a contractors guide to superplastisizers

Yo u’ve won the job.Whether it be placing awall with heavy re i n f o rc e-ment, pumping a founda-

tion mat, or overlaying a bri d g edeck with silica-fume concre t e,you’ve decided to use a high-rangewater reducer (HRWR), also know nas a superplasticizer, or “super” fors h o rt. But what types of superplas-t i c i zers are there, and how do yo uchoose the best type for the job?When should you add superplasti-

c i zers to the mix? How will achange of brands affect set timeand slump loss?

Superplasticizer Types

Hi g h - range water reducers areclassified by ASTMC 494, “St a n d a rdSpecifications forChemical Ad m i x-t u res for Co n c re t e”( Ref. 2), as Types Fand G. Type F is aw a t e r- re d u c i n g ,h i g h - range admix-t u re, and Type G isa water- re d u c i n g ,h i g h - ra n g e, re t a rd-ing admixture. It isi m p o rtant for thec o n t ractor to un-derstand the dif-f e re n c e. The im-p roper use of a

Type G admixture may re q u i re afinishing crew to wait three addi-tional hours until the re t a rder hasw o rn off.

Ty p i c a l l y, within the differe n tt y p e s, superplasticizers are salts ofsulfonated melamine-form a l d e-h yde condensates (melamine),salts of sulfonated naphthalene-f o rm a l d e h yde condensates (naph-thalene), or lignosulfides. Mo s tH RWRs used in cast-in-place con-c rete today are naphthalenes. Lig-

n o s u l f i d e s, typically Type A waterre d u c e r s, can also be used asH RWRs by increasing their dosagera t e, but their use is no longer re c-ommended because they cancause serious set re t a rd a t i o n .

Within the different ASTM types,m a n u f a c t u rers offer many differe n tb ra n d s. Di f f e rent brands may pro-duce different re s u l t s.

Plant or Site Addition

One admixture manufacture rstates that if haul times are part i c-ularly long, a Type G HRWR addedat the plant can minimize pro b-lems associated with ASTM C 94’slimitations on delive ry time andmixer re vo l u t i o n s. The re t a rder in aType G will also delay the set forthat long run. If unexpected delaysa re encountered in transit, howe v-e r, the effects of the superplasticiz-er may not last until the contra c t o r

A Contractor’s Guide to Superplasticizers

BY TIMOTHY S. FISHER

Various brands affect set time and slump loss differently

Superplasticized concrete pumps easily and arr i v e sworkable. For maximum effectiveness, however,superplasticizers must be added to the mix at the right time.

THE USE OF A TYPE G ADMIXTURE MAY REQUIRE A FINISHING

CREW TO WAIT THREE ADDITIONAL HOURS BECAUSE

THE ADMIXTURE RETARDS THE SET.

is ready to place the load. Wo rk e r smay then be unhappy with thes l u m p, add water, and, conse-q u e n t l y, reduce stre n g t h s. This canlead to rejection of the concre t e.

Su p e r p l a s t i c i zers can also beadded on-site from an extern a ltank on the ready mix truck or us-ing on-site dispensing equipment.One manufacturer packages its su-pers in bags that can be tossed inthe drum in the quantities needed.Ad vantages of adding supers on-site are that the contractor can seethe effects, the inspector is assure dthat the super is going in, and thec o n t ractor can add the super whenit will be most effective. After intro-ducing a super on-site, rotate thed rum 70 re volutions to ensurep roper mixing. If problems are

f o reseen with meeting ASTM C 94’sre q u i rement of concrete placement

b e f o re 300 re vo l u t i o n s, the contra c-tor should bring this up in the pre-pour conference and request addi-

tional re vo l u t i o n s.Some specifications call for con-

c rete to be placed within 60 minutesafter batching during hot we a t h e r. Insome cases, the use of a superplasti-c i zer with a re t a rder will allow thattime limit to be extended to 90 min-u t e s. Bring this to the attention of theengineer at the prepour confere n c e.

Delayed Addition

De l a yed addition or redosing ofs u p e r p l a s t i c i zers is not a pro b l e mfor most pro d u c t s. One manufac-t u rer recommends limiting re d o s-ing to two times after initial dose.Another manufacturer does not puta limit on the amount of additionaldoses but warns that too much su-p e r p l a s t i c i zer can cause delaye dset times. Ma n u f a c t u rers re c o m-mend checking slump and air con-tent after each redosing to be surethey are within project specifica-tion limits. Ty p i c a l l y, within a fewre d o s e s, the slump will reach thatof the original dose.

Additional doses can be costly,ranging from $3 to $5 a cubic yard .A contractor that redoses oftenshould evaluate why. Are the tru c k scoming too soon and waiting inline on-site while the effects ofplant-added superplasticizer arewe a ring off? Is enough manpowe ror the proper equipment ava i l a b l eon-site when the trucks arri ve ?

Set Time

Your ready mix producer is sup-plying quality superplasticized con-c rete and your finishers are quickly

CH E C K S L U M P A N D A I R C O N T E N T A F T E R E A C H R E D O S I N G O F

S U P E R P L A S T I C I Z E R T O B E S U R E T H E Y A R E W I T H I N P R O J E C T

S P E C I F I C AT I O N L I M I T S.

