What is concrete mix design and proportions?

- Concrete mix design may be defined as the art of selecting suitable ingredients of concrete and determining their relative proportions with the object of producing concrete of certain minimum strength & durability as economically as possible.
- Objectives of Mix Design: The purpose of concrete mix design is to ensure the most optimum proportions of the constituent materials to fulfil the requirement of the structure being built. Mix design should ensure the following objectives. (a) To achieve the designed/ desired workability in any given stage. (b) To achieve the desired minimum strength in the hardened stage. (c) To achieve the desired durability in the given environment conditions . (d) To produce concrete as economically as possible.
- Basic considerations of ready mix concrete designs: The following points must be considered while designing concrete mixes: (a) Cost (price per m3 ) (b) Specification (c)Workability (d) Strength and Durability
- The cost of concrete is made up of material Cost, equipment Cost (ready mix concrete trucks, plant, loaders), labour cost. The variation in the cost of materials arises from the fact that cement is several times costlier than aggregates. So it is natural in mix design to aim at as lean a mix as possible. Therefore, all possible steps should be taken to reduce the cement content of a concrete mixtures without sacrificing the desirable properties of concrete such as strength and durability.
- The following point may be kept in mind while designing concrete mixes , Minimum Compressive Strength required, Minimum water/ cement ratio, Maximum cement content to avoid shrinkage cracks, Maximum aggregate / cement ratio, Maximum density of concrete in case of gravity dams.
- The following points related to workability shall be kept in mind while designing concrete mixes. The consistency of concrete should no more than that necessary for placing, compacting and finishing. For concrete mixes required high consistency at the time of placing, the use of water-reducing and set-retarding admixtures should be used rather than the addition of more water. Wherever possible, the cohesiveness and finishibility of concrete should be improved by increasing sand/ aggregate ratio than by increasing the proportion of the fine particles in the sand.
- Strength and durability require lower water cement ratio. It is usually achieved not by increasing the cement content, but by lowering the water at given cement content. Water demand can be lowered by throughout control of the aggregate grading and by using water reducing admixtures.
- The concrete shall be in grades designated in group grade designation Characteristics compressive strength of 150 mm cube at 28 days, N/mm2 Ordinary Concrete M10 M15 M20 10 15 20 Standard Concrete M25 M30 M35 M40 M45 M50 M55 25 30 35 40 45 50 55 High Strength Concrete M60 M65 M70 M75 M80 60 65 70 75 80. The environment where your concrete cures is important, your test cubes strength may vary from the test you may get on the actual job simple because your test cubes has been curing in water which is different from the actual job. This does not mean your job will be inferior than the test cubes, by time it will reach that strength, hence its advisable to put some water just after 24 hours on the concrete for the first 3 days or so for it to cure fast.
- What is M 20 ? M refers to Mix , 20 refers to characteristic compressive strength of 150 mm cube at 28 days in N/mm2. The minimum Grade of Plain Concrete (PCC) shall be 15 N/mm2. The minimum grade of reinforced Concrete ( RCC) shall be 20 N/mm2
- The wide use of concrete as construction materials has led to the use of mixes of fixed proportion, which ensures adequate strength. These mixes are called nominal mixes. They offer simplicity and Under normal circumstances, has margin of strength above that specified. Nominal mix concrete may be used for concrete of grades M5, M 7.5, M10, M15 and M20.
- The concrete mix produced under quality control keeping in view the strength, durability, and workability is called the design Mix. Other factors like compaction equipment’s available, curing method adopted, type of cement, quality of fine and coarse aggregate etc. have to be kept in mind before arriving at the mix proportion. The design mix or controlled mix is being used more and more in variety of important structures, because of better strength, reduced variability, leaner mixed with consequent economy, as well as greater assurance of the resultant quality.
- Factors Influencing Choice of Mix Design: The important factors influencing the design of concrete mix are: Grade of Concrete (10mpa, 15 mpa, 20mpa, 25mpa, 30mpa, 35mpa etc), type of Cement (32.5N, 42.5N, 52.5N), Maximum nominal Size of Aggregate (6mm -22mm), Grading of combined aggregate, Maximum water/ Cement ratio, workability, durability.
