In this article we will discuss about:- 1. Meaning of Super Plasticizers 2. Classification of Super Plasticizers 3. Uses.
Meaning of Super Plasticizers:
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.
Classification of Super Plasticizers:
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Following polymers are commonly used as base for super plasticizers:
1. Sulphonated melamine formaldehyde condensate (SMF)
2. Sulphonated naphthalene formaldehyde condensate (SNE)
3. Modified Lignosulphonates (MLS)
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4. Other types.
In addition to the above, following new generation super plasticizers are also used in other countries:
(a) Multi-carboxylatethers (MCE)
(b) Copolymer of carboxylic acrylic acid with acrylic ester (CAE)
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(c) Poly carboxylate ester (PC)
(d) Cross linked acrylic polymer (CLAP)
(e) Acrylic polymer based (AP)
(f) Combination of the above.
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The first-four super Plasticizers differ from one another due to the base component or due to different molecular weight. Probably the Sulphonated napthalene formaldehyde condensate (SNF) is more effective in dispersing the cement. It is also found to have some retaining properties. The dispersing action mainly is developed by the sulphonic acid. The sulphonic acid gets adsorbed on the surface of cement particles, making them negatively charged and mutually repulsive.
This action increases the workability at a given water/cement ratio, increasing the slump from 75 mm to 200 mm. The resulting concrete is cohesive and not subjected to excessive bleeding and segregation. The cohesiveness is found if fine aggregate is increased by 4 to 5% and very angular and flaky coarse aggregate is avoided.
Each commercial product has been found to have different action on cements. The dosage of conventional plasticizers does not exceed 0.25% by weight of cement in case of lignosulphanates and 0.1% in case of carboxylic acids. The dosages of super plasticizers like SMF and NSF are used as high as 0.5 to 3.0% as they do not entrain air, but at the relatively high dosage they can produce undesirable effects as acceleration or retardation in setting time. However they increase the air entrainment in concrete.
Plasticizers and super plasticizers are water based. The solid contents may vary to any extent in a product manufactured by different firms.
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New Generation Super Plasticizers:
The super plasticizers are used for the following purposes:
1. To increase the workability of concrete without any change in the composition of the mix.
2. To reduce the water content of mixing water, to reduce the water/cement ratio resulting in increase of strength and durability of concrete.
3. To reduce cement and water contents in the concrete to reduce the cost of production of concrete. The reduction in cement and water contents reduce the creep, shrinkage and heat of hydration.
One of the most important draw backs of the traditional super plasticizers as sulphonated melamine formaldehyde condensates (SMF) or Sulphonated naphthalene formaldehyde Condensates (SNF) and Modified ligno-sulphate (MLS) is the slump loss. The slump loss with time presents a serious limitation on the advantages of these super plasticizers. To overcome this limitation, experiments were conducted in Europe and Japan and they discovered new generation of super plasticizers recently.
The more recently developed all new generation super plasticizers are based on the family of acrylic polymers (A, P). Out of these super plasticizers, the properties of the following two super plasticizers namely carboxylated acrylic ester (CAE) Copolymer and multi-carboxylatether (MCE) are discussed here.
The carboxylated Acrylic Ester Contains Carboxylic (COO) instead of sulphoric (SO3) groups present in the SNF and SMF. In the case of SNF and SMF dispersion of cement grain was considered to be caused by the electrostatic repulsion, but the recent experiments carried out by M. Collpardi et al and Y-0 Tanaka et al did not confirm the above hypothesis for the mechanism of plasticizing action of the acrylic polymers.
The production of repulsive force known as zeta potential produced by acrylic polymers (AP) and in SNF based super plasticizers is shown in Table 6.9, From the table it will be seen that AP based super plasticizers produce negligible zeta potential change (0.3 to 5 mv) in contrast to SNF based super plasticizers (23.0-28.0 mv) in aqueous suspension of cement particles as reported by Sakai E and Daiman M. in 1997.
Fig. 6.21 shows the adsorption of CAE and SNF on cement as a function of polymer dose, as reported by Collpardi M. From the Fig. 6.21 it will be seen that the adsorption of CAE is about 85% and that of SNF as 75% i.e. the adsorption of CAE is much more than SNF. Fig. 6.22 shows the zeta potential of CAE and SNF on cement particles as a function of polymer dose. From this Fig. 6.22 it will be seen that the zeta potential of cement particles mixed with CAE is much lower than SNF.
Super plasticizers based on acrylic polymers show the following characteristics:
1. Flowing concrete can be produced at lower water/cement ratio.
2. The effectiveness of super plasticizer does not depend on the addition procedure (immediate or delayed) as shown in Table 6.10 below.
3. The slump loss is much reduced in comparison of traditional sulphonated super plasticizers.
From table 6.10 it will be seen that slump i.e. workability of concrete increases markedly by adding NSF after mixing for 1 minute. Slump increase is found from 100 to 230 mm, while in case of CAE plasticizers the effect of delayed addition of super plasticizer is inappreciable i.e. the increase is from 230 to 235 mm only.
Fig. 6.23 shows the slump loss of CAE and NSF family super plasticizers. From this Fig. 6.23 it will be seen that with the use of (0.3%) CAE plasticizers the loss in slump is negligible for agitating mix for150 minute where as in case of (0.4%) NSF slump reduces from 230 mm to about 25 mm for agitating about 120 minutes.
Fig. 6.24 shows the effect of curing time on the 28 days compressive strength of concrete. It will be seen from the Fig. 6.24 that in case of CAE super plasticizers the gain in strength is mere even with reduced water/cement ratio than the NSF family super plasticizers. Thus it can be said that the increase in compressive strength is due to the ability of CAE to reduce more water content for the same workability. The effect of super plasticizers is shown with different water/cement ratio on 1 to 3 days strength in Fig. 6.25.
Multicarboxylatether:
For the production of high performance concrete new generation super plasticizer named multicarboxylatether (MCE) is found more effective.
The properties of these super plasticizers are found as follows:
1. The water reduction is found high.
2. At low water/cement ratio their flow ability is found excellent.
3. Lower slump loss with time is observed.
4. Shorter retardation time is observed.
5. Early strength is found very high.
6. The quantity of dose is low.
Problems Encountered at Site in the Use of Super Plasticizers:
Some of the practical problems encountered at site in the use of super plasticizers are as follows:
1. Slump of concrete without plasticizer is known as reference mix.
2. Inefficient laboratory mixer used for trial.
3. Selection of plastizer and super plasticizer.
4. Determination of dosage.
5. Slump loss
6. Reduction of slump loss.
7. Casting of cubes
8. Compaction of concrete at site
9. Segregation and bleeding
10. Sequence of adding plasticizer
11. Problem with crushed dust
12. Problem with crushed sand
13. Grading of coarse aggregate
14. Compatibility with cement
15. Finishing
16. Removal of form work etc.
Out of the above problems maintenance of proper slump at the point of placing concrete is very important.
Hence reduction in slump may be checked by taking any one or more of the following steps:
1. Keeping initial slump high.
2. Using retarders.
3. Keeping low temperature.
4. By using retarder plasticizers or super plasticizers.
5. By using super plasticizer at final point.
6. By repetitive dose etc.
Use of Super Plasticizers:
The super plasticizers are used for the following purposes:
1. To produce more workable concrete than the concrete without the use of super Plasticizer at the same water/cement ratio.
2. For the same workability, the use of super plasticizer permits the use of lower water/cement ratio.
3. The use of super plasticizer also permits the reduction in cement content due to the increase in strength.
4. The use of plasticizers also produces a homogeneous and cohesive concrete without any tendency of segregation and bleeding.