Everyone likes a short cut. It’sjust plain tempting. That’swhy men once searched fora means of tra n s f o rm i n g

base metals into gold. It seemed somuch easier to make it than earn it.

In the area of concrete constru c-tion, where much depends onk n ow - h ow, patience and care f u lw o rk m a n s h i p, it was only natura lthat short cuts should be offered inthe form of admixture s. Of all themany products put on the market inthe last few decades, one has ri s e nhead and shoulders above the oth-ers in its effective n e s s. It is, ofc o u r s e, the air entraining agent.

Even this admixture is no won-d e r- w o rk e r, howe ve r. It is not in-tended to replace correct pro c e-d u res in mix design or concre t i n g .Ra t h e r, it complements these quali-ty concrete practices to produce afinished product that will be mored u rable and satisfactory to all con-c e rned. Let’s take a closer look atthis pre-eminent admixture.

What air does

The introduction of air into con-c rete mixes has some pro n o u n c e deffects on the chara c t e ristics ofboth the plastic and the hard e n e dconcrete.

In fresh concre t e, the tiny air bub-bles act as a lubricant in the mixwhich improves its workability andi n c reases its slump. Also, in a sense,the bubbles function as a third ag-g re g a t e. Because of their small size,the bubbles act as fines, there bycutting down the amount of sandneeded. Because air entra i n m e n ta f f o rds increased slump, it is possi-ble to decrease the amount of waterto get higher strengths without af-fecting work a b i l i t y. Less watermeans less drying shrinkage —al-

ways a desirable feature. Bleeding inc o n c rete is cut approximately inhalf by entrained air. This re d u c e sc o n s i d e rably the adverse effects ofa higher water/cement ratio at thes u rface of slabs and of laitancef o rming on concrete surf a c e s. Air al-so produces stickier, more cohesivec o n c rete; as a result, less segre g a-tion is experienced and more attra c-t i ve surfaces are achieve d .

In hardened concre t e, the out-standing attributes of air entra i n-ment are the enhanced we a t h e r-ability and resistance to scalinga f f o rded. Damage to concrete byf re ezing and thawing is causedwhen enough ice forms in the cap-i l l a ries to create a pre s s u re gre a t e rthan the tensile strength of the ce-ment paste. This disrupts the capil-l a ry walls. Where enough air vo i d sa re present, howe ve r, hyd ra u l i cp re s s u re created by fre ezing forc e sthe remaining water into the air

vo i d s. These air voids in air en-t rained concrete act thus as a safetyva l ve by giving the water a re s e rvo i rinto which to flow. When thawingo c c u r s, compressed air in the vo i d sf o rces the water back into the capil-l a ri e s, there by freeing the voids foruse again during the next fre eze.

Test sections of pavement in Ne wYo rk State built with and without aire n t rainment showed dra m a t i c a l l ythe value of air in exposed work. Af-ter 14 years of exposure to the ri g-ors of this nort h e rn climate and fre-quent salting, the non-air- e n t ra i n e dc o n c rete was badly scaled; in somecases 100 percent of the surface wasaffected. The air entrained pave-ment, laid next to the other sec-t i o n s, showed absolutely no scalingor D-cra c k i n g .

Du ring October 1942, 94 columnscast of concrete made with seve ra ltypes of cements and va ry i n gamounts of air entrainment we re

Air entrainment and concreteSome things every contractor should know about a combination that assureslonger-lasting and more attractive concrete

This photomicrograph of air entrained plastic concrete suggests how minutebubbles of air function as very small ball bearings which make the concretemore workable than a non-air entrained mix.

set on the exposure rack at Treat Is-land, Ma i n e. At the end of 1 ye a r(188 fre eze- thaw cycles) 10 perc e n tof the columns had failed; nonewe re air entrained concrete speci-m e n s. After 5 years of exposure (663f re eze-thaw cycles) 26 percent of thecolumns had failed; again, no air en-t rained specimens had failed.

