CVS 518: MECHANICS OF COMPOSITE MATERIALS
UNITS: 3 UNITS
LECTURER: MRS VIOLAH BUNDOTICH
Course Purpose
The purpose of the course is to introduce the students to the properties of composite materials as
applied in civil and structural engineering. The course also deals with analysis of laminates.
Course Objectives
Upon completion of this course, the student should be able to:
1. Distinguish between the different types of composite materials
2. Understand the properties of existing composites
3. Perform analysis of laminates
Course Content
Types of composite materials such as fibre‐reinforced concrete, ferrocement, plywood, glue‐
laminated timber and laminated plastics and their applications. Properties of existing composites:
thermal, mechanical and ply properties. Composite macromechanics: ply uniaxial analysis,
transformation, failure criteria, behaviour under combined stresses. Laminate production:
manufacture; fabrication process. Storage conditions. Laminate Properties: thermal and mechanical;
residual stresses, warpage. Laminate Analysis: configuration, interply contribution, transformation,
behaviour under combined stresses, failure mechanisms, failure criteria. Safety margins. Structural
analysis. Boundary conditions. Load conditions: static, cyclic and impact.
Laboratory Work:
Bending, tensile and shear tests for laminates.
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1.0.INTRODUCTION
1.1.What is a composite?
A composite is a structural material that consists of two or more combined constituents that
are combined at a macroscopic level and are not soluble in each other. One constituent is
called the reinforcing phase and the one in which it is embedded is called the matrix. The
reinforcing phase material may be in the form of fibres, particles, or flakes. The matrix phase
materials are generally continuous.
Examples of composite systems include concrete reinforced with steel and epoxy reinforced
with graphite fibres, etc
The reinforcement is discontinuous, stiffer, and stronger whereas the matrix less stiff and
weaker phase. Sometimes, because of chemical interactions or other processing effects, an
additional distinct phase called an interphase exists between the reinforcement and the
matrix. The properties of a composite material depend on the
- Properties of the constituents; one of the most important parameters is the volume (or
weight) fraction of reinforcement or fibre volume ratio. The distribution of the
reinforcement determines the homogeneity or uniformity of the material system. The
more non-uniform the reinforcement distribution, the more heterogeneous the
material, and the higher the scatter in properties and the probability of failure in the
weakest areas.
- Their geometry; the geometry and orientation of the reinforcement affect the
anisotropy of the system.
- The distribution of the phases.
1.1.1. Role and Selection of fibers
The points to be noted in selecting the reinforcements include compatibility with matrix
material, thermal stability, density, melting temperature etc. The efficiency of
discontinuously reinforced composites is dependent on tensile strength and density of
reinforcing phases. The compatibility, density, chemical and thermal stability of the
reinforcement with matrix material is important for material fabrication as well as end
application. The thermal discord strain between the matrix and reinforcement is an
important parameter for composites used in thermal cycling application. It is a function of
difference between the coefficients of thermal expansion of the matrix and
reinforcement. The manufacturing process selected and the reinforcement affects the
crystal structure.
Also the role of the reinforcement depends upon its type in structural Composites. In
particulate and whisker reinforced Composites, the matrix are the major load bearing
constituent. The role of the reinforcement is to strengthen and stiffen the composite through
prevention of matrix deformation by mechanical restraint. This restraint is generally a
function of the ratio of inter-particle spacing to particle diameter. In continuous fibre
reinforced Composites, the reinforcement is the principal load-bearing constituent. The
metallic matrix serves to hold the reinforcing fibres together and transfer as well as distribute
the load. Discontinuous fibre reinforced Composites display characteristics between those of
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continuous fibre and particulate reinforced composites. Typically, the addition of
reinforcement increases the strength, stiffness and temperature capability while reducing the
thermal expansion coefficient of the resulting MMC. When combined with a metallic matrix
of higher density, the reinforcement also serves to reduce the density of the composite, thus
enhancing properties such as specific strength.
1.1.2. Functions of a Matrix
In a composite material, the matrix material serves the following functions:
• Holds the fibres together.
• Protects the fibres from environment.
• Distributes the loads evenly between fibres so that all fibres are subjected to the same
amount of strain.
• Enhances transverse properties of a laminate.
• Improves impact and fracture resistance of a component.
• Helps to avoid propagation of crack growth through the fibres by providing alternate
failure path along the interface between the fibres and the matrix.
• Carry interlaminar shear.
The matrix plays a minor role in the tensile load-carrying capacity of a composite structure.
However, selection of a matrix has a major influence on the interlaminar shear as well as inplane shear properties of the composite material. The interlaminar shear strength is an
important design consideration for structures under bending loads, whereas the in-plane shear
strength is important under torsion loads. The matrix provides lateral support against the
possibility of fibre buckling under compression loading, thus influencing to some extent the
compressive strength of the composite material. The interaction between fibres and matrix is
also important in designing damage tolerant structures. Finally, the processability and
defects in a composite material depend strongly on the physical and thermal characteristics,
such as viscosity, melting point, and curing temperature of the matrix.
1.2.Advantages of composites over other materials
1. High resistance to fatigue and corrosion degradation.
2. High ‘strength or stiffness to weight’ ratio. As enumerated above, weight savings are
significant ranging from 25-45% of the weight of conventional metallic designs.
3. Due to greater reliability, there are fewer inspections and structural repairs.
4. Directional tailoring capabilities to meet the design requirements. The fibre pattern
can be laid in a manner that will tailor the structure to efficiently sustain the applied
loads.
5. Fibre to fibre redundant load path.
6. Improved dent resistance is normally achieved. Composite panels do not sustain
damage as easily as thin gage sheet metals.
7. It is easier to achieve smooth aerodynamic profiles for drag reduction. Complex doublecurvature parts with a smooth surface finish can be made in one manufacturing operation.
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8. Composites offer improved torsional stiffness. This implies high whirling speeds,
reduced number of intermediate bearings and supporting structural elements. The overall
part count and manufacturing & assembly costs are thus reduced.
9. High resistance to impact damage.
10. Thermoplastics have rapid process cycles, making them attractive for high volume
commercial applications that traditionally have been the domain of sheet metals.
Moreover, thermoplastics can also be reformed.
11. Like metals, thermoplastics have indefinite shelf life.
12. Composites are dimensionally stable i.e. they have low thermal conductivity and low
coefficient of thermal expansion. Composite materials can be tailored to comply with a
broad range of thermal expansion design requirements and to minimise thermal stresses.
13. Manufacture and assembly are simplified because of part integration (joint/fastener
reduction) thereby reducing cost.
14. The improved weatherability of composites in a marine environment as well as their
corrosion resistance and durability reduce the down time for maintenance.
15. Close tolerances can be achieved without machining.
16. Material is reduced because composite parts and structures are frequently built to shape
rather than machined to the required configuration, as is common with metals.
17. Excellent heat sink properties of composites, especially Carbon-Carbon, combined with
their lightweight have extended their use for aircraft brakes.
18. Improved friction and wear properties.
19. The ability to tailor the basic material properties of a Laminate has allowed new
approaches to the design of aeroelastic flight structures.
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1.3.Disadvantages of composites
High cost of fabrication of composites is a critical issue. For example, a part made of
graphite/epoxy composite may cost up to 10 to 15 times the material costs.
Mechanical characterization of a composite structure is more complex than that of a
metal structure. Composite materials are not isotropic and therefore, they require
more material parameters. For example, a single layer of a graphite/epoxy composite
requires nine stiffness and strength constants for conducting mechanical analysis.
Repair of composites is not a simple process compared to that for metals. Sometimes
critical flaws and cracks in composite structures may go undetected
Composites do not have a high combination of strength and fracture toughness*
compared to metals.
Composites do not necessarily give higher performance in all the properties used for
material selection. (parameters — strength, toughness, formability, joinability,
corrosion resistance, and affordability).
1.4.Types of composite materials.
Composite materials are commonly classified at following two distinct levels:
• The first level of classification is usually made with respect to the matrix constituent. The
major composite classes include Organic Matrix Composites (OMCs), Metal Matrix
Composites (MMCs) and Ceramic Matrix Composites (CMCs). The term organic matrix
composite is generally assumed to include two classes of composites, namely Polymer
Matrix Composites (PMCs) and carbon matrix composites commonly referred to as
carbon-carbon composites.
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• The second level of classification refers to the reinforcement form - fibre reinforced
composites, laminar composites and particulate composites. Fibre Reinforced
composites (FRP) can be further divided into those containing discontinuous or
continuous fibres.
• Fibre Reinforced Composites are composed of fibres embedded in matrix material. Such a
composite is considered to be a discontinuous fibre or short fibre composite if its
properties vary with fibre length. On the other hand, when the length of the fibre is such
that any further increase in length does not further increase, the elastic modulus of the
composite, the composite is considered to be continuous fibre reinforced. Fibres are small
in diameter and when pushed axially, they bend easily although they have very good
tensile properties. These fibres must be supported to keep individual fibres from bending
and buckling.
