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Concrete 1 common cement is Type I. Type III cement is used Concrete if more rapid strength development is required. The A composite material that consists essentially of a other types are characterized by either lower heat binding medium, such as a mixture of
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  Concrete  1 Concrete  A composite material that consists essentially of abinding medium, such as a mixture of portland ce-ment and water, within which are embedded parti-clesorfragmentsofaggregate,usuallyacombinationof fine and coarse aggregate.Concrete is by far the most versatile and most widely used construction material worldwide. It canbeengineeredtosatisfyawiderangeofperformancespecifications, unlike other building materials, such as natural stone or steel, which generally have to beused as they are. Because the tensile strength of con-crete is much lower than its compressive strength, itis typically reinforced with steel bars, in which caseit is known as reinforced concrete.  See  REINFORCEDCONCRETE . Materials.  A composite material is made up of var-iousconstituents.Thepropertiesandcharacteristicsof the composite are functions of the constituentmaterials’ properties as well as the various mix pro-portions. Before discussing the properties of thecomposite, it is necessary to discuss those of theindividual constituents as well as the effects of themixproportionsandmethodsofproduction. See COMPOSITE MATERIAL . Cement.  Therearemanydifferentkindsofcements.In concrete, the most commonly used is portlandcement, a hydraulic cement which sets and hardensby chemical reaction with water and is capable of doing so under water. Cement is the “glue” thatbindstheconcreteingredientstogetherandisinstru-mental for the strength of the composite. Although cements and concrete have been around for thou-sands of years, modern portland cement was in- vented in 1824 by Joseph Aspdin of Leeds, England.The name derives from its resemblance of the natu-ral building stone quarried in Portland, England.  See CEMENT .Portland cement is made up primarily of four mineral components (tricalcium silicate, dicalciumsilicate, tricalcium aluminate, and tetracalcium alu-minoferrite), each of which has its own hydrationcharacteristics. By changing the relative proportionsof these components, cement manufacturers cancontrol the properties of the product.The primary product of cement hydration isa complex and poorly crystalline calcium-silicate-hydroxide gel (or CSH). A secondary product of hy-drationiscalciumhydroxide,ahighlycrystallinema-terial. A category of siliceous materials known aspozzolans have little or no cementitious value, butin finely divided form and in the presence of mois-ture will react chemically with calcium hydroxide toform additional CSH. This secondary hydration pro-cess has a generally beneficial effect on the final con-crete properties. Examples of pozzolans are fly ash,groundgranulatedblast-furnaceslag,andmicrosilicaor silica fume.The American Society for Testing and Materials(ASTM) defines five types of cement, specifying for each the mineral composition and chemical andphysical characteristics such as fineness. The mostcommon cement is Type I. Type III cement is usedif more rapid strength development is required. Theother types are characterized by either lower heatof hydration or better sulfate resistance than that of Type I cement.  Aggregate.  Theaggregateisagranularmaterial,such as sand, gravel, crushed stone, or iron-blast furnaceslag. It is graded by passing it through a set of sieves with progressively smaller mesh sizes. All materialthat passes through sieve #4 [0.187 in. (4.75 mm)openings] is conventionally referred to as fine ag-gregate or sand, while all material that is retainedon the #4 sieve is referred to as coarse aggregate,gravel, or stone. By carefully grading the materialand selecting an optimal particle size distribution,a maximum packing density can be achieved, wherethe smaller particles fill the void spaces between thelarger particles. Such dense packing minimizes theamount of cement paste needed and generally leadsto improved mechanical and durability properties of the concrete.Theaggregateconstitutestypically75%ofthecon-crete volume, or more, and therefore its propertieslargelydeterminethepropertiesoftheconcrete.For the concrete to be of good quality, the aggregate hastobestronganddurableandfreeofsilts,organicmat-ter, oils, and sugars. Otherwise, it should be washedprior to use, because any of these impurities may slow or prevent the cement from hydrating or re-duce the bond between the cement paste and theaggregate particles.  