Application of deicers to pave-ments and other flatwork hascaused great damage to concre t ethat is not air entrained. The effec-t i veness of entrained air in re d u c i n gor pre venting such damage—calledsalt scaling—is illustrated in the ac-companying photographs of labora-t o ry test specimens. These speci-mens we re cove red with water ontheir top surfaces (the surf a c e ss h own in the photos) and subjectedto repeated cycles in which theywe re frozen and subsequentlyt h a wed while deicers we re pre s e n ton the surface of the ice. Sp e c i m e n swe re rinsed with clear water beforebeginning each new cyc l e.

Sulfate resistance is also height-ened by the use of air entra i n i n ga g e n t s. Although low water- c e m e n tratios and the use of Type II or V ce-ments are of pri m a ry importance inlessening the effects of sulfate attackon concre t e, air entrainment is alsoh e l p f u l .

Time and again, in labora t o ri e sand in the field, tests have unequiv-ocally shown that air entra i n m e n tb rings about a tremendous incre a s ein the durability of concre t e.

How to entrain air

As mentioned earlier, certain al-t e rations in mix design are neededwhen air is added. The air should bec o n s i d e red as a fifth ingre d i e n twhen pro p o rtioning the mix. Thefirst matter to be considered is thep e rcentage of air desired (see table).

Tests have disclosed that im-p rovement in we a t h e rability is ex-p e rienced starting at approx i m a t e l y2 percent air. The lowest amountusually specified, howe ve r, is 3 per-cent. At the other extre m e, the max-imum air content usually called foris 6 percent. In specifications aircontent is generally stated as a give n

f i g u re plus or minus a certain per-c e n t a g e, quite commonly 11⁄2 p e r-cent. It is not feasible to hold thep e rcentage constant from batch tobatch because air content is affect-ed by so many factors (includingmaximum size of coarse aggre g a t e,amount of air entraining agent, typeand gradation of aggre g a t e, hard-ness of water, length and means of

mixing, brand of cement, and con-c rete tempera t u re ) .

Air can be entrained in concre t eby either of two methods: the use ofair entraining agents or of air en-t raining cements. The former in-clude such products as natura lwood re s i n s, animal or ve g e t a b l ef a t s, wetting agents and water solu-ble soaps of certain acids. These are

The laboratory specimen at the left with no entrained air was severelydamaged in laboratory tests of salt scaling. The one at the right, withan adequate amount of air, was undamaged.

If nominal maximumcoarse aggregate sizea is... Specify...

3⁄8 inch (10 millimetres) 8 ± 2 percent air1⁄2 inch (13 millimetres) 7 ± 2 percent air3⁄4 inch (19 millimetres) 6 ± 2 percent air

1 inch (25 millimetres) 5 ± 1.5 percent air

11⁄2 inches (38 millimetres) 4.5 ± 1.5 percent air

2 inches (51 millimetres) 4 ± 1.5 percent air

3 inches (76 millimetres) 3 ± 1.5 percent air

a Normal weight aggregate concretes only. For lightweight aggregateconcretes specify 7.5 ± 1.5 percent air for resistance to freezing andthawing; for workability, specify 5.5 ± 1.5 percent for lightweight ag-gregate concretes in the 3000- to 4500-psi (21- to 31-megapascal)strength range and 4.5 ± 1.5 percent in the strength range above4500 psi (31 megapascals).

i n t roduced to the concrete when itis batched. Air entraining cementsa re those made by interg rinding anair entraining agent with the ce-ment clinker during manufacture.

The air entraining component, byw h a t e ver method introduced, low-ers surface tension of the water andg e n e rates a stable foam that re s u l t sin formation of microscopic airvo i d s. Labora t o ry calculations indi-cate that a cubic yard of concre t ewith an air content in the range of 3to 6 percent contains from 400 to600 billion air bubbles.

Choosing the amount and meansby which air is to be added to themix depends on a great number ofva ri a b l e s. (In some areas the natu-rally occurring fine aggregate con-tains organisms which add 4 to 5p e rcent accidental air to the mix.) Ing e n e ral, it is easiest and most pro f-itable to consult the supplier of theready mixed concre t e. He is familiarwith aggre g a t e s, cements and otherp e rtinent factors in the locality andwill be able to deliver an air en-t rained concrete that is consistentand best suited to the job at handand there by save some headachesfor the contra c t o r.

