ad mixers on concrete

8/12/2019 Ad mixers on Concrete

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CONCRETE ADMIXTURES

Adm ixtures are chemicals wh ich are added to

concrete at the mixing stage to modify some of

the proper t ies of the mix. Admixtu res should

never be regarded as a subst i tute for good

mater ia ls, good mix design and good

workmanship.

1. Uses of admixtures

The most common reasons for using admixtures inconcrete are:

To increase workability without changingwater content.

To reduce water content without changingwork ability.

To effect a combination of the above.

To adjust setting time.

To reduce segregation and/or bleeding.

To improve pumpability.

To accelerate the rate of strengthdevelopment at early ages.

To increase strength.

To improve potential durability and reducepermeability.

To reduce the total cost of the materialsused in the concrete.

To compensate for poor aggregateproperties.

2. Types of Admixtures

Admixtures are normally categorized according totheir effect:

Plasticizers (water-reducing agents)

Superplasticizers

Air entrainers

Accelerators

Retarders

Others

Many admixtures provide combinations ofproperties such as plasticizer / retarders orplasticizer / air entrainers. Each admixture type isdiscussed in the following sections.

2.1 Plasticizers

When added to a concrete mix, plasticizers (water-reducing agents) are absorbed on the surface ofthe binder particles, causing them to repel eachother and deflocculate. This results in improvedworkability and provides a more even distribution ofthe binder particles through the mix.

The main types of plasticizers are lignosulphonicacids and their salts, hydroxylated carboxylic acidsand their salts, and modifications of both.

Dosage

The typical dosage of a plasticizer varies from200ml 450 ml per 100 kg of cementitiousmaterial.

Uses

Plasticizers usually increase the slump ofconcrete with a given water content.

Plasticizers can reduce the waterrequirement of a concrete mix for a givenworkability, as a rule-of-thumb, by about10%.

The addition of a plasticizer makes itpossible to achieve a given strength with alower cement content.

Plasticizers may improve pumpability.

Practical considerations

A number of plasticizers contain a retarderand can cause problems if overdozed.

Some plasticizers contain chlorides whichmay increase the danger of corrosion ofreinforcing steel.

While some plasticizers entrain varyingamounts of air, others are reasonablyconsistent in the amount of air they entrain.

Where plasticizers are used to increaseworkability, the shrinkage and creep willinvariably be increase.


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2.2 Superplasticizers

These admixtures are chemically distinct fromnormal plasticizers and although their action is

basically the same, it is more marked.

When they are used to produce flowing concrete arapid loss of workability can be expected andtherefore they should be added just prior to placing.

Superplasticizers are usually chemical compoundssuch as sulphonated melamine formaldehyde,sulphonated naphthalene formaldehyde, andmodified lignosulshonates.

Dosage

The normal dosage of a superplasticizer is between750ml and 2500 ml per 100 kg of cementitiousmaterial.

Uses

Superplasticizers are used to best advantage:

In areas of congested reinforcement.

Where a self-levelling consistencefacilitates placing.

For high-strength concretes by decreasingthe water: cement ratio as a result ofreducing the water content by 15-25%.


Special mixes must be designed forsuperplasticizers and their use must becarefully controlled.

The effect of a superplasticizer maydisappear as soon as 30 minutes aftermixing.

They have a relatively high unit cost.

Where superplasticizers are used to producevery high workability, the shrinkage andcreep will be increased.

2.3 Air entrainers

An air-entraining agent introduces air in the form ofminute bubbles distributed uniformly throughout thecement paste. The main types include salts ofwood resins, animal or vegetable fats and oils andsulphonated hydrocarbons.

Dosage

Typical dosage for air-entraining agents is between

50ml and 150ml per 100kg of cementitiousmaterial.

Uses

Where improved resistance of hardenedconcrete to damage from freezing and

thawing is required.

For improved workability, especially inharsh or lean mixes.

To reduce bleeding and segregation,especially when a mix lacks fines.


Air entrainment may reduce the strength ofconcrete and overdosing can cause major

loss of strength. As a rule-of-thumb, 1% airmay cause a strength loss of 5%. It istherefore important that mixes be speciallydesigned for air entrainment and that thepercentage of air entrained duringconstruction be monitored.

Because the doses are so small, specialdispensers and accurate monitoring arerequired.

Different types and sources ofcement/cement extenders may result in theentrainment of different amounts of air for

the same dose and mix proportions.

A change in cementitious content, in thegrading or proportions of the fine fractionsof sand will normally alter the volume of airentrained.

The amount or air entrained may dependon the source and grading of sand inconcrete.

Forced-action mixers entrain largervolumes of air than other types.

Increasing ambient temperature tends toreduce the volume of air entrained.

The use of ground granulated blastfurnaceslag (GGBS) and fly ash (FA) tend toreduce the amount of air entrained.

Duration of mixing can also affect aircontent.

2.4 Accelerators

These admixtures speed up the chemical reaction

of the cement and water and so accelerate the rateof setting and/or early gain in strength of concrete.


