MUD BRICK (ADOBE) CONSTRUCTION
Contents
Background .......................................................................................................................................... 2
Resources ............................................................................................................................................ 3
Advantages and Disadvantages ...................................................................................................... 4
Structural Considerations ............................................................................................................... 6
Resistance to Erosion by Rainfall .................................................................................................. 8
Case Studies ..................................................................................................................................... 12
The information presented in this module is for general guidance only for those considering the
use of mud brick construction and Engineers Without Borders strongly recommends that advice
be sought from a suitably qualified engineer for particular projects.
This module has been prepared by Dr Kevan Heathcote (Kevan.Heathcote@gmail.com) based on
his experience in earth building and his research into earth wall durability. He was a member
of the standards committee which produced New Zealand standard NZS 4297 Engineering
Design of Earth Buildings. The sections on structural considerations and resistance to erosion
have been based on that standard and on Dr Heathcotes experience and have been simplified
to reflect the low technology inherent in mud brick construction.
Background
Unstabilised mud bricks are made by mixing wet clayey soil with straw and forming them into
brick shaped molds. The molds are then removed and the bricks allowed to dry in the sun.
Adobe bricks are typically 250mm by 350mm by 100mm and are laid flat to produce 250 mm
thick walls. Their mass is around 14 kg (density around 1600 kg/m
3
) and mortar of similar
material is used to build walls. Mud bricks are made from low clay content soils stabilised with
bitumen emulsion in New Mexico but in other areas unstabilised bricks are made from higher
clay content soils.
Next to timber mud brick construction is the oldest construction technique in the world,
originating in biblical times in Mesopotamia. Whilst it is a common form of low cost
construction in low rainfall areas such as the Middle East (Egypt, Yemen, Iran, Irak, Pakistan,
India and Afganistan) and parts of China it is also historically common in higher rainfall regions
such as the Andes regions of South America, Spain and Portugal. Adobe itself is an Arabic
word and we can assume that this type of construction was transferred to Spain by the Arab
conquest and was then taken by the Spanish to their colonies in South America. Mud brick
construction is prevalent in New Mexico and the style of construction there is commonly
referred to as the Adobe style. More recently mud brick construction has been revived as an
environmentally sympathetic material in places such as the southern states of Australia and
New Zealand.
900 Year Old Taos Pueblo in New Mexico Mud Brick House in Australia
Traditionally mud brick construction has relied on external renders for protection from the
weather in relatively low rainfall areas (repaired each year in the case of the Taos Pueblo) or
on large roof overhangs in higher rainfall areas to prevent rainfall damage.
Resources
Building with earth bricks & rammed earth in Australia, Earth Building Association of
Australia (EBAA), no date. EBAA has been established for some time and has a lot of
experience in this area. See http://www.ebaa.asn.au/
Bulletin 5 Earth-Wall Construction (4
th
Ed) National Building Technology Centre, 1987.
Used to be referenced in the Building Code of Australia as the de-facto standard but
removed in 2008. Earth building for BCA now requires an earth building to be certified
by an expert person
Adobe bricks in New Mexico, Edward W. Smith, New Mexico Bureau of Mines and
Mineral Resources, 1982 Comprehensive technical coverage of adobe building in New
Mexico.
The Earthbuilders Encyclopedia by Joseph M. Tibbets, Southwest Solaradobe School,
New Mexico, 1988. Detailed coverage of all aspects of earth building with particular
reference to New Mexico.
Dirt Cheap The Mud Brick Book, John and Jerry Archer,Compendium, 1976. Practical
construction handbook from Australia
Making the Adobe Brick, Eugene H. Boudreau, Fifth Street Press, 1971. Practical
construction handbook from the USA
New Zealand Earth Building Standards NZS 4297:1998 Engineering Design of Earth
Buildings, NZS 4298:1998 Materials and Workmanship For Earth Buildingsand NZS
4299:1998 Earth Buildings Not Requiring Specific Design very comprehensive set,
NZS 4297 probably too detailed for mud brick construction though.
http://www.youtube.com/watch?v=qCEdDM-c7RQ Video of owner builder construction
of mud brick house in South Africa.
Mud Bricks Drying (Source virginmedia.com)
Advantages and Disadvantages
The advantages of mud brick construction are
Well designed mud brick buildings look warm and inviting
Mud brick manufacture utilises the natural material of the site and requires no
artificial additives (except bitumen when that is used as a stabiliser).
Mud brick manufacture need not require mechanical equipment and has negligible
embodied energy if material sourced at site.
The technology is very simple and almost anyone can build in this material.
