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PCA R&D Serial No. 2951

Environmental Performance of Concrte

by William C. Panarese

©ASM International 2005 www.asminternational.org All rights reserved

Reprinted with permission of ASM International

http://www.asminternational.org/

Environmental Performance of ConcreteWilliam C. Panarese, Portland Cement Association

PORTLAND CEMENT CONCRETE haslow environmental impact, versatility, dura-bility, and economy, which make it the mostabundant construction material in the world. Inthe United States more than 300 million m3

(400 million yd3) of ready-mixed concreteis used each year in airports, roads, bridges,stadiums, low- and high-rise buildings, dams,homes, and numerous other structures.

Concrete is a mixture of two main compo-nents: aggregate and paste. The paste binds theaggregates (usually sand and gravel or crushedstone) into a rocklike mass as the paste hardensthrough the chemical reaction of the cement andwater.

The paste is composed of water and entrappedair or purposely entrained air, and sometimeschemical admixtures. The paste constitutesabout 25 to 40% of the total volume of concrete.Figure 1 shows the range of component volume.

Because aggregates make up about 60 to 75%of the total volume of concrete, their selection is

important. Aggregates should consist of particleswith adequate strength and resistance to envir-onmental conditions and should not containmaterials that will cause deterioration of theconcrete. A continuous gradation of aggregateparticle sizes is desirable for efficient use ofthe paste. The quality of any concrete dependson the quality of the paste and aggregate and thebond between the two. In properly madeconcrete, each particle of aggregate is coatedcompletely with paste, and all of the spacesbetween aggregate particles are filled completelywith paste.

Durability of concrete is based on its abilityto resist weathering action, chemical attack,and abrasion while maintaining its desiredengineering properties. The ingredients, pro-portioning of those ingredients, interactionsbetween the ingredients, placing and curingpractices, and the severity of the environmentdetermine the durability and useful life of theconcrete.

Types and Causes ofConcrete Degradation

The exceptional durability of portland cementconcrete is a major reason for its wide use.Material limitations, poor design and construc-tion practices, and severe exposure conditionscan cause concrete to deteriorate, resulting inaesthetic, functional, or structural problems.Following are potential causes of concretedeterioration and the factors that influence them.

Corrosion of reinforcing steel and otherembedded metals is the leading cause ofdeterioration in concrete. When steel corrodes,the resulting rust (iron oxide) occupies a greatervolume than the steel does. This expansion cre-ates tensile stresses in the concrete that caneventually cause cracking, delamination, andspalling (Fig. 2). Corrosion of embedded metalsin concrete can be reduced greatly by placingcrack-free concrete with low permeability andsufficient concrete cover over the metal. Low-permeability concrete can be attained bydecreasing the water to cementitious materialsratio of the concrete and by the use of pozzolansand slag admixtures (Ref 1). These admixturesreduce the corrosion rate even after it initiates.Minimum concrete cover requirements for dif-ferent exposure conditions are set by buildingcodes (Ref 2). Additional measures to mitigatecorrosion of steel reinforcement in concreteinclude coating of reinforcement with epoxyresin and the use of sealers and membranes onthe concrete surface.

Alkalinity. The alkaline environment of con-crete—a pH of 12 to 13—protects the steel. Atthis high pH level, a thin oxide layer forms on the

Air-entrainedconcrete

Non-air-entrainedconcrete

Cement15%

Water18%

Fine aggregate28%

Coarse aggregate31%

Air8%

7% 14% 4% 24% 51%

15% 21% 3% 30% 31%

7% 16% 1% 25% 51%

Mix 1

Mix 2

Mix 3

Mix 4

Fig. 1 Range in proportions of materials used in concrete (by absolute volume). Mixes 1 and 3 represent rich mixes withsmall-size aggregates. Mixes 2 and 4 represent lean mixes with large-size aggregates.

Steel Corrosionby-products(rust)

Fig. 2 The expansion of corroding steel creates tensilestresses in the concrete, which can cause

cracking, delamination, and spalling.

© 2005 ASM International. All Rights Reserved.ASM Handbook Volume 13B, November, 2005.

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steel surface, preventing further reaction. Steelcorrosion is not an issue in the majority of con-crete structures. However, corrosion can occurwhen the passivity layer is destroyed; thisdestruction occurs when the alkalinity of theconcrete is reduced or when the chloride con-centration in concrete is increased to a certainlevel.

Chlorides. Exposure of reinforced concrete tochloride ions is the primary cause of prematurecorrosion of steel reinforcement. The intrusion ofchloride ions—present in deicing salts, seawater,and some admixtures—into reinforced concretecan cause steel corrosion if oxygen and moistureare also available to sustain the reaction. Chlor-ides dissolved in water can permeate throughsound concrete or reach the steel through cracks.Table 1 shows the maximum permissible water-soluble chloride-ion content for prestressed andreinforced concretes in various exposure condi-tions (Ref 2).

Carbonation occurs when carbon dioxidefrom the air penetrates the concrete and reactswith hydroxides, such as calcium hydroxide, toform carbonates (Ref 3). In the reaction withcalcium hydroxide, calcium carbonate is formed.This reaction reduces the pH of the pore solutionin the concrete to as low as 8.5, a level at whichthe passive film on the steel is not stable.

Carbonation is generally a slow process. Inhigh-quality concrete, it has been estimated thatcarbonation will proceed at a rate up to 1.0 mm(0.04 in.) per year. The amount of carbonation isincreased significantly in concrete with a highwater-to-cement ratio, low cement content, shortcuring period, low strength, and a highlypermeable or porous paste.

Carbonation of concrete lowers the amount ofchloride ions needed to promote corrosion. Innew concrete with a pH of 12 to 13, about 7000 to8000 ppm of chlorides are required to start cor-rosion of embedded steel. If, however, the pH islowered to a range of 10 to 11, the chloridethreshold for corrosion is significantly lower—ator below 100 ppm (Ref 4). Like chloride ions,however, carbonation destroys the passive filmon the reinforcement but does not influence therate of corrosion.

