Journal of Natural Disaster Science, Volume 32, Number 1, 2010, pp1-21

Earthquake Resistance of Adobe Reinforced by Low Cost Traditional Materials
Mohammad Shariful ISLAM*
Kazuyoshi IWASHITA***Department of Civil Engineering, Bangladesh University of Engineering and Technology**Professor, Department of Civil and Environmental Engineering, Saitama University(Received December 25 . 2008 accepted July 8 . 2010)
ABSTRACT
Adobe houses are expected to continue providing the housing needs of rural poor
people because of their energy efficiency, comfort, and cost effectiveness. Unfortunately
traditional adobe constructions are very poor against earthquakes and cause significant lossof life and property. Seismic deficiencies of such structures are mainly due to brittlenessand low strength of the blocks; low strength as well as poor bonding of mortar; and lackof structural integrity. Considering affordability, availability, environmental friendlinessand easy construction, the current study adopts natural fibers (straw, hemp and jute) andcementing materials (cement and gypsum) as reinforcing materials to improve the strengthand ductility of adobe material. Straw improves the ductility of adobe block at the cost ofcompressive strength. Jute is the most effective among the selected fibers to improve both
the ductility and toughness of adobe block. Gypsum improves the strength of adobe block
but not the ductility. Similarly, cement is effective for improving the mortar strength only.
On the contrary, jute is effective for improving the mortar strength as well as coherence
between block and mortar. Higher flexibility, tensile strength and friction of jute might be
the reason for its best performance.
Keyword: adobe, ductility, low cost, toughness, and natural fiber1994). However, traditional adobe construction responds very poorly to earthquakes; suffers serious
structural damage or collapse and causes significant
loss of life and property. The 2001 Gujarat earthquakein India and the December 26, 2003 Bam earthquake
in Iran bear ample testimony to this fact. In spite ofthis, due to different socio-economic reasons and unavailability of suitable alternatives, these structures
are expected to continue to exist for decades to come,
especially in developing countries. But unfortunately
only a few studies have focused on the developmentof earthquake resistant reinforcement of such struc-

1. INTRODUCTION
Adobe is one of the oldest and most widely usedbuilding materials for its manifold advantages suchas, low cost, easy availability, easy construction, lowenergy requirement, and environment friendliness.
Moreover, its excellent thermal and acoustic properties make it appropriate for areas with severe weather
conditions and a high range of temperature variation. Approximately 30% of the world’s populationand 50% of the population in developing countries
still live in earthen dwellings (Houben and Guillaud,1

MOHAMMAD SHARIFUL ISLAM , K. IWASHITA

tures.

crete beam and wire mesh. The cost of reinforcing an
adobe structure of an ideal configuration (9.0 m × 6.0
m × 2.9 m) using these four methods has been estimated based on the local price (PWD Schedule, 2002)
in Bangladesh, which is a developing country. Detailsof the reinforcements are available in Blondet et al.
(2003). The cost of reinforcement varies between
150~580 US $. Comparison of the costs of such reinforcements using the Gross National Income (GNI)
per person of the developing countries (e.g., GNI of
Bangladesh is 394 US $, World Bank Statistics, 2004)
indicates that none of these methods would be affordable for low income people. PP-band and polymer
meshes are cost effective solutions. But these are notbiodegradable; therefore, if disposed of after failure,these meshes may produce natural hazards.

So, why do adobe structures fail? The responseof adobe structures in past earthquakes and thecauses of their failure are described in many reports(e.g., Boduroglu, 1989; Tomaževič, 1999; Tolles etal., 1996). These reports indicate that the damage toadobe structures is mainly due to tensile failure of themortar. However, past earthquakes indicate that adobe
structures generally fail due to the following reasons:● Adobe is a brittle material although it may havelarge compressive strength. Once yielding occurs,
cracks develop in the adobe and a complete loss oftensile strength results.
● Unreinforced mortar is extremely weak and theconnection between block and mortar is poor. This
results in a partial or total disintegration under afew cycles of shaking of a moderate earthquake.
● Heavy mass inducing great inertial force and configuration of the walls derives an unstable structure.
● Lack of structural integrity (i.e., sufficient strong
connections between various elements of the structure).

Therefore, the structural reinforcement techniques described above are not feasible methods toapply to adobe structures for developing countries dueto their high cost, requirement for skilled design andconstruction as well as environmental hazard. Hence,finding a feasible and an implementable solution for
the earthquake resistant reinforcement of adobe structures for developing countries is a very important issue.

State-of-the-art research studies and field applications indicate that seismic resistance of adobe
structures can be improved in two ways: 1) improving
the structural integrity using structural reinforcementand (2) improving block and mortar properties (i.e.,
strength, ductility, toughness as well as bonding between block and mortar).

Adobe home-makers have been using natural
fibers such as straw and grass traditionally with adobe
as reinforcement material for centuries (Hossein-Javaheri, 1972; Hammond, 1973). Straw is an agricultural
waste, very cheap, easily available worldwide andenvironmentally friendly. However, although used
for millennia, there are only a few analytic studies
on earth/straw mixtures (Vargas et al., 1986; Morris,
1993; Warren, 1999; Islam, 2002; Binici et al., 2005;
Islam et al., 2006). These studies mainly focused onstrength characteristics, crack control, and moistureresistance of adobe material. However, short discontinuous fibers have the advantage of mixing with thematrix homogeneously. Therefore, fibers might beeffective for improving the properties of adobe blockand mortar in the following way:
● Fibers might be effective for increasing the tensile
strength and elasticity of the material. They will
also help to prevent the cracks in adobe block andmortar during drying.
● The sewing action of fibers may provide better co-

Efforts have been made by different researchers in the past to improve the seismic resistance ofadobe structures using structural reinforcement suchas concrete beams, anchored roof beams, horizontal
and vertical steel rods, welded wire mesh, steel meshwith cement mortar, tensile steel bars, etc. (Meli etal., 1980; Scawthorn and Becker, 1986; Ottazzi et al.,
1988; Bariola and Sozen, 1990; Ginnell et al., 1995;
Tolles et al., 2000). Possibilities of using locally available cane and wood beams have also been explored(Arya, 2000). The effectiveness of PP-Band mesh andpolymer mesh has been evaluated recently (Paola etal., 2006; Blondet et al., 2006). Among several methods available for the seismic reinforcement of adobe
structures, the most common are―horizontal and vertical bamboo/cane reinforcement, timber beam, con2

EARTHQUAKE RESISTANCE OF ADOBE REINFORCED BY LOW COST TRADITIONAL MATERIALS

herence between the block and the mortar.
● Fibers in the matrix will provide a means to preventthe crack growth in the shear band when cracks
propagate due to the imposed loads. Thus fibers canimprove the ductility, toughness, or both, and flexural strength of the block.
● Moreover, the addition of fibers will reduce thedensity of the material, which will help to reducethe inertia forces.

be effective for preventing the shrinkage cracks inadobe block and mortar during drying.
● Both gypsum and cement might be effective for improving the strength of adobe block and mortar aswell as coherence between block and mortar due totheir cohesive properties.
None of these past studies stated above clarified
the effectiveness of cementing materials on ductility,
toughness, and the coherence of block and mortar.

