evaluation of strawbale as a building material
DESCRIPTION
A FINAL YEAR THESIS ON THE USE OF STRAWBALE AS A BUILDING MATERIAL, TROPICS AS A CASE STUDY. WRITTEN BY LAWALE FAWALE, ET ALTRANSCRIPT
EVALUATION OF THE EFFECTIVENESS OF STRAW BALE IN
BUILDING CONSTRUCTION
BY
FAWALE, BABATUNDE LAWALE
(041434)
AJAYI, JOHNSON OLUWAFEMI
(041108)
OYEDEMI, PETER OLUWATOSIN
(032723)
BEING A PROJECT
SUBMITTED TO
DEPARTMENT OF CIVIL ENGINEERING
FACULTY OF ENGINEERING AND TECHNOLOGY
LADOKE AKINTOLA UNIVERSITY OF TECHNOLOGY,
OGBOMOSO OYO, STATE.
IN PARTIAL FULFILMENT OF THE AWARD OF BACHELOR OF
TECHNOLOGY (B.TECH) IN CIVIL ENGINEERING
OCTOBER , 2008.
1
CERTIFICATION
This is to certify that this project was carried out by AJAYI
JOHNSON OLUWAFEMI (041108), FAWALE BABATUNDE LAWALE,
(041434), OYEDEMI, PETER OLUWATOSIN (032723), of the department
of Civil Engineering, Ladoke Akintola University Technology, Ogbomoso,
Oyo State.
……………………. …………………….
Supervisor Date
Mr. O. Ajamu
……………………. …………………….
Head of Department Date
Engr. O.S. Oladejo
2
DEDICATION
This project is dedicated to Almighty God for His guidance and
inspiration from the beginning of this project to the end.
3
ACKNOWLEDGEMENT
We wish to express our sincere gratitude to our parents Chief & Mrs.
Fawale, Chief and Mrs. D. O. Ajayi and Mr. And Mrs. Thomas Oyedemi for
their relentless efforts towards the success of this project and throughout our
undergraduate programme.
Our profound gratitude also goes to our able and dynamic supervisor
Mr. S. O. Ajamu for His attention, supports contribution in diverse ways
towards making this project a successful one.
We wish to also appreciate Dr. A.A. Adegbola (Dean Faculty of
Engineering) for His intervention on our project. Location, Dr. A. A.
Adedeji, Engr. S. O. Ojoawo, Engr. Oladejo, Engr. A.P. Adewuyi, Dr. A.
Raheem, Engr. Osuolale, Mr. Akinleye and Mr. Abogunde for their support
in different capacity towards this project.
We are also indebted these people: Mr. Gideon Bamigboye, Mr. Julius
Odetunde, Elder & Mrs. Solademi, Barr. Awe, Engr. Adedeji Sanmiwo,
Bello Taofeek, Tunmise, Adedoyin.
We cannot but appreciate our siblings, cousins and friends for their
prayers and moral supports. You are too much!
4
TABLE OF CONTENT
TITLE
PAGE………………………………………………………………….. i
CERTIFICATION…………………………………………………. ..ii
DEDICATION…………………………………………………………iii
ACKNOWLEDGEMENT………………………………………….. iv
TABLE OF CONTENT…………………………………………………..v
LIST OF TABLES ……………………………………………………vi
LIST OF FIGURES …………………………………………………. vii
LIST OF PLATES …………………………………………………… ix
ABSTRACTS ……………………………………………………………x
CHAPTER ONE
1.0 INTRODUCTION………………………………………………..1
1.1 BACKGROUND OF THE STUDY ….…………………………..1
1.2 AIMS AND OBJECTIVES………………………………………4
1.3 SCOPE……………………………………………………………5
1.4 METHODOLOGY……………………………………………….5
CHAPTER TWO
2.0 LITERATURE REVIEW ……………………………………….6
2.1 HISTORICAL BACKGROUND ………………………………6
5
2.2.0 MATERIAL PROPERTIES ……………………………………7
2.2.1 THERMAL INSULATION ………………………………………7
2.2.2 SOUND INSULATION ………………………………………..8
2.2.3 STRUCTURAL STRENGHT ………………………………….8
2.2.4 FIRE RESISTANCE ……………………………………………..8
2.2.5 MOISTURE RESISTANCE ……………………………………9
2.2.6 PEST RESISTANCE …………………………………………….10
2.2.7 COST AND AVAILABILITY…………………………………10
2.3 SRAW BALE CONSTRUCTION PROCESS ………………..10
2.3.1 FOOTING ……………………………………………………..11
2.3.2 BALE STACK UP …………………………………………….12
2.3.3 STRAW BALE WALL HEIGHT ……………………………..13
2.3.4 PLASTER ………………………………………………………14
2.3.5 PINNING ………………………………………………………16
2.4 BASIC TESTS ON STRAW BALE …………………………..16
2.4.1. THERMAL INSULATION TEST RESULT ………………….16
2.4.2. TEST ON PLASTERED BALE ……………………………….18
2.5.0 APPLICATION ………………………………………………..19
CHAPTER THREE
3.0 METHODOLOGY ……………………………………………...21
6
3.1 COMPRESSIVE STRENGHT TEST …………………………..21
3.1.1 UNIT DENSITY AND DRY UNIT WEIGHT …………………21
3.1.2 THERMAL INSULATION TEST ……………………………..21
3.2.0 CONSTRUCTION OF PROTOTYPE PLASTERED STRAW
BALED WALL……………………………………………………….22
3.2.1 MATERIALS USED FOR CONSTRUCTION ………………….22
3.2.2 MATERIALS QUALITY AND ACQUISITION …………….…23
3.3.0 CUTTING AND BAILING OF STRAW ……………………..24
3.4.0 BATCHING …………………………………………………25
3.4.1. MORTAL MIXING ……………………………………………25
3.5.0. MOULD FOR STRAW BALE PLASTER …………………….26
3.5.1 SRAW BALE WALL LYING …………………………………….28
3.5.2 PLASTERING OF SRAW BALE ………………………………28
3.5.3 PLASTERED STRAW CURING ………………………………29
3.5.4 ROOFING OF PROTOTYPE BUILDING …………………….. 30
