Abstract: it can’t fully replace to steel and

Abstract: Construction
industry acts as an important role in the world and it consumes more material
and energy to fulfil the all needs. Concrete and steel are the most common
materials use in construction industry and price of steel is increasing day by
day. So alternative materials should be found to face that problem and reduce
the cost of construction. Using timber in construction is new trend and
researchers are trying to find the suitability of using timber materials like
bamboo and palmyrah as reinforcement in concrete. In this paper a study was
carried out to find an approach to design a hybrid reinforced concrete slab by
physical modelling and finite element modelling using Midas software. Bamboo is
selected to use as reinforcement with steel because it is rapid growing and
easily available material. Bamboo has higher strength compare with other timber
materials and has higher strength to weight ratio.  Due to its brittle nature it can’t fully
replace to steel and properties of bamboo also vary with its age and species.
Still now there are no any code practice to design hybrid reinforcement element
and because of that designers are avoid using timber for reinforcement in
concrete.       

Keywords:  Bamboo;
Concrete; Reinforcement; Hybrid reinforcement

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1.   
Introduction

Researchers
are trying to find suitable replacement for conventional materials of building
construction for sustainable development. Need of construction activities
increase day by day with increase in population. Provide a structure with
acceptable cost and prevent environmental degradation by construction process
are the responsibilities of people involved in construction industry. Concrete
and steel are most commonly used materials in construction and it consumes high
percentage cost of total budget. In reinforced concrete structures steel acts
as very important role to solve the problems in concrete. Concrete is very
strong in compression and have high durability. But the problem is it is weak
in tension. So steel is used with concrete to solve the problem because steel
is strong in tension. That’s why steel has higher demand in the world and it
causes to increase the cost of steel.

Researchers
found that timber like bamboo, palmyrah are good replacement material for steel
and many researches also carried out on this subject. But they found many
problems in using timber as reinforcement material and gave a solution to
overcome some of these problems. Use timber as a part of the reinforcement with
steel (hybrid reinforcement) in concrete is best solution to get rid of the
disadvantages in using timber. In this case, bamboo is selected as
reinforcement material with steel because it reaches its full growth in a few
months and has higher tensile strength compare with other timber materials.

It
is already reported that bamboo can be used as reinforcement material in
concrete for light weight elements. It is because of the brittle nature of
bamboo, low stiffness and low compressive bearing capacity. Water absorption of
bamboo is one of the major problems and it causes to swelling and shrinkage of
bamboo in concrete. Study on effect of water absorption of bamboo in concrete
and treatment methods were already investigated by researchers (Munasinghe,
2015).
Some coating is used to prevent the water absorption of bamboo in concrete and
varnish is one of the best water repellent for this. Bonding strength is
another important factor in concrete and bond between bamboo and concrete is
relatively low compare with that of steel and to improve the bond between
bamboo and concrete bamboo should be treated (Alireza
Javadian a, 2016).    

2 Bamboo as a construction material

Many years ago, study on the use of
bamboo as reinforcement to replace steel material was started (Lopez, 2003). Before that bamboo
was used to some construction work like scaffolding, shuttering and dwelling. Even
concrete is generally the most economical material of construction and will
provide long life and low maintenance costs, it can’t withstand the tension. So
that reinforcement is provided to solve this problem and steel is the best
solution because it is strong in tension. But due to the economical problem and
high energy consumption there is a need to find a suitable alternative for
steel. Bamboo has been used as a construction material due to its specific
properties like low weight, high tensile strength and biodegradable. There are
many species of bamboo are growing in the world and there was a report that in
Sri Lanka 14 species of bamboo was growing (Senerathne, 1956).

Bamboo is not a tree and it is a rapid
growing grass type. It was selected as reinforcement in concrete because of its
high tensile strength. It can reach strength of 370 MPa (Ghavami K. , 1995) and it makes bamboo
as an attractive alternative material in tensile loading elements. Many researches
were already conducted to find the mechanical properties of bamboo and Table 1
shows a summary of these details which they found strength and stiffness
properties of bamboo (Dias, Munasinghe, &
Baskaran, 2015).