• Laminar Composites are composed of layers of materials held together by matrix.
Sandwich structures fall under this category.
• Particulate Composites are composed of particles distributed or embedded in a matrix
body. The particles may be flakes or in powder form. Concrete and wood particle boards
are examples of this category.
1.4.1. Organic Matrix Composites
1. Polymer Matrix Composites (PMC)/Carbon Matrix Composites or CarbonCarbon Composites
Polymers make ideal materials as they can be processed easily, possess lightweight, and
desirable mechanical properties. It follows, therefore, that high temperature resins are
extensively used in aeronautical applications.
Two main kinds of polymers are thermosets and thermoplastics. Thermosets have qualities
such as a well-bonded three-dimensional molecular structure after curing. They decompose
instead of melting on hardening. Merely changing the basic composition of the resin is
enough to alter the conditions suitably for curing and determine its other characteristics. They
can be retained in a partially cured condition too over prolonged periods of time, rendering
Thermosets very flexible. Thus, they are most suited as matrix bases for advanced conditions
fiber reinforced composites. Thermosets find wide ranging applications in the chopped fiber
composites form particularly when a premixed or moulding compound with fibers of specific
quality and aspect ratio happens to be starting material as in epoxy, polymer and phenolic
polyamide resins.
Thermoplastics have one- or two-dimensional molecular structure and they tend to at an
elevated temperature and show exaggerated melting point. Another advantage is that the
process of softening at elevated temperatures can be reversed to regain its properties during
cooling, facilitating applications of conventional compress techniques to mould the
compounds.
Thermosets are the most popular of the fiber composite matrices without which, research and
development in structural engineering field could get truncated. Aerospace components,
automobile parts, defense systems etc., use a great deal of this type of fiber composites.
Epoxy matrix materials are used in printed circuit boards and similar areas.
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2. Metal Matrix Composites (MMC)
Metal matrix composites are not as widely in use as their plastic counterparts. High strength,
fracture toughness and stiffness are offered by metal matrices than those offered by polymer.
They can withstand elevated temperature in corrosive environment than polymer composites.
Most metals and alloys could be used as matrices and they require reinforcement materials
which need to be stable over a range of temperature and non-reactive too. However the
guiding aspect for the choice depends essentially on the matrix material. Light metals form
the matrix for temperature application and the reinforcements in addition to the
aforementioned reasons are characterized by high moduli.
Most metals and alloys make good matrices. Titanium, Aluminium and magnesium are the
popular matrix metals currently in vogue, which are particularly useful for aircraft
applications. If metallic matrix materials have to offer high strength, they require high
modulus reinforcements. The strength-to-weight ratios of resulting composites can be higher
than most alloys.
The melting point, physical and mechanical properties of the composite at various
temperatures determine the service temperature of composites. Most metals, ceramics and
compounds can be used with matrices of low melting point alloys. The choice of
reinforcements becomes more stunted with increase in the melting temperature of matrix
materials.
3. Ceramic Matrix Materials (CMM)
Ceramics can be described as solid materials which exhibit very strong ionic bonding in
general and in few cases covalent bonding. High melting points, good corrosion resistance,
stability at elevated temperatures and high compressive strength, render ceramic-based matrix
materials a favourite for applications requiring a structural material that doesn’t give way at
temperatures above 1500ºC. Naturally, ceramic matrices are the obvious choice for high
temperature applications.
High modulus of elasticity and low tensile strain, which most ceramics posses, have
combined to cause the failure of attempts to add reinforcements to obtain strength
improvement. This is because at the stress levels at which ceramics rupture, there is
insufficient elongation of the matrix which keeps composite from transferring an effective
quantum of load to the reinforcement and the composite may fail unless the percentage of
fiber volume is high enough. A material is reinforced to utilize the higher tensile strength of
the fiber, to produce an increase in load bearing capacity of the matrix. Addition of highstrength fiber to a weaker ceramic has not always been successful and often the resultant
composite has proved to be weaker.
The use of reinforcement with high modulus of elasticity may take care of the problem to
some extent and presents pre-stressing of the fiber in the ceramic matrix is being increasingly
resorted to as an option.
When ceramics have a higher thermal expansion coefficient than reinforcement materials, the
resultant composite is unlikely to have a superior level of strength. In that case, the composite
will develop strength within ceramic at the time of cooling resulting in microcracks extending
from fiber to fiber within the matrix. Microcracking can result in a composite with tensile
strength lower than that of the matrix.
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1.4.2. Classification Based on Reinforcements
Introduction to Reinforcements
Reinforcements for the composites can be fibres, fabrics particles or whiskers. Fibres are
essentially characterized by one very long axis with other two axes either often circular or
near circular. Particles have no preferred orientation and so does their shape. Whiskers have a
preferred shape but are small both in diameter and length as compared to fibres. Figure
Reinforcing constituents in composites, as the word indicates, provide the strength that makes
the composite what it is. But they also serve certain additional purposes of heat resistance or
conduction, resistance to corrosion and provide rigidity. Reinforcement can be made to
perform all or one of these functions as per the requirements.
A reinforcement that embellishes the matrix strength must be stronger and stiffer than the
matrix and capable of changing failure mechanism to the advantage of the composite. This
means that the ductility should be minimal or even nil the composite must behave as brittle as
possible.
1. Fibre Reinforced Composites/Fibre Reinforced Polymer (FRP) Composites
Fibres are the important class of reinforcements, as they satisfy the desired conditions and
transfer strength to the matrix constituent influencing and enhancing their properties as
desired. Glass fibres are the earliest known fibres used to reinforce materials. Ceramic and
metal fibres were subsequently found out and put to extensive use, to render composites
stiffer more resistant to heat.
Fibres fall short of ideal performance due to several factors. The performance of a fibre
composite is judged by its length, shape, orientation, and composition of the fibres and the
mechanical properties of the matrix. The orientation of the fibre in the matrix is an indication
of the strength of the composite and the strength is greatest along the longitudinal directional
of fibre. This doesn’t mean the longitudinal fibres can take the same quantum of load
irrespective of the direction in which it is applied. Optimum performance from longitudinal
fibres can be obtained if the load is applied along its direction. The slightest shift in the angle
of loading may drastically reduce the strength of the composite.
Unidirectional loading is found in few structures and hence it is prudent to give a mix of
orientations for fibres in composites particularly where the load is expected to be the
heaviest.
2. Laminar Composites
Laminar composites are found in as many combinations as the number of materials. They
can be described as materials comprising of layers of materials bonded together. These may
be of several layers of two or more metal materials occurring alternately or in a determined
order more than once, and in as many numbers as required for a specific purpose.
Clad and sandwich laminates have many areas as it ought to be, although they are known to
follow the rule of mixtures from the modulus and strength point of view. Other intrinsic
values pertaining to metal-matrix, metal-reinforced composites are also fairly well known.
Powder metallurgical processes like roll bonding, hot pressing, diffusion bonding, brazing
and so on can be employed for the fabrication of different alloys of sheet, foil, powder or
sprayed materials. It is not possible to achieve high strength materials unlike the fiber
version. But sheets and foils can be made isotropic in two dimensions more easily than fibers.
Foils and sheets are also made to exhibit high percentages of which they are put. For instance,
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a strong sheet may use over 92% in laminar structure, while it is difficult to make fibres of
such compositions. Fibre laminates cannot make over 75% strong fibres.
The main functional types of metal-metal laminates that do not possess high strength or
stiffness are single layered ones that endow the composites with special properties, apart from
being cost-effective. They are usually made by pre-coating or cladding methods.
Pre-coated metals are formed by forming by forming a layer on a substrate, in the form of a
thin continuous film. This is achieved by hot dipping and occasionally by chemical plating
and electroplating. Clad metals are found to be suitable for more intensive environments
where denser faces are required.
There are many combinations of sheet and foil which function as adhesives at low
temperatures. Such materials, plastics or metals, may be clubbed together with a third
constituent. Pre-painted or pre-finished metal whose primary advantage is elimination of final
finishing by the user is the best known metal-organic laminate. Several combinations of
metal-plastic, vinyl-metal laminates, organic films and metals, account for upto 95% of
metal-plastic laminates known. They are made by adhesive bonding processes.
3. Particulate Reinforced Composites (PRC)
Microstructures of metal and ceramics composites, which show particles of one phase strewn
in the other, are known as particle reinforced composites. Square, triangular and round shapes
of reinforcement are known, but the dimensions of all their sides are observed to be more or
less equal. The size and volume concentration of the dispersoid distinguishes it from
dispersion hardened materials.