3.6.1 SCALE FACTOR FOR PLASTERED STRAW BALE…………32
3.6.2 COST AND AVAILABILITY ………………………………….32
CHAPTER FOUR
4.0 RESULTS …………………………………………………….. 33
4.1.0 RESULTS ON PLASTERED STRAWBALE PRISM……….. 33
7
4.1.1 UNIT DENSIY OF PLASTERED STRAWBALE PRISM….. 33
4.1.2 COMPRESSIVE STRENGTH TEST……………………….. 36
4.2.0 THERMAL INSULATION TEST ON PROTOTYPE
PLASTERD STRAWBALE BUILDING AND PLASTERED
SANCRETE BLOCK BUILDING ……………………………. 38
CHAPTER FIVE
5.0 CONCLUSION AND RECOMMENDATION …………….. 40
5.1 CONCLUSION ……………………………………………. 40
5.1.1 UNIT DENSITY TEST ……………………………………… 40
5.1.2 COMPRESSIVE STRENGTH TEST………………………….. 40
5.2.0 RECOMMENDATIONS ………………………………………. 41
REFERENCES 47
8
LIST OF TABLES
Table 2.1 Comparism of U valve of strawbale with other
conventional wall materials 17
Table 2.2: Result of analysis for maize and sorghum 17
Table 4.1: Unit density of plastered Strawbale prism 33
Table 4.2: Compressive Strength test results on plastered strawbale
Prisms 36
Table 4.3: Inside temperature (0C) of strawbale and sandcrete block
building models exposed to similar conditions 38
9
LIST OF FIGURES
Figure 4.1: Graph of Average Density against plaster mixtures - 34
Figure 4.2: Compressive Strength versus plastered mixtures - 37
Figure 4.3: Relationship between inside temperatures of
Strawbale and sandcrete block models - 39
10
LIST OF PLATES
Plate 1: Building with Rammed tyre footing - 12
Plate 2: Strawbale Building Stacked - 13
Plate 3: A – 3 Storey Strawbale Building - 14
Plate 4: Cement Plastered Strawbale Building - 15
Plate 5: A retrofit building (Porta cabin/office) - 20
Plate 6: Plastered Strawbale Prism under a compression
Machine - 21
Plate 7: Hot bucket of water with thermometer inside
Sandcrete block building - 22
Plate 8: Hot bucket of water with thermometer inside
Strawbale building - 22
Plate 9: Stock of Guinea Corn (Straw) - 22
Plate 10: Strawbale prism mould - 24
Plate 11: Foundation Footing - 26
Plate 12: Laying of Foundation Footing - 27
Plate 13: Stacking of strawbale building - 27
Plate 14: Casting of plastered strawbale prisms - 27
Plate 15: Sandcrete Block - 29
11
Plate 17: Plastered prototype strawbale & Sandcrete building -
30
Plate 16: Roofed Prototype buildings - 30
Plate 18: Casting of Plastered Strawbale prisms - 30
Plate 19: Global Buckling failure mode of plastered
strawbale prism - 35
12
ABSTRACT
This project is an evaluation of the effectiveness of strawbale in
building construction.
It introduced the use of straw, which when baled could be used as a
resource, that is, as a walling material, more economically than other
conventional walling materials.
This study shows the edges that strawbale has over other conventional
walling materials (sandcrete block), thermal insulating property, availability
ease of construction, economical amongst others.
The minimum plaster thickness (coating) which when applied to the
strawbale wall that can give the optimum strength was found to be 15mm
this is obtained from compressive strength test.
It also displays the graphical representation of the thermal insulation
tests carried out on the two prototype buildings (strawbale and sandcrete
block), in which strawbale building retained more heat than the sandcrete
block building.
13
CHAPTER ONE
1.0 INTRODUCTION
1.1 BACKGROUND OF THE STUDY
The need to construct buildings with viable low cost material has
become a necessity in our fast growing society. It goes without saying that
the growing population of the people of Nigeria (most populous black nation
on earth) and the escalating cost of conventional building construction
materials is now a problem in the housing sector. With this in mind, there is
need to look for alternative, which have to be cost effective and readily
available. Hence, the need for this study
A material that meets up with such requirements is straw bale. Straw
bale construction is a method of building that incorporates the use of straw
bales as structural elements. It has been found to be suitable for living
houses, offices and other purposes.
Straw is the springy tubular stalk of grasses like wheat, rice, sorghum
that are high in tensile strength. It is not hay, which is used for livestock
feeding and includes the grain head. Straw is composed of cellulose, hemi-
cellulose liquid and silica. It is non rigid it is flexible and easy to work with.
