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Arghhhh...sibuk buat macam-macam assignment dan mo siapkan thesis..even baru 3 chapter, tp mo pecah da kepala memikirkan...ni mau share one of my assignment:

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
Tests	Exposure Period	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.

Along The Journey

Friday, December 23, 2011

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Thursday, December 22, 2011

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Tuesday, December 20, 2011

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