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Mechanics of straw bales for building applications

Authors:
Thibaut Lecompte at University of Southern Brittany
Thibaut Lecompte
Antoine Le Duigou at University of Southern Brittany
Antoine Le Duigou
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Abstract

Straw bales are seen as increasingly viable for building insulation and even for the construction of small load-bearing straw houses in the last decades, especially in view of the need to seek low environmental footprints. Straw bales can be used as load-bearing structures but they are currently mainly used as a filler insulation material associated with a timber structure. Up to present, very few studies are available concerning the mechanical behaviour of straw bales in buildings. This study aims at investigating the behaviour of straw bales and leads to recommendations for required bales densities. This allows to derive compression models which describe their behaviour in a wall. Therefore, the results show that, in the density range 90–110 kg/m³, the elastic and strength characteristics are similar whatever the position of the bales (laid flat or on their edge).
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

Univ. Bretagne Sud, FRE CNRS 3744, IRDL, F-56100 Lorient, France
*thibaut.lecompte@univ-ubs.fr
Abstract:
             
 !
  "           
#
 $!  

      %      
 !
!&' (('")*!
+,-

  "       
         ".   ,!  /
0
!.+(-"
!
"1+2-!

Keywords: !!3!!!"
1-Introducon
0 4!  5'6          
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:;!
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<(44A"+B-
C          
!<7*8$7580$D!
7=8 "7>8
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          
7&!('8  
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E 
7(5!(=8
    7(>8 H   7(?8    
 "! #L  / 
/

  %</!7(@8
  
  !  M""   72'8  N   72(8
 "!"

 E7(*!(@8  
!
0!#" 


+ -!3
    >=  ((= ")*    3  !
7(*!(&8#
  !         

,

!    /   
 %
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2 Materials and methods
2.1 Material
"J9!<
      (     !   
,
(.
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J(2'(*22'(5!3#!
3.2@')R
<*

J    7*8       
@'")*(2'")*!
J(
J

0!<*! 3
! "
+S>6-!
#
3
#!3/
 .
2@')R!#
!!
 3
 !  3          7(>8
<!#+&6!
:-9
+ 2'6  -!         
3"
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+(-"
!"0
"
//3.
         !    
!(     
! E3+
-!!
,9!": 7(>80!
2
N
0! "

<*.
2.2 Experimental device and data processing
!
()4%.
,      !   
+<5-
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<5.3#+F-!
1+-,!'??
C!

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and the stress as the true stress

4%2.
ε=ln ⁡
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Oh h'
h +<5-
σ=Force
Contact Area
+2-
"
,
3- Results and interpretaon
*(C
<=.!+
,-
T! 3.-
L#!-% 
('6!-% +<=-
4,!
3.," 
"J,
0
7(*8! !
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!
O !
"!
  0,!
9.
σ=F
w .(l0+h0−h)
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OF!wl0
!hh0
h!
0! !
%
<!+<=-
.

'( ('6!
4
    / #  0 
>6
('6
'(
  
   L<! 
 ! 
 L!
3!
  ,  3      
! !

"
! @'6+<5>-;<>!
,3.U>2")*U&@")
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"7(?!2(8!
 /!  %"
K!    "    !
    F  !    
7(*8!!
       !    / 
           !  
7(>8 "
*2C
< ?        ('6     !
!#+
,-0 @' (''")*!3
#
.  !   ,/   ! 
    ,!  
 7228
K!+>' ?'")*-!
3     
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
    #L       !
       V 72*8  2  
VWa+2-
   XY 0 VW !     
!
7258XY'*2*!
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2.J #L      
4/"I!/")*
I '(67"I8
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!
 ,    MR 0       /  
       9   0 , ! 

+   -      
/

0!!

,
  + -       
0,!
S
 F.<2!

!)!