Bales are masses of straw compressed into rectangular blocks and
bound with steel wire or propylene twine.
14
The typical straw bale is bound with two or three strings of
polypropylene twine though sometimes wire or fibre twine is used, and it is
relatively easy for one man or woman to handle. In other words bales as
farmers have always made them are generally just fine for construction. The
biggest concern for bale holders is the type of combine used to harvest the
straw -conventional combines that leaves long straw fibres (which is good
for building), and rotary combines that chop the straw into short fibre (which
makes for unstable crumbly bales).
With the increased industrialization of farms, other types of the bales
are beginning to predominate in many areas:
1. Jumbo bale: Rectangular blocks bound with six to ten string of a
typical size like 3ft x 4ft x 8ft (1m x 1.3m x 2.2m), which can only be
handled by mechanized equipment.
2. Circular bales: Disks bound with twine of typical dimension 3ft thick
and 6ft in diameter. Also machine handled.
3. Super compressed bales: Ordinary bales compressed to roughly twice
the original density.
There are two primary ways of building with straw bales.
i Load bearing or Nebraska style.
ii Post and beam / In - fill / non- load bearing method
15
In load bearing straw bale construction, bales are stacked and
reinforced to provide structural walls that carry the roof load. While in post
and beam method, a wood, metal or masonry structural frame supports the
roof and bales are stacked to provide non- structural in insulating walls.
With either alternative, the bale walls are plastered or stuccoed on both the
interior and exterior of the wall. The study will be based on the former.
The load bearing straw bale construction employs relatively simple
techniques that are forgiving to novice builder and yet have sufficient
flexibility to allow the creation of design features such as curved walls.
Moreover, a straw bale demonstrates excellent insulative properties
such as thermal, sound and fire resistance. Also the structural capacity of the
straw bale construction is surprisingly good.
It should be added that the technology of straw bale construction is
still rapidly evolving .It is highly rated for ‘buildability’ because it can be
very straightforward. And it is important to keep bales during storage and
construction and to try and eliminate vermin. During construction, tarpaulins
or plastic sheets should be kept ready for covering otherwise up protected
walls. Although it may not be dried if bales do get wet slightly, they can
often be dried out sufficiently to be usable.
16
Lastly, it has been found that straw bale walls are very resilient and in
the event of damage they can be repaired. Wet bales can be taken out and
replaced. (Harvest Homes; 2004)
1.2 AIMS AND OBJECTIVES
i. To evaluate the Compressive Strength of straw bale as a walling
material. (Plastered straw bale wall and unplastered straw bale wall).
ii. To assess the thermal insulating property of Straw bale in building
construction.
iii. To carry out comparison of thermal insulating property between straw
bale and sandcrete blocks.
iv To establish the ease at which straw bale material can be used in
building construction.
v. To make appropriate recommendation.
17
1.3 SCOPE
- Evaluation of the strength of straw bale (load – bearing type) in
building construction.
- Determination of the plaster thickness that can be applied on straw
bale wall.
- Assessing thermal Insulation as a property in straw bale building
construction.
- Comparative evaluation of thermal insulating property of straw bale
and sandcrete block.
- Comparison of the cost of using straw bale in building construction to
sandcrete block.
1.4 METHODOLOGY
Basically, this research work will require getting samples of straw
from local source and compressing it to form bale. Comparative test will be
carried out by constructing a prototype enclosed wall using straw bale and
sandcrete block; thereby authenticating basic properties such as strength,
thermal insulation, fire resistance e.t.c
Thus, further search will be made to evaluate the affordability of straw
bale in terms of its cost.
18
CHAPTER TWO
2.0 LITERATURE REVIEW
2.1 HISTORICAL BACKGROUND ON STRAW BALE
Straw is one of the finest renewable building materials available and
is found around the world in abundance. It is the strong stalk of tall grain
plants such as guinea corn, maize, wheat, hemp, rye or rice that remain in
the field after the seed grains have been harvested.
Its chemical composition is primarily cellulose, just like trees. When
bundled together into a bale it becomes. Solid block that is highly resistant
to decomposition. When assembled together and covered with a plaster skin,
straw bales make a beautiful, strong, energy – efficient and ecologically
sound house.
Early settlers of the grasslands of Nebraska and Western Canada
discovered the beauty of building with straw bale over 100 years ago.
Necessity is the germ of their discovery. Confronted with cold winters a
lacks of trees and a need for a quickly built out – building, the settler turned
to the abundant straw to house their animals. They piled the bales, put on a
roof and added earthen plaster. The result was superb. The insulation against
the cold was found and exceeded any wood building. They then proceeded
to build their homes with straw bales too. In fact, many of the original
19
settlers’ farm buildings and homes are still intact standing as a testament to
the materials strength and longevity. (Harvest Homes coy, 2003).
Contemporary builders are waking up to its virtues too. With every year,
people around the world are discovering the benefit of buildings with the
straw bales (Harvest Homes Company Journal, 2003.).
2.2.0 MATERIAL PROPERTIES
The choice of building construction materials is governed largely by
the properties of the materials that are pertinent to what the builder has in
mind. Straw bale as a construction material is not an exception.
