The admixtures can be broadly classified according to their characteristic effect as follows: 1. Accelerating Admixtures 2. Accelerator Plasticizer 3. Retarders 4. Oil Wells 5. Retarding Plasticizer (Water Reducing Admixtures) 6. Air Entrainment 7. Air Contents 8. Gas Forming Agents 9. Damp and Water Proofing Admixtures 10. Workability Agents and a Few Others.
1. Accelerating Admixtures:
Sometimes conditions require to shorten the time of set and to increase the rate of hardening for early strength development in concrete. This effect can be obtained by using certain substance in the concrete known as accelerator. An accelerator also serves as an antifreeze agent by increasing the rate of heat evaluation and causing concrete to set before frost-damage could occur.
Accelerators are used to increase the rate of early hardening for the following purposes:
1. Permitting earlier removal of form work.
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2. Shortening the curing period.
3. Opening the structure for use at the earliest opportunity.
4. Off-setting the retarding effects of low temperatures.
5. For compensating the retarding effects of some other admixtures.
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Accelerating Agents:
Usually chlorides of calcium, aluminium and sodium, the sulphates of sodium and potassium, caustic soda, caustic potash, certain silicates, carbonates, triethanolamine and fluosilicates etc. are used for accelerating and hardening agents in concrete. Out of the above substances calcium chloride is the most commonly used accelerating and antifreeze admixture. It has also been found that the use of CaCI2 increases the resistance of concrete to erosion and abrasion and improvement persists at all ages.
Calcium chloride may be added to rapid hardening as well as ordinary Portland cement. The more rapid the natural rate of hardening of the cement, the greater is the action of the accelerator. However calcium chloride should not be used with high alumina cement. Most probably the action of calcium chloride is that of a catalyst in the reactions of hydration of C3S and C2S. The reduction in the alkalinity of the solution promotes the hydration of the silicates. By the addition of calcium chloride the hydration of C3A is delayed somewhat, but the normal process of hydration of cement is not changed.
As per IS 456-1964, 1½ % of calcium chloride by weight of cement is sufficient to increase the hardness of concrete, but many other researchers have advocated the use upto 2 % of calcium chloride to produce the desired results. Although 2% is a conservative maximum value, but during freezing weather with cold materials, calcium chloride upto 4% has been used. However, more than 4% of calcium chloride has been found to have adverse effect on the properties of concrete. The maximum amount of calcium chloride in relation to temperature is suggested as follows Table 6.1.
The most effective amount of calcium chloride was found 2 percent at 40 to70°F and 1½ percent at 90°F. Further increase in workability was found with the addition of calcium chloride upto 3 percent.
By the use of 2% calcium chloride increase in compressive strength of concrete was found 17% at 1 day, 30% at 7 days and about 10% at 28 days. The increase of about 10% in compressive strength is found at all ages (upto 2 years). The increase in cement mortar of 1:2.6 mortar mix was found as shown in the following Table 6.2.
Fig. 6.1 shows the effect of adding 2% calcium chloride on the compressive strength of concrete for two different curing conditions.
Further it has been observed that the use of 2% calcium chloride increases the rate of heat evolution at early ages and reduces the setting time to about 1/3rd of the normal setting time of cement. The addition of each 1 % anhydrous calcium chloride affects the rate of hardening as much as a rise of 6°C (11°F) temperature. Fig. 6.2 shows the relation between the setting time of portland cement and calcium chloride content (percentage by weight of cement).
Some researchers have suggested the acceleration in setting time of cement by the use of CaCl2 as shown in Table 6.3.
Using 2% calcium chloride at 20°C or high early strength cement, concrete roads can be opened for traffic after 1 day curing, while under normal conditions they cannot be opened before 4 days curing. Raising temperature higher than 20°C, the reduction in duration of curing can further be reduced, which is more important in the case of aerodrome and bridges.
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Calcium chloride can be used most advantageously in cold weather to reduce the time of wet curing at normal temperatures. However concrete with calcium chloride should not be used during hot weather as rapid evaporation at exposed surfaces may cause shrinkage and cracking. Hence it is essential to protect the surface with in first few hours. Later when concrete cools, larger thermal tensile stresses develop due to the contraction of concrete and cause rupture in the body of the structure.
