Adobe   Stone   Brick   Concrete Block             Improved Adobe construction       Strengthening of Adobe houses       Glossary, notations and useful knowledge       References       Adobe houses are normally classified as class A to class B on the EMS scale for vulnerability of masonry strucures. However adobe houses can resist earthquakes provided the requirements outlined below concerning the planning and construction process are met. When the details for complete load path and components strength are adequately implemented, life safety can be achieved even under an earthquake with an intensity of the order of IX grade on the MSK scale. This grade intensity corresponds to average values of peak ground acceleration of 0.4g. To achieve improvement of the adobe houses three key factors are discussed: Building materials and construction process Planning and layout Details for seismic resistance Building materials and construction process   1. Foundations A trench of min depth 0.8 m and min width equal to the adobe wall thickness but not less that 0.4 m is excavated. The trench can be filled with field stones and cement grout in layers to form strip foundations underneath walls. In case the plinth course is constructed in concrete, a formwork can be erected for the plinth course, and can be cast together with the foundations. The vertical wall reinforcement should be erected in place and be embedded in the foundations min 400 mm. The canes should be attached with nylon threads or other means to the formwork to ensure correct position see Figure 1 below. Field stone- max size 250mm Grout- cement based mixed in proportion 1:4 (cement:sand) by volume Vertical reinforcement- bamboo or canes sticks with diameter around 20mm. The length should be determined as follows: H=400 + wall height +bond beam depth(but min 200) ,[mm] Figure 1- Foundation construction (Dowling,2002)   In case of construction of non-load bearing gable-end walls the length of the canes should be determined to include the height of the gable-end walls. 2. Plinth course The first few courses of wall above the foundations should be constructed using clay brick or stone. The height of plinth should be above the flood water line or a minimum of 350mm above ground level. Good connection with the foundations are ensured by the vertical cane reinforcement. The plinth level can also be cast in concrete. When concreting care should be taken to ensure compaction and correct position of the vertical canes. After completing the plinth masonry a damp proof course should be installed. A damp proof course can be achieved using rich cement mortar ie. 1:3 (cement:sand). Masonry units- cut stone(square dressed units) or burnt clay bricks laid in cement or cement-lime mortar Mortar- possible mixing proportions are 1:1:6 (cement:lime:sand) or 1:4 (cement:sand) 3. Adobe walls Different bonds are possible for the adobe masonry wall construction. English or Flemish bonds are recommended with 1 brick/block wall thickness see Figure 2 and Figure 3. For vertical and horizontal reinforcing of walls by means of canes see the part of this document- Details for seismic resistance. Horizontal reinforcement should be installed every fourth course of adobe. Lintels over openings should be embedded 500 mm in adobe walls. Figure 2- Flemish bond for one brick thick wall Figure 3- English bond for one brick thick wall   Adobe bricks/blocks Made of clayey earth consisting of 30-40% clay and 70-60% earth which is mixed with water. Straw is added to the mixture of clay and earth -7-10 kg per m3 of dry earth. This amount of straw is equivalent to 0.5% of mass of the dry soil. Then the mud mixture is allowed to 'sleep" (soaking the earth mixture in water at least a couple of days to activate the clay) before the fabrication of adobe bricks or mortar. It seems that this procedure allows for a better dispersion and thus for a more uniform action of the clay particles. To make a decision regarding the suitability of a soil for adobe production perform the following test: Dry Strength Test: Figure 4- Dry strength test (PUCP/CIID,1995)   Five or Six small balls of soil of approximately 20 mm in diameter are made. Once they are dry (after 48 hours), each ball is crushed between the forefinger and the thumb. If they are strong enough that none of them breaks, the soil has enough clay to be used in the adobe construction, provided that some control over the mortar micro-fissures caused by the drying process is exercised.If some of the balls break, the soil is not considered to be