+ All Categories
Home > Documents > Mix Design

Mix Design

Date post: 22-Nov-2014
Category:
Upload: goran-adil
View: 130 times
Download: 5 times
Share this document with a friend
Popular Tags:
22
Mix selection (mix design) Content: 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14.
Transcript
Page 1: Mix Design

Content:

1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14.

Page 2: Mix Design

Introduction

How do we decide what concrete we need in any particular case? The required properties of hardened concrete are specified by designer of the structure and the properties of fresh concrete are governed by the type of construction and by the techniques of placing and transporting.

These two sets of requirement make it possible to determine the composition of the mix .taking also account of the degree of control exercised on site .mix design can , therefore ,be defined as the process of selecting suitable ingredients of concrete and determining their relative quantities with the purpose of producing an economical concrete which has certain minimum properties ,notably workability ,strength and durability .

1. Purpose of mix designFrom the definition of mix design, it can be seen that the purpose of the

mix design is twofold as follow:1. Achieve the stipulated minimum strength and durability.2. To make concrete in the most economical way. Cost wise all

concrete depend mainly on two factor a. Cost of material and:b. Cost of labour

Factors to be considered

The economic and technical factors and the procedures of estimating the mix quantities will now be outlined. Virtually always, the strength of concrete has to be considered .the actual cost of concrete is related to the material required to produce a certain mean strength but it is the minimum strength which is specified by the structural designer, normally, the strength for structural purpose is required at 28 days .but other consideration may dictate the strength at other ages, e.g. formwork striking time .the expected or known control techniques, the variability of strength can be minimized so that the lowest mean strength is needed for a given minimum specified strength .however, the cost of implementing and operating a more elaborate quality control scheme has to be weighed against the possible saving in cement resulting from a lower mean strength .

Page 3: Mix Design

2. Object of mix design The object of mix design is to decide the proportions of material which

will produce concrete of produced properties. The mix proportion should be selected in such a way that the resulting concrete is of desired workability while fresh and it could be placed and compacted easily for the intended purpose. The fresh concrete should be fluid enough to fill the form work and surround the reinforcement fully and the hardened concrete should develop required strength and durability.

Factor affecting the choice of mix proportion

1. Grade designation 2. Type of cement 3. Max. nominal size 4. Grading of combined aggregate 5. Water /cement ratio 6. Workability 7. Quality control

1. Grade designation

Grade designation specifies the characteristic compressive strength of concrete. The characteristic compressive strength is the value of concrete strength below which not more than 5% of the test results are expressed to fall. It is the major factor which influences the mix design depending upon the degree of control at site, the concrete mix should be designed for the mean compressive strength, which is little higher than characteristic strength.

2. Type of cement

Page 4: Mix Design

The type of cement is important as the rate of strength development depends on cement, the choice of the type of cement depend on requirements of its performance. When very high compressive strength is required, Portland cement is suitable. In case where an early strength required, rapid hardened Portland cement is suitable, for mass concrete work, low heat Portland cement suitable.

3. Max. nominal sizeThe maximum nominal size of coarse aggregate is determined by sieve analysis and is designed by the sieve size higher than the largest size on which 15% or more the aggregate is retained. It is governed by the size of section and spacing of the reinforcement. According to ACI code 318.05 Nominal maximum size of coarse aggregateShall be not larger than: (a) 1/5 the narrowest dimension between sides of forms, nor (b) 1/3 the depth of slabs,

(c) 3/4 the minimum clear spacing between individual reinforcing bars or wires, bundles of bar, individual tendons, bundled tendons, or ducts.

4. Grading of combined aggregate

In a concrete mix the relative proportions of the fine and coarse aggregate is one of the important factor which affects the strength of the concrete. For dense concrete, it is essential that the fine and coarse aggregate be well graded. The locally available aggregate generally do not conform to the standard grading. In such situations the aggregate should be combined in suitable proportions, so that the resulting grading is close to the desired grading. The aim of combining the grading is to obtain a grading close to coarsest grading of the standard grading curves, the most economical min having highest permissible aggregate cement ratio. The aggregate can be combined either by analytical calculations or graphically.

5. Water /cement part

The compressive strength of concrete at a given age under normal condition of curing mainly depends upon water/cement ratio. Lower water cement ratio with limits, greater the compressive strength and vice-verse. A number of relationships between compressive strength and water cement ratio

Page 5: Mix Design

are available which are valid for a wide range of conditions. The generalized relationship between water/cement ratio and compressive strength of concrete is shown in table1.