, !"
!!%,
,#**
** C,
O    !        
!
         !  F
/./
 !     /   
,
,
!
/! !
"
!!
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      .  +"   
7(>8-! +,9!:!-
"
.!""
"   3  + (-!    
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VW  72*8!     C  4   
.       "     .
!!!!"<
!C4
"!
    !       
:
0)"
 
   3           

7(=87258K!
9   !     " !  
'(
('6!
'(.
1−ρ
bulk
ρ
0.1
1−ρ
bulk
ρ
S
=aexp
(
−k
1
σ
0.1
)
+
(
1−a
)
exp
(
−k
2
σ
0.1
)
 +5-

bulk"!"
S
# !"+(=''
")*-! a,k1 k2 1 k1 + k2-!"I!
+-" 
0.1
('6!4%*.
ρ
0.1
=ρ
0
l
0
h
0
(l
0
+0.1 h
0
)( h
0
−0.1 h
0
)
+=-
h0  l0 
'<@
/!""(/
7(*!(@8
04%5
!>2")*+(-
<>!"('6!!

4%*.
((
2@2
2@*
2@5
2@=
2@>
2@?
2@@
2@&
2&'
2&(
2&2
2&*
2&5
2&=
2&>
2&?
2&@
2&&
*''
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*'*
*'5
*'=
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*'@
*'&
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*((
*(2
*(*
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22
ρ=ρ0
l0
l0+∆ h
h0
h0−∆ h
+>-
O
h.
h=h0-h
#/+<@-C4W
         %  N  Z'52!
"(Z'@5"I"2Z5&"I!SMRZ'@*(
MRZ'==2"2/
5&"I!0S
 <
7(>8!,/ (?I
>>I!0!
"!3<!
E!S
       5& "I     
)
" 
2(= !  "   =* ")*!    
    ((@ ")*  : ,     
 !.(2'
")* "(C4W
 7(=!2580
 "       

(2
*(?
*(@
*(&
*2'
*2(
*22
*2*
*25
*2=
*2>
*2?
*2@
*2&
**'
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**2
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**5
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2*
25
<@.+-4/>2
")*!C[41+-4/
#C[4!Z'52!"(Z'@5"I"2Z5&"I
P=69"
/<@@
7(*87(@80"=*")*
!/;
      #  < @  
""P
=6<@@>6(?*62!
)
%
*5 C
'(
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</!
+<&-
<&.4('6
,.
E0.1, flat =11.8σ0.1 , flat
+?-
E
0.1, edge
=11.4σ
0.1,edge
+@-
4%+?-  +@-     K"W +
'(Z'(4-!   
        !  >6  ('6

*= N! 
 ,!
#!<('
0 ,>/>R,+:
-!%2"H)
$!2.
´
σ=2μ´
ε+λ tr (´
ε)´
I
+&-
O
´
σ=
[
σxx 0
0−P
]
!
´ε=
[
0 0
0εz
]
.
(5
*=*
*=5
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−P=
(
2μ+λ
)
εz
+('-
I     !   \   #   ]
S! 0       +/! :-!  I  
4%*4%>!
,!I+/!:- 
+<('-.
νzx=−εx
εz
=−∆ h
l0
.(−h0
∆ h )=h
0
/l0
+((-
!
0!!
"
<('.

+('-!+((-]^W
ISZ4)72+(B-8Z4.
P
ε
z
=−E
r
(
r−1
)
(
r+1
) (
r−2
)
∧q
ε
z
=−wE r
(
r−1
)
(
r+1
) (
r−2
)
+(2-
r  +r=l0/h0-!w%
   +"H) - 4% (2    /    ^
    0            
3.,!
3  !  % 5! =! ?  (2!      
9%
</!(''") *!
"  =* ")*   Z'5@/'*@/'& *    
!       !    
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"!%(6
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4%54%=.
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'(Z2&!&"II%
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!
4'(+2?'"I-0!#
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     +< >- "  

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9
  !           
< ! /     
   3           , < 
!,3&' (('")*!
3  %   # O  ,    /
!.

," "
""
"!:

