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Concrete yield is an imp ortant con sideration on

many jobs. An engineer may compute the

nu m ber of cubic yards of concrete needed for

a 500,000 square foot floor, and the ready m ix

produ cer may deliver the con crete in com plete accor-

dan ce with the specifications. Yet wh en the specified

nu m ber o f cubic yards of concrete h as been delive red,

the floor ma y be far from com pleted. Since this short age

can r esult from a n um ber of factors, it will be helpful to

take a look at the m eth ods of calculating yield, th e fac-

tors which will caus e va riations in yield an d th e tech -

niqu es for po licing yield.

CALCULATING YIELD

Yield is defined in ASTM C 138 as the volum e of con -

crete (per un it volum e of ceme nt) de live red to t he job-

site in response to a request for a specified nu m ber of

cub ic yards of con crete.

The volum e of concrete produ ced per batch can b e

figured out w ith this form ula:

S = (NX94) + Wf + Wc + Ww_______________________W

Where :

s = volume of concrete produced per batch, in cubic

feet;

n = number of bags of cement , in the ba tch;

94 = net weight of a bag of cement, in pounds;

Wf = total weight of fine aggregate in batch in con di-

tion used, in poun ds;

Wc = total weight of coarse aggregate in batch in con-

dition used, in pou nds;

Ww = tota l weight of mixing water adde d to batch , in

pound s; and

W = weight of concrete, in pou nds pe r cubic foot.

To calculate yield the following formula should beused:

Y = s___ , in wh ichn

Y = yield of concrete produced per 94-pound bag of

cem ent, in cubic feet;

S = volume of concrete produced per batch in cubic

feet;

N = number of bags of cement in the batch.

Re l a ti ve yield, which is th e rat io of actua l volum e o f

c o n c rete o btained to the volum e as designed for the

batch , can b e calculated with th e following formula:

Ry = S_________ , in which

Vd x 27

Ry = re lative yield;

S = volume of concre te produced per ba tch in cubic

feet;

Vd = volume of concre te which the ba tch was de-

signed t o prod uce in cubic yards.

NOTE: A value for Ry greater th an 1.00 ind icates a n ex-

cess of concrete being produ ced, whereas a value less

than this ind icates the ba tch to be sho rt of its designed

volum e.

The accom pan yin g Yield De t erm in a t io n table can

be used a s a guide in dete rmin ing yield.

FACTORS AFFECTING YIELD

Yield ca n be affected while the st ructure is still in th e

de sign stage. It is not u nu sual to encou nter m ix de signs

which sim ply will not yield 27 cub ic feet of concrete p er

cub ic yard of concrete ordered. When the concrete pro-

d ucer re c e i ves the app roved m ix design, he shou ld

check it for yield. This can be accom plished b y applying

the form ulas in the p receding section. If the m ix design

is incom patible with correct yield, this fact shou ld be

called to the attention of the app ropriate party and a re-

vised de sign ob tain ed wh ich will produ ce correct yield.

Usually, the con crete prod ucer is in an excellent position

to ad vise on alterations in th e mix design to get th e best

results with h is materials and eq uipm ent. The following

inform ation is necessary for a workable m ix design:

(1) Dry weights p er cub ic yard of cem ent a nd ea ch ag-

gregate used in th e m ix.(2) Tota l weight of water in dry aggregate m ix.

(3) Loose dry un it weigh ts or free m oisture in a ggre-

gate used in th e mix.

(4) Adm ixture and/ or air-entraining agent used in the

mix in p oun ds or oun ces per cubic yard.

(5) Plastic unit weight of con crete in pou nd s per cu-

bic foot, as determin ed in th e laboratory.

(6) Slum p in inches.

(7) Entrained a ir cont ent o f concrete (volum etric).

Ho w to d e te r m in ec on c r e t e yie ld

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(8) 7- and 28- da y com pre s s i ve stren gths as deter-

mined in th e laboratory.

(9) 28-day air dry un it weight o f con crete, if required

by specifications.

(10) Tem pe ra t u res of fresh con crete an d of air when

sam pled.

After t he correct m ix design has been pre p a red and

approved, prob lems with yield can develop at the ba tch-

ing plant. Con tam ination o f the aggregates by materials

of differen t weight m ay result in c han ges in yield, sincein b atchin g by weight the volum e of aggregates will

change. For examp le, if lightweight organ ic ma terial was

to con tam inate regular weight aggregates, a given weight

of the aggregates would produ ce a greater volum e than

anticipated . Although com plaints are ra rely voiced for

receiving more concrete t han bargained for, stren gth will

inevitably be affected. When th e cemen t conten t is kept

constan t and yield increases, the ceme nt factor is, in ef-

fect, lowe red an d th e result will be lowe red stre n g t h .

