Process of Ready Mixed Concrete

7/27/2019 Process of Ready Mixed Concrete

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Ready Mixed Concrete ProcessesReady mixed concrete, which is identified as an industrial product, is delivered to the consumer in

fresh condition after some production processes in the ready mixed concrete plant and it gains its

hardened characteristics in course of time. This section aims to describe these processes from

concrete design to maintenance. These processes are analyzed under six main parts being design,production, transport (dispatch), pouring, placement and maintenance of ready mixed concrete.2.1. DESIGNThe calculation of concrete mixture is described as the calculation made to determine the amounts

of aggregate, water, air and additives, where necessary, needed for obtaining the most economical

concrete with the desired viscosity, workability, resistance, durability, volume consistency and

other characteristics required (TS 802, 2009). To be able to make a concrete mixture calculation,

two sets of data containing qualities of the concrete to be produced and the capacity of theproduction tools and specifications of the inputs such as cement and aggregate to be used inproduction are needed to be known. Those needed to be known in the first group of data are thedosage of binding material, requirement of mineral and/or chemical additives, water/binder

proportion and viscosity. In the second data group, the cement resistance, specific weight ocement, aggregate unit volume weight, specific weight of aggregate and aggregate gradation,

which allow calculation are included (Akman, 1987). The components amounts in the concrete

mixture should be expressed as the component mass needed for concrete of 1 m3. Therefore, the

variables such as cement paste - aggregate proportion, water-cement proportion, sand- rough

aggregate proportion and use of additives are of importance for the persons designing the mixture

(Mehta and Monteiro, 2006). For instance, if the aggregate component in 1 m3 of concrete is

increased, the cement paste component should be decreased.Selection of the suitable components is the first step in design to achieve the concrete of desired

specifications and performance. The second step is determining the mixture proportions. Since the

concrete composition affects the cost and specifications of the product, the engineers designing

the ready mixed concrete mixture should take as basis the commonly used procedures. Theconcrete mixture calculation principles used in Turkey (TS 802), the normal, heavy and mass

concrete mixture proportions used in the United States of America (ACI 211-1) and British Ready

Mixed Concrete Association mixture procedure (BRMCA) are just a few examples.Here, first the mixture principles mentioned in TS 802 Concrete Mixture Calculation Principlesstandard used for years in the ready mixed concrete industry shall be dealt with and then, the

procedure of concrete mixture design shall be described on the basis of the procedures

recommended by the British Ready Mixed Concrete Association and American Concrete

Institution respectively.

2.1.1. Concrete Mixture Design According to TS 802When making the design according to this standard; the dimensions of the structural component,

environmental and chemical effects likely to be exposed by the concrete, physical effects such as

frost, excessive heat and wear, the impermeability, resistance, durability, density, workability and

volume stability required from the concrete shall be taken into account. The concrete design in

accordance with TS 802 consists of eight steps and can be summarized as shown in the flow chart

below:


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Figure 2.1 - Flow Chart of Concrete Mixture Design According to TS 802

Step 1: Selection of the Greatest Aggregate Particle Size:

According to TS 802, the mixture calculations start with selection of the greatest aggregate

particle size (Dmax). Dmax is one of the aggregate specifications described in the part four and is

defined as the smallest sieve size that the entire aggregate may pass through. The greatest particle

size of the aggregate to be used in concrete design is closely related to the type of the structural

component where the concrete will be used, the dimensions of this structural component and the

location of the equipments in it. The greatest particle size of the aggregate to be used in concrete

design should be selected smaller than 1/5 of the width of the concrete form, 1/3 of the thickness

of the flooring and 3/4 of the smallest equipment size. If the concrete is to be poured by pump,

then the greatest particle size should be smaller than 1/3 of the inner diameter of the pump pipe.

The greatest aggregate particle sizes for various structural components and the size of th section o

the structural component are summarized in tale 2.1.

Table 2.1 - The greatest aggregate particle size to be used according to the dimensions for various

structural components

The narrowest size

of the section of

the structuralcomponent

Greatest aggregate particle size (maximum) (mm)

Reinforced

curtains, joists and

columns

Densely

reinforced

flooring

Loosely

reinforced and

non-reinforced

Non-reinforcedcurtains

1. Selection of the greatest aggregate

particle size

2. Selection of particle size distribution

(granulometry)

3. Selection of water/cement proportion

4. Selection of water quantity

5. Selection of air quantity

6. Selection of viscosity

7. Making the mixture calculation

8. Verification of mixture calculationsby experiments


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(mm) flooring

60-140 16 16 32 16

150-290 32 32 63 32

300-740 63 63 63 63

Step 2: Selection of Particle Distribution:

The gradation of the aggregate to be used in concrete design directly affects the workability,durability and economy of the concrete. The gradation of the aggregate in the mixture should be

selected within the limits mentioned in Figure 2.2 and Figure 2.5, in relation to the greatest

particle size, and the mixtures should be prepared in accordance with these limits. The aggregate

gradation of the mixture being within the area number 3 shown in the figures should be preferred

since it will contribute to the workability, resistance and economy of the concrete, and, if this not

possible, the area number 4 should not be exceeded. However in compulsory cases, the gap-graded

particle distributions within the area number 2 might be used. In these figures (Figure 2.2-2.5), the

area number 1 represents very coarse gradation where area number 2 represents graded, number 3

represents suitable, number 4 represents finer than area number 3 and area number 5 represents a

very fine gradation.

Figure 2.2 - The aggregate gradation curves determined for concrete with the greatest aggregate

particle size (dmax) of 8 mm


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For concrete to be poured by pump, the limits of suitable gradation for mixtures made by

composition of fine and coarse aggregate classes are listed in Table 2.2 and these curves are shown

in Figure 2.6 and Figure 2.7 (TS 802, 2009).

