πBefore you take a job in thefall or winter, you need toexamine how yo u’ll dealwith problems like froze n

s u b g ra d e s, extended concrete set-ting times, slower stre n g t h - g a i nra t e s, and decreased worker com-fort (and efficiency).

All are common to the placingand finishing of ready mixed con-c rete in cold we a t h e r. If you don’ta d d ress these cold-weather pro b-l e m s, the concrete may fre eze beforeit attains an adequate compre s s i ve

s t rength of at least 500psi, weakening or eve nd e s t roying your work.

The American Co n-c rete Institute (ACI) de-fines cold weather ast h ree successive days inwhich the ave rage dailyair tempera t u re dro p sb e l ow 40°F and fails torise above 50°F form o re than half a day( Ref. 1). To obtain high-quality concrete underthese conditions, it’si m p o rtant to take thef i ve general pre c a u-tions listed below. Fo rm o re detailed inform a-tion, consult Re f e r-ences 1 and 2.

1. Plan Ahead

Because cold we a t h-er can complicate your work, addi-tional planning is re q u i red. For ex-a m p l e, you should never placec o n c rete on a frozen subgra d e. Bu th ow will you thaw the subgrade? Ifyou decide to use ground heaters,h ow many heaters will you needon-site? Will a compactor be need-ed to reconsolidate the thawed ma-t e rial? Will the weather be cold

enough to warrant enclosing andheating the placement area? Bes u re to make arrangements for cov-e ring, insulating, housing, or heat-ing newly placed concrete beforep l a c e m e n t .

Make sure all the equipment yo uneed is on-site before you begin toplace concre t e. It may not be long be-f o re concrete tempera t u res appro a c h

BY DAVE IMSE, JERRY KRUEGER, AND NEAL R. MOSS

With a few basic precautions, you can producehigh-quality concrete even in severe weather

Placing and finishing concrete in the bone-chillingwinters of Michigan’s Upper Peninsula requirescareful planning. Knowing what needs to beaddressed before you tackle a job like this oftenmeans the difference between success and failure.

Robert J. Ryan

Te m p e r a t u re A p p roximate ( ° F ) Setting Time

( H r s . )

7 0 6

6 0 8

5 0 1 02⁄3

4 0 1 41⁄3

3 0 1 9

2 0 C o n c rete fre e z e sand doesn’t set.

Source: Concrete Construction,March 1990.

TABLE 1. EFFECT OF TEMPERATURE

ON CONCRETE SETTING TIME

The Musts of Successful

COLD-WEATHERCONCRETEING

d a n g e rously low leve l s. To ease finish-ing operations in cold we a t h e r, yo umay need additional equipment. In-sulating blankets, for example, maybe needed to cover the subgrade be-f o re the pour. Or wooden floats maybe needed to avoid sealing the con-c rete surface before bleeding stops.Wi n d b reaks or eva p o ration re t a rd e r smay also be needed, depending onambient conditions.

2. Choose Cold-Weather Concrete Mixes

Because cement hyd ration iss l owed in cold we a t h e r, the concre t ecools quickly, extending setting timeby about one-third for each 10°F re-duction in tempera t u re (see Table 1)and reducing compre s s i ve stre n g t h sat early ages. The result: delayed fin-ishing and form stri p p i n g .

To expedite your finishing opera-tions and speed form re m oval andre u s e, ready mixed concrete pro-ducers usually heat the mixing wa-ter or aggre g a t e s. You may also wantto consider ord e ring a mix with in-c reased cement content, faster- s e t-ting cement, or an accelerating ad-m i x t u re, since these adjustmentscan speed setting time, incre a s eearly and ultimate strength, and re-

duce the time re q u i red for pro p e rc u ring and pro t e c t i o n .

In c reasing cement content ra i s-es concrete tempera t u re duri n gc u ring because of the heat pro-duced by cement hyd ration. Andadding cement without incre a s i n gwater content reduces the water-cement ra t i o, partially offsettingthe lower rate of strength gaind u ring cold we a t h e r.

Using Type III (high-early-s t rength) cement speeds concre t e’ssetting time and strength gain.Ground more finely than Type I or IIcement, Type III cement may alsocontain more tri-calcium silicate.Both the fine gri n dand higher tri c a l c i-um silicate contenti n c rease the rate atwhich heat is gen-e rated by hyd ra-tion. Re m e m b e r,h owe ve r, that Ty p eIII cement may notbe available in alllocal mark e t s, or itsuse may be limitedby the number ofsilos at the concre t ep ro d u c e r’s yard .

