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Concrete Mix Design
INTRODUCTION: Concrete as we know is the basic construction material. Its versatility and mouldability has lent it the advantage of being used in practically all kinds of environment. Concrete can be given any given strength by changing the various percentages of its constituent materials. Mix design is a process to arrive at the percentages of various constituents of the mix to achieve the design strength. Concrete mix design was in started in India with the publication of IS 10262:1982. At that time the code for concreting used to be IS 456:1978 so the mix design method was kept in view of the codal provisions of the 1978 code. With the publication of IS 456:2000 a great many changes have come about in concrete technology. These days mix design is being done using the method of IS 10262:1982 and as far as possible the provisions of IS 456:2000 are being followed. However the 1982 code is under revision and it is widely believed in the engineering fraternity that the new code is going to encompass all possible scenarios where concreting operations are done. Concrete mix design as per IS 10262:1982 As per IS 10262:1982 the method of concrete mix design is as follows: 	Collecting data for mix design. a)	Determining characteristic compressive strength (target mean of concrete) b)	Degree of workability required. c)	Water cement ratio and minimum cement content. d)	Type of aggregate and max. size of aggregate to be used. e)	Standard deviation of compressive strength of concrete. 	Selection of mix proportions. a)	Selection of water cement ratio: The graph for strength of concrete vs. different water cement ratio should be drawn. The graph can also be plotted for different strength of cement and correlating it to strength of concrete as given in Fig. 2 of 10262:1982 b)	Estimation of air content w.r.t max. size of aggregate used. c)	Selection of water content and fine to total aggregate ratio w.r.t max. size of aggregate used. d)	Calculation of cement and aggregate content as per logical formulas given in the code. 	Preparation of Trial mixes.

Preparing the trial mixes and assessing the quality and strength of final product. However the above summarized method has to be modified in line with the developments that have taken place since 1982. Particularly the workability of the mix in this method is given in terms of compacting factor but as per new code it is given in terms of Slump value. Further the ratio of fine to coarse aggregate has also been fixed between 1:1½ to 1:2½ as per the new code. So the design of mix concrete is to be carried out in accordance with the new provisions of 10262:1982 and suitable adjustments have to be made as and when necessary. CONCRETE MIX DESIGN- IT’S PRESENT AND FUTURE At present concrete mix designing is not done in many organizations of our country. In fact wherever concrete is required in bulk quantity, batching plants are installed that produce nominal mix concrete on a large scale. Therefore, only the scale of concrete production gets bigger but the method employed remains the same. This practice cannot be carried on in future. Concrete mix design has some advantages that have been recognized by a number of good organizations and as such they have introduced this method in their construction practices. Some of the reasons why concrete mix design must be carried out are as follows: 1.	By using nominal mix concrete (1:1.5:3) we get no idea of the actual strength of concrete. Many a times the strength may be 50 N/mm2	but we treat it to be 20 N/mm2. The extra strength therefore, goes waste. 2.	All our specifications (BIS and CPWD) restrict the use of nominal mix concrete to minor repair and maintenance work. Its use for RCC is always discouraged. In fact the maximum strength that may be considered for nominal mix has been specified as 20 N/mm2. As such we cannot believe our structures to carry loads more than 20 N/mm2. 3.	As 20 N/mm2 is the maximum strength that we can use for concrete, in places where high strength is required, we are left with no option but to construct heavy structures. This makes our construction heavy and uneconomical. As such our structures never fail due to less strength but due to miscellaneous factors like dampness etc. In an economical structure the failure due to strength and serviceability must occur at the same but this does not happen in our case. 4.	A great number of construction projects are coming up in our country. In such a scenario the quality of aggregates (both coarse and fine) has greatly deteriorated. As companies are willing to buy low quality aggregates, in order to meet their time and cost targets, the suppliers make no effort to improve the quality of materials. In coming times the shortage of good quality materials that can confirm to our requirements is going to get worse. In this situation unless we design the concrete mix we will never be able to achieve the required strength. Even in present times we notice that many construction materials that can confirm to our requirements are not available near our site. So these have to be brought from faraway places. This is making our construction work uneconomical. 5.	With the advent of high strength cements the quality of concrete has undergone a vast change. For making high strength cement, the cement companies have resorted to fine grinding of clinker. Today the fineness of cement is not less than 300m2/kg whereas earlier it used to be of the order of 225 m2/kg. Moreover for achieving high strength greater quantity of C3S is admixed in the cement. This leads to high heat of hydration while ensuring high early strength. So even though concrete may appear strong initially, it develops minor cracks that get enlarged with time and ultimately lead to loss of durability of the concrete. With this kind of cement it is necessary that not more than the requisite amount of cement is used in concrete. However by using nominal mix concrete we have to add 400kg cement in 1 cum of concrete and this is high enough to reduce the durability of the structure. 6.	Lastly all future concrete operations are going to be done after mix design. The specifications are also going to confirm to mix design procedures only. As such sooner or later we will have to introduce mix design in our corporation as well.