Working with it

As can be judged from a listing ofp rofound effects produced by theinclusion of air in a mix, it is neces-s a ry to modify somewhat the man-ner in which the resulting concre t eis handled. Co n t ractors who areunfamiliar with these differe n c e ssometimes blame poor perf o r-mance on the use of air. In re a l i t y,air entrainment will produce sig-nificantly better looking, longerlasting concrete when it is handledc o r re c t l y.

When the mixer truck pulls inw i th a load of air entrained concre t eit is especially important that formsbe ready to re c e i ve it. Althoughopinions va ry somewhat, it is gen-e rally conceded that the amount ofintentionally entrained air incre a s-es noticeably with mixing duri n gthe first 5 minutes. It then re m a i n sre l a t i vely constant for about 5 ad-ditional minutes, after which timethe air content decreases gradually.

After 40 to 60 minutes of mixing thep e rcentage of air will be aboutequal to that at 1 minute of mixing.It is apparent, therefore, that a con-t ractor must be ready to re c e i veready mixed concrete when it is de-livered if he wishes to ensure a con-stant percentage of air. Since hight e m p e ra t u res tend to dispel airf rom the mix, prolonged mixingd u ring the summer months wouldalso tend to cause va riations in aircontent. The type of mixer usedcauses differences in the amount ofair entrained; but with ready mixedconcrete this problem is avoided bythe close control afforded by tru c kmixers.

Since air entrained concrete ism o re cohesive and workable thannon-air entrained, there is not asmuch danger of segregation and airp o c k e t s. Ac c o rd i n g l y, vibration neednot be prolonged. Vi b ration timewith air entrained mixes should bes h o rtened and kept reasonably con-stant since it tends to dispel the air.A normal amount of vibra t i o n(about 1⁄2 minute) reduces the initialair content about 10 percent. Whenv i b rated 1 minute, air content isl owe red 15 to 20 percent. In addi-tion, internal vibration reduces airm o re than does external vibra t i o n .Slump should also be watched sinceair content increases as the slumpi n c re a s e s.

As to appeara n c e, air entra i n e dc o n c rete has an unusually cre a m y,fatty look. This, coupled with thev i rtual absence of bleeding, some-times results in incorrect handlingof the concre t e. In stru c t u ral work ,difficulties are not usually experi-enced because the mix easily as-sumes even intricate shapes. It alsof l ows easily around closely spacedre i n f o rcing bars. In slabs re q u i ri n gfinishing, howe ve r, a change oftechnique is re q u i re d .

When working with non-air- e n-t rained concre t e, it is customary towait until the water at the surface ofthe slab (caused by bleeding) hase va p o rated before finishing opera-tions are begun. This classic ap-

p roach can cause troubles if appliedto air entrained concrete slabw o rkb e c a u s e, as previously stated, aire n t rained concrete undergoes littlebleeding. Often workers have wait-ed for bleeding to appear until thec o n c rete has set up to an extent thatmakes finishing difficult or impossi-b l e. Ge n e rally speaking, finishingshould always commence soonerwith air entrained concre t e. Earlyfinishing is even more import a n twhen the air tempera t u re is high,humidity is low or a wind is blow i n g .Finishing need not be carried on aslong as with ord i n a ry concre t e. Allthis means that air entrained con-c rete can be protected and curi n gcan start earlier than with ord i n a ry

Another dramatic illustration of howconcrete pavements are protected byair entrainment. The pavement shownhere is at a busy street intersectionsubjected to heavy and frequentapplications of salt. The concrete inthe unscaled portion to the left of thejoint is identical to the heavily scaledconcrete to the right except for theinclusion of entrained air.

+ Numbers in parentheses are references to themetric equivalents listed with this article.

c o n c re t e.Sometimes there is a tendency for

air entrained concrete pave m e n ts u rfaces to tear when using me-chanical screeds or finishing ma-c h i n e s. This can be corrected bys h o rtening the amount of forw a rdmotion per tra n s verse stroke or bymaintaining a 21⁄2 to 3-inch ( 1 )+s l u m p. In addition, a longitudinalfloat as well as a tra n s verse scre e dshould be used. Tra n s verse finishingmachine screeds should be adjustedfor a lower tilt than is generally usedwith non-air- e n t rained concre t e.Usually 1⁄8- i n c h( 2 ) b a c k w a rd tilt or liftof the front screed cutting edge anda level rear screed prove most satis-f a c t o ry with air entrained concre t ep a ve m e n t s. Co n c rete pushed aheadof the front screed should have adepth of from 3 to 6 ( 3 ) inches duri n gthe first pass of the finishing ma-c h i n e. In hand finishing, better re-sults will usually be encountere dwith a wood rather than a metalf l o a t .