Table 2.2 - Gradation limits of aggregate mixtures which are recommended to be used for concrete

transferred by pump and the greatest particle sizes of which are 31,5 mm and 22,4 mm

Sieve aperture size, (mm)

Passed through the sieve, % (cumulative)

Greatest particle size

31,5 mm

Greatest particle size

22,4 mm

45 100 -

31,5 90-97 100

22,4 80-90 89-96

16 68-82 73-86

8 52-69 54-71

4 37-56 37-562 26-43 25-43

1 17-33 16-32

0,5 10-23 10-22

0,25 6-16 6-15

0,15 3-10 3-10

0,063 1-5 1-5

Pan 0 0


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ekil 2.6- Pompa ile dklecek betonlarda agrega en byk tane boyutu (Dmaks) 22,4 mm iinagrega karmnn nerilen gradasyon erisi

Figure 2.7 - Recommended gradation curve of aggregate mixture for the greatest aggregate

particle size (Dmax) of 31,5 mm in concrete to be poured by pump

In addition to these, the particle distribution of fine aggregate is of more importance for

pumpability of concrete to be poured by pump. The gradation of the fine aggregate to be used for

concrete mixture suitable for pouring by pump is summarized in Table 2.3. The information in this

table is presented in graphical form in Figure 2.8. Furthermore, the fineness module of the fine

aggregate in such concrete being between the limits of 2,30 - 3,10 provides great convenience in

pouring concrete.

Table 2.3 - Limits of the gradation curve recommended for the fine aggregate to be used foconcrete to be transferred by pump


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Sieve aperture size, (mm)

Passed through the sieve, (%)

(cumulative)

8,0 100

5,6 95-1004,0 85-98

2,0 69-90

1,0 44-74

0,50 20-50

0,250 8-25

0,150 3-10

0,063 0-3

Pan 0

Figure 2.8 - Gradation curve recommended for the fine aggregate to be used for concrete to be

poured by pump

Step 3: Selection of Water/Cement Proportion:

The Water/Cement (w/c) proportion is directly related to the class of the concrete and the intensity

of the environmental and chemical impacts that concrete may be exposed to. Taking into accountthe climate conditions and environmental impacts for the concrete to be designed, the

environmental impact class, the minimum cement dosage, the lowest characteristic pressure

resistance and the greatest water/cement proportions should be determined. Table 2.4 shows the

characteristic pressure resistances (fck) by classes of concrete and the target pressure resistances

(fcm) to be taken as basis in mixture calculations. Table 2.5 shows the greatest water/cement

proportions that can be selected according to the concrete pressure resistance for 28 days for both

air-entrained and non-air-entrained concrete. Water/cement proportion is shown as one of the most

important factors affecting the concrete resistance and durability (Erdoan, 2004). In general, asthe water/cement proportion increases, the reasistance and durability of concrete is adversely

affected. However, in very low water/cement proportions, the workability of concrete decreases

and undesired gaps may occur in concrete.Table 2.4 - The targed pressure resistance to be taken as basis for mixture calculation according to


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the concrete classes (fcm) and the average pressure resistances required for experiment samples

Concrete

class

Characteristic pressure resistance,

fck

(MPa)

Target pressure resistance, fcm

(MPa)

Characteristic

cylinder

(150x300 mm)pressure

resistance,

fck

(MPa)

Equivalent

cube (150 mm)

pressure

resistance

fck

(MPa)

Standard

deviation

known

Standard deviation unknown

Cylinder

150x300 mm)

Equivalent

cube (150

mm)

C14/16 14 16

fcm=fck+1.48

18 20

C16/20 16 20 20 24

C18/22 18 22 22 26

C20/25 20 25 26 31

C25/30 25 30 31 36

C30/37 30 37 36 43

C35/45 35 45 43 53

C40/50 40 50 48 58

C45/55 45 55 53 63

C50/60 50 60 58 68

C55/67 55 67 63 75

C60/75 60 75 68 83

C70/85 70 85 78 93

C80/95 80 95 88 103

C90/105 90 105 98 113

C100/115 100 115 108 123

Note 1 - When determining fcm target pressure resistance, the coefficient of 1,48 was taken from

TS EN 206-1 standard with 95% reliability.

Table 2.5 - Approximate w/c proportions according to the pressure resistance of concrete which is

28 days old

Pressure resistance (28

days)

(150x300 mm)Cylinder

(MPa)

Water/cement proportion

Non-air-entrained concrete Air-entrained concrete

45 0,37 -

40 0,42 -

35 0,47 0,39

30 0,54 0,45

25 0,61 0,52

20 0,69 0,6015 0,79 0,70


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Step 4: The required quantity of water in concrete mixture can be defined as the sum of the surface

humidity water of the aggregate the "saturated surface dry" (DYK) status of which is taken into

account for the mixture and the water to be added forthe requird reactions and workability of theconcrete. The concrete mixing water is related to the viscosity class of the desired concrete, the

gradation of the aggreagate used for concrete, the shape and type of the aggregate, fine aggregate /rough aggregate proportion and the quantity of air in the mixture. The mixing water quantity in the

concrete mixture significantly affects the workability, resistance and durability of the concrete. In

Figure 2.9 and 2.12, various slump values desired in 1 m3 of concrete mixture made with natural

and angular aggregate and the graphics where the approximate water quantities that can be used

for the greatest aggregate particle size are shown. The water quantities that can be calculated by

these graphics are for the concrete mixtures to be produced without any chemical additives except

for air-entraining additive. When chemical additives are used as plasticizer, a certain quantity of

water may be decreased in the mixing water quantities calculated by the graphics depending on the

additive type and dosage.

Figure 2.9 - Approximate mixing water quantities of non-air-entrained concrete produced with

natural aggregate without chemical additives


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Figure 2.10 - Approximate mixing water quantities of air-entrained concrete produced with natural

aggregate without chemical additives

Figure 2.11

- Approximate mixing water quantities of non-air-entrained concrete produced with angular

aggregate without chemical additives


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Figure 2.12 - Approximate mixing water quantities of air-entrained concrete produced with

angular aggregate without chemical additives

5. Selection of Air Quantity

The gaps arising as result of incomplete compaction in fresh state of the concrete which consists of

aggregate, water and cement and is a composite material are defined as air voids. These air voids

adversely affect the resistance and durability characteristics of the concrete. On the other hand, air

bubbles may be entrained into the concrete by chemical additives to increase freezing-thawingresistance. These gaps are called entrained air voids. The air quantity of the concrete should be

determined by taking into account the greatest aggregate particle size and climate conditions for

non-air-entrained concrete and air-entrained concrete to be poured in various climate conditions.