adequate, because it does not have enough clay and should be discarded. The recommended sizes of adobe bricks(blocks) are 300x130x100 mm and 400x180x100 mm. The storage of the produced adobe bricks should be under a shelter to ensure protection from rain. Mortar To guarantee the bond between blocks and mortar, the micro-fissures of the latter should be avoided. The properties of the mortar are very important because of the fact that the mortar gets in contact with blocks which readily absorb moisture and also they restrict the drying contraction. This produces the above-mentioned micro-fissures, which consequently weaken the masonry. The joint mud should normally be the same as used to manufacture the block. Some coarse sand could also be added, the adequate proportion being given by the fissuring control test. For clayey soils, the adobe blocks should be moistened for a few minutes before placing them. Also it will be useful to moisten the previous layer of blocks before placing the joint mortar. For sandy soils, it will be enough only to moisten the preceding layer of blocks. To test the suitability of the earth for mortar perform the following test: Fissuring control test This is a test to determine clay content in the soil and whether problems from drying shrinkage can occur. Make a sandwich with two bricks and mortar. After 48 hours separate the bricks and observe whether there are cracks in the mortar. If that is the case add sand in proportion 1:5 up to 1:3. The sandwich with the least amount of sand that shows no visible cracking after opening 48 hours after manufacturing indicates the soil/sand proportion to be used for adobe construction. The entire wall should be protected against water damage by suitable facia or plaster. A water drain should be made slightly away from the wall to save it from seepage. Figure 5- View of an adobe house wall reinforcement (Blondet et al, 2002)   4. Bond beam at roof level Bond beam a top of all walls should be constructed to provide restraint and add stiffness to the walls. Bond beams can be constructed in timber or reinforced concrete. In the case of RC care should be taken to ensure continiouty at wall corners of bond beam reinforcement and adequate connection between the cast concrete and adobe wall. When using timber, since the thickness of the walls is usually 400 mm, several timber members are used to assemble the bond beam. These should be connected to the vertical wall reinforcement. As shown below a solution forming a truss with the lintel is also possible and is particularly recommended. Figure 6- Timber lintel forming a truss with the timber bond beam (Blondet et al, 2002)   Reinforcement steel and steel connection components Mild steel should be used with min. tensile yield strength of 240MPa 5. Construction of roof The roof should be ideally as light as possible. This would not only reduce lateral seismic loads, but would also reduce the risk of casualties in the event of roof collapse or partial collapse. Earthquake resistant roof type is the pyramidal low-pitch roof, with four inclined planes, resting on horizontal bond beam constructed over all walls. However simpler roof types like single sloped timber roofs and low pitched couple roofs are also recommended. Slopes up to 1:5 (11 degrees) are recommended in order to facilitate gable-end walls support. In all types of roofs the structure should not transfer thrust onto walls. The roof structure should transfer only vertical reaction at the supports. Timber plate or timber wedges must be used for uniform load distribution onto bond beam. The roof over-hanging will depend on local climatic conditions. Overhang of 500 mm is recommended. Board wood sheating or plywood as well as corrugated sheets can be used for roof cladding. Clay tiles are not so suitable due to their comparatively high mass. 6. Plastering of walls For protection adobe walls should be plastered with permeable soft plaster. Hydraulic lime or mud-lime based plasters are recommended. The plaster can be improved by reinforcing it with straw culms. Straw can be cut in 50 mm (2") lengths and added in the plaster mix. Impermeable (water tight) materials like portland cement should not be used for plaster since the adobe wall should be able to breathe. Plasters need regular maintenance every 5 to 10 years.   