Class Note - Concrete (Moddified 10-01-2069)

CLASS NOTES

ON

CONCRETE TECHNOLOGY

Class notes on Concrete Technology (P -1) Er. Sudip Karanjit – Khwopa Engineering College

1.0 INTRODUCTION

1.1 CONCRETE AS STRUCTURAL MATERIAL:

At present concrete is one of the most widely used material in the world (˜12 billion ton per

year). Human consume no material in such a tremendous quantity except water. Due to its

ease of use and wide range of applicability in structural & nonstructural component, its

quantity & application area is growing more & more. Presently some of the areas of

application are listed below:

i) Building: Various structural & nonstructural components.

ii) Bridge: Including pier & super structures.

iii) Steel structure: Foundation base for various steel structures.

iv) Tunnel / Underground Structures.

v) Slope Stabilization: Shotcreting / Ground retaining structures.

vi) Offshore Structure: Various marine structures for oil/gas extraction.

vii) Nuclear Shield: As concrete is excellent in protection of radiation.

viii) Refractory: High temperature application.

ix) Fire Proofing: Widely used in fireproofing of steel structure.

x) Reinforced/ precast/ prestressed concrete components: Various new technology of

use of concrete, making its use more popular & wider. Etc.

1.2 COMPONENT OF MODERN CONCRETE:

i) Cement.

ii) Aggregate (Fine / Coarse)

iii) Water.

iv) Admixture.

v) Other additives.

1.3 TYPES OF CONCRETE:

I) Based on compressive strength:

a) Low Strength: fck < 20 Mpa

b) Medium Strength: fck = 20 – 40Mpa

c) High Strength: fck >40 Mpa

fck = 28days characteristic strength

of concrete.


II) Based on Unit weight:

a) Normal weight concrete: Concrete having unit wt. in between 1800 – 3200kg/m3 is

treated as normal wt concrete. Mostly used concrete in our practice have in average

2400kg/m3.

b) Light weight concrete: Concrete having unit wt. in between 300 – 1800kg/m3. Used

for non structural & lightly loaded structural component. Can further classified into :

Light wt. Aggregate concrete: Produced using light wt. aggregate.

Light wt. Aerated concrete: Produced introducing air bubble in concrete using

chemicals.

Light wt. No fine concrete: Produce without using fine aggregate.

c) Heavy weight concrete: Concrete having unit wt. > 3200 kg/ m3. Use heavy wt.

natural aggregates as well as steel aggregate. Used in nuclear plant as radiation shield.

# Other Special types (depending upon material used):

III) Sulphur infiltrated conctere: This concreting process involves the impregnation of

sulphur into concrete by various methods. Improvement in physical property has been

reported several hundred times. Can be used in precast concrete elements.

IV) Fibre reinforced concrete: As we know concrete is fairly good in compression but

its tensile strength is very less so as to improve the same various types of fibers are being

used in concreting (these fibers are mixed with other ingredient in green stage). Normally

have diameter 0.25 – 0.75mm & length to dia. ratio 30 – 150. Eg: steel fiber, glass fiber,

carbon fiber etc.

V) Polymer Concrete: This concreting process involves the impregnation of polymer

into concrete or by mixing it in green stage. Can be used in precast concrete elements &

marine structure where high durability required against corrosive environment.

VI) Ferrocement: Ferrocement is considered as a new material consists of cement sand

mortar & wiremesh. The cement mortar used is with cement: sand ratio 1:2 to 1:3 with

W/C 0.40 to 0.45 & wiremesh 0.5 to 1.0mm dia @ 5 – 10mm spacing. Very thin section

can be formed with this material (2-3cm thk.). Now a days its widely used in construction

of water tanks, manway cover, pressure pipes, various architecture shape etc.

VII) Bacterial Concrete: Bacteria called bacillus pasteruii develops calcite (CaCO3)

during its metabolism which can be utilized as cementing material for sealing crack in

concrete & rock structures. As calcium carbonate is the natural & environmental friendly

cementing / bounding agent, above bacteria is utilized to fill the crack ( in natural rock

strata / oil well etc ) mixing with sand and silica fume.


# Other Special types (depending upon application method):

VIII) Guniting / Shotcreting: Concreting process in which concrete is pneumatically

projected to surface at high velocity. Normally used in tunnel lining, refractory

concreting, slope stabilization & numerous other application where placing of formwork

is difficult.

IX) Vacuum Concrete: As we know higher the W/C ratio higher the workability of

concrete but it results low strength concrete in harden concrete. In this concreting process,

concrete is initially placed with higher W/C ratio & immediately after placing, excess

water is removed through special equipment setup (through vacuum process).

