Lecture 1-Proportioning Concrete Mixtures

7/29/2019 Lecture 1-Proportioning Concrete Mixtures

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Proportioning Concrete Mixtures


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What is concrete?

Concrete, is a product or mass made by the

use of a cementing medium.

Generally, this medium is the product of

reaction between hydraulic cement and water

(Neville and Brooks, 1987).


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HOW??

Concrete is prepared by mixing cement (binder), sand (fineaggregate), gravel (coarse aggregate) and water with specificproportions.

The cement and the water will react through the hydration process.The hydrated cement composed of four major compounds namely,tri-calcium silicate (C3S), di-calcium silicate (C2S), tricalciumaluminate (C3A) and tetracalcium aluminoferrite (C4AF). The mostimportant products of the hydration reaction are the calcium silicatehydrate (C-S-H) and the calcium hydroxide (CH).

The hydration reactions of the major compounds can be written asbelow:

23233 )(362 OHCaHSCHSC

23232 )(42 OHCaHSCHSC

633 6 AHCHAC

636324 10)(2 FHCAHCHOHCaAFC


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Proportioning of concrete mixtures

The proportioning of concrete mixtures is the process ofarriving at the right combination of cement, aggregates,water, and admixtures for making concrete according togiven specifications.

The process is considered an art rather than a science Although many engineers do not feel comfortable, with an

understanding of the underlying principles and, with somepractice, the art of proportioning concrete mixtures can bemastered.

Given an opportunity, the exercise of this art is veryrewarding because the effect of mix proportioning on thecost of concrete and several important properties of bothfresh and hardened concrete can be clearly seen.


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The mix design process


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Components of Modern Concrete

Concreteis a composite material that consistsessentially of a binding medium within whichare embedded particles or fragments ofaggregate.

To obtain concrete with certain desiredperformance characteristics the followingsteps are essential:-

1. The selection of component materials2. A process called mixture proportioning, which

means achieving the right combination ofcomponents.


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Purpose of mix proportioning

Conventionally, the two most essential requirements ofmix proportioning are the workability of fresh concreteand the strength of hardened concrete at a specifiedage.

Durability is another important property, but it isgenerally assumed that under normal exposureconditions durability will be satisfactory if the concretemixture develops the necessary strength.

Another purpose of mix proportioning is to obtain aconcrete mixture satisfying the performancerequirements at the lowest possible cost.


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Purpose

The overall objective of proportioning concrete

mixtures can therefore be summarized as selecting thesuitable ingredients among the available materials anddetermining the most economical combination thatwill produce concrete with certain minimum

performance characteristics. The tools available to the engineer to achieve this

objective are limited. An obvious constraint in concretemixture proportioning is that within a fixed volume youcannot alter one component independent of others.

For example, in a m3 of concrete, if the aggregatecomponent is increased, the cement paste componentdecreases.


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Purpose

With concrete-making materials of given

characteristics and with given job conditions (i.e.,structural design, and equipment for handlingconcrete), the variables generally under thecontrol of a mix designer are as follows:

The cement paste-aggregate ratio in the mixture, thewater-cement ratio in the cement paste, the sand-coarse aggregate ratio in the aggregates, and the useof admixtures.

The task of mixture proportioning is complicated by

the fact that certain desired properties of concretemay be oppositely affected by changing a specificvariable.

The process of mixture proportioning boils down tothe art of balancing various conflicting requirements.


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Approaches for concrete mix design

The weight method and the absolute volumemethod.

In the weight method, the unit weight of freshconcrete is known from previous experience forthe commonly used raw materials and is used tocalculate the weight of the last unknowncomponent of concrete, usually the sand.

