CONCRETE MIX DESIGN

 Concepts of mix design – Statistical quality control of concrete – Mix design as per IS and other methods of mix design.

 CONCEPTS OF MIX DESIGN: It will be worthwhile to recall at this stage the relationships between aggregate and paste, which are the two essential ingredients of concrete. Workability of the mass is provided by the lubricating effect of the paste and is influenced by the amount and dilution of paste. The strength of the concrete is limited by strength of paste, since mineral aggregates with rare exceptions, are far stronger than the paste compound. Essentially the permeability of concrete is governed by the quality and continuity of the paste, since little water flows through aggregate either under pressure or by capillarity. Further, the predominant contribution to drying shrinkage of concretes is that of paste.

Since the properties of concrete are governed to a considerable extent by the quality of paste, it is helpful to consider more closely the structure of the paste. The fresh paste is a suspension, not a solution of cement in water.

The more dilute the paste, the greater the spacing between cement particles, and thus the weaker will be the ultimate paste structure. The other conditions being equal, for workable mixes, the strength of concrete varies as an inverse function of the water/cement ratio. Since the quantity of water required also depends upon the amount of paste, it is important that as little paste as possible should be used and hence the importance of grading. 

 VARIABLES IN PROPORTIONING

With the given materials, the four variable factors to be considered in connection with specifying a concrete mix are:

(a)    Water – cement ratio

(b)   Cement content or cement – aggregate ratio

(c)    Gradation of the aggregates

(d)   Consistency.

 In general all four of these inter-related variables cannot be chosen or manipulated arbitrarily. Usually two or three factors are specified, and the others are adjusted to give minimum workability and economy. Water/ cement ratio expresses the dilution of the paste- cement content varies directly with the amount of paste. Gradation of aggregate is controlled by varying the amount of given fine and coarse aggregate. Consistency is established by practical requirements of placing. In brief, the effort in proportioning is to use a minimum amount of paste (and therefore cement) that will lubricate the mass while fresh and after hardening will bind the aggregate particles together and fill the space between them. Any excess of paste involves greater cost, greater drying shrinkage, greater susceptibility to percolation of water and therefore attack by aggressive waters and weathering action. This is achieved by minimizing the voids by good gradation.

VARIOUS METHOD OF PROPORTIONING

(a)    Arbitrary proportion

(b)   Fineness modulus method

(c)    Maximum density method

(d)   Surface area method

(e)    Indian road congress, IRC 44 method

(f)     High strength concrete mix design

(g)    Mix design based on flexural strength

(h)    Road note No.4 (Grading Curve method)

(i)      ACI Committee 211 method

(j)     DOE method

(k)   Mix design for pump able concrete

(l)      Indian standard Recommended method IS 10262-82

          Out of the above methods, some of them are not very widely used these days      because of some difficulties or drawbacks in the procedures for arriving at the satisfactory proportions. The ACI committee 211 method, the DOE method and Indian standard Recommended method of mix design of pump able concrete has become important. Therefore, only the more popular and currently used methods are described.

Before we deal with some of the important methods of concrete mix design, it is necessary to get acquainted with statistical quality control methods, with are common to all the methods of mix design.

STATISTICAL QUALITY CONTROL OF CONCRETE

            Concrete like most other construction processes, have certain amount of variability both in material as well as in constructional methods. This results in variation of strength from batch to batch and also within the batch. It becomes very difficult to assess the strength of the final product. It is not possible to have a large number of destructive tests for evaluating the strength of the end products and as such we have to resort to sample tests. It will be very costly to have very rigid criteria to reject the structure on the basic of a single or a few standard samples. The basis of acceptance of a sample is that a reasonable control of concrete work can be provided, by ensuring that the probability of test result falling below the design strength is not more than a specified tolerance level.

            The aim of quality control is to limit the variability as practicable. Statistical quality control method provides a scientific approach to the concrete designer to understand the realistic variability of the materials so as to lay down design specifications with proper tolerance to cater for unavoidable variations. The acceptances criteria are based on statistical evaluation of the rest result of samples taken at random during execution. By devising a proper sampling plan it is possible to ensure a certain quality at a specified risk. Thus the method provides a provides a scientific basis of acceptance which is not only realistic but also restrictive as required by the design requirements for the concrete construction.

            The quality of concrete will be of immense value for large contracts where the specifications insist on certain minimum requirements. The efforts put in will be more than repaid by the resulting saving in the overall concreting operations.

            The compressive strength test cubes from random sampling of a mix, exhibit variations, which are inherent in the various operations involved in the making and testing of concrete. If a number of cube test results are plotted on histogram, the results are found so follow a bell shaped curve known as “Normal Distribution Curve”. The result are said to follow a normal distribution curve if they are equally spaced about the mean value and if the largest number of the cubes have a strength closer to the mean value, and very few number of results with much greater or less value that the mean value. However, some divergence from the smooth curve can be expected, particularly if the number of results available is relatively small. Figure shows the histogram and the normal distribution curve respectively. The arithmetic means or the average value of the number of test result gives no indication of the extent of variation of strength. However, this can be ascertained by relating the individual strength to the mean strength and determining the variation from the mean with the help of the properties of the normal distribution curve.