Galvanic corrosion can occur when twometals are in contact within concrete becauseeach metal has a unique electrochemical

potential. This situation can occur in balconieswhere embedded aluminum railings are in con-tact with the reinforcing steel (Ref 5). Followingis a list of metals in order of their electrochemicalactivity in concrete:

Active or anodic

ZincAluminumSteelIronNickel (active)TinLeadBrassCopperBronzeStainless steel (passive)Gold

Noble or cathodic

When two metals are in contact in an activeelectrolyte, the more active metal corrodes.

Freeze-Thaw Deterioration. When waterfreezes, it expands about 9%. As the water inmoist concrete freezes, it produces pressure inthe capillaries and pores of the concrete. If thepressure exceeds the tensile strength of the con-crete, the cavity will dilate and rupture. Theaccumulative effect of successive freeze-thawcycles and disruption of paste and aggregate caneventually cause expansion and cracking, scal-ing, and crumbling of the concrete (Fig. 3).

Air Entrainment. The severity of freeze-thawexposure varies with climate. The resistance ofconcrete to freezing and thawing in a moistcondition is improved significantly by the use ofintentionally entrained air (Ref 6). The tinyentrained air voids act as empty chambers in thepaste for the migrating water to enter, thusrelieving the pressure in the capillaries and poresand preventing damage to the concrete. Concretewith a low permeability (that is, a low water-cement ratio and high compressive strength) isbetter able to resist freeze-thaw cycles. Recom-mended concrete air-content requirements,water-cement ratios, and compressive strengthsfor various exposure conditions are given inRef 2 and 7.

Deicing chemicals used for snow and iceremoval, such as sodium chloride, can aggra-vate freeze-thaw deterioration. In addition,

because salt absorbs moisture, it keeps the con-crete more saturated, increasing the potential forfreeze-thaw deterioration. However, properlydesigned and placed air-entrained concrete canwithstand deicers for many years. Deicers con-taining ammonium nitrate and ammonium sul-fate should be prohibited because they rapidlyattack and disintegrate concrete.

Freezing Temperatures. Concrete gains verylittle strength at low temperatures (Ref 8), sofreshly placed concrete must be protected againstfreezing until the degree of saturation of theconcrete has been reduced sufficiently by cementhydration. The time at which this reduction isaccomplished corresponds roughly to the timerequired for the concrete to attain a com-pressive strength of 3.5 MPa (0.5 ksi) (Ref 9).Concrete that is to be exposed to deicers shouldattain a strength of 28 MPa (4 ksi) prior torepeated cycles of freezing and thawing (Ref 10).

Chemical Attack. Concrete performs wellwhen exposed to various atmospheric condi-tions, water, soil, and many chemicals. However,chemical environments that degrade even high-quality concrete are given in Table 2. Concrete israrely attacked by solid, dry chemicals. To pro-duce significant attack, aggressive chemicalsmust be in solution and have a certain minimumconcentration (Ref 11).

Acids. In general, portland cement concretedoes not have good resistance to acids. In fact, nohydraulic cement concrete, regardless of itscomposition, will hold up for long if exposed to asolution with a pH of 3.0 or lower. However,some weak acids can be tolerated, particularly ifthe exposure is occasional. Acid rain, whichoften has a pH of 4 to 4.5, can etch concreteslightly, usually without affecting the perfor-mance of the exposed surface.

Acids react with the calcium hydroxide of thehydrated portland cement. In most cases, thechemical reaction forms water-soluble calciumcompounds, which are then leached away byaqueous solutions (Ref 12). To minimize orprevent deterioration from acid attack, surfaceprotective treatments are available (Ref 11, 12).Properly cured concretes with reduced perme-ability experience a slightly lower rate of attackfrom acids.

Salts and alkalis also can be a problem. Thechlorides and nitrates of ammonium, magne-sium, aluminum, and iron cause concrete dete-rioration, those of ammonium producing themost damage. Most ammonium salts aredestructive because, in the alkaline environmentof concrete, they release ammonia gas andhydrogen ions. These are replaced by dissolvingcalcium hydroxide from the concrete. The resultis a leaching action, much like acid attack. Strongalkalis (over 20%) also can cause concretedegradation (Ref 13).

Sulfates can attack concrete by reactingwith hydrated compounds in the hardenedcement. Naturally occurring sulfates of sodium,potassium, calcium, or magnesium are some-times found in soil or dissolved in groundwater.These reactions can induce sufficient pressure to

Table 1 Maximum chloride ion (Cl )content of concrete

Type of member

Maximum water-solublechloride ion (Cl�) content in

concrete, % by mass ofcement

Prestressed concrete 0.06Reinforced concrete exposed to

chloride in service0.15

Reinforced concrete that will bedry or protected frommoisture in service

1.00

Other reinforced concreteconstruction

0.30

Source: Ref 2 Fig. 3 Freeze-thaw cycles can cause scaling of con-crete surfaces as shown on this pavement.

580 / Environmental Performance of Nonmetallic Materials



disrupt the cement paste, resulting in loss ofcohesion and strength.

Environmental conditions have a great influ-ence on sulfate attack. Seawater also containssulfates, but seawater is not as severe an expo-sure as sulfates in groundwater (Ref 14, 15). Aridareas have a particular problem. Sulfate attack isgreater in concrete exposed to wet/dry cycling.When water evaporates from concrete, sulfatescan accumulate at the surface, increasing thesulfate’s concentration and its potential forcausing damage.

Resistance to sulfates is best achieved byusing a low water-to-cement ratio and cementwith a limited amount of tricalcium aluminates(Ref 16). ASTM C 150 type II cement containsless than 8% C3A and type V contains less than5% (Ref 17). ASTM C 1157 type MS cement(moderate sulfate resistant) and type HS cement(high sulfate resistant) can also be used (Ref 18),as well as moderate sulfate-resistant cementsmeeting ASTM C 595 (Ref 19).

Studies have shown that some pozzolans andground-granulated blast-furnace slags increasethe life expectancy of concrete exposed to sul-fates (Ref 20, 21). Good results have beenobtained with fly ash meeting the requirementsof ASTM C 618 Class F (Ref 1). Slags shouldconform to ASTM C 989 (Ref 22). However,some pozzolans, especially some Class C flyashes, decrease sulfate resistance. Therefore,pozzolans chosen to improve sulfate resistanceshould be tested to confirm their behavior.Reference 23 provides the requirements forconcrete exposed to sulfates.

