CHEMICAL ADMIXTURES IN CONCRETE-Part 2

CHEMICAL ADMIXTURES IN CONCRETE- Part 1

Accelerators:

        Accelerating admixtures are added to concrete to increase the rate of early strength

development in concrete to:

·         Permit earlier removal of formwork.

·         Reduce the required period of curing.

·         Advance the time that a structure can be placed in service.

·         Partially compensate for the retarding effect of low temperature during cold weather concreting.

·         In the emergency repair work.  

                                        

              Calcium chloride                               saraswati accelerators

                                              

Pg-14

                                              Quikrete

                              

                                                 Larsen rapid hardner

Air-entraining admixture:

        Air entraining admixture incorporates millions of non-coalescing air bubbles which act as ball-bearings and modify the properties of concrete like workability, segregation, resistance to frost action, etc. These air bubbles are in the range of 5µ to 80µ which are distributed throughout the concrete mass.The common air-entraining agents in United States are Vinsol resin, Darex, N Tair, Airalon, Orvus, Teepol,Petrosan and Cheecol.

    In India these air entrainers are not that popular. Only in case of dams these are used and that too for workability purposes. Certain indigenous air-entrainers are Aerosin-HRS, Rihand A.E.A., Koynaea, Ritha powder, MC-Mischoel LP MC-Michoel AEA, Complast AE 215, Roff AEA 330 and hico.

                          

  Super air plus                        slick pak                                aqualac

                                                     

                                                     Vinsol resin

Effect on properties of concrete:

v  Resistance to freezing and thawing:

The greatest advantage derived from the use of air entrained concrete is the high

resistance of hardened concrete to scaling due to freezing and thawing. It is found that when ordinary concrete is subjected to a temperature below freezing point, the water contained inthe pore of the concrete freezes. It is well known that the volume of ice is about 10 per centhigher than the corresponding volume of water. Hence, the ice formed in the pores ofhardened concrete exerts pressure. The cumulative effect of this pressure becomesconsiderable, with the result that surface scaling and disruption of concrete at the weakersection takes place.In air-entrained concrete though the air voids are more they are in form of discreet air bubbles of uniform shape and size which inhibits formation of large ice crystals. Secondly, they are interconnected due to which the pressure is relieved. The below figure shows that an excellent concrete can withstand 2000 cycles of frost action at 4% air entrainment.

scaling

                                       

Damage caused by frost action

                        

v  Effect on workability:

The entrainment of air in fresh concrete by means of air entraining agent improves

workability. The placeability of air entrained concrete having 7.5 cm slump is

superior to that of non-air entrained concrete having 12.5 cm slump. This easier placeabilityof a lower slump should be recognised by the people concerned with concrete constructionin difficult situations. Better placeability of air entrained concrete results in more homogeneousconcrete with less segregation, bleeding and honeycombing. The concrete containingentrained air is more plastic and ‘fatty’ and can be more easily handled than ordinary concrete.The pumpability of the mix also increases enormously.

v  Effect on strength:

Due to the increase in air content the strength of concrete normally reduces. But as the air-entraining admixture enhance the workability it is possible to reduce the w/c ratio which results in an increase of strength which compensates some of the loss.

v  Effect on Segregation, Bleeding and Laitence:

Segregation, bleeding and consequent formation of laitance are reduced greatly by air entrainment. These actions probably result from physical phenomena due to the incorporationof a system of air bubbles.

1.      The bubbles buoy up the aggregates and cement and hencereduce the rate at which sedimentation occurs in the freshly placed concrete.

2.      Thebubbles decrease the effective area through which the differential movement of water mayoccur.

3.      The bubbles increase the mutual adhesion between cement and aggregate.

4.      The surface area of voids in the plastic concrete is sufficiently large to retard the rate atwhich water separates from the paste by drainage.

                A test performed at College of Military Engineering, Pune by using Ritha powder and Vinsol resin has been shown below. It can be seen that air-entrained concrete shows reduced bleeding. A good mix having only a slight possibility of segregation is not affected by air-entrainment. Only a poor mix shows improved results.

v  Effect on permeability:

     The air entrained concrete affects the permeability due to following reasons:

1.      Greater uniformity of concrete with entrained air due to its increased workability;

2.      modified pore-structure of the air entrained concrete;

3.      reduction of water channel due toreduction in bleeding.

Cement stored in silos made of Air-entrained concrete does not show any Caking.

Some other properties like Alkali-Aggregate rection, Elasticity and Abrasion resistance are also improved with Air-Entrainment.

Optimum air content:

        It depends on :

1.      The purposefor which the concrete is used and its location and climatic condition (b) the maximum;

2.      size of aggregate;

3.      the richness of the mix.

   For floors it is 4% even in cold countries, for RCC it is 3-4%, for mass concrete with 160mm size aggregate it is 2.5-3%.

