C6 – Barrel Vault Country : Tunisia

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Building Techniques : C6 – Barrel vault Country : Tunisia

PRÉSENTATION

Geographical Influence

Definition

Barrel vault

- Horizontal framework, semi-cylindrical shape resting on load-bearing walls

- For the building, use or not of a formwork or formwork supports.

- Used as passage way or as roofing (in this case, the extrados is protected by a rendering).

Environment

One finds the barrel vault in the majority of the Mediterranean countries studied. This structure is usually used in all types of environment: urban, rural, plain, mountain or seaside. Associated floors:

Barrel vaults are used for basement, intermediary and ground floors for buildings and public structures. This technique is sometimes used for construction of different floors. In Tunisia, stone barrel vaults are only found in urban environment, they are common in plains and sea side plains, exceptional in mountain. The brick barrel vault is often a standard in mountain environment. The drop vault (rear D' Fakroun) is often a standard everywhere.

Associated floors: In Tunisia, this construction technique is used to build cellars, basements, cisterns (“ fesqiya ”, ground floors and first floors (“ ghorfa ”). Drop arches and brick vaults can sometimes be found on top floors.

Illustrations

General views : Detail close-up :

This project is financed by the MEDA programme of the European Union. The opinions expressed in the present document do not necessarily reflect the position of the European Union or of its member States 1/6

C6 Tunisia – Barrel vault

CONSTRUCTION PRINCIPLE

Materials Illustrations

Nature and availability (shape in which it is found) For the construction of barrel vaults, the most often used materials are limestone and terracotta brick in all the studied countries. Other kinds of stones are also used according to the local availability in materials ( schist and granite...). The materials added for the filling of the interstices of the extrados, or for the composition of mortars are: broken brick, lime, gypsum plaster, gravel or pieces of broken pots and coal debris. Earth and stone are the materials used in Tunisia to build stone barrel vaults, the filling materials are plaster, lime mortar and coal debris. The brick vault is built with solid terracotta, plaster, lime mortar and hollow brick. The vault in D' Fakroun is carried out with the same materials as above.

Modules, Dimensions, Dose ratio The barrel vault is masonry laid in mortar: the modules are assembled one against the other according to a sloped plane determined by a radiant wire that shows the angle of each section from the bottom of the vault to the key, and from the center of the arc. The stone or brick modules which make up the vault can be cut so as to form a specific key for the vault (2 tilted Barrel vault in stone faces make up both a stone course and a bearing surface). They are often hardly outlined in flat stone: mortar catches up any irregularities showing on the surface.

Type of laying

Type of laying, use of a formwork, formwork supports The building of the vault is generally carried out using a wooden formwork. The latter is composed of at least two trusses (section of the vault), on which one fixes laths of wood (sole-piece), the outside shapes the intrados of the vault. The formwork can also produce a more approximate shape, which is then corrected with a bed of earth or sand... Anchored directly in the walls (cross beam, retractable) or supported by bolders (projecting stones left in place after laying ), this unit constituting " the formwork " is held by supports. Barrel vault in cave The laying of stones and bricks is made from the springer to the key by balancing the loads on Mosquee in Kasba (Sfax) both sides of the vault axis. In the Mediterranean area, some techniques are freed from this formwork system (low wood supplies? tools are considered as overly cumbersome?, particular and local know-how?...). Building without formworks is possible with regular shaped modules (terracotta brick, cut or not), well prepared mortars (adherence of bricks, hardening speed...) and with a particular laying method. The latter seeks to limit the overhang of a key stacking, by creating intermediary support joints, by slanting the laying axis (springer/key/crown) towards the walls. In Tunisia, the barrel vaults can be up to six meters long, they are laid with mortar, using a formwork "built from scratch" : this formwork is a full length structure, made with wooden elements, superimposed stone, rubble and sand to adjust the shape. The brick barrel vault one can be built without a formwork if plaster is the binding material, or with formwork if the binding material is lime mortar. The drop vault “ Dh' har Fakroun ” (literally: tortoise back), whose layout arrow is lower by half a span, is build with or without a formwork according to the nature of the binding material. Details in " Realization " section.

