Reinforced Concrete Concrete

Added on -2020-02-18

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Reinforced ConcreteConcrete is manufactured by mixing aggregates like crushed stone, gravel and sandwith a binding agent like cement, water and chemical agents. When water is added to cement,cement hydrate crystals are formed and the crystals lock the gravel and cement together. Thisinterlocking provides the necessary strength for concrete and water provides for the chemicalreactions and hydrates the cement. The concrete gains strength and hardens for at leastanother 5 years. (ACI committee 116, 1967). The composition of concrete could be varied toproduce concrete that sets and weathers very well, appears different in color or flows quicker.Manufacturing of reinforced concrete is fairly simple but in order to understand theprocedure first we should know how concrete is manufactured as reinforced concrete isnothing else other than concrete with steel reinforcements. The first process in themanufacturing of concrete is the preparation of Portland cement which is processed by dryingand then powdering alumina, silica and limestone into fine powder and mixing them togetherin definite proportions (Reynolds, et al, 2008). It is then preheated, calcined and burned at2550º F in large rotary kiln to form clinker, which is then cooled and grinded in a rotatingdrum to form Gypsum which consists of tetra-calcium alumino-ferite, tricalcium aluminate,decalcium silicate and tricalcium silicate and this is called cement (McCormac and Brown,2014).This cement is then mixed with crushed stone, gravel, sand, water, fibers andadmixtures so as to form a uniform blend. Fibers are added using hand laying, impregnating,premixing or direct spraying techniques with the help of a dispersing agent like silica fume.Now this concrete is transported to the work site using pumps, buckets, wheelbarrows orspecial trucks. The next step would be placing and compacting, but during the manufacturingof reinforced concrete this step is slightly modified by casting the wet concrete aroundreinforced steel bars. The concrete is uniformly placed inside the formwork using bull floatsand magnesium floats. The surface of concrete is smoothened using trowels, float blades,edgers, brooms and polishers. The final step is curing, where once the compacted concrete should be cured in orderto make sure that the concrete does not dry quickly as the strength of concrete depends on the
moisture level while it hardens. Concrete has good compressive strength but poor tensilestrength and steel reinforcement helps to improve the tensile strength of concrete (Skalny andMindess, 1989). Steel is preferred for reinforcement because the expansion and contraction ofsteel due to heat and cold is same as that of concrete (Neville and Brooks, 1987). The steelbars are held firmly inside concrete because they are made by twisting strands with ridges ornobbles so as to eliminate the risk of slippage inside the concrete. Reinforced concrete is subjected to a lot of tests to evaluate its structural, fire andsafety integrity. The structural integrity of concrete is evaluated based on the following test,slump test is used to find out the workability of concrete, a compressive strength test isperformed to determine the compression strength of concrete, initial surface absorption, rapidchloride ion penetration, water absorption and water permeability tests are performed to findout the durability of concrete. Then there are tests for determining the density, compactionfactor, air content, flow, vebe time, static and dynamic modulus of elasticity, flexural strengthand tensile splitting strength of concrete. All tests are performed as per BS 1881. Some of thenondestructive tests used to evaluate concrete are as follows.Concrete Cover Measurement by Laser Based Instrument.Core Extraction for Compressive Strength Test.Ultrasonic Pulse Velocity (UPV) Test.Rebound Hammer (RH) Test.Ingredient Analysis of Concrete Core.Combined UPV & RH Test.The advantages of using reinforced concrete in building construction are as follows.Use of reinforced concrete gives rigidity to the building.The compressive strength per unit cost of reinforced concrete is lower than similarmaterials.The erection of reinforced concrete does not require highly skilled labor as comparedto similar materials such as structural steel.It requires very low maintenance.It can resist fire and water better than similar materials as it suffers only surfacedamages in case of a fire outbreak.
Most of the materials used in the manufacture of reinforced concrete are relativelyinexpensive such as water, gravel and sand and it requires relatively small amounts ofthe expensive reinforcing steel and cement.It can be manufactured in a variety of shapes like shells, columns, beams, slabs andarches.As far as piers, walls, basement, floor slabs and footings are concerned reinforcedconcrete is the only economical option.It has very long service life so it could be used almost indefinitely without anydecrease in its load carrying capability. As cement takes years to solidify the strengthof concrete only increases with time.The disadvantages of using reinforced concrete in building construction are as follows.In order to set concrete in place formworks are required and without this formworkconcrete would not harden properly. Additional shoring is required to keep theformwork in place for floors, walls and roofs until the structure attains the requiredstrength to support itself. The tensile strength of concrete is low and therefore it requires reinforcement.Large structures tend to be heavier due to the low strength per unit weight and thisincrease in weight may bend large structures. In order to reduce the weight ofconcrete lighter materials could be used but this in turn increases the cost ofconstruction.Placing and Curing of concrete is usually not controlled properly resulting inunevenness.Concrete structures will be very large because of the relatively low strength per unitvolume (Mindess, 1991).If the concrete is not mixed proportionately then its properties may vary and it wouldnot meet the desired target.The cost of formworks is high which in turn increases the cost of construction.Structural SteelThe process of manufacturing steelwork to form the structural frame by assemblingand jointing individual steel components is known as fabrication. The entire steel structure is
made from locally available standard sections, protective coatings and bolts. Theeffectiveness of steel construction depends on the efficiency of fabrication and erection.Structural engineer holds the responsibility of erection and fabrication of steel structures.Structural steel fabrication can be executed on site or in shop. Fabrication completedin shops is specific and of assured quality, whereas fabrication carried out on site iscomparatively inferior (Bowles, 1980). The procedure adopted for on-site fabrication issimilar to that used in shop but the equipment used are less sophisticated and so is the talentof employees and hence the quality of finished product tends to be fairly inferior. The type offabrication determines the significance and sequence of operation followed for fabricatingstructural steel. The sequence of procedures followed during structural steel fabrication is asfollows (Brockenbrough and Frederick, 2011).Cleaning the Surface is used to eliminate mill scales before the procedure offabrication starts. Cleaning can be carried out either by hand or manually, butfabrication shops prefer blast cleaning. Blast cleaning is carried out either bycompressed air or by centrifugal impeller for removing rust and roughening thesurface before applying coating on the surface. Other methods of cleaning used byfabrication shops are flame cleaning using oxy-acetylene torch and by immersingsteel in acids.Cutting the steel to required lengths follows surface cleaning and this process iscarried out with the help of any of the following procedures cropping and shearingusing hydraulic shears, cold sawing, arc plasma cutting using an electric arc, burningor flame cutting using propane like gases.Drilling and Punching used to punch holes of desired size in flanges and webs of thesteel components.Rolling, Bending and Straightening of the steel components is the next procedure infabrication. Straightening is done by gag pressing, pattern heating is carried out toremove misalignments. Reaming and Fitting the steel components is done temporarily using bolts, rivets andwelds. Riveting is best done manually.Proper fastening using bolts and rivets gives strength to the entire structure.Finishing is one of the final steps of fabrication and it is carried out to transmit thebearing load so as to support each other. It is carried out using milling or sawing.After finishing the steel surface is treated by coating it with paints or metallic coatings(Gehringer, 1990).

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