Alkali-Aggregate Reaction (AAR). In mostconcrete, aggregates are chemically inert. How-ever, some aggregates react with the alkalihydroxides in concrete, causing expansion andcracking over a period of many years. This AARhas two forms: alkali-silica reaction (ASR) andalkali-carbonate reaction (ACR).

Alkali-silica reaction is of more concernbecause aggregates containing reactive silicamaterials are more common. In ASR, aggregatescontaining certain forms of silica will react withalkali hydroxide in concrete to form a gel thatswells as it draws water from the surroundingcement paste or the environment. In absorbingwater, these gels can swell and induce enoughexpansive pressure to damage concrete.

Typical indicators of ASR are random mapcracking and, in advanced cases, closed jointsand attendant spalled concrete (Fig. 4). Crackingdue to ASR usually appears in areas with a fre-quent supply of moisture, such as close to thewaterline in piers, near the ground behindretaining walls, near joints and free edges inpavements, or in piers or columns subject towicking action.

Alkali-silica reaction can be controlled usingcertain mineral admixtures (Ref 24). Silica fume,fly ash, and ground-granulated blast-furnace slaghave reduced ASR significantly. Class F flyashes have reduced reactivity expansion up to70%, or more, in some cases. In addition, lithiumcompounds have been shown to effectivelyreduce ASR (Ref 25). Although potentiallyreactive aggregates exist throughout North

America, ASR distress in concrete is not thatcommon because of the measures taken to con-trol it. It is also important to note that not all ASRgel reactions produce destructive swelling.

Alkali-carbonate reactions are observed withcertain dolomitic rocks. Dedolomitization, thebreaking down of dolomite, is normally asso-ciated with expansion. This reaction and sub-sequent crystallization of brucite may causeconsiderable expansion. The deteriorationcaused by ACR is similar to that caused by ASR;however, ACR is relatively rare because aggre-gates susceptible to this phenomenon are lesscommon and are usually unsuitable for use inconcrete for other reasons.

Abrasion, Erosion, and Cavitation. Abra-sion damage occurs when the surface of concreteis unable to resist wear caused by rubbing andfriction. As the outer paste of a concrete surfacewears, the fine and coarse aggregate are exposed;abrasion coupled with impact will cause addi-tional degradation that is related to the aggre-gate-to-paste bond strength and hardness of theaggregate. The two most damaging forms ofabrasion occur on vehicular traffic surfaces andin hydraulic structures, such as dams, spillways,and tunnels.

Abrasion of floors, pavements, and othertraffic surfaces are the result of productionoperations or vehicular traffic. Many industrialfloors are subjected to abrasion by steel or otherhard-wheeled traffic, which can cause significantrutting and joint damage. Tire chains and studdedsnow tires can cause considerable rutting onconcrete pavements. Compressive strength is themost important factor controlling the abrasionresistance of concrete, and hard aggregate ismore wear resistant than soft aggregate. Inaddition, a hard-steel-troweled surface resistsabrasion better than a surface that has not beentroweled resists. Figure 5 shows results of abra-sion tests on concretes of different compressive

Table 2 Chemicals that deteriorateconcrete

Promote rapid deterioration of concrete

Aluminum chlorideCalcium bisulfiteHydrochloric acid (all concentrations)(a)Hydrofluoric acid (all concentrations)Nitric acid (all concentrations)Sulfuric acid, 10%–80%(a)Sulfurous acid

Promote moderate deterioration of concrete

Aluminum sulfate(a)Ammonium bisulfateAmmonium nitrateAmmonium sulfate(a)Ammonium sulfideAmmonium sulfiteAmmonium superphosphateAmmonium thiosulfateCastor oilCocoa bean oil(a)Cocoa butter(a)Coconut oil(a)Cottonseed oil(a)Fish liquor(b)Mustard oil(a)Perchloric acid, 10%Potassium dichromatePotassium hydroxide (420%)Rapeseed oil(a)Slaughterhouse waste(c)Sodium bisulfateSodium bisulfiteSodium hydroxide (420%)Sulfite liquorSulfuric acid, 80% oleum(a)Tanning liquor (if acid)Zinc refining solutions(d)

(a) Sometimes used in food processing or as food or beverage ingredient.Ask for advisory opinion of Food and Drug Administration regardingcoatings for use with food ingredients. (b) Contains carbonic acid, fishoils, hydrogen sulfide, methylamine, brine, and other active materials.(c) May contain various mixtures of blood, fats and oils, bile and otherdigestive juices, partially digested vegetable matter, urine, and manure,with varying amounts of water. (d) Usually contains zinc sulfate in sul-furic acid. Sulfuric acid concentration may be low (about 6% in “lowcurrent density” process) or higher (about 22 to 28% in “high currentdensity” process)

Fig. 4 Typical indicators of alkali-silica reactivity aremap cracking and, in advanced cases, closed

joints and attendant spalled concrete.

0

2

4

6

8

10

20 30 40 50 60 70

3 4 5 6 7 8 9 10

Abr

asio

n-er

osio

n lo

ss, m

ass%

Compressive strength, MPa

Compressive strength, ksi

Limestone

Traprock

Chert

Quartzite

Fig. 5 Effect of compressive strength and aggregatetype on the abrasion resistance of concrete.

High-strength concrete made with a hard aggregate ishighly resistant to abrasion (Ref 26, 27).

Environmental Performance of Concrete / 581



strengths and aggregate types (Ref 26, 27). Theservice life of warehouse floors subjected tohard-wheeled traffic abrasion is increasedgreatly by the use of special hard or toughaggregates used in toppings or dry-shakes(Ref 28).

Erosion damage in hydraulic structures iscaused by the abrasive effects of water-borne silt,sand, gravel, rocks, ice, and other debrisimpinging on the concrete surface. Althoughhigh-quality concrete can resist high-watervelocities for many years with little or nodamage, the concrete may not withstand theabrasive action of debris grinding or repeatedlyimpacting on its surface.

In such cases, abrasion erosion ranging froma few millimeters (inches) to several meters(feet) has resulted, depending on flow con-ditions. Dam spillway aprons, stilling basins,sluiceways, drainage conduits or culverts, andtunnel linings are particularly susceptible toabrasion erosion. As is the case with traffic wear,erosion damage in hydraulic structures can bereduced by using strong concrete with hardaggregates.