Tools

In addition to traditional mason tools, no particular tool is necessary for the building of the vault in throughout the studied countries.

Physical (span...) Space covered with barrel vaults at the clothing souk.

C6 Tunisia – Barrel vault

CONSTRUCTION PRINCIPLE (CONTINUED)

Trades Illustrations

Trade, Number of people necessary The barrel vault is exclusively carried out by experts, mason, mason/stone cutter, or stone cutter helped by assistants (significant handling, mortar to prepare...), and sometimes assisted by a carpenter for the realization of the formwork. In Tunisia, the barrel vault is carried out by a mason helped by two workmen.

Performances

Physical (span...). This construction technique leads to span widths that range from 1 to 7 meters, for a thickness that varies from 30 to 50 centimeters and can even reach 1 meter on the edges of the vault in Tunisia. In Tunisia, the usual span of for stone barrel vaults found in various types of construction Construction principle : construction (cellars, rooms, shops, ghorfa on a higher floor) is between 2 and 3 meters. As for the technique of a vault without armature thickness of this framework, it is about 40 cm on the level of the axis of the vault and can reach 1 meter on its sides. The common span for drop vaults and brick barrel vaults is between 3 and 6 meters, for an average thickness of about 25 cm.

Thermal - Acoustic. For a stone construction, limestone in particular, the thermal and acoustic performances have been reported from rather good to excellent by all the countries concerned. Considering the significant mass of the vault, its thickness, the materials used in its construction, this piece of work has remarkable thermal and acoustic insulation qualities. In Tunisia, this enormous horizontal structure is a relatively light mass thanks to the hollow bodies which fill its sides, framed by a " Nasriy a " masonry: it is laid in stone with lime mortar and ensures excellent thermal and acoustic performance. Regarded as the best technical solution in the areas concerned. Brick barrel vault without armature

Construction principle : thermal - acoustic

Construction principle : materials, thermal - acoustic – vertical section

C6 Tunisia – Barrel vault

ASPECT, PATHOLOGY

Aspect Illustrations

Finishing, associated roofing. The intrados of the vault is generally covered with a rendering. However, in the case of ashlar vaults or carefully laid bricks, the facing can be left bare. In the case of a vault for a passage way, the extrados of the vault generally does not get any particular processing; haunches are frequently filled with various materials (quarry rubble, building site residue), the unit gets a system to seal the ground. In the case of a roofing vault, the extrados of the vault has regular lines, and receives an elaborate waterproofing rendering. In Tunisia, the barrel vault stone is never apparent outside, it is always protected by flat a terrace known as " tah ". It is initially surrounded by an enclosing wall called " Nasriya ". One fills the wings of the vault with unusable pottery: this contributes to greatly reducing the weight of the roof terrace. One then determines the shape of the slope with three successive sand layers of wadi, lime concrete, and finally a layer of smoothed rendering. Two types of coating are reported for the brick barrel vault: either it gets a light renewal rendering, or it is completely embanked and the roof is thus transformed into a flat terrace. As for the drop vault, shape contributes to the protection and the waterproofing, allowing Ageing Pathology rainwater run-off; but this waterproofing is also ensured by two to three layers of whitewash applied on the second stone or brick layer. This is renewed each year, but one can also transform the flat roof by filling it with earth or light " quarry rubble ": one then applies the layers of mortar for the floor, or shapes the slope if this roof is used as a superstructure.

Ageing pathology

Linked to materials and climatic conditions : As for stone or brick masonry, pathologies related to materials originate in water (capillarity, infiltration, condensation...), which dissolves the mortars and weakens them. The control of water infiltration is more significant in the case of the vaults; Beyond the dissolution of the mortars it is necessary to note direct erosion, that can affect the rendering mortars. In Tunisia, one has seldom found a completely collapsed barrel vault, which proves its great resistance. But the sagging of load-bearing walls or a lack of terrace maintenance are factors which can deteriorate and partially destroy the vault. The sagging of load-bearing walls can come from a water leakage at the base, from faulty construction or a compressing of the courses.

