The Construction Material Concrete Construction Essay

The Construction Material c everywhere Construction EssayThe emergence of readiness cover, handling it in distinct climatic conditions and the miscellaneous diseases that plunder arise in cover structures atomic recite 18 the key points of this paper.The word cover is derived from a latin word concretuswhich means compact or condensed. cover is a thick composite artificial st matchless like solid which is made by meld cementum (generally Portland cement) and various other hoard ups, such(prenominal)(prenominal) as sand, pebbles, gravel, shale, and so on, with urine and allowing the com sectionalisationmentalisation to harden by hyd dimensionn. Since the ancient generation it had been apply in constructing structures and to daylight excessively it is use much than either other man made construction material in the world. writingDepending upon the reqirement of the structure,the proportions of the main ingredients discussed below be varied to avail different types of cover. By doing so the finished product howevert joint be tailored to its cover program with varying metier, density, or chemical and thermal resistance properties.CEMENTPortland cement is a basic ingredient of cover. It consists of a cockleture of oxides of calcium, silicon and aluminium. Portland cement and similar materials ar made by heating systeming limestone (a source of calcium) with clay, and contriteness this product (called clinker) with a source of sulfate (most commonly gypsum).WATERWhen pissing is added in a cementitious material, cement paste is varianceed through the make for of hydration. The cement paste glues the aggregate to halther, fills voids within it, and allows it to flow more(prenominal) smoothly. irrigate should be added in the mix very cargon fully as less irrigate in the cement paste bequeath cushion a stronger, more durable cover more wet exit give an free-flowing cover with a postgraduate impression. Impure body of water utilise to make cover provoke caseful problems when setting or in ca victimisation premature failure of the structure.In the act of hydration m whatsoever(prenominal) different re serves occur at the comparable cartridge clip. As the re transactions proceed, the products of the cement hydration process piecemeal bond to bring ab discloseher the individual sand and gravel particles, and other components of the concrete, to grad a solid compact mass.AGGREGATESThe nominal head of aggregate greatly forthgrowths the boldness of concrete higher up that of cement.Following be the different items which could be utilize as aggregatesSand, natural gravel and crushed stoneRecycled aggregates (from construction, demolition and excavation waste) and fabricate aggregates like nisus-cooled blast furnace slag and bottom ash ornamental stones such as quartzite, small river stones or crushed glass atomic number 18 some successions added to the come forth of concrete for a decorative exposed aggregate finish.CURING OF concreteThe term readiness concrete signifies the process of taenia freshly poured concrete from drying fall out too quickly. Unless the concrete mass is non cured, or if it is left out to dry out on its own, it leave not develop the full bond between all of its ingredients. In order to form a water cement paste, water is added in Portland cement aft(prenominal) which chemical chemical reactions take place in the mixture turns the paste into a bonding performer. This reaction is cognise as hydration and it produces a stone-like compact substancethe harden cement paste. twain the ramble and degree of hydration, and the conduceing peculiarity of the final concrete, depend on the solidifying process that follows placing and consolidating the fictile concrete. As long as the mixture contains water and the temperature conditions atomic number 18 favorable, the process of Hydration continues ,though at a decreasing rate . The volume of concrete is dependent on crystal upriseth within the concrete matrix finished the process of hydration If water is not adequate, the crystals cigarettenot grow, due to which the concrete is unable to attain the want strength. The presence of water through proper stage set of the concrete facilitates the crystal growth that premises the gravel and sand mix, causing interweaving with each other..The cartridge clip-span of the process of set begins from the clock of consolidation and ends at the point where the concrete reaches its design strength. The per centage point female genitalia vary from a few days to a month or long-dated. For most structural use, the hardening check for cast-in-place concrete is normally 3 days to 2 weeks. During this period, the concrete mass should be unbroken moist and as near 73F as practicable. Bridge decks and other slabs exposed to hold up and chemical attack usually require longer bent periods. The time span of the curing period also depends on the factors such as type of cement utilise mix proportions infallible strength size and shape of the concrete mass digest conditions and temperature future painting conditionsSIGNIFICANCE OF CURINGA proper and effective curing process garters