1.0 INTRODUCTION_______________________________________________________Concrete is a composite construction material composed primarily of aggregate, cement, and water. There are many formulations, which provide varied properties. The aggregate is generally a coarse gravel or crushed rocks such as limestone, or granite, along with a fine aggregate such as sand. The cement, commonly Portland cement, and other cementitious materials such as fly ash and slag cement, serve as a binder for the aggregate. Various chemical admixtures are also added to achieve varied properties. Water is then mixed with this dry composite, which enables it to be shaped (typically poured) and then solidified and hardened into rock-hard strength through a chemical process called hydration. The water reacts with the cement, which bonds the other components together, eventually creating a robust stone-like material. Concrete has relatively high compressive strength, but much lower tensile strength. For this reason it is usually reinforced with materials that are strong in tension (often steel). Concrete can be damaged by many processes, such as the freezing of trapped water.Concrete is widely used for making architectural structures, foundations, brick/block walls, pavements, bridges/overpasses, motorways/roads, runways, parking structures, dams, pools/reservoirs, pipes, footings for gates, fences and poles and even boats. Famous concrete structures include the Burj Khalifa (world's tallest building), the Hoover Dam, the Panama Canal and the Roman Pantheon. Concrete technology was known by the Ancient Romans and was widely used within the Roman Empirethe Colosseum is largely built of concrete. After the Empire passed, use of concrete became scarce until the technology was re-pioneered in the mid-18th century. The environmental impact of concrete is a complex mixture of not entirely negative effects; while concrete is a major contributor to greenhouse gas emissions, recycling of concrete is increasingly common in structures that have reached the end of their life. Structures made of concrete can have a long service life. As concrete has a high thermal mass and very low permeability, it can make for energy efficient housing.

2.0 OBJECTIVE____________________________________________________________Durability is the ability of a material or structure to withstand its design service conditions for its design life without significant deterioration. A durable material helps the environment by conserving resources and reducing wastes and the environmental impacts of repair and replacement. Construction and demolition waste contribute to solid waste going to landfills. The production of new building materials depletes natural resources and can produce air and water pollution. The design service life of most buildings is often 30 years, although buildings often last 50 to 100 years or longer. Most concrete and masonry buildings are demolished due to obsolescence rather than deterioration. A concrete shell can be left in place if a building use or function changes or when a building interior is renovated. Concrete, as a structural material and as the building exterior skin, has the ability to withstand natures normal deteriorating mechanisms as well as natural disasters. Durability of concrete may be defined as the ability of concrete to resist weathering action, chemical attack, and abrasion while maintaining its desired engineering properties. Different concretes require different degrees of durability depending on the exposure environment and properties desired. For example, concrete exposed to tidal seawater will have different requirements than an indoor concrete floor. Concrete ingredients, their proportioning, interactions between them, placing and curing practices, and the service environment determine the ultimate durability and life of concrete.Concrete will remain durable if: 1. The cement paste structure is dense and low of permeability.2. It has entrained air to resist freeze-thaw cycle under extreme condition.3. It is made with graded aggregate that are strong and inert.4. The ingredients in the mixed have minimum quantity of impurities such as alkalis, chlorides, sulphates and silt.

3.0 MAIN INGREDIENTS PROPORTION IN CONCRETE_______________________There are many types of concrete available, created by varying the proportions of the main ingredients below. In this way or by substitution for the cemetitious and aggregate phases, the finished product can be tailored to its application with varying strength, density, or chemical and thermal resistance properties.3.1 CementPortland cement is the most common type of cement in general usage. It is a basic ingredient of concrete, mortar and plaster. English masonry worker Joseph Aspdin patented Portland cement in 1824; it was named because of its similarity in color to Portland limestone, quarried from the English Isle of Portland and used extensively in London architecture. It consists of a mixture of oxides of calcium, silicon and aluminium. Portland cement and similar materials are made by heating limestone (a source of calcium) with clay and grinding this product (called clinker) with a source of sulfate (most commonly gypsum).In recent years, alternatives have been developed to help replace cement. Products such as PLC (Portland Limestone Cement), which incorporate limestone into the mix, are being tested. This is due to cement production being one of the largest producers of global green house gas emissions.3.2 WaterCombining water with a cementitious material forms a cement paste by the process of hydration. The cement paste glues the aggregate together, fills voids within it, and makes it flow more freely. A lower water to concrete ratio will yield a stronger, more durable concrete; while more water will give a freer-flowing concrete with a higher slump. Impure water used to make concrete can cause problems when setting or in causing premature failure of the structure. Hydration involves many different reactions, often occurring at the same time. As the reactions proceed, the products of the cement hydration process gradually bond together the individual sand and gravel particles and other components of the concrete, to form a solid mass.