Linked to the technique: The vaults produce lateral thrusts, that are weak in the case of the barrel vault. However, not taking this strain into account in dimensioning load bearing walls leads to a cracking of the Openings forming light well vaults, sometimes even collapse. No ageing pathology linked specifically to the construction technique of the vault was reported in Tunisia.

ASSOCIATED WORKS

Openings

Small openings can be made to ventilate the rooms (in Palestine and Israel) or to give light, particularly in the souks (Tunisia). The rectangular openings are between 0,5 and 1 meter. A peripheral frame (rafter) distribute the loads. In Tunisia, openings are rare and limited to a few cases, to bring ventilation or lighting solutions. Most common examples are limited to souks, which can have rectangular openings and dimensions ranging between 0,5 and 1 meter, or cistern openings which are generally square, 0,6 meter per side.

Connection framework - Vertical Structure (wall):

No type of connection was reported: the vault is assembled in the continuity of the walls, at the same time and by the same people. The two works are indissociable. In Tunisia, no type of connection was reported: the vault is assembled in the continuity of the walls, at the same time and by the same people. The two works are indissociable.

Connection with structure – vertical structure

C6 Tunisia – Barrel vault

REALIZATION DESCRIPTION

In Tunisia : (Text in French)

Les voûtes en berceau, dont la longueur peut atteindre les six mètres, sont pos ées au mortier au moyen d’un coffrage « construit à blanc » sur toute leur longueur (ou partie) avec des pièces en bois, de la pierre superposée, du tout venant et du sable pour ajuster la forme. En achevant la construction des murs porteurs, le maçon prend soin de ne pas liaisonner au mortier les moellons constituant la dernière rangée interne de ces murs. Ces moellons restent ainsi libres mais parfaitement stabilisés au moyen de petits cailloux. Le coffrage est construit en plaçant les planches en bois sur les moellons libres. On procédera ensuite à la superposition des pierres (sans liant) de moins en moins espacées au fur et à mesure que se dessine le profil de la voûte . Les vides entre les pierres des couches supérieures sont comblés par du tout-venant. Une couche de sable fin et tamisé est enfin étendue sur le coffrage pour lui donner une forme régulière. La première couche de la voûte proprement dite est une couche de plâtre coulée sur le coffrage. Après la prise du plâtre, on entreprend la construction, par dessus, de la voûte avec du mortier de chaux et en blocage de pierres disposées verticalement et en quinconce pour assurer à l’ensemble la meilleure cohésion possible. Après un long séchage, on retire les moellons laissés libres et sans liant sous le coffrage, la voûte présente à l’intérieur une surface lisse et régulière, aucun enduit ni aucune autre forme de finition ne lui est nécessaire.

La voûte en berceau de briques peut être construite sans coffrage si on utilise le plâtre comme liant ou avec coffrage si le liant est un mortier de chaux.

