Concrete 1

ConcreteA composite material that consists essentially of abinding medium, such as a mixture of portland ce-ment and water, within which are embedded parti-cles or fragments of aggregate, usually a combinationof fine and coarse aggregate.

Concrete is by far the most versatile and mostwidely used construction material worldwide. It canbe engineered to satisfy a wide range of performancespecifications, unlike other building materials, suchas 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 REINFORCED

CONCRETE.Materials. A composite material is made up of var-

ious constituents. The properties and characteristicsof 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 ofthe mix proportions and methods of production. SeeCOMPOSITE MATERIAL.

Cement. There are many different kinds of cements.In concrete, the most commonly used is portlandcement, a hydraulic cement which sets and hardensby chemical reaction with water and is capable ofdoing so under water. Cement is the “glue” thatbinds the concrete ingredients together and is instru-mental for the strength of the composite. Althoughcements 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. SeeCEMENT.

Portland cement is made up primarily of fourmineral 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-dration is calcium hydroxide, a highly crystalline ma-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,ground granulated blast-furnace slag, and microsilicaor silica fume.

The American Society for Testing and Materials(ASTM) defines five types of cement, specifying foreach the mineral composition and chemical andphysical characteristics such as fineness. The most

common 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 ofType I cement.

Aggregate. The aggregate is a granular material, suchas sand, gravel, crushed stone, or iron-blast furnaceslag. It is graded by passing it through a set of sieveswith 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 ofthe concrete.

The aggregate constitutes typically 75% of the con-crete volume, or more, and therefore its propertieslargely determine the properties of the concrete. Forthe concrete to be of good quality, the aggregate hasto be strong and durable and free of silts, organic mat-ter, oils, and sugars. Otherwise, it should be washedprior to use, because any of these impurities mayslow 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 largenumber of mineral and chemical admixtures that maybe 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 witheach freeze–thaw cycle. If the air voids created bythe air-entraining agent are of the right size and av-erage spacing, they give the freezing water enoughspace to expand, thereby avoiding the damaging in-ternal stresses.

2. Water-reducing admixtures, also known as su-perplasticizers, are chemicals that lower the viscos-ity of concrete in its liquid state, typically by creatingelectrostatic surface charges on the cement and veryfine 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 ofstrength or rigidity.

4. Color pigments in powder or liquid form maybe 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 relativelylow tensile strength, it is typically reinforced withsteel 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-ameter expressed in eighths of an inch. For example,a number 6 bar has a diameter of 6/8 = 0.75 inch. Theavailable bar sizes range in general from 2 to 18. Re-inforcing steel usually has a nominal yield strengthof 60,000 lb/in.2 (414 MPa). To improve the bondstrength between the bars and the concrete, thebars are fabricated with surface deformations or ribs.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 porestructure or traffic-induced cracks that permit the de-icing chemicals used in winter to penetrate the pro-tective concrete cover. Additional protective mea-sures may be necessary, such as using epoxy coatingson the bars, noncorrosive steels, or nonmetallic rein-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 iscalled mortar or concrete matrix. Mortar plus coarseaggregate 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 strengthand 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 anyair-filled pores, do not contribute to the materialstrength. The result is a drastic decrease in strengthas a function of increasing the w/c ratio. On the otherhand, too low w/c ratios cause poor workability ofthe concrete. For practical reasons, the w/c ratio typ-ically varies between 0.4 and 0.6. The other impor-tant mix design variables are the cement-to-aggregateratio and the fine-to-coarse aggregate ratio. Also, themaximum aggregate size is of importance. And sincecement is the most expensive bulk ingredient, themix design will generally aim at the least amount ofcement 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-crete results when the cement hydration process hasadvanced sufficiently to give the material mechani-cal strength. Concrete that is batched and mixed ina plant and then transported by truck in its fresh, orplastic, 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-site or off-site. Precast concrete refers to any struc-ture or component that is produced at one site, typi-cally in a precasting plant, and then transported in itshardened state to its final destination. The controlledenvironment of a precasting plant generally permitshigher quality control of the product than is possiblewith 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 iswell 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 orby pumping through pipelines. In modern high-risebuilding construction, concrete has been pumped ashigh as a thousand feet (330 m).

During placement, large amounts of air are en-trapped in the mix, which lowers the strength ofthe 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 excessivevibration; 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, witha 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 througha nozzle directly into place to form the desired sur-face.

Before the concrete sets and hardens, it is relativelyeasy 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, orsandblasting.

Curing. Once the concrete has been placed andcompacted, it is critical that none of the mixingwater needed for cement hydration is lost. This isthe objective of curing. For example, in hot or dryweather large exposed surfaces will lose water byevaporation. This can be avoided by covering suchsurfaces with sheets of plastic or canvas or by period-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, whichpermits 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 byaccredited 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. (PortlandCement Association)

Properties of fresh concrete. The most importantproperty of fresh concrete is its workability or flowa-bility, because this determines the ease with whichit 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 ofthe 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 thecake formed by the fresh concrete (or mortar) is mea-sured.