The dispersed size in particulate composites is of the order of a few microns and volume
concentration is greater than 28%. The difference between particulate composite and
dispersion strengthened ones is, thus, oblivious. The mechanism used to strengthen each of
them is also different. The dispersed in the dispersion-strengthen materials reinforces the
matrix alloy by arresting motion of dislocations and needs large forces to fracture the
restriction created by dispersion.
In particulate composites, the particles strengthen the system by the hydrostatic coercion of
fillers in matrices and by their hardness relative to the matrix.
Three-dimensional reinforcement in composites offers isotropic properties, because of the
three systematical orthogonal planes. Since it is not homogeneous, the material properties
acquire sensitivity to the constituent properties, as well as the interfacial properties and
geometric shapes of the array. The composite’s strength usually depends on the diameter of
the particles, the inter-particle spacing, and the volume fraction of the reinforcement. The
matrix properties influence the behaviour of particulate composite too.
1.5. Examples for composite materials:
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Fibre reinforced plastics:
o Classified by type of fibre:
- Wood (cellulose fibres in a lignin and hemicellulose matrix)
- Carbon-fibre reinforced plastic (CRP)
- Glass-fibre reinforced plastic (GRP) (informally, "fiberglass")
o Classified by matrix:
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-
•
•
•
•
•
•
•
Thermoplastic Composites
• short fibre thermoplastics
• long fibre thermoplastics or long fibre reinforced
thermoplastics
• glass mat thermoplastics
• continuous fibre reinforced thermoplastics
- Thermoset Composites
Reinforced carbon-carbon (carbon fibre in a graphite matrix)
Metal matrix composites (MMCs):
o White cast iron
o Hardmetal (carbide in metal matrix)
o Metal-intermetallic laminate
Ceramic matrix composites:
o Bone (hydroxyapatite reinforced with collagen fibers)
o Cermet (ceramic and metal)
o Concrete
Organic matrix/ceramic aggregate composites
o Asphalt concrete
o Dental composite
o Syntactic foam
o Mother of Pearl
Chobham armour (see composite armour)
Engineered wood
o Plywood
o Oriented strand board
o Wood plastic composite (recycled wood fiber in polyethylene matrix)
o Pykrete (sawdust in ice matrix)
Plastic-impregnated or laminated paper or textiles
o Arborite
o Formica (plastic)
1.6. Composite materials in Civil and Structural engineering.
1.6.1. Fibre-reinforced concrete
In conventional concrete, micro-cracks develop before structure is loaded because of drying
shrinkage and other causes of volume change. When the structure is loaded, the micro cracks
open up and propagate because of development of such micro-cracks, results in inelastic
deformation in concrete. Fibre reinforced concrete (FRC) is cementing concrete reinforced
mixture with more or less randomly distributed small fibres. In the FRC, a numbers of small
fibres are dispersed and distributed randomly in the concrete at the time of mixing, and thus
improve concrete properties in all directions. The fibers help to transfer load to the internal
micro cracks. FRC is cement based composite material that has been developed in recent
years. It has been successfully used in construction with its excellent flexural-tensile strength,
resistance to spitting, impact resistance and excellent permeability and frost resistance. It is
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an effective way to increase toughness, shock resistance and resistance to plastic shrinkage
cracking of the mortar. The failure modes of FRC are either bond failure between fibre and
matrix or material failure.
Fibres are produced from different materials in various shapes and sizes. Typical fibre
materials are:
1. Steel Fibres; straight, crimped, twisted, hooked, ringed, and paddled ends. Diameter
range from 0.25 to 0.76mm.
2. Glass Fibres; Straight, diameter ranges from 0.005 to 0.015mm (may be bonded
together to form elements with diameters of 0.13 to 1.3mm).
3. Natural Organic and Mineral Fibres; Wood, asbestos, cotton, bamboo, and rockwool.
They come in wide range of sizes.
4. Polypropylene Fibres; Plain, twisted, fibrillated, and with buttoned ends.
5. Other Synthetic Fibres; Kevlar, nylon, and polyester. Diameter ranges from 0.02 to
0.38mm.
A convenient parameter describing a fibre is its aspect ratio (L/D), defined as the fibre length
divided by an equivalent fibre diameter. Typical aspect ratio ranges from about 30 to 150 for
length of 6 to 75mm.
Types of fibre-reinforced concrete.
1. Steel Fibre Reinforced Concrete (SFRC)
Steel fibre reinforced concrete is a composite material which is made up from cement
concrete mix and steel fibres as a reinforcing. The steel fibres, which are uniformly
distributed in the cementations mix .This mix, have various volume fractions, geometries,
orientations and material properties. It has been shown in the research that fibres with low
volume fractions (<1%), in fibre reinforced concrete, have an insignificant effect on both the
compressive and tensile strength.
The types of steel fibres are defined by ASTM A820:- Type I: cold-drawn wire
- Type II; cut sheet
- Type III: melt-extracted
- Type IV: mill cut
- Type V: modified cold-drawn wire
Figure 1.1: Steel Fibres
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Generally SFRC is very ductile and particularly well suited for structures which are required
to exhibit;
- High fatigue strength resistance to impact, blast and shock loads
- Shrinkage control of concrete
- Tensile strength, very high flexural, shear
- Erosion and abrasion resistance to splitting
- Temperature resistance, high thermal
- Earth quake resistance
The degree of improvement gained in any specific property exhibited by SFRC is dependent
on a number of factors that include:- Concrete mix and its age
- Steel fibre content, volume fraction
- Fibres geometry, its aspect ratio (length to diameter ratio) and bond characteristics
volume fraction
2. Glass Fibre Reinforced Concrete (GFRC)
Glass fibre–reinforced concrete is (GFRC) basically a concrete composition which is
composed of material like cement, sand, water, and admixtures, in which short length discrete
glass fibres are dispersed. Inclusion of these fibres in these composite results in improved
tensile strength and impact strength of the material. GFRC has been used for a period of 30
years in several construction elements but at that time it was not so popular, mainly in nonstructural ones, like facing panels (about 80% of the GRC production), used in piping for
sanitation network systems, decorative non-recoverable formwork, and other products.
At the beginning age of the GFRC development, one of the most considerable problems was
the durability of the glass fibre, which becomes more brittle with time, due to the alkalinity of
the cement mortar. After some research, significant improvement have been made, and
presently, the problem is practically solved with the new types of alkali-resistant (AR
resistance) glass fibres and with mortar additives that prevent the processes that lead to the
embrittlement of GFRC.
Figure 1.2: Glass Fibres
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3. Polymer Fibre Reinforced Concrete
Civil structures made of steel reinforced concrete normally suffer from corrosion of the steel
by the salt, which results in the failure of those structures. Constant maintenance and
repairing is needed to enhance the life cycle of those civil structures.
There are many ways to minimize the failure of the concrete structures made of steel
reinforce concrete. The custom approach is to adhesively bond polymer fibre composites onto
the structure. This also helps to increase the toughness and tensile strength and improve the
cracking and deformation characteristics of the resultant composite. But this method adds
another layer, which is prone to degradation. These fibre polymer composites have been
shown to suffer from degradation when exposed to marine environment due to surface
blistering. As a result, the adhesive bond strength is reduced, which results in the delamination of the composite.
A uniform distribution of fibres throughout the concrete improves the homogeneity of the
concrete matrix. It also facilitates reduced water absorption, greater impact resistance,
enhanced flexural strength and tensile strength of concrete. The use of polymer fibres with
concrete has been recognized by the Bureau of Indian Standards (BIS) and Indian Road
Congress and is included in some of their Standard documents.
Figure 1.3: Polymer Fibres
4. Natural Fibre Reinforced Concrete
The first use of fibres in reinforced concrete has been dated to 1870‟s. Since then, researchers
around the world have been interested in improving the tensile properties of concrete by
adding, iron and other wastes. In addition to industrial fibres, natural organic and mineral
fibres have been also investigated in reinforced concrete. Wood, sisal, jute, bamboo, coconut,
asbestos and rockwool, are examples that have been used and investigated.
5. Synthetic Fibre
Synthetic fibres are no substitute for primary reinforcement in concrete because they add
little or no strength. But structural reinforcement doesn’t provide its benefits until concrete
hardens. That’s why some contractors add synthetic fibre to concrete as secondary. Unlike
structural reinforcement, synthetic fibres provide benefits while concrete are still plastic.
They also enhance some of the properties of hardened concrete.
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Synthetic fibres commercially available are polypropylene, polyester, and nylon. Though the
fibres within each type come in various lengths, thicknesses, and geometries, synthetic fibres
provide similar benefits when used as secondary concrete reinforcement.
Mixture Compositions and Placing
Mixing of FRC can be accomplished by many methods. The mix should have a uniform
dispersion of the fibres in order to prevent segregation or balling of the fibres during mixing.