Reaction:Cement chemist notation: C3S + H C-S-H + CHStandard notation: Ca3SiO5 + H2O (CaO)(SiO2)(H2O)(gel) + Ca(OH)2Balanced: 2Ca3SiO5 + 7H2O 3(CaO)2(SiO2)4(H2O)(gel) + 3Ca(OH)23.3 AggregatesFine and coarse aggregates make up the bulk of a concrete mixture. Sand, natural gravel and crushed stone are used mainly for this purpose. Recycled aggregates (from construction, demolition and excavation waste) are increasingly used as partial replacements of natural aggregates, while a number of manufactured aggregates, including air-cooled blast furnace slag and bottom ash are also permitted.Decorative stones such as quartzite, small river stones or crushed glass are sometimes added to the surface of concrete for a decorative "exposed aggregate" finish, popular among landscape designers. The presence of aggregate greatly increases the robustness of concrete above that of cement, which otherwise is a brittle material and thus concrete is a true composite material. Redistribution of aggregates after compaction often creates inhomogeneity due to the influence of vibration. This can lead to strength gradients. 3.4 ReinforcementConcrete is strong in compression, as the aggregate efficiently carries the compression load. However, it is weak in tension as the cement holding the aggregate in place can crack, allowing the structure to fail. Reinforced concrete solves these problems by adding either steel reinforcing bars, steel fibers, glass fiber, or plastic fiber to carry tensile loads. Thereafter the concrete is reinforced to withstand the tensile loads upon it.

3.5 Chemical AdmixturesChemical admixtures are materials in the form of powder or fluids that are added to the concrete to give it certain characteristics not obtainable with plain concrete mixes. In normal use, admixture dosages are less than 5% by mass of cement and are added to the concrete at the time of batching/mixing. The common types of admixtures are as follows. Accelerators speed up the hydration (hardening) of the concrete. Typical materials used are CaCl2, Ca(NO3)2 and NaNO3. However, use of chlorides may cause corrosion in steel reinforcing and is prohibited in some countries, so that nitrates may be favored. Retarders slow the hydration of concrete and are used in large or difficult pours where partial setting before the pour is complete is undesirable. Typical polyol retarders are sugar, sucrose, sodium gluconate, glucose, citric acid, and tartaric acid. Air entrainments add and entrain tiny air bubbles in the concrete, which will reduce damage during freeze-thaw cycles, thereby increasing the concrete's durability. However, entrained air entails a trade off with strength, as each 1% of air may result in 5% decrease in compressive strength. Plasticizers increase the workability of plastic or "fresh" concrete, allowing it be placed more easily, with less consolidating effort. A typical plasticizer is lignosulfonate. Plasticizers can be used to reduce the water content of a concrete while maintaining workability and are sometimes called water-reducers due to this use. Such treatment improves its strength and durability characteristics. Superplasticizers (also called high-range water-reducers) are a class of plasticizers that have fewer deleterious effects and can be used to increase workability more than is practical with traditional plasticizers. Compounds used as superplasticizers include sulfonated naphthalene formaldehyde condensate, sulfonated melamine formaldehyde condensate, acetone formaldehyde condensate and polycarboxylate ethers. Pigments can be used to change the color of concrete, for aesthetics. Corrosion inhibitors are used to minimize the corrosion of steel and steel bars in concrete.

Bonding agents are used to create a bond between old and new concrete (typically a type of polymer) . Pumping aids improve pumpability, thicken the paste and reduce separation and bleeding.

