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Reinforced Concrete(专业外语).ppt

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Reinforced Concrete(专业外语).ppt
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REINFORCED CONCRETE by Vincent McKinnone­mail: vincentmckinnon@yahoo.comSYLLABUS§ 1 Materials – Concrete & Reinforcement§ 2 Beams§ 3 Slabs§ 4 Columns§ 5 Walls§ 6 Bases & FoundationsTypes of Cement§ OPC – the most common§ Rapid hardening Portland Cement§ Low heat Portland Cement§ Sulphate resisting Portland CementAggregates§ Course aggregate§ Fine aggregateWater§ Water/cement ratio§ Minimum is 0.23 weight of water§ Normally, ranges from 0.45 – 0.6Admixtures§ Hardening/setting accelerators or retarders§ Water reducing to improve workability§ Air­entraining improve damage resistance§ Superplasticisers for complicated sectionsREINFORCED CONCRETE DESIGN§Reinforced concrete is a composite material of steel bars embedded in a hardened concrete mix. Concrete, assisted by the steel carries the compressive forces whilst the steel resists the tensile forces.CONCRETE & REINFORCEMENT§ Concrete itself is a composite material. The dry mix consists of cement together with course and fine aggregates. § Water is added and this reacts with the cement which hardens and binds the aggregate into the concrete matrix which sticks or bonds onto the reinforcing bars.CONCRETE & REINFORCEMENT§ Knowledge of the properties and an understanding of the behavior of concrete is an important factor in the design process.Cement § Ordinary Portland cement is the commonest type in use. § raw materials of OPC: lime, silica, alumina and iron oxide. § These constituents are crushed and blended in the correct proportions and then burnt in a rotary kiln. § The clinker is then cooled, mixed with gypsum and ground to a fine powder to produce cement. § The main chemical compounds in cement are calcium silicates and aluminates.Aggregates Aggregates are classed into the following two sizes:§ course aggregate – gravel or crushed rock greater than 5mm in size§ fine aggregate – sand, less than 5mm in sizeAggregates§ Aggregates should be chemically inert, clean, hard and durable. § alkali­silica reaction: Some aggregates containing silica may react with alkalis in the cement causing the concrete to disintegrate. § the presence of chlorides in the aggregate – eg salt in marine sands will cause corrosion of the steel reinforcement. Excessive amounts of sulphate will also cause the concrete to disintegrate.Water § The water to cement ratio: most important factor in affecting concrete strength. § For full hydration, cement absorbs 0.23 of its weight of water. But this amount of water would produce a very dry mix and so extra water is therefore required to improve workability. § The actual water/cement ratio used is generally from 0.45 – 0.6.Admixtures Admixtures are substances added to concrete mixes in very small amounts in order to improve certain properties by their chemical or physical effects.Types of admixture§ hardening or setting accelerators or retarders§ water-reducing which gives an increase in workability with a lower water/cement ratio§ air-entraining which increases resistance to damage from freezing or thawing§ superplasticisers which are used in complicated sectionsConcrete Mix Designtwo types of mixes are used:§ Design Mix. Strength forms an essential part of the requirement for compliance§ Prescribed Mix. Here, proportions of the constituents needed to give the required strength and workability are specified. Strength testing is not required.CONCRETE PROPERTIES§ The COMPRESSIVE strength is the most important property of concrete. The characteristic strength is measured by the 28 day cube strength. § standard cubes of 100mm or 150mm are crushed to determine the strength. § Typical strength values are 30 N/mm2 or 40 N/mm2. These are the 28 day cube strength.CONCRETE PROPERTIES§ TENSILE strength This is normally assumed to be around 10% of the concrete’s compressive strength. So, for a grade 30 concrete –ie one who’s compressive strength is 30 N/mm2, the tensile strength would be assumed to be 3 N/mm2 and for a grade 40 concrete, it would be assumed to be 4 N/mm2.§ The tensile strength is measured by loading a concrete cylinder across a diameter as shown in the diagram (see Dia 1).SHEAR A vertical shear force in a beam causes complimentary shear stresses and diagonal tensile and compressive forces of the same magnitude to occur. This is shown in the next diagram (see Dia 2) where the stresses are shown in a small element near the neutral axis. The maximum shear stress at the neutral axis of the section is –vmax = 1.5*V/(b*h) You will note an important point about the shear strength of a section. It depends upon the amount of reinforcement present, on the grade of concrete and upon the section’s depth.MODULUS OF ELASTICITY § The short term stress­strain curve for concrete in compression is also shown in Dia 1. This is known as the short term value of Young’s Modulus. (See also Dia 3).CREEP § This is the gradual increase in strain with time in a member subjected to prolonged stress. When loaded, it is observed that the creep strain is much larger than the elastic strain. If the specimen is unloaded, there is an immediate elastic recovery and a slower recovery in the strain due to creep. Both amounts of recovery are much less than the original strains under load.CREEP § The main factors affecting creep strain are, the concrete mix and strength, the type of aggregate, the curing, the ambient relative humidity and the magnitude and duration of the sustained loading.SHRINKAGE § Shrinkage, or drying shrinkage is the contraction which occurs in concrete when it dries and hardens. § The aggregate type and content are the most important factors affecting shrinkage. § The larger the size of aggregate, the lower is the shrinkage. At the same time, the lower the workability and water/cement ratio the lower is the shrinkage.REINFORCEMENT§ Reinforcing bars are produced in two grades – hot rolled mild steel bars have a yield strength of fy = 250 N/mm2. § Hot rolled or cold worked bars have a yield strength of fy = 460 N/mm2. § Steel fabric is made from cold drawn steel wires to form a mesh. It has a yield strength of fy = 460 N/mm2. REINFORCEMENT§ The stress­strain curve for steel is shown in Dia 3. The Young’s Modulus or the Modulus of Elasticity for steel is 200 kN/mm2. § The behaviour in tension and compression is assumed to be the same.§ Mild steel bars are produced as round bars. But high yield steel bars are produced as deformed bars with transverse ribs.COVER§ The cover (including links) to a reinforcing bar will depend upon the exposure conditions. These are as shown in the table of Dia 4. § Notice also the exposure conditions which, as you will note, range from mild to extreme. For severe exposures in a marine environment, 75mm is considered appropriate 2 LIMIT STATE DESIGN AND STRUCTURAL ANALYSIS § The British code (BS8110) for reinforced concrete design states that the aim of the design is the achievement of an acceptable probability that the structure will perform satisfactorily throughout its lifetime. It must carry the applied loads safely, it must not deform excessively and it must have adequate resistance to effects of misuse and fire
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