Five factors affecting the workability due to admixture in concrete pdf

Download Now
Five factors affecting the workability due to admixture in concrete pdf
Five factors affecting the workability due to admixture in concrete 

Factors Effecting the Workability due to Admixture in Concrete

Concrete chemical admixtures are used to improve the fresh and hardened properties of mortar or concrete in different applications. Their addition can allow for the reduction of the water to cement ratio (w/c), without affecting the workability of the mixture.

  • Type of superplasticizers
  • Dosage 
  • Mix composition 
  • Variability in cement composition and properties 
  • Mixing procedure 
  • Equipments 
  • Others 

Type of Superplasticizers 

It is a well established fact that the average molecular weight of the plasticizer is of primary importance for its efficiency as plasticizer in concrete. The higher the molecular weight, the higher is the efficiency. However, it should be noted that there is a maximum value of molecular weight beyond which efficiency is expected to decrease. It may be further noted that several intrinsic properties of the superplasticizers may influence the performance. Therefore, it is difficult to compare the efficiency of one plasticizer from the other in the absence of number of related properties of superplasticizers.

Dosage. 

It has been already explained while describing the Marsh cone test that the dosage of superplasticizer influences the viscosity of grout and hence the workability of concrete. The optimum dosage can be ascertained from Marsh cone test if brand of cement, plasticizer and w/c ratio is already fixed. Simple Marsh cone test can give realistic dosage than manufacturers instructions which is general in nature. In our country generally low dosage is adopted for normal concreting operations. 

types of superplasticizers effect on concrete
types of superplasticizers effect on concrete

A dosage more than 2.5% by weight of cement is rarely used. But in other countries much higher dosages up to 4 to 5% are used in special situations. It has been reported in literatures that upto a dosage of about 3% there are no harmful effect on the hardening properties of concrete. Higher dosage is said to have affected the shrinkage and creep properties. 

Mix Composition. 

The mix composition particularly the aggregate/cement ratio or richness of the mix, w/c ratio, and use of other supplementary cementing materials like fly ash or silica fume affects the workability. Wetter the mix better is the dispersion of cement grains and hence better workability. The size and shape of aggregate, sand grading will also have influence on the fluidifying effect. 

Read also - portland pozzolana cement pdf (PPC GRADE)

Variability in Cement Composition. 

The variability in cement with respect to compound composition, in particular C3A content, C3S/C2S ratio, fineness of cement, alkali content and gypsum content are responsible for the lack of compatibility with a particular type of superplasticizer and their performance in concrete. Out of the above C3 A content will have over-riding influence on the performance of superplasticizer.

Mixing Procedure 

Plasticizer must be properly and intimately mixed in concrete to bring about proper dispersion with cement particles. Therefore, hand mixing is out of question. When you use concrete mixer, generally about 80% of the total water is added to the empty drum and then materials are loaded into the drum by hopper. When you use superplasticizer, it is better to add all the water to the drum keeping about one litre of water in spare. The exact quantity of superplasticizer is diluted with that one litre of water and thrown into the drum in two or three installments over the well mixed concrete so that proper dispersion of plasticizer actually takes place in the drum. Having added the plasticizer, the concrete must be mixed for about one more minute before discharging. The practice of adding superplasticizer along with the bulk mixing water is not giving good results. Experimental result showed that adding plasticizer after three minutes of mixing has yielded better results. 

Variability in cement composition and properties

It has been found that all over India, electrically operated small laboratory mixer is used for conducting laboratory trials. These laboratory mixers are inefficient and they do not mix the concrete ingredients thoroughly, leave apart the efficient mixing of superplasticizers. The results obtained from the trials, using laboratory mixer, is far from realistic. In such situations the following procedure gives better consistent results. 

Add all the calculated quantity of water into the drum. Then add all the quantity of cement and sand. Mix these ingredients very well. When they are well mixed, add the calculated quantity of plasticizer and thoroughly mix them together. You will notice the full action of plasticizer in fluidifying the mix. Then you add the course aggregates and mix them for another one minutes. When the mixer is not efficient as in the case of laboratory mixer, the above procedure of mixing will give good results.

Equipment. 