Special applications

Purposefully entrained air in con-c rete has been found to be of va l u ein a number of special applications.In most cases, its excellent we a t h e r-ing qualities and increased cohe-s i veness render it decidedly superi-or to ord i n a ry concre t e.

L i g h t weight concre t e. Light-weight aggregate concretes havebeen enjoying mounting accep-tance in recent ye a r s. As concre t ebuildings rise higher and spansg row longer, weight reduction hasbecome increasingly important. Al-most all lightweight aggregates arem a n u f a c t u red. They are formed byheating clay, shale, slag, slate, micaand pumice materials to a point thatis below fusion but that is sufficientto dri ve off water explosive l y, thuse f f e c t i vely expanding the aggre g a t e.

The resulting material is light-weight; but it is also angular, poro u sand cellular—all of which can re s u l tin harsh, unworkable concre t e s. Thequantity of water of conve n i e n c e(that needed beyond the amountn e c e s s a ry for cement hyd ration) re-q u i red in lightweight aggregate con-

c rete work is usually considera b l eThis usually means a high shri n k a g era t e, laitance because of consider-able bleeding and lower strengths orhigher cement contents than withn o rmal weight concre t e s. Air en-t rainment, by producing a cre a m i e rmix, reduces the amount of waterneeded for handling. This re d u c t i o namounts to as much as 335 poundsof water per cubic yard(4) of light-weight concre t e. Thus compre s s i ves t rength, homogeneity and imper-meability are all greatly improve d .Since the air bubbles have noweight, unit weight is decreased andinsulating value is increased. It iso bvious that lightweight aggre g a t ec o n c retes are benefitted consider-ably by air entra i n m e n t .

The amount of air to be entra i n e ddepends upon the intended use andd e s i red weight of the concre t e.St ru c t u ral concretes should haveless air than insulating types.Amount of air is usually inve r s e l yp ro p o rtional to the weight of thec o n c rete— the lighter the concre t e,the more air is re q u i re d .

Un f o rt u n a t e l y, some pro p e rties ofl i g h t weight aggregates conspireagainst entrainment of air. The gre a ta n g u l a rity of lightweight aggre g a t e sreduces the number of air bubblese n t rained because the sharp pointsand edges break the bubbles whilemixing is taking place. Also, aggre-gate angularity results in large vo i ds p a c e s. The consequent large airbubbles break down easily or rise tothe surface of the concre t e. Ord i n a r-ily this means that greater quantitiesof air entraining admixtures mustbe used in lightweight concre t e s.Un f o rt u n a t e l y, if ve ry large amountsa re used, the organic makeup of thea d m i x t u res might result in delaye dsets or lowe red strength, especiallyin winter. Howe ve r, powe rful air en-t raining agents that largely elimi-nate these troubles have been de-veloped especially for lightwe i g h ta g g regate concrete work .

Tremie concre t e . Tremie concre t e( c o n c rete placed under water byg ravity feed through a ve rtical pipe)has been in use for over a century asa means of placing concrete under

difficult conditions—for coffer-d a m s, caissons and underw a t e rfoundations and in certain hard - t o -reach re i n f o rced concrete situa-t i o n s. It offers the advantages ofeasy and rapid placement, elimina-tion of dewatering, perfect curi n ge n v i ronment and reduction of hon-e yc o m b i n g .

Along with these adva n t a g e scome a number of dra w b a c k s, in-cluding nonuniform i t y, high slumpsn e c e s s a ry for flow and high inci-dence of laitance. In addition, highcement contents are often re q u i re d .The use of air entrainment has beenfound to reduce all of these com-p l a i n t s. The greater cohesive n e s sand workability of air entra i n e dc o n c rete results in greater uniform i-ty and less segregation. Slumps canbe reduced and the formation of lai-tance is greatly re t a rded. Since thecement content can be reduced, in-t e rnal heat caused by hyd ration islessened. Also, gentler slopes are theru l e, thanks to the high flow a b i l i t yof air entrained concre t e.