If the unit weight of fresh concrete (wet concrete)

is known, we have

admixturesandaggregatewatercementconcretewet WWWWWW


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Absolute volume method

In the absolute volume method, the total

volume (1m3) is equal to the sum of volume of

each ingredient given by:-

1)( airvolumeWWWWW

admixture

admixture

sand

sand

aggregate

aggregate

water

water

cement

cement


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Density (unit weight) of fresh concrete

= density (unit weight) of fresh concrete, kg/m3

a = weighted average bulk specific gravity (SSD) ofcombined fine and coarse aggregate

A = air content, %

C = cement content, kg/m3

= specific gravity of cement (generally 3.10 for Portlandcement)

W = mixing water requirement, kg/m3

31110010 mkgWCA aaa


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Rough estimate Density (unit weight) of

fresh concrete

31110010 mkgGWG

GCAGU am

c

amam

where Um is the weight of fresh concrete, kg/m3; Ga

is the weighted average bulkspecific gravity (SSD) of

combined fine aggregate and coarse aggregate,

assuming reasonable weight proportions; Gc is thespecific gravity of cement; A is the air content, %;

Wm is the mixing water content, kg/m3; Cm is the

cement content, kg/m3.


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Mass of fine aggregate

The volume method is an exact procedure forcalculating the required amount of fineaggregate. Here, the mass of fine aggregate,Af, is given by:

3/101000 mkgAACWAc

cff

where

Ac = coarse aggregate content, kg/m3

f= bulk specific gravity (SSD) of fine aggregate

c = bulk specific gravity (SSD) of coarse aggregate


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General Considerations

When the aggregate under consideration

contains alkali-reactive minerals, the use of

pozzolanic admixtures in combination with a

high-alkali cement may turn out to be themore cost-effective alternative, at times.


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General

Further cost reduction is possible, without

compromising the essential performance

characteristics of a concrete mixture, if

cheaper and suitable materials are found toreplace a percentage of Portland cement.


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Specific Principles

When reviewing the following specific principlesfor selecting concrete mixture proportions, it willbe helpful to remember again that the underlyinggoal is to strike a reasonable balance between

the workability, strength, durability, and cost ofconcrete.

A key consideration governing many of theprinciples behind the procedures forproportioning concrete mixtures is therecognition that; cement costs much more thanaggregates.


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Specific

Among all the constituents of the concrete,the admixture has the highest unit cost,followed by cement.

When a material is available from two or moresources and a significant price differentialexists, the least expensive source of supply isusually selected unless there are

demonstrable technical reasons that thematerial will not be suitable for the job athand.


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Workability

Workability embodies certain characteristics of freshconcrete, such as consistency and cohesiveness.

Consistency, broadly speaking, is a measure of the wetnessof the concrete mixture, which is commonly evaluated interms ofslump.

To obtain the specified slump, the mixture waterrequirement generally decreases as:1. The maximum size of a well-graded aggregate is increased;

2. The content of angular and rough-textured particles in the

aggregate is reduced;3. The amount of entrained air in the concrete mixture isincreased; and

4. Coal fly ash is used as a partial replacement for a cement.


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Cohesiveness

Cohesiveness is a measure of compactibility andfinishability, which is generally evaluated by

trowelability and visual judgment of resistance to

segregation.

In trial mixtures when cohesiveness is judged as

poor, it can usually be improved by taking one or

more of the following steps:

increase the sand/coarse aggregate ratio, partially replacethe cement or sand with coal fly ash, and

increase the cement paste/aggregate ratio.


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Cohesiveness

Obviously, due to its lower density, fly ash has the

ability to increase the cement mortar/aggregate ratio

by volume without an increase in the cement, water,

or sand content of the mixture. There are no standard requirements for workability

because they may vary from one job to another,

depending on the type of construction and the

equipment used to transport and consolidateconcrete.


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Strength

From the standpoint of structural safety, the

strength of concrete specified by the designer is

treated as the minimum required strength.

Therefore, to account for variations in materials;methods of mixing, transportation, and

placement of concrete; and curing and testing of

concrete specimens, ACI Building Code 318requires a certain degree of strength overdesign,

which is based on statistical considerations.


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Strength

In other words, depending on the variability oftest results, the mixture proportions selectedmust yield a mean or average strength higherthan the minimum or the specified strength.

It should be noted that the average strength, notthe specified strength, is used in mixture designcalculations.

Although other factors also influence strength,

the tables and charts used for the purposes ofmixture proportioning assume that strength issolely dependent on the water-cement ratio andthe content of entrained air in concrete.