Planning and layout To beginning of document   Build one storey houses Provide foundations Provide buttresses at all corners and junctions of walls Roughly squared rooms Provide at least 4 courses of plinth masonry above foundations Build light low pitched couple roof, four sloped roof or single sloped roof The roof should transfer only vertical loads to the walls.Include a collar beam when constructing pitched roofs. The height of wall should not be greater than 8 times its thickness. The length of a wall, between two consecutive walls at right angles to it, should not be greater than 10 times the wall thickness 't' nor greater than 64t2/h where h is the height of wall. When a longer wall is required, it should be strengthened by an intermediate vertical buttress. The width of an opening should not be greater than 1.20 m. The distance between an outside corner and the opening should not be less than 1.20 m. The sum of the widths of openings in a wall should not exceed 1/3 of the total wall length. The bearing length (embedment) of lintels on each side of an opening should not be less than 0.5 m. An adequate configuration is shown on the Figure below. Figure 7- Improved adobe house planning   L < 10*T or 64*T*T/(h2+h3) L1 ~ L L2 ~ L l1 >= 1.2m l2 > 0.5m l3 >= 1.2m l4 > 0.5m l5 >= T l6 <= h4/3 h1 >= 0.8m h2 > 0.35m h3 < 8*T-h2 h4 <= L/12 T > (h2+h3)/8 T1 ~ T D < 1.2m W < 1.2m R < 1:6 (10 degrees)   Details for seismic resistance To beginning of document   Concept The performance of the building subject to an earthquake motions is governed by the inter-connectivity of structural components as well as the individual component's strength, stiffness and ductility. Thus the details to provide seismic resistance will be separated in two categories: 1. Details for complete load path Provide wall to wall connection ie. tying of walls Provide means for walls to foundations connection Provide connection of bond beams to roof Provide connection of walls to bond beams Provide stiff in their plane floors/roofs 2. Details to improve structural component's strength and ductility Means to increase the compressive strength of component Means to increase the flexural strength of component Means to increase the shear strength of component Means to increase the ductility factor, m of the component   1. Details for complete load path 1.1 Bond beam at roof level The continous beam that is constructed on top of the finished adobe walls is called a bond beam(see Glossary for other widely used terms).The bond beam has to be connected with the wall reinforcement and with the roof top timber plate -see Figure 7. The bond beam can be constructed in timber or reinforced concrete. Timber bond beam specification Figure 8- Timber bond beam over pillastered walls   The parallel joists(2) and braces(4) are recommended to be 100x75 and the blocking members(3)- 100x40 or 75x40. Figure 9- Timber bond beam connection to wall centre bamboo reinforcement (PUCP/CIID, 1995) Figure 10- Timber bond beam incorporating links to lintel (PUCP/CIID, 1995)   RC bond beam specification In order to enable satisfactory performance of the RC bond beams, a number of strutural measures needs to be followed. Concrete of at least C15 should be used. According to specification given in EC8 the min cross section is to be 150x150 mm. However the thickness of the bond beam should be equal to the thickness of the wall. Therefore bond beam cross section should be min 250x150 mm. At least 4 mild steel rebars with diameter 10 are recommended. Sixty rebar diameters splices are required by EC8 for the longitudinal reinforcement to ensure continiouty. Stirrups of mild steel diameter 6 should be provided every 200 mm. Adequate connection between the bond beam and masonry walls is required. For this purpose at the last masonry course adobe blocks can be omit at regular intervals. In this way, after concreting shear connectors will be created in the bond beam for better connection to walls. Figure 11- RC bond beam-splicing of rebars at wall corners   L >= 0.25m l1 = 0.2m l2 >= 0.5m l3 = L-60mm l4 = H-60mm l5 = 80mm l6 >= 0.2m l7 = 2*sqrt(2)*l3 H >= 0.15m   1.2 Tying of walls with rebars bands External reinforcement can be used for reinforcing wall to wall connection in both existing and new houses. Different techniques were tested in PUCP. Different reinforcement materials were tested, including wooden boards, ½-inch rope, chicken wire mesh, and welded mesh. Seismic simulation tests were performed on 'U'-shaped walls, with and without reinforcement, as shown in Figure 12. Figure 12- Dynamic Testing of 'U' Shaped Walls (Zegarra et al, 1997)   Broad horizontal bandages at lintel level and cill level are recommended below. The mesh should be fixed to the adobe wall by means of flat steel ties at 400 mm horizontal