Figure 2.13 shows the graphics by which we can calculate the air quantity to be selected for non-

air-entrained and air-entrained concrete to be poured in various climate conditions.


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Figure 2.13 - Air content in concrete mixture

6. Selection of Viscosity

Viscosity is defined as the level of wetness of fresh concrete. In other words, viscosity expresses

the level of dryness or wetness of the concrete. Viscosity is usually determined by slump test in

ready mixed concrete industry and today the slump value of concrete is mentioned in the project in

advance according to the construction technique and construction type where concrete will bepoured. The viscosity may be increased or decreased according to the conditions in the

construction site where the concrete will be poured. By means of the advances in the concrete

technology and the use of chemical additives, high viscosity concrete can be transferred to pumps

without any decomposition and can be settled easily. In works and projects where the viscosity has

not been mentioned in any way, the suitable slump values for various structural components can

be determined by use of Table 2.6.Table 2.6 - Suitable slump values for various structural components

Structural component

Slump, mm

Minimum MaximumConcrete foundation walls

and feet30 80

Non-reinforced concrete

foundations, caissons and

sub-structure walls

30 80

Joist, column, concrete

curtains, tunnel side and

cincture concrete

50 100

Flooring concrete 30 80

Tunnel floor coating

concrete 20 50


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Dam mass concrete 20 50

7. Calculation of Mixture

The quantities of the materials to be used in 1 m3 of compressed concrete can be calculated by the

formula below:

Here;

: cement mass in mixture (kg)

: mineral additive (pozzolana) mass in mixture (kg)

: chemical additive mass in mixture (kg)

: cement, mineral additive, chemical additive and aggregate density (kg/dm3)

: volume of water in mixture (dm3)

: aggregate mass in mixture (kg): total air quantity in concrete (%)

After the water/cement proportion is found by step 3 and the water quantity is found by step 4, the

cement mass in the mixture can be calculated by the formula below:

Here; : water/cement proportion.

Since the volume left after te cement, water, chemical additive, mineral additive and air in the

mixture will be filled by aggregate, the formula above can be re-written as follows to determine

the aggregate voluma and thus the aggregate quantity in the mixture:

After the aggregate volume has been found by means of this formula, the density of each particle

class is found to determine the aggregate mass to be used for 1 m3 of concrete and the average

density of the aggregate to be used for the concrete mixture is calculated by means of the formula

below:

Here is the weighted average relative density value of the aggregate

and, , , and are the mixture proportions of various particle classes. The total mass o

the aggregate to be used in 1 m3 of concrete is calculated by multiplying the weighted average

relative density value calculated by the aggregate volume;

is the total mass of the aggregate to be used for 1 m3 of concrete.

The masses of aggregates in different particle size classes are calculated by multiplying this value

by the mixture proportions of different aggregate particle classes. The density values of the


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aggregates used in these formulae are Saturated Dry Surface (DKY) values. However, since

aggregates are not usually included in the concrete mixture as DKY, aggregate humidity rates

should be checked regularly and the necessary corrections in the mixture calculations should be

made. When the humidity rates and water absorption values of the aggregates are known, the

correction in the water quantity and aggregate quantity in the mixture is made by means of the

formulae below:

Here;

: Water quantity after correction (kg/m3)

: Water quantity before correction (kg/m3)

: Quantity of aggregate class after correction (kg/m3)

: Quantity of aggregate class before correction (kg/m3)

: Water absorption rate of aggregate class (%)

: Humidity rate of aggregate class (%)

Step 8: Verification of the Mixture Calculation by Experiments

Since the values given for factors such as the suitable particle distribution, water/cement

proportion and water quantity which are used in determining the concrete mixture proportions and

which directly affect the characteristices of fresh and hardened concrete are values obtained from

the results of many experiments, a trial mixture should be prepared using the aggregate, water,

cement, chemical additive, mineral additive and air values calculated for any specific mixture and

the calculated values should be verified by experiments. If there are differences between the fresh

and hardened concrete characteristics forecasted and foreseen prior to the experiments and thecharacteristics observed during the experiments, the mixture calculations should be repeated. After

the trial mixtures are prepared, the viscosity, unit volume weight and air content of the fresh

concrete shuld be measured and the mixture proportions of actual concrete should be determined

on the basis of these values.2.1.2. Concrete Mixture Design According to BRMCA (British Ready Mixed Concrete

Association)

The main levels of the concrete mixture design according to British Ready Mixed Concrete

Association (BRMCA) are summarized in Figure 2.14. This method is widely used by the

members of British Ready Mixed Concrete Association Quality Community (Dewar and

Anderson, 1992). According to this method, there are two important matters in determining the

proportions of the concrete mixture components which are workability of fresh concrete and

resistance of hardened concrete. Another purpose of determining the proportions of mixture

components is obtaining a concrete mixture to provide the sufficient characteristics with the

lowest cost (Mehta and Monteiro, 2006). Figure 2.14 - Main levels of BRMCA (British Ready

Mixed Concrete Association) mixture design method

The most important step of BRMCA method is the 2nd step. Mixtures should be designed in such

manner to show the optimum performance in plastic viscosity suitable for transportation,

processing, compacting and finishing stages. The fine/total aggregate percentage should be

selected such that it will provide the enough cohesion for every mixture and minimize the risk o

segregation. The steps to be followed in design of plastic concrete characteristics of BRMCAmethod are provided in Figure 2.15. The analysis of the trial mixture and the resistance test data


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are provided in Level 4 and here the cement quantity is essential. BRMCA mixture design method

Level 3 Hardened concrete performance can be seen in Figure 2.16. (Dewar and Anderson, 1992).

Figure 2.15 - BRMCA (British Ready Mixed Concrete Association) mixture design method -

Level 2 - Plastic characteristics design

Figure 2.16 - BRMCA (British Ready Mixed Concrete Association) mixture design method -

Level -3 - Hardened concrete performance

BRMCA (British Ready Mixed Concrete Association) mixture design method Level-4-Analysis

and presentation, use of the mixture design data can be seen in Figure 2.17 (Dewar and Anderson,

1992).