Because they have high thermal mass mud brick buildings are very effective in reducing
temperature swings in climates where the daily temperature swings are large. When
combined with good passive solar design mud brick houses are extremely comfortable,
warm in winter and cool in summer.
Unreinforced round mud brick buildings have good earthquake resistance. See B.
Samali, W. Jinwuth, K. Heathcote, and C. Wang, Seismic Capacity Comparison between
Square and Circular Plan Adobe Construction, The Twelfth East Asia-Pacific
Conference on Structural Engineering and Construction, January 26-28, 2011, Hong
Kong SAR, China
The main disadvantages are
Unreinforced rectangular mud brick buildings perform extremely poorly in earthquakes.
In the 2003 earthquake in Bam, Iran many mud brick buildings were completely
destroyed. Note however that Dominic Dowling of UTS in Sydney has devised a system
of reinforcing rectangular mud brick buildings with bamboo to withstand earthquakes.
See http://www.thinkbigmagazine.com/mindset/258-earthquake-proof-housing and
www.world-housing.net/uploads/WHETutorial_Adobe_English.pdf
The insulation properties of earth walls is not good (R approximately 0.5 m
2
K/W for
250 thick walls) and therefore thermal performance is poor in very hot or very cold
climates with small daily temperature fluctuations.
Local soils are not always suitable for use or straw or bitumen may not be readily
available.
Can require frequent maintenance if exposed to extreme weather unless stabilised or
rendered.
Drying of bricks prior to laying can be a problem in areas of high humidity.
Structural Considerations
Soil Composition In general mud bricks can be made with most soils having clay
contents between 5% and 45% and with at least 30% sand and not more than 50% silt.
In Australia and New Zealand mud brick soils tend to have clay contents in the upper
range with straw added to reduce shrinkage cracks. In New Mexico there is a
requirement for soils to have at least 50% sand to reduce cracking and clay contents
are generally in the lower range. These bricks are typically stabilised with bitumen
emulsion (5-12% by mass) to prevent excessive water absorption. The figure below
shows the general range of soil types used but in the end an acceptable soil is one that
produces bricks which are suitable for their exposure environment (see rainfall erosion
resistance below)
Foundations Foundations can be the most expensive part of the construction if made
of concrete. For stable soils 400 wide by 300 deep footings are needed with 2 /12 mm
diameter reinforcement bars top and bottom. For moderately reactive soils the depth
needs to be 600 mm with 4 bars top and bottom. For more detail see AS2870
Residential Slabs and Footings. Alternatives to concrete on stable soils can be stone
slabs or small pieces placed in wire boxes (gabions) and in some cases the natural soil
stabilised with 10-15% cement may be suitable. For all footings a durable plinth such as
fire clayed bricks, rendered stone or mud bricks with 10-15% cement should be
constructed to provide protection from splashing rain at the base of the wall. Such a
plinth should be at least 225 mm above ground level.
Compressive Strength of Walls the overall compressive strength of load bearing
walls is generally not a problem with light roof loads. The compressive strength of mud
brick walls may be assumed to be 0.5 MPa, provided individual bricks do not break when
dropped from waist high onto a hard surface (See EBAA reference in Resources
section). Load factors of 1.2 for dead load and 1.5 for live load should be used together
with a capacity reduction factor () of 0.6 (If all loads are dead loads this gives an
allowable stress of 0.25 MPa). Reduction factors of [1-2.3(eccentricity of load/wall
thickness)] should be applied for eccentric loads and [1.18-0.03(wall height/wall
thickness)] for wall height (maximum 1.0). Wall heights should not be greater than [12
wall thickness] for walls supported at the top and bottom and 6 for free standing walls.
For localised bearing stresses such as lintels compressive stress of 1 MPa can be
assumed but it must be remembered that lintels should be stiff enough to provide an
even bearing. A timber top plate is usually provided on the top of walls to spread the
roof loads and concentrated loads should receive special attention.
In-Plane Shear Strength of Walls a wall shear stress of 0.035 MPa can be assumed
for determining resistance to wind loading (shear walls) and low earthquake loading.
Out-Of-Plane Loads - for determining resistance to wind loading it is suggested that a
yield line approach be used with = 0.6 and a flexural tensile strength of 0.1 MPa
(100 kPa). [For a long wall the yield line method gives M
y
= wH
2
/8 and for a square
panel M
y
= wH
2
/24]
As an example consider a 3 metre by 3 metre panel which is 250 thick
M
capacity
= 0.61000.25
2
/6 = 0.625 kN.m/m
M
yield
= w3
2
/24 = 0.375w
Equating the two gives an ultimate wind load resistance of 1.67 kPa (0.5 kPa for a long
3 metre high wall).
