A Study of Mix Design Adopted in RMC

Ready mix concrete (RMC) is defined as a concrete manufactured as per the specifications of purchaser at a centrally located batching plant under controlled conditions and delivered at construction, site in a truck mixer, capable of mixing the ingredients of concrete and agitating the mixed concrete during the transportation. In India, the present consumption of cement through these plants is only 5 per cent of the total production of cement. Whereas, in US, 75% of the cement produced is used in RMC. Though delayed, the acceptance of RMC in India is constantly increasing among the end users. Once it is tried, there is high degree of return of customers. There are two principal categories of RMC as under. a) Centrally mixed concrete: In this, the mixing is done at a central plant and the mixed is done at a central plant and the mixed concrete is then transported through a transmit mixer which revolves slowly so as to prevent segregation and undue stiffening of concrete. b) Truck mixed concrete: In this, the materials are batched at a central plant but are mixed in a mixer truck either in transit to the site or immediately prior to the concrete being discharged. This permits a longer haul and is less vulnerable is case of delays.

Use of ready mix concrete, RMC as it is popularly known, started only in early 90‘s in India. Although this concept was very old in developed countries, but it took off in India after the decontrol of cement. Plants that were setup initially were producing RMC for the specific projects only. But now RMC is available to any individual interested to use it in most of the major towns. There are about 90 RMC plants in existence in India producing approximately 40 lakh cubic meter of concrete, setting up of many more plants is in pipe line. Still we can say that RMC is in its infancy in India.

Mix design can be defined as the process of selecting suitable ingredients of concrete and determining their relative proportions with the object of producing concrete of certain minimum strength and durability as economically as possible.

The purpose of designing of mix is two-fold. The first object is to achieve the stipulated minimum strength and durability. The second object is to make the concrete in the most economical manner. Cost wise all concretes depend primarily on two factors, namely cost of material and cost of labor. Labor cost, by way of formworks, batching, mixing, transporting, and curing is nearly same for good concrete and bad concrete. Therefore attention is mainly directed to the cost of materials. Since the cost of cement is many times more than the cost of other ingredients,

The advantages due to good grading of aggregate can also be viewed from another angle. If concrete is viewed as a two phase material, paste phase and aggregate phase, it is the paste phase which is vulnerable to all ills if concrete. Paste is weaker than average aggregate in normal concrete with rare exceptions when very soft aggregates are used. The paste is more permeable than many of the mineral aggregate. It is the paste that is susceptible to deterioration by the attack of aggressive chemicals. In short, it is the paste which is a weak link in a mass of concrete. The lesser the quantity of such weak material, the better will be the concrete. This objective can be achieved by having well graded aggregates. Hence, the importance of good grading.

Grading pattern of a sample of coarse aggregate or fine aggregate is assessed by sieving a sample successively through all the sieves mounted one over the other in order of size, with larger sieve on the top. The material retained on each sieve after shaking, represents the fraction of the aggregate coarser than the sieve in question and finer than the sieve above. Sieving can be done either manually or mechanically. In the manual operation the sieve is shaken giving movements in all possible direction to give chance to all particles for passing through the sieve. Operation should be continued till such time that almost no particle is passing through. Mechanical devices are actually designed to give motion in all possible direction, and as such, it is more systematic and efficient than hand sieving. For assessing the gradation by sieve analysis, the quantity of material to be taken on the sieve is given Table 1.1

F.M. is a ready index of coarseness or fineness of the material. Fineness modulus is an empirical factor obtained by adding the cumulative percentages of aggregate retained on each of the standard sieves ranging from 80mm to 150micron and dividing this sum by an arbitrary number 100. The larger the figure, the coarser is the material. The following limits may be taken as guidance. Fine sand Fineness modulus : 2.2-2.6 Medium sand Fineness modulus : 2.6-2.9 Coarse sand Fineness modulus : 2.9-3.2 Sand having a fineness modulus more than 3.2 will be unsuitable for making satisfactory concrete.

The grading patterns of aggregate can be shown is table or charts. Expressing grading limits by mean of a chart gives a good pictorial view. The comparison of grading pattern of a number of samples can be made at one glance. For this reason, often grading of aggregate is shown by means of grading curves. One of the most commonly referred practical grading curves are those produced by Road Research Laboratory (U.K.) On the basis of large number of experiments in connection with bringing out mix design procedure, Road Research Laboratory has prepared a set of type grading curve for all in aggregate graded down from 20mm 40mm. they are shown in fig. 1.1 and fig. 1.2 respectively .Similar curves for aggregate with maximum size of 10mm and downward have been prepared by McIntosh and Erntory. It is shown in fig.1.3, fig. 1. 4 show the desirable grading limit for 80mm aggregate.