Fig; Vacuum Concreting.

X) Roller Compacted Concrete: Roller compacted concrete is mainly no slump, almost

dry concrete compacted by vibrator roller. This type of concrete is mainly used in dam &

other mass concreting process. Concrete is placed in thin layers (20-30cm) to allow

complete compaction. Cement content ranges from 60 - 360kg/m3.


XI) Self Compacted Concrete: This type of concrete is designed to flow under its own

weight with very high degree of workability. Max. aggregate size is limited to 20mm &

uses HRWR (High Range Water Reducing admixture). This type of concrete is

extensively used in many projects due to: it easiness & speed in placing, reduced

manpower, good surface finish and durability etc.

1.4 PROPERTIES OF HARDEN CONCRETE:

i) Strength (Compressive & Tensile)

ii) Durability

iii) Modulus of Elasticity

iv) Poisons ratio

v) Thermal Property


2.0 AGGREGATE

Aggregate is one of the main constituent of concrete which occupy around 70 – 80% of the

body of concrete. Variation in aggregate property also affects the property of concrete

(Strength, workability & durability).

2.1 SOURCE AND CLASSIFICATION:

I) Depending upon its Origin aggregate can be classified into:

a) Natural aggregate: Aggregate derived from natural source (eg, quarries or river) by

blasting, crushing or screening. Can be further classified into:

Igneous: Normally all type of aggregate from igneous rock source is considered

suitable for concreting.

Metamorphic: As aggregate from metamorphic rock source shows weak plane of

foliation is not considered very suitable but aggregate like quartzite & gneiss still

produce good concrete of its class.

Sedimentary: The quality of aggregate from sedimentary rock source varies

considerably depending upon the pressure in which original rock is compacted &

cementing material in it. Normally limestone, siliceous sandstone can produce good

quality concrete.

b) Artificial aggregate: Artificially made aggregate for special concrete or industrial

byproducts.

Clean broken bricks: Obtained by cleaning broken bricks. Not suitable for wear & tear

surface & can be used in low to medium strength concreting.

Blast furnace slag: This is the byproduct of pig iron in blast furnace.

Steel shots: Steel aggregate made for high density concrete. Used in nuclear plants.

II) Depending upon its Unit wt.:

a) Light wt.: Aggregate with sp. gravity< 2.5. Used to produce concrete with unit weight

upto 1200kg/m3.

b) Medium wt.: Aggregate with sp gr. 2.5-2.7. Commonly used in construction. Produce

concrete with unit wt. 2300-2600kg/m3.

c) Heavy wt.: Aggregate with sp gr. >2.7. Normally used in radiation shield. Eg, ferro-

phosphorus 5.8-6.8, magnetite 4.2-5.2, iron shots 6.2 - 7.8 sp gr.

III) According to its Size:

a) Fine Aggregate: Normally aggregate passing through 4.75mm sieve is considered as

fine aggregate. IS383:1970 further classified this into various zone depending upon its

fineness.

b) Coarse Aggregate: Normally aggregate retained on 4.75mm sieve is considered as

coarse aggregate. Coarse aggregate is normally represented by is nominal max size.


IV) According to its Shape:

a) Round: Aggregate with round particles, directly obtained from river or quarries

without any processing. Due to its minimum surface area these type of aggregate

gives good workability with lower W/C ratio also & require minimum cement paste

for bonding. But due to its poor interlocking unsuitable for high strength concrete.

b) Angular: Aggregate with angular particles, obtained by manual or machine crushing.

Due to its angular & rough surface gives good interlocking bond. But gives less

workability & requires more cement paste due to its greater surface area. Suitable for

high strength concrete.

c) Irregular: Aggregate comprising irregular aggregate type (between round & angular

aggregate). Normally obtained from natural quarries. These type of aggregate gives

workability & interlocking in between round & angular aggregate. These are also

considered unsuitable for high strength concrete.

d) Flaky: Aggregate having its least dimension less than 0.6 times its mean dimension.

Relatively comprising thin particles. Due to its poor bonding & low strength is not

considered suitable for medium or high strength concrete.

e) Elongated: Aggregate having its larger dimension greater than 1.8 times its mean

dimension. Relatively comprising long particles. Due to its poor bonding character is

not considered suitable for medium or high strength concrete.