- The grade of concrete gives characteristic compressive strength of concrete. It is one of the important factor influencing the mix design. The grade M 20 denotes characteristic compressive strength fck of 20 N/mm2. Depending upon the degree of control available at site, the concrete mix is to be designed for a target mean compressive strength (fck) applying suitable standard deviation.
- The rate of development of strength of concrete is influenced by the type of cement. The higher the strength of cement used in concrete, lesser will be the cement content. The use of 42.5 grade and 52.5 grade of cement, gives saving in cement consumption as much as 15 % and 25 % respectively, as compared to 32.5 grade of cement. For concrete of grade 25mpa-60 mpa it is advisable to use 42.5 and 52.5 grade of cement.
- For heavily reinforced concrete structures as in the case of ribs of main beams, the nominal maximum size of the aggregate should usually be restricted to sum less than the minimum clear distance between the main bars or 5 mm less the minimum cover to the reinforcement, whoever is smaller. The workability of concrete increases with an increase in the maximum size of aggregate. But the smaller size of aggregates provide larger surface area for bonding with the mortar matrix which gives higher strength.
- The relative proportions of the fine and coarse aggregate in a concrete mix is one of the important factors affecting the strength of concrete. For dense concrete, it is essential that the fine and coarse aggregate be well graded. In the case when the aggregate available from natural sources do not confirm to the specified grading, the proportioning of two or more aggregate become essential
- The water/Cement ratio, for any given condition of test, the strength of a workability concrete mix is dependent only on water/cement ratio. The lower the water/Cement ratio, the greater is the compressive strength. Workability of fresh concrete determines the case with which a concrete mixture can be mixed, transported, placed, compacted and finished without harmful segregation and bleeding.
- Durability require low water/Cement ratio. It is usually achieved not by increasing the cement content, but by lowering the water demand at a given cement content. Water demand can be lowered by through control of the aggregate grading and by using water reducing admixtures
- Some of the commonly used mix design methods are: I.S. Method, A.C.I method, Road Note 4 method ( U.K. Method), IRC 44 method, Arbitrary method, Maximum Density method, Fineness modulus method, Surface area Method, Nix design for high strength concrete, Mix design for pumpable Concrete, DOE (British) Mix design method.
- Different bureau of standards recommend different sets of mix designs and procedures for design of concrete mix.
- IS Method of Mix Design: The target average compressive strength (fck) of concrete at 28 days is given by : Fck= f ck + t.s Where, Fck= target average compressive strength at 28 days, F ck= characteristics compressive strength at 28 days, s= Standard deviation, t= a stastical value, depending upon the accepted proportion of low results and the number of tests. According to Is 456: 2000 and IS 1343:1980 the characteristic strength is defined as the value below which not more than 5 percent of results are expected to fall. In such cases the above equation reduced to, Fck= fck + 1.65 s.
- The ACI Method of Mix Design: This method assumes that the optimum ratio of the bulk volume of coarse aggregates and on the grading of fineness aggregates regardless of shape of particles. This method also assumes that even after complete compaction is done, a definite percentage of air remains which is inversely proportional to the maximum size of aggregate.
- The ACI Method of Mix Design :The steps by steps operation in the ACI method are Step-1 Data to be collected. Fineness modulus of FA: Unit weight of dry CA, Specific gravity of FA and CA saturated surface dry condition. Specific gravity of Cement, Absorptions characteristics of both CA and FA
- The ACI Method of Mix Design Step-2. Calculation mean design Strength, from the minimum strength specified, using standard deviation: fm= fmin + K.S Where, F m= Specified minimum strength (Characteristic Strength) K= Constant dependency upon the probability of certain no of results likely to fall. The ACI Method of Mix Design Step-3 Estimation of Water-Cement Ratio, Water Cement ratio is estimated.
- The ACI Method of Mix Design Average Compressive Strength at 28 days Effective Water-Cement Ratio (By Mass) Non-Air Entrained Concrete Air-entrained Concrete 45 0.38 – 40 0.43 – 35 0.48 0.4 30 0.55 0.46 25 0.62 0.53 20 0.7 0.61 15 0.8 0.71
- The ACI Method of Mix Design, The water Cement ratio obtained from Strength point of view is to be checked against maximum W/C Ratio given for special exposure condition given in table 9.11 and minimum of the two is to be adopted. Requirement of ACI for W/C Ratio and Strength for Special Exposure Condition.