PROPERTIES OF MIX INGREDIENTS- While designing the concrete mix it is very essential to consider the different properties of mix ingredients.

AGGREGATE:

Size: The largest maximum size of aggregate practicable to handle under a given set of conditions should be used. Perhaps, 80 mm size is the maximum size that could be conveniently used for concrete making. Using the largest possible maximum size will result in: ● Reduction of the cement content ● Reduction in water requirement ● Reduction of drying shrinkage. However, the maximum size of aggregate that can be used in any given condition may be limited by the following conditions; ● Spacing of reinforcement; ● Clear cover ● Mixing, handling and placing techniques. Generally, the maximum size of aggregate should be as large as possible within the limits specified, but in any case not greater than one-fourth of the minimum thickness of the member. Aggregates are divided into two categories from the consideration of size ● Coarse aggregate and ● Fine aggregate. The size of aggregate bigger than 4.75 mm is considered as coarse aggregate and aggregate whose size is 4.75 mm and less is considered as fine aggregate. Shape: The shape of aggregates is an important characteristic since it affects the workability of concrete. It is difficult to really measure the shape of irregular body like concrete aggregate which are derived from various rocks. Not only the characteristic of the parent rock, but also the type of crusher used will influence the shape of aggregates, e.g., the rocks available round about Pune region are found to yield slightly flaky aggregates, whereas, good granite rock as found in Bangalore will yield cubical aggregate. The shape of the aggregate is very much influenced by the type of crusher and the reduction ratio i.e., the ratio of size of material fed into crusher to the size of the finished product. From the standpoint of economy in cement requirement for a given water/cement ratio, rounded aggregates are preferable to angular aggregates WATER: Water is an important ingredient of concrete as it actively participates in the chemical reaction with cement. Since it helps to form the strength giving cement gel, the quantity and quality of water is required to be looked into very carefully. In practice, very often great control on properties of cement and aggregate is exercised, but the control on the quality of water is often neglected. Since quality of water affects the strength, it is necessary to go into the purity and quality of water. The permissible limits for solids as per IS 456 of 2000 are as follows:

A mineral additive also called a supplementary cementitious material (pozzolana) is a finely ground siliceous material which, as such, does not possess cementitious property in itself, but reacts chemically with calcium hydroxide Ca (OH)2 released from the hydration of Portland cement at normal temperature to form compounds of low solubility having cementitious properties. The action is termed pozzolanic action. These materials are often added to concrete to make concrete mixtures more economical, reduce permeability, increase strength, or influence other concrete properties, which can be used individually or in combination with Portland or blended cement or as a partial replacement of Portland cement. The pozzolanic materials can be divided into two groups namely, natural pozzolanas and artificial pozzolanas. The typical examples of natural pozzolana are: clay, shales, opaline cherts, diatomaceous earth, and volcanic tuffs and pumicites. The commonly used artificial pozzolanas are: fly ash, blast-furnace-slag, silica fume, rice husk ash, metakaoline, and surkhi.

Strength of concrete primarily depends upon the strength of cement paste. The strength of cement paste depends upon the dilution of paste or the strength of paste increases with cement content and decreases with air and water content. In 1918 Abrams presented his classic law in the form:

S=A/B X

Where, 14,000 Ibs/sq. in. and 7 respectively. Abrams water/cement ratio law states that the strength of concrete is only dependent upon water/cement ratio provided the mix is workable.

The methods that are commonly used to design a concrete mix are: ● American concrete institute method ● Indian standard recommended method ● Department of environment, UK method ● Road note no.4 method ● IRC 44 method In these most favorable method used to design ready mixed concrete method is DOE method using fly ash as a mineral additive.

DOE (DEPARTMENT OF ENVIRONMENT,U.K.) METHOD

The DOE method was first published in 1975 and then revised in 1988. The DOE method is applicable to concrete for most purposes, including roads. The method can be used for concrete containing fly ash. DOE method presently is the standard British method of concrete mix design, the procedure involved in this method is described as follows. The following are the steps to design a concrete mix: Step 1: Find the target mean strength from the specified characteristic strength Target mean strength = specified characteristic strength + Standard deviation x risk factor. Risk factor is on the assumption that 5 percent of results are allowed to fall less than the specified characteristic strength.

