S ilica fume produces concretethat is stronger and more dura-

ble than conventional concrete. Fieldstrengths of 14,000 psi have beenachieved with this highly reactivepozzolan. Also, rebar corrosion is re-duced because the reaction productsof the extremely fine silica fume par-ticles plug internal pores. This slowscarbonation and helps keep chloridesout of the concrete. Because of thesebenefits, many engineers are nowspecifying silica-fume concrete forhigh-strength structural applica-tions, abrasion-resistant surfaces,

Reprinted with permission from Concrete Construction,published by The Aberdeen Group, 426 South Westgate,Addison, Illinois 60101

Working withsilica-fume concreteImproving concrete with silica fume requires usingthe correct placing, finishing, and curing procedures

Construction workers must becarefully trained to get the bestresults with silica-fume concrete.There is little or no bleeding inflatwork, so finishers must adjustthe timing of finishing operations.

and structures exposed to deicingagents or salt water.

Adding silica fume isn’t a substi-tute for good concreting practices,however. For best results, concretesuppliers must pay close attention toseveral production details. Also, con-tractors need to know how placing,finishing, and curing procedures dif-fer from those used for conventionalconcrete.

Choosing the product formSilica fume is available commer-

cially in several forms: dry, densifieddry, and slurried, each with or with-out chemical admixtures. Silica fumeor products containing silica fume

are available in bulk, drums, andbags, depending on the supplier. Theform of the material impacts thehandling of materials and the pro-duction of concrete, but has no sig-nificant effect on the properties ofthe fresh or hardened concrete.

Silica fume is often available inbulk or bags near the smelters whereit is produced. Bulk silica fume canbe transported and handled likeportland cement or fly ash. Baggedsilica fume can be used by emptyingthe bags directly into truck mixers.Bagged silica fume isn’t commonlyused, though, because of the dust itcreates. Bulk and bagged silica fumeare also costly to transport. Little fitsin a truck because silica fume weighsonly 12 to 15 pounds per cubic foot,compared with cement which weighs94 pounds per cubic foot.

Transporting silica fume as a wa-ter-based slurry is more economical.The slurries offer ease of use oncethe required dispensing equipment isavailable at the batch plant. Becauseof the nature of the slurry and be-cause the quantities used per cubicyard of concrete are greater, the dis-pensing equipment is larger andmore complex than that used forchemical admixtures. Because theslurry may stiffen in storage, dispens-

By Terence C. HollandConcrete ConsultantCoraopolis, Pennsylvania

CONCRETE CONSTRUCTION / MARCH 1987 261

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ers may be equipped with agitators.Where installing dispensing equip-

ment and using slurry is not prac-tical, a dry, densified silica fumeproduct with chemical admixtures isavailable. It weighs 35 to 40 poundsper cubic foot and is cost effective totransport. The densified product al-so creates little dust.

Producing the concreteThree critical tasks must be per-

formed with care when producingconcrete containing silica fume: mea-suring, adding, and mixing. The rou-tine aspects of concrete productionalso require extra care. For example,the amount of wash water in thetruck should be accounted for inmixture calculations. Drivers shouldbe careful not to add water to thedrum when they wash dust from atruck after loading.

Measuring. The correct amount ofsilica fume must be added by the con-crete supplier. In some specificationsthe silica fume is required as an ad-dition to the portland cement whilein others it is a replacement for thecement.

Measure silica fume with the samedegree of accuracy as other concreteingredients. Typically, accuracies ofplus or minus one percent by mass orvolume are specified. Recognize whatis being specified and what is beingmeasured: the silica fume itself or thecommercial product containing sili-ca fume. For example, the slurriedproducts contain about 50 percentsilica fume by weight while the dryproducts may be 100 percent silicafume.

The amount of water in a slurrymust also be accounted for in themixture proportions. You must re-duce the weight of batched water tocorrect for the water in the slurry.

Adding. The type of material be-ing used determines when to add thesilica fume product. Make a few tri-al batches to find the best batchingsequence for a particular plant. Drysilica fume can be added anytimeduring the production process, par-ticularly if the batch plant can han-dle the dry material in bulk.

Add slurried products to a truckmixer first since these products con-tain much of the batch water. Add-

ing slurried products to a truck lastcan result in head pack or poor dis-tribution of silica fume throughoutthe load. For central-mixed concrete,add the slurry after all other mate-rials are in the mixer.

Mixing. The silica fume must beuniformly distributed throughout theconcrete. Within a single load, com-pressive strength variations of 3000psi can result from poor mixing.