in improving various properties of concrete such as cover and unblock resistance strength watertightness wear resistance volume stability. serviceability and apperanceThe compressive strength of concrete that has been properly cured is 75 to 95 per cent more than the concrete that has not been cured. Figure 1 illustrates this by comparability the compressive strength of concrete(at 180 days) for which the surface has been either kept moist for the broad(a) duration of 180 days or is kept moist for various periods of time and allowed to dry out or is allowed to dry out from the time it was first made.It is quite clearly depicted that the compressive strength of the concrete is highest when it is fully cured for the undefiled period of 180 days.Fig 1dampish curing effect on compressive strength of concrete sourcewww.tpub.comThe permeableness and absorptivity of concrete mix depends upon the porosity of the mix ie whether the pores and capillaries are discrete or interconnected. The porosity of the mix is reduced overtime through proper and effective curing of concrete at that placeby increasing the enduringness of concrete. withal, proper curing of concrete mix exit reduce crazing, dusting and measure of the slab in that locationby ensuring better serviceability and appearance.STAGES IN CURING PROCESS afterward the concrete mix have been placed, it is imperative to continue it moist and oblige specified concrete temperatures.The process of curing starts immediately after the place of concrete and it goes through two stages Initial curing stage time of placement to sign set.This stage would embroil all the deliberate action taken between placement and final finish of conc rete. Approximate conditions during the initial curing period should be forecast prior to construction. chthonian this stage,the rate of strength slang of the concrete is minimal.The various aspects to be dealt with construction initial curing period as shown in diagram below are verifying dehydration conditions concrete temperatures, wind velocity, air temperature, and telling humidness are required to be taken into account during placement. These elements are used to make nomograph to calculate the dehydration rate and to determine whether precise drying rates exist. Using the concrete placing temperature, the time of initial setting support be estimated. The time of initial setting is the optimal time for application of final curing.Major items requiring attention during construction-initial curing period. Source www.fhwa.dot.govtOnsite adjustments for Excessive drying In order to reduce vaporisation rates of phlebotomize water the future(a) two adjustments could be ma de-reducing concrete placing temperatures- Cooling aggregate stockpiles, temperature reduction meld water, or using ice for mixing water are very effective ways of reducing concrete temperatures.use of evaporation reducers. Evaporation reducers are water emulsions of film-forming compounds. The film-forming compound is the active ingredient that slows wipe out evaporation of water. There is also a benefit from the water portion of the evaporation reducers, in that it compensates to a small degree for losses of mixing water to evaporation.Evaporation reducers tweedthorn need to be applied several measure, depending on the conditions.Concrete that are liable to quick drying is required to be cover with wet gunny bag or wet hessian cloth properly squeezed, so that the water does not drip and at the akin time, does not allow the concrete to dry.This condition should be maintained for 24 hours or at least till the final setting time of cement at which duration the concrete will h ave assume the final volume.The Final curing stage The final curing period is defined as the time interval between application of curing procedures and the end of deliberate curing. Final curing methods fall into four categoriesWater curing Water curing give the gate be done in various ways like immersion,ponding, spraying or fogging and wet masking piece.This is by far the best method of curing as it satisfies all the requirements of curing, namely, forwarding of hydration, elimination of shrinkage and absorption of the heat of hydration.Curing a set up slab by flooding. Source http//www.builderbill-diy-help.com/curing-concrete.htmlThe precast concrete items are normally immersed in curing tanks for a certain duration. Pavement slabs, roof slab etc. are covered under water by making small ponds. tumid retaining wall or plastered surfaces or concrete columns etc. are cured by spraying water. In some cases, wet coverings such as wet gunny bags, hessian cloth, jute matting, str aw etc., are wrap to vertical surface for go alonging the concrete wet. For horizontal surfaces saw dust, domain or sand are used as wet covering to keep the concrete in wet condition for a longer time so that the concrete is not unduly desiccate to go along hydration. The diagram below summarises the major(ip) criteria for using water-added curing methods.Major features of curing with added water.Membrane curing this curing method is mostly used when there is less availability of water in the region where curing is done. Under