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Among the main types of accelerators are calciumchloride, calcium formate, soda ash, potassiumchloride and a number of organic materials.

Calcium chloride appears to be the only one that isreasonably predictable in its performance, but ittends to promote corrosion of steel in concrete.

Dosage

Calcium Chloride: 2% by mass of all cementitiousmaterial. Calcium chloride powder or flakes shouldbe added to and dissolved in water which mustthen be kept agitated. This solution should bedispensed with the mixing water. Calcium chloridesolution is also available.

Non-chloride accelerators: 500ml to 2000ml per100kg of cementitious material.

Shotcrete accelerators: react almostinstantaneously, causing stiffening, rapid set andrapid hardening of the shotcrete.

Uses

Where rapid setting and high earlystrengths are required (eg in shaft sinking).

Where rapid turnover of moulds orformwork is required.

Where concreting takes place under verycold conditions.


Certain accelerators may increase dryingshrinkage, cracking and creep.

They may cause lower flexural strengths.

Many chloride-based accelerators promotecorrosion of reinforcing steel.

Calcium chloride should not be used in:

- reinforced concrete- water-retaining structures

- prestressed concrete

- steam-cured concrete

Overdosing with these materials cancaused marked retardation.

Accelerators work more effectively at lowerambient temperatures.

2.5 Retarders

These admixtures slow the chemical reaction of thecement and water leading to longer setting timesand slower initial strength gain.

The most common retarders are hydroxylatedcarboxylic acids, borax, lignins, sugar and somephosphates.

Dosage

Typical dosages for retarders are between 150mland 500ml per 100kg cementitious material.

Uses

When placing concrete in hot weather,particularly when the concrete is pumped.

To prevent cold joints due to duration ofplacing.

In concrete which has to be transported fora long time.


If a mix is overdosed beyond the limitrecommended by the supplier, retardationcan last for days.

Retarders often increase plastic shrinkageand plastic settlement cracking.

Delayed addition of retarders can result inextended retardation.

2.6 Other admixturesOther admixtures with different chemicalcompositions and effects are available (eg pumpingaids, pigments, expansion aids and groutingadmixtures). These are beyond the scope of thisleaflet, and information should be obtained from theadmixture suppliers.

3. Design of mixes

A laboratory test programme should be carried outto optimize dosages of admixtures with the

cements/blended cements and aggregates whichwill be used on the site. This programme should bebased on good concrete mix design and correctlaboratory methods. It is also essential to verify theproposed mixes under site conditions.

The control mix (ie one without admixture), shouldbe prepared before any other to prevent tracecontamination with the admixture under test.

Changes in materials or ambient conditions (egtemperature) may also change the effectiveness of

admixtures; trials are essential if such changes are

envisaged.If more than one effect is desired, then more thanone admixture may be required. The compatibilityof admixtures, particularly if from different


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suppliers, needs to be tested using materials fromthe site.

4. Storage and handling

Most admixtures are aqueous solutions of activechemicals that are biodegradable. These twofactors define the general rules for storage.

Admixtures should be protected from frost, as mosthave freezing points at or below 0C. When anadmixture freezes, the chemicals crystallized out ofthe solution and do not readily go back into solutionon thawing.

It is also necessary to protect admixtures from heat

because their degradation is accelerated atelevated temperatures. They should be stored outof direct sunlight and away from other heatsources. Many admixture companies provide light-colored or reflective packaging for admixtures toreduce heat absorption.

Admixtures generally contain preservatives toreduce biodegradation and extend shelf life.Despite this, the activity and effectiveness of anadmixture will gradually decrease with time.Therefore they should be used on a first-in-first-outbasis.

Admixtures are normally stored on site in 200drums or in bulk storage tanks. Bulk liquid storagetanks are often supplies, serviced and maintainedby the admixture supplier. Drums should behandled with care.

5. Dosage and dispensing

The correct dosage of admixture is crucial for

satisfactory mix performance and the proper use ofwell maintained and calibrated dispensingequipment is essential.

Admixture suppliers normally provide, install andservice dispensers which are either manual orautomatic.

Admixtures should be added to a concrete duringmixing with the last portion of the mix water in order

to ensure even dispersion of the admixturethroughout the concrete, unless delayed addition isnecessary for a specific purpose. The admixtureshould not be added directly to the dry cement oraggregates.

For the majority of admixtures it is not necessary tochange the concrete mixing procedure. Anyspecial requirements will be given in the admixturemanufacturers product literature.

6. Safety

While most admixtures are not hazardous to health,certain admixtures are caustic in nature and somemay be flammable. The following safetyprocedures apply to all admixtures.

Avoid eye, mouth and skin contact as all chemicals

should be regarded as toxic and corrosive.Contamination should be washed off immediatelywith plenty of fresh water. Contaminated clothingshould be removed and washed.

Seek medical advice in cases of serious eyecontamination, ingestion or excessive inhalation offumes.

Admixture spillage will cause floor areas to becomeslippery and unsafe. Spillage should immediatelybe hosed down with water to prevent accidents.

ad mixers on concrete

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