V) According to its Surface texture:

a) Glassy: Aggregate having glassy surface. Eg, black flint

b) Smooth: Aggregate with smooth surface texture. Eg, chert, slate, marble etc

c) Crystalline: Aggregate with crystalline surface texture. Eg, basalt, dolerite, granite etc

d) Granular: Aggregate formed essentially with granular material. Eg, sand stone, oolite

etc

e) Honeycombed or porous: Aggregate with porous surface texture. Eg, pumice


2.2 GRADING OF AGGREGATE (IS Code):

Particle size distribution of aggregate is also termed as grading of aggregate. According to IS

code there is two types of aggregate according to their size.

a) Fine Aggregate: Aggregate passing through 4.75mm sieve is considered as fine

aggregate. IS383:1970 further classified this into various zone depending upon its

fineness. Zone I represents the course while Zone IV represents the finer sand.

Workability & uniformity of concrete is affected by its grading.

IS 383:1970 – Aggregate Grading Table.

Fine Aggregate

b) Coarse Aggregate: Normally aggregate retained on 4.75mm sieve is considered as

coarse aggregate. There is two types of grading as per IS IS383:1970, Single sized &

Graded coarse aggregate.

Single sized coarse aggregate may have its nominal size 63, 40, 20, 16, 12.5 and 10 mm.

While graded coarse aggregates are comprise of various size to have densely packed

system. Gradiation limit for single sized & graded aggregates are given below:


Coarse Aggregate- Single Sized aggregate & Graded aggregate As Per IS 383:1970

All in aggregate: Aggregate having different fraction of fine & coarse aggregate to achive minimum void thus

max. compaction while concreting. Grading limit of all in aggregate as given by IS383:1970

is given below:

Grading Limit of all in Aggregate aggregate As Per IS 383:1970

2.3 AGGREGATE PROPERTIES & METHOD OF TESTING:

Refer related books/class notes for detail mechanical/thermal properties. Here we discuss

only about some important / significant properties that affects concrete in green & its harden

stage.


FLAKYNESS INDEX: can be define as percentages by weight of particle with least

dimension (thickness) less than 0.6 times its mean dimension. (Not applicable for particle size

less than 6.3mm)

ELONGATION INDEX: can be define as percentages by weight of particle with largest

dimension (length) more than 1.8 times its mean dimension.

BULKING OF SAND:

Free moisture content in aggregate results in the increase in volume of aggregate also

termed as bulking of aggregate. Any moisture content at the surface of aggregate cause

repulsive force to neighboring aggregate (due to surface tension) which cause increase in

volume of aggregate. This force mainly depends upon the moisture content & size of the

aggregate,

Higher the aggregate size less the effect of bulking phenomenon.

While at particular moisture content bulking take its highest value.

Due to bulking, aggregate shows very unrealistic volume which affects the proportioning of

aggregates. Fine aggregate may have volume increse upto 40% of its actual volume due to

bulking. So the extreme care needs to be taken while performing volumetric proportioning of

concrete.


ALKALI AGGREGATE REACTION:

Aggregates are normally inert material but some aggregate may contain reactive silica which

reacts with alkalies (Sodium / Potasium oxide) present in cement called alkali aggregate

reaction.

In USA must of the structure failures are reported due to this type of reaction.

Reaction stars with attack on reactive siliceous mineral in aggregate by alkaline hydroxide

derived from alkalies in cement. This produce alkali-silica gel with unlimited swelling which

results spreading patterned crack on concrete.

Factors affecting alkali aggregate reaction:

a) Reactive aggregate type: The main cause of alkali aggregate reaction is the presence of

reactive silica in aggregate. Without the presence of reactive silica alkali aggregfate

reaction will not be possible.

b) Alkali content in cement: Another factor that initiates reaction is alkali content in cement.

Lower alkali content greatly reduce the possibility of alkali aggregate reaction. For the

same reason IS Code has limited the alkali content of cement to 0.6%.

c) Moisture availability: Alkali aggregate reaction requires moisture for its initiation, lesser

the available moisture less the chances of this reaction.

d) Optimum temperature: Alkali aggregate reaction finds temperature of 10 – 38 deg C

much favorable for its reaction.


3.0 CEMENT

3.1 DEFINATION AND MANUFACTURING PROCESS:

Cement is considered as one of the most important constituent of concrete as it acts as

binding agent to whole concrete mass.

Property of cement greatly affects the property of concrete; in harden stage as well as in

green stage.

Modern Cement also called Ordinary Portland Cement (as it resembles in colour &

quality with Portland Stone in Dorset, UK).