!!,3
 .3
JC 4!#
     S3    
#"
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"2(=
    !        
3
!"
 !  %7(>8
H7(?8""
/
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... In short, higher density results in greater stiffness but worse insulation, and vice versa [1,4]. Studies on compression properties of straw bales show nonlinear and anisotropic characteristics of the strain-stress relation, and the deformation under loading recovers as the loads are removed [5][6][7]. The elastic moduli of straw bales with initial densities ranging from 60 kg/m 3 to 120 kg/m 3 cover a range from 60 kPa to 320 kPa as the load is perpendicular and parallel to the straw direction [6]. ...
... Studies on compression properties of straw bales show nonlinear and anisotropic characteristics of the strain-stress relation, and the deformation under loading recovers as the loads are removed [5][6][7]. The elastic moduli of straw bales with initial densities ranging from 60 kg/m 3 to 120 kg/m 3 cover a range from 60 kPa to 320 kPa as the load is perpendicular and parallel to the straw direction [6]. The thermal conductivities that present the insulation properties of straw bales have also been sufficiently studied in previous works. ...
... The bending strength f b and elastic modulus E b can be calculated by Eqs. (5) and (6). ...
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... Although the price of agricultural straw is very low compared to other lignocellulosic materials, the composites fabricated using rice straw pocess good mechanical properties (Ghazilan et al. 2016, Shang et. al 2012, Lecompte and Le-Duigou 2017 The composites fabricated using cement based materials and rice straw fibers with flame retardant treatment showed significant improvements on thermal stability and flame retardancy (Jiang et al. 2021). The fiber composition and the color of the wheat straw change during the long storage of wheat straw. ...
Influence of rice straw and wood fiber combination on physical and mechanical properties of rice straw pulverized composite board
Article
Full-text available
  • Mar 2024
The production of particleboards from various kinds of ligno cellulosic materials are successful on commercial scale in various parts of the country. However, much success is not seen in use of crop residues for particleboards. Rice straw and wheat straw are one such materials that are abundantly available in India that can be used to produce particleboard. The present work investigated the effect of Rice straw particles and wood fibers combination on physical and mechanical properties of the composite board. The pulverized rice straw composite boards were produced with three different composition of rice straw material (100 %, 70 %, 50 %) and wood Fibers (0 %, 30 %, 50 %). Melamine Urea Formaldehyde (MUF), Phenol Cardanol Formaldehyde (PCF), Blocked Isocyanate resin combined with PCF (BPCF), Blocked Isocyanate combined with Urea Melamine Formaldehyde (BUMF) and Blocked Isocyanate combined with PF (BPF) resins were synthesized. The resin system (10%,12%,14%) with requisite additive was admixed with rice straw and wood fibers. According to the resin system employed, the hot press temperature, specific pressure and curing time were worked out for compression of the boards. The boards were produced with 780 - 850 kg/m3 nominal density and prior to testing, the test specimens were exposed to an atmosphere maintained at a relative humidity of 65 ± 5 percent and at a temperature of 27 ± 2 °C until their masses are nearly constant. The boards made were subjected for evaluation of physical and mechanical properties according to IS 3087. It was found that the Rice Straw particles with Wood Fibers of 50-50 combination bonded with PCF and also with MUF adhesive of 12 % resin solids confirms to the requirement of physical and mechanical properties as per IS 3087 particleboards of wood and other lignocellulosic materials (medium density) for general purposes Specification. Comparison with resin for hybrid boards of rice straw particles with wood fiber, BPCF has shown excellent properties than BPF and BUMF. The study indicated that increasing the percentage replacement of wood fibers with rice straw particles by more than 50 % decreased the internal bond strength. It can be concluded that rice straw pulverized particleboard can be successfully made with the combination of 30 % - 50 % wood fibers using PCF resin or BPCF resin.
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... The deformation is accompanied with a small increase in stress. By analogy with research on straw bales [54] and hemp-lime concrete [55], this slight hardening is supposed to be due to two physical mechanisms accompanying the decrease of the inter-granular porosity: a progressive failure of binder and a deformation of straw fibers with bending and a progressive ovalization of their cross section under compression. In the last stage, the straw walls come into contact and the stiffness increases rapidly. ...
Effect of aggregate/binder ratio on mechanical and energy absorption properties of a straw-lime composite material
Article
  • Jul 2023
  • CONSTR BUILD MATER
This study investigates the mechanical and energy absorption properties of an eco-friendly straw-lime composite. Compression tests were performed to explore the effect of the straw-to-binder ratio (S/B = 0.2, 0.3, and 0.4), as well as the influence of compression speed, and test direction, on the mechanical properties and capabilities to dissipate energy. The Digital Image Correlation (DIC) technique was applied to analyze strain field homogeneity during the deformation process and to evaluate Young’s modulus and Poisson’s ratio. Results show decreasing strength and energy absorption with higher straw to binder ratios. Samples tested orthogonal to straw fiber orientation exhibited an increase of over 100% in absorbed energy per unit volume compared to parallel samples. Physical properties are sensitive to strain rate, with values up to 0.72 MPa for the plateau stress and 300.40 kJ/m3 for the absorbed energy, for a 0.2 S/B ratio at a loading speed of 100 mm/min. The material has shown greater energy absorption than lightweight foam concrete of equivalent density. These results are promising for the development and new use of straw-lime biocomposites as energy absorbing protective materials.