Cavitation is damage caused by the formationand collapse of vapor bubbles in liquid. Inhydraulic structures, bubbles form where thelocal pressure drops, causing the water tovaporize at the prevailing fluid temperature. Thevapor cavities collapse, causing very highinstantaneous pressures that impact on concretesurfaces, causing pitting, noise, and vibration.

Once cavitation damage has altered water flowsubstantially, other degradation mechanismscome into play. Fatigue due to vibration, rushingwater striking irregular surfaces, and mechanicalfailure due to vibrating reinforcing steel cancause significant concrete damage. Althoughproper materials selection can increase theresistance of concrete to cavitation, the solutionis to design hydraulic structures with flow pat-terns that reduce or eliminate cavitation.

Fire and Heat. Concrete performs excep-tionally well at the temperatures encountered inmost applications. Nonetheless, when exposed tofire or unusually high temperatures, concrete canlose strength and stiffness. The effect of hightemperatures on the compressive strength, flex-ural strength, and modulus of elasticity of curedconcrete has been determined by various inves-tigators (Ref 29). Modulus of elasticity is themost sensitive to elevated temperature, followedby flexural strength and compressive strength.Many factors influence the performance of con-crete at elevated temperatures. Numerous studies(Ref 30–34) have found these general trends:

� Concrete that undergoes thermal cycling suf-fers greater loss of strength than concrete thatis held at a constant temperature, althoughmuch of the strength loss occurs in the firstfew cycles.

� Concrete that is under design load whileheated loses less strength than unloaded con-crete loses. The reason: imposed compressivestresses inhibit development of cracks that

would be free to develop in unrestrainedconcrete.

� Concrete that is allowed to cool before testingloses more compressive strength than con-crete that is tested hot.

� Concrete containing limestone and calcareousaggregates performs better at high tempera-tures than concrete containing siliceousaggregates.

� Strength loss is not proportional to compres-sive strength of concrete.

� Concrete with a higher aggregate-cementratio suffers less reduction in compressivestrength; however, the opposite is true formodulus of elasticity.

� If residual water in the concrete is not allowedto evaporate, compressive strength is greatlyreduced. If heated too quickly, concrete canspall as the moisture tries to escape.

Restraint to Volume Changes. Concretechanges slightly in volume because of fluctua-tions in moisture content and temperature of theconcrete. Restraint to volume changes, espe-cially contraction, can cause cracking if thetensile stresses that develop exceed the tensilestrength of the concrete.

Plastic shrinkage cracking can occur whenwater evaporates from the surface of freshlyplaced concrete faster than it is replaced by waterbleeding to the surface. Because of the restraintprovided by the concrete below the drying sur-face layer, tensile stresses develop in the weak,stiffening plastic concrete, resulting in shallowcracks of varying length and depth. Plasticshrinkage cracking can be curtailed by takingmeasures to prevent rapid water loss from theconcrete surface; such measures include, forexample, using fog nozzles, plastic sheeting,windbreaks, sunshades, or placing concrete atnight when it is cooler with no sun.

Drying shrinkage cracking occurs becausealmost all concrete is mixed with more waterthan is needed to hydrate the cement. Much ofthe excess water evaporates, causing the concreteto shrink. Restraint to this shrinkage, provided bythe subgrade, reinforcement, or other parts of thestructure, causes tensile stresses to develop thatmay exceed the tensile strength of the hardenedconcrete. Restraint to drying shrinkage is themost common cause of concrete cracking(Fig. 6). Since drying shrinkage cracking isalmost inevitable, control joints are placed inconcrete to predetermine their location and toconceal any cracks. Drying shrinkage can belimited by minimizing the water content ofconcrete and maximizing the coarse aggregatecontent.

Thermal cracking might occur because con-crete expands when heated and contracts whencooled. An average value for the coefficient ofthermal expansion of concrete is 10 · 10�6/�C(5.5 · 10�6/�F). This amounts to a length changeof about 5 mm in 10 m (2/3 in. in 100 ft) ofconcrete when concrete is subjected to a rise orfall of 50 �C (90 �F). Thermal expansion andcontraction of concrete varies with factors such

as aggregate type, cement content, water-cementratio, temperature range, concrete age, andrelative humidity. Of these factors, aggregatetype has the greatest influence (Ref 35). Tominimize the effects of temperature variations,designers should allow for thermal movement byproviding proper expansion or isolation jointsand correct detailing.

Overload and Impact. Properly designedand constructed concrete members are usuallystrong enough to support the loads for which theyare intended. Nevertheless, overloading canoccur for a variety of reasons: a change in use of astructure without proper structural upgrade;unintentional overloading; and other unusualcircumstances, such as earthquakes beyondspecified design (a classic example of over-loading of concrete structures).

Overload damage can occur during con-struction when concrete has not yet reacheddesign strength. Early removal of formwork orthe storage of heavy materials or operation ofequipment on and around the structure can resultin the overloading of certain concrete elements.A common error occurs when precast concretemembers are not properly supported duringtransport and erection. Errors in post-tensionedconstruction, such as improperly timed orsequenced strand release, can also cause over-load cracking.

Impact damage is another form of overload.A common form of impact damage occurs at slabedges of joints in industrial floors (Ref 28). Evenin properly designed reinforced concrete, load-induced tensile stresses can occur. This point isreadily acknowledged and accepted in concretedesign. Current design procedures use reinfor-cing steel to not only carry tensile loads but toobtain both an adequate distribution of cracksand a reasonable limit on crack width.

Loss of support beneath concrete structuresusually is caused by settlement or the washingout of soils and subbase support materials. Thisloss can cause a variety of problems in concretestructures, from cracking and performance pro-blems to structural failure. Loss of support canalso occur during seismic events. During con-struction, inadequate formwork support or pre-mature removal of forms cause loss of support.

Fig. 6 Restraint to drying shrinkage is the most com-mon cause of concrete cracking.




