms of sustainability. A case study shows that for structural work of this type of construction, transport of materials (expressed in t.km), that is to say the amount of mass transported over a given distance, can be reduced five-fold, and embodied energy [MOR 01] can be reduced three-fold – embodied energy is the energy required to manufacture the product from its design to the end of its life. [HAB 10, HAB 12] presents an alternative method for quantifying the impact of construction using earth materials, which offer notable benefits in terms of their low impact. Moreover, another case study shows that work time on the construction site is tripled: this work has a positive impact upon the economy because it requires skills [MAR 09].

1.2.1. Socio-economic aspects

Our context concerns materials and therefore structures for which new constructions are few and far between in 21st Century Europe, as they are transformed and constructed primarily by manual labor. Society’s choice of industrialization is reflected in the price of non-animal energy (electricity, oil, etc.), which is one to two hundred times cheaper than human labor [MAR 02]. Under these conditions, new constructions or restorations are hard to carry out with short term competitive costs, while the need to innovate increases; this also comes at a cost. However, there are hundreds of new structures and many more renovations carried out using masonry with earth materials. These sites provide some figures for numerical analysis. Socioeconomic aspects are the least well-covered to our knowledge, however information on these challenges can be found in [MAR 02, MAR 09, RIG 02].

1.2.2. Environmental impact

Many more studies have been carried out on environmental impact than on socio-economic aspects. General tools currently used for calculating this impact are still being developed, but mature tools already exist for industrial materials and structures. These include lifecycle analysis (LCA). These tools, despite their complexity, are inadequate for technologies and architectures using earth materials [HAB 12]. In addition, the databases necessary for their use are not yet available in the case of earth materials.

The two best-covered areas in the published literature are embodied energy and energy expended during service (mainly for heating buildings). In France, thermal regulations (RT) only apply to the consumption of service energy. With increasing generalization of construction of passive buildings that do not consume external service energy or energy efficiency, the concept of embodied energy should become more important.

[HAB 12] covers works of art, particularly dry stone retaining walls, and the case study [BAU 12] covers rubble stone masonry bridges. For earth material buildings, see [MOR 01, MAR 02, MAR 09].

1.2.3. Sustainability

Sustainability is an important point in works on earth materials, but it depends on the geology of the site, and as for all structures, the quality of maintenance. It also depends on the design, the quality of implementation and therefore on cultural, economic or political criteria. However, it should be noted that paradoxically, the trend continues to favor conventional industrial approaches and therefore constructions with low durability by, for example, decreasing the thickness of concrete and the quantities of steel used.

Compared to current design criteria, monumental constructions such as Roman bridges appear to be oversized, while more vernacular structures are undersized, for example dry stone retaining walls used in farming. In the first case, the aim was to limit maintenance work in accordance with the available manual labor force; in the other case, it was to save manual labor on rebuilding collapsed parts after a storm, if necessary.

Unsuitable contemporary renovations have a more or less significant effect on reducing the lifecycle of earth materials and can even cause their sudden destruction. These consequences are the result of two main families of errors: coatings that are not sufficiently porous, and mechanical reinforcements that are too rigid to survive earthquakes. This last point is further detailed in [FER 05]. Further details concerning the case of industrial coatings which can not be applied to earthen material walls may be found in [ECO 13].

For sustainability issues concerning physicochemical actions, such as those caused by pollution or salt, readers may wish to consult the thesis [GRO 09] for the particular case of earthen structures. There are also many publications in this field regarding stone constructions. Organizations such as ICCROM [ICC 14] (International Centre for the Study of the Preservation and Restoration of Cultural Property), the Paul Getty Trust [GET 13], have studied this area extensively.

1.2.4. Recycling and reuse

By definition, the non-industrial use of an earth material allows direct reuse (of rubble stone masonry, earth mortar and earth masonry). This practice has been used for thousands of years and is still widespread. Reuse of a material qualifies it as renewable, even if the resource is not infinitely renewable. This reuse of an earth material neither affects biodiversity nor does it tie down agricultural land, unlike intensive forestry, for example. This reusability aspect is a key advantage of these earth materials in terms of sustainability.