6. Workability

For satisfactory placing and compacting of concrete the workability is controlled many factors as shape and size of the section to be concreting, quantity and spacing of reinforcement, method of transportation, placing and compaction of concrete etc . The aim should be to have minimum possible workability consistent with satisfactory placing and compaction of concrete. The insufficient workability will result in complete compaction, resulting in less strength and durability. There is various method of determining the workability of concrete, but there is no rigid correlation between workability of concrete measured by different methods. Hence it is desirable to decide the test method before starting the concrete work. For comparable concretes workability measured by different method is shown in table1

Table1. Recommended workability value

No degree of workability

values of workabilities in terms of

slump in mm

compacting factor

vee bee time in secs

drop table revolutions

1 extremely low(very stiff) — ≤0.70* 30—20 96—48

2 very low(stiff) 0—25 0.75—0.80 20—10 48—24

3 low(stiff plastic) 25—50 0.80—0.85 10—5 24—12

4 medium(plastic) 50—75 0.85—0.92 5—2 12—6

5 high(flowing) 75—150 >0.92 2—0 6—0

*compacting factor test is not used for concrete prepared with aggregate having nominal size of 40 mm and higher

7. Durability

It can be defined as the resistance to internal and external deteriorating influence. The requirements of durability may be achieved by restricting the minimum cement content and minimum water-cement ratio as given in table 2

Page 6: Mix Design

8. Quality control

The strength of concrete is never constant. It varies from batch to batch. This variation may be due to the variation in the quality of constituent materials, variations in mix proportions, variation in quality of batching and mix equipment available, quality of workmanship and supervision etc. controlling these factors is important to minimize the difference between the minimum strength and characteristic mean strength of the mix thus reducing the cement content. The method of controlling these differences is quality control.

Page 7: Mix Design

Trial mixes

The calculated mix proportions should be checked by making trial mixes, only a sufficient amount of water to produce the required workability should be used, regard less of the amount calculated. The trial mix should be tested for workability, cohesiveness, finishing properties and air content, as well as for yield and density (unit weight). If any one of these properties, except the last tow, is unsatisfactory, adjustments to the mix proportions are necessary. For example lack of cohesiveness can be corrected by increasing the fine aggregate content at the expense of the coarse aggregate content. The ACI 211.1-91 “rule of thumb) are as follow:

a) The estimated mixing water to produce the same slump as the trial batch will be equal to the net amount of mixing water used divided by the yield of

The trial batch in m3. If slump of the trial batch was not correct, increase or decrease the re-estimated water content by 2 kg/m3 of concrete for each increase or decrease of 10

b)To adjust for the effect of incorrect air content in a trial batch of air-entrained concrete on slump, reduce or increase the mixing water content of A1.5.3.9.1 by 3 kg/m’ of concrete for each 1 percent by which the air content is to be increased or decreased from that of the trial batch.

American method of selection of mix proportions

Page 8: Mix Design

The ACI standard practice ACI 211.91 describes a method of selection of mix proportions of concrete containing Portland cement alone or together with other cementitious materials, and containing also admixtures, it should be emphasized that the method provides a first approximation of mix proportions to be used in trail mixes. In essence the method of ACI 211.91 consists of a sequence of logical straightforward steps which take into account the characteristics of the materials to be used. These steps will now be described.

Step1: choice of slump

At the time of mix proportioning, the slump will have been determined by the exigencies of construction. It should be noted that slump should be specified not only at the minimum end, but a maximum value should also be specified. This is necessary to avoid segregation when the mix has not been selected to have a higher slump, suddenly become wet.

Table: RECOMMENDED SLUMPS FOR VARIOUS TYPES OF CONSTRUCTION

Step2: choice of maximum size of aggregate

This too, will have to decided, usually by the structural designer, bearing in mind the geometric requirements of member size and spacing of reinforcement or alternatively for reasons of availability.

Page 9: Mix Design

Table3: Approximate mixing water and air content requirements for different slumps and nominal maximum sizes of aggregates:

Page 10: Mix Design
Page 11: Mix Design

Step3: estimate of water content and air content

Water content required to produce a given slump depends on several factors; the maximum size of aggregate, its shape, texture, and grading; the content of entrained air; the use of admixture with plasticizing or water reducing properties; and temperature of concrete table1 relating slump to these properties have to be used.

Step 4: selection of water cement ratio

There are two criteria for selection of the water /cement ratio: strength and durability. As far as the compressive strength is concerned the average value aimed at must exceed the specified ’minimum “strength by an appropriate margin. The term “cement” refers to the total mass of cementitious materials used; their choice is governed by numerous factors, such as heat development rate of gain of strength, and resistance to various forms of attack, so that the type of blended cement to be used has to be established at the outset of mix proportioning . It is for the actual cement to be used that the relation between strength and the water cement ratio has to be established over a certain range of strengths.

Table: relationship between water cement ratio or water-cementitious material ratio and compressive strength of concrete.

Page 12: Mix Design

Step 5: Calculation of cement content

The outcome of steps 3 and 4 gives the cement content directly: it is the water content divided by the water-cement ratio. If, however, from durability considerations, there is a requirement for certain minimum cement content, the large of two values must be used.