Most balling occurs during the fibre addition process. Increase of aspect ratio, volume
percentage of fibre, and size and quantity of coarse aggregate will intensify the balling
tendencies and decrease the workability. To coat the large surface area of the fibres with
paste, experience indicated that a water cement ratio between 0.4 and 0.6, and minimum
cement content of 400 kg/m3 are required. Compared to conventional concrete, fibre
reinforced concrete mixes are generally characterized by higher cement factor, higher fine
aggregate content and smaller size coarse aggregate.
A fibre mix generally requires more vibration to consolidate the mix. External vibration is
preferable to prevent fibre segregation. Metal trowels, tube floats, and rotating power floats
can be used to finish the surface.
Mechanical Properties of FRC
Addition of fibres to concrete influences its mechanical properties which significantly depend
on the type and percentage of fibres. Fibres with end anchorage and high aspect ratio have
been found to have improved effectiveness. It is shown that for the same length and diameter,
crimped-end fibres can achieve the same properties as straight fibres using 40 per cent less
fibres. In determining the mechanical properties of FRC, the same equipment and procedure
as used for conventional concrete can also be used. Some properties of FRC determined by
different researchers are:
1. Compressive Strength; the presence of fibres may alter the failure mode of cylinders,
but the fibre effect will be minor on the improvement of compressive strength values
(0 to 15 per cent).
2. Modulus of Elasticity; modulus of elasticity of FRC increases slightly with an
increase in the fibre content. It was found that for each 1 per cent increase in fibre
content by volume there is an increase of 3 per cent in the modulus of elasticity.
3. Flexure; the flexural strength was reported to be increased by 2.5 times using 4 per
cent fibres.
4. Toughness; for FRC, toughness is about 10 to 40 times that of plain concrete.
5. Splitting Tensile Strength; the presence of 3 per cent fibre by volume was reported to
increase the splitting tensile strength of mortar about 2.5 times that of the
unreinforced one.
6. Fatigue Strength; the addition of fibres increases fatigue strength of about 90 per cent
and 70 per cent of the static strength at 2 x 106 cycles for non-reverse and full reversal
of loading, respectively.
7. Impact Resistance; the impact strength for fibrous concrete is generally 5 to 10 times
that of plain concrete depending on the volume of fibre.
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8. Corrosion of Steel fibres; a 10-year exposure of steel fibrous mortar to outdoor
weathering in an industrial atmosphere showed no adverse effect on the strength
properties. Corrosion was found to be confined only to fibres actually exposed on the
surface. Steel fibrous mortar continuously immersed in seawater for 10 years
exhibited a 15 per cent loss compared to 40 per cent strength decrease of plain mortar.
Structural Behaviour of FRC
1. Flexure; the use of fibres in reinforced concrete flexure members’ increases ductility,
tensile strength, moment capacity, and stiffness. The fibres improve crack control and
preserve post cracking structural integrity of members.
2. Torsion; the use of fibres eliminate the sudden failure characteristic of plain concrete
beams. It increases stiffness, torsional strength, ductility, rotational capacity, and the
number of cracks with less crack width.
3. Shear; addition of fibres increases shear capacity of reinforced concrete beams up to
100 per cent. Addition of randomly distributed fibres increases shear-friction strength,
the first crack strength, and ultimate strength.
4. Column; the increase of fibre content slightly increases the ductility of axially loaded
specimen. The use of fibres helps in reducing the explosive type failure for columns.
5. High Strength Concrete; fibres increases the ductility of high strength concrete. The
use of high strength concrete and steel produces slender members. Fibre addition will
help in controlling cracks and deflections.
6. Cracking and Deflection; tests have shown that fibre reinforcement effectively
controls cracking and deflection, in addition to strength improvement. In
conventionally reinforced concrete beams, fibre addition increases stiffness, and
reduces deflection.
Applications
The uniform dispersion of fibres throughout the concrete mix provides isotropic properties
not common to conventionally reinforced concrete. The applications of fibers in concrete
industries depend on the designer and builder in taking advantage of the static and dynamic
characteristics of this new material. The main area of FRC applications are:
1. Runway, Aircraft Parking, and Pavements
For the same wheel load FRC slabs could be about half the thickness of plain concrete slab.
Compared to a 375 mm thickness of conventionally reinforced concrete slab, a 150mm thick
crimped-end FRC slab was used to overlay an existing asphaltic-paved aircraft parking area.
FRC pavements are now in service in severe and mild environments.
2. Tunnel Lining and Slope Stabilization
Steel fibre reinforced shortcrete (SFRS) are being used to line underground openings and
rock slope stabilization. It eliminates the need for mesh reinforcement and scaffolding.
3. Blast Resistant Structures
When plain concrete slabs are reinforced conventionally, tests showed that there is no
reduction of fragment velocities or number of fragments under blast and shock waves.
Similarly, reinforced slabs of fibrous concrete, however, showed 20 per cent reduction in
velocities, and over 80 per cent in fragmentations.
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4. Thin Shell, Walls, Pipes, and Manholes
Fibrous concrete permits the use of thinner flat and curved structural elements. Steel fibrous
shortcrete is used in the construction of hemispherical domes using the inflated membrane
process. Glass fibre reinforced cement or concrete (GFRC), made by the spray-up process,
have been used to construct wall panels. Steel and glass fibres addition in concrete pipes and
manholes improves strength, reduces thickness, and diminishes handling damages.
5. Dams and Hydraulic Structure
FRC is being used for the construction and repair of dams and other hydraulic structures to
provide resistance to cavitation and severe erosion caused by the impact of large waterborne
debris.
6. Other Applications
These include machine tool frames, lighting poles, water and oil tanks and concrete repairs.
1.6.2. Ferrocement
It is a composite structural material comprising thin sections consisting of cement mortar
reinforced by a number of closely spaced layers of steel wire mesh. (ACI Concrete
Terminology) Ferrocement is a type of thin-wall reinforced concrete commonly constructed
of hydraulic-cement mortar reinforced with closely spaced layers of continuous and relatively
small wire mesh. The mesh may be made of metallic or other suitable materials. Ferrocement
has a very high tensile strength-to-weight ratio and superior cracking behaviour in
comparison to conventional reinforced concrete. Unlike conventional concrete, ferrocement
reinforcement can be assembled into its final desired shape and the mortar can be plastered
directly in place without the use of a form.
1.6.2.1. Constituents of ferrocement
Matrix
The matrix of ferrocement is usually cement mortar, consisting of cement, sand, water and
additive. The matrix should have some or all of the following requirements, depending on the
use of the structure, high compressive strength, impermeability, and hardness, resistance to
chemical attack, low shrinkage, and workability. Most of the available specifications
concerning the properties of the mortar used in ferrocement depend on observation and
practical consideration of the ferrocement uses, with some aid from the knowledge on
concrete technology. The main factors which affect the properties of the mortar are:
water/cement ratio; sand/cement ratio; gradation, shape and maximum size of aggregates;
quality, age, and type of cement; admixtures; curing condition; and mixing, placing and
compaction of mortar.
The limits of the above factors are affected by the requirements of the mortar which in turn
depend on the use of ferrocement.
1. Cement
Ordinary Portland cement is used in making of mortar. The cement should be fresh and free
from lumps.
2. Aggregates
Normal-weight fine aggregate is the most common aggregate used in ferrocement. The
aggregate consists of well graded fine aggregate that passes a 2.34 mm sieve; and since salt15
free source is recommended, sand should preferably be selected from river-beds and be free
from organic or other deleterious matter and relatively free from silt and clay. Good amount
of consistency and compaction is achieved by using a well-graded, rounded, natural sand
having a maximum top size about one-third of the small opening in the reinforcing mesh to
ensure proper penetration.
3. Water
The mixing water should be fresh, clean, and potable
The mix proportion ranges of the mortar for ferrocement application are sand/cement ratio by
weight of between 1 to 2.5 and Water-cement ratio by weight of 0.30 to 0.5. The amount of
water used should be the minimum consistent with compatibility. The mix should be as stiff
as possible, provided it does not prevent full penetration of the mesh. Normally the slump of
fresh mortar should not exceed 2 in. (50 mm). For most applications, the 28-day compressive
strength of 75 by 150-mm moist-cured cylinders should not be less than 35 MPa.
Reinforcement for Ferrocement
Different types of meshes are available almost in every country in the world. Two important
reinforcing parameters are commonly used in characterizing ferrocement and are defined as
Volume fraction of reinforcement; it is the total volume of reinforcement per unit volume of
ferrocement. Specific surface of the reinforcement, it is the total bonded area of reinforcement
per unit volume of composite. The principal types of wire mesh currently being used are
given below: Hexagonal wire mesh Welded wire mesh, Woven wire mesh, expanded metal
mesh and three dimensional meshes.
