1.0 INTRODUCTION
Concrete is the mainly and widely used to construct
the structures such as highways, streets, parking lots, parking garages,
bridges, high-rise buildings, dams, homes, floors, sidewalks, driveways, and
numerous other applications. Concrete’s versatility, durability, and economy
have made it the world’s most used construction material. However, concrete is
an inherently brittle materials with a relatively low tensile strength as compared
to its compressive strength, requiring a lot of reinforcement.
Basically, concrete obtained by mixed the cement,
water and aggregate with the proper proportion on mix design. For advance
condition, the concrete possibly made from other cementitious material. These
materials called blended cement where there is a partial replacement of
Ordinary Portland Cement. The two most common replacement materials are Pulverized-Fuel
Ash (pfa) and slag. The use of blended cement in concrete is to improve the properties
of concrete such as compressive strength, workability, heat of hydration and
sulphate resistance.
2.0 PROPOSAL
TO REPAIR THE DAMAGED REINFOCED CONCRETE STRUCTURE
The
common factors which cause concrete to deterioration are corrosion of steel
reinforcement, mechanical wear and tear, chemical attack, use of poor quality
materials, bad workmanship and errors in construction and specifications.
Otherwise, concrete deterioration also due to cracks, spalls and permeability.
Concrete
used in the marine environment faces simultaneously the physical, the chemical
and the mechanical deterioration process. The marine environment is generally
divided into three zones depending upon their effect on the structure. Concrete
in each environment zone is subjected to different types of attack zones.Besides
physical and chemical reactions, the concrete in the marine structure located
in the tidal zone also faces mechanical forces and therefore deterioration is
generally observed to be more severe.
Cracking
of concrete can be defined as a separation of the individual components of
concrete resulting in a discontinuous material. Depending upon the extent of
cracking the cracks can be classified as bond cracks, mortar cracks and
aggregate cracks. Failure occurs when the sufficient interconnected bond cracks
with mortar cracks. When an extensive continuous crack pattern has developed
and the load path has been reduced considerably, the carrying capacity of
concrete decrease, and from this stage, the stress-strain curve begins to
descend.
2.1 Deterioration
of Reinforced Concrete Structures
Analysis by a repair specialist is
important step in the process of deteriorated concrete. The analyst must carry
out the analysis of the nature of the damage, the concrete quality, depth of
steel reinforcement, depth of carbonation and the environmental factors from
the examination. The objectives of any structural repair should be returning
the concrete to a satisfactory condition of structural adequacy, durability and
appearance at a cost.
The deterioration of reinforced concrete structures
or its composite building materials, concrete and steel can be broadly
classified as follow:
i.
Deterioration due to material-related
stresses and
ii.
Deterioration due to function – related
stresses
Material related stresses are caused by internal
chemical reaction within the building material itself which eventually will
result in physical changes. Common symptoms of these stresses are expansion and
swelling. Damage due to these material related stresses are normally discovered
late since these forces normally work from centre of the material outwards.
Function related stresses are stresses relating to design function of the
structure, which impose strain on the building materials or affect its
durability of resistance to external influences associated with normal use or
exposure to the environment. These function related stresses in particular
affect to follow part of the reinforced concrete structure.
2.2 Corrosion
of Steel Reinforcement
Corrosion
of the steel reinforcement is an electro-chemical process that occurs at the
interface between the reinforcing bars and the cementitious matrix. Corrosion
of reinforcing can be simplified into two processes, anodic and cathodic. The
anodic process is the dissolution of iron atoms to ferrous ions when the
protective layer at the surface of the reinforcement has been destroyed. The
cathodic process involves the reduction of oxygen as it reacts with water to form
hydroxyl ions. The anode and cathode are separated by distances that can vary
greatly. The anode and cathodes areas may alternate along a continuous
reinforcing steel bars when areas of the bar become anodic and adjacent areas
become cathodic. Oxygen is only required at the cathode to remove electrons
from the bar that were liberated from the oxidation of the iron. The
equations are as below:
i.
Anodic process Fe ↔ Fe² + 2e
(characterized by pitting)
ii.
Cathodic process H2O
+ 1/2O2+
2e ↔2 (OH) (characterized by rust formation)
The speed at which
corrosion advances depend on:
i.
Local difference in electrical
potential, which nearly always occur in the case of steel owing to the presence
of surface contaminants or variations in the structure of the metals.
ii.
The presence of an electrolyte which is
conductive, as in the form of a thin film of rainwater which always containsa
certain amount of carbonic acid produced by chemical conversion of carbon
dioxide absorbed from the atmosphere.
iii.
The presence of other contaminants such
as sulphate and chlorides which can substantially help to speed up the
corrosion process by destroying the highly alkaline calcium hydroxide
protective film formed by the hydration of a cement.