 

Table
1: Properties of Bamboo

 

 

Ratio of tensile strength to weight is six times greater
than that of steel (Ghavami & Hombeeck, 1981). Researchers found
that modulus elasticity of bamboo is very low compare with concrete and it
can’t contribute to the flexural stiffness of the reinforce element. And also
it is consider approximately 1/15 of that of steel (Nayak, Bajaj, Jain, Khandelwal, & Tiwari, 2013).

 

Bamboo is one of the sustainable construction materials
and no need any fuel energy to produce it. But the problem is properties of
bamboo vary with its age and location. Properties of bamboo mainly depend on
nature of growth and growing is depended on climate condition and soil type.  It is reported that bamboo reaches its maximum
strength between 2.5-4 years of its age and also its strength decreases at a
later age (Amada & Untao, 2001). Colour of bamboo
use to identify the suitability of using construction material and Brown colour
trunk is indicate that the tree is at least three years old.

 

Properties of bamboo vary with species and generally
dendrocallamus gianteus is one of the most usable and high strength species. It
has tensile strength of 120 MPa and compressive strength of 55 MPa. Because of
the low density of bamboo which is about 700 kg/m3, strength to
weight ratio is much higher than that of steel and concrete. Density of steel
is 7800 kg/m3 and that of concrete is 2400 kg/m3 (Baba, 2009).

 

A Material selection method was developed at
Cambridge University from the relationship between modulus elasticity E and
density q. In Figure 1, the line presenting the equation C= E1/2 q
for bamboo. The materials situated above the line have a better performance
than bamboo and materials which are situated below the line have worse
performance than bamboo. It shows that only palm trees and balas are in the
same range as bamboo and steel, concrete are situated far below the line (Wegst, Shercliff, & Ashby, 1993).

 

 

 

Figure
1: Performance of bamboo and other materials, in
relation to their E and q

Durability of bamboo depends on its age, type of species
and curing condition. It can easily attack by insects and it will increase with
the amount of starch plus humidity content. Some treatments are help to reduce
the starch such as heating and immersion. In past years many of the concrete
structures were deteriorated because of corrosion of the steel. But that problem
is avoided when using the bamboo as reinforcement in concrete. There a research
was conducted to check the durability of concrete with steel structure and
concrete with bamboo structure. A reinforced concrete column after 10 years
service which was a part of tunnel structure of Rio’s Metro and bamboo
reinforced concrete beam which has 15 years of service life were tested and
results also compared. The condition of these elements is sown in Figure 2 and
3 (Ghavami & Hombeeck, 1981).

 

 

Figure
2: Condition of bamboo reinforced beam

 

 

Figure
3: Condition of reinforced column

 

It shows that steel was severely corroded and need to be
replaced. But they found that the bamboo reinforce beam was satisfactory
condition. That bamboo bars were treated to getting strong bond and prevent the
attack from insects.

 

But there are some disadvantages in using bamboo as
reinforcement in concrete such as water absorption and shrinkage of bamboo
element. Study on these properties of bamboo was conducted by many researchers
and they recommended suitable treatment methods to solve these problems.

 

2.1 Water absorption of bamboo

 

Generally water absorption of timber is very high and
it causes severe problem in concrete when it used as reinforcement. Many
researches were conducted on properties of bamboo to absorb water. Volume of
bamboo is changed when it absorbed water from concrete. That volumetric change
causes to induced stresses and it will develop crack in concrete. Swelling and
shrinkage of bamboo create several problems in concrete and it is also one of
the major problems in using bamboo as reinforcement in concrete instead of
steel.

 

Reinforcing bamboo absorbs water from concrete when
it’s poured and curing. At the time it expands like shown in Figure 4(a). Then
concrete was pushed away from reinforcement bars due to swelling of bamboo
shown in Figure 4(b). After curing process bamboo starts to loss moisture and
shrinks back to initial condition. Swelling and shrinkage of bamboo cause to
develop crack in concrete as shown in Figure 4(c) (Ghavami & Zielinski, 1988).