Admixtures.  While aggregate, cement, and water arethe main ingredients of concrete, there are a largenumberofmineralandchemicaladmixturesthatmay be added to the concrete. The four most commonadmixtures will be discussed.1. Air-entraining agents are chemicals that areadded to concrete to improve its freeze–thaw re-sistance. Concrete typically contains a large num-ber of pores of different sizes, which may be par-tially filled with water. If the concrete is subjectedto freezing temperatures, this water expands whenforming ice crystals and can easily fracture the ce-ment matrix, causing damage that increases with each freeze–thaw cycle. If the air voids created by the air-entraining agent are of the right size and av-erage spacing, they give the freezing water enough space to expand, thereby avoiding the damaging in-ternal stresses.2. Water-reducing admixtures, also known as su-perplasticizers, are chemicals that lower the viscos-ityofconcreteinitsliquidstate,typicallybycreatingelectrostaticsurfacechargesonthecementandvery fine aggregate particles. This causes the particles torepel each other, thereby increasing the mix flowa-bility, which allows the use of less water in the mixdesign and results in increased strength and durabil-ity of the concrete.  See  FLOW OF SOLIDS .3. Retarding admixtures delay the setting time, which may be necessary in situations where delaysin the placement of concrete can be expected. Ac-celerators shorten the period needed to initiate ce-ment hydration—for example, in emergency repair   2  Concrete situations that call for the very rapid development of strength or rigidity.4. Color pigments in powder or liquid form may be added to the concrete mix to produce coloredconcrete. These are usually used with white port-land cement to attain their full coloring potential. See  PIGMENT (MATERIAL) . Reinforcing steels.  Because of concrete’s relatively low tensile strength, it is typically reinforced with steel bars (  Fig. 1  ). These bars are produced in stan-dard sizes. In the United States, the identificationnumber of a reinforcing bar refers to the nominal di-ameterexpressedineighthsofaninch.Forexample,anumber6barhasadiameterof   6  /  8 = 0.75inch.Theavailable bar sizes range in general from 2 to 18. Re-inforcing steel usually has a nominal yield strength of 60,000 lb/in. 2 (414 MPa). To improve the bondstrength between the bars and the concrete, thebarsarefabricatedwithsurfacedeformationsorribs.The relatively high cost of steel mandates its sparinguse. This means that the concrete is usually assignedthe task of resisting compressive forces, while thesteel carries primarily the tensile forces. The alkalin-ity of the cement paste generally provides sufficientprotection of the steel against corrosion. However,corrosion protection is often breached, for exam-ple, in highway bridge decks with continuous porestructureortraffic-inducedcracksthatpermitthede-icing chemicals used in winter to penetrate the pro-tective concrete cover. Additional protective mea-suresmaybenecessary,suchasusingepoxycoatingsonthebars,noncorrosivesteels,ornonmetallicrein-forcement (for example, fiber-reinforced polymers). See  CORROSION .Other important concrete terminology can be de-fined. A mixture of cement and water is called ce-ment paste. Cement paste plus fine aggregate iscalledmortarorconcretematrix.Mortarpluscoarseaggregate constitutes concrete. Concrete reinforced Fig. 1. Workers placing and vibrating concrete on a bridge deck including epoxy-coatedreinforcing steel. (   Portland Cement Association  )  with steel or other high-strength material is knownas reinforced concrete.  See  MORTAR  . Production of concrete.  The properties of the endproduct depend not only on the various constituentmaterials listed above but also on the way they areproportioned and mixed, as well as on the methodsof placing and curing the composite. Mix design.  