The steel corrosion also can due to carbonation. Carbon
dioxide in the atmosphere can enter the capillaries of the concrete or through
cracks formed by shrinkage, creep, plastic settlement or partial trensile
failure to react with the strongly alkaline calcium hydroxide formed by the
hydration of tricalcium silicate and dicalcium silicate which are the main
constituents of ordinary Portland cement. The carbonation process progressively
lowers the initial alkalinity of concrete which is 12. Below 9.5 the alkalinity
is no longer sufficient to support the passivating oxide film and the access of
moisture and oxygen can cause the steel reinforcement to corrode when the
carbonation depth reached the steel reinforcement.
Other harmful substances also can attack the steel
reinforcement. In highly alkaline environment, corrosion of steel reinforcement
can still occur due to localized attack from a variety of corrosive substances
such as chlorides ion which may be present in the concrete from additives used,
aggregates or water used in the concrete mix. In service, chloride can migrate
into the concrete in marine environment or from exposure to de-icing salts.
Sulphates and sulphites which may be deposited in high density industrial
arteas where the atmosphere is polluted with sulphur dioxide.
The load-bearing capacity of reinforced concrete is
affected by various imposed stresses resulting from loading and deformation of
the structural member those imposed stresses can take various form such as the
dead weight of the member itself, normal service or traffic loads, additional
or exceptional loads or from internal stresses resulting from restrained
shrinkage, restrained thermal contraction and expansion or other causes. Most
common cases of damage under this category are those due to uniform and
unscheduled stress associated with accidents, disasters or subsidence or
changes in the use of the structure. The principal types of accident or
disaster encountered in practice are natural force such as earthquakes and
floods, also explosion such as detonation, war damages or fire.
2.3 Procedures of Repairing the Damaged
Reinforced Concrete Structure
It is important to
consider the steps in the repair process before any structural repair is undertaken.
The following five steps are important to consider:
i.
Evaluating of causes, extend and
consequences of deterioration.
ii.
Selection of suitable repair material.
iii.
Surface preparation.
iv.
Application of repair materials.
v.
Application of protective coatings.
By evaluating of causes,
extent and consequence of deteriorated of concrete, the engineer must be
identified whether the damage will impair the structural performance by assess
the need and urgency for repairs as well as the option available. There are
many types of repair materials such as portland cement mortar, polymer modified
cementitious mortars, epoxy resin mortar to low viscosity epoxy resins. Before
that, there are factors need to be considered such as strength, compability,
appearance and cost. Surface preparation is one of the most important in
concrete repair. Lack of adequate surface preparation has been identified as
the most reason for poor repairs. The concrete substance must be sound, free
from laitance, loose or segregated materials, voids or flaws, and substance
which could decrease the bond between the old and the new concrete. The most
common techniques for preparing the concrete substrate are:
i.
Hammer and chiseling method mainly used
on small localized area.
ii.
Sand blasting normally used for cleaning
large areas where thin layers of material like laitance, paint, coatings and surface
contaminants need to be removed.
iii.
High pressure water jets are also used
for large areas cleaning and removing the surface skin.
The
repair mortar such as acrylic based, polymer latex based and epoxy resin based
bonding aids must be applied when the bonding coat is still tacky to achieve
the bonding effect. Also, the steel reinforcement needs to treat to remove all
the rust or to stabilize the surface by some special treatment. There are two systems
to protecting the steel, based either on reactive resins or polymer modified
cements. Methods of derusting can be achieved by normal wire brushing for
smaller areas or by sand or grit blasting for large areas.
Patching and
resurfacing, pressure grouting, sprayed concrete and crack repairs are the
methods available to repair the concrete. Patching normally refers to repairing
relatively small areas of localized damage using mortar. Resurfacing or reinstatement
refers to the application of mortars to large surface areas. The damaged area
is restored to the profile of the surrounding undamaged concrete. The repair mortar
is mixed in a mortar mixer mounted with a slow speed paddle. The mortar is then
applied using metal trowel or screed and finish with wooden float or sponge
float. While, the cement grout which are of pumpable consistency are injected
to the area enclosed in tight formwork under pressure using a hand operated
grout pump or motorized grout pump. The grout may consist may consist of neat
cement grout with an admixture or may be pre-blended in bags. High strength non
shrink grouts are easily available today. For large voids to be grouted, it is
advisable to include suitable size aggregates in order to achieve better
compressive strength and also to reduce shrinkage of the grout. This method is
known as repacked grouting. Sprayed concrete used when the large areas of
walls, arches and soffit of slabs or deck, repair by sprayed concrete is the
most economical and fast method. In this method, the sand and cement are
pre-mixed and conveyed pneumatically to the nozzle where the gauging water is
introduced under pressure. Crack also can be repair by pressure injection of
epoxy resin adhesive is today a generally accepted technique for repairing of
structural cracks. The procedures involved are as follows:
i.
