Following factors that have been found affecting the workability of concrete: 1. Type of Super Plasticizer 2. Dosage 3. Mix Composition 4. Variation in Cement Composition and Properties 5. Mixing Procedure 6. Equipment 7. Others.
Factor # 1. Type of Super Plasticizer:
It has been well established that efficiency of a super plasticizer is the function of its molecular weight (molecular weight of plasticizer). Higher the molecular weight, the higher is the efficiency. However beyond a maximum value of molecular weight, the efficiency starts to decline. Further it has also been noticed that some intrinsic properties of the super plasticizers also influence the performance.
Factor # 2. Dosages:
It has been established that the dosage of a super plasticizer influence the viscosity of the mix and hence the workability of the concrete. For a given brand of cement, water/cement ratio and plasticizer, the dosage or quantity of super plasticizer may be decided by MARSH CONE TEST.
For normal concrete work, the dosage in India generally is adopted low. A dosage more than 2.5% by weight of cement is rarely used, where as in other countries 4 to 5% dosages are used. Some researchers have advocated the use of super plasticizers upto a dosage of 3.0% without any adverse effect on the properties of hardened concrete. Higher dosages than 3.0% have been reported to affect the shrinkage and creep properties of concrete.
Factor # 3. Mix Composition:
The aggregate/cement ratio, or richness of the mix, water/cement ratio and the use of other supplementary cementing materials like fly ash or Silica fume have been found to affect the workability of the mix. Wetter the mix, better the dispersion of cement particles resulting in better workability. Sand grading, size and shape of aggregate also influence the workability.
Factor # 4. Variation in Cement Composition:
The variation in compound composition of cement, such as C3A content, C3S/C2S ratio, fineness of cement, alkali and gypsum contents have been found to have different effects with different super plasticizers, especially the C3A content has been found to have more influence on the performance of the super plasticizer. The workability of a cement mortar containing 2% calcium lingo-sulphonate has been found to decrease sharply from 27 mm to 5 mm as the C3A content increases from 3% to 14% as shown in Fig.6.18.
Factor # 5. Mixing Procedure:
The sequence of putting materials into the mixer has been found to influence the workability of the concrete to a great extent. When super plasticizers are to be used in the concrete, hand mixing is not permitted. In case the concrete mixer is used, following sequence of putting the materials in the drum of the mixer has been found to give better results.
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Add all water, in the drum holding back one litre of water. The exact quantity of super plasticizer is added to this one litre of water to have well stirred solution and other materials loaded into the drum and mixed well. Now the solution of super plasticizer is added into the drum in two or three installments over the well mixed concrete.
After adding super plasticizer to the mix, it is further mixed in the mixer for about one minute before discharging. This method has proved to produce better dispersion of super plasticizer. The practice of adding super plasticizer with bulk mixing water has not been found giving good results.
Experiments have shown that adding super plasticizer after three minutes of mixing ingredients has yielded better results. Fig.6.19 shows the effect of adding superplasticizer in water three minutes after mixing with the water. It has been found that throughout India for conducting laboratory trials, small electrically operated laboratory mixers are used. Using laboratory small mixers have not yielded satisfactory results. In such situations, following sequence of putting materials should be adopted.
All the quantity of calculated water is put in the drum. Then all the quantity of cement and sand is put in the drum and ingredients mixed well. After mixing the sand and cement well, the calculated quantity of super plasticizer is added to the mix and mixed thoroughly in the mixer. Then coarse aggregate is added and mixed for one minute more. This procedure is found to give good results.
Factor # 6. Equipment:
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Small mixers have not been found efficient to mix the concrete uniformly and properly. Thus they are unable to exploit the action of super plasticizers fully. Generally pan type mixers have shown better efficiency.
Mixers of pan type of half a cubic metre capacity or more have been found very efficient. The mixing time is about 20 seconds and in this short period, the mixing is very intimate. It is observed that for identical parameters concrete mixing in batching plant give about 20 to 30 mm more slump than mix trials carried out in laboratory using small mixers.
Factor # 7. Others:
The slump value of a super plasticizer concrete is also found affected due to the use of other admixtures along with super plasticizers such as fly ash, slag, silica fume and air entraining agents etc.