Pozzolanic Admixtures:

        Usage of pozzolanic material dates back to the era of Romans and Greeks. They used the volcanic ash found near Pozzuoli by adding it to lime and hence the name Pozzolanic materials. The pozzolana when mixed in proper proportion with OPC can improve a number of properties of concrete like:

1.     Lower the heat of hydration and thermal shrinkage.

2.     Increase the watertightness.

3.     Reduce the alkali-aggregate reaction.

4.     Improve resistance to attack by sulphate soils and sea water.

5.     Improve extensibility.

6.     Lower susceptibility to dissolution and leaching.

7.     Improve workability.

8.     Lower costs.

        It is known that when cement and water react calcium hydroxide and C-S-H gel is formed. It is this gel which binds the aggregates together. But the calcium hydroxide has no role and may come out with percolating water in the form of leaching. The pozzolanic materials thus react with this Ca(OH)₂ and form a substance similar to C-S-H gel.

Natural pozzolans:

·         Clay and Shales

·         Opalinc Cherts

·         Diatomaceous Earth

·         Volcanic Tuffs and Pumicites.

Artificial pozzolans:

·         flyash

·         Blast Furnace Slag

·         Silica Fume

·         Rice Husk ash

·         Metakaoline

·         Surkhi.

Flyash:

        Fly ash is finely divided residue resulting from the combustion of powdered coal and transported by the flue gases and collected by electrostatic precipitator. In India, Fly ash was used in Rihand dam construction replacing cement upto about 15 per cent. Apart from technical advantages usage of flyash reduces pollution in two ways:

·         Flyash is a waste from generated power plants. Every year we produce 75MT of flyash which needs to be disposed. Its usage in concrete helps solve this problem to certain extent.

·         7% of worlds CO₂ is attributable to OPC production. This can be reduced by replacing cement with fly ash.

                       Fly ash was used in the Rihand irrigation project.

Usage:

        There are two ways that the fly ash can be used:

·         one way is to intergrind certain percentage of fly ash with cement clinker at the factory to produce Portland pozzolana cement (PPC);

·         the second way is to use the fly ash as an admixture at the time of making concrete at the site of work.

 The latter method gives freedom and flexibility to the user regarding the percentage addition of fly ash. The quality of fly ash is governed by IS 3812 - part I - 2003. High fineness, low carbon content, good reactivity are the essence of good fly ash. One of the important characteristics of fly ash is the spherical form of the particles. This shape of

particle improves the flowability and reduces the water demand.

Effect on fresh concrete:

      

         Use of right quality flyash, results in reduction of water demand for desired slump. With the reduction of unit water content, bleeding and drying shrinkage will also be reduced. Since fly ash is not highly reactive, the heat of hydration can be reduced through replacement of part of the cement with fly ash. Fig shows the reduction of temperature rise for 30% substitution of fly ash.

Effect on hardened concrete:

        Fly ash, when used in concrete, contributes to the strength of concrete due to its pozzolanic reactivity. However, since the pozzolanic reaction proceeds slowly, the initial

strength of fly ash concrete tends to be lower than that of concrete without fly ash. Due to

continued pozzolanic reactivity concrete develops greater strength at later age, which may

exceed that of the concrete without fly ash. The pozzolanic reaction also contributes to making the texture of concrete dense, resulting in decrease of water permeability and gas permeability.

Water proofing admixtures:

         Waterproofing admixtures may be obtained in powder, paste or liquid form and may

consist of pore filling or water repellent materials. The chief materials in the pore filling class are silicate of soda, aluminium and zinc sulphates and aluminium and calcium chloride. These are chemically active pore fillers. In addition they also accelerate the setting time of concrete and thus render the concrete more impervious at early age. The chemically inactive pore filling materials are chalk, fullers earth and talc and these are usually very finely ground. Their chief action is to improve the workability and to facilitate the reduction of water for given workability and to make dense concrete which is basically impervious.

        In some kind of waterproofing admixtures inorganic salts of fatty acids, usually calcium or ammonium stearate or oleate is added along with lime and calcium chloride. Calcium or ammonium stearate or oleate will mainly act as water repelling material, lime as pore filling

material and calcium chloride accelerates the early strength development and helps in efficient curing of concrete all of which contribute towards making impervious concrete.  

                               

          Densi-proof                             cetex                                  water-seal

Gas forming admixtures:

     A gas forming agent is a chemical admixture such as aluminium powder. It reacts with

the hydroxide produced in the hydration of cement to produce minute bubbles of hydrogen

gas throughout the matrix. The extent of foam or gas produced is dependent upon the type

and amount of aluminium powder, fineness and chemical composition of cement, temperature and mix proportions. Usually unpolished aluminium powder is preferred. The

amount added are usually 0.005 to 0.02 per cent by weight of cement which is about one

teaspoonful to a bag of cement. Larger amounts are being used for the production of light

weight concrete.