this method, concrete could be covered with membrane which will effectively cachet off the evaporation of water from concrete. A continuous seal over the concrete surface is maintained by means of a soused impervious film to prevent wet in concrete from escaping by evaporation. Some of the materials, which good deal be used for this purpose, are bituminous compounds, polyethylenePLASTIC SHEETING sourcewww.builderbill-diy-help.comor polyester film, w aterproof paper, rubber compounds etc. When waterproofing paper or polyethylene film are used as membrane, care must be taken to see that these are not punctured eachwhere and also see whether adequate lapping is given at the junction and this lap is effectively sealed.Application of heat When concrete is subjected to higher temperature it accelerates the hydration process eventing in smart development of strength. Concrete tooshienot be subjected to dry heat to accelerate the hydration process as the presence of wet is also an essential requisite. Therefore, subjecting the concrete to higher temperature and maintaining the required wetness can be achieved by subjecting the concrete to steam curing.The exposure of concrete to higher temperature can be done by Steam curing at ordinary pressure, Steam curing at high pressure ,Curing by Infra-red radiation ,Electrical curing.Traditional steam curing of concrete pipes (www.construction-int.com)Miscellaneous calcium chloride is us ed either as a surface coat or as an admixture. It has been used satisfactorily as a curing medium. Both these methods are based on the fact that calcium chloride being a salt shows affinity for moisture. The salt not only absorbs moisture from atmospheric state only if also retains it at the surface. This moisture held at the surface prevents the mixing water from evaporation and thereby keeps the concrete wet for a long time to promote hydration. Formwork prevents escaping of moisture from the concrete, particularly, in the case of beams and columns.Keeping the form work intact and sealing the joint with wax or any other sealing compound prevents the evaporation of moisture from the concrete. This procedure of promoting hydration can be get winded as one of the miscellaneous methods of curing.At the end of the curing process majority of cement gets hydrated.There is slow rate of compressive strength gain of around 50- one hundred psi/hours and little exothermic heat generation .In the entire process of curing if one of the curing procedures is used initially, it may be replaced by one of the other procedures after the concrete is 1 day old, provided thatthe concrete surface is not permitted to become dry at any time.After the termination of the curing process the adequacy of a curing program could be verified.Although strength is the primary variable around which curing specifications are based several other approaches can also be used likeSurface Water Absorption the amount of water a dry concrete specimen absorbs in the first split second or so after contact with liquid water is at one time related to the timbre of the curing of the near-surface zone of the concrete. This method has direct pertinence to verifying curing. The method is reasonably applied to cores, which can be dried to a constant low moisture content before examen. beat up Hammer The rebound hammer method basically measures the modulus of elasticity of the nearsurface concrete. This may actually recommend the method for use in evaluating the curing of concrete pavements, where near-surface effects are considered most important. The test method is suitable for in-place measurements. research laboratory work has shown that rebound numbers of uncured concrete exposed to modestly severe drying are reduced by about 50 part at 7 days relative to well-cured concrete.Strength of Cores The strength of concrete is strongly affected by inadequate curing, and, in theory, could be find by measuring strength of cores. the effects of poor curing are only strongly apparent in the properties of the top 50 mm of concrete, and sometimes even less.Ultrasonic Pulse Velocity The ultrasonic thrill velocity (UPV) method is an indirect measure of the modulus of elasticity of concrete. The modulus of elasticity of concrete tends to increase with increasing hydration (or quality of curing) of the cement paste fraction of the concrete. UPV interrogatory can be set up in a number of c onfigurations, each of which tends to focus on slightly different features of the concrete. A simplistic pulse velocity taken through a piece of concrete, which is the tralatitious way of using UPV to investigate concrete properties, gives information on the come quality of the concrete.Abrasion Resistance The degree of curing has been shown in numerous research publications to be strongly reflected in the scrawl resistance of the cement-paste fraction of concrete. This truth is easily verified qualitatively using an electrically powered wire brush and observing the ease with which the near-surface mortar can be removed from a small spot of concrete. Poorly cured concrete is easily abraded away(predicate), while well-cured concrete is quite difficult to abrade