Density (kg/m3)

Fine aggregate rate (%)

Aggregate/cement

proportion

Water/cement proportion

(free or total water)

Weight of all materials in

1 m3

Average resistance of 28

days

Cement quantity requiredfor the characteristics of

the design

Production mixtures

Cement quantity for

design resistance


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Figure 2.17- BRMCA (British Ready Mixed Concrete Association) mixture design method Level-

4-Analysis and presentation, use of the mixture design data

2.1.3. Concrete Mixture Design According to the American Concrete Institute (ACI)The method recommended by the American Concrete Institute (ACI) is widely used in America

and many other countries. The concrete mixture design according to ACI method consists of 9

steps as shown in Figure 2.18. Before starting the calculations, sieve analysis for fine and coarse

aggregate, fineness module, coarse aggregate dry loose unit weight, specific weight of materials,

aggregate water absorption capacities and current humidity proportions, differences in the mixing

water need depending on the air percentage aand aggregate gradation, the relation between the

resistance and water-cement proportion, the maximum water-cement proportion, the minimum air

percentage, the minimum slump quantity, the greatest aggregate particle size an target resistance

values are needed to be known (Mehta and Monteiro, 2006).

Figure 3- Flow chart of the mixture design method recommended by ACI 211.1-91Step 1: Slump value selectionThe first step of the mixture design is, just as in BRMCA method, determining the slump value o

the concrete. This value will ve determined according to the site conditions, however Table 2.7

shows the values that can be used where this cannot be determined. On the other hand, the slump

range of a concrete mixture should generally be between 100 -150 mm. (Mehta and Monteiro,

2006).Step 2: Selection of the maximum aggregate particle size

For a regular aggregate gradation, as the maximum aggregate pzerticle size increases, the amount

of gaps and also the amount of mortar in concrete decreases. The American Concrete Institution

recommends that the maximum aggregate particle size does not exceed 1/5 of the narrowest size

between the form corners, 1/3 of the flooring depth and 3/4 of the minimum distance between the

reinforcements (Mehta and Monteiro, 2006).Table 2.7 - Selection of slump value according to various structure types* (ACI Committee 211)

3. Mixing water and air quantity estimates

1. Selection of slump value

2. Selection of the greatest aggregate particle size

6. Estimated quantity of coarse aggregate

8. Aggregate humidity correction

7. Determining the quantity of fine aggregate

5. Calculation of cement quantity

4. Selection of water/cement proportion

9. Making trial mixtures


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Structure type

Slump value (mm)

Maximum** Minimum

Reinforced foundations and ve istinat duvarlar 75 25Non-reinforced foundations , caissons and

foundation walls 75 25

Joists and curtain walls 100

Columns 100 25

Coating and flooring 75 25

Mass concrete 50 25

* This value may be increased when chemical additives are used and the water/binder

proportion is not changed or decreased by use of additives.

** It can be increased 25 mm when any other compaction than vibration is applied.

Step 3: Mixing water and air quantity estimate

The mixture water quantity required for 1 m3 of concrete according to ACI depends on the

greatest aggregate particle size, aggregate shape and gradation, concrete temperature, entrained airquantity and use of chemical additives. Although the mixing water and air quantity estimate given

in Table 2.8 change a bit depending on the aggregate type and gradation, it provides enough

information for the start.

Step 4: Water/cement proportion selection

The proportion of water/cement or water/binder used in concrete mixture significantly affects the

resistance and durability of the concrete. It is true that different aggregates may generally provide

different resistances in a fixed proportion of water/cement or water/binder of a concrete produced

with cements and other binder materials. Therefore it is not very easy to create a relation between

the pressure resistance of concrete and water/cement proportion and use it in concrete mixture

design. So, the proportions in Table 2.9 may be used where these relations do not exist and cement

of a type not containing mineral additive is used. On the other hand, rather than resistance to someenvironmental impacts, durability is of importance. In such cases it shall be more suitable to selcet

and use the proportions in Table 2.10.Table 2.8 - Mixing water and air quantity estimate

a) Concrete containing non-entrained air

Slump (mm)

Water quantity (kg / m3)

Mixumum aggregate particle size (mm)

9.5 12.5 19 25 37.5 50

25-50 207 199 190 179 166 154

75-100 228 216 205 193 181 169

150-175 243 228 216 202 190 178

Air quantity (%) 3 2.5 2 1.5 1 0.5

b) Concrete containing entrained air

Slump (mm)

Water quantity (kg / m3)

Mixumum aggregate particle size (mm)

9.5 12.5 19 25 37.5 50

25-50 181 175 168 160 150 142

75-100 202 193 184 175 165 157150-175 216 205 197 184 174 166


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Air quantity according

to environmentsal

impact severity (%)

Light 4.5 4.0 3.5 3.0 2.5 2.0

Medium 6.0 5.5 5.0 4.5 4.5 4.0

Heavy 7.5 7.0 6.0 6.0 5.5 5.0

Table 2.9 Relation between the water/cement proportions and the pressure resistance

Concrete pressure resistance

(MPa)

Water/cement proportion (in weight)

Non-air-entrained concrete Air-entrained concrete

41 0.41 -

34 0.48 0.40

28 0.57 0.48

21 0.68 0.59

14 0.82 0.74

Table 2.10 - Maximum water/cement proportions in concrete under severe environmental impacts

Structure Type

Concrete Which is Always Wet

or Frequently Exposed to

Freezing-DefreezingConcrete Exposed to Sea Water

or Sulphated AmbianceConcrete wihc has thin section

or less cover than 25 mm on the

reinforcement

0.45 0.40

Other structures 0.50 0.45

Step 5: Calculation of cement quantity

The cement quantity is calculated by division of the mixing water quantity determined in Step 3 by

the water/cement proportion.