       The action of aluminium powder, when properly controlled causes a slight expansion in

plastic concrete or mortar and this reduces or eliminates the settlement and may, accordingly,

increase the bond to reinforcing bars and improve the effectiveness of grout, in filling joints.

It is particularly useful for grouting under machine bases. Because very small quantity of aluminium powder is used and as it has a tendency to float on the water, the powder is generally pre-mixed with fine sand and then this mixture is added to the mixer.

Alkali aggregate reaction inhibitors:

        There are some evidences that air entraining admixture reduces the alkali-aggregate reaction slightly. The other admixtures that may be used to reduce the alkali-aggregate reaction are aluminium powder and lithium salts.

Grouting aadmixtures:

        Grouting under different conditions or for different purposes would necessitate different qualities of grout-mixture. Sometimes grout mixtures will be required to set quickly and sometimes grout mixtures will have to be in fluid form over a long period so that they may

flow into all cavities and fissures. Sometimes in grout mixtures, a little water is to be used but at the same time it should exhibit good workability to flow into the cracks and fissures. There are many admixtures which will satisfy the requirements of grout mixture. Admixtures used for grouting are:

    (a) Accelerators

     (b) Retarders

     (c) Gas forming agents

     (d) Workability agents

     (e) Plasticizers.

      Accelerating agents may be used in grout to hasten the set in situation where a plugging

effect is desired. In such a case calcium chloride or triethanolamine can be used.

      Retarders and dispersing agents may be used in a grout to aid pumpability and to effect

the penetration of grout into fine cracks or seams. They include mucic acid, gypsum and a

commercial brand known as RDA (Ray Lig Blinder) etc.

     Gas forming admixtures can be used while grouting in completely confined areas, such

as under machine bases. Aluminium powder is the most commonly used agent.

                      

Bina non-shrinkage                    feb-grout                        cia-grout

Corrosion inhibitors:

        The problem of corrosion of reinforcing steel in concrete is universal. But it is more acute in concrete exposed to saline or brackish water or concrete exposed to industrial corrosive fumes. A patented process by Dougill was used for the North Thames Gas Board in UK, in which sodium benzoate was used as corrosion inhibiting admixture to protect the steel in reinforced concrete. In this process 2 per cent sodium benzoate is used in the mixing water or a 10 per cent benzoate cement slurry is used to paint the reinforcement or both. Sodium benzoate is also an accelerator of compressive strength.

        It is found that calcium lignosulphonate decreased the rate of corrosion of steel embedded in the concrete, when the steel reinforcement in concrete is subjected to alternating or direct current.

Fungicidal, Germicidal and Insecticidal Admixtures:

       

        It has been suggested that certain materials may either be ground into the cement or added as admixtures to impart fungicidal, germicidal or insecticidal properties to hardened cement pastes, mortars or concretes. These materials include polyhalogenated phenols, dieldren emulsion or copper compounds.

Damp proofers:

     (a) Accoproof: It is a white powder to be mixed with concrete at the rate of 1 kg per bag

          of cement for the purpose of increasing impermeability of concrete  structures.

     (b) Natson’s Cement WaterProofer: It is a waterproofing admixture to be admixed at the rate of 1.5 kg per bag of cement.

     (c) Trip-L-Seal: It is a white powder, the addition of which is claimed to decrease permeability of concrete and mortars and produce rapid hardening effect.

     (d) Cico: It is a colourless liquid which when admixed with concrete, possesses the

properties of controlling setting time, promoting rapid hardening, increasing strength and

rendering the concrete waterproof.

    

     In addition to the above the following are some of the commercial waterproofing

admixtures:

    (a) Arzok

    (b) Bondex

    (c) Impermo

    (d) Luna-Ns-1

    (f ) Arconate No. 2

    (e) Sigmet

Surface hardeners:

·         Metal Crete: Metal crete is a metallic aggregate which is tough, ductile, specially processed, size graded iron particles with or without cement dispersing agent. It is claimed that it gives greater wear resistance, corrosion resistance, non-dusting and non-slipping concrete surface.

·         Ferrocrete No. 1: It is a surface hardener and makes the concrete surface compact, dense and homogeneous.

·         Metal Crete Steel Patch: It is a surface hardener. When added 20 per cent by weight of cement, it is supposed to increase the compressive strength and abrasion resistance.

·         Arconate No. 1: It is a black powder composed of iron filings. It is used as surfaceArconate hardener in concrete.

Conclusion:

        Thus each and every property of traditional concrete can be varied as one desires by the addition of any or combination of above stated admixtures. Within a few years there is a possibility that the idea of concrete having only four ingredients will vanquish and the addition of one of the admixtures will become necessary as a fifth ingredient. It will become necessary that all the IS codes regarding concrete will require a revision with the inclusion of these admixtures.