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... It provides effective insulation and can be combined with render or another dense covering to improve its performance. While there is no reliable empirical relationship between moisture content and the various approaches for monitoring it, a study has found an empirical equation that relates to moisture resistance (Lecompte & Le Duigou, 2017;Robinson et al., 2017).This makes straw bale construction suitable for use in areas where moisture resistance is required with a low GHG footprint. Additionally, researchers have found that straw insulation performs substantially better at reducing heat transfer compared to wood frame walls (Cascone et al., 2019) 4 Cross-laminated timber (CLT) Cross-laminated timber (CLT) is a sustainable engineered wood product that has a small carbon footprint and is strong, durable, lightweight, easy to install, low GWP, and energy efficient (Younis & Dodoo, 2022). ...
Sustainable development by carbon emission reduction and its quantification: an overview of current methods and best practices
Article
Full-text available
  • Jun 2023
The construction sector accounts for 36% of global energy consumption and 39% of global carbon dioxide emissions. Sustainable development, which entails reducing and quantifying carbon emissions, is essential to address climate change and the depletion of non-renewable resources. This review paper examines a range of strategies and methodologies, including green building rating systems, sustainable materials and renewable energy, smart building management systems, carbon capture and storage, life cycle assessments, and greenhouse protocols, that can facilitate sustainable development by reducing and quantifying carbon emissions. The paper underscores the need for up-to-date information and more precise techniques to achieve sustainable development goals. Additionally, the paper highlights the potential advantages of sustainable construction practices, such as lower energy costs, better indoor air quality, and more efficient use of resources.
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Experimental investigation of the biaxial load-bearing and deformation behaviour of wheat straw big bales for construction
Article
  • Jun 2024
  • CONSTR BUILD MATER
Growing awareness of the impacts of anthropogenic climate change is driving the demand for sustainable alternatives to conventional building materials. Alongside modern high-performance materials, construction methods utilising traditional renewable raw materials and techniques are garnering increasing attention within society. Square straw bales can offer environmentally friendlier building solutions due to their excellent thermal properties and their ability to withstand mechanical loads when sufficiently compacted. This article focuses on experimental investigations into the biaxial load-bearing behaviour of vertically loaded wheat straw big bales intended for use in load transfer within buildings. The primary emphasis lies on analysing the short-term load-bearing behaviour under cyclic loads and the short-term creep behaviour under permanent loads. Utilising electromechanical and optical metrology, both vertical and horizontal deformation states are determined, forming the basis for establishing biaxial stress-strain relationships. It can be demonstrated that big bales display a notably stiffer behaviour in contrast to the small bales which have predominated scientific studies thus far. The stiffness characteristics are heavily influenced by bale density and the inhibition of pronounced lateral expansion behaviour. However, the creep behaviour is positively impacted by bale compaction due to mechanical preloading. Furthermore, the article aims to comprehensively document all pertinent specimen properties (such as appearance and straw structure), growth, manufacturing, and storage conditions (including environmental factors and pressing technology), as well as experimental parameters (such as test setup and metrology), test procedures (including load regimes and visual observations), and evaluations (such as stress-strain relationships and creep curves). This documentation aims to ensure traceability and reproducibility, facilitating comparisons with future test series and contributing to the development of standardised test methods for load-bearing straw bales.
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A Review on Sustainability: Straw Bale Usage as Building Material
Article
  • Mar 2024
This study thoroughly investigates the use of straw bales as a building material, capitalizing on their natural and sustainable properties. Straw, utilized in construction for centuries, offers environmental and structural benefits, aligning with sustainability practices. The global adoption of straw bale building emphasizes its advantages in thermal insulation, sustainability, durability, low embodied energy, and sound insulation. While recognizing these benefits, the study addresses difficulties in moisture management and mold growth, stressing the need for careful consideration in implementation. Thus, this paper aims to contribute to the ongoing discourse on straw bale building, offering valuable insights and recommendations for architects seeking sustainable alternatives in the construction industry. Furthermore, the study underscores the importance of continuous research and innovation to overcome limitations and enhance the efficiency of straw bale architecture. By fostering a deeper understanding of the problems and opportunities associated with this eco-friendly building method, this paper tries to encourage the building industry to embrace sustainable practices and contribute to a greener future.