Step 6: Coarse aggregate quantity estimate

The more is the coarse aggregate quantity the less will the cost be in concrete production. The

finer is the sand and the higher is the coarse aggregate particle size the higher will the coarse

aggregate volume be and thus, a workable concrete will be produced. The coarse aggregate

volume in 1 m3 of concrete can be determined by means of the data in Table 2.11 using the

maximum aggregate particle size and the fineness module of the fine aggregate. This volume is

multiplied by the dry loose unit weight and converted to coarse aggregate dry weight (Mehta and

Monteiro, 2006).Table 2.11 - Finding the Volume of Coarse Aggregate in 1 Cubic Meter of Concrete

Maximum

Aggregate Particle

Size (mm)

Dry-Skewered Big Aggregate Volume According to Various Fineness

Modules of Fine Aggregate (m3)

Fineness Modules of Fine Aggregate

2.40 2.60 2.80 3.00

9.5 0.50 0.48 0.46 0.44

12.5 0.59 0.57 0.55 0.53

19.0 0.66 0.64 0.62 0.60

25.0 0.71 0.69 0.67 0.65

37.5 0.75 0.73 0.71 0.6950.0 0.78 0.76 0.74 0.72


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Step 7: Determining the fine aggregate quantity

After Step 6 has been completed, the fine aggregate quantity may be determined according to the

weight or volume method. However, with the use of automation in ready mixed concrete sector in

recent years, today calculations are usually made according to volume method. In this method, the

volume of water, air, cement and coarse aggregate is substracted from the volume of 1 m3 and the

volume of fine aggregate is found. The weight of the fine aggregate can be determined bymultiplication of this value by the density of the fine aggregate.Step 8: Aggregate humidity correction

In concrete mixture calculations, the aggregates are assumed to be in saturated-surface dry status.

However, aggregates may be more humid or dry in comparison with the current weather

conditions. If the humidity correction is not made in such a case, the actual water-cement

proportion of the trial mixture will behigher or lower than the water-cement proportion selected in

Step 4. The coarse and fine aggregate weights calculated as described above (in Steps 6 and 7) are

valid for dry aggregate. These weights should be included into the calculations as saturated-

surface dry as well.Step 9: Preparation of trial mixture

After the calculations above, a trial mixture of nearly 20 dm3 should be prepared and slump testshould be carried out in fresh concrete. The unit weight and air quantity of the fresh concrete

should also be measured. After the samples taken from the fresh concrete are cured under certain

conditions and for certain periods, they should be tested to find their pressure resistances in the

determined ages. When the desired workability and resistance has been obtained after a few trials,

the mixture proportions obtained in the laboratory should be reflected to the site applications

(Mehta and Monteiro, 2006).2.2. ProductionThere are two separate processes in ready mixed concrete production which are preparation and

actual production stages. At the preparation stage, the sandpits near the site are analyzed and the

suitable source of aggregate is determined. Cement is usually supplied in bulk in silos. After the

materials to be used in the concrete mixture are determined, the concrete mixture calculations

described at the first stage of this part are made (Akman, 1987).

The actual production stages of concrete consist of measurement, mixing, transportation,

placement and curing stages. The measurement is carried out according to the weight or volume

method. The weight method which is more widely used today can only be applied in concrete

plants with automatic scales. In automatic scales, measurements with different sensitiveness are

performed for each component. These sensitivenesses are usually selected as 20 g for chemical

additives, 1 kg for water, cement and mineral additives and 5 kg for aggregates. The allowed

deviations in weighing for mixture blends of component materials according to TS EN 206-a

standard should not exceed the limit values provided in Table 2-13 for 1 m3 or more of concrete.

Table 2.13 - Tolerances in the mixtures of component materials

Component

material

Tolerance

Cement

3% of required quantityWater

Total aggregate

Mineral additive

Chemical additive 5% of required quantity

2.2.1. Production procedure of ready mixed concrete


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There are two main objectives in ready mixed concrete production. The first one is that the

concrete has the desired characteristics (required quality) and the other is that the concrete at the

desired quality is prouced in the most economic way (Usta, 2005). There are two types of Ready

Mixed Concrete production produced by mixing of the materials brought together at the desired

proportions by computer control and delivered to the consumer as fresh concrete names as dry

and wet system depending on whether the water measuring and mixing operations are performedin the plant or in the transmixer.The ready mixed concrete with dry mixture is the concrete of which the aggregate and cement is

measured in the concrete plant and mixed in the plant or transmixer and the water and chemical

additives, if any, are measured and mixed at the place of delivery. For ready mixed concrete with

dry mixture, the water quantity given at the site (not more than that provided for in the formula)

and the mixing period (sufficient period for a homogenous mixture) should be given importance.

The ready mixed concrete with wet mixture is the ready mized concrete of which all components

including water is measured and mixed in the concrete plant and, it must be poured into the form

within a limited period of time.

In both systems, the aggregate brought to the production facility is classified according to size and

stored in star or bunker type tanks. The cement and additives are stored in special silos and tanks.The quantities of raw materials to be used according to the class and type of concrete are

determined in advance and the relevant data are stored in the computer on the automation system.

The production is performed by means and under control of these computers.To analyze the quality and compatibility of the materials to be used in ready mixed concrete

production (cement, aggregate, water, additives), laboratory experiments are carried out at the first

stage. In order to avoid adverse changes in the course of time in the materials used in these

experiments, continuous quality inspections should be performed. Having stocked the materials o

sufficient quantity with quality complying with the standards for production of concrete with the

desired characteristics, precise measurement and use of the materials to be used in the concrete

mixture, performance of the mixing operation in a suitable manner and for a sufficient period o

time significantly affects the characteristics of the concrete (Usta, 2005).The mixing period of concrete is defined as the period elapsed from the moment when all

materials have been placed in the plant mixer and the mixing operation has started until the

completion of the mixing operation. The production stage of the ready mixed concrete starts with

the plant operator determining the number of the formula defining the concrete to be produced and

operating the computer system. After the first command, the aggregate, cement, mineral additive,

water and chemical additive stored in separate compartments are scaled in different scales. Then,

the scaled aggregate is coveyed to the mixer tank by belt or hopper. The volume of a blend o

concrete is usually between 1 and 3 m3 according to the volume of the mixer. The mixing

operation in the plant should be continued until the concrete gains a uniform appearance. The

sufficiently mixed blend is transferred to the transmixer and the same operation is carried out untilthe transmixer capacity is reached.The star type plant is a plant which has a star shaped storing area in front of the plant and where

the aggregate is transferred to the mixing tank behind by means of the hopper. The bunkered plant

is a concrete plant where the aggregate and sand is stored in the bunkers in front of the plant and

transferred to the mixing tank in front of the plant by means of a belt system (Kafal, 2004).Today, production of various concretes according to the desired characteristics is possible by

virtue of an automation system to be established in the plant where various concrete mixture

formulae are entered into the system and automatic dosing is provided and, production is carried

out under computer control. In a ready mixed concrete system; dosing, process monitoring,

production reporting, alarm monitoring, order management, production planning, stock control,

truck scale, transmixer monitoring, quality control laboratory reporting are the parts where theautomation system can be established. In the dosing automation which must exist in a ready mixed