1. Hexagonal or Chicken Wire Mesh
This mesh is readily available in most countries and it is known to be the cheapest and easiest
to handle. The mesh is fabricated from cold drawn wire which is generally woven into
hexagonal patterns. Special patterns may include hexagonal mesh with longitudinal wires.
2. Welded Wire Mesh
In this mesh a grid pattern is formed by welding the perpendicular intersecting wires at their
intersection. This mesh may have the advantage of easy moulding into the required shape; it
has the disadvantage of the possibility of weak spots at the intersection of wires resulting
from inadequate welding during the manufacture of the mesh. Welded-wire fabric normally
contains larger diameter wires 2 mm or more spaced at 25 mm or more. Welded-wire fabric
could be used in combination with wire mesh to minimize the cost of reinforcement. The
minimum yield strength of the wire measured at a strain of 0.035 should be 414 MPa.
3. Woven Wire Mesh
In this mesh, the wires are interwoven to form the required grid and the intersections are not
welded. The wires in this type of mesh are not straight. They are bent in the shape of zigzag
lines and large angle of bending might cause cracks along the mesh However, the moulding
performance of this mesh is as good as the hexagonal and the welded wire mesh.
4. Expanded Metal Mesh
This mesh is formed by cutting a thin sheet of expanded metal to produce diamond shape
openings. It is not as strong as woven mesh, but on cost to strength ratio, expanded metal has
16
the advantage. This type of mesh reinforcement provides good impact resistance and crack
control, but they are difficult to use in construction involving sharps curves.
Figure 1.4: Types of mesh
1.6.2.2. Construction Methods and Applications
17
Construction process is important for ferrocement construction. Since the ferrocement
elements are very thin in the order of 10-25 mm, considerable care is to be taken to maintain
minimum cover of 3 mm.
1. Armature System
The armature system is a framework of tied reinforcing bars to which layers of reinforcing
mesh are attached on each side. Mortar is then applied from one side and forced through the
mesh layers towards the other side. The skeletal steel can assume any shape. Diameter of the
steel bars depends on the size of the structure. Skeletal steel is cut to specified lengths, bent to
the proper profile, and tied in proper sequence. The required numbers of layers of mesh are
tied to each side of the skeletal steel frame. Mortar is then applied preferably from one side
and forcing it to penetrate through the wire mesh openings of many layers till the time slight
excess quantity of mortar appears on the other side. After that the excess mortar is pressed
back and the remaining quantity is struck off. The skeletal steel is placed in both the
directions. They are not considered as structural reinforcement, they just add to the dead
weight of structure. Skeletal steel act as spacers rods to the wire mesh reinforcement.
Skeletal steel
2. Closed Mould System
The mortar is applied from one side through several layers of mesh or mesh and rod
combinations that have been stapled or otherwise held in position against the surface of a
closed mould. The mould may remain as a permanent part of the finished ferrocement
structure. If removed, treatment with release agents may be needed. The use of the closedmould system represented tends to eliminate the use of rods or bars.
3. Integral mould systems:
As the name implies in this method the mould used becomes integral part of the Ferrocement. A semi-rigid framework with some minimum layer of wire mesh or by using rigid
foam insulation material such as polystyrene or polyurethane can act as the integral mould.
After the placing of the mould wire meshes are fixed from both the sides and mortar is
applied.
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4. Press Fills Method
In ferrocement construction, mortar plastering and penetration on to the mesh plays a crucial
role. The mortar is usually applied in the mesh reinforcement either by hand or shot through a
spray gun device in order to get a homogeneous mixture of ingredients and produces almost a
fabric of mesh coated and well packed with mortar
5. Lay- Up Technique
Lay-up technique which involves placing the mesh in the mortar rather than the mortar in the
mesh; and successive layers of mesh are placed in layers of freshly sprayed, or manually
placed, mortar. To assure that mesh layers do not pop out, a thin mortar cover layer is placed
first and allowed to set, but not dry completely, prior to application of a second mortar layer
and the first mesh layers. This first layer of mortar cover is generally about 3 mm. A major
advantage of the lay-up technique is that each layer of mesh is placed under full visual
contact any gap in the mortar is immediately apparent and instantly corrected.
Figure 1.5: Typical cross-sections of ferrocement.
1.6.2.3.Properties of Ferrocement
Ferro-cement is an extremely thin reinforced member versatile material with depth of around
25mm but the properties which it has got it has with respect to material behaviour and its
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suitability for structural application is quite unique. Compared to other form of concrete
construction Ferro-cement possesses greater degree of toughness, durability, strength & crack
resistance.
1. Tensile strength of Ferro-cement is limited to the tensile strength of reinforcement
taken alone the direction of loading as mortar layer above itself has very less tensile
strength. Tensile strength in Ferro-cement is of same order as that of compressive
strength in it. Depending upon the orientation of the wire mesh tensile strength of the
Ferro-cement varies. Square mesh at 0° or 90° is equally efficient, while the
efficiency of square meshes in tension get reduced upto 50% if the wires are arranged
at 45° to the loading axis. Meshes with smaller opening decreases the size of the
cracks formed and thus indirectly helps out in obtaining high strength concrete. The
tensile strength of Ferro-cement is directly dependent on the volume of reinforcement
present in it. The more the reinforcement (wire meshes), the higher the tensile
strength. Transverse reinforcement helps in increasing tensile strength and safety of
structure.
2. Compression characteristics of the Ferro-cement are mainly due to high compressive
strength of cement mortar, though reinforcement also contributes to the compression
strength to Ferro-cement. But with the increase in the number of layers of wire mesh
(volume of reinforcement) the compressive strength increases. Compressive strength
ranges from 30 Mpa to 70 Mpa for typical Ferro-cement products.
3. Durability of any material depends upon the type of environment under which it is
placed/used. The durability issues related with Ferro-cement are quite similar to those
of reinforced concrete like permeability, corrosion, external causes from outside
which can be in any form physical, chemical or mechanical and due to internal causes
like differential temperature, alkali aggregate reaction and so on. The most important
property (constituent) which determines the durability of Ferro-cement in any kind of
environment is the cement mortar used. More impervious the mortar layer is, the more
durable the Ferro-cement is. The measures required to be applied in reinforced
concrete to insure its durability should also be applied to Ferro-cement. But Ferrocement being a thin reinforced structure there are certain unique factors which affects
it durability properties.
- The cover in Ferro-cement is abnormally low and thus it is relatively easy for
the corrosive agents to reach the reinforcement.
- Ferro-cement consists of greater volume of small diameters wire mesh as
compared to reinforced concrete. The specific surface area of the wire mesh is
unusually high and thus providing large area of contact on which corrosion
can take place hence potentially increasing the rate of corrosion in it.
- Most of the reinforcing meshes used in Ferro-cement are galvanized to prevent
corrosion but the zinc coating can have some adverse effects from gas bubble
formation.
In order to reduce the corrosion risk in Ferro-cement and make it more durable the
mortar which is used should have low water to cement ratio. Porosity and
permeability of the mortar should be reduced. Proper compaction of the mortar should
20
be carried out. And use of mineral additives and admixtures in Ferro-cement should
be made. Depending on the exposure condition, coating layer should be applied.
4. Liquid Retaining Capacity; Ferro-cement has got wide application in retaining water
like in liquid storage tanks. Leakage starts in the tank made up of Ferro-cement after
the first cracking has occurred. The leakage increases in the cracked reinforcement as
average crack width increases and with decrease in the wall thickness. With increase
in the volume or specific surface area of the wire mesh the leakage decreases as the
crack width and number of cracks formed decreases. The crack widths formed in
Ferro-cement are much smaller in size as compared to those formed in normal
reinforced concrete as a result of it the leakage occurring in Ferro-cement is
comparatively less.
5. Fire resistance; based on various studies carried on Ferro-cement by different
researchers on its fire resistance capability different results were obtained. Most of the
scholars agreed on that by providing a Ferro-cement jacket on reinforced structures
their resistance to fire increased. This property of Ferro-cement was due to its specific
heat capacity which was slightly higher than those of concrete cover and thus it can
absorb more heat as compared to that of concrete cover. Structure members are
weakened when exposed to high temperatures causing the structures to collapse. It
was found from the studies that by using thin layer of Ferro-cement as jacketing to the
reinforced members the surface spalling reduced due to reinforcing wire mesh.
Increase in wire mesh content in Ferro-cement significantly improved flexural and
toughness under normal conditions, after fire exposure the wire mesh content had no
longer significance on the two mechanical properties. Moreover, by increasing the
wire mesh content the insulation property becomes poorer which is basically due to
decrease in specific heat capacity of Ferro-cement. On the other hand increasing the
mortar covering resulted in improved insulation performance.
