Ch- 6 Special Concreting Techniques and Mix Design

8/18/2019 Ch- 6 Special Concreting Techniques and Mix Design

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Prepared By:- Prof. Anuj Chand

Special concreting

techniques and Mixdesign

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• A concrete which can be pushed through a pipeline is called a pumpable concr

• The concrete mix is designed in such a manner that it does not wedge whiland its friction at the inner wall of the pipe line does not become very high.

• Pumpable concrete emerging from a pipe line flows in the form of a plug

separated from the pipe wall by a thin lubricating layer consisting of cement p

water in the paste hydraulically linked with the interparticle water layer in the

Pumped concre e

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• Shotcrete or gunite is a mortar or a fine concrete that is pneumatically tran

through a hose and projected on to a surface at a high velocity.

• This system is called by different names is different countries such as

!uncrete "et#crete $ucrete spraycrete etc. though the principle is essen

same. This system is very well suited for construction of lightly reinf

section.

• Shotcrete is more economical than conventional concrete because of less form

re%uirements re%uired a small portable plant for manufacture and placement.

"ho cre e or #uni in$

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• The various application of shotcrete are &

Thin overhead vertical or hori'ontal surface.

Swimming pools and prestressed tanks.

(anal and tunnel lining.

)epair of damaged concrete.

*verlays on concrete roads.

)efractory lining works.

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• Several forms of e%uipment are available for shotcreting by this techni%ue.

• A common layout includes an air#compressor am material hose air and water ho

gun and a pressure tank or pump for water supply.

• The various stages involved in the dry#mix process are as follows

+.The cement and sand are thoroughly mixed. The mixture of cement#sand is fed int

air#pressuri'ed mechanical feeder termed as ,cement gun-.

.The mixture is metered into the delivery hose by a feed wheel or distributor.

/. This material is carried by compressed air though the delivery hose to a special no

no''le is fitted with a perforated manifold through which water is introduced under

and intimately mixed with other ingredients.

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0. The mortar is jetted from the no''le at high velocity on to the surface t

shotcreted. Any alternation in the %uantity of water can be easily accomp

by the no''leman.• Advantages of 1ry#mix process& #

• The dry mix process is preferred for light weight aggregate concrete.

• The lower w2c ratio obtained with the dry process probably accounts f

lesser crep higher strength and greater durability of concrete.

• The dry process e%uipment can convey the material to distance of /33

433m hori'ontally and 04 to +33 m vertically.

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,e mi) proce**

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• 5n this process cement sand small si'ed coarse aggregate and water are m

entering the chamber of delivery e%uipment.

• The )eady 6ixed (oncrete 7)6(8 is received into a feeding chamber from which

concrete is blown by compressed air at a pressure of 4.4 to 9 atmospheres t

rubber hose.

• :%uipments are available which can place concrete at the rate / to ; cubic me

hour. Additional air is injected at the no''le to increase the velocity and impr

gunning patterns.

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• Placing concrete under water

• Special precautions should be taken whenever concrete is to be placed under water.

• Such a concrete should have cement content atleast 043


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• 5n some situations gunny bags are filled about two#thirds full with dry o

dry mixture of cement fine and coarse aggregate. They are lowered inwater and placed carefully in a header and stretcher fashion like that of

masonry construction with the help of divers.

• This method does not give satisfactory concrete as the concrete mass

full of voids.

Ba$$ed concre e

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• *ther method of placing concrete under water or in a trench filled wi

bentonite slurry is by bottom dump bucket method.

• 5n the bottom dump bucket concrete is taken through the water in a wate

bucket of box.

•

*n reaching the final position the bottom of the bucket is made to open bymechanism and the whole concrete is dumped slowly in water.

• There are chances of washing away of some %uantity of cement when con

dumped from the bucket.

Bo om dump uc e

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• The most satisfactory method of placing concrete under water is by the u

tremie pipe.• The word tremie is derived from the >rench word hopper.

• 5n this method tremie pipe of 33 mm to 43 mm diameter is used.

• The length of pipe can be easily increased or decreased by using copulings.

• A funnel is provided at the top end of pipe to facilitate pouring of concrete.