and 400 mm vertical (every 4th course). The steel ties should be embedded in the adobe masonry during construction. After completion of the wall a base of cement:sand mortar, mixed in 1:4 proportion and thickness 15mm, for the steel mesh should be installed. The welded steel mesh (diameter min 2mm @ 50x50 pitches) should be attached to the already cast strip of mortar by means of the embedded ties. Finally a second mortar layer of 15mm thickness is installed to complete the reinforcing bandages. See Figure 13 and Figure 14 for specifications. Figure 13- Tying of walls with steel mesh reinforcement Figure 14- Tying of walls with steel mesh reinforcement-a detail   L >= 0.5m l1 >= 50mm l2 >= T+l4 l3 = 50mm l4 = 15mm T >= 0.25m D >= T+0.6m H = 0.4m 1.3 Tying of walls to foundations The connection between adobe masonry walls and foundations can be greatly improved when bamboo cane wall reinforcement is being used. The vertical wall reinforcement is recommended to be erected in place and be embedded in the foundations min 400 mm. The canes should be attached with nylon threads or other means to the formwork to ensure correct position. The plinth level can be constructed in concrete together with the foundations as shown in Figure 15 Figure 15- RC bond beam -splicing of rebars at wall corners (Dowling, 2002) Figure 16- Construction of adobe masonry wall after completing the damp proof course over plinth (Dowling, 2002)   1.4 Anchoring of timber roof to the bond beam Figure 17- Anchoring of timber roof to RC bond beam   1.5 Securing gable-end walls Figure 18- Gable-end walls attachment to roof rafters   2. Details to improve structural components strength 2.1. Vertical and horizontal wall centre core cane reinforcement The most effective vertical reinforcement will be in the form of wood posts, bamboo or cane. For vertical wall reinforcement is recommended to be with diameter approximately 20 mm. For horizontal reinforcement is recommended crushed cane usually several shafts being assembled together by means of threads. The masonry the units are arranged around the vertical reinforcement as the wall costruction progresses. The vertical shafts should be built into the foundations and plinth and continued through and tied to the roof bond beam by binding wire, nylon thread. Vertical reinforcement helps to tie the wall to the foundation and to the bond beam. The poor flexural strength of the adobe walls is improved and thus restrains out-of-plane bending. The shear strength of the walls is also greatly improved. Detail A and Detail B are shown respectively on Figure 20 and Figure 21. Figure 19- Recommended distribution of vertical bamboo cane reinforcement Figure 20- Detail A Figure 21- Detail B   After completion of 4 courses of brickwork a band of horizontal reinforcement is installed. Several canes are assembled with threads and laid on each side of the core reinforcement see Figure 22 and Figure 23. These are embedded in the mud mortar. Horizontal reinforcement helps to transmit the bending and inertia forces from the out-of-plane walls to the in plane walls. The horizontal and vertical reinforcement should be tied together by means of wire or nylon string- Figure 21. This attachment provides a stable matrix, which is inherently stronger than the individual components. Figure 22- Assembling of crushed canes for the horizontal wall reinforcement (PUCP/CIID,1995) Figure 23- Assembling of crushed canes for the horizontal wall reinforcement (Blondet et al, 2002) Figure 24- Vertical canes for wall reinforcement (Dowling, 2002)   2.2. Use of Steel reinforcement for adobe walls The vertical and horizontal wall reinforcement can also be provided from steel rebars. However placing vertical rebars in adobe walls can be difficult particularly because the bars needs to be grouted to ensure bond with adobe. Consequently a special block with a wedge opening (see Figure 24) is required. The vertical holes with the rebars are later grouted with cement-sand based mixture. For regions of high seismicity (zones 3 and 4 for according to UBC) the UBC requires min reinforcement in the horizontal and vertical direction of not less than 0.07% of gross cross section of the wall and the min sum of the horizontal and vertical reinforcement to be min 0.2%. To beginning of document For information about this web site, click here. Purpose of this web site and disclaimer, click here. To find out How to use this web site, click here. The layout of this web site has been tested using Internet Explorer 6.0, Navigator 7.0 and Firefox 0.9.