Step 2: Calculate the water/cement ratio. This is done in a rather round about method, using Table 1.3 and fig. 1.5. Table 1.3 gives the approximate compressive strength of concretes made with a free w/c ratio of 0.5.Using this table find out the 28 days strength for the approximate type of cement and types of C.A. Mark a point on Y axis in fig.1.5.equal to the compressive strength read from the table 1.3. Through this intersection point, draw a parallel dotted curve nearest to the intersection point. Using this new curve, we read off the w/c ratio as against target mean strength. Step 3: Next decide the water content for required workability, expressed in terms of slump or vebe time, taking into consideration the size of aggregate and its type from Table 1.4. Step 4: Find the cement content knowing the water/cement ratio and water content, Cement content is calculated simply dividing the water content by W/C ratio. The cement content so calculated should be compared with the minimum cement content specified from the durability consideration and higher of the two should be adopted. Sometime maximum cement content is less than the specified maximum cement content. Step 5: Next find out the total aggregate content. This requires an estimate of the wet density of the fully compacted concrete. This can be found out from fig. 1.6for approximate water content and specific gravity of aggregate. If specific gravity is unknown, the value of 2.6 for uncrushed aggregate and 2.7 for crushed aggregate can be assumed. The aggregate content is obtained by subtracting the weight of cement and water content from weight of fresh concrete. Step 6: Then, proportion of fine aggregate is determined in the total aggregate using Fig. 1.7. Fig. 1.7(a) is for 10 mm size, 1.7(b) is for 20 mm size and Fig. 1.7(c) is for 40 mm size coarse aggregate. The parameters involved in Fig. 1.7 are maximum size of coarse aggregate, the level of workability, the water/cement ratio, and the percentage of fines passing 600 micron sieve. Once the proportion of FA is obtained, multiplying by the weight of total aggregate gives the weight of fine aggregate. Then the weight of the C.A. can be found out. Course aggregate can be further divided into different fractions depending on the shape of aggregate. The proportion so worked out should be tried in a trial mix and confirmed about its suitability for the given concrete structure. Table 1.5 gives the reduction of free water contents from the figures given in Table 1.4 when fly ash is used in the mix.

Design of concrete mix by DOE method using Fly Ash:

DATA:

M20 Grade concrete Crushed Aggregate are used slump = 80mm Cover of Reinforcement = 25mm Grading F.A. shows that 40% passes through 600micron sieve Moderate exposure conditions. Sp. Gravity of FA = 2.6

CA = 2.7

Fly ash used = 25%

Cement content C = ( 100-P) W/ {( 100-0.7P)( W/C+0.3F)} Fly Ash Content F = PC/ (100-P) Where, P = 100F/( C+F) P is the percentage of flyash in total cementitious material W is free water content. W/( C+0.3F) is the free water/ cementitious ratio for design strength. From table 9. 1 compressive strength of ordinary cement with fly ash with a free W/(C+0.3F) ratio of 0.5 is 4.9 Mpa. Target mean strength = 20+1.65*4.2 = 27Mpa For target mean strength of 27Mpa, fig. 11.3 gives, a free W/( C+0.3F) ratio of 0.7 Referring to Table 9.2,

For slump of 80mm, for maximum size of aggregate of 20mm, in case of crushed sand, the approximate water content is 225 kg/m3 Since 25% of fly ash is used, referring to table 9.3 water content is reduced by, 17.5kg/m3

The water content = 225-17.5 = 207.5 kg/m3 Cement content = (100-25) 207.5/ {(100-.7*25)(0.7)} Cement content = 270kg/m3 And the fly content = 25*270/(100-25) = 90.0 kg/m3 Hence, total cementitious material content is 270+90 = 360kg/m3 The free water/ cementitious material ratio is,

270/360 = .58

For moderate exposure condition, concrete cover of 25mm the maximum free water/ cementitious material ratio is 0.5 and minimum cement content is 350 kg/m3 Cement content satisfies the durability requirement satisfies durability requirement. Therefore adopt as 0.5. Water content = 360*0.5 = 180 l/m3 From fig 9.2 for water content 180 l/m3, average specific gravity of 2.65 of aggregate, wet density of concrete comes to 2445 kg/m3 Total weight of aggregate = 2445 – (270+90+180) = 1905 kg/m3 From fig 9.3 ( b) For water/ cementitious material ratio of 0.5 and for FA, 40% passing through 600 micron sieve and for slump of 80mm the proportions of FA is = 40% Weight of FA = 40/100*1905 = 762 kg/m3 Weight of CA = 1905-762 = 1143 kg/m3 Estimated quantities of materials in kg/m3 Cement = 270kg/ m3 Fly ash = 90 kg/m3 Fine aggregate = 762 kg/m3 Coarse aggregate = 1143 kg/m3 Water = 180 l/m3 Since, required w/c ratio is 0.58 and provided w/c ratio is 0.5. therefore, it will be harsh concrete. Therefore, for workable concrete, proper dosage of water reducing admixture is used.