 AMERICAN CONCRETE INSTITUTE METHOD OF MIX DESIGN:

            This method of proportioning was first published in 1944 by ACI committee 613. In 1954 the method was revised to include, among other modifications, the use of entrained air. In 1970, the method of mix design became the responsibility of ACI committee 211. ACI committee 211 have further updated the method (ACI-211.1) of 1991. Almost all of the major multipurpose concrete dams in India built during 1950 have been designed by using then prevalent ACI committee method of mix design.

            We shall now deal with the latest ACI committee 211.1 of 1991 method. It has the advantages of simplicity in that it applies equally well, and with more or less identical procedure to rounded or angular aggregate, to regular or lightweight aggregates and to air-entrained or non-entrained concretes. The ACI Committee mix design methods assume certain basic facts, which have been substantiated by field experiments or large works. They are:

(a)    The method makes use of the establish facts, that over a considerable range of practical proportion, fresh concrete of given slump and containing a reasonably well graded aggregate of given maximum size will have practically a constant total water content regardless of variations in water/cement ratio and cement content, which are necessarily interrelated.

(b)   It makes use of the relation that the optimum dry rodded volume of coarse aggregates per unit volume of concrete depends on its maximum size and the fineness modulus of the fine aggregate as indicated in Table 11.4 regardless of shape of particles. The effect of angularity is reflected in the void content, thus angular coarse aggregates require more mortar than rounded coarse aggregate.

(c)    Irrespective of the methods of compaction, even after complete compaction is done, a defined percentage of air remains, which is inversely proportional to the maximum size of the aggregate.

 The following is the procedure of mix design in this method:

 (a)    Data to be collected:

(i)                  Fineness modulus of selected F.A.

(ii)                Unit weight of dry rodded coarse aggregate.

(iii)               Sp. gravity of coarse and fine aggregates in SSD condition.

(iv)              Absorption characteristics of both coarse and fine aggregates.

(v)                Specific gravity of cement.

Table 11.4. Dry Bulk Volume of Coarse Aggregate Per Unit Volume of Concrete as given by ACI 211.1-91

Note: The values given will produce a mix that is suitable for reinforced concrete construction. For less workable concrete the values may be increased by about 10 percent. For more workable concrete such as pump able concrete the values may be reduced by up to 10 per cent.

(b)   From the minimum strength specified, estimate the average design strength either by using standard deviation or by using coefficient of variation.

(c)    Find the water/cement ratio from the strength point of view from Table 11.5. Find also the water/cement ratio from durability point of view from Table 11.6. Adopt lower value out of strength consideration and durability consideration.

Table 11.5. Relation between water/cement ratio and average compressive strength of concrete, according to ACI211.1-91

            Note: Measured on standard cylinders. The values given are for a maximum size of aggregate of 20 or 25mm and ordinary Portland cement and for recommended percent of air entrainment shown in Table 11.8.

Table 11.6. Requirements of ACI 318-89 for W/C ratio and Strength for Special Exposure Condition

Table 11.8. Approximate requirements for mixing water and air content for different workability’s and nominal maximum size of aggregates according to ACI211.1-91

    (d)   Decide maximum size of aggregate to be used. Generally for RCC work 20 mm and prestressed concrete 10 mm size are used.

(e)    Decide workability in terms of slump for the type of job in hand. General guidance can be taken from table 11.7.

Table 11.7. Recommended Values of Slump for various types of Construction as given by ACI 211.1-91

Note: The upper limit of slump may be increased by 20 mm for compaction by hand.

Table 11.9. First estimate of density (unit weight) of fresh concrete as given by ACI 211.1-91

(f)     The total water in Kg/m³ of concrete is read of concrete is read from table 11.8 entering the table with the selected slump and selected maximum size of aggregate. Table 11.8 also gives the approximate amount of accidentally entrapped air in non-air-entrained concrete.

(g)    Cement content is computed by dividing the total water content by the water/cement ratio.

(h)    From table 11.4 the bulk volume of dry rodded coarse aggregate per unit volume of concrete is selected, for the particular maximum size of coarse aggregate and fineness modulus of fine aggregate.

(i)      The weight of C.A. per cubic meter of concrete is calculated by multiplying the bulk volume with bulk density.

(j)     The solid volume of the coarse aggregate in one cubic meter of concrete is calculated by knowing the specific gravity of C.A.

(k)   Similarly the solid volume of cement, water and volume of the air is calculated in one cubic meter of concrete.

(l)      The solid volume of the sand is computed by subtracting from the total volume of the concrete the solid volume of the concrete, coarse aggregate, water and entrapped air.

(m)  Wight of fine aggregate is calculated by multiplying the solid volume of fine aggregate by specific of F.A.

Table 11.10 required increased in strength (mean strength) for specified design strength (specified characteristic strength) when no tests records are available, according to ACI 318-89