away with such equipment.DISEASES OF CONCRETEDeterioration of concrete mental synthesiss is becoming a cause of business sector now a days. Often the structures located along unfastened water, lakes, rivers and oceans are extremely vulnerable to attack from the harsh environment i.e. salt, moisture, humidity, speed of light dioxide, etc. cracks, spalls and rust stains are some of the visual symptoms of the deterioration of concrete. pickCrack is the most common diseases of concrete structures and it occurs as a result of material discontinuity. Thermal and shrinkage cracks can be associated with high cement content and high strength concrete. Usually cracks occur in the concrete structures due to various reasons such as excess water in the mix ,rapid drying of the concrete, improper strength concrete poured on the job, lack of conquer joints, if Concrete is poured on frozen solid ground, alkali-silica reactionetc.In order to prevent cracks in concrete structures, following preventive measures could be taken Estimate in advance the quantity of water to be mixed to get the required concrete mix. Adding too much water in the mix is one of the main causes of cracked concrete Do not pour concrete on the frozen ground .A compacted subgrade could be used to pour concrete upon to prevent cracking Cut control joints into the concrete so that the slab will be able to shrink and expand with temperature changes. Control joints should be put down the same depth of the slab and spaced a maximum of three times the thickness of the concrete. The slab must retain enough moisture so that the drying and shrinking happens as slowly as workable in the days and weeks after pouring. Curing helps the concrete to retain moisture in the concrete so that it can continue to gain strength to resist cracking.CRACKS ON CONCRETE SURFACESourcewww.homebuildinginformation.comFROSTWhen the concrete is critically saturated i.e. slightly 91% of its pores are filled with water then deterioration of concrete from freeze thaw actions may occur. When water freezes to ice it occupies 9% more volume than that of water.In case there is no space in concrete for this expanded volume , a kind of distress is cause d which will result in loss of concrete surface.A surface active agent should be added to the concrete mixture in order to prevent frost in a concrete structure. This creates a large number of close spaced, small air bubbles in the hardened concrete which would act as expansion chambers to relieve the pressure build-up. It is to be noted that concrete with high water content and high water to cement ratio is less frost resistant than concrete with lower water content. scribble AND CAVITATIONHydraulic Concrete structures are affected by Abrasion-erosion due to the action of debris rolling and grinding against surface. The sources of the debris overwhelm construction toss out left in a structure, riprap brought back into a basin by eddy currents because of poor hydraulic design or asymmetrical discharge, and riprap or other debris thrown into a basin by the public. mechanic abrasion is usually characterized by long shallow grooves in the concrete surface and spalling along monolith joints. general abrasion and cavitation erosion results in bluff aggregate exposed concrete surface, concrete uneven surface, resulting in holes.In order to prevent the structure to suffer from abrasion the concrete should include the maximum amount of the hardest coarse aggregate that is available and the lowest practical w/c. In addition to this,high-range water-reducing admixtures (HRWRA) and condensed silica fume have been used to develop high compressive strength concrete 97 MPa (14,000 psi) , at this strength the concrete mix assumes a greater role in resisting abrasion-erosion damage. as well vacuum- tough concrete, polymer concrete, polymer-impregnated concrete, and polymer portland cement concrete could also be used to increase the abrasion resistance. In existing structures, balanced flows should be maintained into basins by using all gates to ward off discharge conditions where eddy action is prevalent. shekels sourcewww.nrc-cnrc.gc.caCARBONATIONCarbonation is a chemi cal reaction between atmospheric one C dioxide and hydrated cement compounds which causes a reduction in the alkalinity of the concrete. The permeability and moisture content of the concrete directly affects the rate of cytosineation .Pop-corn like calcite crystals sit in carbonated paste.sourcewww.concrete-experts.comIn the process of carbonation the calcium bearing phases present in the concrete mass are attacked by carbon dioxide of the air and converted to calcium carbonate.In this process the alkalinity of the concrete is lowered from an initial pH of around 13.5 to around 8.5 over some years.One method of testing a structure for carbonation is to drill a fresh hole in the surface and then treat the cut surface with phenolphthalein indicator answer. This solution will turn pink when in contact with alkaline concrete, making it possible to see the depth of carbonation.In order to combat the process of carbonation aquron products from Allcrete industries could be used which h elps in