Floors. Air entrainment is oftenomitted in floor slab constru c t i o nbecause this type of flatwork is noto rd i n a rily exposed to we a t h e ri n g .Howe ve r, as we have seen, re s i s-tance to we a t h e ring is only one ofmany advantages of air entra i n e dc o n c re t e.

In slab-on-grade work in locali-ties where sulfate attack is likely too c c u r, air entrainment can helpm i n i m i ze or entirely eliminate dete-ri o ration of the concre t e. Since seg-regation is reduced, it is easier toobtain surfaces able to resist thea b rasion of heavy tra f f i c. Be t t e rw o rkability can be had with less wa-ter resulting in less finishing andhigher strength slabs. On occasion,inside floor slabs are subjected tof re ezing and thawing because of de-lay in the completion of other por-tions of a stru c t u re. In such cases, ofc o u r s e, the margin of safety prov i d-ed by air entrainment is most desir-a b l e. It should also be re m e m b e re dthat on large jobs there is always thehuman element to be considere dwhen concretes with and withoutair are being used on the same job. It

sometimes happens on such jobsthat non-air- e n t rained concrete isused for exterior work either inad-ve rtently or because workers are notready to use it in flatwork at the timeit is delive red, and it is dive rted tothe exteri o r.

For these reasons many specifiersp refer to use entrained air in all one-course floors and for the bases oftwo-course slabs. There is no re a s o nfor using it, howe ve r, for toppings ofheavy duty two-course floors. Si n c ethese toppings are of such low wa-t e r-cement ratios and contain suchhigh quality aggre g a t e s, air entra i n-ment is not re q u i re d .

Co l o red concre t e. The last decadehas seen a considerable increase inthe use of exposed concrete as ana rc h i t e c t u ral feature of buildings.Right along with this trend there hasbeen an increase in the use of color-ing agents in concre t e. Un f o rt u n a t e-l y, field experience and labora t o rytests have shown that certain color-ing agents (especially carbon blackand black iron oxide) result in a low-e ring of the we a t h e rability of thec o n c re t e.

It appears that this adverse effectis a direct result of the action of col-o ring agents in reducing the air con-

tent of fresh mixes. As little as 3 per-cent by weight of a coloring materi-al in a mix containing 5 percent airresults in a reduction in air contentto less than 1⁄2 p e rcent. When suffi-cient air entraining agent is used tore s t o re an air content of from 3 to 6p e rcent, colored concrete resists theaction of fre ezing and thawing ands u rface scaling to the same extent asn o n - c o l o red air entrained mixes.Many producers of concrete color-ing agents now grind an air entra i n-ing agent in with their pro d u c t .

St rength of air entrained con-c re t e. “But air bubbles don’t haveany strength!” And so starts many adiscussion on the question ofwhether air entrainment has an ad-verse effect on concrete strength. Inre a l i t y, this question is now re s o l ve dwith a high degree of certainty to asimple yes and no.

The answer is yes if you refer toidentical mixes except for inclusionof entrained air in one of them. As-suming a concrete containing atleast 560 pounds of cement per cu-bic yard( 5 ) and a maximum coarsea g g regate size of 11⁄2 to 2 inches, eachp e rcentage increase in air contentwill cause a reduction in flexura ls t rength of 2 to 3 percent and a low-

e ring of 3 to 4 percent in compre s-s i ve strength. The richer the mix, theg reater will be the reduction ins t rength by use of air entra i n m e n t .

The answer is no, howe ve r, ifp roper adjustment of the mix ism a d e. Because air entra i n m e n tg reatly improves the workability ofc o n c re t e, the mix design should bea l t e red by reducing the amount ofsand and water. If advantage is tak-en of this chara c t e ristic, stre n g t h swill be equal to non-air- e n t ra i n e dc o n c retes or, in some cases, eve nh i g h e r. Ex p e riments have indicatedthat air entrainment mixes are alsosomewhat less susceptible to thea d verse effects of improper curi n g .

Air entrained mixes are subject tothe water-cement ratio law in thesame manner as their non-air- e n-t rained counterparts; any desire ds t rength possible with ord i n a ry con-c rete can be achieved by observ i n gthe usual rules of quality concre t e.