None of these past investigations describedabove examined the effectiveness and mechanismsof the addition of straw to adobe material on ductility, toughness and bonding characteristics, which
are the key issues to improve their earthquake resistance. However, different kinds of natural fibers areavailable in different parts of the world. It is worth
comparing the effectiveness of the performance ofthe straw reinforced adobe with that of other natural
fibers to select the most suitable fiber type. Natural
fibers such as jute and hemp are stronger, more flexible than straw and have high friction to soil particles.
They are easy to find, cheap and do not produce any
environmental hazard. Huge quantities of jute areavailable in India and Bangladesh where adobe houses are being used significantly. Hemp is produced inmany countries such as China, Hungary, Canada, andthe USA.

Considering the affordability, availability, environmental friendliness, and ease of construction thecurrent study focuses only on the improvement of
adobe block and mortar properties using natural fibers(i.e., straw, hemp and jute) and cementing materials(i.e., gypsum and cement). These materials have beenused with adobe traditionally and will be accepted byhome-makers if there are benefits.
Adobe home-makers use the locally available soil to make adobe. Therefore, it is necessary
to investigate the effect of soil composition on theperformance of the proposed reinforcing materials,
especially the clay content, which gives the maximum
strength to the soil.
There is always a gap between the material properties determined from the laboratory tests and thereal material properties for several reasons (e.g., size,
shape, making condition). Among these reasons, difference in the size of the specimen and size of the realunit in the structure is the most important.

Cementing materials such as cement, lime andgypsum are also mixed traditionally with adobe to
improve strength characteristics. Gypsum stabilized
adobe showed better performance in past earthquakes(Brown et al., 1978). Gypsum is being used with
adobe in Turkey (Hammond, 1973; Isik et al., 1999).
The cost of gypsum depends on the availability. Although gypsum is cheap and easily available in somecountries, it might be costly and not available in othercountries. However, gypsum is related to cement interms of its cementing effect. Although cement isexpensive, it is readily available worldwide. The effect of gypsum on adobe was investigated recently
by Isik et al., 1999; Islam, 2002; and Binici et al.,2005. However, these studies focused mainly on thestrength characteristics. Gypsum and cement might beeffective in the following way:
● Gypsum expands in volume while drying. Therefore, a judicial blend of gypsum with earth mightThis study focuses on the improvement of blockand mortar properties of adobe structures. Uniaxial
compression tests were conducted on block andsandwich specimens (two adobe blocks joined withmortar) to evaluate the effectiveness of the proposed
reinforcing materials on adobe block and mortar
properties (strength, ductility, toughness and coherence between block and mortar), respectively. Therefore, the test results describe the behavior of blockand mortar only. Further tests (cyclic or dynamic) are
required to obtain parameters that can be used for the
analysis of adobe structures under earthquake excitation. However, better strength and ductility of blockand mortar mean that the structure will perform better
3

MOHAMMAD SHARIFUL ISLAM , K. IWASHITA

the stress-strain curve up to 15% strain, then stresscorresponding to the 15% strain is taken as the compressive strength. Failure strain is defined as the strain
corresponding to the peak stress and 15% if there isno peak before 15% strain. Figure 1a shows the compressive strength, failure strain, and ultimate failure
point of the stress-strain relationship according to JIS
A 1216 (JGS, 1999). As presented in Fig. 1a, the ultimate failure point corresponds to the 2/3qu (where, quis compressive strength), at 2.0% from the peak and15.0% for Case I, II and III, respectively.

under earthquake loading. The main objectives of thisinvestigation are:a) To evaluate the effectiveness and mechanismof straw as an earthquake resistant reinforcing
material for adobe. To investigate the influenceof quantity, size in length and type (crushed or
uncrushed) of straw on the seismic behavior of
adobe block.
b) To compare the characteristics of straw reinforced adobe with that of jute and hemp reinforced adobe to select the best suitable natural
fiber type as the reinforcing material and thus tounderstand its mechanism. To determine the optimum quantity and length of the most suitablefiber.
c) The effect of soil composition and scale of test
specimens on the effectiveness of the proposed
reinforcing materials.
d) The effectiveness of cementing materials (gypsum and cement) to improve the earthquake resistance of adobe.

(2) Ductility
Ductility (μD) is defined as the relationship of a
total elasto-plastic response deformation related to thedeformation in the elastic limit (Krätzig and Meskouris, 1998) as stated in Equation 1. This can only be
defined for linear elastic, fully plastic responses. But
for real materials, the response graphs become curvilinear and the fixing of the elastic limit involves considerable uncertainty (Fig. 1b). As a result, defining
ductility contains many ambiguities. In spite of this,ductility was estimated using Equation 1 to compare
the performances of the proposed reinforcing material. Displacement (i.e., strain) corresponding to thequ/2 is taken as the elastic limit of displacement. In
the case of soils, elastic modulus (E50) is determinedfrom the ratio of the qu/2 and the strain corresponding
to it. Hence, choosing the strain corresponding to thequ/2 as elastic limit is reasonable. Displacement (i.e.,
strain) equal to the ultimate failure point was taken as
the ultimate limit of displacement.

2. EXPERIMENTAL PROGRAM
2.1 Testing and Indices2.1.1 Test Procedure
Uniaxial compression tests were conducted toevaluate the effectiveness of the proposed fibers andcementing materials on the earthquake resistance ofadobe. All tests were conducted by a chain of devices
composed of a loading device, a digital acquiring
device, and a personal computer. Load values were
measured with a load cell of 20 kN capacity. Strain
was calculated from the strain rate and time. For eachgroup, at least 3 specimens were tested to check therepeatability of the test results. All tests were conducted in accordance with JIS A 1216 (JGS, 1999).
Compressive strength, failure strain, and ultimate
failure point were determined from the stress-strain
relationships. To compare the effectiveness of the proposed reinforcing materials, ductility and toughness
were estimated from the stress-strain relationships.

D

u ult
u el

(1)

where, uel and uult are elastic limit and ultimate
limits of displacements (i.e., strain), respectively.
Ductility has also been estimated using the energy concept. In this method, elastic energy and plastic
energy were calculated from the stress-strain relationship as shown in Fig. 1c. Definition of ductility (μE)
according to this method is presented in Equation 2.

2.1.2 Definition of Parameters(1) Compressive strength, Failure strain, Ultimate Failure Point
Compressive strength is defined as the peakstress on the stress-strain curve. If there is no peak in

Wp
E

We

(2)

where, We and Wp are elastic and plastic energy,
respectively.
4EARTHQUAKE RESISTANCE OF ADOBE REINFORCED BY LOW COST TRADITIONAL MATERIALS

qu
600
1/3qu

400

（b）

2%

(I)

200

(II)

ε
0
0

5

εf 10

Force variable（a）

Axial stress, σa (kPa)

800
real
fy
ideal

(III)

ult

15

uel

20

u
Dispalcement variable

Axial strain (%)

ult

9Unloading path

Straw length= 1.0 cm
Straw content= 0.0 ~ 3.0%

A
Plastic energy
W = Area OABD

E

Parallel lines

B
Elastic energy
W e= Area BCD

63

μE= W p/W e

p

O

（d）

Ductility , μ（c）Axial stress, σaInitial tangent

0

DC
Axial strain, ε

0

a

3

6
9
Ductility, μD

1215Fig. 1 (a) Definition of compressive strength, failure strain and ultimate failure strain (point marked by arrow [↓]on the stress-strain curve indicates the compressive strength and corresponding strain is failure strain;strain corresponding to the dot [•] on the stress-strain diagram is ultimate failure strain) according to JIS
A1216 (JGS, 1999). Definition of: (b) ductility, μD; (c) ductility, μE and (d) comparison of μD and μE.