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Experimental investigation of the biaxial load-bearing and deformation behaviour of wheat straw big bales for construction
Preprint
Full-text available
  • Feb 2024
Growing awareness of the impacts of anthropogenic climate change is driving the demand for sustainable alternatives to traditional building materials. Alongside modern high-performance materials, construction methods utilising traditional renewable raw materials and techniques are garnering increasing attention within society. Square straw bales can offer environmentally friendlier building solutions due to their excellent thermal properties and their ability to withstand mechanical loads when sufficiently compacted. This article focuses on experimental investigations into the biaxial load-bearing behaviour of vertically loaded wheat straw big bales intended for use in load transfer within buildings. The primary emphasis lies on analysing the short-term load-bearing behaviour under cyclic loads and the short-term creep behaviour under permanent loads. Utilising electro-mechanical and optical metrology, both vertical and horizontal deformation states are determined, forming the basis for establishing biaxial stress-strain relationships. Alongside exploring fundamental aspects of the non-linear load-bearing behaviour, the influence of bale bulk density and horizontal deformation constraints is examined. Furthermore, the article aims to comprehensively document all pertinent specimen properties (such as appearance and straw structure), growth, manufacturing, and storage conditions (including environmental factors and pressing technology), as well as experimental parameters (such as test setup and metrology), test procedures (including load regimes and visual observations), and evaluations (such as stress-strain relationships and creep curves). This documentation aims to ensure traceability and reproducibility, facilitating comparisons with future test series and contributing to the development of standardised test methods for load-bearing straw bales.
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Study of lime hemp composite precasting by compaction of fresh mix — An instrumented die to measure friction and stress state
Article
Full-text available
  • May 2014
  • POWDER TECHNOL
This paper deals with the processing of lime hemp composite blocks by compaction. A rigid compression die has been developed to reach relatively high compression pressure. This has been fitted with force and strain transducers to measure the stress state during compaction. It provides some means of understanding the effect of mix design and stress level on the compressibility and homogeneity of hardened composites. It is shown that moisture globally increases both friction and packing compressibility. Proper moisture content and paste-to-shiv mass ratio (P/S) can be chosen to reduce the heterogeneity. The combination of friction at wall level and stress transmission is the most detrimental for a moisture content of 20% and a P/S = 1.
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Performance of straw bale wall: A case of study
Article
Full-text available
  • Aug 2011
  • ENERG BUILDINGS
This research aimed to evaluate a straw bale house located in Bavaria, Germany. An extensive test program was carried out. The experimental work includes compression tests, moisture content, thermal stability of bales and pH. The in situ work includes temperature and relative humidity inside the straw bale wall. The stress–strain behavior of straw bales was investigated including nonlinearity and anisotropy. Thermal stability of bales under constant temperature and relative humidity was studied considering time dependence. The moisture content of straw bale was about 11%, while pH value inside the bale was about 7.29. Moreover, the temperature and the relative humidity between the interior (inside straw bale wall) and the exterior were investigated.
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An investigation of the moisture content in the walls of a straw-bale building
Article
Full-text available
  • Dec 2004
  • BUILD ENVIRON
This paper examines the use and accuracy of a moisture probe used in the walls of a straw-bale building. The performance of the moisture probe is assessed against laboratory-based measurements. The measurements from a number of moisture probes placed in the walls of a case study straw-bale building over a 2-year period are presented. The moisture content measurements from the building are discussed in the light of laboratory findings and the condition of the straw in the lower part of the most exposed wall. Potential influences upon the readings obtained from the moisture probes along with possible alternative assessment methods are discussed.
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Mechanical analysis of the bending behaviour of plant stems
Article
  • Jan 2015