Another way to improve con-c re t e’s rate of setting and stre n g t hgain is by using chemical admix-t u re s. To accommodate differe n tjob re q u i re m e n t s, they’re ava i l a b l ein va rious classes. If re i n f o rc i n gsteel won’t be needed, the most ef-f e c t i ve mix adjustment is the addi-tion of either calcium chloride or ana c c e l e rating admixture containingcalcium chlori d e. If the stru c t u recontains re i n f o rcing steel, nonchlo-ride accelerating admixtures are al-so ava i l a b l e, including one type de-signed for concrete placed ins u b f re ezing tempera t u re s.

In addition to accelerating ad-m i x t u re s, air- e n t raining admix-t u res may also be necessary. Us i n gthese admixtures is critical if yo uexpect the concrete to be exposedto fre eze-thaw cycles while it’s ins e rvice or in a saturated condition.Sa t u rated concrete shouldn’t be al-l owed to fre eze and thaw before itd e velops a compre s s i ve strength of3500 to 4000 psi (Ref. 2).

3. Maintain Adequate ConcreteTemperatures

It’s important to maintain ade-quate concrete tempera t u res dur-ing mixing and placing and in thefirst 24 hours after placement. If thet e m p e ra t u re of undisturbed plasticc o n c rete drops below about 29°F,f re ezing may occur, reducing the28-day compre s s i ve strength by asmuch as 50% (see gra p h ) .

To pre vent early-age fre ez i n g ,p rotection must be provided imme-

Section Size, Minimum Dimension, In.

Line Air < 12 12 to 36 36 to 72 > 72Temperature

Minimum concrete temperature as placed and maintained

1 — 55°F 50°F 45°F 40°F

Minimum concrete temperature as mixed for indicated air temperature*

2 Above 30°F 60°F 55°F 50°F 45°F

3 0 to 30°F 65°F 60°F 55°F 50°F

4 Below 0°F 70°F 65°F 60°F 55°F

Maximum allowable gradual temperature drop in first 24 hours after end of protection

5 — 50°F 40°F 30°F 20°F

TABLE 2. RECOMMENDED CONCRETE TEMPERATURES

* For colder weather a greater margin in temperature is provided between concrete as mixedand required minimum temperature of fresh concrete in place.

Source: Behavior of Concrete under Temperature Extremes, SP-39,American Concrete Institute.

diately after concrete placement. Insome cases, you may only need toc over the concrete with insulatingm a t e rials and use the natural heat ofh yd ration to maintain the tempera-t u re at the recommended leve l s.Useful insulating materials includep o l y s t y rene foam sheets, ure t h a n efoam, foamed vinyl blankets, miner-al wool or cellulose fibers, blanket orbatt insulation, and hay cove re dwith plastic. In more extreme cases,consider building enclosures andusing heating units to maintain thed es i red tempera t u re s.

Table 2 provides AC I ’s re c o m-mended tempera t u res for concre t eas it is mixed, placed, and main-tained. Two important points: Ke e p

c o n c re t e, subgra d e, and ambientt e m p e ra t u res within a range of2 0 ° F, and don’t allow concrete tem-p e ra t u res to rise more than 20°Fa b ove these minimum va l u e s. Hi g hc o n c rete tempera t u res don’t signif-icantly protect the material fro mf re ezing. In fact, the rate of heat lossis even more rapid when concre t et e m p e ra t u res are higher than am-bient tempera t u re s.

Consult Tables 3 and 4 to helpd e t e rmine the proper length ofp rotection time for concrete madewith Type I or Type II cement. No t ethat the use of Type III cement, ad-m i x t u re s, or additional Type I or IIcement shortens the concre t e - p ro-tection period.

4. Maintain Proper Curing Conditions

Proper curing of concrete is cri t i-cal to normal strength deve l o p m e n t .Cooler ambient tempera t u res gener-ally re q u i re longer, more care f u l l yc o n t rolled concrete curing. You wantto ensure that the concrete deve l o p senough strength for safe re m oval off o rm s, shore s, and re s h o re s, and forsafe loading of the stru c t u re duri n gand after constru c t i o n .