Further the use of calcium chloride has been found to increase the corrosion of reinforcing steel and increase of drying shrinkage upto 40% of normal concrete. It may also increase the efflorescence on the surface of the concrete. Resistance to sulphate attack is also found to be reduced by the addition of calcium chloride.
Thus CaCl2 should not be used in the following situations:
1. In pre-stressing concrete specially concrete in contact with pre-stressing wires.
2. Reinforced concrete to be steam cured, as the use of calcium chloride has been found to have severe corrosion of reinforcement steel.
3. Concrete having alkali reactive aggregate.
Thus it is advised that test should be carried out on concrete before use.
Calcium chloride is available in anhydrous (water free) form or in a hydrated form (CaCl2 2H2O), containing 67 to 72% of calcium chloride by weight and the remaining 33 to 28% water. The amount of water should be taken into account while using it in concrete. It should be remembered that CaCl2 should be uniformly distributed throughout the mix. It can be achieved by dissolving it in the mixing water before use in the mixer. CaCl2 being hygroscopic should be stored in dry place.
2. Accelerator Plasticizer:
To accelerate the strength development of concrete some ingredients are added to the plasticizers or super plasticizers. Such admixtures are known as accelerator plasticizers. The addition of such accelerator plasticizers to the concrete results in the development of strength in the concrete at a faster rate.
The accelerating materials added to plasticizers or super plasticizers are as follows:
1. Calcium nitrite.
2. Nitrates and flousilicate.
3. Triethenolamine chloride.
The accelerating plasticizers or super plasticizers manufactured by reputed firms are chloride free.
Till recent past calcium chloride (CaCl2) was one of the commonly used material as accelerator. Nowadays it is not used and some of the soluble carbonates, silicates, flousilicate and some organic compounds such as triethenolamine are used. Flousilicates and triethenolamine are comparatively costly accelerators.
The recent studies have shown that calcium chloride (CaCl2) as accelerators is harmful to the reinforced and pre stressed concrete. However it can be used for plain cement concrete in comparatively high dose.
Some of the new accelerators produced are so powerful that with the use of these accelerators it is possible to get the cement set as hard as a stone within 5 minutes or less.
With the availability of such powerful accelerates concreting under following situations has become very easy, as:
1. Concreting under water.
2. Repair work to the water front structures in the region of tidal variations.
3. Basement water proofing operations
4. In fabricating factories.
5. Concreting in cold weather conditions, as such accelerators can be used upto – 10°C temperatures.
3. Retarders:
Admixtures capable of delaying or prolonging the setting of cement paste in concrete are called retarders. The use of retarders slows down the chemical process of hydration, so that concrete may remain in plastic state and workable for longer period. Retarders are used primarily to offset the accelerating and damaging effect of high temperature and keep concrete workable during the whole period of placing so that construction joints may not develop.
Retarders are used usually in the following situations:
1. In situations where concreting is to be done in large number of lifts without development of construction joints.
2. Pumping cement grouts in oil wells.
3. In situations where concrete has to be hauled for long distances and it is required to keep concrete in plastic and place-able condition.
4. To eliminate the tendency of some cements to develop false set.
5. To obtain aggregate look on the exposed face of the concrete.
Materials Used as Retarder:
A number of substances can be used as retarders. Perhaps the most common retarder is calcium sulphate. It is inter ground to retard the setting of cement. The use of gypsum for retarding setting time should only be recommended when adequate control and inspection is available. The quantity of gypsum to be used should be determined carefully for the given job, otherwise it may cause undesirable expansion and undue delay in setting of concrete.
Another most effective retarding agent is common sugar. It can be used for delaying the setting time of concrete without any harmful effect on the ultimate strength of concrete. Some researchers say that at normal temperature addition of 0.05% sugar by weight of cement retards the setting time up to 4 hours and 0.2% sugar by weight of cement hydration is retarded to such an extent that final set may not take place even within 72 hours.
While some other researchers say that sugar from 0.05% to 0.15% by weight of cement retard the setting time of cement and early strength of concrete also is reduced but the 28 days strength improves. According to them the use of 0.2% sugar by weight setting is accelerated. Further increase in sugar quantity results in rapid setting and reduction in 28 days strength.
4. Oil Wells:
Sometimes oil wells are drilled upto a depth of 6 kilometer (6000 m) depth, where the temperature may be about 205°C. Some times through this depth stratified, fissured or porous strata may be encountered, which needs to be cement grouted.