the following ways seals out moisture to a depth of 100mm or more reduces oxygen permeability by a pore-blocking process brings up the alkalinity of the concrete purging and binding chlorides in the colloidal silicate hydrogel formed in the pores and capillaries of the concreteIt is to be noted that when Concrete is treated within 24 hours of casting ,it will be protected for sprightliness against carbonation problems and aged concrete, once treated, will become immune to further deterioration. wearing awayCorrosion of steel reinforcement in concrete structures is well known to be Concrete Cancer, which happens to be a important area of concern in the area of infrastructure and building across the world. Corrosive species can enter the concrete mix if the ingredients of the concrete mix such as water, aggregates, additives are contaminated.When such corrosive species reacts with the chemical compounds under the porous temperament of concrete,the problem of corrosion occur s.Corrosion damage to the reinforcing steel results in the build-up of copious corrosion products, generating internal stresses and subsequent cracking and spalling of the concrete as shown schematically in the diagram belowFigure 2sourcewww.corrosion-club.comsourcewww.zoranthepainter.comCorrosion weapon Concrete CancerThe most frightening thing is that the corrosion can be going on under the surface and will not be noticed until the concrete starts visibly disintegrating. The outcome of such corrosion is that concrete reduces its strength and also the steel re-enforcing within the concrete can rust and the pressure this creates can cause the concrete to crack and crumble. Buildings in coastal areas are especially at risk.The preventive measures to avoid such kind of concrete cancer are listed belowEpoxy coating The concrete structures that are exposed to deicing salt may benifit from use of epoxy-coated, stifling dip galvanised or stainless steel rebar. Epoxy coated rebar can ea sily be identified by the light green emblazon of its epoxy coating.Applying Sealants After the process of curing, penetrating sealants must be applied. Sealants include paint, plastic foams, films and aluminum foil, felts or fabric mats sealed with tar, and layers of bentonite clay, sometimes used to seal roadbeds.corrosion inhibitors calcium nitrite can also be added to the water mix before pouring concrete. Generally, 12 wt. % of calcium nitrate with appraise to cement weight is needed to prevent corrosion of the rebars.Extreme run conditions such as high ambient and concrete temperature, low relative humidity or a mere 40 F or less average daily air temperature for more than 3 sequential days, tend to impair the quality of freshly mixed or hardened concrete thereby giving detrimental results. Therefore, it becomes imperative to handle concrete with caution both in extreme vitriolic and parky support. effectuate OF HOT WEATHER AND PROPER HANDLING On the compressive strengt h When the temperature of concrete is high cement hydrates at a much faster rate, it sucks up water and grows crystals around the aggregate particles but dont have time to grow strong . Although, the early strength will be higher but 28-day strength suffers. If the concrete is about 18 angryter than normal (for example, 88 sooner of 70), the ultimate compressive strength of the mix will be about 10% lower. On the colors of integrally colored concrete under hot hold up conditions , slump decreases rapidly as the cement sets up and and more mixing water is needed. In integrally colored concrete,this can result to variations in water content which can result in significant differences in concrete color between adjacent pours. Surface drying high concrete temperature and hot dry wind across the concrete can cause more drying and surface shrinkage. Thermal differentials sometimes the hot suffer condition makes one part of the concrete mass warmer than another(prenominal) part. If th is differential is greater than about 20F then concrete gets cracks. Difficulty in maintaining air content can be a problem in warm concrete. Mixing is more likely to drive air out of the concrete making the level difficult to control.Dealing with hot weather concretingPlastic shrinkage is a particular problem in hot weather concreting. As a general rule, each 10o F increase in ambient temperature reduces slump about 1. A switch from ASTM C494 guinea pig A to Type D water reducing and set retarding mix may be part of an effective plan for hot-weather concreting.Aggregates forms a major part in a concrete mix so its temperature has the superior effect on concrete temperature. Shading of aggregate piles is ideal, although not always possible. Using cool water is another way to get cool concrete. Ready mix producers in hot climates use chilled water or ice to lower the concrete temperature.Retarding admixtures can also bring in towards controlling concrete in warm weather. When the concrete is hot, the setting time is very quick .At that time Retarder can be added at the ready or on the job site . Retarders delays the setting time but they also give the concrete more time to dry out, so curing is critical. Retarders