3.6 Mineral Admixtures And Blended CementsThere are inorganic materials that also have pozzolanic or latent hydraulic properties. These very fine-grained materials are added to the concrete mix to improve the properties of concrete (mineral admixtures),or as a replacement for Portland cement (blended cements). Fly ash: A by-product of coal-fired electric generating plants, it is used to partially replace Portland cement (by up to 60% by mass). The properties of fly ash depend on the type of coal burnt. In general, siliceous fly ash is pozzolanic, while calcareous fly ash has latent hydraulic properties. Ground granulated blast furnace slag (GGBFS or GGBS): A by-product of steel production is used to partially replace Portland cement (by up to 80% by mass). It has latent hydraulic properties. Silica fume: A by-product of the production of silicon and ferrosilicon alloys. Silica fume is similar to fly ash, but has a particle size 100 times smaller. This results in a higher surface to volume ratio and a much faster pozzolanic reaction. Silica fume is used to increase strength and durability of concrete, but generally requires the use of superplasticizers for workability. High reactivity Metakaolin (HRM): Metakaolin produces concrete with strength and durability similar to concrete made with silica fume. While silica fume is usually dark gray or black in color, high-reactivity metakaolin is usually bright white in color, making it the preferred choice for architectural concrete where appearance is important.

4.0 FACTORS AFFECTING DURABILITY OF CONCRETE_____________________ Durability of concrete depends upon the following factors 1) Cement content

Mix must be designed to ensure cohesion and prevent segregation and bleeding. If the quantity of cement is reduced, then at fixed w/c ratio the workability will be reduced leading to inadequate compaction. However, if water is added to improve workability, w/c ratio increases and resulting in highly permeable material.

2) Compaction

The concrete as a whole contain voids can be caused by inadequate compaction. Usually it is being governed by the compaction equipments used, type of formworks, and density of the steelwork.

3) Curing

It is very important to permit proper strength development aid moisture retention and to ensure hydration process occur precisely.

4) Cover

Thickness of concrete cover must follow the limits set in codes.

5) Permeability

It is considered the most important factor for durability. It can be noticed that higher permeability is usually caused by higher porosity. Therefore, a proper curing, sufficient cement, proper compaction and suitable concrete cover could provide a low permeability concrete

5.0 METHOD OF TESTING DURABILITY OF CONCRETE_____________________There are three durability index tests that have been developed, namely :1. Oxygen Permeability Test.2. Water Sorptivity Test.3. Chloride Conductivity Test.Each test measures a different transport property of fluids or ions through the concrete cover layer, typically covering the main mechanisms related to deterioration. The tests have been developed and proved in the laboratory, and increasingly are being applied on site in actual construction. They have progressed to the point of being in regular use, and specifications are being written around their site application. At the same time, the performance of structures built using the index approach is being monitored as far as possible to validate the approach and implement improvements. 5.1 Oxygen Permeability TestThis involves a falling head permeameter in which oven-dried (50 C for 7 days) concrete samples, generally 68 mm diameter and 25 to 30 mm thick, are placed in rubber collars secured on top of a permeability cell3. The cell is pressurised with oxygen to 100 kPa before being isolated, after which the pressure decay is monitored, from which the Darcy coefficient of permeability, k, may be determined. The oxygen permeability index (OPI) is defined asOxygen permeability index = -Log (k) (1)Oxygen permeability indexes are logarithmic values and range generally from 8 to 11, i.e. three orders of magnitude; the higher the index, the less permeable the concrete. A diagram of the test apparatus is shown in Figure 1.

Figure 1: Schematic diagram of oxygen permeability apparatus5.2 Water Sorptivity TestSorptivity is defined as the rate of movement of a wetting front through a porous material. The water sorptivity test involves the uni-directional absorption of water into one face of a pre-conditioned concrete disc sample. At predetermined time intervals, the sample is weighed to determine the mass of water absorbed, and the sorptivity is determined from the plot of mass of water absorbed versus square root of time. The lower the water sorptivity index, the better is the potential durability of the concrete. Sorptivity values typically vary from approximately 5 mm/h, for well-cured M30-M50 concretes, to 15 20 mm/h for poorly cured M20 concrete. A diagram of the test is shown in Figure 2.