2.2.1 THERMAL INSULATION
Straw bale demonstrates excellent insulative properties; it is possibly
the most cost effective thermal insulation available. Straw has a similar
insulation value to fiberglass batts. Straw bales wall has insulation value that
greatly exceeds that of any conventional construction. Downton (2003).
Likewise, Adedeji (2007) said ‘all new building must be energy efficient
with straw walls; the insulation (straw) is also the building block.’
Also, Lee (2001), said “Straw compacted into bales offers much better
insulation with its high thermal resistance”.
20
2.2.2 SOUND INSULATION
Paul Downton (2003),stated that ‘the effect of sound insulation
contributes to the livability of straw bale construction and can be quite
marked. Even walking into the space created by an unfinished straw bale
structures, one can appreciate the quietness and hear the difference
compared with conventional building.
In the same vein, Adedeji (2007) said “there is over-whelming
experimental evidence that straw walls offer far more sound insulation than
20th century wall building materials”.
2.2.3 STRUCTURAL STRENGTH
Unlike wood from construction that has low tolerance to extreme
lateral and lift force such as tornadoes or earthquakes. Straw bale building’s
continuous monolithic plaster provides extra ordinary strength and elasticity
in the face of such force. They can actually flex and bend, but resist collapse
making straw bale building ideal for quake, hurricane or tornado prone
areas. (Harvest Homes, 2003).
2.2.4 FIRE RESISTANCE
Straw bale are lightly packed and covered with a skin of cement
render. Fire cannot burn without oxygen and the dense wall provides a
nearly airless environment. So the fire resistance of compacted straw is very
21
good. Conclusive evidence of its good fire resisting performance can be
found in laboratory fire tests conducted at the Richmond field station in
1997 by students at University of Berkeley. These rated a straw wall at 2
hours. Straw bale home survived Californian burn fire that destroyed
conventional building. (Downton, 2003).
Adedeji (2007), said it is popular misconception that straw bale
buildings are a fire risk. In a fire, its chars on the outside and then the
charring itself protect the straw from further having. When the wall is
plastered in both sides, the risk of fire is reduced.
Compactness of the straw, combined with plaster coating does not
provide air to support combustion. Canada Mortgage and Housing Company
(C.M.H.C) demonstrated a lower fires risk when compared to conventional
construction materials and method, straw bale walls came out with 2 hours
or commercial fire rating (Harvest Homes, 2003).
2.2.5 MOISTURE RESISTANCE
Paul Downton (2003) profers that provided the straw is reasonably
well protected and is not allowed to become water logged, it can last many
years with moderate maintenance. Bales during construction should have
moisture content not exceeding 15% and not below 10%. Straw bales should
not get wet inside, should it be wet it will not be a problem since straw does
22
not wick water like concrete. If rain is driven into the sides of bales the
natural air movement around the bales is able to dry them out and this cycle
of wetting and drying does not damage the bale.
The California straw bale code (2001) asserted that the moisture content of
the bales at the time of installation should not exceed 20% of the total
weight of the bale.
2.2.6 PEST RESISTANCE
Straw is ideal for areas at high risk of termites since it does not suit
their palette. (Harvest Homes, 2003).
2.2.7 COST AND AVAILABILITY.
Straw bale is a low cost material. It is readily available of an
affordable cost. It is can be obtained from both near and far sources.
2.3.0 STRAW BALE CONSTRUCTION PROCESS
Straw bale construction is still a very much a developing technology,
nonetheless, a distinct trend has become apparent: There are two basic styles
of construction, the load bearing in which the weight of roots and entirely by
bales; and the post and beam straw bales is that in which bales are used as
in-fill panels between or around structural frame of any structural materials.
23
Emphasis will be laid on the former in this study. A little will be discussed
on the construction process.
2.3.1 FOOTINGS
A straw bale wall requires footings with a similar load – carrying
capacity to that required for a masonry wall, although a straw wall is lighter
(one mud brick weights about the same as a straw bale.). The footings
usually used are concrete strips or slabs. There have been successful
experiments with rubble trench and rubber tyre footings and there are several
straw bale buildings built on piers, beam and joist. (Paul Downton, 2003).
The California straw bale code (2001), has it that support for bale
walls shall extend to an elevation of at least six inches above adjacent
ground at all points, and at least one – inch above floor surfaces.
King (2001) opines that the bottom of the bale wall must be well separated
from the foundation and at the very moisture water- proof barrier should be
laid over moisture below. Additionally, many builders are placing a layer of
pea gravel between wood sill plates along the inside and the outside faces so
that bales will never be sitting in water.
Likewise, California straw bale code (2001) recommends that, a
moisture barrier shall be used between foundation top and the bottom of bale
24
walls to prevent moisture from migrating through the foundation so as to
come into contact with the bottom course of bales.
Plate 1: A building with rammed tyre footing
2.3.2 BALES STACK UP
Typically, the bales are stacked in running or stack bond on top one
another; early experiments with adding cement mortar between courses
created thermal bridges while giving no clear benefits. In load bearing
building, the bales are laid flat, that is with the longest dimension parallel to
the wall and the shortest vertical. In other application, the bales can be
stacked on edge that is with the shortest dimension horizontal. This saves
interior space with slimmer wall, and, interestingly appears to offer the same
net insulation value due to the slightly different orientation of the fibers.
(Bruce king, 2001).