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concrete plant; it is ensured that, when the concrete mixture formulae have been entered into the

system and the production command have been given, the sufficient quantities of materials

required by the desired mixture formula (cement, aggregate, additives, mixing water, etc.) are

automatically scaled and mixed at suitable proportions (Kafal, 2004).2.2.2. Mixing

What is expected from the mixing operation is that the surfaces of all aggregate particles arecovered by cement paste and a uniform mixture is obtained. Today, there are four types of mixers

widely used in ready mixed concrete plants. These are tiltable rotary tub, non-tiltable rotary tub,

pan type and double compartment mixers (Neville and Brooks 2001; Erdoan 2007).Tiltable rotary tub mixers are suitable for mixtures with low workability and mixtures containing

coarse size aggregate. In non-tiltable rotary tub mixers, the axis of the mixer is always horizontal

and the discharge is performed by connecting a pipe to the tub where mixing is performed. Since

the discharge is slow, a certain extent of decomposition may occur. In double tub mixers generally

used in road construction, there are two compartments in a single tub. The materials are mixed for

a short period of time after they have been placed in the first compartment and the mixing

operation is concluded after the mixed materials have been transferred to the second compartment.

Thus, the capacities of these mixers operating more rapidly and faster are higher (Neville andBrooks, 2001).Mixing period: The producers of ready mixed concrete intend to mix the concrete as fast as

possible in the plant to increase the capacity. So, the minimum mixing period for producing a

uniform concrete must be known. The optimum mixing period depends on the type and size of the

mixer, the rotation speed and the mixing quality of the components during loading. (Neville and

Brooks, 2001). The minimum recommended mixing periods are provided in Table 2.14.Table 2.14 - Minimum recommended mixing periods (ACI 304-89 and ASTM C94-92)

Mixer capacity (m3)Mixing period (minutes)

0,8 1

1,5 1 2,3 1 3,1 1 3,8 2

4,6 2 7,6 3 Water should be added after 1/4 of the mixing period has been elapsed. The values in the table are

for usual mixers. However, in many modern larger mixers, the mixing period is between 1 and 1,5

minutes. The mixing period in high speed pan type mixers is 35 seconds. On the other hand, the

mixing period should not be less than 5 minutes when light aggregate is used. The optimummixing period depends on the mixer type, mixer status and rotation speed of the mixer tank. The

mixing period for concrete produced with angular aggregate should be longer than the mixing

period for concrete produced with uncrushed natural aggregate. The mixing period for 1 m3 o

concrete should at least be 1 minute. The mixing period should be increased by 1/4 minute for

each additinal 1 m3 of concrete (Mindess and Young, 1981).Mixing for long time: If the mixing lasts long, the water in the mixture evaporates and the

workability decreases. A secondary effect of mixing for long time is that, especially due to the

decomposition of weak aggregate, the aggregate gradation becomes finer and the workability

decreases. Furthermore, the friction between the component materials increases the mixture

temperature. Mixing for long time in air entrained concrete decreases the air quantity by 1/6

depending on the air entraining additive type. Adding water to increase workability decreasesresistance and increases shrinkage (Neville and Brooks, 2001).


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2.2.3. Checking the ready mixed concrete quantityThe quantity of ready mixed concrete produced or delivered may be checked by the seller,

composer by various methods. One of these methods may be the calculation of the concrete

volume which is found by division of the weight of all materials by the fresh density of concrete.

This check may be performed by comparing the unit volume weight of the fresh concrete obtainedby using the weight of the transmixer before and after filling and/or filling reports with the unit

volume weight of the fresh concrete obtained in the laboratory by experimental procedures.

Estimation of the fresh density of the concrete or assumption by selecting a value might lead to

significant mistakes.2.3. TRANSPORTATIONA transmixer is a vehicle similar to a truck which was designed specially for transporting the fresh

concrete to the place where it will be used without any decomposition in the characteristics of the

concrete. Ready mixed concrete is a construction material which must be consumed within

maximum two hours after production. Therefore, transmixers are among the most important

production tools of a ready mixed concrete plant. Transmixers may have various carrying

capacities of 4, 6, 8 and even 12 cubic meters. Many ready mixed concrete plants in Turkey havetransmixers and pumps with the latest technology. Technology has a very important role in quality

ready mixed concrete production (Kafal, 2004).2.3.1.Impact of transportation on workability of ready mixed concreteReady mixed concrete is affected not only from time but also from the transportation method. The

mixing and transportation methods of ready mixed concrete are provided in Table 2.15 (Dewar

and Anderson, 1992).

Table 2.15 - Mixing and transportation of ready mixed concrete (Dewar and Anderson, 1992)

MethodMixing in the plant Transportation in mixerMixing on site

1Mixing completely in the

plant or mixer

Mixer tub rotates at a

certain speed

Re-mixed for a short period

of time only

2

Mixing partially in the plant

or mixer using a certain part

of the water

Mixer tub does not rotate

Remaining water is added

and concrete is re-mixed

for a few minutes

3(a)Mixing completely in the

plant or mixerMixer tub does not rotate

Re-mixed for a short period

of time only

3(b)Mixing completely in the

mixerDumper truck Nothing needs to be done

The transportation of ready mixed concrete must be performed as rapidly as possible. Under

normal conditions, a negligible loss of viscosity occurs within the first 30 minutes after the start ocement hydration. If concrete is mixed periodically, a loss of viscosity occurs in the course of time

however this normally does not lead to any problems in placement and settlement within 90

minutes (Mehta and Monteiro, 2006). Today, losses of viscosity have become easily controllable

parameters as result of the development of chemical additive systems.The transportation of ready mixed concrete is usually performed by transmixers with capacities o