In addition to this, annular spacing between the tube and the walls of the well will have to be sealed with cement grout to prevent the entry of oil and gas into tome strata. For all these works cement grout should remain in fluid or mobile condition for about 3 to 4 hours even at high temperature without getting set. For this purpose sugar is one of the good retarder.
Other materials used successfully as retarding agents are as follows:
1. Ligno-sulphonic acids and their salts.
2. Hydroxylated carboxylic acids and their salts.
These agents in addition to the retarding effect also reduce the water quantity required for a given workability, which increases the 28 days compressive strength of concrete by 10 to 20%. In addition, materials like calcium acetate, Mucic acid, starches, carbohydrate derivate, soluble Zinc salts and borates ammonia, sodium bicarbonate and some commercial products are used as retarding agents.
These days’ admixtures are manufactured to combine set retarding and water reducing properties. These admixtures are the combination of conventional water reducing agents and sugar or hydroxylated carboxylic acids or their salts. These materials affect both setting time and rate of development of strength.
These are shown in Table 6.6 below:
5. Retarding Plasticizer (Water Reducing Admixtures):
All the plasticizers and super plasticizer developed show retardation to some extent by themselves.
Many a times the extent of reduction of setting time developed by admixtures is found inadequate. Instead of adding retarders separately, they are mixed with plasticizers or super plasticizers at the time of commercial production. Such productions are known as retarding plasticizers (ASTM Type-D) or retarding super plasticizers (ASTM Type-G).
Retarding plasticizers or super plasticizers are used in the ready mixed concrete industry for retaining the slump loss during high temperatures and along transportation to avoid construction joint, regulation of heat of hydration and slip form construction etc.
The selection of such readymade retarding admixtures should be done very carefully. Due to the heterogeneous nature and different molecular weight of different retarders used with plasticizers they tend to separate out or settle down. Generally it happens when sugar solution is used as retarder, but when gluconate is used as retarder, such separation or settlement does not take place. Thus such retarding plasticizers should be well shaken or stirred before use. There are cases when settlements of retarders from the rest of the ingredients have caused excessive retardation resulting failure of the structure.
Further it has been observed that the effectiveness of an admixture depends on the time when it is added to the mix. A delay of even 2 minutes after water has come into contact with the cement increases the retardation. The increased retardation occurs specially with cements which have a high C3A content, because once some C3A has hydrated, it does not absorb the admixture and the more of it is left to retard the hydration of the calcium silicates (C3S and C2S). Retarders tend to increase the plastic shrinkage due to extended plastic stage, but drying shrinkage is not affected.
Certain materials are also available as surface retardants. These are applied as a liquid to the inside of the form work. When the shuttering is stripped, the soft surface of the cement is brushed to give the exposed aggregate surface. Thus an architectural finish of exposed aggregate surface can be obtained. The main difficulty in this method is to produce uniformity over the surface and the effect is often patchy. Great care must be exercised while using such materials and advance experiments must be conducted before using them.
6. Air Entrainment:
Since 1930, one of the most advancement made in concrete technology is the discovery of air entrained concrete. Since then there has been an increasing use of air entrained concrete all over the world, especially in U.S.A. and Canada. Due to the merits of air entrained concrete, about 85%. Concrete manufactured in U.S.A. contains air entraining agent. Now air entraining agent is considered as a fifth ingredient of concrete.
Air entrainment in concrete can be defined as the deliberate incorporation of air by means of a suitable air entrain agent. The entrained air should be clearly distinguished from accidentally entrapped air. The difference between the entrained air and entrapped air can be recognised from the difference in the size of their bubbles. The size of the entrained air varies from 5 micron to 80 micron while the size of entrapped air may vary- from 10 to 1000 micron.
Secondly the entrained air is deliverable incorporated in the concrete in the shape of spherical bubbles distributed evenly in the entire mass of the concrete, while entrapped air is present in the voids of concrete due to insufficient compaction. The entrapped air voids may be of any shape and size. Normally they are formed in between aggregate surfaces. Also they are not uniformly distributed throughout the concrete
7. Air Contents:
For each mix there must be a minimum volume of voids required for protection from frost. On the basis of his experiments, KLIEGER found that the minimum volume of voids should be 9 percent of the volume of mortar and the air must be distributed uniformly throughout the cement paste. The actual controlling factor is the spacing of air bubbles i.e. the thickness of cement paste between adjacent air voids. A spacing of 0.25 mm between the voids is found satisfactory for full protection from frost damage. In Germany it is taken as 0.2 mm.