Conversely, if aggregates are con tam inated with m ateri-

al of great er weight, yield will be redu ced a n d strengt h

i n c reased. As an examp le, assum e that a stockpile of

s tru ctural light weight aggregates acciden tally beco m escontam inate d with regular weight aggregates. When the

aggregates are batched , the weight will no t sup ply ne ar-

ly the volum e an ticipated an d th e yield will be reduced.

In ad dition, if yield is redu ced to 25 cubic feet an d ce-

me nt con tent m aintained, 564 poun ds of cemen t wil l

be equal to 610 pounds in 27 cubic feet. This is, of

course, an u necono mic use of cemen t.

If yield is to b e main tained, the m oisture con tent o f

both the fine and coarse aggregates m ust be care f u l l y

watch ed. If weights in the m ix design are pre dicated on

a g g regates in one condition of saturation and they are

batched in some other condition (without appro p ri a t e

adjustm ents), t he result will be a chan ge in aggre g a t evolum e batched and, therefore, in yield. Failure to adjust

the am oun t of water in th e mix will also cause loss of

slump an d workability or lowered strength.

This m atter is espe cially critical in lightweight a ggre-

gate concrete where absorption can run as high as 12

p e rcent. Unwat ched, this can spell ma jor variations in

yield. For exam ple, a ssum e th e following lightwe i g h t

concrete m ix design is b ased on bon e-dry aggregates:

564 lb. cem en t

1300 lb. norma l weight sand

900 lb. lightweight coarse aggregate

29 gal. water

6 percent en trained air

4 in. slum p

In th e field, however, we find t hat the san d h as a m ois-

ture conten t of 5 percent an d th e lightweight aggregate

has a total of 15 percent absorbed and su rface mo isture

conten t. App lying a com pensa ting adjustmen t, the m ix

should be batched with 564 pound s of cemen t, 1365

pou nd s o f sand , 1035 pou nd s of lightweight a ggre g at e

an d 15 gallons of water. However, if the m ix is batch ed

a d h e ring to the weights given in t he m ix design 564,

1300 and 900the actu al ceme nt an d aggregate weights

wou ld be 564, 1235 an d 765. This would occur b ecau se

there would be 65 poun ds of water in the sand, an d 135

pou nds in th e lightweight aggregatea tota l of 200

po un ds or 24 gallons of extra water. Only abou t 12 addi-

tional gallons o f water would be n eeded to produ ce the

required 4-inch slum p. In terms of yield, we would en d

up with a batch t otaling 25 cub ic feet rath er than 27aloss of 8 percen t.

Althou gh the effect of aggregate moisture con tent o n

yield in norm al weight concrete would n ot be as dra-

m atic, it could neve rth eless be sub stan tial eno ugh to

cause com plaints.

Air entrainme nt can also cause variations in yield. In

regular weight concrete each percen t of air entra i n e d

causes one pou nd per cub ic foot reduction in concrete

weight. It also causes an increase in yield. Whe n p repar-

ing a m ix design for regular weight co ncrete, the e ffect

of air entrainm ent on yield sh ould be considered.

Ge n e rally speaking, the ready m ix produ cer is h eld

respon sible for su pp lying 27 cubic feet of concret e fore ve ry cub ic yard ord e red. Howe ve r, whe n tru cks are

f o rced to wait long p eriods b efore t hey can discharg e

con cre te, there is boun d to be a reduction in yield due

to loss of air and wat er. In this case, the con tractor m ust

assum e the respon sibility for loss in yield, as well as the

other degradation s of qu ality tha t accom pan y ove r l y

long waits at the jobsite.

Other practices that can result in loss of yield are: over-

vibration this drives out air; soup y mixesthese ten d

to lose air content readily; and allowing concrete to b e-

com e hotthis dri ves o ut ent rain ed air. Any pra c t i c e

which affects the water content of a mixsuch as

addin g water at the job, or allowing concrete to dry out will affect yie ld.

Certain t ypes of concrete ca stings will result in loss of

yield, even with excellent pra c t i c e s. For exam ple,

colum ns, high walls, m ass concrete an d oth er structura l

app lications involvin g considerab le con crete p re ssu re

will result in loss of yield, often a s mu ch a s 10 perce n t.