Ductility estimated from the displacement (i.e.,
strain) and energy based on Equations 1 and 2 isdenoted as μD and μE hereafter. A comparison ofductility estimated from both equations is presented
in Fig. 1d. Both ductilities showed a similar trend.
Therefore, ductility estimated from the displacement
was only used for the comparison of performances.

nese soils were used to make adobe. To determine thegeneral adobe composition, some old adobe collectedfrom Choga Zanbil Temple (which is a world heritagesite located in the western part of Iran) was studied.
Dry density and water content of the old adobe arein the range of 2.71~2.76 g/cm3 and 1.40~5.10%,
respectively. More details about the old adobe areavailable in Islam (2002). To attain similar grain size
distributions of the old adobe, locally available Japanese Acadama clay, Toyoura sand, and Bentonite weremixed with a weight ratio of 2.5:1.0:0.6. This mixture
was named the ‘soil-sand mixture’ (SSM) in the current investigation. To understand the effect of the soil
composition on the behaviour of the adobe, specimens were also prepared using Kaolin clay instead of
Bentonite clay. Physical properties of the soils used
for making adobe are presented in Table 1. Chemicalcomposition of the soils is available in Islam (2002).
Grain size distribution of the ‘soil-sand mixture’ andthose of the old adobe are compared in Fig. 2. The
grain size distribution of the ‘soil-sand mixture’ issimilar to that of the old adobe specimens.

(3) Toughness
Toughness is a measure of the combination ofductility and strength. The ability of a material toabsorb energy before fracture is termed as toughness.
Toughness is calculated as the area under the stressstrain curve up to the ultimate failure point as shownin Fig. 1a.
2.2 Materials Used
2.2.1 Soils
Ideally, the soil to make adobe must contain four
elements: coarse sand or aggregate, fine sand, silt,
and clay. Since this investigation was conducted at
Saitama University in Japan, locally available Japa5

MOHAMMAD SHARIFUL ISLAM , K. IWASHITA

Table 1. Physical properties of soils
Soil

Specific
gravity

Liquid limit（%）

Plastic limit（%）

Plasticity index（%）

Acadama clay

2.65

145.0

67.0

78.0

Toyoura sand

2.51

―

―

―

Bentonite

2.51

232.0

31.0

201.0

Kaolin

2.65

84.5

34.5

50.0

SSM

2.62

70.0

35.0

35.0

(3) Hemp
Hemp is also collected from the hemp plant,
which is an annual plant. It grows about 3~4 m in 110
days and its stem diameter is 2~3 cm. Hemp originates from central Asia, and is now available worldwide. Its production per unit area is relatively high. In
this case, commercially available hemp was also used.
Hemp was also used in the form of jute as shown in
Fig. 3b.

% Finer by weight

100
80
MB-1

60

MB-2

40

MB-3

20

MB-4
SSM

0
-4
10

-3

10

-2

10

-1

10

0

10

1

10

2

10

Grain size (mm)Fig. 2 Grain-size distributions of old adobe collectedfrom Iran (MB) and soil-sand mixture (SSM)
used for making adobe.
2.2.2 Reinforcing materials(1) Straw
Straw is an agricultural by-product, the dry stalkof a cereal plant, after the nutrient grain or seed has
been removed. Straw makes up about half of theyield of a cereal crop such as barley, oats, rice, rye or
wheat. The internal structure of a single straw is tubular, tough, flexible, and has high tensile strength. In
this investigation, fully dried rice straw was used after
chopping into short pieces as shown in Fig. 3a.

（a）(2) Jute
Jute fiber is collected from jute stems. Jute fiber
is flexible, rough and has high tensile strength. Jute,
which is an annual plant produced in tropical countries such as Bangladesh, India, grows about 2~3 m in
4 months. Since jute is not produced in Japan, commercially available jute was used in this investigation
after chopping it into short pieces as shown in Fig. 3b.

（b）Fig. 3 Photographs of chopped: (a) straw and (b) jute.

6

EARTHQUAKE RESISTANCE OF ADOBE REINFORCED BY LOW COST TRADITIONAL MATERIALS（a）（b）
Fig. 4 Photographs of: (a) soil-sand slurry and (b) steel mould.
(4) Gypsum
Gypsum is a very soft mineral composed of calcium sulphate dihydrate, with the chemical formula
CaSO4.2H2O. Commercially available gypsum wasused in this investigation. The specific gravity of the
gypsum used is 2.30.

were removed from the mould and kept open in a tin
shed room for 45-60 days to dry.
The final water content of the specimens prepared in this way was in the range of 8.38~10.30%,
dried for 6 to 20 weeks in Japanese weather conditions. For the measurement of the water content of
adobe block, 3 specimens were collected from thetop, bottom and centre of the specimen. The reported
water content is the average of these three water contents. However, to understand the real behavior of
adobe, water content is very important because theshear strength and stiffness of adobe significantly
depend on the water content as indicated by Warren(1999). In Japanese weather conditions, it is impossible to reduce the water content up to 3.0% by natural
drying for the particular soil composition. As a result,
specimens were dried using an oven. To dry specimens using an oven, at first the adobe specimens withmould were kept in the oven for 3 days at 40°C. Thistemperature is selected because Iran’s daytime temperature (where adobe houses are being used significantly) is similar and no cracks occur in adobe at thistemperature. But it was found that the water content
did not decrease below 7.0% at 40°C (Fig. 5a). To
reduce the water content further, specimens were thenkept in the oven at 60°C for 1 more day after removalfrom the mould. The water content of the specimen
at this stage was reduced up to 3.0%. Therefore, todry the specimens using an oven, at first the specimen
with mould was kept in the oven for 3 days at 40°C
and after that the specimen was removed from themould and kept 1 more day at 60°C.

(5) Cement
In the case of cement, ordinary Portland cement
was used. The specific gravity of the cement is 3.16.
2.3 Preparation of Specimens
Adobe is a sun dried brick. Generally, it takes
2~3 weeks to obtain dried adobe. The water content
of dried adobe is about 3.0%. Cylindrical block andsandwich specimens were prepared to evaluate the effectiveness of the proposed reinforcing materials on
block and mortar characteristics.
2.3.1 Block Specimen
First of all, Acadama clay was cleaned of unwanted materials, air-dried, and then reduced to powder. After mixing the clay with reinforcing materials
and soils in dry conditions, water content equaling 1.5
times the liquid limit (LL) of the ‘soil-sand mixture’
was added. Then it was mixed vigorously by hand to
make the mixture homogeneous. The slurry (Fig. 4a)
was kept for 1 day for the complete dispersion of theclay particles. On the following day, the mixture wasremixed and poured into a steel mould 5 cm in diameter and 10 cm in height (Fig. 4b) in three layers. Eachlayer was compacted equally to remove the void fromthe specimen. The moulds were kept for 10-15 days
to reduce the water content such that the specimens
could stand without any slump. Then the specimens