Curling is a common problem related to lossof support in floor slabs (Ref 28). Curling is therise of the edges and corners of a slab due todifferences in the moisture content or tempera-ture between the top and bottom of a slab. Thetop dries out or cools and contracts more than thewetter, warmer bottom. Curling results in a lossof contact between the slab and its subbase andcan lead to cracking, slab deflection, joint dete-rioration, and problems with vehicular traffic.

Surface defects can occur on the surfaces offormed concrete (for example, walls and col-umns) or finished concrete (for example, floorsand pavements). Many of these defects areavoidable by using proper materials and con-struction practices, while others are difficult toeliminate.

Air voids in formed surfaces, also known asbug holes, are small cavities that form in thevertical surface during placing and consolidationof formed concrete (Fig. 7). They can be up to25 mm (1 in.) wide but are usually no more than15 mm (9/16 in.) wide. These defects are morelikely to occur when sticky or stiff concretemixes of low workability are used. Such mixesmay have an excessive amount of fine aggregate,entrapped air, or both. Improper use of vibratorsand form-release agents also may contribute tothe bug hole problem.

Honeycomb in formed surfaces occurs whenmortar fails to fill all the spaces between coarseaggregate particles in concrete. Congested rein-forcement, segregation, and insufficient fineaggregate content can contribute to the problem.Higher concrete slumps and proper vibration willassist in preventing honeycomb by increasing theflowability of the concrete.

Cold joints in formed surfaces are dis-continuities in concrete members resulting froman excessive delay between placements of twosuccessive lifts of concrete. In other words,visible lines in the surface indicate the presenceof a joint where one layer of concrete had har-dened before subsequent concrete was placed.Aside from their appearance, cold joints can be aconcern if they allow moisture penetration or ifthe loss of tensile strength of the concrete acrossthe joint is deemed detrimental to the perfor-mance of the structure.

Delamination in finished surfaces (slabs)occurs when air and bleed water become trappedunder a prematurely finished (densified) mortarsurface. The trapped air and bleed water separatethe uppermost 3 to 6 mm (1/8 to 1/4 in.) layer ofmortar from the underlying concrete. Delami-nation is very difficult to detect during finishingand becomes apparent after the concrete surfacehas dried and the delaminated area crushes outunder traffic.

Blisters are a smaller and more noticeableform of delamination; they form at the con-crete surface due to trapped air and bleed water.The primary cause of delamination: finishing theslab surface before bleeding is complete.

Dusting of finished slab surfaces is thedevelopment of a fine, powdery material thateasily rubs off the surface of hardened concrete.It is the result of a thin, weak surface layer, calledlaitance, which is composed of water, cement,and fine particles. The finishing operations offloating and troweling with bleed water on thesurface are the usual causes of dusting. Othercauses include using a too-wet mix, spreadingdry cement over the surface to accelerate fin-ishing, and allowing rapid drying of the surface.

Popouts in finished surfaces are fragments ofaggregates that break out of the surface of con-crete, leaving a hole usually 6 to 50 mm (1/4 to2 in.) in diameter (Fig. 8). The cause of a popoutusually is a piece of porous rock having a highrate of absorption and relatively low specificgravity. As the offending aggregate absorbsmoisture or freezes under moist conditions, itswells, creating internal pressures sufficient torupture the concrete surface. Pyrite, hard-burneddolomite, coal, shale, soft fine-grained lime-stone, or chert commonly cause popouts.

Subsidence cracks in finished surfaces maydevelop over embedded items, such as reinfor-cing steel, as the concrete settles or subsides(Fig. 9). Subsidence cracking results frominsufficient consolidation (vibration), highslumps (overly wet concrete), or a lack of ade-quate concrete cover over embedded items.

Crazing in finished surfaces is a maplikepattern of fine cracks that do not penetrate muchbelow the surface and are usually of cosmeticconcern only. They are barely visible, exceptwhen the concrete is drying after the surface hasbeen wet. Preventing excessive evaporationduring placement and proper curing can preventcrazing. Reference 36 provides additionalinformation about surface defects in slabs.

Addressing Durability with thePrescriptive Approach

Durability of concrete can be addressed bytwo approaches. The first is known as the pre-scriptive approach, where designers specifymaterials, proportions, and construction methodsbased on fundamental principles and practicesthat exhibit satisfactory performance. The sec-ond is called the performance approach, wheredesigners identify functional requirements suchas strength, durability, and volume changesand rely on concrete producers and contractorsto develop concrete mixtures to meet thoserequirements. The prescriptive approach is basedon what one might call “old technologies”because these requirements have been known formany years. Following are some prescriptiveprinciples and practices that improve resistanceto degradation (Ref 11):

� Water-cement ratio (w/c), or the water-cementitious materials ratio (where applic-able), should not exceed 0.45 by weight (0.40for corrosion protection of embedded metal inreinforced concrete). For severe conditions,w/c often range from 0.25 to 0.35.

� Cement content should be at least 335 kg/m3

(564 lb/yd3) of cementitious material forconcrete exposed to corrosive environments.

� Cement type should be suited to the exposure.There are eight types of portland cementspecified by ASTM (Ref 17), and there areother blended, modified, and special purposecements (Ref 18, 19, 37–43). Some, such assulfate-resistant cement, are formulated toprevent specific attack (Ref 11). Sulfate-resistant cements, like other portland or

Fig. 7 Surface air voids, called bug holes, are smallcavities of entrapped air bubbles in the surface

of formed concrete.Fig. 8 Some aggregates absorb water and, upon

freezing, expand to produce a popout.Fig. 9 Subsidence cracks can develop over reinforcing

steel as the concrete settles or subsides.




blended hydraulic cements, are not resistant tomost acids or other highly corrosive sub-stances.

� Aggregates that are not prone to freeze-thawdeterioration, chemical attack, or AAR shouldbe used. References 11 and 24 offer guidanceon susceptibility to AAR.

� Mixing water should not contain impuritiesthat can impair basic concrete propertiesor reduce resistance to chemical attack(Ref 44, 45). Water that is safe to drink is safefor use in concrete.

� Air entrainment in the proper amount—dependent on exposure conditions and max-imum aggregate size—should be used (Ref 2).Air entrainment makes concrete resistant todeicers and scaling due to freezing andthawing; it also improves sulfate resistance,watertightness, and workability.