25
In the same vein, the California straw code holds the view that in load
bearing walls of bale, bale should be laid flat and be stacked in a running
bond, where possible with each bale overlapping the two bales beneath it.
Overlaps shall be a minimum of 12 inches.
Plate 2: Straw bale building being stacked
2.3.3 STRAW BALE WALL HEIGHT
Australia straw bale expert’s recommends a maximum wall height of
2.5 meters when using standard sized bales in load bearing construction.
The California straw bale code (2001) has it that building with load
bearing bale walls shall not exceed one story height without substantiating
calculations and design by a California civil engineer, and the bale portion of
the load bearing walls shall not exceeds a height to width ratio of 5.6:1 (for
26
example the maximum height for a wall that is 23 inches thick would be 10
feet 8 inches or 3.27 meters).
Whereas, the harvest homes (2003) holds the view that “ given that
straw bale walls have been shown to be at least 4 times stronger than a
conventional 2x6 frame wall there are no special limitation to the heights of
a building.
Plate 3: A 3-storey straw bale building
2.3.4 PLASTER
The California straw code (2001) has it that minimum bale thickness
shall be 13inches. Also plaster can be lime-gypsum, lime – cement plaster.
Bruce king (2001) held the view that it is essential that, once plaster is
applied directly to either or both of the straw bale surfaces, the completed
27
wall assembly is now a hybrid of straw and plaster, in other words a
sandwich panel. Plaster is used generically to include traditional earthen
plasters, lime and gypsum plasters shot Crete or gucite, cement stucco.
Also, Paul Downton (2003) said “ that three layers of render should be
progressively ‘weaker’ to reduce the potential for cracking caused by having
too brittle an external layer.
Plate 4: Cement plastered straw bale building
2.3.5 PINNING
Straw bales are comparatively soft and do not behave like bricks.
Except when surrounded by a sturdy frame of post and beam the bales must
be braced or pinned during stacking for stability and alignment. Internal
pinned of the walls (with bamboo or dowels) has been prescribed in early
28
straw bale codes, but it as falling out of favour, for it is under whether
internal pins contribute appreciably to the strength of the finished wall
assembly. Bruce king (2001).
Likewise, Downton (2003) said ‘the vertical and horizontal stability of
straw bale walls generally needs to be guaranteed by tying bales to structural
frames or pinning between bales and structural elements, however there is a
growing consensus that the extensive use of reinforce steel bars and
excessive pinning that characterized early straw bale construction is not
necessary.
2.4.0. BASIC TESTS ON STRAW BALE
2.4.1 THERMAL INSULATION TEST RESULT
Adedeji, A (2007) has it that the u-value or thermal transmittance of a
material the amount of heat transmitted per unit area of the between inside
and outside environment. It is measured in watt per square meter per degree
of temperature difference (Kelvin) N/m2k. Simply put, it is a measure of
how much heat materials allows to pass through it. The lower the u – value,
the greater the insulation of the materials.
The tables below display the comparison of u-value of straw with
other materials and the result of analysis for different types of straw.
29
Table 2.1: COMPARISON OF U VALUE OF STRAW WITH OTHER
CONVENTIONAL WALL MATERIALS
MATERIALS U-VAL
105mm brick work, 75mm mineral fibre
100mm light concrete block, 13m light weight plaster
0.33
100mm heavy weight concrete block, 75mm mineral fibre,
100mm heavy weight concrete block, 13mm light weight plaster,
0.40
100mm heavy weight concrete block, 75mm mineral fibre,
13mm light weight plaster.
0.29
450mm straw wall
Source: A.A Adedeji, 2007.
Table 2.2 Result of analysis for maize and sorghum
Mean values (1, 2)
0C
Thermal conductivity
(Nmk-1)
Thermal receptivity Example
Thickness d(m)
Maize Sorghum Maize Sorghum Maize Sorghum
1. 0.016 51.00 50.65 0.090 0.1439 11.00 6.95
2. 0.034 50.85 54.73 0.085 0.0820 12.10 12.19
3. 0.038 51.30 52.58 0.082 0.0769 13.00 13.00
4 0.042 53.00 61.60 0.0083 0.0758 12.92 13.21
30
2.4.2 Test On Plastered Bale
Zhang John (2002) carried out “ Load carrying characteristics of a
single straw bale under compression. Two–string wheat bales were tested
flat and on edge, plastered and unplastered, in some cases under low
frequency cyclic loading.
Both cement and earthen plasters were tested, there were two weeks
between cement coats with average thickness of 1.6”. From the test, the
following were observed.
i. There is an initial set phase of the test in which the ‘fluff’ between
bales is compressed; the author identified this as 3 to 4% of height. In
other words, at least for this particular bales 3 to 4% of its height
before plastering.
ii. There is always a delayed effect on the recovery of the deformation as
the unplastered straw is unloaded.
David Mar. (2003), carried out test on the bearing capacity of
plastered straw bales.
Rice straw half- bales were fabricated and stacked flat giving a cross –
section of 23” x 23” plus 1.5” plaster skin each side. Plasters were cured at
least one month and load was applied via a stiffened plywood plate covering
the plaster edges; ultimate loads were recorded as follow.
31
Lime – cement stucco w/2” x2”x14 gauge mesh/avg of 3:2810lbs12.5ka)
High straw fibre earthen plaster w/ coconut fiber mesh /avg of
3:2340lbs10.4480). Low straw fiber earthen plaster w/coconut fibre mesh/
avg of 2 1575lbs(7kw).