Figures 5b and 5c show the dependency of7

MOHAMMAD SHARIFUL ISLAM , K. IWASHITA

compressive strength and failure strain on the watercontent for both the oven dried and naturally dried
unreinforced specimens, respectively. With the increase of water content the compressive strength ofadobe decreases linearly. The compressive strength of
the naturally dried specimens is also close to that ofoven dried samples for the same water content. The
failure strain does not depend on the water content for
both the naturally and oven dried specimens. A similar trend was observed for the dependence of compressive strength and failure strain of the reinforced
specimens. This means that the compressive strength
decreases linearly with the increase of the watercontent but the failure strain does not depend on thewater content. Therefore, the control of the humidity

content of adobe may significantly contribute to theirresistance in seismic conditions.
2.3.2 Sandwich Specimen
Several cylindrical specimens were tested under
uniaxial compression; the specimens failed at the angle of 60-70 degrees. Therefore, such specimens prepared with a similar soil composition will fail at thisangle. Since it was not confirmed for different soil
compositions, it cannot be said that this is a generaltendency for adobe material. Therefore, block partsfor making sandwich specimens were obtained from acylindrical block specimen by cutting it into two partsat an angle 60°to the horizontal. Then 0.5 cm thickmortar was laid between the two parts to make thesandwich specimen. Figures 6a and 6b present two
parts of the specimen after cutting and a sandwich
specimen, respectively. The top and bottom surfacesof the specimens were made flat using sandpaper anda sharp anti-cutter to avoid bedding errors.

（a）Water content (%)

25
20

Oven dry (40o C)

15
10
5
0
0

5

10

15

20

25

Drying time (hour)（b）Comp. strength (kPa)2000
Natural dry
Oven dry

1500
1000
500

（a）

0
0

5
10
15
20
Water content (%)

25

（c）

Failure strain (%)

4

――――――― Mortar

Natural dry

3Oven dry2

――――――― Block

1
0
0

5

10
15
20
Water content (%)（b）
Fig. 6 Photographs showing: (a) two parts of a specimen after cut and (b) a sandwich specimen.

25

Fig. 5 Variation of: (a) water content with drying
time; (b) comp. strength with water content;and (c) failure strain with water content.
8EARTHQUAKE RESISTANCE OF ADOBE REINFORCED BY LOW COST TRADITIONAL MATERIALSStraw reinforced and gypsum stabilized specimens were prepared at the first phase in the naturally
dried condition. Jute reinforced and sandwich specimens were prepared in the second phase using anoven. The water content of the naturally dried andoven dried specimens was in the range of 8.0~14.0%
and 2.0~5.5%, respectively. Dry density of the unreinforced and fiber reinforced specimens was in therange of 1.05~1.30g/cm3 and 0.82~1.11g/cm3, respectively. Dry density decreases slightly with the increaseof the fiber content. However, the density of thespecimens is significantly lower than that of the usual
adobe material. Since the specimens were preparedfrom slurry without providing consolidation pressure,density became significantly low. This is not expected
to affect the main objectives of this investigation.

the straw length, compressive strength decreased but
the ductility increased with the increase of the straw
content in general. The ductility of the specimens
containing 1 cm long straw increased linearly with
the increase of straw content. Ductility (μD) of thespecimens containing 2 cm and 3 cm long straw also
increased with the increase of the straw content. But
the improvement in ductility became almost constantafter 1.5% straw content (Fig. 7b and 7c).
Figures 7d, 7e and 7f present the dependency of
toughness on the straw content for specimens containing 1 cm, 2 cm and 3 cm long straw, respectively. For
specimens containing 1 cm long straw, there is no
particular dependency of toughness on straw content.
However, specimens containing 1.5% straw had the
maximum toughness, which is slightly higher than theunreinforced case. For specimens containing 2 cm and3 cm long straw, toughness decreased up to a straw
content of 1.0% and after that toughness increased
again and attained a toughness value close to that of
unreinforced adobe. In the case of specimens containing 2 cm long straw, toughness increased after 1.5%
straw content. Again, in the case of specimens containing 3 cm long straw, toughness decreased slightly
after 1.5% straw content. From the above discussions,the following conclusions can be drawn:● Strength of adobe block decreases with the increaseof straw content irrespective of the straw length.
● Ductility increases with the increase of straw content. But the rate of improvement of ductility after
1.5% straw content is not significant for specimens
containing straw longer than 1 cm.
● The addition of straw in adobe block below 1.5%
causes decrease in toughness. But by adding straw
1.5% or more toughness can be achieved similar tothat of unreinforced adobe block.
● Although the addition of this amount of straw(1.5%) causes a decrease in compressive strength,
toughness is similar (or slightly higher than) to theunreinforced case. But the improvement in ductility
is significant. This means that by using 1.5% straw,
collapses of adobe structure might not be prevented
but they can be delayed. Therefore, 1.5% straw by
weight is optimum for improving the ductility of
adobe material.

3. TEST RESULTS AND DISCUSSIONS
3.1 Effect of Fibers on Adobe Block3.1.1 Effectiveness of Straw
The main objective of this section is to present
the effectiveness and mechanism of straw as a reinforcing material for adobe. Adobe home-makers use
an arbitrarily selected amount and length of straw
without knowing the effect quantitatively. Therefore, it is necessary to find the optimum content andlength of straw to obtain the best seismic resistanceif there is any positive effect. Crushed straw is also
used instead of whole straw. So, there is potential toinvestigate the difference between the performanceof crushed and whole straw. The size of the laboratory specimen is different from the real adobe block.
Therefore, it is necessary to know the scale effect of
the test specimen to simulate the test result for the real
design purpose. The influence of straw content, sizein length, type (i.e., crushed and uncrushed straw) andscale effect of the test specimens is described in thefollowing sections.
(1) Straw Content
To investigate the effect of straw content, specimens were prepared from 1, 2 and 3 cm long straw
and ‘soil-sand mixture’ with varying straw content
from 0.0 to 3.0% by weight. Figures 7a, 7b and 7c
present the variation of compressive strength and ductility with straw content for specimens containing 1
cm, 2 cm and 3 cm long straw, respectively. Whatever

The mechanisms behind the above-mentioned
9

MOHAMMAD SHARIFUL ISLAM , K. IWASHITA

30

20

10

200

0

0
0.0

1.0

2.0

SL= 1 cm

15
10
5
0

3.0

0.0

1.0

（a）

（d）

30
SL= 2 cm

20

qu

600

μ

20

D

400
10

200
0

0
0.0

SL= 2 cm

15
10
5
0
0.0

1.0
2.0
3.0
Straw content, SC (%)

1.0

20

400
10

200

1.0

2.0

3.0

D

u

μD

0

Toughness (kPa)20

Ductility, μComp. strength, qu (kPa)30
q

SL= 3 cm

3.0

（e）

800
600

2.0

Straw content, SC (%)

（b）

0.0

3.0

Straw content , SC(%)

800

0

2.0

Straw content , SC (%)

Toughness (kPa)uToughness (kPa)μD

Ductility, μD

600
400

Comp. strength, q (kPa)

20

qu

D

u

SL= 1 cm

Ductility, μComp. strength, q (kPa)800

SL= 3 cm

15
10
5
0

0.0

1.0

2.0

3.0

Straw content, SC(%)

Straw content, SC (%)