6. Impact resistance; Due to its higher ability of absorbing impact energy Ferro-cement
is very adequate to resist the impact as compared to that of conventional reinforced
concrete. The reason for high impact energy of Ferro-cement is due to high specific
surface area and large volume reinforcement in it. With addition of fly-ash and silica
fume to the matrix energy absorbing capacity due to impact increases. Thickness of
Ferro-cement also affects the impact resistance. Impact damage is generally localised
and occurs at the point of contact. Spalling of internal mortar layer and delamination
of the mesh layers may occur due to the impact. Punching failure may occur in Ferrocement having high impact energy due to highly reinforcement with wire mesh.
1.6.2.4. Applications of Ferrocement
1. Floating marine structures. Construction of Ferro-cement boats has been found
attractive because it can be fabricated into any shape and traditional designs could be
reproduced and often improved, besides being more durable and cheaper than wooden
boats. Ferro-cement is preferred for boat construction as it has got high impact
resistance. Use of Ferro-cement in boat building was one of its first applications.
2. Secondary roofing slab.
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3. Water tank construction. Ferro-cement can be used for the storage of water or other
liquid as it is strong, durable & water-tight. Leakage in such type of tank is very less
as compared to that or normal reinforced concrete structure.
4. Silos construction. The problem of food storage in the developing countries is
emerging as a major subject of attention from technical assistance organizations.
Advantage of Ferro-cement in building food-storage facilities in developing countries
is its adaptability to an almost unlimited range of curved shapes and local conditions.
Ferro-cement silos require little maintenance and they offer protection against
rodents, birds, insects, water, and weather.
5. Maintenance and repair of deteriorated structures. Cracking and spalling are some of
the major reasons for the deterioration of RCC structures. With time the cracks get
deepen and peeling of concrete starts. A good repair improves the function and
performance of structures, restore and increase its strength and stiffness, enhances the
appearance of the concrete surface, provides water tightness and prevents ingress of
the aggressive species to the steel surface durability. Ferro-cement can be used for
repair purpose of deteriorated RCC structures. Use of Ferro-cement can increase the
strength up to 30% and along with that it prevents formation of cracks. Confinement
of Ferro-cement around defective columns can enhance the strength, ductility and
energy absorption capacity of the existing structure. This confinement work also
protects the existing reinforcement, provides water tightness and prevents ingress of
the aggressive species to the surface of original concrete or steel surface.
6. Used in constructing members, hollow columns, wall, beams.
1.6.3. Ply-wood
Plywood is a building material consisting of veneers (thin wood layers or plies) bonded with
an adhesive. There are two types of plywood: softwood plywood and hardwood plywood.
Softwoods generally correspond to coniferous species. The most commonly used softwoods
for manufacturing plywood are firs and pines. Hardwoods generally correspond to deciduous
species. For hardwood plywood, commonly used wood species include oak, poplar, maple,
cherry, and larch.
Softwood plywood is manufactured by gluing several layers of dry softwood veneers
together with an adhesive. Softwood plywood is used for wall siding, sheathing, roof
decking, concrete form-boards, floors, and containers. Hardwood plywood is made of
hardwood veneers bonded with an adhesive. The outer layers (face and back) surround a core
which is usually lumber, veneer, particleboard, or medium density fibreboard. Hardwood
plywood may be pressed into panels or plywood components (e.g., curved hardwood
plywood, seat backs, chair arms, etc.). Hardwood plywood is used for interior applications
such as furniture, cabinets, architectural millwork, panelling, flooring, store fixtures, and
doors.
22
Figure 1.6: Plywood.
Types of Plywood
1. Structural plywood: Used in permanent structures where high strength is needed. This
includes flooring, beams, formwork, and bracing panels. It can be made from
softwood or hardwood.
2. External plywood: Used on exterior surfaces where a decorative or aesthetic finish is
important. It is not used to bear loads or stress, such as on exterior door surfaces, and
wall cladding.
3. Internal plywood: This has a beautiful finish, for non-structural applications like wall
panelling, ceilings, and furniture.
4. Marine plywood: It is specially treated using preservatives, paint, or varnish, to resist
water damage. It is used in shipbuilding, resists fungal attacks and does not
delaminate.
1.6.3.1.Manufacture
Plywood consists of the face, core, and back. The face is the surface that is visible after
installation, while the core lies between the face and back. Thin layers of wood veneers are
glued together with a strong adhesive. This is mainly a phenol or urea formaldehyde resin.
Each layer is oriented with its grain perpendicular to the adjacent layer. Plywood as a
building material is generally formed into large sheets. It may also be curved for use in
ceilings, aircraft, or ship building.
The manufacture of softwood or hardwood plywood consists of nine main processes: log
storage, log debarking and bucking, heating the logs, peeling the logs into veneers, drying the
veneers, gluing the veneers together, pressing the veneers in a hot press, plywood cutting,
and other finishing processes such as sanding.
The initial step of debarking is accomplished by feeding logs through one of several types of
debarking machines. The purpose of this operation is to remove the outer bark of the tree
without substantially damaging the wood. After the bark is removed, the logs are cut to
23
appropriate lengths in a step known as bucking. The logs (now referred to as blocks) then are
heated to improve the cutting action of the veneer lathe or slicer, thereby generating a product
from the lathe or slicer with better surface finish. Blocks are heated to around 93°C (200°F)
using a variety of methods - hot water baths, steam heat, hot water spray, or a combination of
the three.
After heating, the logs are processed to generate veneer. For most applications, a veneer lathe
is used, but some decorative, high quality veneer is generated with a veneer slicer. The slicer
and veneer lathe both work on the same principle; the wood is compressed with a nose bar
while the veneer knife cuts the blocks into veneers that are typically 3 mm (1/8 in.) thick.
Decorative hardwood veneers are usually sliced much thinner than 3 mm (1/8 in.) thick. The
veneer pieces are then clipped to a usable width, typically 1.37 m (54 in.), to allow for
shrinkage and trim. Veneers are taken from the clipper to a veneer dryer where they are dried
to moisture contents that range from around 1 to 15 per cent, dry basis. Face veneer moisture
contents can range up to 25 per cent, dry basis. Veneer moisture is checked against the target
moisture level as the veneer exits the veneer dryer. Veneer re-dryers may be used to re-dry
the veneer that did not reach the target moisture content.
After drying, veneers sometimes are glued together on the edges to form larger sheets of
veneer. This process is called composing. Narrow veneer slices must be composed before
they are used in plywood panels or other products requiring wider veneer sheets.
When the veneers have been dried to their specified moisture content, they are conveyed to a
layup operation, where a thermosetting resin is spread on the veneers. The two main types of
resins are phenol-formaldehyde, which is used for softwood plywood and exterior grades of
hardwood plywood, and urea-formaldehyde, which is used to glue interior grades of
hardwood plywood. The resins are applied by glue spreaders, curtain coaters, or spray
systems. Spreaders have a series of rubber-covered grooved application rolls that apply the
resin to the sheet of veneer. Generally, resin is spread on two sides of one ply of veneer,
which is then placed between two plies of veneer that are not coated with resin.
The laid-up assembly of veneers then is sent to a hot press in which it is consolidated under
heat and pressure. Hot pressing has two main objectives: (1) to press the glue into a thin layer
over each sheet of veneer; and (2) to activate the thermosetting resins. Typical press
temperatures range from 132° to 165°C (270° to 330°F) for softwood plywood, and 107° to
135°C (225° to 275°F) for hardwood plywood. Press times generally range from 2 to 7
minutes. The time and temperature vary depending on the wood species used, the resin used,
and the press design. The plywood then is taken to a finishing process where edges are
trimmed; the face and back may or may not be sanded smooth. The type of finishing depends
on the end product desired.
Composite plywood can also be engineered with a core of solid timber pieces or
particleboard, with a wood veneer for the face and back. Composite plywood is preferable
when thick sheets are required. Additional materials can be added to the face and back
veneers to improve durability. These include plastic, resin-impregnated paper, fabric,
24
Formica, or even metal. These are added as a thin outer layer to resist moisture, abrasion and
corrosion. They also facilitate better binding of paint and dyes.
Grades of Plywood
Plywood grades are determined by strength, discolorations, surface defects, and resistance to
moisture, among other properties. The quality of surface veneer, type of wood, and strength
of adhesive, will then be allocated a particular rating. Each rating will determine the type of
application the plywood is suited for.
Plywood grades are N, A, B. C, and D. The D grade has several surface defects such as
graining and knotting, while the N grade has few of these. An “interior C-D” rating for
example, indicates the plywood has a grade C face, and a grade D back. It also means the
adhesive is suited for interior applications.
1.6.3.2. Properties of Plywood
High Strength: Plywood has the structural strength of the wood it is made from. This is in
addition to the properties obtained from its laminated design. The grains of each veneer are
laid at 90 degree angles to each other. This makes the whole sheet resistant to splitting,
especially when nailed at the edges. It also gives the whole sheet uniform strength for
increased stability. Furthermore, plywood has a higher strength to weight ratio as compared
to cut lumber. This makes it ideal for flooring, webbed beams, and shear walls.