In order to optimise the processing of stem crops, insight into the deformation behaviour of the crop and the interaction between crop and machine is essential. Most research in the area of mechanical and physical properties of crop stems is focused on characterising the agricultural products to the point of failure using mechanical parameters and empirical relations. No studies have been conducted on the processes which lead to failure of stems and on the processes that take place after failure. In this paper it is shown that the bending behaviour of wheat and barley stalks is very similar to that of steel tubes. Two consecutive phases can be distinguished: ovalisation and buckling. During ovalisation the forces on the wall tend to flatten the cross-section. When this process continues the flexural stiffness is reduced until the structure becomes unstable and buckles. The cross-section locally completely flattens. This deformed cross-section offers virtually no resistance to bending. Mechanical models described in literature have successfully been adapted to describe the bending behaviour of crop stalks during both phases (R2>0.98R2>0.98 for ovalisation and R2>0.97R2>0.97 for buckling). The crop species, growing conditions, stem diameter and wall thickness were found to influence the bending process significantly. The presence of a core-rind structure increases the bending resistance of the stems.
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Durability characteristics of straw bales in building envelopes
Article
  • Oct 2014
  • CONSTR BUILD MATER
Cereal straw, including wheat, barley and rice, offers a renewable and sustainable resource stream for a variety of construction products, including compressed board panels, thatched roofing and bales. The successful use of straw bales as thermal insulation within the external envelope of buildings has been demonstrated by the increasing number of successful contemporary projects around the world. However, the warranty, insurance and financing of such projects is often still not as straightforward as competing solutions, which can be attributed to concerns relating to the long-term durability of the straw. This paper presents findings from an on-going experimental study into the condition monitoring of modern straw bale construction, and also reports on a study investigating the degradation behaviour of wheat straw cyclically exposed to elevated humidity levels. The findings of the study provide encouraging insight into the robustness of straw bale construction.
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A review of the mechanical behaviour of straw when compressed to high densities
Article
  • Sep 1989
The present state of knowledge is reviewed concerning the high density compression of straw in relation to the production of briquettes for fuel or other purposes. Consideration is given to the extensive body of experimental data and theoretical analyses of the mechanics of the compression of fibrous materials. The available information on the formation of high density briquettes in closed and tapered dies is discussed. The information is classified under four broad aspects, namely: (1) the physical properties of straw, (2) the compression characteristics, (3) the formation of briquettes, and (4) theoretical considerations of compression. The many variables involved are discussed in relation to die pressure, specific energy required and the relaxed density and durability of the straw briquette. Particular attention is given to factors of briquette formation such as die size, die charge, maximum pressure, dwell time, straw chop length and moisture content, and type of straw. Some consideration is given to relevant information from work on the compression of grass and lucerne. Work on the effects of temperature and additives is considered and the mechanism of briquette formation discussed. The theoretical work on the compression of fibrous materials in dies is examined in relation to empirical formulae relating pressure and density. These include studies of pressure distributions within the material and rheological models based on spring and damper elements. The value of approaches using dimensional analysis and multivariate analysis is also discussed. Areas of work are identified where further research is required to advance the design of machines for producing high density straw briquettes.
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Development of a cost effective probe for the long term monitoring of straw bale buildings
Article
  • Jan 2011
  • BUILD ENVIRON
This paper reviews current methodologies for measuring the moisture content of straw bale walls in buildings. It discusses the development of an affordable and accurate moisture probe that has been designed to be easily assembled by the builder or owner of a straw bale building from items readily available in the United Kingdom (UK). The probe uses a timber block inserted into the wall, relying upon the measurable moisture variances of the timber and relating this to the surrounding straw. The probes are designed to be used in pairs of varying length, taking measurements at different depths to give an estimate of the moisture gradient through the wall. In order to properly calibrate this device, a full set of sorption and desorption isotherms were established for both Oat and Wheat straw and three different timber species. The results from an environmental chamber have been compared to readings from specimens of the new probe installed in a straw bale house in the south west of the UK. The results were found to be consistent, to within 2%, with the laboratory findings.
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Compaction Behavior of Several Ceramic Powder
Article
The compaction behavior of four ceramic powders of widely different hardness but of essentially the same particle-size fraction (nominally 44 to 62μ) was examined by measuring the travel of a laboratory hydraulic press. The results revealed that the softer powders yielded a greater fractional volume compaction at a given pressure. An account of compaction by two largely independent probabilistic processes, the filling of large holes and the filling of small pores, led to an expression for the fractional volume compaction, , in terms of applied pressure which, with the proper choice of coefficients, at and kt, was found to be adequate to describe the behavior of all the powders.
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