Co n c rete exposed to cold we a t h-er is normally unlikely to dry at anu n d e s i rable ra t e, so little or no ex-t e rnal supply of moisture is typical-ly re q u i red for curing. But this maynot be true for concrete withinheated pro t e c t i ve enclosure s.When concrete warmer than 60°F isexposed to air at 50°F or higher, it’si m p o rtant to pre vent drying. Us i n gan exhaust steam heater is pro b a-bly the most effective method,since it provides both heat andm o i s t u re. If dry heating is used, bes u re to protect exposed concre t es u rfaces with an impervious cove r-ing or a curing compound meetingASTM C 309 re q u i re m e n t s. Mo n i t o rcombustion heaters care f u l l y. Ift h e y ’re unvented or impro p e r l yvented, they can cause carbonationof the concre t e, resulting in we a k ,dusting surf a c e s.

After re m oving tempera t u re pro-tection, it’s usually not necessary top rovide measures to pre vent exc e s-s i ve drying as long as the air tem-p e ra t u re remains below 50°F, exc e p twhen concrete is placed in extre m e-ly arid climates. If exc e s s i ve dry i n gis anticipated, concrete can be wa-t e r- c u red when no fre ezing is ex-pected. Ot h e rw i s e, use curing com-pounds or an impervious cove r.

5. Limit Rapid TemperatureChanges

It’s also important to limit ra p i dt e m p e ra t u re changes, especiallyb e f o re the concrete has deve l o p e dsufficient strength to withstand in-duced thermal stre s s e s. ACI pro-vides the following maximum al-l owable drops in tempera t u red u ring the first 24 hours after the

Protection period at temperature i n d i c a t e d i n L i n e 1 o f Ta b l e 2 ,d a y s * *

Line Service Type I or Type III cement,category II cement or accelerating

admixture, or 100 lb./cy. of additional cement

1 No load; not exposed 2 1

2 No load; exposed 3 2

3 Partial load; exposed 6 4

4 Full load See Reference 1, Chapter 6.

TABLE 4. LENGTH OF PROTECTION PERIOD*

*For concrete placed during cold weather.**A day is a 24-hr period.

P ro t e c t i o n p e r i o da t t e m p e r a t u re i n d i c a t e din Line 1 of Table 2, days**

Line Exposure Type I or II cement Type III cement, or accelerating admixture, or 100 lb./cy. of additional cement

1 Not exposed 2 1

2 Exposed 3 2

TABLE 3. LENGTH OF PROTECTION PERIOD*

*Required to prevent damage from early-age freezing of air-entrained concrete.**A day is a 24-hr period.

end of the protection period: Fo rc o n c rete that’s less than 12 inchesthick, the maximum allowable dro pis 50°F; 12 to 36 inches thick, 40°F;36 to 72 inches thick, 30°F; andg reater than 72 inches thick, 20°F.

D o n’t forget that the basic re-q u i rements for cold-weather con-c reting are no different than stan-d a rd recommended practice: Use asuitable mix, protect the fresh con-c rete from the elements, and bes u re to cure it pro p e r l y.

Although profitability can be chal-lenged during cold-weather concre t-ing opera t i o n s, it’s not hard to followthe five basic steps described above,all designed to help you better planyour next fall or winter concrete job.K n owing what needs to be ad-d ressed b e f o re you tackle the job willmake the difference between suc-cessful and unsuccessful cold-weather concre t i n g .

References1. ACI Committee 306, Cold WeatherConcreting, ACI 306R-88, AmericanConcrete Institute, Farmington Hills,Mich., 1988.2. S.H. Kosmatka and W.C. Panarese,Design and Control of Concrete Mix-tures, 13th ed., Portland Cement As-sociation, Skokie, Ill., 1988.

Dave Imse, P.E., is technical serv i c eengineer for Holnam Inc., Oak Bro o k ,Ill.; Jerry Krueger is technical ands e rvice manager for Alby MaterialsInc., Wa t e rf o rd, Wis.; and Neal R.Moss is regional accounts managerfor Master Builders Inc., Cleveland. The authors recently presented a sem-inar on cold-weather concreting for Al-by University. These free classes areheld at Alby Materials’ corporate of-fices in Wa t e rf o rd, Wis. For more in-f o rmation, or for registration form s ,call Alby’s Bill Hahn at 414-534-5144.

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