High panel shear: Plywood is made with an odd number of layers, making it tough to bend.
The angle at which the veneer grains are laid against each other may be varied from 90
degrees. Each veneer can be laid at a 45 or 30 degree angle to the next one, increasing the
plywood’s strength in every direction. This cross lamination increases the panel shear of
plywood, important in bracing panels and fabricated beams.
Flexibility: Unlike cut timber, plywood can be manufactured to fit every requirement. The
thickness of each veneer can vary from a few millimetres to several inches. The number of
veneers used also ranges from three to several, increasing the thickness of the sheet. The
extra layers add more strength to the plywood. Thinner veneers are used to increase
flexibility for use in ceilings and panelling.
Moisture resistance: The type of adhesive used to bind the veneers makes the plywood
resistant to moisture and humidity. A layer of paint or varnish can also increase resistance to
water damage. These types of veneers are suitable for exterior use such as cladding, sheds,
and in marine construction. They are also suited for holding concrete while it sets. Moisture
resistance is important in interior applications as well, including on floors. The cross
lamination ensures the veneers do not warp, shrink, or expand when exposed to water and
extreme temperature.
Chemical resistance: Plywood treated with preservative does not corrode when exposed to
chemicals. This makes it suitable for chemical works and cooling towers.
Impact resistance: Plywood has high tensile strength, derived from the cross lamination of
panels. This distributes force over a larger area, reducing tensile stress. Plywood is therefore
able to withstand overloading by up to twice its designated load. This is critical during shortterm seismic activity or high winds. It is also useful in flooring and concrete formwork.
25
Fire resistance: Plywood can be treated with a fire resistant chemical coating. More
commonly, it is combined with non-combustible materials such as plasterboard or fibrous
cement. This makes it ideal for use in fire resistant structures.
Insulation: Plywood has high thermal and sound insulation. This makes it a useful insulating
material for flooring, ceilings, roofing, and wall cladding. Insulation offered by plywood can
greatly reduce heating and cooling costs.
•
•
•
•
•
•
1.6.3.3. Uses of Plywood
To make light partition or external walls
To make formwork, or a mould for wet concrete
To make furniture, especially cupboards, kitchen cabinets, and office tables
As part of flooring systems
For packaging
To make light doors and shutters
1.6.4. Glue-laminated timber
1.6.4.1.Introduction
Glulam (also known as glued laminated timber, laminated wood, glulam beam, or classic
glulam) is a composite material with more uniform distribution and higher values of
mechanical characteristics than wood. Thin laminates are arranged so that the grain is
generally parallel; they are glued together with structural adhesives that are rigid and durable,
water resistant, and resistant to humidity, temperature, and biological factors.
Due to its outstanding elastic and mechanical characteristics, it can be used for production of
individual beams and columns as well as for large-span planar and spatial construction.
Figure 1.7: Plywood
26
Advantages of glued-laminated timber
1. Size
Glued-laminated timber permits the creation of structural members that are much larger than
the trees from which the component lumber is sawn. By combining small pieces of lumber in
the form of glued-laminated timber, large structural members can be created.
2. Architectural Freedom
The long, clear spans afforded by glued-laminated timber allow for open floor plans
unconstrained by columns. Because of their natural beauty, glued-laminated timbers are most
often left exposed as a decorative element in residences, churches, shopping centres and other
public-use structures. By bending the lumber during the manufacturing process, a variety of
architectural effects, including arches and compound curves, can be created that are difficult
or even impossible
3. Kiln-Dried Lumber
The lumber used in fabricating glued-laminated timber must be kiln dried prior to assembly;
therefore, the effect of checks, splits, warpage, and other defects, which normally develop as
sawn timbers dry in service, on the strength and appearance of laminated members is
minimized. In addition, structures built with glued-laminated timber can be designed on the
basis of seasoned wood, which permits the use of higher allowable design values than can be
assigned to unseasoned timber. Thus, there is both an appearance advantage–minimal
checking — and a structural advantage — higher allowable design values–when using gluedlaminated timber compared with sawn timber.
4. Variable Cross Section
Structural members may be designed with a variable cross section along their length, as
determined by the strength and stiffness requirements of the application. For example, the
central section of a glued-laminated timber can be made deeper to account for the increased
stress that occurs in this region. Arches often have a variable cross section for the same
reason.
Figure 1.8: Glulam Cross-sections
27
5. Efficient Use of Lumber Grades
A major advantage of glued-laminated timber is that the laminating process allows for the
strategic placement of different grades of lumber within a member. Typically, the best grades
of lumber are placed in the highly stressed laminations near the top and bottom of the
member, while the lower grades of lumber may make up the inner half or more of the
member. This means that a large quantity of lower grade lumber can be used for these less
highly stressed laminations. Species can also be varied within a member to match the
structural requirements of the laminations.
6. Environmentally Friendly
Much has been discussed and written regarding the relative effects on the environment of
using wood, concrete, steel, and other structural materials. Several analyses have shown that
wood's renewability, relatively low energy consumption during manufacture, carbon storage
capability, and recyclability offers potential long-term environmental advantages compared
with other structural materials.
1.6.4.2.Manufacture
The glued-laminated timber manufacturing process consists of four phases:
1. Drying and grading the lumber
To minimize dimensional change following manufacture, as well as to take advantage of the
higher allowable design values assigned to lumber compared with large sawn timbers, it is
critical that the lumber be properly dried. This generally means kiln drying. For most
applications, the maximum moisture content of laminations permitted under ANSI/AITC
A190.1 is 16 per cent, which results in a member with an average moisture content of about
12 per cent. In addition, the maximum range in moisture content permitted among
laminations is limited to 5 percentage points to minimize differential changes in their
dimensions following face gluing.
2. End jointing the lumber into longer laminations
To manufacture glued-laminated timber in lengths beyond those commonly available for
sawn lumber, laminations must be made by end jointing lumber. The most common end joint
is a structural finger joint about 1.1 inches long. Structural finger joints have the potential to
attain 75 per cent or more of the tensile strength of clear wood for many species of lumber.
The standards require that end joint meet the required strength level of the highest strength
grade of glued-laminated timber it is going to produce. This necessitates that the results of
tensile tests of end-jointed lumber meet certain strength criteria, and that adhesive durability
test results do the same.
Figure 1.8: Plywood.
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3. Face gluing the laminations
The laminates are then assembled by gluing the laminates together by using adhesives that
meet specific shear strength and durability requirements for the species e.g. Phenol
resorcinol. To obtain clean, parallel, and gluable surfaces, the best procedure is to plane the
two wide faces of the laminations just prior to the gluing process. This ensures that the final
assembly will be rectangular and the pressure will be applied evenly. Adhesives that have
been pre-qualified are then spread, normally with glue extruder.
Laminations are assembled into the specified lay-up pattern. Straight beams may or may not
have a camber built in. Laminations wider than about 11 inches (a 2 by 12) are made by
placing narrower pieces side-by-side during face gluing, such that the edge joints in adjacent
laminations are offset. Laminations for curved members and arches are glued and cured in
curved forms that define their shape. The degree of curvature in a glued-laminated timber is
controlled by the thickness of its laminations.
After the adhesive is given sufficient time to begin to penetrate into the wood, pressure is
applied with clamping beds in which a mechanical or hydraulic system brings the laminations
into intimate contact. The adhesive is allowed to cure at room temperature for 6 to 24hours
before pressure is released. Upon completion of the process, the adhesive usually has most of
its ultimate strength. Curing continues for the next few days, but at a much slower rate.
4. Finishing and fabrication
After the glued-laminated timber is removed from the clamping system, the wide faces
(sides) are planed or sanded to remove beads of adhesive that have squeezed out between
laminations and smooth any slight irregularities between the edges of laminations. This
results in the finished glued-laminated timber being slightly less in width than the dimension
lumber from which it was made.
The next step in the glued-laminated timber manufacturing process is fabrication. Here final
cuts are made, holes drilled, connectors added, and, if specified, a sealer or finish is applied.
Depending on the member’s intended use, some prefabrication of parts may be done at this
point.
1.6.4.3.Properties of Glue-laminated Timber
1. Fire
Glulam elements have significantly better fire resistance than is generally attributed to them,
surpassing the fire resistance of steel and reinforced concrete beam structure. The capacity of
wood to conduct heat is insignificant because it conducts heat 300 to 400 times more slowly
than steel. The elements char slowly from the surface towards the inner parts. Charring
reduces heat conduction and prevents oxygen from reaching the wood. In the non-charred
cross-section the beams preserve full load capacity. In the non-charred cross-section the
beams preserve full load capacity. In a normal fire, solid spruce wood burns at a speed of 0.6
to 1.1 mm/min and glulam at 0.1 mm/min. Glulam elements do not change shape during
combustion. As a result, the beams do not exert pressure on peripheral walls and do not cause
them to collapse.