• The bottom end is closed with a plug or thick polyethylene sheet to prevent

of water into the pipe.

remie

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• The pipe is lowered and made to rest at the point where the concrete is going to b

The concrete having a very high slump of about +4 to 3 cm is poured into the fun

• Precautions to be taken during concreting by tremie method

a8(offerdams or forms should be sufficiently tight to reduce the flow of water to les

m per minute through the space to be concreted.

b8All the time the lower end of the tremie pipe should be kept well embedded

concrete.

c8*nce concreting has started the tremie should not be moved laterally through the d

concrete as this will again disturb the concrete. 5f it is necessary to shift the

should be lifted out and moved to the new position.

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d8 Pumping of water should not be allowed whilst concrete is being placed nor

thereafter. 5f simultaneous pumping is done it may suck the cement particles.

e8 ?nder water concreting need not be compacted as concrete gets auto

compacted by the hydrostatic pressure of water.

f8 @igh water2cement ratio is re%uired for high consistency which reduces the

concrete. But at present with the use of super plastici'ers it is possible to

concrete with as low a water2cement ratio as 3./.

g8 hen large %uantities of concrete are re%uired to be placed over an extensive

advisable to use a number of tremies.

Pile (oncreting 7tremie method8.mp0

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• Another method of placing concrete under water is the grouting of prepacked aggre

• (oarse aggregate is dumped in the forms to assume full dimension of the concrete m

• (ement mortar grout is injected through pipes which extend up to the botto

aggregate bed. The pipes are alowly withdrawn as the grouting proceeds.

• The grout forces the water out of the forms and fills the interstices in the aggreg

method however has been used very little. >or plugging the well foundation this m

often adopted.

#rou ed a$$re$a e*

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• (oncrete pumps and pipes can also be used for placing concrete under wate

pipeline is plugged at the end and lowered until it rests on the bottom.

• Pumping is then started when the pipe is completely filled the plug is force

concrete surrounding the lower end of the pipe seals the pipe.

• The pipe is held in this position until the pressure becomes too great. Then the

withdrawn and the operation is repeated. This process is repeated until

reached the level above water.

Concre e pump

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• Prepacked concrete is a special techni%ue of placing concrete under water.

• hen ,tremie method- or ,bottom dump bucket method- are not found feasible this

is adopted.• This techni%ue also called ,grouted concrete- consists of placing the coarse aggreg

in the form and thoroughly compacting it to form a prepacked mass.

• This mass is then grouted with the cement mortar of the re%uired proportions.

• This process can be employed for both plain or reinforced cement concrete.

• This method is employed where the reinforcement is very complicated or where

arrangements like pipes conduits openings are re%uired to be incorporated in the

Pre pac ed concre e

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• The production of concrete in cold weather introduces many problems such as delay

and hardening damage to concrete in plastic condition due to the formation of ice le

• @ence it is essential to maintain the temperature of the concrete above 4 ( it

accepted that there is little cement hydration and strength gain if concrete is fro'en

fro'en below +3 (.

• Therefore fresh concrete must be protected against disruptive expansion by free

ade%uate strength has been gained.

• Cower concrete temperatures are permitted for massive sections because the amo

generated is very large.

Cold wea her concre in$

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• The effects of cold weather on concrete are as follows&

A.Delayed setting:-

• The rate of hydration of cement depends upon the temperature. 5f temperature is low

process will go slow and concrete takes a longer time to set to develop strength.

• The setting period necessary before removal of formwork is thus increased.

B.Freezing of concrete at early age:-

•

hen the temperature of concrete falls below free'ing point the free water held in thconcrete free'es.

• 1ue to free'ing of water concrete expands and hydration of cement will be stopped.

• This will result in considerable loss of strength.

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C. Freezing and thawing:-

• hen concrete is subjected to alternate cycle of free'ing and thawing its durab

is greatly impaired.

• 5t has been found that even one cycle of free'ing and thawing durin

prehardening period may reduce the compressive strength to 43D of what wou

expected for normal temperature concrete.

D. Stresses due to temperature differential:-

• 5n case of mass concreting in cold weather there will be a large temp

differential due to higher temperature inside the mass which may promote

cracking and has harmful effect on durability of concrete.

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• (oncreting in hot weather especially in tropical countries and desert areas w

temperatures above 03 ( are reached posses some problems.

• @igh temperature and reduced relative humidity are the main climatic factors

concrete.

• 5n 5ndia most of the areas are in tropical regions.