To be more effective against frost damage, the air bubbles should be as small as possible. Their size depends to a large degree on the foaming process. Also the voids are not all of one size therefore it is convenient to express their size in terms of specific surface. For air entrained concrete of satisfactory quality the specific surface (square mm per cubic mm) of voids may vary from 15 to 24 mm-1 and the specific surface of accidental air is less than 12 mm-1.
The size of entrained air bubbles varies from 0.05 mm to 1.25 mm. For a given air content of the concrete, the spacing of air bubbles depends on water/ cement ratio of the mix. Typical values of the amount of air required for 0.25 mm spacing for different mixes are given in the following table based on powers results.
8. Gas Forming Agents:
Gas forming agent in concrete is a chemical admixture. Aluminium powder, Zinc, magnesium powder and hydrogen peroxide may be used as gas forming agents in concrete. Generally aluminium powder is used as a gas forming agent in concrete. Aluminium powder reacts with hydroxide produced during the process of hydration of cement resulting in the production of minute hydrogen gas bubbles throughout the cement paste.
The extent of gas production depends upon the following factors:
1. Type and amount of aluminium powder
2. Chemical composition and fineness of cement
3. Mix proportion
4. Temperature
The amount of aluminium powder usually added varies from 0.005 to 0.02% by weight of cement i.e., one tea spoonful powder per bag of cement.
Properly controlled action of aluminium powder causes slight expansion in plastic concrete or mortar.
The expansion of plastic concrete has the following effects on the concrete properties:
1. It reduces the settlement of concrete ingredients.
2. It increases the bond of cement paste with reinforcing bars.
3. It improves effectiveness of cement grout filling in joints.
It is particularly useful for grouting under machine bases. The effect on strength depends whether or not the concrete is restrained from expansion. If it is restrained, the effect on strength is negligible and if not restrained the loss of strength is considerable. Thus to get good results it is important that form work be tight and the grout confined completely.
In hot-weather, the action of aluminium powder is very quick and its beneficial effect is lost, where as in cold weather the action is very slow and desired results may not be produced before the concrete has set.
To produce the same amount of expansion at 4°C as at 21°C, twice the amount of aluminium powder is required. As the very small quantity of aluminium powder is used, and it has a tendency to float on water, it should be pre mixed with sand and then this mix should be added to the mixer.
Aluminium powder is also used for the production of light weight concrete. For this purpose about 100 gram of aluminium powder is used per bag of cement. Sometimes Sodium hydroxide or tri sodium phosphate is used to accelerate the reaction.
By varying the proportion of aluminium powder depending upon the temperature and controlling the gas formation, light weight concrete in a wide range of density, may be produced.
9. Damp and Water Proofing Admixtures:
In practice one of the most important requirements of concrete is that it must be impervious to water under the following conditions:
1. When concrete surface is subjected to water pressure on one side.
2. The concrete should be impervious to the absorption of surface water by capillary action.
To achieve the above objectives, the use of well-chosen admixtures has been accepted. Here it is to be noted that the use of admixture is not the substitute for bad materials, bad design or workman ship. An admixture cannot compensate for cracks or large voids in concrete causing permeability.
Water proofing agents may be obtained in liquid, paste or powder form and may consist of water repellent or pore filling materials.
Pore Filling Materials:
Following are the main pore filling materials:
(a) Silicates of soda
(b) Aluminium and Zinc sulphates
(c) Calcium and aluminium chlorides.
These materials are chemically active pore fillers. In addition to this, they also act to accelerate the setting time of concrete, rendering the concrete more impervious at early age.
The chemically inactive pore filling materials are earth fullers, chalk and talc etc. These materials are ground very finely.
Their main action is:
1. To improve the workability
2. To reduce the water requirement for the given workability
3. To make the concrete dense this is basically impervious
Water Repelling Materials:
In this group of admixtures, following materials are included.
Resins, fats, waxes, soda, potash and calcium soaps and coaltar residue etc.