6 m3 or 8 m3. The rotation speed of the cylinder tub in the transmixers is usually low such as 1-2

rpm. In low viscosity mixtures, dmper equipments are used. Very high viscosity concrete may

restrict carrying capacity. While mixers of 6 m3 are normal size mixers, some companies use

larger mixers (e.g. 8 m3 or 10 m3). Concrete mixers of 12 m3 capacity are also used in our

country. For smaller works, mixers of 2-3 m3 capacity can aldo be used (Newman and Choo,

2003).2.3.3. Uniformity of the Mixture


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The effectiveness of the transmixer in the uniformity of the mixture can be measured by taking

various samples from the mixture. According to ASTM C94-92a standard, samples should be

taken from 1/6 and 5/6 of the concrete. The differences between the two samples should not

exceed 16 kg/m3 in the concrete density, 1% in the air percentage, 25 mm in the viscosity value,

6% in the aggregate left on sieve of 4,75 mm, 1.6% in the density of the mortar and 7,5% in the

pressure resistance (average of 3 cylinders for 7 days) (Neville and Brooks, 2001)2.3.4. Delivery of fresh concreteThe user shouls agree with the producer on matters such as the delivery date and time, the concrete

quantity to be supplied in unit time (speed), the special method of carriage in the site, special

placement methods of the fresh concrete, type (mixing/non-mixing equipment), size, height or

gross weight of the delivery vehicles (TS EN 206-1, 2002).

2.4. PLACEMENTThe next operation following the transportation of the ready mixed concrete is placement of the

concrete into the forms where it will be left for hardening and be placed during its service life. A

concrete which was designed and transported very well but was not placed properly since the

requirements were not respected might not fulfil its functions. Therefore, the placement of the

fresh concrete to its place in the construction is at least as important as the design andtransportation. There are many methods for moving the concrete from the transmixer to it place.

The choice of method depends on the economic factors and the quantity of the concrete to be

carried. Wheelbarrows, discharge chute, conveyor belts and pumps can be listed as the most

widely known and used methods. The most important matter in all conditions is the selection and

production of the concrete complying with the method chosen for placement and in a manner to

avoid cohesion and segregation (Neville and Brooks, 2001).2.4.1. Setting time and loss of workabilityReady mixed concrete starts hardening within approximately four hours depending on the ambient

conditions and the mixture characteristics. However, the loss of viscosity is very important and

concrete must be placed before its viscosity decreases (Newman and Choo, 2003). The fresh

concrete temperature should not bebelow 5 C at the time of delivery. Where there are conditionsfor the lowest and highest concrete temperatures differnt from this temperature, these temperatures

should be mentioned along with the deviation limits. Any conditions regarding the heating or

cooling of fresh concrete before delivery should be determined by mutual agreement of the

producer and the user (TS EN 206-1, 2002).2.4.2. Placement and compactionThe equipments used widely today for placement of concrete are truck-mounted chutes, conveyor

belts and pumps. Concrete should not be poured from excessive height to minimize segregation.

Generally, the concrete mixture is placed in horizontal layers such that the thickness will be

uniform and each layer should be compacted before the next layer is placed. he placement

operation should be performed at a sufficient speed. Thus, the layer below should not have lost itsplasticity when a new layer is placed on it. This prevents cold joint (Mehta and Monteiro, 2006).Placement and compaction is the process of forming of concrete. Here, the purpose is avoiding

entrapped air in concrete. Today, by use of vibrators, placement of mixtures with low water-

cement proportion or high aggregate quantity can easily be performed. High viscosity mixtures

should be compacted carefully without allowing any segregation. Vibrators should only be used

for compaction operation and should not be used for moving the concrete horizontally (Mehta and

Monteiro, 2006).Vibration is a widely used method for compaction of concrete. The internal friction between the

coarse aggregate particles decreases at a high rate by vibration. As a result, the mixture acts like a

fluid and starts flowing to empty areas. The primary purpose of the use of vibrators is avoiding

entrapped air. For this purpose, the vibrator is immersed into the concrete rapidly and removedslowly. Internal vibrators are used for compaction of column, joist, wall and flooring concrete


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(Mehta and Monteiro, 2006).Placement and compaction operations are independent from each other and carried out

simultaneously. These operations are of great importance for the required resistance,

impermeability and durability of the hardened concrete in the construction. Concrete should not be

poured in columns and walls in such a manner that they will rise 2 meters an hour and delays

should be avoided since long delays may lead to cold joints. In order to prevent segregation indeep sections tremie pipe may be used (Neville and Brooks, 2001).2.4.3. VibratorsAs result of compaction of the concrete by vibration, the entrapped air gets out of the concrete and

the particles get closer to each other. The vibration application gives excellent results especially in

dry and solid mixtures. Insufficient vibration or excessive vibration leads to non-uniform

compaction and concrete with the necessary resistance cannot be obtained with insufficient

compaction. On the other hand, excessive vibration may cause segregation (Neville and Brooks,

2001).Internal vibrators are most widely used vibrators in practice. In such vibrators, a steel tube called

diver (dalc) is connected to a motor by means of a flexible hose. A weight connected

excentrically in the steel tube creates high frequency vibrations by rotation and these vibrationscompacts the concrete (Neville and Brooks, 2001).External vibrators are fixed on the form and, when vibration is applied, both the form and the

placed concrete are exposed to vibration. External vibrators are often used perticularly in precast

concrete production.

The user might need some information regarding the composition of the concrete to use the

suitable method in placement and curing of fresh concrete and estimate the resistance development

of the concrete. The information regarding the type and resistance class of the cement, type of the

aggregates, type of the chemical additives, type and approximate quantity of mineral additives, i

any, target water/cement proportion, resistance development and sources of the component

materials should be provided to the user by the producer (TS EN 206-1, 2002).