• The procedure of concreting in hot#weather is et out in 5S & 9EF+#+;94 7Part#58

• The effects of hot#weather are as follows

o wea her concre in$

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A. Rapid rate of hydration:-

• A higher temperature results in a more rapid hydration leading to %uick sett

reducing the handling time of concrete and also lowering the strength of h

concrete.

• ith the increase in the temperature of concrete the workability of concrete de

and water demand increases.

• The addition of water without proper adjustments in the mix proportions ad

affects the ultimate %uantity of concrete.

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B. Rapid e aporation of mi!ing water:-

• As mentioned earlier due to high ambient temperature the water mixed

concrete to give the re%uired workability will be lost by evaporation. T

workability of concrete will be reduced.

• Such concrete cannot be properly compacted and it will result in redu

strength.

• The rate of evaporation depends on the ambient temperature relative hum

wind speed.

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C. Rapid e aporation during curing:-

• @ot weather re%uires early and a continuous effort for curing particularly w

grade cement is used.

• 5f there is only lapse the concrete surface dries up fast interrupts the cont

hydration.

• The subse%uent wetting does not fully contribute to the development

strength.

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D. Air- entrainment:-

• At higher temperature it is difficult to control the air content in air#ent

concrete.

• >or a given amount of air#entraining agent hot concrete entrains less air tha

concrete at normal temperatures.

". #ncreased tendency of crac$ing:-

• )apid evaporation of mixing water leads to plastic shrinkage cracking

subse%uent cooling of hardened concrete introduces tensile stresses.

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• A concrete whose constituents are weight#batched at a central batching plan

either at the plant itself or in truck mixers and then transported to the construand delivered in a condition ready to use is known as ready mixed concrete

• This techni%ue is very useful in congested sites or at diverse work places and

consumer from the botheration of procurement storage and handling of

materials.

• The use of ready mixed concrete is also advantageous when only small %u

concrete are re%uired or when concrete is placed only at intervals.

2eady 3i) Concre e 23C

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Concre e 3i) (e*i$n

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• (oncrete mix design may be defined as the art of selecting suitable ingredients of c

and determining their relative proportions with the object of producing concrete o

minimum strength and durability as economically as possible.• *bjectives of mix design

• The purpose of concrete mix design is to ensure the most optimum proportion

constituent materials to fulfill the re%uirements of the structure being built.

• 6ix design should ensure the following objectives.

• To achieve the designed2desired workability in the plastic stage.

• To achieve the desired minimum strength in the hardened stage.

Concre e 3i) (e*i$n

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• To achieve the desired durability in the given environmental conditions.

• To produce concrete as economically as possible.

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• The following points must be considered while designing concrete mixes.

+.(ost

.Specifications.

/. orkability

0.Strength and durability.

Ba*ic con*idera ion*

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• The cost of concrete is made up of 7+8 material cost

•

7 8 e%uipment cost• 7/8 labour cost.

• The variation in the cost of materials arises from the fact that cement is sever

costlier than aggregate.

• So it is natural in mix design to aim at as lean a mix as possible.

co*

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• The following points may be kept in mind while designing concrete mixes.

• 6inimum compressive strength re%uired.

• 6aximum water2cement ratio.

• 6aximum cement content to avoid shrinkage cracks.

• 6aximum aggregate2cement ratio.

• 6aximum density of concrete in case of gravity dams.

"peci ca ion*

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• The following points related to workability shall be kept in mind while designing

mixes.

• The consistency of concrete should be no more than that necessary for placing c

and finishing.

• >or concrete mixes re%uiring high consistency at the time of placing the us

reducing and set#retarding admixtures should be used rather than the addition of mo

• henever possible the cohesiveness and finishibility of concrete should be imp

increasing sand2aggregate ratio rather than by increasing the proportion of the fin

in the sand.

,or a ili y

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• Strength and durability re%uire low w2c ratio.

•

5t is usually achieved not by increasing the cement content but by lowering demand at a given cement content. ater demand can be lowered by through con

the aggregate grading and by using water reducing admixtures.

" ren$ h and dura ili y

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• The wide use of concrete as construction material has led to the use of mixes

proportions which ensure ade%uate strength. These mixes are called nomi• They offer simplicity and under normal circumstances have a margin of streng

that specified.

• $ominal mix concrete may be used for concrete of grade 6 4 6 9.4 6 +3 6

6 3.