1.3. Past (civil engineering and architecture), present and future (design tools) practices

1.3.1. Architectural heritage

There is a rich heritage of masonry using earth materials from all over the world. We will limit ourselves to mentioning a few key works here. However, a large number of inventory documents are available, and are often produced by State agencies: ministries of agriculture, transport, and housing.

On the one hand, archaeologists and historians have carried out a lot of work in this area, see for example [BAR 08] for Roman bridges; architects [LOT 11] have worked on European vernacular earthen heritage. Studies coordinated by Ferrigni [FER 05] and Klein [DEC 03, DEC 07, DEC 11] take a trans-disciplinary approach, respectively in the fields of stone masonry and earthen construction.

Dry stone retaining walls are mainly found in France on county roads, rural roads, agricultural terraces and footpaths. Statistics relating to the national network only [ODE 00] include 2,100 studies unevenly distributed over three-quarters of the French departments. In the UK, studies emphasize the economic challenges: the road network is supported by dry stone walls over 2,000–3,000 km long [PRE 00]. It would be economically unfeasible to replace these walls which, although often stable, no longer meet regulatory safety requirements. This has motivated renewed research into this field over the last twenty years.

Tunnels lined with stone, like bridges, are also currently being studied as part of new PhD theses because of their economic importance [DOM 06, STA 11].

1.3.2. Cultural heritage

Architecture is a natural part of culture. However, in the context of our chosen subject, it is interesting to highlight that there are two cultural components in masonry constructions with earth materials: the most obvious is the structure, once it is built, but there is also the skill set required for construction. These skills constitute an intangible heritage, leading toward a more sustainable dimension in our changing society.

This last point implies that regulations are not sufficient to ensure construction quality; we must also ensure that the skills of stakeholders are recognized. This requires special procedures, including training of masons (e.g. through apprenticeships), see section 1.5.

1.3.3. Rehabilitation, strengthening

Renovation of buildings include earth material masonry currently provides the largest bulk of work, while new buildings are rare. Intervention in these structures requires a thorough knowledge of the mechanical and hydrothermal behavior of these structures.

Interventions fall into three broad categories: modifications to suit current tastes (or the tastes of the client), maintenance or repair (Figures 1.1 and 1.2), and compliance with thermal or seismic regulations (resistance to horizontal stress).

Figure 1.1. Dent repairs on two dry stone retaining walls. Left, repair in progress, granite (photo: Bruno Durand); right, after restoration, limestone (lighter part on the right) (photo: Paul McCombie)

Figure 1.2. Renovation of a rammed earth house, raising walls by the Nicolas Meunier company (photo: Nicolas Meunier)

1.3.4. New constructions

New constructions with earth materials are still rare in Europe, but the last decade has seen a marked increase in interest in this subject, measurable by the increase in scientific publications concerning them. These constructions have found a place in the niche of ‘green building’ (eco-construction). This type of construction is carried out by clients and contractors involved in pioneering works (Figure 1.3). Other notable contributions have been made by self-builders.

Figure 1.3. New constructions: load-bearing rammed earth and timber frame in Chasselay (France), contractor: Nicolas Meunier; right, rubble stone masonry bridge at St. Andeol de Clerguemort (France), contractor: Thomas Brasseur and Marc Dombres (photo: Paul McCombie)

1.4. Durability, deformation and possible movement

Masonry that is sustainable over several decades, even centuries, often undergoes changes of use over time. These changes can create problems because they were not taken into account by the original designers. This type of pathology is similar to those relating to improper restoration, often coupled with a change of use. In the case of road works, dry stone retaining walls and arched bridges including stone masonry, the increased weight and speed of vehicles is a very common source of problems, as these structures were not designed to cope with these heavy weights. In the case of buildings, including earthen structures, changes in human behavior are associated with renovation. For example, the conversion of a barn into a house completely changes the hydrothermal transfers within the walls and can quickly (in less than 10 years) generate significant increases in the quantities of liquid water in the walls. Implementation of waterproof coatings, for example, can cause significant loss of strength, and may even lead to collapse (Figure 1.4).