Concrete which can be pushed through a pipe line is called a pumpable concrete. It is proportioned in such a manner that its friction at the inner wall of the pipeline does not becomes so high to prevent its movement at the pressure applied by the pump. A pump concrete is no special concrete. It is a standardized good concrete with certain content of fines to offer lubrication at the inner wall of pipe line. The pump able concrete has:

of 32 mm C.A.. In case of very angular, flaky aggregates this quantity is to be increased by approximately 10%. ● a minimum cement content of approximately 240 kg/m3 for maximum size of 32 mm C.A.. It must be increased by 10% in case of maximum size aggregate of 16 mm. ● a water/cement ratio of 0.42 to 0.65.

● a grading of aggregate typically as shown in Fig. 10.1 is to be used. Assuming the pump is mechanically sound, there are two reasons why blockage occur. The plug of concrete will not move because either (a) Water is being forced out of the mix creating bleeding and blockages by jamming, or (b) There is too much frictional resistance due to the nature of ingredients of the mix. A good grading is very important to produce pumpable concrete. Elongated and flaky aggregate will make the concrete harsh for the given cement content and water/cement ratio. The typical grading curves are shown in Fig. 1.8 for pumpable mixes. The aggregates for the proposed mix should have a grading parallel to these curves, but not coarser than a 2. Adjustments to bring the grading parallel to the curves can be made by altering proportions between C.A and FA. It is recommended that 10-20% of the fine aggregate should pass through 300 micron sieve. Sometimes 3-4% extra sand is added to safeguard against understanding.

The following is the procedure: Fig. 1.9 shows the more acceptable grading limits for pumpable concrete using 20 mm maximum size aggregates. The grading pattern should fall within the curve envelope of Fig. 1.9. If necessary the proportions of fine and coarse aggregate can be readjusted. ● The limits of the FINES content should be as given in Table 1.7. ● Mix proportions for 75 to 100 mm slump, are given in Table 1.8. ● The mix proportions given in Table 1.8 is based on the grading of aggregate as given in Table 1.10. ● The absolute density of material used is given in Table 1.9. ● The calculation of grading of combined aggregate on the basis of proportions given in Table 1.8 is given in Table 1.10 These combined grading figures should fall within the limits indicated in Fig. 1.9. ● The FINES content should then be worked out. ● Cement content = 310 kg (From Table 10.3) ● Fine aggregate grading shows that 18% passing through 0.3 mm sieve and 5% passing through 0.1 5 mm by interpolation probable percentage passing through 0.25 mm sieve = 15%.

● Fine particles in sand =15/100 x 880 = 132 kg/m3 ● Therefore, Total FINES =310+ 132 = 442 kg/m3

● Referring to Table 10.2 for 190 l/m3 of water content, the range of FINES is 330 to 465 kg/m3. Therefore, FINES of 442 kg/m3 is considered suitable.

Ready mixed concrete plant has to follow up the good concrete mix design. It can be achieved by proper grading of aggregates and designing concrete mix using appropriate method. DOE (Department of Environment, U.K.) method using fly ash and water reducer admixture gives better result. To design a

durability can be achieved. At the RMC plant, it is possible to achieve the concrete mix under close quality control.

M.L. Gambhir (2006). ― Concrete Technology‖, Tata McGraw Publication, 2006, page no. 198-209, 281-284. M.S.Shetty (2006). ―Concrete Technology‖, S. Chand Publication, page no. 64-73. 119-123,219-221, 459-488. P.D. Kulkarni and R.K. Ghosh (2006). ―Textbook of concrete technology‖, Laxmi publication, page no. 177-18. Sunil Gupta and Vikas Ahluvalia (2006). ―RMC- Need of hour‖, Civil Engg. Construction review, page no 85, 86, 88, 90.

Asst. Prof. Dr. D. Y. Patil Institute of Engineering and Management Research