7ominal 3i) concre e

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• The concrete mix produced under %uality control keeping in view the

durability and workability is called the design mix.• *ther factors like compaction e%uipment available curing methods adopte

cement %uality of fine and coarse aggregate etc. have to be kept in mi

arriving at the mix proportion.

• The design mix or controlled mix is being used more and more in v

important structures because of better strength reduced variability le

with conse%uent economy as well as greater assurances of the resultant %

(e*i$n mi) concre e

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• According to 5S 04F# 333 and 5S +/0/#+;E3 the important factors influencing the design of concret

• !rade of concrete

• Type of cement

• 6aximum nominal si'e of aggregate

• !rading of combined aggregate

• 6aximum water2cement ratio

• orkability• 1urability

• Guality control

8ac or* in9uencin$ he choice of mi)de*i$n

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3e hod* of Concre e 3i) (e*i$n


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• 5.S. method

• A.(.5. method

• )oad $ote#0 method

• 5)( #00 method

• Arbitrary method

• 6aximum density method

• >ineness modulus method

• Surface area method

• 6ix design for high strength concrete

• 6ix design for pumpable concrete

• 1*: mix design method

3e hod of Concre e 3i) (e i$n

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• The Bureau of 5ndian Standards recommended a set of procedure for design of

mix.

• The mix design procedures are covered in 5S & +3 F #+;E .

• The 5S recommended guidelines for mix design include the design of normal

mixes for both medium and high strength concretes.

.". 3e hod of 3i) (e*i$n

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(a a for mi) de*i$n


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• (haracteristic compressive strength of concrete at E days.

• 1egree of workability desired.

• Cimitations on the water2cement ratio and the minimum cement to ensure

durability.

• Type and maximum si'e of aggregate to be used.

• Standard deviation of compressive strength of concrete.

(a a for mi) de*i$n

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.". 3e hod (e*i$n " ep*

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" ep-1 ar$e * ren$ h for mi) de*i$n


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• The target average compressive strength of concrete at E days is given by

fck H fck I 7t . S8 where fck H target average compressive strength at E

fck H characteristic compressive strength at E days.

s H standard deviation

t H a statistical value depending upon the accepted

of low results and the number of tests.

ep 1 ar$e ren$ h for mi) de i$n

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• Jalue of t

Accepted proportion of low results %alue of t

+ in 4 3.E0

+ in +3 +. E

+ in +4 +.43

+ in 3 +.F4

+ in 03 +.EF

+ in +33 .//

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• According to 5S & 04F# 333 and 5S & +/0/# +;E3 the characteristic strength i


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According to 5S & 04F# 333 and 5S & +/0/# +;E3 the characteristic strength i

the value below which not more than 4 D 7 + in 38 results are expected fal

cases the above e%uation reduced to&

• fck H fck I +.F4 . S

Grade of concrete Assumed standard deviation(s) N/mm2

3 10; 3 15 %.53 !0; 3 !5 4.03 %0; 3 %5; 3 40; 3 45; 3 50 5.0

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" ep-! "elec ion of wa er-cemen


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ep-! elec ion of wa er-cemenra io

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• Approximate amount of entrapped air to be expected in normal concrete is gi

K.

" ep-%


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• Approximate sand and water contents per cubic meter of concrete for grade up t

• w2c H 3.F3 workability H 3.E3 (.>.

ep 4 elec ion of wa er con enand ne o o al a$$re$a e ra io

&ominal ma!imum

size of aggregate 'mm(

)ater content per cu*ic

meter of concrete '$g(

Sand as + of total

*y a*solute olum

+3 3E 03

3 +EF /4

03 +F4 /3

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• Approximate sand and water contents per cubic meter of concrete for grade above

• w2c H 3./4 workability H 3.E3 (.>.

• Adjustment of values in water content and sand D for other conditions.7P.T.*.8

&ominal ma!imum

size of aggregate 'mm(

)ater content per cu*ic

meter of concrete '$g(

Sand as + of total

*y a*solute olum

+3 33 E

3 +E3 4

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(hange in condition stipulated Adjustment re%uired in


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ater

content

Perc

total

>or sand confirming to grading 'ones 5 555 or 5J of 5S &/E/#+;93

7standard 'one is 558

3 I+.4#+.4

#/.3

5ncrease or decrease in value of compacting factor by

+.3 7Standard value H 3.E38

L /D 3

:ach 3.34 increase or decrease in free water cement

ratio. 7standard value H 3.F38

3 L +D

>or rounded aggregate #+4


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• The cement content per unit volume of concrete may be calculated from the

cement ratio obtained in step# and the %uality of water per unit volume o

obtained in step#0 .