2.5.0 APPLICATION
Today, straw bale building has become very popular. Everything from
garden, Sheds, cottages, studios, modest houses, grand mansions to large
commercial building can be found in most Canadian provinces united states,
Mexico, Australia, Europe and far beyond.
They are also ideal for retrofit applications. It has been used as wrap
on the exterior of old, poorly insulated houses or in the interior of large
commercial building for warehouses in order to dramatically improve energy
efficiently.
32
Plate 5 : A retrofit building (Porta Cabin/Office)
33
CHAPTER THREE
3.0 METHODOLOGY
3.1.0 UNIT DENSITY AND DRY UNIT WEIGHT
Weight of Six consecutive plastered straw bale prisms were measured
with weighing balance and the dry unit weight were determined.
Plastered straw bale unit density is the gross density of the complete
dry plastered straw bale mass divided by its gross volume. This is done with
a view to knowing the load the sample can carry and handling requirements.
3.1.1 COMPRESSIVE STRENGTH TESTS
This is the performance of plastered straw bale wall under loading
with varying plaster thickness. The modes of failure of the six prisms were
noted.
Plate 6. Plastered Prism under a Compression Machine
34
3.1.2 THERMAL INSULATION TEST
The thermal insulation test was carried out on a prototype straw bale
building and sandcrete block. Built in the open with and the same floor area
of 1.sq.meter and identical floors and roofs.
A bucket of containing 10 litres of hot water at say 65- degree
centigrade was put into each of the prototype buildings. The roots of the
models were then wrapped with thick polythene to prevent the loss of heat
through the roof opening. The temperatures of the each of the models were
taken at regular intervals with thermometer to ascertain the heat losses in
each model.
Plate 7.Hot bucket of water with thermo Plate 8.Hot bucket of water with
-meter inside sandcrete model thermometer inside strawbale model
35
3.2.0 CONSTRUCTION OF PROTOTYPE PLASTERED STRAW
BALED WALL
3.2.1 MATERIALS USED FOR CONSTRUCTION
The construction of a prototype straw bale wall will be carried out
with the following materials. Mature straw of guinea corn stalks (cut to size
and baled) ordinary Portland cement, fine aggregates, steel wire/ twine, wire
mesh and water.
3.2.2 MATERIAL QUALITY AND ACQUISITION
Guinea corn is the straw type used in this project. It was left in the
open space to dry for several weeks after acquisition from a local farm. The
straw is clean, free from debris and other leaves. This is usually obtained
between December and February when farmers are preparing to clear their
farms for the raining season.
Ordinary Portland cement was used and mixed with water fit for
drinking, clean, contaminant free, and free from organic material, coupled
with dissolved or suspended solid which could affect the mortar strength
adversely.
The fine aggregate used was clean, soft, well -graded natural sand free
from salt and organic contaminants.
36
Plate 9. Stalks of Guinea Corn (Straw)
3.3.0 CUTTING AND BALING OF STRAW
The straws were cut to size 120mm using cutlass and were held
together with wooden battens. Compressive strength machine was used in
lieu of baling machine, since it is not reputed to be available in Nigeria. The
baled straws were compressed and tightly bound with twine. This was done
to avoid looseness, thus, increasing the density of bale.
Plate 10.Bailing of Straw using a Compression Machine.
37
3.4.0 BATCHING
Proportioning of cement to fine aggregate mix can either be done by
volume or weight.
As far as this project is concerned, batching was made by weight. The
mix ratio adopted is 1:8, that is one part of cement to eight parts of fine
aggregate.
3.4.1 MORTAR MIXING
Mixing of mortar may be done manually or mechanically. Here,
mixing is done manually using shovel, the mixture of cement and sand was
done twice dry and twice wet thoroughly.
3.5.0 MOULD FOR STRAW BALE PLASTER PRISMS
Construction of wooden moulds of inner sizes of 80mm x 120mm x
200mm, 110mm x 150mm x 230mm and 140mm x160mm 240mm for
plastered straw bale prisms and different mould were constructed according
to variation in plaster thickness of 10mm, 15mm and 20mm.
For easy removal of the specimen, lubrication oil and polythene were used
38
Plate 11. Moulds for straw bale Prisms.
3.5.1 STRAW BALE WALL LAYING
The foundation footing was laid in trenches by pouring concrete in it.
Then wooden joist (150 x 150mm) were laid on the concrete footing and
floorboards were laid across. Thereafter, a stud is driven into the joist
vertically this could be bamboo.
Afterwards, the bales were stacked in a running or stack bond flat,
directly a top one another that is, the longest dimension parallel to the wall
and the shortest vertical.
39
Plate 12. Laying of foundation footing
Plate 13 & 14. Stacking of strawbale walls
40
3.5.2.0 CONSTRUCTION OF PROTOTYPE SAND CRETE BLOCK
WALL
3.5.2.1 MATERIALS USED FOR CONSTRUCTION
The construction materials are sharp sand and ordinary Portland
cements and water.
3.5.2.2 MOULDING OF SAND CRETE BLOCKS
The cement and sharp sand used was batched by volume 1:8 mixed to
a dry and twice wet it was then placed into steel moulds (100 x 100 x
200mm size) rammed. Thereafter it was sieved for seven days.