� Mixing must be thorough and should continueuntil the concrete is uniform in appearancewith all materials evenly distributed. Mixturescontaining silica fume may require a longermixing period to thoroughly distribute theadmixture.

� Workability requires the avoidance of mixesthat are so harsh and stiff that honeycombingmay occur and mixes so fluid that excess bleedwater rises to the surface. Slump (a measure ofthe consistency of freshly mixed concrete)should generally be 50 to 125 mm (2 to 5 in.).If necessary, water reducers and super-plasticizers can be used to make mixes moreworkable (higher slump).

� Finishing of slabs should not begin until allbleed water has left the surface. Supplemen-tary cementitious materials, such as fly ash,slag, silica fume, or blended cements, mayaffect the bleeding characteristics of concrete.For instance, silica fume mixtures bleed verylittle and slag mixtures may bleed longer dueto the retarding effect slag has on setting time.Placing concrete at the proper temperaturehelps control finishing operations.

� Jointing, the proper use of isolation, contrac-tion, and construction joints, helps controlcracking. Contraction joints in slabs onground should be spaced about 30 times theslab thickness. In some cases, joints mustbe sealed with a sealant capable of enduringthe environment. Water stops, if used, must beplaced properly. Special construction meth-ods, such as the use of heavily reinforced slabsor post-tensioned slabs, are helpful in redu-cing the number of joints in areas where jointsare undesirable.

� When curing, either additional moistureshould be supplied to the concrete during theearly hardening period or the concrete shouldbe covered with a water-retaining material.Curing compounds may be used but not onsurfaces that are to receive protective surfacetreatments. Concrete should be kept moist andabove 10 �C (50 �F) for the first week or untilthe desired strength is achieved. Longer cur-ing periods increase concrete’s resistance tocorrosive substances.

� Chemical admixtures, such as water reducers(Ref 46) and superplasticizers (Ref 47), canbe used to reduce the water-cementratio, resulting in reduced permeability andless absorption of corrosive materials intoconcrete. Polymer admixtures, such as styr-ene-butadiene latex, greatly reduce the per-meability of concrete to many corrosivesubstances. Admixtures containing chlorideshould not be used for reinforced or pre-stressed concrete, although corrosion inhibi-tors are available to reduce chloride-inducedsteel corrosion. Shrinkage-reducing admix-tures can reduce the formation of shrinkagecracks through which aggressive materialspenetrate concrete.

� Supplementary cementitious materials, suchas fly ash and metakaolin (Ref 1), slag(Ref 22), and silica fume (Ref 48), reducepermeability and by-produce additionalcementitious compounds that increasestrength. Dosages for these materials byweight of cementitious material range from15 to 40% for fly ash, 35 to 50% for slag, and50% for silica fume.

Addressing Durability with thePerformance Approach

During the last several decades, advances inconcrete technology have widened the appli-cation and use of this material. New devel-opments in admixture technology includehigh-performance concrete (HPC) (Ref 49–51),self-consolidating concrete (SCC) (Ref 52),high-strength concrete (HSC) (Ref 53–57), fiber-reinforced concrete (FRC) (Ref 58–61), andnumerous special types of concrete (Ref 7). Inorder to ensure adequate durability, perform-ance-based specifications are used. Designersspecify the functional requirements such asstrength, density, permeability, and volumestability depending on project requirements. Abrief discussion of these innovative technologiesfollows.

High-performance concrete exceeds theproperties and constructability of normal con-crete. Special ingredients, mixing, placing, andcuring practices may be needed to produce andhandle HPC. Extensive performance testsusually are required to demonstrate compliancewith specific project needs.

High-performance concrete characteristics aredeveloped for particular applications and envi-ronments, such as in tunnels, bridges, roads,streets, and tall structures (Fig. 10). Propertiesthat may be required include:

� High strength� High early strength� High modulus of elasticity� High abrasion resistance� High durability and long life in severe envir-

onments� Low permeability and diffusion

� Self consolidation� Resistance to chemical attack� High resistance to frost and deicer scaling

damage� Toughness and impact resistance� Volume stability

Typically, HPC concretes have a low w/c(0.20–0.45). Plasticizers are used to make theseconcretes fluid and workable. High-performanceconcrete usually has a higher strength than nor-mal concrete; however, strength is not always theprimary required property. A normal strengthconcrete with very high durability and very lowpermeability is considered to have high-perfor-mance properties. It has been demonstrated that40 MPa (6 ksi) HPC for bridges could be eco-nomically made while meeting durability factorsfor air-void system and resistance to chloridepenetration (Ref 62).

Table 3 lists materials often used in high-performance concrete and why they are selected.High-performance concrete specifications ide-ally should be performance oriented. Unfortu-nately, many specifications are a combination ofperformance requirements, such as permeabilityor strength limits, and prescriptive requirements,such as air-content limits or dosage of supple-mentary cementitious material (Ref 63).

Self-Consolidating Concrete. The con-struction industry has always longed for a high-performance concrete that can flow into tightspaces without requiring vibration. The need forthis technology has grown over the years as

Fig. 10 The Two Union Square building in Seattle,WA, used concrete with a design compressive

strength of 131 MPa (19 ksi) in its steel tube and concretecomposite columns.




designers specify more heavily reinforced con-crete members and ever more complex form-work. Until recently, the industry usedsuperplasticizing admixtures in conventionalmixes in an attempt to duplicate the advantagesof a true self-consolidating concrete. Thisallowed the use of 200 mm (8 in.) slump con-crete, or more, but some vibration was stillrequired for adequate consolidation. While highdoses of superplasticizer create a very fluidconcrete that flows readily, the mixes oftensegregate because the mortar is too thin to sup-port the weight of the coarse aggregate. The keyto creating SCC is to produce a very flowablemortar that still has a viscosity high enough tosupport the coarse aggregate. Today, newadvances in admixtures and mix proportioningare making SCC a reality. Developed in Japanduring the 1980s, this technology is now gainingconsiderable attention in Europe and NorthAmerica (Ref 52).

High-Strength Concrete. The definition ofHSC has changed over the years as strengthshave increased. Most HSC applications aredesigned for compressive strengths of 70 MPa(10 ksi) or greater. To get these strengths, strin-gent application of the best practices is required.Compliance with strict guidelines and com-mendations for preconstruction laboratory andfield-testing is essential (Ref 64).