Nous rencontrons les voûtes surbaissées Dh’har Fakroun (traduction littérale : dos de tortue) essentiellement dans la région du Cap Bon, mais aussi dans d’autres régions et notamment dans les montagnes du sud-est Tunisien. Le tracé de cette voûte est un arc ayant une flèche inférieure à la demi portée, et elle se construit avec ou sans coffrage selon la nature du liant. Dans les exemples du sud-est tunisien, les voûtes surbaissées sont exécutées avec un coffrage assez sommaire fait de bois de palmier. Pour améliorer son allure, on couvre ce coffrage de nattes, elles mêmes recouvertes de sable. On construit ensuite la voûte proprement dite au moyen de lamelles de pierres posées sur un lit d’enduit, perpendiculairement à la courbe de la voûte, l’étanchéité étant assurée par une deuxième couche d’enduit. Dans la région du Cap Bon, Cette voûte est constituée par une première couche faite de briques en guise de coffrage perdu. Celles-ci sont à trois trous dans les exemples datés du XXème siècle, ce qui contribue à un allégement considérable de la voûte. Ces briques sont assises sur les murs porteurs latéraux ou sur des poutres et sont assemblées par un mortier de plâtre appliqué à la main sur les deux champs de la brique. Pour maintenir la linéarité horizontale de la voûte, le maçon se sert d’une ficelle accrochée au bout des deux tampons opposés de l’espace à couvrir, cette ficelle mobile servant à dessiner la forme de l’arc de cercle au fur et à mesure de l’avancement des travaux. Une fois la voûte terminée, on pose une couche d’enduit de chaux et on laisse sécher pendant quelques jours pour procéder ensuite à la mise en œuvre de la deuxième couche : celle-ci est constituée soit de lamelles de pierres de 5x10 cm posées sur champ perpendiculairement à la courbe de la voûte en briques, soit de briques de six par trois mises à plat.

C6 Tunisia – Barrel vault

USE, EVOLUTION AND TRANSFORMATION

Use

Types of buildings This framework is used for all types of construction : housing, monuments and buildings public, facilities (mosques, souks, hammam, trade, craft industries, caravanserais) In Tunisia, the brick barrel vault is used for particularly long or deep areas such as souks or Maskabât (spans) or balâlât (nave) prayer rooms in mosques: in the latter, the vault contributes to the orientation of religious areas.

Period when the technique first appeared. Period when the technique is in use – still used today or disappeared : This construction technique has existed since antiquity. It is on the verge of disappearing everywhere, when it hasn’t already completely disappeared. It is used, however, in the restoration of certain buildings. In Tunisia, the oldest examples of stone barrel vaults are still visible in Sfax, in Borj Gannouri, and go back approximately to 1700. The brick barrel vault or drop vault, go back several centuries in Tunisia and Cape Bon: it seems to have been introduced by the Andalusians in the XVIth century, together with the other techniques related to brick.

Reasons why the technique disappeared or has been modified : The reasons evoked by all the countries are mainly; high cost, slowness of work and skilled labour required for the technique, and a loss of know- how related to this technique.

Evolution / Transformation

The materials The stone barrel vault with lime mortar is used only for rare works of restoration. We can consider that it is not completely lost as far as the shape is concerned, and it simply changed because of the emergence of brick, especially hollow brick, that generalised and replaced stone. In Tunisia, the drop vault was perfected in the XXth century, particularly thanks to the introduction of perforated industrial brick, which allowed for a considerable weight reduction of the vault. The introduction of cement and reinforced concrete were also beneficial for this framework: the use of anchoring, for example, makes it possible to better contain lateral forces, and avoid enlarging the load-bearing walls or building buttresses. Its presence is however rather modest compared with the other vaults known in Tunisia.

The technical aspects The hollow bodied floor is the covering system that replaces the barrel vault. The barrel vault is built as a whole with the load bearing wall, without interruption, whereas new floors are built with reinforced concrete beams that are not embedded in the walls: they thus constitute mere filling walls, made with hollow bricks, and do not play any structural role. It is therefore recommended to embed the b eams using pins, and sink the steel elements in an uninterrupted reinforced concrete blanket over the whole floor frame.

Evaluation of materials and replacement techniques - On an economic level and given the speed of production, the use of reinforced concrete beams is amply satisfactory - Reinforced concrete beams undergo rather significant strain under the effect of temperature variation - Replacement materials are more vulnerable to water infiltration, and less resistant in time - On an aesthetic level, the aspect of finishing of contemporary reinforced concrete vaults is completely different from traditional crossed vault in stone or brick. - Mechanically, the reinforced concrete floor framework undergoes traction, which is not compatible with the bearing structures in masonry which undergo compression: cracks appear at the contact point of the two structures .