This approach can also be adopted to give a guide to the resistance of rectangular
buildings to low earthquake loading, bearing in mind that the corners of rectangular
buildings are the main weak points in out of plane loading (and need to be well
reinforced).
For example the above wall has a mass of around 400 kg/m
2
(Density of 1600kg/m
3
)
In this case w = 400a/9.8 = 40.82a giving M
yield
of 15.3a kN.m which when equated
to a wall capacity of 0.625 kN.m gives an earthquake resistance of 0.04g
For a more detailed analysis of earthquake loading refer to Appendix B of NZS
4287:1998
Quality Control at least one brick for every 1000 used should be subjected to the
drop strength test (see above) and one brick in every 2500 should be assessed for
resistance to rainfall (see below) where walls are to be exposed to the weather.
Resistance to Erosion by Rainfall
The resistance of mud brick walls to rainfall depends on the composition of the soil, the
intensity of wind-driven rain and on the exposure of the wall. In general it is best to avoid
exposure by providing wide eaves to buildings, although some soils produce bricks that are
remarkably resistant to erosion by wind-driven rain. External coatings such as renders or
linseed oil may be used to improve erosion resistance but these may require frequent renewal.
Walls that are not protected from the weather or otherwise coated with a proven erosion
resistant coating shall be of sufficient durability to withstand the effect of driving rain
appropriate to their exposure situation. Such durability may be verified as follows.
a) Wall exposure situations shall be classified as Exposure Category 1 to 7 according to the
following Table.
Typical Wind Speed During Rainfall
Annual Rainfall (mm) Low Medium High
<600 mm 1 3 5
600-899 mm 2 4 6
900-1300 mm 3 5 7
Where eaves extend away from the wall greater than one quarter of the wall height an
Exposure Category one less than given by the above table may be assumed with a minimum
Exposure Category of 1. Where eaves extend away from the wall greater than one half of
the wall height an Exposure Category two less than given by the above table may be
assumed with a minimum Exposure Category of 1.
b) Resistance of walls to erosion due to driving rain shall be indicated by a Test
Resistance Number (TRN) determined for samples subjected to the Geelong Drip test in the
case of walls of Exposure Categories 1-3 and by the UTS Spray Test for Exposure
Categories 4-7 Wall samples shall have a TRN greater than or equal to their Exposure
Category to be considered acceptable.
NOTE: To the best of available knowledge walls that meet the above criteria may in
normal circumstances be expected to have an acceptable (less than 15 mm) erosion over a
50 year period. In the event that the Designer of the building requires a better
performance the required TRN of the wall material shall be stated on the drawings.
GEELONG DRIP TEST
a) The specimen is mounted inclined at 27 degrees to the horizontal.
b) The specimen shall be tested without any surface coating.
c) lOO ml of water is discharged vertically 400mm onto the sloped surface of the
sample.
d) The water must be discharged onto the sample within 20 - 60 minutes.
e) The pit depth is to be measured with a cylindrical probe with an end diameter of
3mm.
(Sourced from EBAA Publication)
Classification of Results
Pit Depth <5 mm TRN = 3
Pit Depth > 5mm but < 10 mm TRN = 2
Pit Depth >10 mm but< 15 mm TRN = 1
Pit Depth >15 mm Sample suitable for Internal Use Only
10
UTS SPRAY TEST
a ) G e n e r a l
The test consists of spraying the face of a sample of the material (brick or rammed earth
wall) with water for a period of one hour
b) Test Detai l s
The spray nozzle to be used shall be a Fulljet 1550 nozzle (Available from
Spraying Systems Company)
Water pressure shall be 70 kPa measured with a reliable pressure guage
The specimen shall be positioned 350 mm from the nozzle and at right angles to
it. The nozzle shall be horizontal and the specimen face vertical.
The specimen is to be protected with a 150 mm diameter shroud with the gap
between brick and shroud sealed with a gasket
The erosion depth shall be measured by placing a straight edge across the erosion
crater and measuring the deepest extent of the crater using a pencil as a depth
guage.
Classification of Results
Depth of Erosion <5 mm TRN = 7
Depth of Erosion <10 mm TRN = 6
Depth of Erosion >10 but < 20mm TRN = 5
Depth of Erosion >20 but <30mm TRN = 4
Depth of Erosion >30 mm Suitable for Internal Use Only
11
12
Case Studies
Mud Brick Building in Gaza
See http://ingaza.wordpress.com/2009/05/07/denied-cement-re-building-with-mud/
Small Mud Brick Test Building