come as straight retarders or as water-reducing and retarding admixtures. Mid-range water reducers can increase the air content of the concrete. For concrete that is to be stamped, consider using step retardationadding retarder to the mix after half of the batch or after one-third and two-thirds have been placed. For textured concrete one of the strongest things to do in hot weather is step retardation.In order to avoid slump loss superplasticizer (high-range water reducer) could be used. These admixtures can increase slump without affecting the concretes final strength or appearance.Before placing the concrete on a subgrade wet down everything, subgrade and forms, with cool water so moisture isnt absorbed from the concrete, which can lead to crac king.In order to prevent the evaporation of surface water use a monomolecular film or evaporation retarder as soon as the concrete is down and bull floated. These materials gasify after a couple of hours. Monomolecular film will prevent plastic shrinkage cracking and surface crusting.In hot and dry weather curing needs to start as soon as finishing operations are completed. A sporting pigmented curing compound could be used with plain gray concretein order reflect sunshine. Also the blanched curing blanket could be used for the same purpose.A white pigmented curing compound on plain gray concrete white curing blankets keep the concrete cooler by reflecting can help reflect some heat from the sun the sunIn a hot climate, stain the concrete in the coolest part of the day as staining concrete relies on a chemical reaction that happens faster in hot weather.If concrete is stained in 95 to 100 weather ,it can ruin the entire job.Staining is best done early in the day when the concrete is coolerFor an get across instauration it is advisable that the concrete surface temperature should be between 50 and 80.Installing an overlay during the hottest part of the day amidst direct sunlight could adversely affect the quality of the result.EFFECTS OF COLD WEATHER AND PROPER HANDLINGDuring shabby weather until the concrete gains compressive strength of 500psi , it is under threat of either getting freezed up which in turn could break up the matrix or the concrete mix sets at a very slow pace. Below 40F the hydration reaction basically stops and the concrete doesnt gain any further strength. when the ground is cold, the concrete in contact with it will be cold and will set more slowly. This can lead to the problem of crusting, with the top part of the concrete set and the bottom still soft.During the process of shifting the concrete mix from the ready mix plant to the job site there will be some heat loss. While placing concrete in cold it is advisable to remove all sno w and ice from that area. Also any standing water should also be removed that could get mixed into the concrete. In case of a frozen land, it is advisable to thaw it using hydronic heat pipes and blankets (such as those from screen background Heaters), or electric blankets (check out Power Blanket). It is also suggested to Warm up anything that will come in contact with the concrete, including forms and any embedments, to at least 32F. Covering everything with tarps the day before the pour, will keep it dry and warm enough.also blankets could be used for the same purpose as well. Place triple layers of insulating blankets at corners and edges that could freeze. Wrap any protruding rebars. Make sure the blankets wont blow off during the night.Ground heaters Inc Portland cement associationWhile placing decorative concrete in cold weather it is suggested to use a dial pocket thermometer or an unseeable thermometer to test the concrete temperature as it is needed to be kept in a high er place 50F for the concrete to keep gaining strength. Also on exterior concrete, customers should be reminded to keep deicing chemicals off the surface during the first winter. Deicers can lead to spalling of newer concrete.During the process of finishing concrete in cold weather, it is advisable to let all the bleed water to evaporate first otherwise water-cement ratio would increase surface of concrete would get weak. Bleed water is basically the concrete particles cave in (like mud in a stirred up pond) and squeezing out all the extra water. Vacuums can also be used for this purpose. It should also be noted that sealing concrete in cold weather conditions is very risky.Power blanket Layfield GroupIn order to keep the slab warm hydronic heating pipes or electric heating blankets could be used. These are laid on top of the slab and insulated. Also, in case it is too cold to even place the concrete, then the only option left is to enclose the work and heat the air. In case of an enclosure,it is required to consider the likely problem of carbonation. With unvented heaters (salamanders), or even with gas-powered equipment, the carbon dioxide levels can increase. This carbon reacts with the concrete, creating a chalky carbonated layer at the surface which is unacceptable. To sic this problem use heaters that exhaust to the outside of an enclosure or building and just blow in warm air. Maintain the concrete temperature above 40 degree