Cultural aspects should also be taken into consideration, for example, the perception of cracks. Cracks occur systematically in old and new masonry, yet seem to be difficult for buyers to accept when purchasing a new house. The appearance of cracks is not necessarily a symptom of structural stability issues, but shows the masonry adapting to movement, for example at foundation level.

Figure 1.4. Collapse of a rammed earth house wall due to excessive water content of the walls at the base as a result of capillary action which was then unable to evaporate

In this book, we will not go into detail concerning the movement behaviors which precede perfect plasticity or fracturing, which can be understood using the material’s behavior, that is to say, the relationship between stress and strain (rheology). Generally, inclusion of the behavior of the materials is limited to elasticity or elastoplasticity.

In this book, we have chosen only to take account of strength or stress stability, without consideration of movements and strains. This choice is based on the fact that, like the soils, masonry material exhibits a behavior that is close to perfect plastic behavior, and it resists very little, or not at all, to traction. This choice will be detailed in the following chapters. Of course, the designer should always remember that structural strains must remain below a certain limit.

1.5. Importance of expertise (complexity of cases and history of the structure, evolution over time)

The importance of experience and empirical knowledge of local masonry materials is essential for the reasons discussed above, that is to say, because of the variability of materials, architectures, heterogeneity of blocks and the strength of underlying assumptions. For complex structures, it is useful that the engineer, architect and mason cooperate to better understand the study of the structure. This cooperation may also extend to other disciplines, such as anthropology, sociology and archaeology (see section 1.3).

1.6. Rationalization and calculation methods

We have therefore chosen to use strong hypotheses, which nevertheless seem realistic, and which, in any case, will provide rigorous results. In Part 3, we use yield design, which solves a stability problem with equilibrium equations and failure criterion of the material only. The resulting solution is correct if we assume a material behaves with perfect plasticity. We discuss yield design with, on the one hand, the mechanics of rigid bodies in the formalism of the point mass mechanics, and on the other hand, the formalism of continuum mechanics.

Local materials used for vernacular buildings have been largely ignored in academic research; our scientific approach focused on finding models in the literature that were adaptable to our study, then modifying or limiting their use as necessary. Our contribution will therefore make use of rustic models that have already been tested, allowing an initial interpretation of the mechanical stability of masonry using earth materials and the possibility of expanding to operational contexts.

The pragmatic approach taken in this book is the result of efforts to respond to a relatively urgent need. The skill sets of artisan builders are endangered and earth materials heritage is deteriorating. At the same time, the modern use of local materials constitutes a response to the challenges of reducing all kinds of industrial pollution.

Throughout this book, we borrow ideas or models from different fields of mechanics:

– from rigid body mechanics via graphic statics, a simple approach which gives quantitative results. This concept will be associated with yield design (defined in Chapter 9) to reach conclusions of the stability of masonry systems of elements that are considered dimensionally stable (Chapter 10);

– from soil mechanics for which yield design has been developed in a continuum mechanics formalism (Chapter 11). Masonry materials come from the soil (including rock), so obey complex material behavior for which much research is still ongoing. Under these conditions, yield design methods make it possible to obtain a good level of connection between theory and experiments;

– from studies on masonry structures using periodic regular blocks, which have been subject to a number of developments (Chapter 11).

1.7. Presentation of the outline of this book

In this book, we will follow the chronology of a masonry construction. We will begin by considering the blocks, then the mortar and finally, masonry. The quality of its components and their interaction will determine the quality of the structure they form.

This book is composed of three complementary parts: the first part deals with technologies and construction process (Chapters 1 to 4), the second with graphic statics (Chapters 5 to 8) and the third part deals with the implementation of yield design applied to masonry structures