Sealing along the cracks leaving
injection ports at centres equal to the depth of crack
ii.
Injecting the liquid epoxy so that air,
water vapour and water are displaced.
iii.
Curing
iv.
Removing the surface seal where aesthetics
require.
For
non-structural crack permits ingress of contaminants that may accelerate deterioration
of steel reinforcement and concrete. A non-structural crack may eventually
become structural if not repaired. The objective of repair is to install a
barrier to corrosive elements and the best time to repair such cracks is when
they first occur since contaminants can block the flow of epoxy and impair the
bonding surface.
After the completion of the repair,
it is advisable to paint the repaired areas and the non-repaired areas with a
protective coating. The objective is to give the whole structure a uniform
appearance and to reduce the permeability of the remaining sound concrete to
ingress of oxygen which is essential to the process of corrosion of steel
reinforcement, water and aqueous solution such as sea water and carbon dioxide,
to prevent further carbonation of the concrete. The important thing is the
protective coating must have a good resistance to UV radiation and atmospherically
attack. Other than that, the coating must be good on penetration and adhesion
on alkaline and non-alkaline. Also, it must low on permeability for water,
vapour and carbon dioxide to prevent the carbonation attack.
3.0 THE
SELECTION OF CONSTRUCTION MATERIAL
The structure which is built in certain condition
such as facing the sea in the earthquake zone must have a special construction
material. The building must be sulphate resistant and not easily cracked. Thus,
construction material selection for this type of building must be accurate to
avoid the problems in the future.
3.1 Palm
Oil Fuel Ash (POFA)
Palm Oil Fuel Ash or common known as POFA have the
potential to be used as recycle construction materials as pozzolans. POFA is
the ashes produced from husk fiber and shell of palm oil burning by generation
plant boiler which generate energy to be used in palm oil mill in order to
extract palm oil. POFA is found having a high pozzolanic material and it is not
just can be used as partial cement replacement but also can increase the
compressive strength and durability of concrete. The applications of pozzolans
in concrete give better result in 30% optimum mixing which is more 10% better
than the normal concrete.
|
Tests
|
OPC
|
POFA
|
|
Physical
Properties:
Fineness – Sp.surface area (m²/kg)
Specific gravity
Chemical
Analysis (%)
Silicon dioxide (SiO2)
Aluminium Oxide (Al2O3)
Ferric Oxide (Fe2O3)
Calcium Oxide (CaO)
Magnesium Oxide (MgO)
Sulphur Trioxide (SO3)
Alkalies
Loss on Ignition (LOI)
|
225-300
3.15
20
6
3
60-63
1.5
2.0
1.0
2.0-2.7
|
500-725
2.22-2.64
40-55
11-13
4.5-8.0
8.5-10
4-5
1-3
2.5-4.0
4-18
|
|
Pozzolanic Activity Index with OPC
|
----
|
112
|
Table
3.1: Physical properties and chemical composition of typical OPC and POFA
POFA are suitable to use as cement replacement
materialto suit the marine condition. The table below shows the
characterization of ageing performances of concrete exposed to marine
environment.
|
Tests
|
Exposure
Period
|
Types of
Concrete
|
|
|
OPC
|
POFA
|
||
|
Visual Observation
|
2-year
|
Almost intact with few bore holes on the
surface of the concrete specimens
|
Same as OPC concrete
|
|
Compressive Strength (MPa)
|
6-month
1-year
2-year
|
56.8
59.9
60.7
|
58.1
59.5
61.9
|
|
Flexural Strength (MPa)
|
6-month
1-year
2-year
|
6.85
6.95
6.90
|
7.70
7.00
7.50
|
|
Chloride Penetration (mm)
|
6-month
1-year
2-year
|
21.0
23.5
28.0
|
13.5
17.0
20.5
|
|
Carbonation (mm)
|
6-month
1-year
2-year
|
0.0
1.5
2.0
|
0.5
1.5
1.5
|
Table 3.2: Characterization of
ageing performances of concrete exposed to marine environment.
Various aspects of POFA were proven
that ability of this cement replacement material can be used to improve the
quality and resist the corrosion of reinforcement. Other than that, POFA can
give the higher ultimate strength to the concrete and enhance the durability of
concrete.
3.2 Fibre Reinforced Concrete (FRC)
Fiber
Reinforced Concrete (FRC) is made from hydraulic cements with or without
aggregate of various sizes and incorporating in the main, discrete fibre
reinforcements. The concept of using fibers as reinforcement is not new. Fibres
have been used as reinforcement since ancient times. Historically, horsehair
was used in mortar and straw in mud bricks. In the early 1900s, asbestos fibers
were used in concrete, and in the 1950s the concept of composite materials came
into being and fiber-reinforced concrete was one of the topics of interest.