FEASIBILITY OF STARTING THIS METHOD

Why has it not been started yet? Concrete mix designing is a complex process that requires knowledge of concrete technology. For making concrete by this method a number of graphs and tables are to be prepared as detailed in 10262:1982. In this situation it is important that a permanent employee of the corporation will have to be deployed for this purpose. Given the shortage in the number of people posted at site, it is very difficult to employ a person permanently for mix design work. Further, mix design has been introduced only in those organizations where mass concreting is done. In our works very little concrete is required and so uptill now mix design has been considered to be uneconomical. How can it be started? The major reason for not starting concrete mix design is that very little concrete is required in our works. However looking at the changing times, immense advantages offered by this method and the cumulative losses that are building up by not using this method, it is natural that sooner or later we will have to shift to this method of concreting. If we examine the present concreting system then we shall notice the following: 1.	The contractor supplies best quality sand from far away locations as local sand does not confirm to our specifications. 2.	The aggregates are generally available nearby. If good quality aggregates are not available, they are also procured from faraway places. 3.	Cement is procured only from approved manufacturers. 4.	Generally the source of sand and coarse aggregate is approved at the starting of the project and all further procurements are made from that source only.

If we carry out mix design we will not have to go to far off places for good quality aggregates as we shall be able to make good concrete (of desired strength) with the locally available aggregates. Further once mix design is done, it remains valid for one year (Cl. No.9.2.3 Pg No.22, 456:2000) provided the sampling of the aggregates has been done efficiently and the source does not change. More often than not the amount of cement required is not as much as is required in nominal mix and so considerable economy can be affected in saving of the cement also. So in all aspects of concrete making we can affect economy by design mix: - use of lesser quantity of cement and use of locally available aggregates while maintaining the required strength of final product.

However, a great number of changes will have to be brought about for carrying out mix design. For this purpose separate agency will have to be deployed for concrete making that will be given the task of supplying concrete to all the other agencies. The agency will have to be financially strong so that work is not held up due to non-availability of concrete. However, deploying separate concrete making agency may not be as difficult as it may seem to be. As we have discussed earlier since the versatility of material to be used for concrete making is more the availability of such materials is never an issue. Moreover only skilled labour is required for concrete making and such labour is easily available. Normally we notice at site that labour for concrete transporting is more in demand as they are required in large numbers but skilled labour is more readily available.

By introducing this method maximum advantage can be affected in mass concreting works. Overall speed of such works will increase. CONCLUSION As is evident there are many advantages in changing our method of concrete making. Given the readiness of corporations of our times to accept new working methods it is natural that mix design shall also be started in everywhere soon. As it is evident, mix design method is itself very old and needs suitable revision in line with the developments of the last 25 years. Further testing equipments that have to be made available for mix design are heavy and bulky. The testing of water content, compressive strength etc. also takes considerable time. Moreover mix design method itself places restrictions on the quality of the construction materials. These and other technical difficulties have to be addressed by the code revision committee to make this method more practical and useful. References: 1.) IS 456:2000	   2.) IS: 10262:1982 3.) IS: 1199:1959 S A F Zaidi Engineer