It has been discussed above that inefficient spot mixer at site or laboratory that is small mixer does not exploit the action of superplasticizer fully, often they do not even mix the concrete uniformly and properly. The fabrication of concrete mixer is not a simple job. The shape of the drum, its bottom diameter and shape, number of blades, the angle of blades, length and depth of blades, the space between drum and blade and space between the blades will have lot to do with the mixing efficiency. The manufacturers often neglect these details. Generally pan mixer show better efficiency particularly in case of small scale laboratory mixers. Such small capacity pan mixers are not generally available for trial. Some manufacturers of concrete mixers have now started fabrication and supply of pan mixers.

The mixers in the batching plant are of capacity half a cubic meter and above. They are generally of pan type. They are well designed and fabricated and as such every efficient. The mixing time is around 20 seconds. Within this short spell of 20 seconds, very intimate mixing is done. It is observed that for identical parameters concrete mixed in the batching plant gives about 20 to 30 mm more slump than trial mix carried out in laboratory using small, inefficient mixers. 

Others. 

The slump value of a superplasticized concrete may also be affected on account of other admixtures used concurrently in concrete such as air-entraining agent, fly ash, slag or silica fume. The temperature and relative humidity also affect the result. 

Read also - Theory of magnetic compass & requirements of a magnetic needle -pdf

Site Problems in the use of Superplasticizers 

Some of the practical site problems in the use of superplasticizers are listed below: 

  • Slump of reference mix. (i.e., concrete without plasticizer) 
  • Inefficient laboratory mixer for trial.
  • Sequence of addition of plasticizer.  
  • Problem with crusher dust.
  • Problem with crushed sand.  
  • Importance of shape and grading of coarse aggregate. 
  • Compatibility with cement. 
  • Selection of plasticizer and superplasticizer. 
  • Determination of dosage.
  • Slump loss. 
  • How to reduce slump loss.
  • Casting of cubes. 
  • Compaction at site. 
  • Segregation and bleeding. 
  • Finishing. 
  • Removal of form work

It is not intended to discuss each of the above points separately as some of the problems are self explanatory. The points mentioned above are discussed in general and where applicable, solution to the problems are also indicated, for general guidance. 

It is mentioned earlier that very stiff zero slump concrete can not be perceptibly improved at nominal dosage. Although there is improvement in rheology of matrix with the use of superplasticizers, it does not become perceptible and measurable by slump test. If the initial concrete mix is designed in such a way as to have about 2 to 3 cm slump, then only the slump could be enhanced to a high level. High dosage of plasticizer may give good slump in case of a stiff mix. But it is uneconomical. Initial slump in the reference concrete is an important consideration. 

Generally available laboratory mixers are inefficient especially when small quantity of plasticizers are used for trial mix. Use of pan mixer will give better results. 

The sequence and method of addition of superplasticizer is described earlier for good results. 

In the years to come for large projects one will have to go for crushed sand. In spite of the modern well designed crushers, higher quantity of dust in generally present. This dust interferes with plasticizing properties of mix and hence anticipated results are not obtainable. For the construction of Mumbai-Pune express highway, they specified only the crushed sand. The initial trials presented lot of problems with the presence of excess of crusher dust when reasonable doses of superplasticizer were used. In some sectors, some contractors had to go for combination of natural sand and crushed sand. Whereas some other contracting firm managed to use only crushed sand. Whatever it is, the dust in crushed sand affects the performance of plasticizers. Incidentally, excess of crusher dust increases drying shrinkage.

For normal strength concrete upto 30 or 40 MPa, the shape of aggregate is not of primary importance. For production of high strength concrete of the order of 50 MPa or 60 MPa, the w/c ratio become so low that shape of aggregates becomes very important and also the use of superplasticizers becomes essential for the requirement of workability, particularly when concrete is to be transported over long distance and pumped. In one situation in the construction of high rise building at Mumbai where 60 MPa concrete was used, well graded, cubical shaped aggregate, specially manufactured, could solve the problem. In one of the project sites at Delhi, where aggregates flakiness index was very high, particularly in 10 mm aggregate, the achievement of high slump was found to be difficult in spite of using high dosage of superplasticizers. 

Site Problems in the use of Superplasticizers

In many sites, compatibility problem with cement and plasticizer becomes primary considerations. This can be solved by simple Marsh cone test. Marsh cone test also indicates the economical dosage. Superplasticisers are costly and they operate at higher dosages. If the slump value at a point of batching and the slump value at a point of placing is known, by conducting a few field trials, it is possible to arrive at a decision whether plasticizer would be sufficient or one should go for superplasticizer. Site trials are also required to find out the dosage, the slump value and probable slump loss. 

Post a Comment

0 Comments