CONCRETE MIX DESIGN – A COMPLETE GUIDE

AN IN DEPTH EXPLANATION ABOUT CONCRETE MIX DESIGN

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Abstract

The concrete mix design in its essential sense refers to the proportioning of the ingredients of concrete(sand,cement,coarse aggregates and water). This knol deals with concreteb mix design methods, what is mix design, different methods of mix design, computer aided mix design etc……

INTRODUCTION

Of course, ours is the age of concrete. The concrete technology is becoming a major branch of civil engineering. It is becoming the backbone of infrastructural development of every country. It has made tremendous advancement in western & eastern world. Though India is lagging behind, we are catching up fast with the rest of world.

                        Concrete and steel are the two most commonly used structural materials. They sometimes complement with one another and sometimes compete each other. Today we are the second largest producers of cement in the world only behind to china. In 2004, the production of cement has crossed 120 million tons. quality of concrete and other cement products made , utilizing over 120 million of cement , to cater for the tremendous infrastructural development, that is taking place in the country making our concrete industry  one of the biggest in monetary terms

                        As engineers, we have to concentrate more on the quality of concrete. Seen against a background of rising awareness about the quality of concrete used in the construction and growing needs to effect economy in the face of ever rising cost of materials. Mix design of concrete assumes great importance. Properly designed mix for a particular application, keeping in mind the specific requirements in terms of workability, durability and strength, and knowing the source and properties of aggregates and type of cement consumption and effect substantial saving in cost. The age old method of specifying concrete by volume is replaced by specifying concrete in terms of grades i.e., M20, M25, and so on. Thus, concrete technology is the science, as well as the art of making concrete to its desirable qualities at the cheapest prices.

APPROCH TO MIX DESIGN

Concrete is essentially a mixture of Portland cement, water, coarse and fine aggregate which consolidates into a hard mass due to chemical reaction between the cement and water. Each of the four constituents has specific function. The coarse aggregates act as filler. The fine aggregates coated with cement fill up the voids between coarse aggregates and, cement in conjunction with water act as the binder.

                  There are numerous methods available for concrete mix design for ordinary concrete. The major methods are listed below

o   IS method

o   ACI method

o   DOE method

o   USBR method

o   Arbitrary proportion

o   Fineness modulus method

o   Maximum density method

o   Surface area method

o   IRC-44 method

o   RRL – method

o   Minimum void method

 

Rational proportioning of the ingredients of concrete is the essence of concrete mix design and its purpose is to ensure most optimum proportions of the constituent materials to meet the requirements of the structure being built

 

The mix design should ensure that, the concrete,

                                                       I.            Complies with the specification of structural strength laid down, which is usually stated in terms of the compressive strength of standard test specimens.

                                                    II.            Complies with the durability requirements to resist the environment in which the structure will serve its functional life

                                                 III.            Be capable of being mixed, transported, compacted and placed as efficiently as possible

                                                 IV.            And last but not the least, be as economical as possible

 It should however be stressed that the data used in selecting should be expected merely to serve as a guide; they should be backed up by personal experience and knowledge of basic principles of concrete mix design. Based on the observations made on the trial mixes, the mix proportions have to be adjusted and the refinement carried to the stage where the optimum proportion have been attained.

                        To sum up “concrete mix design” is still very much a problem of trial-and-error and any calculations based on design data are really only a means of providing, at best, a starting point so that the first test can be conducted.

MAIN METHODS IN DETAIL

    The main methods practiced all over the world are 1). IS Method, 2). USBR Method, 3). DoE Method, 4). ACI Method. We will now investigate in detail the above methods.

AMERICAN CONCRETE INSTITUTE METHOD OF MIX DESIGN

                         This method of proportioning was published by ACI committee 613. In 1954 the method was revised to include the use of air entrained concrete among other modification. It has the advantages of simplicity in that it applies equally well, and with more or less identical procedures to rounded or angular aggregate, to regular or light weight aggregates and to air entrained or non air entrained concretes.

Procedure

  1. Data to be collected

1.      Fineness modulus of selected fine aggregates

2.      Unit weight of dry rodded coarse aggregate

3.      Specific gravity of coarse and fine aggregates in SSD condition

4.      Absorption characteristics of both fine and coarse aggregates

5.      Specific gravity of cement

  1. From the minimum strength specified, estimate the average design strength
  2. Find the water-cement ratio from the strength point of view, find also water- cement ratio from durability point of view from table
  3. Decide the maximum size of aggregate to be used
  4. Decide workability in terms of slump
  5. The total water in Kg/m3 of  concrete is read from table, entering the table with selected slump and selected maximum size of aggregates
  6. Cement content is computed by dividing the total water content by the water- cement ratio
  7. From the table, the bulk volume of dry rodded coarse aggregates per unit volume of concrete is selected for the particular maximum size of coarse aggregate and fineness modulus of fine aggregate
  8. The weight of C.A per cubic meter of concrete is calculate by multiplying the bulk volume with bulk density
  9. The solid volume of C.A in one m3 of concrete is calculated by knowing the specific gravity of C.A
  10. Similarly the solid volume of cement, water and volume of air is calculated in on cubic meter of concrete
  11. The solid volume of sand is computed by subtracting from the total volume of concrete, the solid volume of cement, coarse aggregate, water and entrapped air
  12. Weight of F.A is calculated by multiplying the solid volume of fine aggregates by specific gravity F.A

 

DOE METHOD OF CONCRETE MIX DESIGN

 

                         The DOE method was first published in1975 and then revised in 1988.While the road note No.4 or grading curve method was specifically devel­oped for concrete pavements. The DOE method is applicable to concrete for most purposes, including roads. The method can be used for concrete containing fly ash.