- Exposure Condition Maximum W/C ratio, normal density aggregate concrete Minimum Design Strength, low Density aggregate Concrete, MPA Concrete Intended to be Watertight (a) Exposed to fresh Water (b) Exposed to brackish or sea Water 0.5 0.45 25 30 Concrete Exposed to freezing and Thawing in a moist Condition: (a) Kerbs, gutters, guard rails or thin sections 0.45 30 Other elements 0.5 25 In presence of de-icing chemicals 0.45 30 For corrosion protection of reinforced concrete exposed to de- icing salts, brackish water, sea water or spray from the sources.
- Decide maximum size of aggregate to be Used. Generally RCC work 20 mm and Pre-stressed Concrete 10 mm Size are used. Decide Workability in terms of slump for the type of job in hand.
- The ACI Method of Mix Design Type of Construction Range of slump mm Reinforced foundation walls and footings 20-80 Plain footing, cassions and substructure wall 20-80 Beams and Reinforced Wall 20-100 Building Column 20-100 Pavement and Slabs 20-80 Mass Concrete 20-80
- The ACI Method of Mix Design Step-4 Minimum Water Content and entrapped air content: • Decide maximum size of aggregate to be used. Generally for RCC work 20 mm and for pre-stressed concrete 10 mm size are used. Decide workability in terms of slump for the type of job in hand. The ACI Method of Mix Design Step-5 Cement Content • Cement Content is computed by dividing the water content by the water/ Cement Ratio Step-6. Bulk Volume of Dry Rodded Coarse Aggregate per Unit Volume of Concrete. Decide the value of slump and maximum size of aggregate, decide the mixing water content and entrapped air content.
- The British Method: The traditional British method has been replaced by the department of the environment for normal mixes, known as DOE(British) mix design method. The following steps are Involved in DOE Method Step-I. Find the target mean strength from the specified Characteristic Strength. ft= fck + k.S Where, ft= target mean strength fck= characteristic Strength S= Standard Deviation K= risk factor or probability factor.
- ROAD NOTE No. 4 METHOD OF MIX DESIGN Proposed by the Road Research Laboratory, UK (1950) Introduction In this method, the aggregate to cement ratios are worked out on the basis of type of aggregate, max size of aggregate and different levels of workability. The relative proportion of aggregates is worked on basis of combined grading curves. This method facilitates use of different types of fine and coarse aggregates in the same mix. The relative proportion of these can be easily calculated from combined grading curves. The values of aggregate to cement ratio are available for angular rounded or irregular coarse aggregate. CONCRETE MIX DESIGN 156
- Drawbacks Of Road Note No. 4 Method: This method leads to very high cement contents and thus is becoming obsolete. ?In many cases use of gap graded aggregate becomes unavoidable. In many parts of the country the practice is to use 20mm coarse aggregates without 10mm aggregates. This is because of quality of 10mm aggregates produced from jaw crusher is very poor .Gap grading does not fit in to the standard combined grading curves of RRL method. ?Sand available in some parts of country is graded that it is high on coarse fraction (1.18mm and above) and low on fines (600micron and below). It is difficult to adjust the sand content to match any of the standard combined grading curves .The combined grading curve often cuts across more than one standard curves in such cases. Different aggregate to cement ratios are given for different levels of workability ranging from low to high. But these levels of workability are not defined in terms of slump, compaction factor or Vee Bee time as in case of other methods. The fine aggregate content cannot be adjusted for different cement mix contents. Hence the richer mixes and leaner mixes may have same sand proportion, for a given set of materials. More info is available concrete mix designs and propotions, and ref 2. THIS IS OUR OWN RESEARCH AND IS NOWHERE COMPREHENSIVE BUT JUST AN ILLUSTRATIVE TO THE PRODUCTION OF BETTER CONCRETE. SEE ALSO OUR CONCRETE MIX DESIGN TABLE BELOW.