Traditionally, the specified strength of con-crete has been based on 28 day test results.However, in high-rise buildings, where HSC iscommonly used, the process of construction issuch that the structural elements in lower floorsare not fully loaded for periods of a year or more.For this reason, compressive strengths based on56 or 91 day test results are commonly specified;this achieves significant economy in materialcosts. The time dependence of compressivestrength is seen in Fig. 11. When later-agestrengths are specified, supplementary cementi-tious materials usually are incorporated into theconcrete. This produces additional benefits in theform of reduced heat generation during cementhydration.

With the use of low-slump or no-slump mix-tures, high-compressive-strength concrete hasbeen produced routinely under careful control inprecast and prestressed concrete plants for dec-ades (Ref 65). These stiff mixes are placed inrugged forms and consolidated by prolongedvibration or shock methods. However, typicalcast-in-place concrete uses more fragile formsthat do not permit the same compaction proce-dures; hence, more workable concretes withsuperplasticizers usually are necessary toachieve the required compaction and to avoidsegregation and honeycomb.

Fiber-Reinforced Concrete. Fibers havebeen used in construction materials for centuries.The last three decades have seen a growinginterest in the use of fibers in ready-mixed con-crete, precast concrete, shotcrete, plaster, andstucco. Steel, plastic, glass, and natural material(such as wood cellulose) fibers are availablein a variety of shapes, sizes, and thicknesses.They may be round in cross section or flat,crimped, and deformed with typical lengths of6 to 150 mm (0.25–6 in.) and with thicknessesranging from 0.005 to 0.75 mm (0.0002 to0.03 in.).

Fibers are added to concrete during mixing.Like any composite, factors that control theperformance of FRC are the physical propertiesof the fibers and matrix and the strength of thebond between fibers and matrix. Although thebasic principles are the same, there are severalcharacteristic differences between conventionalreinforcement and fiber systems:

� Fibers are generally distributed throughoutthe entire cross section of a member, whereasreinforcing bars or wires are placed onlywhere required.

� Most fibers are relatively short and closelyspaced as compared with reinforcing bars orwires.

� It is generally not possible with achieve thesame ratio of area of reinforcement to area ofconcrete using fibers as compared with anetwork of reinforcing bars or wires, without

reducing workability and affecting fiber dis-persion.

Fibers are typically added to concrete in low-volume dosages (often less than 1%) and havebeen shown to be effective in reducing plasticshrinkage cracking. Fibers do not significantlyalter the free shrinkage of concrete; however, athigh-enough dosages they can increase theresistance to cracking and decrease crack widths(Ref 66).

Special types of concrete are those withunusual properties or those produced by unusualtechniques. Table 4 lists many special types ofconcrete made with portland cement and somemade with binders. In many cases, the namedescribes the use, property, or condition of theconcrete. Brand names are not given here, butsome of these special concretes are identified bybrand name. Some of these special concreteswere discussed previously; following are briefdescriptions of others.

Structural lightweight concrete is similar tonormal weight concrete except that it has a lowerdensity. It may be made with all lightweightaggregates or with a combination of lightweightand normal-weight aggregates. Most of thestructural lightweight concrete used today is ofthe sanded variety made with coarse lightweightaggregate and natural sand. ASTM C 567 (Ref68) provides a test for density. This concrete isused to reduce the dead load weight of floors inhigh-rise buildings.

Insulating lightweight concrete has an ovendry density of 800 kg/m3 (50 lb/ft3) or less. It ismade with cementitious materials, water, air, andwith or without aggregate and chemical admix-tures. The oven dry density ranges from 240 to800 kg/m3 (15–50 lb/ft3) with a 28 day com-pressive strength between 0.7 and 7 MPa (0.1and 1 ksi). Cast-in-place insulating concrete isused primarily for thermal and sound insulation,roof decks, fill for slabs cast on grade, levelingcourses for floors or roofs, firewalls, and under-ground thermal conduit linings.

Table 3 Materials used in high-performance concrete

Material Primary contribution/desired property

Portland cement Cementing material/durabilityBlended cement Cementing material/durability; high strengthFly ash Cementing material/durability; high strengthSlag Cementing material/durability; high strengthSilica fume Cementing material/durability; high strengthCalcined clay Cementing material/durability; high strengthMetakaolin Cementing material/durability; high strengthCalcined shale Cementing material/durability; high strengthSuperplasticizers FlowabilityHigh-range water reducers Reduce water to cement ratioHydration control admixtures Control settingRetarders Control settingAccelerators Accelerate settingCorrosion inhibitors Control steel corrosionWater reducers Reduce cement and water contentShrinkage reducers Reduce shrinkageAlkali-silica reaction (ASR) inhibitors Control ASRPolymer/latex modifiers DurabilityOptimally graded aggregate Effective use of binder/strength and durability

60

50

40

30

20

10

007 28 91 365

8

6

4

2

0

Age at test, days

Moist-cured entire time

In air after moist curing 28 days

In air after moist curing 7 days

In laboratory air entire time

Com

pres

sive

stre

ngth

, MP

a

Com

pres

sive

stre

ngth

, ksi

Fig. 11 Concrete strength increases with age as long asmoisture and a favorable temperature are

present for hydration of the cement. The effect of moistureduring cure is evident.




Moderate-strength lightweight concrete has adensity of 800 to 1900 kg/m3 (50–120 lb/ft3)oven dry and compressive strength of approxi-mately 7 to 15 MPa (1–2.2 ksi). It is made withcementitious materials, water, air, and with orwithout aggregates and chemical admixtures. Atlower densities, it is used as fill for thermal andsound insulation of floors, walls, and roofs and isreferred to as fill concrete. At higher densities, itis used in cast-in place walls, floors and roofs,and for precast wall and floor panels. Moreinformation can be found in committee reports ofthe American Concrete Institute (Ref 69–71).