（c）

（f）

Fig. 7 Dependence of compressive strength and ductility on straw content for adobe reinforced with (a) 1 cm longstraw, (b) 2 cm long straw, and (c) 3 cm long straw. Dependence of toughness on straw content for adobe reinforced with (d) 1 cm long straw, (e) 2 cm long straw, and (f) 3 cm long straw (SL= straw length).
observations can be explained as follows:● Straw replaces the soil-sand part of the mixture.
As a result, contacts between soil to soil decreases.
This causes reduction in friction and thus the decreases in strength. Since straw is not flexible
enough to move in phases with soil particles during
the drying shrinkage of adobe, separation occurs
between soil and straw. This causes micro-cracks inadobe. This might be another reason for decreasing

the strength due to the addition of straw. Strength
decreases with the increases of straw content since
the cohesion decreases and the number and depth ofcracks increase with the increase of straw content.
● Straw transmits the stresses across the cracked surface and prevents the crack from opening. In thisway, straw improves the ductility of adobe material.
● Toughness is a measure of both the strength andductility. Since the decreases in strength are more10

Toughness (kPa)

SC= 0.5%

μD

20

400
10
200

D

qu

600

0
-1

20

30

800

Ductility, μComp. strength, qu (kPa)

EARTHQUAKE RESISTANCE OF ADOBE REINFORCED BY LOW COST TRADITIONAL MATERIALS

4

SC= 0.5%

12
8
4
0
-1

0
0
1
2
3
Straw length, SL (cm)

16

0
1
2
3
Straw length, SL (cm)（d）

30SC= 1.5%

qu

600

μD

20

400
10
200
0
-1

20

Toughness (kPa)800

Ductility, μD

Comp. strength, qu (kPa)

（a）

0
0
1
2
3
Straw length, SL (cm)SC= 1.5%

15
10
5
0
-1

4

0
1
2
3
Straw length, SL (cm)30
SC= 3.0%

20

qu

600

μ

20

D

400
10
200
0
1

2

3

Toughness (kPa)

800

0

4

（e）

Ductility, μD

Comp. strength, qu (kPa)

（b）

0
-1

4

16
12
8
4
0
-1

4

SC= 3.0%

0

1

2

3

Straw length, SL (cm)

Straw length, SL (cm)

（c）

（f）

4

Fig. 8 Dependence of compressive strength and ductility on straw length for specimens containing (a) 0.5%
straw, (b) 1.5% straw, and (c) 3.0% straw. Dependence of toughness on straw length for specimens containing (d) 0.5% straw, (e) 1.5% straw, and (f) 3.0% straw (SC= straw content by weight).
significant than the improvement in ductility up
to 1.0% straw content, toughness decreases up to
this straw content. After that the rate of decrease instrength decreases but the ductility increases significantly with the increase of straw content and thus
the toughness increases.

of compressive strength and ductility on straw lengthfor specimens containing 0.5%, 1.5% and 3.0% straw(by weight), respectively. Compressive strength decreases with the increase of straw length for all cases.
Ductility does not improve with the increase of straw
length for the specimens containing 0.5% straw. For
the other cases, ductility increases with the increaseof straw length. In the case of specimens containing
1.5% straw, the improvement in ductility for speci-2) Straw Length
Figures 8a, 8b and 8c present the dependency
11

MOHAMMAD SHARIFUL ISLAM , K. IWASHITA

（a）
（b）
（c）
（d）
Fig. 9 Photographs of specimens containing 1.5% straw of: (a) 1 cm long whole straw, (b) 2 cm long wholestraw, (c) 3 cm long whole straw and (d) 3 cm long crushed straw.
mens containing 3 cm long straw from the specimens
containing 2 cm long straw is not significant if the
one extreme case is ignored (Fig. 8b). The reason forthe large variation of the one case is not clear and thatis why it was ignored in the discussion. The ductility of the specimens containing 3.0% straw becomes
constant after 2 cm long straw except for one extreme
case (Fig. 8c).

As the bonding length increases with the increaseof the straw length, the capacity of straw to hold theshear band increases. Thus the ductility increases with
the increase of straw length.
3) Crushed Straw
This section describes the effectiveness ofcrushed straw. To do so, the original straw was cut
into 4 pieces for a particular straw 3 cm in length
along the diameter. Typical stress-strain relationshipsof specimens containing whole diameter straw andcrushed straw are compared in Fig. 10. The compressive strength of the specimen prepared using crushedstraw is higher than that of specimens containing
whole straw. However, the ductility and toughness ofboth the crushed and whole straw reinforced adobe
are similar. This means that crushed straw is effective
for improving the strength characteristics of adobe.

Figures 8d, 8e and 8f present the dependence of
toughness on straw length for specimens containing
0.5%, 1.5% and 3.0% straw, respectively. Toughness
decreases with the increase of straw length for specimens containing 0.5% straw. Toughness of the specimens containing 1.5% and 3.0% straw are almost independent of the straw length and the toughness value
is similar to that of the unreinforced case. Therefore,on the basis of the dependency of compressive
strength, ductility and toughness on straw length, theoptimum length of straw is 2 cm to improve the seismic performance of adobe block.

Original straw
Crushed straw

400a

Axial stress, σ (kPa)

500

Photographs of specimens containing 1.5%
straw are presented in Fig. 9. The number and depthof cracks in the adobe specimens increase with the increase of straw length for the particular straw content.
This might be the reason for the decrease in the compressive strength with the increase of straw length fora particular straw content.

300
200
100
0
0.0

2.0

4.0
6.0
Axial strain, εa (%)

8.0

Fig. 10 Comparison of stress-strain relationships ofspecimens containing crushed and wholestraw (straw content= 1.5%; straw length= 3cm).

Ductility of adobe blocks increases with the increase of straw length for the specimens containing
1.5% straw or more. Although the number of straws
in the shear band is higher for the shorter straw, thebonding length is not sufficient to hold the shear band.

12

1000

u

A photograph of a specimen prepared withcrushed straw is presented in Fig. 9d. A comparisonof Figs. 9c and 9d shows that the number and depth ofcracks in the specimens containing crushed straw aresmaller than those of the specimens containing wholestraw.

Comp. strength, q (kPa)

EARTHQUAKE RESISTANCE OF ADOBE REINFORCED BY LOW COST TRADITIONAL MATERIALS（a）

The effectiveness of crushed straw is described
below:● Crushed straw is more flexible than whole straw.
That’s why the elasticity of the matrix with crushedstraw becomes higher, which helps to prevent the
micro-cracks in adobe due to drying shrinkage.
Since the cracks decrease, the strength increases.
Again, due to higher flexibility, the binding of
crushed straw with soil is better than that of wholestraw, which helps to prevent the shear band more
efficiently.
● Whole straw is tubular; therefore, the volume ofcrushed straw is smaller than that of the wholestraw for a particular weight. Therefore, the amountof soil replacement will be less for crushed straw.
● However, since the number of cracks is smaller anddepth is lower in adobe block prepared with crushedstraw, this will help to prevent rain infiltration thatwill aid its longevity.

Small
Large

800
600
400
200
0

0.0
1.0
2.0
3.0
Straw content, SC(%)（b）

Ductility , μ

E

25
Small
Large

20
15
10
5
0

0.0

1.0

2.0

3.0

Straw content, SC(%)

Fig. 11 Comparison of dependency of: (a) compressive strength and (b) ductility on straw
content of large and small specimens (straw
length = 3 cm).
3.1.2 Selection of Best Fiber
The main objective of this section is to compare
the effectiveness of straw with that of other fibers inorder to select the most suitable fiber type. Jute andhemp were selected since these have high tensile
strength and high friction. A comparison of the effectiveness of different fibers, optimum quantity andlength of the most suitable fibers and their mechanism
is presented in this section. To select the best fiber,
specimens were prepared by mixing the soil-sandmixture with 1.0% fiber (by weight) 1.0 cm in length.
Although the best value of the straw was known,
the best value of other fiber was not, thus tests wereconducted on specimens prepared with arbitrarily selected fiber content (1.0%) and fiber length (1 cm).