3.5.2.3 LAYING OF SANDCRETE BLOCKS
After digging of trenches, foundation was laid. Afterwards, the blocks
were laid on the stretcher side up to seven courses.
3.5.2.4 RENDERING AND PLASTERING OF SANDCRETE BLOCKS
The fine sand was sieved and batching was carried out by volume
(1:8). Both cement and sand) were mixed well with water and the water is
applied the walls’ surface and smoothened.
41
Plate 15. Sandcrete Blocks
3.5.3 PLASTERING OF STRAW BALE
The straw bale wall was plastered with the aid of hand trowel. It was
worked in to the bale surface, inside and outside layers and it was given a
smooth finish.
Also, for the compression tests, the bales are encased in mixed mortar
placed in mould and compacted to eliminate voids. Prior to this chicken
mesh cut to size, placed as the stretcher side of the bale to reinforce the
plaster skin.
42
Plate 16. Plastered Prototype Straw bale and sandcrete buildings
3.5.4 ROOFING OF PROTOTYPE BUILDINGS
The materials used for the rooting are: plank {2” x2”, 2 x4” and 2”
x4”), corrugate iron sheet and nails. The some materials were used for both
buildings. Roof structure was constructed with a gentle slope to give room
for minor headroom.
43
Plate 17. Roofed prototype buildings
3.6.0 CASTING AND CURING OF PLASTERED STRAWBALE
PRISMS
Fine aggregate and cement were batched by volume in the ratio 1:8,
they were mixed thoroughly dried and with water.
Thereafter, the hated straws (80 x 120 x 200mm) were placed inside
the mould(s) and wire mesh was provided at the stretcher faces. The mould
was then filled up with the mixed mortar with thickness varying according to
sizes of the moulds, see plate 18.
The prisms were demoulded the following day and they were cured.
The curing method adopted is the blanket type, in which dry straw
was spread over the plastered straw bale surface. This is to provide
insulation and act as moisture holding medium, the specimens was wetted
for 7 days and were left for 28 days, ready for testing.
Plate 18. Casting of Plastered Strawbale prism
44
3.6.1 SCALE FACTOR FOR PLASTERED STRAW BALE
Scale factor shows a geometric relationship between a model and a
prototype straw bale. This is taken as 1/5 model of 584.2x406.4x1066.8mm
size.
3.7.0 COST AND AVAILABILITY
In actual term, cost of bales varies from one place to another. The
cheapest way to buy bales may be from the field after they have been made
or differ harvest by farmers. In Nigeria buying directly from farmers may be
very easy, while some will be willing to give it away without any attached
cost. Since it will save them the stress of clearing the stalks after harvesting.
It should be noted however, that straw is present everywhere, the cost
incurred will to an extent depend on the nearness to the source, the cost may
be attached to means of transporting, if to the site, baling and stacking
etc.(A.A. Adedeji.). Thus, if compared with the cost of conventional
material like local bricks. Builder, who employ bale in his construction will
safe a huge sum of money than if he had made use of bricks and other
materials.
45
CHAPTER FOUR
4.0 RESULTS AND DISCUSSION
4.1.0 RESULTS ON PLASTERED STRAW BALE PRISM
Sequel to the test carried out on the 28- day plastered straw bale
prisms, the Results are presented in the following subsections.
4.1.1 UNIT DENSITY OF PLASTERED STRAW BALE PRISM
The unit density of the cured-plastered 28- days prism is obtained by
dividing its unit mass by the gross volume.
Table 4.1: UNIT DENSITY OF PLASTERED STRAW BALE PRISM
S/N Length Breadth Height Mass (kg)
Load bearing surface area 10 4 x mm2
Gross Volume x 103 mm3
Gross density x 103 kg/mm3
Average
density
kg/mm3
1 220 140 100 4.09
4.03
3.08 3.08 1.33
1.31
1.32
2 230 150 110 5.57
5.65
3.45 3.80 1.47
1.49
1.48
3 240 160 120 6.49
6.54
3.84 4.61 1.41
1.42
1.42
46
FIG 4.1: AVERAGE DENSITY AGAINST PLASTER THICKNESS
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
0 5 10 15 20 25
Plaster thickness (mm)
Ave
rag
e d
ensi
ty (
kg/m
m3)
Series1
From Table 4.1. above it can be observed that the mass of the
plastered straw bale prism increases with increase in plastered thickness (10-
20mm). While the density increases first, from (10-15mm) then falls at
20mm plastered thickness. Thus optimum density can be obtained at 15mm
thickness of plaster on straw bale.
4.1.2 COMPRESSIVE STRENGTH TEST
This test was carried out on the six plastered straw bale prism at 28-
day. Each of the prisms with varrying plaster thickness was loaded to failure.
The failure points were noted for each of the prism.
47
Likewise, the mode of failure for each of the six prisms was found to be
global buckling type that is the prism bent and broke.
This is the behavior of a typical wall that is well built, but with eccentrically
applied load, bending was induced. See plate 19.