A short while after casting, the concrete stiffensand loses its plasticity. The time of setting can be de-termined by repeatedly dropping a calibrated needleinto the fresh concrete and measuring the time whenthe 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. Inthe United States, the strength is determined 28 daysafter casting by loading standardized test cylindersup to failure. In Europe, test cubes are often used.Most commercially produced concrete has compres-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 theorder of 10% of the compressive strength. Because ofthis low (and unreliable) tensile strength, concreteis usually reinforced with steel bars. See STRESS AND

STRAIN.During hydration and especially if allowed to

dry after hardening, the concrete volume decreasesby a small amount because of shrinkage. If this

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shrinkage is restrained somehow, it can lead to crack-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-portional to the amount of applied load. If these loadsremain in place for an appreciable time (months oryears), these deformations will increase due to a ma-terial property called creep. Even for regular con-crete mixes, creep deformations can be two or threetimes as high as the initial elastic deformations, es-pecially if the concrete is loaded at a very young age.When designing concrete structures, such creep andshrinkage deformations must be accounted for. SeeCREEP (MATERIALS); ELASTICITY.

Durability. Durability is the ability of a material (orstructure) to maintain its various properties through-out its design or service life. Some concrete struc-tures built by the Romans served for over 2000 years.A material that loses its strength in time, for whateverreason, 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 orpore structure. Most concrete structures contain nu-merous cracks. But as long as these remain small (ofthe 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, therebypromoting corrosion. Since the steel corrosion prod-ucts occupy a larger volume than sound steel, theyproduce internal pressure during expansion and canspall off the protective concrete cover, the loss ofwhich 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 ofinternal cracking. In addition, carbon in the atmo-sphere can react chemically with the cement hydra-tion products. This process is known as carbonation.It lowers the pH of the concrete matrix to the pointwhere it can no longer protect the steel against cor-rosion.

Most types of aggregate used for concrete pro-duction are inert; that is, they do not react chemicallywith the cement or hydration products. However,there are various aggregate types, including thosecontaining amorphous silica such as common glass,which react chemically with the alkali in the cement.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, structures

built with well-designed concrete mixes, having lowporosity 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 itselfmay 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 ofconcrete [its specific gravity is typically 2.4 g/cm3

(145 lb/ft3)] 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, togetherwith 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-though steel reinforcing bars conduct electricity andinfluence 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 ofspecialty products designed for specific applications.Some important ones are described below.

Lightweight concrete. Although the heavy weight orlarge 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 chemicaladmixtures are added, which produce large porosity.Such high porosity (in either the matrix or the aggre-gate particles themselves) improves the thermal re-sistance of the concrete as well as sound insulation,especially for higher frequencies. However, becauseweight density correlates strongly with strength, ul-tralightweight concretes [1.1 g/cm3 (70 lb/ft3) 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 nuclearreactor 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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Fig. 3. Glass concrete tile. (C. Meyer)

Other surface textures are obtained by sandblasting,bush-hammering, and similar treatments. Ordinaryportland cement gives concrete the typical graycolor. By adding color pigments to the mix, a large va-riety of colors can be produced, especially in combi-nation with white portland cement. Concrete mixedwith specialty aggregate, such as marble, and groundsmooth is known as terrazzo concrete, which is verypopular for decorative surfaces on floors and walls.Recently, crushed postconsumer glass has been usedas aggregate for decorative applications because ofthe esthetic possibilities, provided suitable coun-termeasures against alkali–silica reaction are taken(Fig. 3).

Fiber-reinforced concrete. The concrete matrix canbe reinforced with short, randomly distributedfibers. Fibers may be metallic (primarily steel), syn-thetic (such as polypropylene, nylon, polyethylene,polyvinyl alcohol, and alkali-resistant glass), or nat-ural (such as sisal, coconut, and rice husk). Suchfibers are typically used in addition to conventionalsteel reinforcement, but in some applications asits replacement. For example, precast glass-fiber-reinforced building façade elements are widely usedin the United States. By being uniformly distributedand randomly oriented, the fibers give the concretematrix tensile strength, ductility, and energy absorp-tion capacities that it otherwise would not have. Inparticular, when these fibers are engineered to opti-mize the fracture energy, so-called high-performancefiber-reinforced concrete is obtained, which has re-markable deformational characteristics and extraor-dinary resistance to blast and impact loads. In theconcrete industry, it is very common to add smallamounts of polypropylene fibers to reduce the ex-tent of shrinkage cracking.