It is not possible to predict the strength and other concrete properties solely based on theproperties and proportions of the mix components.Therefore, mixes are designed on an empirical basis,often with the help of trial mixes. The objectiveof the mix design is to assure that the product hasspecified properties in both the fresh and hardenedstate. The most important mix design variable isthe weight ratio between water and cement, re-ferred to as the w/c ratio. There is a theoreticalminimum amount of water needed for the cementto completely hydrate, which can be determinedusing the equations of hydration chemistry. Any ex-cess water creates pores which, together with any air-filled pores, do not contribute to the materialstrength. The result is a drastic decrease in strength asafunctionofincreasingthew/cratio.Ontheother hand, too low w/c ratios cause poor workability of theconcrete.Forpracticalreasons,thew/cratiotyp-ically varies between 0.4 and 0.6. The other impor-tantmixdesignvariablesarethecement-to-aggregateratio and the fine-to-coarse aggregate ratio. Also, themaximumaggregatesizeisofimportance.Andsincecement is the most expensive bulk ingredient, themix design will generally aim at the least amount of cement necessary to achieve the design objectives. Construction practice.  The material obtained immedi-ately upon mixing of the various concrete ingredi-ents is called fresh concrete, while hardened con-creteresultswhenthecementhydrationprocesshasadvanced sufficiently to give the material mechani-cal strength. Concrete that is batched and mixed ina plant and then transported by truck in its fresh, or plastic, state to the construction site for final place-ment is called ready-mixed concrete. If the result-ing structure or highway pavement, for example,remains in place after placement, the concrete is re-ferred to as cast-in-place concrete, whether mixedon-siteoroff-site.Precastconcretereferstoanystruc-ture or component that is produced at one site, typi-callyinaprecastingplant,andthentransportedinitshardenedstatetoitsfinaldestination.Thecontrolledenvironment of a precasting plant generally permitshigherqualitycontroloftheproductthanispossible with cast-in-place concrete produced at a construc-tion site.  See  CONSTRUCTION METHODS ;  PAVEMENT .Code-writing organizations, such as the AmericanSociety for Testing and Materials, the AmericanConcrete Institute (ACI), and the American Asso-ciation of State Highway and Transportation Offi-cials (AASHTO), have published detailed specifica-tions and recommendations for measuring, mixing,transporting, placing, curing, and testing concrete. A proper mix design assures that the concrete mix is well proportioned. The mixing time should be suffi-cient to assure a uniform mixture. When placing the  Concrete  3 concrete, care should be taken to avoid segregation.For example, if dropped too far, the heavy or bigaggregate particles can settle and lighter mix com-ponents, such as water, tend to rise. The concreteis conveyed from the mixing truck to its final desti-nation in dump buckets by cableways or cranes or by pumping through pipelines. In modern high-risebuildingconstruction,concretehasbeenpumpedashigh as a thousand feet (330 m).During placement, large amounts of air are en-trapped in the mix, which lowers the strength of the hardened concrete. Much of the air is removedby compaction, which is achieved by either immers-ing high-frequency vibrators into the fresh concreteor attaching them to the outside faces of the form- work (Fig. 1). Care must be taken to avoid excessive vibration; otherwise the heavy aggregate particlessettle down and the light mixing water rises to thesurface.For underwater construction, the concrete isplaced in a large metal tube, called a tremie, with a hopper at the top and a valve arrangement at thesubmerged end. For so-called shotcrete applicationssuch as tunnel linings and swimming pools, the con-crete mixture is blown under high pressure through a nozzle directly into place to form the desired sur-face.Beforetheconcretesetsandhardens,itisrelatively easy to give its exposed surfaces the desired finish.Surfaces cast against forms can be given various tex-tures by using form liners or treating the surfacesafter forms are removed. Hardened surfaces can betextured by grinding, chipping, bush-hammering, or sandblasting. Curing.  Once the concrete has been placed andcompacted, it is critical that none of the mixing water needed for cement hydration is lost. This isthe objective of curing. For example, in hot or dry  weather large exposed surfaces will lose water by evaporation. This can be avoided by covering such surfaceswithsheetsofplasticorcanvasorbyperiod-ically spraying them with water. In precast concreteplants, concrete elements are often steam-cured, be-cause the simultaneous application of hot steam andpressure accelerates the hydration process, which permits high turnover rates for the formwork instal-lations. Quality control.  