Some times in some of the water proofing admixtures inorganic salts of fatty acids usually ammonium or calcium stearate or oleate along with lime and calcium chloride are added. Calcium or ammonium stearate or oleate act mainly as water repelling material, lime as pore filling material while calcium chloride acts as an accelerator for the development of early strength and also helps in efficient curing of concrete. All these factors contribute for the development or making concrete imperious.
Heavy mineral oil free from fatty or vegetable oil has been found to be effective in making concrete water proof. The use of asphalt cut back has been tried in quantities of 2.5, 5 and 10% of the weight of cement. The strength and workability has not been found affected.
10. Workability Agents:
Workability of concrete is one of the most important characteristics.
The improvement of workability has two distinct aspects as follows:
1. It makes the mix more fluid.
2. It improves the character of the mix, so that it becomes more cohesive.
Workability is most important under the following situations:
1. If the concrete is to be placed in deep beams, thin sections and closely spaced reinforcement.
2. If the concrete is harsh due to poor grading or poor characteristics of aggregates.
3. For making high strength concrete with low water/cement ratio.
4. Under special circumstances where special means of placement are required as tremie, chute or pumping equipment etc.
There are many materials available which can improve the workability of concrete.
They can be divided into the following groups:
1. Finely divided materials or powders.
2. Surface active materials including air entraining agents.
3. Plasticizers and super plasticizers.
1. Finely Divided Materials:
These materials are chemically inert. Their main function is to increase the cohesiveness of concrete, if used in appropriate quantity. If used excessively they will stiffen the mix, resulting in lowering the strength. The use of these materials improves the workability, reduces the bleeding, increases the strength of lean concrete and may not increase water requirement and drying shrinkage.
Under this category many materials can be used. The common materials used are bentonite, Kaolite, hydrated lime, fly ash, talc, finely divided silica, diatomaceous earth etc.
The use of these admixtures is desirable under the following conditions:
(a) The aggregate is deficient in grading i.e., deficient in fine material.
(b) The cement has a marked tendency to bleed.
These admixtures are often added to the concrete at the mixer. Hydrated lime usually is used in mortars for brick work. Hydrated lime is also used if the sand is more coarse or sharp.
2. Surface Active Materials:
Surface active materials are of greater importance in practice as they have better effects on the other properties of concrete also. These materials are similar to detergents. These materials reduce the surface tension of water and behave as surface wetting agents. Along with wetting the surfaces of the solid particles more easily, they also produce lathering effect, resulting in the production of minute air bubbles ranging from 0.05 mm to 1.55 mm diameter in the cement paste. Air bubbles make the mix more cohesive and more fluid.
The surface active admixtures are used in small quantities and usually ground with cement or sometimes dissolved in the mixing water for the batch. Usually calcium chloride, some oils, stearates are used as surface active agents. Some proprietary products are also used as wetting agents.
3. Plasticizer and Super Plasticizers:
Almost in all the situations the use of these admixtures is for improving the workability of Concrete.
11. Grouting Agents:
Grouting under different situations needs different qualities of grout mix. Sometimes the grout mixture will be required to set quickly as in situations where the plugging effect is desired. On the other hand as in the case of oil wells, grout mixtures should be in fluid state over a long period, so that it may flow into all cavities and fissure.
Following admixtures are used for grouting mixtures:
(a) Accelerates
(b) Retarders
(c) Gas forming agents
(a) Materials to be used:
For accelerators calcium chloride or tri-ethanolamine can be used. Accelerators are used where a plugging effect is desired.
(b) For Retarders:
Gypsum, mucic acid and RDA (Ray Lig Binder) may be used. They are used where grout is required in fluid state for longer time.
(c) Gas Forming Agents:
Gas forming admixtures are used in grout while grouting is to be done in completely confined areas, such as under machine bases. For gas formation agent, aluminium powder is the most commonly used material, which chemically reacts to form small bubbles of hydrogen and produces expansion of the grout. This expansion eliminates the shrinkage and settlement.
Plasticizers and super plasticizers in powder form have been always used as one of the ingredients of the grout mixture for effective flow-ability and obtaining high strength.