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Selection of design strength

The mix design process will deal with a targetaverage strength. The average strength

selected must take into account:

1. The degree of variability anticipated.

2. The degree of certainty of avoiding rejection

required.

3. Any early age strength requirement.

4. The required durability.


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Target mean strength

Characteristic strength is a lower limit ofstrength to be used in structural design.

As with all materials, concrete has an

inherent variability of strength, and anaverage cube compressive strength (or

target mean strength) somewhat above

the characteristic strength is therefore

required.


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deviationdards

strengthSpecifiedF

strengthaveragerequiredX

where

kFX

tan


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Variability

Knowledge of the is required to determinethe target strength. which is a measure of thespread of results assuming concrete strength

to be a normally distributed variable.

For an existing concrete production facility will be known from previous tests.

Where limited or no data are available, the

upper values given in the Figure below whichhas been derived from analysis of the data

from many production facilities can be used.


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Variability

Values ofcan range from less than 2.0 MPa

to more than 6.0 MPa so that the required

target average strength can vary by 6 MPa or

more according to the degree of controlachieved.


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The normal distribution


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k value

k = a constant depending on the proportion ofresults permitted to be below F.

The difference between the characteristic andtarget mean strength is called the margin; a 5%

failure rate is normally chosen for concrete

In the USA the permissible percentage defectiveis usually 10% giving a k value of 1.28.

In most of the rest of the world the percentage is5% giving a k value of 1.645 (which in the UK isrounded to 1.64 and in Australia to 1.65).


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Durability

when concrete is subject to normal conditions

of exposure, the mix-proportioning procedures

ignore durability because strength is

considered to be an index of general durability.

However, under conditions that may tend to

shorten the service life of concrete, its

durability may be enhanced by specialconsiderations in mixture proportioning.


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Durability

For example, entrained air is required with allexposed concrete in climates where freezing andthawing cycles occur.

Concrete exposed to chemical attack by deicingsalts or acidic or sulfate waters may require theuse of water-reducing and mineral admixtures.

In such a situation, although a higher water-

cement ratio would have satisfied the strengthrequirement, a lower water-cement ratio isusually specified considering the exposureconditions.


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Procedures

Mathematical approaches to determine the

correct proportion of component materials of

a concrete mixture meeting a given set of

specifications generally do not work becausethe materials vary widely in their

characteristics large number of empirical

methods based on extensive test datadeveloped from local materials.


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Procedures

The method recommended by ACI Committee211, is popular in the USA and many othercountries in the world.

To the extent possible, the following

background data should be gathered beforestarting the calculations:

1. Sieve analysis of fine and coarse aggregate;

fineness modulus2. Dry-rodded unitweightof coarse aggregate

3. Bulk specificgravityof materials


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Procedures

4. Absorptioncapacity or free moisture in the aggregate

5. Variations in the approximate mixing waterrequirement with slump, air content, and grading ofthe available aggregates

6. Relationship between strength and W/Cfor availablecombinations of cement and aggregate

7. Job specifications if any [e.g., maximum water-cement ratio, minimum air content, minimum slump,maximum size of aggregate, and strength at earlyages (normally, 28-day strength is specified)].


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Steps in ACI 211 method

Step 1: Choice of slump

Step 2: Choice of maximum size of aggregate

Step 3: Estimation of the mixing water content

and air content Step 4: Selection of water-cement ratio

Step 5: Calculation of the cement content

Step 7: Estimation of the fine aggregate content Step 8: Adjustments for the aggregate moisture

Step 9: Trial batch adjustments


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Flow chart forselection and

documentation

of concrete

proportions


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Examples

Concrete is required for a column that will bemoderately exposed to freezing and thawing.

The cross section of the column is 300 300

mm. The smallest spacing between reinforcingsteel is 30 mm. The specified compressive

strength of concrete at 28 days is 40 MPa with

a slump of 80 to 100 mm. The properties of

materials are as follows:


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Cement used is type I Portland cement with aspecific gravity of 3.15.

The available coarse aggregate has a maximumsize of 20 mm, a dry-rodded unit weight of 1600kg/m3, a bulk specific gravity (SSD) of 2.68,absorption capacity of 0.5%, and moisturecontent (oven-dried, OD) of 0.25%.