 

 

 

Figure
4: Behavior of bamboo reinforcement in concrete

Coeffient of thermal expansion of bamboo is as low
as that of concrete and shrinkage of bamboo is higher than that of concrete.
These differential properties of concrete and bamboo are also serious
limitation in use of bamboo as reinforcement. To find the water absorption
capacity of bamboo, a test was carried out on 63 treated and untreated bamboo
samples which are getting from top and bottom part of the trunk of the tree.
These samples were submerged in to the water for 96 hours and weighted. Sand sealer
with single coat, sand sealer with double coats, varnish with single coat,
varnish with double coat, black oil with single and double coat were used to
treat the bamboo samples and results were compared. Untreated bamboo samples
shown higher amount of absorption and varnish coating have the potential to
limit water absorption to 8% by mass and limiting the volume change of bamboo
element (Dias, Munasinghe, & Baskaran, 2015).

 

2.2 bonding strength of bamboo

 

In a reinforced concrete element bonding between concrete
and reinforcement bars is important factor and if the bonding is weak when
apply the load then concrete element goes to failure. So it is very important
to ensure that the bonding strength is adequate to withstand the loading when bamboo
is used as reinforcement to replace the steel. Some studies were conducted on
this chapter and they found that water-repellent treatment needs to improve the
bonding between bamboo segment and concrete. Several types of coating were
applied on the bamboo segments and series of pull out test were conducted to
find out the bonding behavior of bamboo with concrete. Sand was also adds on
top of the coating for some samples to increase the friction. But in this study
they found that bond strength of bamboo with no coating is comparable to coated
reinforcement. And there were no any significant difference in using fine sand
and coarse sand to increase the friction (Javadian,
Wielopolski, Smith, & Hebel).

 

In a study they found that Sikadur 32-Gel is the best
product which used to prevent the corrosion of reinforcing bars, it has
increased the bonding strength of treated bamboo segments (Ghavami K. , 2005). Environmental factors such as
temperature variation, moisture content, pH value of concrete and surface
texture are also related to the boding strength. When apply protective coating
like water proofing to control the water absorption it should be applied as a
thin layer. Otherwise it affects the bonding between concrete and bamboo
segment.

Bamboo has higher bonding capacity compare with palmyrah
strips because it has nodes along the length of the trunk.

 

 

 

 

2.3 Hybrid reinforcement

 

Concrete has lower ductility and it breaks without any
warning signs. Because of that the reinforcing material which use in concrete
should have higher ductile behavior. Steel has high ductile behavior and it
increases the ductile behavior of concrete when using as reinforcement.
Generally timber materials are in brittle nature and bamboo also brittle nature
material. When using bamboo alones in concrete as reinforcement failure of
element   may breaks suddenly without
warning signs. Hybrid reinforcement arrangement is the best solution to solve
that problem. So using steel and bamboo in a proper proportion is help to reach
a effective usage of timber in concrete element.    

3 Conclusions

Bamboo material is one of the
best solutions to face the increasing demand of steel and it has to promote and
researches should be try to make people 
aware about the use of bamboo as reinforcement in concrete. Even there
is some limitation in using bamboo as reinforcement in concrete; there are
several methods to solve these problems. By suitable chemical treatment attack
from insects can prevented. To improve the bonding strength and to avoid the
higher amount of water absorption many coating materials can be used. But still
there is a problem that no one getting idea to use bamboo in concrete because
there are no any guidelines or specific method to do the design. So it is the important
thing and needed thing that find an approach to design a hybrid reinforcement
concrete element. 

4 Acknowledgements

 

I would like to express my gratitude to my research
supervisor, senior lecturer Dr.K.Baskaran and Department of Civil Engineering,
University of Moratuwa to give the financial support. And also I would like to
thank my friends who giving support to continue the research.