Figure 2: Schematic diagram of water sorptivity test

5.3 Chloride Conductivity TestStreicher developed a rapid chloride conductivity test in which virtually all ionic flux occurs by conduction due to a 10 V potential difference between the two faces of a sample. The apparatus consists of a two-cell conduction rig, each cell containing a 5M NaCl solution so that there is no concentration gradient across the sample and chloride migration is the result of conduction from the applied potential difference see Figure 3. The concrete disc sample is pre-conditioned by vacuum saturation with a 5M NaCl solution.

Figure 3: Schematic diagram of chloride conductivity apparatusDiffusion and conduction are related by Einsteins equation, allowing the conductivity test to be used as an index of concrete diffusivity. The test is sensitive to changes in the pore structure and cement chemistry (mainly binder type), which might appear to be insignificant when using the permeation process. Typical chloride conductivity index values range from > 3 mS/cm for M20 M30 OPC concretes, to < 0.75 mS/cm for M40 M50 slag or fly ash concretes. The lower the index, the better is the potential durability of the concrete.

6.0 DISCUSSION___________________________________________________________Durability is a subject of increasing importance for concrete design, where longer design life cycles for concrete are demanded for more sustainable development. The external and internal mechanisms that accelerate concrete deterioration are now well understood. These are:1) Chloride penetration.2) Sulphate attack.3) Alkali-silica reaction.Ground granulated blast-furnace slag-based (GGBS) is effective in preventing all these forms of deterioration, and is now routinely specified for infrastructure projects, where a long service life of concrete is essential. Latest Irish research from Trinity College Dublin on durability of GGBS concrete exposed to Silage Effluent demonstrates that concrete with 50% GGBS is more durable than concretes made with OPC alone in aggressive environments under the action of acids and salts such as those produced by silage. The use of GGBS in concrete causes different reactions and results in different hydraulic products than concrete made with Portland cement only. In particular, the hydration products of the GGBS concrete serve to block the open pore structure that characterises Portland cement concrete. The result is that GGBS concrete has fewer larger pores, and far lower permeability than Portland cement concrete. Sufficiently cured and hardened GGBS structural concrete is much more impermeable to water and reduces ion diffusion by a factor of 30 or so in comparison with Portland cement concrete. This low permeability is the key to GGBS concrete being able to resist attack from sulphates and weak acids.GGBS cement paste is more effective at binding chlorides than an equivalent Portland cement paste, thus GGBS concrete offers much superior protection to reinforcement from corrosion due to chloride ion penetration. In addition, concrete made with GGBS is more chemically stable than concrete made with Portland cement only. It contains much less free lime, which in concrete made with Portland cement leads to the formation of further reaction products such as ettringite or efflorescence. In addition, GGBS contains no C3A; making GGBS concrete much less reactive to sulphates.

7.0 CONCLUSION__________________________________________________________In brief, advances in concrete durability for the 21st century will come from carefully selecting materials to control and optimize their properties; reducing variability in the mixing, transport, placement, and curing of concrete; and creating and using more performance-based specifications to evaluate in-situ concrete. The use of real-time feedback and control from non-destructive evaluation techniques, further automation of production and placement, and improved understanding of the interaction between concrete and its environment will aid in the achievement of these advances.

8.0 REFERENCES__________________________________________________________

1. courses.washington.edu/cm425/durability.pdf2. www.sciaust.com.au/pdfs/durabilityParta.pdf3. www.concretethinker.com/solutions/Durability.aspx4. www3.imperial.ac.uk/concretedurability5. www.cur.nl/upload/documents/duracrete/BE1347R14b.pdf6. www.concretenetwork.com/concrete/concrete_patio/durability.htm7. www.woodheadpublishing.com/en/book.aspx?bookID=8128. Sakkai, K. (ed.) (1996) Integrated Design and Environmental Issues in Concrete Technology. Taylor and Francis, London.9. Marios Soutsos. (2010) Concrete Durability. Thomas Telford Limited, UK.

1 | Page

6