Occasionally, from heat development considerations the specification imposes maximum cement content. Clearly, this must be observed. Heat development is of particular concern in mass concrete and mix proportioning for that type of concrete specifically covered in ACI 211.1-91.

Step 6: Estimate coarse aggregate content

Here, the assumption is made that the optimum ratio of the bulk volume of coarse aggregate to the total volume of concrete depends only on the maximum size of aggregate and on the grading of fine aggregate. The shape of the coarse aggregate particles does not directly enter the relation because, for instance, a crushed aggregate has a greater bulk volume for the same mass (that is the lower bulk density) than a well-rounded aggregate. Thus, the shape factor is automatically taken into account in the determination of the bulk density. Table 1 gives values of the optimum volume of coarse aggregates of different fineness module. This volume converted into mass of coarse aggregate per cubic meter of concrete by multiplying the value from the table by the oven-dry rodded mass of the aggregate (kg/m3).

Page 13: Mix Design

Table4: Volume of coarse aggregate per unit volume of concrete

Step 7: Estimate of fine aggregate content

At this stage, the mass of fine aggregate is the only remaining unknown quantity. The absolute volume of this mass can be obtained by subtracting the sum of the absolute volume of water, cement, entrained air, and coarse aggregate from the volume of concrete, that is, 1 m3. For each ingredient, the absolute volume is equal to the mass divided by the absolute density of the material (in kg/m3); the absolute density is the specific gravity of the material multiplied by the density of water (1000 kg/m3).

The absolute volume of fine aggregate is converted into mass by multiplying this volume by the specific gravity of the fine aggregate and by the density of water.

Step 8: adjustment to mix proportions

As in any process of selection of mix, trail mixes have to be made. Advice , by way of some rules of thumb, for adjustments to the mix, is given below in ACI 211.1-91. In general terms, it is important to remember that, if workability is to be changed, but the strength is to remain unaffected, the water/

Page 14: Mix Design

cement ratio must remain unaltered. Change can be made in aggregate/ cement ratio or, if suitable aggregate are available, in the grading of the aggregate; make influence of grading on workability in account.

Conversely changes in strength but not in workability are made by varying the water / cement ratio with the water content of mix remaining unaltered. This means that a change in the water/cement ratio must accompanied by a change in the aggregate /cement ratio so that the mass ratio.

Water/ (water+cement+aggregate)

Table: first estimate of mass of fresh concrete

Example

We require a mix with a mean 28-day compressive strength (measured on standard cylinders) of 35 MPa and a slump of 50 mm, ordinary Portland cement being used. The maximum size of well-shaped, angular aggregate is 20 mm, is bulk density is 1600 kg m3, and its specific gravity is 2.64. The

Page 15: Mix Design

available fine aggregate has a fineness modulus of 2.60 and specific gravity of 2.58. No air entrainment is required. For the sake of completeness, all steps even when obvious will be given.

Step1: A slump of 50 mm is specified.

Step2: the maximum size of aggregate of 20 mm is specified.

Step3: From the table for a slump of 50 mm and maximum size aggregate of 20 mm (or 19mm), the water requirement is approximately 190 kg per cubic meter of concrete.

Step4: from past experience, a water cement ratio of 0.48 is expected to result in concrete with a compressive strength, measured on cylinders of 35 MPa. There are no special durability requirements.

Step5: the cement content is 190/0.48= 395 kg/m3.

Step6: from the table when used with a fine aggregate having a fineness modulus of 2.60, the bulk volume of oven-dry rodded coarse aggregate with a maximum size of 20 mm is 0.64. given that the bulk density of the coarse aggregate is 1600 kg/m3, the mass of aggregate is o.64*1600=1020 kg/m3.

Step7: to calculate the mass of fine aggregate, we need first to calculate the volume of all the other ingredients. The required values are as follows.

Volume of water is 190/1000 =0.190 m3

Solid volume of cement, assuming usual specific gravity of

3.15, is 395/ (3.15*100) =0.126 m3

Solid volume of coarse aggregate is 1020*(2.64*1000) =0.396 m3

Volume of entrapped air, given in table is 0.02*1000 =0.020 m3

Hence, total volume of all ingredients except fine aggregate=0.732 m3

Therefore, the required volume of fine aggregate is 1.0-0.732=0.268 m3

Page 16: Mix Design

Hence, the mass of fine aggregate is 0.286 *2.58*1000=690 kg/m3.

From the various steps, we can list the estimated mass of each the ingredients in kg per cubic meter of concrete as follow:

Water 190

Cement 395

Coarse aggregate, dry 1020

Fine aggregate, dry 690

Therefore the density of concrete is 2295 kg/m3


Recommended