R. Preetha, GVVSR Kishore, Scientific Officer, C. S Pillai, Associate Director, Indira Gandhi Centre For Atomic Research, A. K. Laharia, ADDL. Chief Engineer(c), M. Umashankar, Engineer in Charge Quality Assurance Civil, Nuclear Power Corporation Ltd.

General

Fly ash is a good pozzolanic mineral admixture, which can replace large quantity of cement in the concrete. The positive effects of using fly ash in concrete are better quality of concrete, eco-friendly and preservation of resources. Construction of power plants require huge quantity of concrete and use of fly ash, as pozzolanic material, can improve the durability of structure, thereby enhancing the safety of plants and also economy in the life cycle cost of the plant.

Properties of concrete whether strength or durability depends on constituents, mixing, placing, compaction, curing etc. To get a holistic view on behavior of flyash concrete to be used in major construction of safety structures, study of mixing methods were also included. Mixing method is the specified order in which ingredients of concrete are introduced in the mixer along with the stages of addition, if any, and time of agitation at each stage. Ingredients of Fly Ash based concrete are added into the mixer in different stages and in every stage, it is agitated for a specified period of time.

Mixing is important to achieve desirable homogeneity and performance of concrete mix. Success of a mixing method depends on optimal combination of mixing energy, time, and mixing sequence. However, studying of mixing method of concrete is difficult because there has been no consensus on evaluation criteria for quality of concrete mixing. The material and mixing systems usually mutually interact. It was expected that mixing methods have influence on property of concrete. A study was conducted to assess the impact of mixing method on the fresh state properties, strength and durability of the concrete. Laboratory mixer of 10 x 7 tilting drum types was used for this study.

Identification of Mixing Methods

A multistage mixing sequence with varying time of agitation at each stage was adopted for the study. The details of these mixing methods are given in Table 1.

Effect of Mixing Methods in Behavior of Flyash Concrete

Mixing method I & III are three stage mixing sequence while mixing method II, IV and V are two stage mixing sequence. Coarse Aggregate, Fine Aggregate and Cement was always loaded in the mixer in the first stage itself. Variation was made for Flyash, quantity of water and chemical admixture in the different mixing methods. Mixing method IV and V were identical in terms of loading sequence, the only difference being the time of agitation in second stage. Except mixing method IV, total mixing time in all the methods was maintained as 270 seconds. (4.5 minutes.)

Identification of Mixes

Mix proportions of M50, M35 and M20 grade of concrete with 40%, 50% and 60% cement replacement levels (CRL) respectively for carrying out the study on mixing methods are given in Table 2.

Effect of Mixing Methods in Behavior of Flyash Concrete

Results and Discussion

The properties of fresh and hardened state of concrete mixes produced by different mixing methods for M50, M35 and M20 grade identified mixes are given in Table 3. The results for slump are plotted in figure 1.The compressive strength results of mixes at 14, 28 and 56 days produced by different mixing methods for grade M50, M35 and M20 are given is Figure 2. The RCPT result of these mixes at 28 days and 56 days respectively are given in Figure 3.

Effect of Mixing Methods in Behavior of Flyash Concrete

Effect of Mixing Methods in Behavior of Flyash Concrete
Figure 1: Slump for concrete mixes produced by different mixing methods

Effect of Mixing Methods in Behavior of Flyash Concrete
Figure 2: Compressive strength of concrete mixes produced by different mixing methods

Effect of Mixing Methods in Behavior of Flyash Concrete
Figure 3: RCPT results for concrete mixes produced by different mixing methods

From the mixing method trials for TPC candidate mixes, it is seen that the mixing method IV with duration of 210 seconds was not giving satisfactory results for strength and durability tests hence ruled out. Among the other four, though mixing method of I &V were found to be giving good results when strength and durability tests are considered. Mixing sequence V was found to be satisfactory when fresh, hardened properties and durability aspects are considered together.

Summary

As expected, the mixing method and sequencing has noticeable influence on the behavior of the concrete mixes both in fresh and hardened states. Mixing method, V was finalized for further studies on fly ash concrete, as it is a two stage mixing, and results were satisfactory when fresh, hardened properties and durability aspects are considered together. The optimum mixing method for a given batching plant may be developed before start of the work. The duration of the mixing time corresponding to 270 seconds mixing time for laboratory mixer has to be calculated for equivalent mixing time for automatic batching plant, keeping the mixing energy same.

Acknowledgment

Authors are grateful to the management of NPCIL for financing the project "Development of FLY Ash Concrete Suitable for NPP Structures," BHAVINI for providing the facilities and AERB, NPCIL for manpower and guidance.
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