Textile-reinforced concrete. Whereas in fiber-reinforcedconcrete the fibers are short [usually no longer than2 in. (5 cm)] and discontinuous, textile-reinforcedconcrete contains continuous woven or knittedmesh or textiles. Conceptually, such reinforcement

acts similarly to conventional steel reinforcing bars orwelded steel wire fabrics. But these fabric materialsare noncorrosive and can have mechanical proper-ties that are superior to those of steel. The fabricscan be premanufactured in a wide variety of ways,thereby lending themselves to new applications, es-pecially for repairing or strengthening existing con-crete structures. See TEXTILE.

Polymer-modified concrete. In polymer-modified con-crete, also known as latex-modified concrete, a poly-mer is added to improve the material’s strength,imperviousness, or both. In applications such ashighway bridge decks, often a layer of latex-modifiedconcrete is placed on top of a regular reinforced con-crete deck for additional protection of the steel re-inforcement. In polymer concrete, the hydraulic ce-ment is replaced by an organic polymer as the binder.See POLYMER; POLYMERIC COMPOSITE.

Roller-compacted concrete. This type of concrete is for-mulated with very low contents of portland cementand water and therefore is of relatively low-cost. It isoften used for pavements and dams. It can be trans-ported by dump trucks or loaders, spread with bull-dozers or graders, and compacted with vibratoryrollers. Because the cement content is so low, theheat of hydration does not cause the kind of prob-lems encountered in dams built with conventionalconcrete.

Ultra-high-strength concrete. Whereas concretes withcompressive strengths of 6000 to 12,000 lb/in.2 (40to 85 MPa) can now be categorized as high-strength,a new technology has been developed that resultsin strengths of 30,000 lb/in.2 (200 MPa) and higher.The key ingredient of this ultra-high-strength con-crete is a reactive powder; therefore, it is also knownas reactive-powder concrete. Other characteristics ofthis material are low water–cement ratios, carefullyselected high-strength aggregates, and small steelfibers.

Self-leveling concrete. The need for good workabilityhas been mentioned. The need for highly skilledworkers who can properly compact concrete at theconstruction site prompted researchers in Japan tooptimize the mix design such that the fresh concretecan flow into place without the need for further vi-bration. The main challenge was to obtain a low-viscosity mix without the threat of desegregation.This innovation is particularly important in applica-tions with dense steel reinforcement, which tradi-tionally have caused severe difficulties of producinghigh-quality concrete.

“Green” concrete. Concrete is by far the most widelyused building material. Well over 10 billion tons areproduced worldwide each year, requiring enormousnatural resources. Also, it has been estimated thatthe production of 1 ton of portland cement causesthe release of 1 ton of carbon dioxide (CO2) intothe atmosphere, a gas that is known to contribute toglobal warming. Together with the large amounts ofenergy required to produce portland cement, the ce-ment and concrete industry has a major impact onthe environment worldwide. Efforts are underwayto reduce this impact and transform the industry to

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conform to the principles of sustainable develop-ment. The most significant step is the replacementof portland cement by other cementitious or poz-zolanic materials, preferably materials that are by-products of industrial processes, such as fly ash(the by-product of coal-burning power plants) andgranulated blast furnace slag (a by-product of thesteel industry). To reduce the need for virgin aggre-gate, recycled concrete is the most promising ap-proach, because construction debris, in particulardemolished concrete, constitutes a major compo-nent of solid waste that fills up sparse landfill ca-pacity. These recent developments are much moreadvanced in Europe and Japan than in the UnitedStates. But the “green” building movement is gain-ing momentum there as well, and for the concreteindustry to maintain its dominant position within the

construction industry, it is undertaking major effortsto make concrete a more “green” material.

Christian MeyerBibliography. ACI Committee 225, Guide to the

Selection and Use of Hydraulic Cements, ACI Re-port 225R-99, American Concrete Institute, Farm-ingdale Hills, MI, 2001; ACI Committee 304, Guideto Measuring, Mixing, Transporting, and PlacingConcrete, ACI Report 304R-00, American ConcreteInstitute, Farmingdale Hills, MI, 2001; B. Mather andC. Ozyildirim, Concrete Primer, 5th ed., ACI Spec.Publ. 1, 2002; P. K. Mehta and P. J. M. Monteiro,Concrete, 3d ed., McGraw-Hill, 1993; S. Mindess,J. F. Young, and D. Darwin, Concrete, 2d ed., Pren-tice Hall, Englewood Cliffs, NJ, 2003; A. M. Neville,Properties of Concrete, 4th ed., Wiley, New York,1996.

Reprinted from the McGraw-Hill Encyclopedia of

Science & Technology, 10th Edition. Copyright c©2003 by The McGraw-Hill Companies, Inc. All rights

reserved.