Follow American Society for Test-ing and Materials requirements formixing (ASTM C 94) when using sil-ica fume in truck-mixed concrete.The rated mixing capacity of thetruck must not be exceeded. The vol-ume of mixed concrete should not ex-ceed 63 percent of the total volumeof the drum. Equally important, anabsolute minimum of 100 revolutionsat a speed of at least 15 rpm is re-quired. Start measuring mixing timeafter all ingredients, including thesilica fume, are in the drum.

To determine whether you are mix-ing adequately conduct a mixer eval-uation as specified in ASTM C 94.If such a test cannot be conducted,closely watch the concrete dischargefrom the truck mixer. Slump varia-tions during discharge indicate poormixing. For example, if the slumpchanges by several inches from the

front to the back of the load duringa continuous discharge, it’s likely thatthe concrete was not mixed properly.

Concrete balls in the discharge al-so indicate poor mixing. Usually,more mixing or reducing the size ofthe load solves this problem. Truckmixers with worn fins are more proneto producing concrete balls. If thesame truck consistently causes prob-lems, don’t use it.

Transporting, placing,and consolidating

Silica-fume concrete acts much likeconventional concrete during trans-port, placement, and consolidation.However, depending on the amountof silica fume, the fresh concrete canbe more cohesive and less prone tosegregation than conventional con-crete. Because of the increased cohe-siveness, use a slump that is 1 to 2inches higher than normal for thesame type of placement. For overallease of placing and finishing, makethe slump as high as is practical.

Transport silica-fume concrete inany equipment used to transportconventional concrete. After discharge,the equipment should be cleaned thesame way as for conventional con-crete. But clean the trucks prompt-ly! Chipping 12,000 psi concrete

SILICA-FUME CONCRETE NEEDS SUPERPLASTICIZERS

Using, silica fume in concrete re-quires using another innovation inconcrete technology: superplasticiz-ers, or high-range water-reducing ad-mixtures. Silica fume has little usewithout them because its water de-mand is so high when used alone.

Silica fume is about one hundredtimes finer than portland cement. Justas with aggregates, decreasing parti-cle size increases surface area andthe water demand. Without a super-plasticizer, silica fume dries up themix. The extra water required to geta reasonable slump would increasethe water-cement ratio, thus reducingstrength and durability. You’d be bet-ter off using conventional, concretewith a lower water-cement ratio.

Silica fume products are availablewith and without preblended super-plasticizers. If a product without a su-perplasticizer is used, you must addyour own. Check with the silica fume

supplier for recommended admixturesto use. If a product containing asuperplasticizer is picked, it may benecessary to add more of the admix-ture or a standard water-reducing ad-mixture to get the performance youneed. Check with your silica fumesupplier to ensure that the admixtureyou want to use is compatible withthat in the product already.

When adding the superplasticizer,you may be using quantities muchlarger than are used in conventionalconcrete. Don’t worry; remember thatthere is a huge surface area in thesilica fume that must be wetted. Forthe best results, the superplasticizershould be added at the batch plant.Shoot for a 4- to 6-inch slump at thebatch plant to ensure adequate mix-ing of the silica fume into the con-crete. If necessary, final slump adjust-ments can be made at the jobsite byadding more admixture, not water.

CONCRETE CONSTRUCTION / MARCH 1987 263

from the mixing drum is an experi-ence you won’t want to repeat.

Silica-fume concrete can be plac-ed successfully using any placementdevice such as a bucket, pump, ortremie. During placing, don’t addwater to the concrete to improveworkability. Just as with convention-al concrete, too much water reducesstrength and durability. If a higherslump is needed, add controlledamounts of a water-reducing admix-ture at the batch plant or at thejobsite.

Consolidation by vibration is need-ed for silica-fume concrete even whena high-slump mixture is used. Con-crete with a slump of 8 to 10 inchescan be deceptive because it flows sowell that workers may think vibra-tion isn’t needed. However, the in-creased cohesiveness caused by silicafume entraps air which must be re-moved by vibration, regardless of theslump.

FinishingThe biggest difference between con-

ventional concrete and silica-fumeconcrete shows up during finishing.Adding up to 5 percent silica fumeby weight of cement makes little dif-ference, but adding higher amountsof silica fume reduces bleeding andmay eliminate it. This makes silica-fume concrete more susceptible thanconventional concrete to rapid sur-face drying and plastic shrinkagecracking.

Plastic shrinkage cracking can af-fect all concrete at two stages in thefinishing operation. The first is dur-ing the period after initial screedingand bull floating but before final fin-ishing. The second is after final fin-ishing and before curing starts.