Autoclaved cellular concrete (ACC) is aspecial type of lightweight building materialalso known as autoclaved aerated concrete. Itis manufactured from a mortar consisting ofpulverized siliceous material (sand, slag, or flyash), cement, and water; to this material a gas-forming agent, such as aluminum powder, isadded. The chemical reaction of aluminum andalkaline water forms hydrogen gas, whichexpands the mortar; 0.5 to 1.5 mm (0.02–0.06 in.) diameter macropores form. After cast-ing this mixture into forms, the material ispressure steam cured (autoclaved) for a period of6 to 12 h at 190 �C (374 �F) and 1.2 MPa(0.17 ksi). After autoclaving, the hardened

mortar matrix consists of calcium silicatehydrates.

This porous mineral building material hasdensities from 300 to 1000 kg/m3 (19–63 lb/ft3)and compressive strength 2.510 MPa (0.300–0.500 ksi). Due to the high macropore content—up to 80 vol%—ACC has a thermal conductivityof only 0.15 to 0.20 W/(m . K) (1–1.4 Btu . in./[h . ft2 . �F]). Autoclaved cellular concrete ismanufactured into blocks, wall panels, roofand floor slabs, and lintels for constructionof residential and commercial buildings (Ref70–73).

High-density (heavyweight) concrete hasdensity to 6400 kg/m3 (400 lb/ft3). Heavyweightconcrete is used for radiation shielding, coun-terweights, and other applications where highdensity is important. Where space requirementsare not important, normal-weight concrete ismore economical, but where space is limited,heavyweight concrete shields against x-rays,gamma rays, and neutron radiation more effi-ciently. High-density aggregates such as barite,ferrophosphorus, goethite, hematite, ilmenite,limonite, magnetite, steel punchings, or shot areused (Ref 74, 75).

Mass concrete is defined (Ref 67) as: “anylarge volume of cast-in-place concrete withdimensions large enough to require that mea-sures be taken to cope with the generation of heatand attendant volume change to minimizecracking.” Mass concrete includes not only low-cement-content concrete used in dams and othermassive structures but also moderate- to high-cement-content concrete in large structuralmembers of bridges and buildings. Mass con-crete placements require special considerationsto reduce heat of hydration and the resultingtemperature rise to avoid damaging the concrete.Excessive temperatures and temperature differ-ences throughout a concrete placement can resultin thermal cracking (Ref 76).

Precast and Prestressed Concrete. Precastconcrete is cast in forms in a controlled envir-onment and allowed to achieve a specifiedstrength prior to placement on location. Pre-stressed concrete is concrete in which compres-sive stresses are induced by high-strength steeltendons or bars in a concrete element. Thesestresses will balance the tensile stresses that willoccur in the element during service.

Prestressing is accomplished in two ways: forpretensioning, usually done in a plant, the ten-dons are placed and tensioned before the con-crete is placed; for posttensioning, usually doneat the job site, tendons or bars are positionedbefore the concrete is placed and tensioned afterthe concrete has cured, hardened, and reached aspecified strength.

Precast and prestressed concrete offersadvantages for all types of structures (Ref 77).Precast elements are economical and of highquality; they have greater permanence thanother building materials. Colored and texturedprecast panels often are used as the skin of abuilding and can also serve as structural elements(Ref 78).

Sustainability

Sustainable development is a topic of growingimportance. Also known as “green building,”sustainable construction makes it possible to useour natural resources efficiently while stillacknowledging the desire for growth. Sustain-ability balances current and future needs. Sincethe population will continue to increase, sus-tainability will help balance the economic,social, and environmental impact of actions wetake to create the built environment.

A few facts highlight dramatically the need tomodify the way we design and construct build-ings. Research has shown that buildings in theUnited States use 40% of the nation’s materialresources and 39% of its annual energy con-sumption. Even more telling is the fact that U.S.buildings use almost three times the energy of itsEuropean counterparts in similar climates (Ref79); there is a definite imbalance.

Green building programs are becoming pop-ular; the U.S. government is adopting them, andan increasing number of states are offering taxbenefits for them. The U.S. government definesgreen buildings as those that:

� Demonstrate the efficient use of energy,water, and materials

� Limit impact on the outdoor environment� Provide a healthier indoor environment

Concrete buildings offer a number of advan-tages: They are durable and have the longestlifespan of any traditional construction material;they are energy efficient in manufacture and use;are made of locally available raw materials; theydo not rust, rot, or burn, and require less energyand resources over time to repair or replace. Formore information about the sustainable benefitsof concrete, see Ref 80–83.

REFERENCES

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Table 4 Special types of concrete

Made with portland cement

Architectural concrete Nailable concreteAutoclaved cellular concrete No-slump concreteCentrifugally cast concrete Polymer-modified concreteColloidal concrete Pervious (porous) concreteColored concrete Pozzolan concreteControlled-density fill Precast concreteCyclopean (rubble) concrete Prepacked concreteDry-packed concrete Preplaced aggregate concreteEpoxy-modified concrete Reactive-powder concreteExposed-aggregate concrete Recycled concreteFerrocement Roller-compacted concreteFiber concrete Sawdust concreteFill concrete Self-compacting concreteFlowable fill Shielding concreteFlowing concrete ShotcreteFly-ash concrete Shrinkage-compensatingGap-graded concrete Silica-fume concreteGeopolymer concrete Soil-cementHeavyweight concrete Stamped concreteHigh-early-strength concrete Structural lightweightHigh-performance concrete Superplasticized concreteHigh-strength concrete TerrazzoInsulating concrete Tremie concreteLatex-modified concrete Vacuum-treated concreteLow-density concrete Vermiculite concreteMass concrete White concreteModerate-strength lightweight Zero-slump concrete

Made without portland cement

Acrylic concrete Magnesium phosphateconcreteAluminum phosphate

concrete Methyl methacrylate (MMA)concreteAsphalt concrete

Polyester concreteCalcium aluminate concretePolymer concreteEpoxy concretePotassium silicate concreteFuran concreteSodium silicate concreteGypsum concreteSulfur concreteLatex concrete

Most of the definitions of these special concretes appear in Ref 67.




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82. “Concrete Builds the Sustainable Move-ment,” RP417, Portland Cement Associa-tion, Skokie, IL, 2003

83. “Building Green with Concrete: Points forConcrete in LEED (Leadership in Energyand Environmental Design),” IS312, Port-land Cement Association, Skokie, IL, 2003




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