There was a need to find a replacement for the asbestos used in concrete and
other building materials once the health risks associated with the substance
were discovered. By the 1960s, steel, glass (GFRC), and synthetic fibers such
as polypropylene fibers were used in concrete, and research into new
fiber-reinforced concretes continues today.
The main objectives of FRC are to improve the
tensile or flexural strength and impact strength. Other than that, the FRC use
in concrete to control cracking and the mode of failure by means of
post-cracking ductility. FRC also used to change the rheology or flow
characteristics of the material in the fresh state. There are several types of
fiber in the market and before using a material in the concrete mix, it is
important to know the nature of the material. Otherwise, the cost and need of
the fibre also be considered. The table below shows the properties of the
fibres.
|
Fibre
|
Diameter
(µm)
|
Density
x10³ Kg/m³
|
E
KN/mm²
|
Tensile
Strength KN/mm²
|
Elongation
at break (%)
|
|
Asbestos
a) Chrysotile
b) Crocidolite
Carbon
a) Type
I
b) Type
II
Polypropylene
Nylon
(Type
242)
Kevlar
a) PRD
49
b) PRD
29
Sisal
Glass
Steel
|
0.02-20
0.1-20
3
9
20-200
>4
-10
12
10-50
9-15
5-500
|
2.55
3.37
1.90
1.90
0.9
1.14
1.45
1.44
1.5
-2.6
7.8
|
164
196
380
230
5
4
133
69
-
-80
200
|
3.1
3.5
1.8
2.6
0.5
0.9
2.9
2.9
0.8
2-4
1-3
|
2-3
2-3
-0.5
-1.0
-20
-15
2-6
4-8
-3
2-3.5
3-4
|
Table
3.3: Fibre Properties
The factors affecting the durability may be external
or internal causes. The external causes may be physical, chemical and
mechanical which are environmental, such as occurrence of extreme temperatures,
abrasion and electrostatic actions, and chemical attack by natural or
industrial liquids and gases. While the internal causes include the
Alkali-Aggregate reaction and Volume changes due to difference in thermal
properties of the aggregate and cement paste.
Fibre reinforcement has been reviewed to use as the
structural application where this material can provide enhanced stability and
integrity to preserve conventionally-reinforced concrete structures subject to
earthquake and explosive loading.
4.0 CONCLUSION
Reinforced concrete is a very versatile and durable
building material. Reinforced concrete efficiently combines the best properties
of concrete and reinforcing steel into a strong structural element. In
addition, the high alkalinity of concrete helps to protect the embedded steel
from corrosion. However, due to the porosity of concrete, its use in exterior
environments and tendency to be exposed to deleterious chemicals, this material
can easily be subjected to deterioration.
Deterioration of concrete can take the form of
corrosion of the internal reinforcing or degradation of the exposed surface of
the material. Cracks increase the likely hood of the deterioration of concrete
in most environments. Concrete can also become deteriorated via freeze or thaw
cycles, internal aggregate reactions and heat. Due to the porosity of concrete,
its use in exterior environments and tendency to be exposed to deleterious
chemicals, reinforced concrete can easily be subjected to deterioration.
Sources of deterioration include corrosion of the reinforcing steel and
degradation of the exposed concrete surface due to chemical attack. Concrete
deterioration can also occur as a result of freeze/thaw cycles, internal
aggregate reactions and exposure to extreme heat. The presence of cracks both
promotes and helps accelerate deterioration of concrete.
Visual and destructive methods of investigation
should be employed when determining the cause of concrete deterioration. The
results of the physical observations and material tests associated with a
proper investigation should be used to establish the best methods of repair and
prevention of further deterioration. For marine condition and earthquake zone,
POFA and fibre reinforced concrete are suitable to use because of the
characteristic of the material and how the material prevent the chemical attack
and control the crack.
5.0 REFERENCE
i.
Gambhir M.L (2004), Concrete Technology,
Tata McGraw-Hill Publishing Company Ltd, New Delhi, pp170, 354,502.
ii.
Budiea A, Hussin M.W, Muthusamy K and
Ismail M.E, (2010), Performance of High Strength POFA Concrete in Acidic
Environment, Concrete Research Letters Vol 1 (1), Asia Pasific Structural
Engineering Conference (ASPEC 2009).
iii.
Ahmad M.H, (2008), Compressive Strength
of Palm Oil Fuel Ash Concrete, ICCBT 2008 pp297-306.
iv.
Johnston C.D (2001), Fibre Reinforced
Cements and Concretes, Gordon and Breach Science Publisher, Netherlands, pp86, 228,
285.
v.
Christer Sjostrom (1996), Durability of
Building Materials & Components, E & FN Spon, London, pp. 294-296.
vi.
Concrete Technology (SAB4163) Notes.
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