To assure that the finished materialhas the specified properties, quality assurance andquality control procedures need to be implemented.From a public safety viewpoint, strength is the mostimportant property. To assure adequate strength,such as determining the time of safe formwork re-moval, concrete batches are sampled by casting testcylinders at the same time and place as the struc-ture being built. These cylinders are then tested by accredited laboratories to determine their strength.If the in-situ strength of existing structures needsto be evaluated, concrete cores may be drilled fromselected parts of the structure and tested in the labo-ratory. There are also nondestructive test methodsavailable to determine various properties of hard-ened concrete. Fig. 2. Concrete slump test with a slump of 1.5 in., typical for pavement work. (   Portland Cement Association  ) Properties of fresh concrete.  The most importantpropertyoffreshconcreteisitsworkabilityorflowa-bility, because this determines the ease with which it can be placed. It is determined using a slump test,in which a standard truncated metal cone form isfilled with fresh concrete (  Fig. 2  ). The mold is thenlifted vertically, and the resulting loss in height of the concrete cone, or the slump value, is indicativeof the concrete’s workability. For very liquid mixes,the flow test is performed, which is similar to theslump test, except that the mean diameter of thecakeformedbythefreshconcrete(ormortar)ismea-sured. A short while after casting, the concrete stiffensand loses its plasticity. The time of setting can be de-terminedbyrepeatedlydroppingacalibratedneedleintothefreshconcreteandmeasuringthetimewhenthe needle no longer sinks in. Properties of hardened concrete.  By far, the mostimportant property of hardened concrete is its com-pressive strength. Since this strength continues toincrease with continuing cement hydration, it is afunction of age which is the time after casting. IntheUnitedStates,thestrengthisdetermined28daysafter casting by loading standardized test cylindersup to failure. In Europe, test cubes are often used.Mostcommerciallyproducedconcretehascompres-sive strengths between 3000 and 6000 lb/in. 2 (20and 40 MPa). If loaded in tension, the material failsat a stress much lower than that, typically of theorderof10%ofthecompressivestrength.Becauseof this low (and unreliable) tensile strength, concreteis usually reinforced with steel bars.  See  STRESS ANDSTRAIN .During hydration and especially if allowed todry after hardening, the concrete volume decreasesby a small amount because of shrinkage. If this  4  Concrete shrinkageisrestrainedsomehow,itcanleadtocrack-ing. Shrinkage deformations caused by drying canbe reversed only partially upon wetting. A concretemember or structure subjected to external load willundergo deformations which, up to a point, are pro-portionaltotheamountofappliedload.Iftheseloadsremain in place for an appreciable time (months or  years), these deformations will increase due to a ma-terial property called creep. Even for regular con-cretemixes,creepdeformationscanbetwoorthreetimes as high as the initial elastic deformations, es-pecially if the concrete is loaded at a very young age. Whendesigningconcretestructures,suchcreepandshrinkage deformations must be accounted for.  See CREEP (MATERIALS) ;  ELASTICITY  . Durability.  Durability is the ability of a material (or structure)tomaintainitsvariouspropertiesthrough-out its design or service life. Some concrete struc-turesbuiltbytheRomansservedforover2000years. Amaterialthatlosesitsstrengthintime,forwhatever reason, cannot be considered durable.There can be numerous causes for loss of durabil-ity or deterioration of concrete structures. The mostcommon one is an excessive amount of cracking or pore structure. Most concrete structures contain nu-merous cracks. But as long as these remain small (of the order of 0.25 mm or less), they are generally in- visible to the naked eye, and the concrete remainsbasically impermeable to salts and other aggressiveagents, so that it can continue to protect the rein-forcing steel against corrosion. Larger cracks pro- vide easy access for such agents to the steel, thereby promotingcorrosion.Sincethesteelcorrosionprod-ucts occupy a larger volume than sound