Concrete No. Admixture Time of Set (Min.)Initial Final

1 none 281 382

2 naphthalene 355 435

3 melamine 350 440

4 calcium lignosulfonate 625 770with air-entraining

depressant

5 calcium lignosulfonate 430 532

6 naphthalene and calcium 390 475lignosulfonate

Set Times of Concrete Treated with Various HRWR

gaining experience on how thisc o n c rete sets. Then somethingchanges and the concrete is not set-ting like it usually does. Either it setstoo fast and is getting away from thef i n i s h e r s, or it’s delayed and thec rew must work ove rt i m e. Whatcauses this change in set time?

ASTM C 494 allows a Type FH RWR to exhibit initial set within a31⁄2-hour window, 1 hour earlier or21⁄2 hours later. Si m i l a r l y, for a Ty p eG HRWR, initial set times can va ryf rom 1 hour later to 31⁄2 hours later.Similar time deviations are allowe dfor final set. What does this mean?If for some reason a ready mix sup-plier switches superplasticize rb rands in the concrete loads toyour jobsite or between pro j e c t s,the set time can va ry by 31⁄2 h o u r s.Set times also can va ry if the con-t ractor buys a different superplasti-c i zer brand thinking all brands arethe same. The table lists typical settimes for different superplasticiz-e r s. By using the super, initial settime has increased at least 69 min-utes with melamine or as much as344 minutes with lignosulfonate.Final set times increased as little as53 minutes or as much as 388 min-u t e s. The use of superplasticize r sand their effect on set time doesnot affect finishing opera t i o n s, butcan change the timing of finishing.

Slump Loss

Su p e r p l a s t i c i zers normally in-c rease concrete slump for 30 to 45m i n u t e s, depending on brand anddosage ra t e. This usually gives thec o n t ractor enough time to placeand finish the concre t e. Howe ve r,when using a super to increase theslump from 4 inches to 7 or 8i n c h e s, which is a typical situa-

tion, the concrete may lose thatadditional slump after 30 minutes.Fi g u re 1 shows slump loss at va ri-ous mixing times of concre t e st reated with different superplasti-c i ze r s. In the 90-minute timef ra m e, the smallest slump loss is 1inch compared to the larg e s tslump loss of over 4 inches.

References1. ACI 212R-91,“Chemical Admixturesfor Concrete,” Manual of ConcretePractice, Vol. 1, American Concrete In-stitute (ACI), Detroit, 1994.

2. ASTM C 494,“Standard Specifica-tions for Chemical Admixtures in Con-crete,” ASTM, Philadelphia, 1993.

3. Vance Dodson, Concrete Admix-tures, Van Nostrand Reinhold, NewYork, 1990.

4. John Gardner, “Pressure of Con-crete on Formwork—A Review,” ACIJournal, September-October 1985

Using high-range water re d u c e r sto increase slump may be neces-s a ry for difficult wall placements.Conditions like narrow form s ,heavy re i n f o rcement, and sectionswith many blockouts, penetra-tions, or embedded items re q u i rethe use of high-slump concre t e .Superplasticized concrete alsoworks well for pumping opera-tions. The fluid concrete consis-tency reduces pump pre s s u re sand eliminates the need for alter-ing mix designs for pumping greatheights. It can also eliminate theneed to use staging pumps.

A c o n t r a c t o r c a n o f t e n t u rn i n al o w e r b i d a n d p ro d u c e a h i g h e r-q u a l i t y w a l l b y u s i n g s u p e r p l a s t i-c i z e d c o n c re t e . U s u a l l y w a l l p l a c e-m e n tl i f t h e i g h t s a re re s t r i c t e d t o 2o r 3 f e e t a n d f ree fall i s h e l d t o u n-d e r 3 t o 5 f e e t . Wi t ht h eu s eo fp ro p-e r l y p ro p o rt i o n e d s u p e r p l a s t i c i z e dc o n c re t e , i t i s a c c e p t a b l e t o i n-c re a s el i f th e i g h t st o 7 f e e ta n df re ef a l l t oo v e r 1 5 f e e t .

Because superplasticizers allowc o n c rete to be placed faster dueto its flowing consistency, allow theplacement of higher lifts, and re-t a rd set, the conservative designa p p roach would be to use full liquidhead when determining wall formp re s s u res. ACI committee re p o rt347R-88 “Guide to Formwork forC o n c rete” does not include equa-tions for concrete mixes with su-perplasticizers. Reference 4 off e r sa less conservative equation to cal-culate form pre s s u res with the useof superplasticized concre t e .

The paste in superplasticizedc o n c rete can leak through evensmall form joints and cause finsand discoloration. There f o re, for ar-chitectural concrete, it is import a n tthat the formwork joints be tight.

Wall Pours and Superplasticizers

PUBLICATION #C940547Co py right © 1994, The Ab e rdeen Gro u pAll rights re s e rve d