4) Scale Effect of Test Specimen
In the current study, diameter and height of thespecimens were 5 cm and 10 cm, respectively. But
adobe structures are generally made from blocksof larger sizes (e.g., length= 50 cm, width= 25 cm,
height= 10 cm) (Clifton, 1977; Hakuno, 1992). In thecase of laboratory specimens, fiber length varied between 1~3 cm. However, the size of fiber that is usedin real construction is different and is in the range of
5~10 cm (Islam, 2002). To study the effect of thesefactors, specimens were prepared in a larger mould 10
cm in diameter and 20 cm in height. Figures 11a and11b show the comparison of dependency of compressive strength and ductility on straw content for small
and large specimens, respectively. The dependency ofboth the compressive strength and ductility on straw
content is similar for the same straw length. Thismeans that strength properties determined from laboratory tests using the small specimens can be used directly in estimating the design strength of real adobe
structures.

Figure 12 presents the typical stress-strain relationships of reinforced and unreinforced adobe. The
failure of unreinforced adobe is due to brittleness asobserved in past earthquakes. The failure of hemp reinforced adobe is also due to brittleness. On the con13

MOHAMMAD SHARIFUL ISLAM , K. IWASHITA

ness of adobe block.

Axial stress, σa (kPa)

1600
1200

Unreinforced
Jute
Hemp
Straw

Figure 13 shows the failure pattern of unreinforced and reinforced specimens. The photographs
show that the unreinforced adobe block has been
separated into two parts (Fig. 13a). But the reinforced
adobe blocks were not separated completely (Figs.
13b~13d). Fibers in the shear band protect the separation. However, parts of the hemp reinforced adobe
specimen could be separated by a small push of thehand at the shear band. A closer view of the shear
band of the jute reinforced adobe (Fig. 13e) reveals
that jute resists the failure of the shear band and transfers the load across the opening. In this way, fiber
increases the ductility of adobe block.

800
400
0
0.0

1.0
2.0
Axial strain, ε (%)

3.0

aFig. 12 Stress-strain relationships of fiber reinforced
adobe.
trary, jute reinforced adobe shows ductile behavior as
observed in the case of straw reinforced adobe. However, although both the jute and straw are effective
for improving the ductility of adobe, the compressive
strength of the straw reinforced adobe is significantly
lower than that of the jute reinforced version. Again,
the toughness of the jute reinforced adobe (13.0~14.8kPa) is higher than that of unreinforced adobe(7.6~10.3 kPa) and other reinforced cases (6.6~9.6
kPa). It seems that jute is the most effective among
the selected fibers to improve the ductility and tough-Jute is flexible and can bind with the soil easily
which gives higher elasticity and tensile strength tothe matrix. It helps the matrix to prevent the development of micro-cracks due to drying shrinkage as happened in the straw reinforced case. Again, the frictionof jute to soil is higher than that of straw and hemp.
These are the reasons for the higher strength of jutereinforced adobe block. Since jute is effective for im-

（a）

（b）

（c）

（d）
（e）
Fig. 13 Photograph showing failure pattern of: (a) unreinforced adobe; (b) straw reinforced adobe; (c) hemp reinforced adobe; (d) jute reinforced adobe and (e) close view of shear band of jute reinforced adobe.
14EARTHQUAKE RESISTANCE OF ADOBE REINFORCED BY LOW COST TRADITIONAL MATERIALS

proving the ductility with a slight decrease in strength,
jute fiber reinforced adobe has significantly higher
toughness than the unreinforced type. It is clear thatfiber like jute with high flexibility, friction and tensile
strength is the most suitable fiber for improving theductility and toughness of adobe block.

observations, it is clear that 2.0% jute by weight is
optimum for improving the ductility and toughness of
adobe block.
Strength and toughness of unreinforced adobe
mainly come from the soil to soil friction and cohesion between soil to soil. But in the case of the fiber
reinforced type, they come from the composite bonding of soil with jute and friction between soil to fiber
as well as fiber to fiber. After 2.0% jute content, thebonding between fibers might become prominent,
which is weaker than the bonding between soil tofiber and soil to soil. This means that the composite
matrix of the soil and jute fiber up to 2.0% is optimum for improving the toughness of adobe block.

Since jute is the most effective for improving theseismic resistance of adobe material, the influence of
jute content and size in length on the seismic performance of adobe was investigated.

μ

20

D

1000
10

500
0

（a）

0
0.0

1.0

2.0

3.0

1500

20
15
10

500

5

0

0
0.0

4.0

1.0

2.0

3.0

Jute length, JL (cm)
40

20

（b）

10

Toughness (kPa)30

Toughness (kPa)

μD

1000

Jute content, JC (%)

（b）

qu

D

D

1500

25

2000

Ductility, μ30
quComp. strength, qu (kPa)

2000(2) Jute Length
To investigate the effect of the jute length, specimens were prepared using 2.0% jute for different
jute lengths using the ‘soil-sand mixture’. Jute length
varied from 0.5 to 3 cm. Figure 15a presents thedependency of the compressive strength and ductility on jute length. Figure 15b shows the variation of
toughness with jute length. The compressive strengthof the jute reinforced adobe is lower than that of theunreinforced adobe. But the compressive strength ofDuctility, μ（a）Comp. strength, qu (kPa)(1) Jute Content
Specimens were prepared using 1 cm long juteby varying the jute content from 0.5 to 4.0% by
weight. Figure 14a shows the variation of compressive strength and ductility on jute content. Compressive strength decreases with the increase of the jutecontent. However, the rate of decrease in strength beyond 2.0% jute content is not remarkable. Again, we
see that ductility increases with the increase of jutecontent. But the rate of improvement after the 2.0%
jute content is very low. Figure 14b shows the variation of toughness with jute content. The toughness increases with the increase of the jute content up to 2.0%
and after that toughness decreases. From the above

30
20
10
0

0
0.0

1.0

2.0

3.0

0.0

4.0

Jute content, JC(%)

1.0

2.0

3.0

Jute length, JL (cm)Fig. 14 Dependence of: (a) compressive strength andductility; and (b) toughness on jute content(jute length= 1 cm).

Fig. 15 Dependence of: (a) compressive strength andductility; and (b) toughness on jute length(jute content= 2.0%).
15

MOHAMMAD SHARIFUL ISLAM , K. IWASHITA

the jute reinforced adobe does not change with theincrease of jute length. Both the ductility and toughness increase with the increase of jute length up to
2 cm and after that both the ductility and toughness
decrease. Therefore, to obtain the maximum ductility
and toughness the length of jute should be 2 cm. The
mechanism behind this phenomenon is yet to be clarified. However, the size in length might depend on thesize of the block.

sum gives cohesion to the soil and that’s why strength
increases. Figure 16b shows a comparison of the typical stress-strain relationship of specimens containing
1.5% straw and specimens containing 1.5% straw as
well as 13.0% gypsum together. For both cases, thestraw length is 3 cm.
The strength of the specimens containing gypsum and straw together is higher than that of the specimens containing straw only. There was no crack inthe specimens containing gypsum and straw together.
But the specimens containing only straw had significant cracks (Fig. 9c). This means that the addition of
gypsum prevents cracking of adobe block. This might
be the reason for the higher strength. Moreover, adobe
reinforced with both the straw and gypsum together
might be more accepted by the user from an aesthetic
viewpoint since there is no crack on the surface ofsuch specimen.