steel, they produce internal pressure during expansion and canspall off the protective concrete cover, the loss of  which may render the structure unsafe to resistloads.The concrete itself may deteriorate or weather, es-pecially if subjected to many cycles of freezing andthawing, during which the pressure created by thefreezing water progressively increases the extent of internal cracking. In addition, carbon in the atmo-sphere can react chemically with the cement hydra-tionproducts.Thisprocessisknownascarbonation.It lowers the pH of the concrete matrix to the point where it can no longer protect the steel against cor-rosion.Most types of aggregate used for concrete pro-ductionareinert;thatis,theydonotreactchemically  with the cement or hydration products. However,there are various aggregate types, including thosecontaining amorphous silica such as common glass, whichreactchemicallywiththealkaliinthecement.In the presence of moisture, the alkali–aggregate re-action products can swell and cause considerabledamage. The deterioration of numerous major struc-tures and highway pavements has been attributedto such reactions, especially alkali–silica reaction,often after years of seemingly satisfactory service.Other common causes of chemical attack are sul-fates found in soils, chlorides in seawater, acid rain,and other industrial pollutants. Generally, structuresbuilt with well-designed concrete mixes, having low porosity or high density and minimal cracking, arelikely to resist most causes of chemical attack, al-though for service in particularly aggressive envi-ronments special countermeasures may have to betaken.Under repeated load applications, structures canexperience fatigue failure, as each successive loadcycle increases the degree of cracking and materialdeterioration to the point where the material itself may gradually lose its strength or the increased ex-tent of cracking is the source of loss of durability. Thermal and other properties.  The heavy weight of concrete [its specific gravity is typically 2.4 g/cm 3 (145 lb/ft 3  )] is the source of large thermal mass.For this reason, massive concrete walls and roof andfloor slabs are well suited for storing thermal energy.Because of this heat capacity of concrete, together  with its reasonably low thermal conductivity, con-crete structures can moderate extreme temperaturecycles and increase the comfort of occupants. Well-designed concrete mixes are impermeable to liq-uids and therefore suitable for storage tanks withoutthe need for impermeable membranes or liners. Al-thoughsteelreinforcingbarsconductelectricityandinfluence magnetic fields, the concrete itself doesneither.  See CONCRETE SLAB ; FLOOR CONSTRUCTION ; ROOF CONSTRUCTION . Special concretes and recent developments.  Con-crete is an engineered material, with a variety of specialtyproductsdesignedforspecificapplications.Some important ones are described below. Lightweight concrete.  Although the heavy weight or large mass of typical concrete members is oftenan advantage, there are situations where this is notthe case. For example, because of the large stressescaused by their own heavy weight, floor slabs areoften made lighter by using special lightweight ag-gregate. To further reduce weight, special chemicaladmixturesareadded,whichproducelargeporosity.Suchhighporosity(ineitherthematrixortheaggre-gate particles themselves) improves the thermal re-sistance of the concrete as well as sound insulation,especially for higher frequencies. However, because weight density correlates strongly with strength, ul-tralightweight concretes [1.1 g/cm 3 (70 lb/ft 3  ) andless] are used only for thermal or sound insulationpurposes and are unsuitable for structural applica-tions. Heavyweight concrete.  When particularly high weightdensities are needed, such as for shielding in nuclear reactor facilities, special heavyweight aggregate isused, including barite, limonite, magnetite, scrapmetal, and steel shot for fine aggregate. Weight den-sities can be achieved that are twice that of normal- weight concrete.  Architectural concrete.  Concrete surfaces that remainexposed may call for special finishes or texturesaccording to the architect’s desires. Textures aremost readily obtained by inserting special form lin-ers before casting the concrete. Sometimes the neg-ative imprint of roughly sawn timber is consid-ered attractive and left without further treatment.
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