 

Steps involved are

a)      Find the target mean strength from the specified characteristic strength.

b)      Calculate the water-cement ratio.

c)      Decide the water content for the required workability expressed in terms of slump or vee bee time, taking into consideration the size of aggregates and its type from table.

d)     Cement content is calculated simply by dividing the water content by water cement ratio.

e)      Find out the total aggregate content. This requires an estimate of the wet density of the fully compacted concrete. This can be found out for ap­proximate water content and specific gravity of aggregate.

f)       Then proportion of F.A is determined in the total aggregate knowing the workability, maximum size of aggregates and percent of fine aggregates passing through 600µ sieve figure. Once the proportion of F.A is obtained. Fine aggregate content can be calculated by multiplying the proportion to the weight of total ag­gregate.

g)      Then the water of C.A can be found out from this

INDIAN STANDARD RECOMMENDED METHOD OF CONCRETE MIX DESIGN (IS 10262-1982)

 

The bureau of Indian standards, recommended a set of procedures for the design of concrete mix mainly based on the work done in national laboratories. The Mix design procedures are sentenced in IS 10262-1982.The methods given can be applied for both medium and high strength concrete.

The following are the steps involved in IS method

a)      Target mean strength for mix design is calculated.

b)      Water-cement ratio can be found out from a graph showing the relation between strength and water cement ratio.

c)      The air content is estimated from table for the nominal maximum size of aggregate used.

d)     The water content in percent of fines in total aggregate by absolute volume is determined  by the maximum size of aggregates

e)      The cement content per unit volume of concrete may be calculated from free water cement ratio and cement content per unit volume.

f)       Calculation of aggregate is done by the following two formulas

 

V= [w+(c/sc) + (I /p)*(1/1000)

 

Ca= [(l-p)/p]* Far *(Sca Sfa)

 

V=absolute volume of fresh concrete, which is equal to gross volume

                                    (m3) -  the volume of entrapped air.

W= mass of water (kg) per m3 of concrete.

 

C=mass of cement (kg) per m3 of concrete

 

Sc=specific gravity of cement

 

P=ratio of FA to total aggregate by absolute volume.

 

Actual qualities required for mix are calcu­lated by adjusting the proportion for the water content and absorption of aggregates. The calculated mix proportions shall be checked by means of trial batches.

a)     

USBR METHOD OF MIX DESIGN

 

                      In this method of mix design, the water content of air entrained con­crete and the proportions of fire and coarse aggregates are determined for a fixed workability and grade of fine aggregates.

 

The step by step procedure of mix proportioning is as follows

a.       The water cement ratios for the target mean 28 day compressive strength of concrete is determined from table.

b.      Approximate air and water contents and the percentages of sand coarse ag­gregate per cubic meter concrete are determined from the table, for con­crete containing natural sand fineness of modulus 2.75 and having worka­bility of 75 to 100m.

c.       Adjustment of values in water content and percentage of sand or coarse ag­gregate are made as provided in table for changes in the fineness modules of sand ,slump of concrete ,air content ,water-cement ratio and sand con­tent other than the reference values in.

d.      The cement content is calculated using the selected water-cement ratio and the final water content of the mix is arrived after adjustments.

e.       Proportions of aggregates are determined by estimating the quality of coarse aggregate from the table or by computing the total solid volume of sand and coarse aggregate in the concrete mix and multiplying the final percentage after adjustment.

COMPUTER AIDED MIX DESIGN

    Now computers have proven their role as time saver in all areas of life. In civil engineering also the aid of computer is inevitable in structural engineering, geotechnical engineering, transportation engineering etc….. In concrete technology also softwares are commonly used. The field of mix design is unaffected by computer softwares till recent times. But although for effective use of materials to produce most brilliant concreta in an economic range can only be achieved by use of softwares. Usually in concrete industry the consultancy who do mix design have their own softwares to proportion the mixes in any convevient methods. Only one commercial software called “First Mix Express”

The above software is an advanced software priced at $440!!

Alternatively there is free open source software called CONMIXER   . This is a software devoloped by a group of enthisuiastic student in INDIA. Still this software is at alpha stage and has got its first version only which is having a simple user interface and a simple code to start with. This software provides support for designing in 3 methods(IS, ACI, and DOE method). Completely coded in Visual Basic has a simple outlook and self explanatory buttons, one who knowws the basic mix design parameters can easily follow the software and get the results. This is at a devolopment stage, the authors are civil engineering students so they not have enough knowledge in software architecture, so good programming and data base system is not used in the program. one who are ready to help us can contact sreejithpro@gmail.com or send a request in the official registered site in sourceforge.net.


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