The fine aggregate has a bulk specific gravity(SSD) of 2.65, absorption capacity of 1.3%, amoisture content (SSD) of 3%, and a finenessmodulus of 2.60.

The aggregates conform to the ASTM C33-84requirements for grading.


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With the given information, the mix design will be carried throughin detail, using the sequence of steps outlined.

Step 1: Choice of slump. The slump is given and consistent with

Table 3-7. Step 2: Maximum aggregate size. The maximum aggregate size is

20 mm, which meets the limitations of 1/5 of the minimumdimension between forms and 3/4 of the minimum clear space.

Step 3: Estimation of mixing water and air content. The concretewill be exposed to freezing and thawing; therefore, it must be airentrained. From Table 3-8, the recommended mixing water amountis 180 kg/m3, and the air content recommended for moderateexposure is 5.0%.

Step 4: Water/cement ratio (w/c).According to both Table 3-1 andTable 3-3, the estimate of the required w/c ratio to give a 28-day

compressive strength of 40 MPa is 0.35. Step 5: Calculation of cement content. Based on the steps 3 and 4,

the required cement contentis 180/0.35 = 514 kg/m3.


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Step 6: Estimation of coarse aggregate content. FromTable 3-11, for fineness modulus of the fine aggregateof 2.60, the volume of dry-rodded coarse aggregate per

unit volume of concrete is 0.64. Therefore, there willbe 0.64 m3 coarse aggregate per m3 volume concrete.And, the OD weight of the coarse aggregate is 0.64 1600 = 1024 kg. The SSD weight is 1024 1.005 = 1029kg.

Step 7: Estimation of fine aggregate content. The fineaggregate content can estimated by eitherthe weightmethod or the volume method.

(a) Weight method. From Table 3-12, the estimated

concrete weight is 2280 kg/m3

. Although for a first trial it is not generally necessary to use

the more exact calculation based on Equation, this valuewill be used here:


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3

3

/2314

167.218015.3

67.21514510067.210

1110010

mkg

mkgGWG

GCAGU am

c

amam

Based on the already determined

weights of water, cement, and coarse

aggregate,the SSD weight of the fine aggregate is

2314 180 514 1029 = 591 kg.


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(b) Volume method. Based on the known

weights and specific gravity of water, cement,

and coarse aggregate, the air volume, the

volumes per m3 occupied by the different

constituents can be obtained as follows:

3

3

3

384.068.21000

1029:

163.015.31000

180:

18.01000

180:

mXG

WSSDAggCoarse

mXG

Wcement

mW

water

wateraggCoarse

aggcoarse

watercement

cement

water

water


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Air: 0.05 m3;

Therefore, the fine aggregate must occupy avolume of 1 (0.180 + 0.163 + 0.384 + 0.05) =

0.223 m3.

The required SSD weight of the fine aggregateis:

0.223 2.65 1000 = 591 kg.


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Step 8: Adjustment for moisture in theaggregate.

Since the aggregates will be neither SSD norOD in the field, it is necessary to adjust theaggregate weights for the amount of water

contained in the aggregate. Since absorbed water does not become part

of the mix water, only surface water needs tobe considered.

For the given moisture contents, the adjustedaggregate weights become:


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Coarse aggregate (stock): From

W(stock) = W(OD)[1 + MC(OD)]

Get: W(stock) = 1024 1.0025 = 1026 kg

The extra water needed for coarse aggregateabsorption is

W(SSD) W(stock) = 1029 1026 = 3 kg

Fine aggregate (stock): 591 1.03 = 609 kg/m

3

Extra water provided by fine aggregate: 609 591= 18 kg

The mixing water is then: 180 + 3 18 = 165 kg.

Thus, the estimated batch weights per m3 are asfollows: water, 165 kg; cement, 514 kg; coarseaggregate, 1026 kg; fine aggregate, 609 kg; total,2314 kg.


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Step 9: Trial mixes. Trial mixes should be

carried out using the proportions calculated.Theproperties of the concrete in the trial mix

must be compared with the desired

properties, and the mix design must becorrected as described.