Plastic shrinkage is caused by rapidmoisture loss from the surface of freshconcrete. If the lost moisture is notreplenished by natural bleeding, ten-sile stresses develop and cracking re-sults. A chart in the American Con-crete Institute report “Hot WeatherConcreting” (ACI 305) can be usedto estimate how fast surface moisturewill be lost. Humidity, wind, and theconcrete and air temperature affectthe evaporation rate.

The ACI report suggests that pre-cautions against plastic shrinkage

cracking are needed if the expectedevaporation rate at the concrete sur-face approaches 0.2 pound per squarefoot per hour. This value is too highfor silica-fume concrete. If the esti-mated evaporation rate approaches0.1 pound per square foot per hour,cracking may occur. Here are someways to prevent plastic shrinkagecracking:

• Erect windbreaks such as support-ed plastic sheets or temporary ply-wood walls. Large vehicles parkednear the slab also can help reducewind velocity.

• Use a light fog-spray of water tokeep the concrete surface moist be-tween finishing operations. Do notfinish the concrete if water fromthe fog spray can be seen on thesurface.

• Cover fresh concrete surfaces be-tween finishing operations, usingplastic sheets or moist burlap cov-ered with plastic.

• Use a proprietary evaporation re-tarder to prevent moisture loss be-tween initial and final finishing.

• Place floors and slabs after thewalls and roof of a structure are inplace.

• Start curing right after you’re donefinishing.

• Use fibers (polypropylene or steel)in the concrete.

• Dampen the subgrade and formsto prevent moisture loss at the baseof the slab.

• Reduce concrete temperature byusing cooled aggregates or wateror by using ice to replace some ofthe batch water.

• Have an adequate crew and all re-quired equipment on hand to placethe concrete quickly and withoutinterruption.

• Place the concrete late in the dayto minimize exposure to directsunlight and higher temperatures.

Sometimes the surface of the silica-fume concrete dries out and hardensfaster than the underlying concretebut doesn’t crack. This gives the freshconcrete a spongy consistency that ishard to finish. The same steps that

prevent plastic shrinkage cracking al-so help prevent surface drying andhardening.

Finishing practices. Use the sametools and procedures that you usewith conventional concrete. Theremay be a difference, however, in thetiming of the finishing operations.The lack of bleeding makes it hardto determine when to get on the con-crete. The chemical admixtures alsoaffect timing because they often in-clude retarders that delay setting.

There are two general recommen-dations regarding finishing. First,under-finish the concrete. This low-ers finishing costs and the potentialfor plastic shrinkage cracking be-cause curing is started sooner. Bylimiting finishing, the potential fordusting, scaling, and surface crazingis also reduced.

Second, conduct a trial placementso the finishing crews can perfecttheir approach to the silica-fume con-crete. This is particularly importantfor finishers who are accustomed toworking on wetter concrete surfaces.Trial placements are mandatory for

CONCRETE CONSTRUCTION / MARCH 1987 266

Superplasticized silica-fume concreteflows well. But even with slumps of 8to 10 inches thorough consolidationby vibration is always needed.

Elkem

Chem

icals, Inc.

floors requiring high silica fume con-tent and a high degree of finishing.If the concrete application requiresmore than 10 percent silica fume,consider screeding, bull floating, andtexturing, then immediately curingthe slab. High quality finishes can beachieved on silica-fume flatwork bymonitoring every detail of the plac-ing, finishing, and curing process.

CuringConcrete won’t perform well un-

less it is cured properly. Impropercuring is more harmful to silica-fumeconcrete than to conventional con-crete. To get the most benefit fromsilica fume, cure the concrete longerthan you would if conventional con-crete were being used. Keep it moistwith wet burlap, sheets of plastic, ora curing compound for at least sevendays.

Start curing immediately after fin-ishing. Remember, high amounts ofsilica fume produce concrete thatdoes not bleed; you do not have towait for the bleeding to stop beforecuring. On some projects where fin-ishing after setting was not required,the curing compound was applied afew minutes after the pass of a vi-brating screed.

If silica fume is being used in con-crete requiring accelerated curing,you may have to modify the curingcycle. The concrete must reach aninitial set before beginning the accel-erated curing. The chemical admix-tures often used with silica fume canaffect the rate of setting.

Good results comefrom good practices

Silica fume offers tremendous po-tential for improving many propertiesof concrete. Silica fume would not bespecified (and paid for) if the specifierdidn’t want to achieve better-than-normal concrete. To achieve highstrength, durable silica-fume concrete,you must use correct practices be-cause this concrete is less forgiving ofany attempts to cut corners than con-ventional concrete.

For more information circle 101 on reader ser-vice card.