15

μD

1000

10
500

5

（a）

0
0

3

6

9

12

Comp. strength, qu (kPa)

qu

D

20

1500

0
-3

3.2.2 Effect of Clay Content
In the previous sections, the effect of different
fibers and gypsum was described. Jute is found to be
the most effective among the selected fibers. In thiscontext, it is necessary to clarify the effect of jute fiber

Ductility, μ（a）Comp. strength, qu (kPa)

3.2 Strength of Adobe Block3.2.1 Effect of Gypsum
It is found that fibers are effective for improving the ductility but not the strength of adobe. Thissection presents the effectiveness of gypsum. Specimens were prepared by varying the gypsum content
0.0 to 13.0% by weight. Variation of the compressive
strength and ductility with the gypsum content isplotted in Fig. 16a. With the increase of the gypsum
content, compressive strength increases but there is
no improvement in the ductility. Therefore, gypsum
is effective for improving the strength characteristics
only. In practical terms, this means that by using gypsum stabilized mud bricks, the thickness of the outerload bearing walls can be reduced substantially. Gyp-

2000
BA

1500

KA

1000
500
0

15

0.0

Gypsum content, GC (%)

GC= 13.0%, SC= 1.5%

400

（b）

200

0
0.0

2.0

3.0

4.0

40
GC= 0.0%, SC= 1.5%

Toughness (kPa)aAxial stress, σ (kPa)

600

（b）

1.0

Jute content, JC (%)

BA
KA

30
20
10
0

2.0

4.0
6.0
8.0
Axial strain, ε (%)

10

0.0

1.0

2.0

3.0

4.0

Jute content, JC (%)

aFig. 16 (a) Dependence of compressive strength andductility on gypsum content and (b) comparison ofstress-strain relationship (GC= gypsum content;
SC= straw content).

Fig. 17 Comparison of dependence, of, (a) compressive
strength and (b) toughness on jute content
for Bentonite Adobe (BA) and Kaolin Adobe(KA) [jute length= 1cm].
16

EARTHQUAKE RESISTANCE OF ADOBE REINFORCED BY LOW COST TRADITIONAL MATERIALS

on adobe with different soil compositions. Specimens
were prepared using Kaolin clay instead of Bentonite
clay. This means that specimens were prepared bymixing Acadama clay, Toyoura sand and Kaolin at theratio of 2.5:1.0:0.6. Figure 17a compares the strengthof ‘Bentonite Adobe’ (BA) and ‘Kaolin Adobe’ (KA).
The strength of BA is significantly higher than thatof the KA. On the contrary, the failure strain of KA
is higher than that of BA (not presented here). The
variation of toughness (Fig. 17b) with jute content
shows that in both cases, specimens containing 2.0%
jute have the maximum toughness. It is clear that 2.0%
jute by weight is optimum for improving the seismic
performance of adobe block. However, it is also clearthat soil composition has a significant effect on theproperties of adobe material. Therefore, suitable soil
selection is very important for the earthquake resistantperformance of adobe. The difference in the properties might be due to the difference in plasticity (Table1). Further investigations are necessary to clarify thiseffect.

3.3 Bonding between Block and Mortar
The improvement of bonding between blockand mortar is the key issue for the improvement ofseismic resistance of adobe structures. The effect ofjute and cement on mortar properties evaluated byuniaxial compression tests is presented in this section.
Compositions, physical properties and test results ofthe sandwich specimens are presented in Table 2 andFig. 18.
3.3.1 Effect of Jute
Both block and mortar of the specimens of the
MJ-1 group are unreinforced. The jute content of
the block part of the specimens of the MJ-2 group is
0.5% by weight but the mortar is unreinforced. Boththe block and mortar part of the specimens of the
MJ-3 and MJ-4 groups contain 1.0% and 2.0% jute byweight, respectively. The effect of jute on the sandwich specimen was investigated up to 2.0% by weightsince it was found optimum for block (Section 3.1.2).
The compressive strength of the unreinforced
block and sandwich specimen is 1,180 kPa and 33
kPa, respectively. This means that the strength of
the unreinforced sandwich specimen is significantly
lower than that of the block specimen. The strength of
the specimens of the MJ-2 and MJ-3 groups is higher
than that of the unreinforced specimens (MJ-1). However, the strength of these groups (i.e., MJ-2 and MJ3) is also significantly lower than the block strength.
The compressive strength and failure strain of the
sandwich specimen of the MJ-4 group are 560 kPa
and 2.43%, respectively. Although the compressive
strength of the sandwich specimen of this group (MJ4) is less than that of the unreinforced block strength,
both the ductility (6.2~13.7) and toughness (9.10~16.5kPa) of this sandwich specimen are higher than that

Axial stress, σa (kPa)

600
MJ-1
MJ-2
MJ-3
MJ-4
MC-1
MC-2

400

200

0
0.0

2.0

4.0

6.0

Axial strain, εa (%)

Fig. 18 Stress-strain relationships of sandwich
specimens (jute length= 1 cm).

Table 2. Description and characteristics of sandwich specimens
Group

Block（%）

Mortar（%）

Jute

Jute

CementStrength（kPa）

Failure strain
（%）

MJ-1

―

―

―

26 〜 400.94 ~ 1.38

MJ-2

0.5

―

―

63 〜 74

1.00 ~ 1.32

MJ-3

1.0

1.0

―

92 〜 175

1.38 ~ 2.15

MJ-4

2.0

2.0

―

455 〜 703

1.88 ~ 2.80

MC-1

1.0

―

9.0

326 〜 482

0.70 ~ 0.80

MC-2

1.0

1.0

9.0

150 〜 262

1.25 ~ 1.65

17

MOHAMMAD SHARIFUL ISLAM , K. IWASHITA

of the block. Thus by adding 2.0% jute in both theblock and mortar, the earthquake resistance of adobe
structures can be improved.

coherence between block and mortar. These might explain the better performance of jute reinforced mortar.
3.3.2 Effect of Cement
The block part of the specimens of the MC-1 andMC-2 groups (Table 2) contains 1.0% jute by weight.
But the mortar part of the MC-1 contains only 9.0%
cement (by weight) while the mortar part of the specimens of the MC-2 group contains 1.0% jute and 9.0%
cement (by weight) together.