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Proportioning of High-Strength

and High-Performance Concrete Mixtures

ACI Definition

HPC is defined as a concrete meeting specialcombination of performance and uniformity

requirements that cannot always be achievedroutinely using conventional constituents andnormal mixing, placing, and curing practices.

Mehta and Aitcin suggested the term HPC

mixtures that possess the following threeproperties: high-workability, high-strength, andhigh durability.


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For a variety of reasons, the ACI211 procedure forconcrete mixture proportioning needs updating.

Since it was developed when concrete mixtures wererequired to meet rather narrow specifications for

compressive strength at 28 days (15 to 45 MPa, Table3-1) and consistency (25 to 175 mm slump).

To satisfy todays high-construction speeds with heavilyreinforced structural elements, concrete placement by

pumping is the common practice now, and this meansthat concrete mixtures are designed to have at least125 to 150 mm slump.


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Also, high-strength and high-performanceconcrete mixtures are being designed forcompressive strength values from 50 to 100 MPa,which is outside the range ofW/C- compressivestrength relationship given byACI-211 (Table 3-1).

The use of mineral admixtures andsuperplasticizers is much more prevalent now,

and ACI 211 guidelines do not adequately dealwith concrete mixtures containing thesecomponents.

M ht d Ait i P d f


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Mehta and Aitcin Procedure of

proportioning HSC/HPC

Mehta and Aitcin developed a sequential, eight-step procedure for proportioning of high-performance concrete mixtures containingsuperplasticizers, mineral admixtures, and 28-day

compressive strength values between 65 to 120MPa.

To provide adequate dimensional stability (e.g.,high elastic modulus, and low drying shrinkage

and creep), the procedure assumes a fixed ratioof 35 to 65% by volume between the cementpaste and the aggregate.


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Mehta

For a 1 m3 batch of concrete containing 0.35 m3cement paste, having known the volume of waterand assuming a certain amount of entrapped orentrained air, the total volume of the

cementitious material can be computed bydifference.

Next, the procedure provides options in thechoice of the cementitious material, i.e, whether

to use Portland cement alone or to use partialreplacement of the cement by one or moremineral admixtures such as fly ash, slag, and silicafume.


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Mehta

To complete the computations for the first trialbatch, a 2:3 ratio by volume between the fineaggregate and the coarse aggregate is assumed.

This trial is used to determine the dosage of the

superplasticizer for obtaining the desiredconsistency and for adjustment of a proper ratiobetween CA : FA.

In general, depending on the type of the

superplasticizer and the physical-chemicalcharacteristics of the cementitious material, thesuperplasticizer dosage may vary from 1 to 3l/m3.


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Mehta

Note that a change of emphasis from the w/c-

strength relation to the water content-

durability relation will provide the necessary

incentive for incorporation of particle packingconcepts into the concrete mixture

proportioning methods.


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Mix proportions and material properties for the

range of RCC concretes used at the Willow Creek

Dam are shown in the Table


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Exercise 1

Use the American method to design a concretemix that is required to have a specified meanstrength of 30 MPa at 28 days. The presence ofreinforcement requires a slump of 75mm and amaximum size of aggregate of 10 mm. The

aggregates are of normal weight, and gradingsconform to the appropriate standard with afineness modulus of 2.8. (Assume that absorptionis 0.7% and moisture condition of the aggregates

is SSD; the bulk density of coarse aggregate is1600 kg/m3; and there will be extreme exposurecondition to freeze-thawing.)


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Exercise 2

Use the American method to design a concrete mixthat is required to have a specified mean strength of 25MPa at 28 days. The presence of reinforcementrequires a slump of 3050 mm and a maximum size ofaggregate of 40 mm. The aggregates are of normalweight and gradings conform to the appropriatestandard with a fineness modulus of 2.8. (Assumethere is negligible absorption and moisture content; adry-rodded bulk density (unit weight) of coarseaggregate is 1550 kg/m3, and there is a bulk specificgravity (SSD) of 2.70; the fine aggregate has a bulkspecific gravity (SSD) of 2.65; and the concrete will bein extreme exposure conditions.)

Lecture 1-Proportioning Concrete Mixtures

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