2.5. MAINTENANCECuring is the process of maintaining the sufficient humidity amount and temperature for a certain

period. The cement hydration is a long process and it requires water and suitable temperature to

continue. So, curing allows continuous hydration for the cement and as a result, the resistance o

the concrete increases. If concrete is not cured after being poured, depending on the ambient

conditions, it gains nearly 50% of the resistance of continuously cured concrete. If concrete is

cured for 3 days only, it gains nearly 60% of the resistance of continuously cured concrete. If it is

cured for 7 days, it gains nearly 80% of the resistance of continuously cured concrete. When the

temperature rises the hydration speed increases and thus the resistance increases. Curing not only

increases the resistance but it also improves other important characteristics of the concrete

regarding durability such as durability, water-tightness, wear resistance, frost-defrost resistance,etc.Curing should start after the setting period of the cement has expired. If the concrete is not cured

after setting, then shrinkage cracks occur. Curing operatin may be performed in three ways. The

first way is covering the surface of the concrete with wet cloth at the early ages to preserve the

water in the concrete. The second one is covering the surface with impermeable paper or plastic

cover or coating it using a chemical additive to prevent loss of mixture water from the concrete.

The third one is the accelerated curing methods to increase resistance by providing heat and extra

humidity such as steam curing, isolation blankets and covers and various heating techniques

(Mamlouk and Zaniewski, 1999).

The curing method to be selected depends on the size and shape of the construction, cost of the

material to be used and availability of the material to be used. In pratice, mostly the water-saturated humidity retainer covers (cotton cloth, curing blanket) are used. The cover can be kept


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wet by watering periodically or covering the cover with polyethylene film. In small works, sand,

chip dust might provide the sufficient wetness. Evaporation from the concrete can be decreased by

use of impermeable paper. Plastic cover or polyethylene film is used for this purpose.

Impermeable ppars are suitable for horizontal surfaces. The use of impermeable paper is also

suitable for simple shaped constructions. Plastic covers are effective in concrete constructions with

varying shapes. Where concrete is expected to gain early resistance, steam curing practice givesgood results (Mamlouk and Zaniewski, 1999).2.5.1. Curing PeriodCuring period should be as long as possible. The cement type, mixture component proportions,

required resistance, suitable air conditions, size and shape of the constructure and curing period

affects the necessary curing period. The curing period at temperatures over 5 C should beminimum 7 days or curing should be continued until 70% of the designed pressure or bending

resistance is achieved. In case of use of cement with high early resistance and at temperatures over

10 C, a curing period of 3 days maybe sufficient (Mamlouk and Zaniewski, 1999).In order to determine the curing period, the information regarding the resistance development o

the concrete may be provided by referring to the table below or drawing the resistance

development curve between 2 days and 28 days at 20 C (TS EN 206-1, 2002). The resistancedevelopment of the concrete at 20 C is shown in Table 2.13.Table 2.13 - Resistance development of concrete at 20 C (TS EN 206-1, 2002)

Resistance developmentEstimated resistance rate fcm,2/fcm,28

Rapid 0,5Medium 0,3


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Figure 2.19 - Estimation of the resistance development of concrete according to various methods

2.5.2. Humidity and its impactHumidity is a very important parameter in hydration process (Mehta and Monteiro, 2006). The

resistance of the concrete cured continuously in humid ambiance may be three times the resistanceof the concrete cured continuously in open ambiance. The rate of water loss from the concrete

immediately after the placement changes depending on not only the surface/volume proportion but

also the temperature, relative humidity and wind (Mehta and Monteiro, 2006).The recommended curing period for normal portland cement concrete is 7 days of curing in humidambiance. The curing period for concrete containing mineral additives such as fly ash should be

longer. Humid curing may be provided by covering the surface of the concrete with wet sand, chip

dust or curing blanket. Since the quantity of water used in the concrete mixture is more than the

quantity of water needed for hydration (30% of the cement weight), the use of impermeable

membrane is an effective way to provide resistance increase (Mehta and Monteiro, 2006).2.5.3. Temperature and its impactSome measures should be taken for pouring concrete in both cold and hot weather. When the

temperature of the fresh concrete drops below 10 C, the resistance gain rate of the concrete slowsdown due to the hydration reactions of cement. On the other hand, as the temperature of the

concrete increases, an increase occurs in the hydration reaction speed as well. While someresearchers express that the hydration speed of the concrete increases until the temperature rises up

to 100 C, some others say that the temperatures over 50 C do not have a significant impact inthe resistance gain speed of the concrete. Furthermore, delayed ettringite which is a durability

problem might also be observed in the concrete at temperatures over 70 C. Therefore, thestandards of many countries require that the temperature of the concrete at the time of placement

to be between 10 and 32 C and, it is expressed that the optimal concrete temperature is between15 and 18C.REFERENCES

ACI Committee 211, ACI 211.1-91: Standard Practice for Selecting Proportions for Normal,Heavyweight and Mass Concrete, ACI Manual of Concrete Practice, American ConcreteInstitute, Farmington Hills, MI, USA, 2006.

Akman, M.S., Yap Malzemeleri, First Edition, stanbul Teknik niversitesi naat Fakltesi


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Matbaas, 1987.Dewar, J.D., Anderson, R., Manual of Ready-Mixed Concrete, Blackie Academic&Professional,

An Imprint of Chapman&Hall, 1992 Taylor&Francis Group, LLC.

Erdoan, T. (2004), Sorular ve Yantlaryla Beton Malzemeleri, Trkiye Hazr Beton BirliiYaynlar, stanbul.

Kafal, M.A., Hazr Beton Sektr Aratrmas, Trkiye Kalknma Bankas A.. AratrmaMdrl, May 2004, Ankara.Mamlouk, M.S., Zaniewski, J.P., Materials for Civil and Construction Engineers, Addison-

Wesley, An Imprint of Addison Wesley Longman, Inc., 1999.

Mehta, P.K., Monteiro, P.J.M., Concrete Microstructure, Properties, and Materials, Third Edition,

McGraw-Hill, 2006.

Mindess, S., Young, J.F., Concrete, Prentice-Hall, Inc. Englewood Cliffs, New Jersey 07632,

1981.

Neville, A.M., Brooks, J.J., Concrete Technology, Revised Edition-2001, Prentice Hall.

Newman, J., Choo, B.S., Advanced Concrete Technology Processes, Butterworth-Heinemann, An

Imprint of Elsevier, 2003, UK.

TS EN 206-1, Beton- Blm 1: zellik, Performans, malat ve Uygunluk, Trk StandardlarEnstits, April 2002, Ankara.TS 802, Beton Karm Tasarm Hesap Esaslar, Trk Standardlar Enstits, June 2009, Ankara.Usta, H., Hazr beton sektr aratrmas, October 2005.

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