This loss is due to th e high de gree of density achieved in

such work an d the att enda nt loss of air content. If the

loss o f inten tionally en trained air is n ot exc e s s i ve, this

will n atu rally p rodu ce su perior concre t e. Howeve r, t he

loss of yield sho uld be kept in m ind a n d the ord er for

concrete should take this into con sideration .

To m inimize problem s with yield, cont ractors shou ld

observe the following ru les:

(1) Make sure th at th e m ix design is consistent with a

yield of 27 cubic feet of con crete per cub ic yard o f de-

signed m ix.

(2) Order enou gh concrete to accomm odate un avoid-

ab le losses in yield for certain types of placemen ts.

(3) Sched ule d elive ries so d elays in discha rgin g con-

crete from trucks will be m inimized.

(4) Do not ad d water to th e m ix at the jobsite.

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(5) Be sure form s are n ot leaking.

(6) Select tra n s p o rting and placing equ ipm ent an d

tech niq ues that will avoid ove r- v i b ration, segre g a t i o n

and other p roblem s.

(7) Do not use an excessively wet m ix. Excessively wet

co ncrete will settle, and a settlemen t of 1/ 8 inch in a 4-

inch slab wou ld constitut e a loss in volum e of 3 percen t.

Resp on sibility for variations in yield can be traced to

conditions or practices before and / or a fter d elive ry of

the concre te. In almost eve ry case, the chan ge in yieldwill be accomp anied by changes in such importan t char-

acteristics as concrete strength and dura bility.

CHECKING YIELD

Yield of the con crete a s delivered at the jobsite can be

checked by the con tractor by regular measu remen t of

unit weight . If this varies m ore than 2 percent, an inves-

tigation should be ma de into such factors as air content,

aggregate weight o r batch weights. When th e un it weight

va ries co ns idera b l y, it will often be found th at yield is

varying (as well as other con crete p ropert ies).

To check th e actu al volum e of con crete bein g deliv-

ered to th e job, divide th e plastic unit weight of the co n-c rete into th e total nu mb er of poun ds of all the m ix in-

gredients.

If the p rope r volum e of con crete was delivered to the

j o b s i t e, and if suitable allowan ces have been m ade for

u n a voidable yield losses in placeme nt, and a size a b l e

shortage of concrete still turn s up, the contracto r can be

certain th at he has h is own p ractices to blame. Or if it de-

velops th at th e job is com pleted b efore a good d eal of the

ant icipate d con crete is delivered, the con tractor cann ot

rest easy; th eres a lways th e kn owledge th at ove r- y iel d

m ean s lowered strengths.

Obviously, it is of the greatest im port ance t o m ainta in

yield cor re c tl y. All part ies b en efit whe n yield is kep t inb ounds, and all stand to lose when it is allowed t o vary

substantially.

YIELD DETERMINATION

Concrete is ordered on the basis of cement content; thesaturated, surface-dry condition of the aggregates andthe total water content. Free moisture content of aggre-gates is compensated for at the ready mix plant. In thefollowing example, 6 bags of cement and 6 gallons ofwater per bag of cement were used.

Weights for 1 cubic yard batch (pounds)

Weights Moisture Adjusted

SSD. Percent Pound Weights

6 bags Cement 564 lb. 564

lb.

Sand* 1250 lb. 5% 62.5 lb. 1312.5lb.

Coarse Agg* 1940 lb. 1% 19.4 lb. 1959.4lb.

36 gal. Water 300 lb. less 81.9 lb. 218.1lb.

________________

Total 4054 lb. 4054.0lb.

* Specific gravity of aggregate is 2.65

Check yield on the job by determining the unit weight ofplastic concrete. For calculation purposes, use the mate-rial weights per batch provided by the ready mixed con-crete producer.

Batch weights for 5 cubic yard batch (pounds)

Cement 2,820 pounds

Sand 6,560 pounds

Coarse aggregate 9,800 pounds

Added water 1,000 (120 gal.) pounds_________________

Total 20,180 pounds

Determined unit weight: 148.7 pounds per cubic foot

Yield = 20,380 136.3 _______ _____= 136.3 cu. ft. = 5.04 cu. yd.148.7 27

Cement

per cu. yd = 2820 559_____ = 559 lb.; ____ = 5.95 bags5.04 94

PUBLICATION#C680361

Copyright 1968, The Aberd een Group

All rights reserved