• The cement content so obtained should be checked against the minimum cement

for the re%uirements of durability as per table 4.0 7table#4 5S &04F# 3338 and

of the two alues adopted .

ep 5 calcula ion of cemen con en

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" ep-6 Calcula ion of a$$re$a e


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ep 6 Calcula ion of a$$re$a econ en

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• H 6ass of water 7kg8 per m= of concrete


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H 6ass of water 7kg8 per m of concrete.

• ( H 6ass cement 7kg8 per m= of concrete.

• Sc H specific gravity of cement say /.+4

• p H ratio of fine aggregate to total aggregate by absolute volume.

• fa (a H total masses of fine aggregate and coarse aggregate 7kg8 per m= o

respectively.

• Sfa Sca H specific gravities of saturated surface dry fine aggregate and coarse

respectively.

• $ormally Sfa H .F and Sca H .9 are used.

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AC 3e hod of de*i$n mi)

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• 5n the ?SA the method suggested by the American (oncrete 5nstitute 7A(58 i


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used.

• The A(5 method is based on the fact that for a given maximum si'e of well

aggregate water content is largely independent of mix proportions i.e. water cconstant regardless of various in water2cement ratio and cement content.

• The method further assumes that the optimum ratio of the bulk volume o

aggregate to the total volume of concrete depends only on the maximum

aggregate and on the grading of fine aggregate regardless of shape of particles.

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" ep-1 (a a o e collec ed


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• >iness modulus of >.A.

• ?nit weight of dry rodded (.A.

• Specific gravity of >A and (A saturated surface dry condition.

• Specific gravity of cement.

• Absorption characteristics of both (A and >A.

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" ep-! Calcula e mean de*i$n * ren$ h;


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p ;from he minimum * ren$ h *peci ed;u*in$ * andard de=ia ion:

Grade of concrete Assumed standard deviation(s) N/mm2

3 10; 3 15 %.53 !0; 3 !5 4.0

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Accepted proportion of low results %alue of t


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+ in 4 3.E0

+ in +3 +. E

+ in +4 +.43

+ in 3 +.F4

+ in 03 +.EF

+ in +33 .//

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" ep- %


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• ater#cement ratio is estimated from the table for the mean design strength.ra ioA erage compressi e strength at , days "ffecti e water-cement ratio '*y mass(

/a &on-air entrained concrete Air-entrained con

04 3./E

03 3.0/

/4 3.0E

/3 3.44

4 3.F

3 3.93

+4 3.E304/15/16


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" ep-4 minimum wa er cemen and


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• 1ecide maximum si'e of aggregate to be used. !enerally for )(( work 3 mm and

prestressed concrete +3 mm si'e are used.

• 1ecide workability in terms of slump for the type of job in hand. )ecommended

of slump for various types of construction as given by A(5 ++#+#;+ is given in

en rapped air con en

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" ep- 5 cemen con en


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• (ement content is computed by dividing the water content by the water2cement

ep 5 cemen con en

" ep- 6 Bul =olume of dry roddedcoar*e a$$re$a e per uni =olume ofconcre e

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" ep-& he wei$h of CA per cu icme er of concre e i* calcula ed ymul iplyin$ he ul =olume wi h ulden*i y of CA

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" ep-'


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concre e

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" ep- + a *olu e =olume*

in$redien * per cu ic me er of concre eare o ained y nowin$ he *peci$ra=i y of cemen ; wa er;

CA and 8A

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" ep-10 rial mi) propor ion* arcalcula ed and adju* men * for lecondi ion* li e free moi* ure and wa ea *orp ion y a$$re$a e are made

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" ep-11 A rial mi) i* hen made


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p )* udy he proper ie* of concre e

re*pec of wor a ili y; cohe*i=ene*ni*hin$ >uali y and !' dacompre**i=e * ren$ h. he propor ionof CA and 8A may e chan$ed o $ede*ired proper ie*.

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Ch- 6 Special Concreting Techniques and Mix Design

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