Measuring air content

A number of tests have been de-vised for measuring the air contentof concre t e. Howe ve r, there are basi-cally only three kinds of methods:g ra v i m e t ric, vo l u m e t ric or pre s s u re.

The gra v i m e t ric method was thefirst kind of test for air content de-vised. It consists of obtaining thesum of the absolute volumes of in-g redients in a known volume of con-c rete and subtracting that sum fro mthe measured gross vo l u m e. Themethod is slow, expensive and ofquestionable accura c y.

The vo l u m e t ric method simplytests fresh concretes containing anytype of aggregate by measuring thevolume of entrained air dire c t l y. Are p re s e n t a t i ve sample of the con-c rete is placed in the testing appara-tus and air is expelled from the con-c rete by manual methods. A dire c treading is made. This technique canalso be employed using only mort a r(cement, sand and water).

The pre s s u re method measure sair content indirectly by noting thevolume change concrete under-goes when subjected to a give np re s s u re. Since the only gas in con-c rete is air, it can be measured by

The roll of concrete ahead of this finishing machine, making its second pass,demonstrates the cohesiveness and workability which are typical of air entrainedconcrete.

applying a pre s s u re and computingthe amount that the concrete con-t ra c t s. With this test, a corre c t i o nmust be made for the porosity ofthe aggre g a t e.

Testing the air content of light-weight aggregate concretes by thep re s s u re method calls for specialp recautions since it measures air inthe aggregate as well as in the ce-ment paste. In addition, compre s-sion may cause the collapse of aircells in certain soft expanded aggre-g a t e s. When the mix is exc e s s i ve l yharsh, high particle interf e re n c eand bridging sometimes causec o m p ression of the concrete whichis not in pro p o rtion to the appliedp re s s u re.

Reducing air content

Too much of anything is bad.Use of air entrainment can re s u l tin a multitude of beneficial effects.But too much can seriously under-mine strengths without improv i n gd u ra b i l i t y.

Ve ry little we a t h e rability is gainedby air contents over 4 percent, but 6p e rcent air does offer better contro lof segregation and water gain. Mi x e swith air contents of up to 12 perc e n tstill exhibit the same resistance towe a t h e ring as those in the 4 to 6p e rcent air ra n g e. The bre a k i n g

point appears to occur at slightlyover 12 percent, at which pointd u rability begins to decre a s em a rk e d l y.

Although durability is not hurt bysuch high air contents, concre t es t rengths are greatly lowe red whenair rises above 7 or 8 percent. Whena b n o rmally high air contents aree x p e rienced or when non-air- e n-t rained concrete is re q u i red and on-ly air entraining cements are ava i l-a b l e, there are materials available toremove air from fresh concrete.

Tributyl phosphate is an odorlesschemical available from mostchemical manufacture r s. Ord i n a ri-ly 10 to 15 millilitres per cubicy a rd( 6 ) a re re q u i red to re m ove all airf rom a typical mix. Another com-monly available chemical, 2-ethylhexanol, will re m ove all air fro mmost mixes by the addition of ap-p roximately 10 millilitres per cubicy a rd .( 7 ) Ex p e rimentation will be re-q u i red to determine the exactamount needed to achieve the de-s i red reduction in air content withspecific batches in the field.

Here to stay

Air entrainment is with us forgood. It has proved its value in somany re s p e c t s, over such a widerange of uses and for so many ye a r s

that its future is secure. It is a deve l-opment in concrete technology thathas enhanced the reputation of con-c rete construction immeasurably byp roviding at little or no extra cost ore f f o rt a material that is mored u rable and attra c t i ve.

Metric equivalents1. 64- to 75-millimetre2. 3-millimetre3. 75 to 150 millimetres4. 198 kilograms per cubic metre5. 332 kilograms per cubic metre6. 13 to 20 millilitres per cubic metre7. 13 millilitres per cubic metre

This is an updating of the article of thesame title published in October 1959,page 1. It is republished here to makethe information more readily availableto current readers.

P U B L I C AT I O N# C 7 6 0 1 0 5

Copyright © 1976, The Aberdeen Gro u p

All rights re s e r v e d