2. Ecology
Although aesthetic and economic considerations are usually the major factors influencing
material selection, the environmental advantages of using wood may have an increasingly
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important effect on material selection. From the environmental viewpoint, wood
biodegradability and the possibility of recycling are important factors. The production of
glulam elements requires approximately twice the energy (1,218 KWh/m³) as a comparable
solid wood construction element (688 KWh/m³). Adhesives are essential for the load-bearing
durability of glulam elements.
Many adhesives contain formaldehyde, which is harmful for health and is a burden on the
environment. However, adhesive accounts for only approximately 1 % of the entire volume
of the construction. As a result, the ecological advantages of glulam constructions are not
significantly lower than those of solid wood constructions, especially when considering the
almost inevitable use of metal connectors.
3. Architecture and construction aspects
With regard to architecture, the main advantage of glulam elements is their versatility in
form, which makes possible various shapes and dimensions. They have an aesthetic appeal
and preserve elegance even with large spans. As regards construction, one of the main
advantages of wood constructions is their high loading capacity in relation to their own
weight (20% of the weight of reinforced concrete).
4. Mechanical properties
Mechanical properties are established or experimentally defined with procedures prescribed
in the HRN EN 408 standard. Glulam strength classes are described in HRN EN 1194 (Wood
Constructions – Glued Laminated Timber – Strength Classes and Determination of
Characteristic Values). The standard categorizes and defines the use of two types of glulam:
homogenous glulam (i.e., GL 24h, GL 28h, GL 32h, and GL 36h) and combined glulam (i.e.,
GL 24c, GL 28c, GL 32c, and GL 36c). The number following the label GL (glued
laminated) is the characteristic bending strength in MPa (N/mm²). The inner part of the crosssection of combined wood is made from lower-strength wood. The minimum thickness of the
outer part is one-sixth of the height of the element or two laminates.
1.6.4.4.Application of Glued-laminated Timber
1. Commercial and Residential Buildings
Rectangular Beams – Glued-laminated timber beams are popular as the main load-carrying
members in flat and low-slope roof systems for single-story warehouses, shopping centres,
and factories.
Pitched and Curved Beams–Roof systems made with pitched and tapered curved gluedlaminated timber beams provide both a pitched roof and an interior space with extra ceiling
height, without increasing the height of the supporting wall. Applications for these members
include office, institutional and other buildings that call for long, clear spans and high
ceilings.
Trusses–Many kinds of trusses are fabricated with glued-laminated timber. The most
common are bowstring, parallel chord, and pitched chord configurations. Glued-laminated
timber is particularly suited to bowstring trusses because the top chord can be manufactured
to the desired curvature.
Multi-storey Heavy Timber-Office and retail buildings up to five stories high have been built
using glued-laminated timber as the main load-carrying members, most often in the exposed
post-and-beam style.
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Arches–Glued-laminated timber arches provide for both the efficient transfer of roof loads
directly to the foundation and dramatic architectural effects. Their numerous shapes allow
architects to create unique structures with sweeping lines. Glued-laminated timber arches
have been used widely in bridges, religious structures, concert halls, swimming pool
enclosures, skating rinks, and other sports venues.
Domes–The high strength-to-weight ratio of glued-laminated timber makes it advantageous
for use in long-span domes for sports arenas, auditoriums, and other assembly-use buildings.
Construction Posts–Post-frame buildings are widely used for farm structures and light
commercial buildings. Glued-laminated timbers are sometimes used for the posts or wall
columns.
2. Bridges
The main use of glued-laminated timber in bridges is for parts of the superstructure, such as
girders and decking. One popular type of bridge consists of glued-laminated timber stringers,
spanning between supports and glued-laminated timber decking placed transverse to them.
For spans up to 25 to 30 feet, a longitudinal glued-laminated timber deck that requires no
stringers is often used. Special architectural effects can be achieved by supporting the main
span with glued-laminated timber arches.
3. Utility Structures
Glued-laminated timber is used in power transmission towers, light standards, and other
utility applications to meet special size and shape requirements that cannot be obtained using
conventional wood poles. Glued-laminated timber cross arms and davit arms are used to
support wires on wood poles.
1.6.5. Laminated plastics
1.6.5.1. Introduction
Laminated plastics are a special form of polymer-matrix composite consisting of layers of
reinforcing materials that have been impregnated with thermosetting resins, bonded together,
and cured under heat and pressure. In a nutshell, plastic laminate is simply multiple layers of
thin kraft’s paper which is soaked in a melamine resin. These impregnated layers of paper are
subject to high pressure and temperature which hardens the paper and resin.
The colours, patterns, etc. that you see are just the top layer of decorative paper. Beneath the
decorative paper layer lie sheets of brown kraft paper. These layers of kraft paper impart
resilience or resistance to impact. The more layers of kraft paper, the better the laminate can
withstand impact.
1.6.5.2. Manufacture
It is made of resins that react with aldehydes during the thermosetting process. Kraft paper is
the same brown paper used in grocery bags. The top two layers of paper are impregnated with
melamine resin, and the lower layers use phenolic. Plastic laminate is a composite building
material made from kraft paper, resin, and adhesives. The process begins by soaking strips of
paper in resin. Decorative plastic laminates can be made in different grades or thicknesses,
depending on its intended use. The kraft paper is run through a "bath tub" or vat containing
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phenolic resins. The paper for the top layer of the sheet is translucent which is run through a
vat of melamine resin. This sheet of paper is printed with the colour or design that will show
through the clear top layer for the desired surface pattern. This sheet is also run through a
melamine vat.
The resin-impregnated sheets are then put into a drying chamber. Next, they are cut and
stacked in layers putting the clear layer and the decorative layer on top of the kraft paper.
Thermosetting; the layers of paper are then loaded onto a flat-bed hydraulic press for final
curing. The press compresses the sandwich of resin-soaked paper at 1,400 psi, while heating
it to a high temperature. The heat catalyses a reaction in the resins where the phenol (or
melamine) and formaldehyde molecules Attach to each other in an alternating chain fashion,
releasing water molecules in the process. Thermosetting converts the paper sheets into one
single, rigid laminated sheet. This sheet is dry and insoluble, and it cannot be shaped or
moulded, even at high temperatures. The dry sheet is cut into the desired size and shape. It
may also be bonded to a building material such as plywood, flakeboard, fibreboard, or metal.
1.6.5.3. Advantages and disadvantages of plastic laminate
Advantages of using plastic laminate
- The laminate is stain and grease resistant and durable if properly cared for.
- Forming shapes with plastic laminate is possible.
- Plastic laminate is relatively easy to manufacture.
- Plastic laminate modular is less expensive than other.
- In situations where the design aesthetic is important, the wide varieties of colours,
textures, and patterns available in plastic laminate make it an attractive choice.
- Has a lower noise level than stainless steel modular works, and is a good choice for
situations that require a quieter atmosphere.
Disadvantages of using plastic laminate
- Tearing of the plastic laminate results in jagged and razor sharp edges and can cause
serious injuries.
- Delamination can be caused by improper fabrication techniques.
- Laminate failure where defective product for manufacturing are used. Some of the
multiple layers from the base layers of paper are not properly bonded to one another
and may be discovered after the product is installed and used.
- Failure of the installed product due to improper attachment, inadequate support, or
environmental changes and poor design choices.
- Delamination and separation caused by improper maintenance and cleaning
techniques.
- Blistering and bubbling caused from exposure to excessive heat generated from
sources such as exterior windows that magnify the outside light from the sun.
- Failure due to water and steam in proximity
- Failure due to surface imbalances and moisture absorption.
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-
They lack the ultra violet protection in most products and the changes in heat and cold
often create differential expansion problems between the substrate and the plastic
laminate itself.
Points to note about adhesive
- In an enclosed area, such as an office or hospital environment, water based contact
cement should be used, if not exclusively required, as fumes from the glue are at a
minimum.
- In respiratory sensitive areas, it is mandatory that certain water based products be
used and well ventilated or filtered while drying. Outgassing of glue vapours as the
product dries often yields an ammonia like scent that can be irritating to sensitive
respiratory conditions.
- In a cabinet shop environment, where ventilation can be controlled, other more
volatile products can be used.
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1.6.5.4. Application
Floors, walls and roofs
Vehicle internal body work
Packages and boxes
Road cases
Residential countertops and cabinetry, Office work areas
Public restrooms
Department store wall panels, Corporate offices, Retail stores and malls,
Hospitality environments, Healthcare settings, Educational institutions, Casino slot
bases, Airplanes, Mass transit seats, Theme park rides, Furniture, Interior doors,
Marker boards, In high-traffic areas, Cashier/checkout stations, Kick plates, Pony
walls, Retail fixtures, Bar tops.
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