Photographs of Fig. 19 show the failure patternof the sandwich specimens. Many cracks are seen in
the unreinforced mortar of the specimen of the MJ-1
group (Fig. 19a). The specimen of the MJ-2 group had
similar cracks. Cracks in the mortar are the main reason for the low strength and poor bonding of unreinforced mortar. On the contrary, however, there are no
cracks in the jute reinforced mortar of the specimenof the MJ-4 group (Fig. 19b). Similarly, the mortar of
the specimens of the MJ-3 group had no cracks. Thismeans that jute is effective for preventing the shrinkage cracks in mortar. The failure surface and patternof the jute reinforced mortar (MJ-4) indicate thatsome parts of the mortar came out during failure. If
the bond/cohesion between block and mortar is weak,
separation between block and mortar occurs. But if
the bond is strong, composite failure (i.e., failures
along the interface between mortar and block, and/or within mortar) occurs. In this case, both failures
along the interface between mortar and block, andwithin mortar have occurred, indicating that the bond
between the block and mortar was stronger in thecase of MJ-4. Thus, the addition of fibers in mortar
provides better coherence between block and mortar.
This might account for the difference in the failure
pattern of MJ-4 and others. This phenomenon can beobserved clearly from the stress-strain relationshipof the specimens of the MJ-4 group (Fig. 18). The
stress-strain relationships of the specimen of the MJ-4
group are different from the others. The specimen of
this group can continue with the peak strength by increasing the strain up to a certain limit and after thatthe strength decreases suddenly. Before bond failure(between block and mortar) the block-mortar system(sandwich specimen) acts as a single system. Whenthe bond between the block and mortar starts to fail,then it goes for immediate failure. The peak of MJ-4
remains constant until the bond failure initiates.

The compressive strength of the sandwich specimen of the MC-1 group (410 kPa) is significantly
higher than that of the unreinforced mortar (33 kPa).
This means that cement is effective for improving the
mortar strength. But the failure strain is significantly
lower (0.70~0.80%) than that of the unreinforced version (0.94~1.38%) and the failure is sudden (Fig. 18).
Many cracks are observed in the mortar of this group(Fig. 19c) indicating that cement cannot prevent
shrinkage cracks in the mortar. Continuous failure occurred through the mortar in this case. Cement gives
cohesion to the matrix and that is why the strength
increases. But poor coherence between block andmortar is responsible for the brittle failure.
The compressive strength of the specimens of
the MC-2 group is 196 kPa, which is higher than thatof the unreinforced mortar strength. But the strengthis significantly lower than the cement reinforced
mortar. However, the failure strain is higher thanthat of unreinforced and cement reinforced mortar.
Figure 19d shows that there is no crack in the mortarof the specimens of the MC-2 group. Thus jute withcement is also effective for preventing cracks in themortar. However, the strength of the MC-2 group is
expected to be higher than that of MC-1. The reason
for the lower strength of MC-2 might be due to thelack of proper hydration of the cement. To make thespecimens, the same amount of water was added tomake the slurry. Since the specimens of the MC-2group contain both jute and cement, both jute and cement absorbed water from the matrix. Therefore, theremaining water (after the saturation of jute) might
not be sufficient for the complete hydration of cementwith time.

From the above observations and discussions, it
is clear that the addition of 2.0% jute by weight to theblock and mortar is the most effective way of improving the ductility and toughness of sandwich specimens. Jute prevents shrinkage cracks and provides

From the above discussion, it is clear that ce18

EARTHQUAKE RESISTANCE OF ADOBE REINFORCED BY LOW COST TRADITIONAL MATERIALS

（a）

（b）

（c）
（d）
Fig. 19 Photographs of sandwich specimens after failure, for Group: (a) MJ-1 (block and mortar: unreinforced),(b) MJ-4 (block and mortar: reinforced by 2.0% jute), (c) MC-1(block: 1.0% jute; mortar: 9.0% cement)and (d) MC-2 (block: 1.0% jute; mortar: 1.0% jute and 9.0% cement).
ment is effective for improving the strength of mortar.
But cement is not effective for preventing crack or
improving the ductility of the sandwich.

(2) Straw is effective for improving the ductility of
adobe at the cost of compressive strength. However, there is an optimal quantity (i.e., 1.5% byweight) and length of straw (i.e., 2 cm) to improve the ductility of adobe block. The strengthof adobe block prepared with crushed straw ishigher than that of the specimens prepared withwhole straw. Straw in the shear band transfers
the stress across the crack and prevents thecrack from opening. Hence, straw improves theductility. Reduction in the friction between soil
to soil due to the replacement of the soil-sandpart by straw and micro-cracks that were causedby straw are responsible for the reduction ofstrength. Since the cracks in the adobe blockincrease with the increase of both the straw
content and length, strength decreases with theincrease of both the straw content and length.
The reason behind the better performance ofcrushed straw might be its higher flexibility,
which helps to prevent the shrinkage cracks inthe adobe block.
(3) Jute is also effective for improving the ductility

4. CONCLUSIONS
The development of cost effective solutions for
improving the seismic resistance of adobe structures
is an important and challenging issue for earthquake
engineers to reduce the earthquake hazard of poor
people in developing countries. Natural fibers (straw,
hemp and jute) and cementing materials (cement
and gypsum) were selected to improve the block andmortar properties of adobe. Uniaxial compression
tests were conducted on cylindrical blocks and sandwich specimens in order to evaluate the effect of the
proposed reinforcing materials on block and mortar,
respectively. The main conclusions obtained are:(1) Compressive strength decreases with the increase of the water content without affecting
the failure strain. So, the control of humidity
content of adobe may significantly contribute totheir resistance in seismic conditions.
19

MOHAMMAD SHARIFUL ISLAM , K. IWASHITA

while hemp is not effective for improving theductility. Jute seems to be the most effective
among the selected fibers since jute improves
both the ductility and toughness of adobe material with slight decreases in compressive
strength. There are also an optimal jute content
and jute length, which are 2.0% by weight and 2
cm, respectively. The higher flexibility, friction
to soil, and the tensile strength of the jute fiberin comparison with those of straw and hemp
might be the reason for its best performance
among the selected fibers.
(4) Gypsum improves the strength of the adobe
block. Gypsum gives cohesion to the matrix andthus the strength increases. Soil composition
also plays a significant role in the seismic resistance of adobe. Both the compressive strengthand failure strain are dependent on the soil composition.
(5) The strength of the ‘block and mortar sandwich’is significantly lower than that of adobe block.
Cracks and poor bonding of unreinforced mortar are the main causes for its poor performance.
Cement improves the strength of the mortar
significantly but cannot improve the ductilityof the sandwich specimen. Jute is effective for
improving both the strength and ductility of the
sandwich specimen. Jute prevents the shrinkage
cracks in the mortar and the sewing action of
jute fibers gives better bonding between blockand mortar. These are the mechanisms behindthe better performance of jute reinforced mortar.
(6) Finally, natural fibers like jute and cementing
materials like cement or gypsum can be added
to block and mortar to improve the earthquake
resistance of adobe structures. The cost of jutereinforcement for a standard adobe house isabout 30 US dollars, which is affordable to low
income people in developing countries.

water into adobe walls, which is under investigation.
However, this study focuses only on the improvementof adobe block and mortar properties. Further investigations are being conducted to improve the structural
integrity using low cost materials (bamboo, cane, andjute mesh).
ACKNOWLEDGEMENTS
The first author is grateful to Prof. H. Watanabeof Saitama University, Japan for his guidance. This
research was partially supported by the Japan Society
for the Promotion of Science (JSPS), Grant-in-Aid
for JSPS Fellows, Grant Number: 17005291. The authors are also grateful to the Iranian Cultural Heritage
Organization for providing old adobe. They express
their deep gratitude to Mr. Honda, Mr. Kin and Mr.
Kamata, former graduate students of Saitama University for their participation during tests.
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