12. Corrosion Preventive Agents:
Though the problem of corrosion of reinforcing steel in concrete is universal, but it is more acute when the concrete is exposed to saline water or to industrial corrosive fumes. Dougill of U.K. got a patent for the use of sodium benzoate as a corrosion preventive agent. In this process 2% sodium benzoate can be added in the mixing water or 10% benzoate cement slurry may be used to paint the reinforcement or both may be applied at a time. Sodium benzoate is also found as an accelerator of compressive strength.
Different agents have been found useful for the prevention of corrosion of concrete reinforcing steel as follows:
1. When the steel reinforcement in concrete is subjected to direct or alternating current, the use of calcium lignosulphonate is found to decrease the rate of corrosion of steel embedded in the concrete.
2. For autoclave products, the use of sodium and calcium nitrate has been found effective to prevent the corrosion of steel of concrete. 2 to 3% of sodium nitrate by weight of cement is found to be quite effective for the prevention of corrosion of reinforcing steel of concrete. There are many commercial admixtures available to prevent the corrosion of steel in the market.
13. Bonding Admixtures:
These admixtures are water emulsion of many organic materials. These admixtures are mixed with cement or mortar grout to be applied to an old concrete surface just before the patching with concrete or mortar to be done. The function of bonding admixtures is to increase the bond strength between the old and new concrete. This procedure is applied for patching up spalled or eroded concrete or for adding thin layers of resurfacing.
The commonly used bonding admixtures are made from natural or synthetic rubber or from organic polymers. Polyvinyl chloride and polyvinyl acetate are important polymers used in the manufacture of bonding admixtures.
Bonding admixtures can be divided into the two groups as follows:
1. Re emulsifiable type
2. Non re-emulsifiable type. This class of bonding admixtures is better suited for external use as it is water resistant.
These emulsions generally are added to the mixtures in proportion of 5 to 20% by weight of cement. Usually bonding admixtures cause entrainment of air and sticky consistency in a grout mixture. The bonding admixtures are effective on sound and clean surfaces.
14. Plasticizers (Water Reducing Agents):
The essence of good quality concrete is the requirement of right workability. Under different situations concrete of different degree of workability is needed. A high degree of workability is required in situations like deep beams, thin sections with high percentage of reinforcement, beam and column junctions, pumping of concrete, tremie concreting, hot weather concreting etc. The conventional methods of improving workability are by improving the gradation or increasing the quantity of fine aggregate or by increasing the cement quantity.
In the field there are limitations and difficulties to obtain high workability under the given set of conditions. In the field in most of the cases generally extra water is added to the concrete unmindful of its ill effects on the properties of the concrete. The use of extra water is very harmful and should never be used. The use of extra water will not improve the inherent good quality such as cohesiveness and homogeneity of the mix, which reduces the bleeding and segregation of the concrete.
Nowadays many water reducing admixtures are available in the market. These admixtures are known as plasticizers and super plasticizers. The combination of organic substances or combinations of organic and inorganic substances which cause reduction in water content for a given workability or give a higher workability at the same water content are known or termed as plasticizer admixtures.
15. Super Plasticizers (High Range Water Reducers):
Super plasticizers are also called high range water reducers. They are of recent origin and relatively are more effective type water reducing admixtures. They were developed in Japan and Germany during 1960 and 1970 respectively. Chemically they are different from normal plasticizers. The dose levels of super plasticizers usually is higher than that of (0.5 to 3.0%) conventional water reducers and (0.18 to 0.47) they have considerably less undesirable side effects on the properties of concrete. As super plasticizers do not reduce the surface tension of water to a great extent, they have not found to entrain significant amount of air in concrete.
The use of super plasticizers can permit the reduction of water in the concrete mix upto 30% without producing any ill effect on the workability of the concrete i.e. without reduction in the workability of concrete, where as normal plasticizers with doses upto 0.1 to 0.4% by weight of cement can reduce water content from 5 to 15%.
Super plasticizers are used for producing flowing concrete to be used in inaccessible locations, floors or where very quick placing is required. A self levelling and self-compacting concrete is called flowing concrete. Super plasticizers are also used for the production of high strength and high performance concrete. With the use of super plasticizers, flowing concrete could be produced with the water/cement ratio as low as 0.25 or even less. The strength of such concrete was found 120 MPa (1200 kg/cm2) or more. The use of super plasticizers also made it possible to use the fly ash, slag and silica fume to produce high quality concrete.