Plate 19. Global Buckling failure mode of plastered strawbale prism
48
Table 4.2 COMPRESSIVE STRENGTH TESTS RESULTS ON
PLASTERED STRAW BALE PRISMS
Sample property 1 2 3 4 5 6
Volume of sample (mm3 x105) 30.8 30.8 30.8 30.8 46.1 46.1
Bearing area of sample (mm2 x104) 30.8 30.8 3.45 3.45 3.84 3.84
Dry weight of sample (kg) 4.09 4.03 5.57 5.65 6.49 6.54
Dry density of sample {/gmm3) 1330 1310 1470 1490 1410 1420
Land of failure (KN) 15.00 14.50 20.00 20.50 40.00 35.50
Compressive strength (N/mm2 0.49 0.47 0.58 0.59 1.04 0.92
Average compressive strength (N/mm2 0.48 0.59 0.98
P
bg Bearing surface Area, Ag = lg x bg
lg= Length of the bearing surface
bg = breadth of bearing surface.
lg P = the load at failure
49
0
0.5
1
1.5
0 10 20 30
Plaster thickness (mm)
Av
era
ge
Co
mp
res
siv
e
Str
en
gh
t (N
/mm
2)
Series1
Fig.4.2. Average Compressive Strength against Plaster thickness
figure 4.2 above it was found out that the more the average thickness
of the plaster, the more the average compressive strength, with 20mm
thickness having 0.98N/mm2 maximum.
So, any plaster thickness from 15mm to 20mm was providing to be stable. See fig 4.2, the 10mm-15mm is almost flat, while the 15mm-20mm is more inclined.
50
4.2.0 THERMAL INSULATION TEST ON PROTOTYPE
PLASTERED STRAW BALE BUILDING AND PLASTERED
SANDCRETE BLOCK BUILDING
The inside temperature of the two prototype buildings were taken as
intervals for 48 hours with both a thermometer for each subjected to same
condition.
TABLE 4.3: INSIDE TEMPERATURE (OC) OF STRAW BALE AND
SANDCRETE BLOCK BUILDING MODELS EXPOSED TO
SIMILAR CONDITION
Time
(Hours)
11.05 am
12.05 pm
1.05 pm
2.05 pm
3.05 pm
4.05 pm
5.05 am
9.05 am
7.05 pm
11.05 am
5.05 pm
8.05 pm
11.05 am
Straw bale building model
65 53 46 40 36 32 29 26 24 23 22 21 20
Sandcrete building model
65 55 46 38 32 28 24 21 19 17 15 13 13
51
FIG 4.3: RELATIONSHIP BETWEEN INSIDE TEMPERATURE OF STRAWBALE
AND SANDCRETE BLOCK BUILDINGS
0
10
20
30
40
50
60
70
1 2 3 4 5 6 7 8 9 10 11 12 13
TIME (HRS)
TE
MP
ER
AT
UR
E (
0C) Straw bale building model
Sandcrete building model
From table 4.3, the temperature of the water dropped about 10 degrees
in the first hour. The temperature in the straw bale model fell slightly faster
because the render inside it was absorbing heat. However during the second
hour there was more heat loss from the sandcrete model.
52
CHAPTER FIVE
5.0 CONCLUSIONS AND RECOMMENDATION
5.1 CONCLUSION
The following conclusions can be reached from the basic test carried
out.
5.1.1 Unit density test
Below are the conclusion reached on unit density test from table 4.1.
The gross density of straw bale wall increased with increasing
thickness (10-15mm) and fall at 20mm it is due to increase in volume
which affects the gross density.
Optimum density can be attained at plaster thickness of 15mm.
Above 20mm plaster thickness; gross volume increased causing
reduction in the gross density.
Thus, for economy 15mm thickness of plaster is desirable.
5.1.2 Compressive strength test
From table 4.2 and figure 4.1
The compressive strength increased with increasing plaster thickness.
The maximum average compressive strength is attained with the
20mm plaster thickness.
The behaviour of the six prisms at failure is global buckling.
53
In which the prism bent approvable broke. This shows that if the straw bale
were well arranged stalled, it will only bend due to eccentrically applied
load.
5.2.0 RECOMMENDATIONS
Based on the results of the various tests and observations the
following are the recommendations:
For comfort, straw bale as a walling material is recommended, owing
to its thermal insulating properties
Since straw is readily available, it is cost- effective as a walling
material.
With the ease of construction, the straw bale as a walling material is
recommended.
For ease of maintenance straw bale is recommended
It is recommended that 15mm plaster thickness when applied both
sides of the wall will give Optimum Compressive Strength.
Straw bale is recommended for Load-bearing walls, Non-load bearing
walls, Retrofit structures, Farm building, and Block of Offices,
Garages amongst other applications.
It is recommended that further research be carried out on the Sound
insulation properties, Fire resistance and the Durability of straw bale .
54
55
REFERENCES
1. Adedeji A.A, 2007, Introduction and design of straw bale masonry,
Olad publishers Ent., Ilorin
2. California straw bale building code, 2001. HS18944
3. Downtown.P, 2003, Australia straw bales. Australia,
www.ausbale.org.
4. Harvest Homes company, 2003 Canada, www.harvesthomes.ca
5 King, B. 2003, Load – bearing straw bale structures, U.S.A
www.ecobuildnetwork.org/strawbale
6. Lacinski,P.,and Bergeron,M. 2000, serious straw bale; a home
construction guide for all climates. Chelsea Green publishing coy.
U.S.A
7. Lee, F, 2001, Straw bale construction, U.S.A www.uheac.org/
links. Html
8. Myhrmam, M, MacDonald, S, 1997 Build it with bales