Primary constituent of Portland cement are:

Calcareous material (Lime Stone, Chalk)

Argillaceous/Siliceous material (Clay, Shale etc)

Process of manufacture of cement consists:

Proportioning of raw material

Grinding & intimate Mixing

Burning in kiln at 1300-1500 deg C (which form fused nodular shape clinker, which

after cooling is ground as fine powder mixing 3-5% gypsum)

Manufacture of cement can be carried out with two method (depending upon mixing &

grinding of raw material) :

a) Wet process: In wet process limestone from quarries crushed to smaller fragments &

is then taken into ball mill where it is mixed with clay or shale & converted to slurry

(creamy liquid) with addition of 35-50% water. Chemical composition of slurry is

then checked & corrected. The slurry is sprayed to rotary kiln against hot surface of

hanging chain. The slurry gets dry & undergoes series of chemical change. The

material gets fused at high temperature & converted into 3-20mm nodular form called

clinker. The clinker so formed is dropped in to cooler & stored in silo/bins. The

clinker is then ground to required fineness in ball-mill in addition of 3-5% gypsum.

b) Dry Process: In dry process raw materials are proportioned & mixing/grinding is

carried out in dry condition. Then is blended/corrected for its composition. By

addition of about 12% of water blended mass is converted in to pellets which permit

air flow & exchange of heat in rotary kiln. The mass is then charged into kiln (1300-

1500 deg C) to form clinker. The clinker so formed is dropped in to cooler & stored in

silo/bins. The clinker is then ground to required fineness in ball-mill in addition of 3-

5% gypsum.

As dry process is quite economic & energy efficient than wet process now a days

most of the plants are upgrading into the dry method of cement manufacture.


Fig: Cement Manufacture Process

Fig: a) Cement Factory, b) Cement Clinker


3.2 OXIDE AND COMPOUND COMPOSITION:

Raw material of cement primarily consists of:

Calcareous material (Lime Stone, Chalk)

Argillaceous/Siliceous material (Clay, Shale etc)

Oxide Composition of which can be represented as:

After fusion the compound composition of Cement/Clinker mainly consists of:

As identification of major compound in cement largely based on RH Bogue’s work its also

called Bogue’s compound & equation to determine quantity of these compound from oxide

composition of raw material is called Bogue’s equation:

Set of Bogue’s equations:

%C3S = 4.071 C – 7.600 S – 6.718 A – 1.430 F – 2.850 𝐒 %C2S = 2.867 S – 0.754 C3S

%C3A = 2.650 A – 1.692 F

%C4AF = 3.043 F

Here,

C – CaO

S - SiO2

A – Al2O3

F – Fe2O3

𝐒 – SO3


Influence of Various compound in Cement:

Tri-calcium silicate C3S:

Mainly contributes the early strength of cement.

Produce high heat of hydration.

Quality/density of C-S-H gel produce from this compound is slightly inferior than from

C2S.

This compound produce more Ca(OH)2 than C2S.

Di-calcium silicate C2S:

Mainly contributes the later strength of cement.

Produce low heat of hydration.

C-S-H gel produce from this compound is dense and with high sp. surface.

This compound produce less Ca(OH)2 than C3S.

More durable than C3S in acidic & sulphur environment.

Tri-calcium aluminate C3A:

Strength contribution of this compound is negligible.

Very high heat of hydration.

This compound is characterized by its very fast reaction leading flash set of concrete; to

control its fast setting character gypsum is added to cement.

Produce C3AH6 cubical compound after hydration.

Harmful for durability as is likely to attack by sulphur.

Tetra-calcium alumino ferrite C4AF:

Strength contribution of this compound is negligible.

Produce high heat of hydration.

Produce C3FH6 after hydration.

Show more resistance to sulphate attack than C3A.


C3A & C4AF in presence of gypsum hydrates to form calcium aluminate trisulphate hydrate

(C6AS3H32) also called “ettringite” & calcium aluminate monosulphate hydrate C4ASH18.

Rate of Hydration of various Compound Rate of Strength gain of various Compound

3.3 HYDRATION OF PORTLAND CEMENT:

Chemical reaction of cement with water is termed as hydration of cement which turns cement

into the binding material with strong adhesive property.

Various compound present in cement reacts individually with water to produce various

hydration products. The major compounds of hydration are C-S-H gel & Ca (OH)2.

a) Hydration C3S & C2S produce C-S-H gel & Ca(OH)2:

b) Hydration of C3A & C4AF:

2C3A+6H = 2C3AH6

C4AF + 2CH + 10H = C3AH6 + C3FH6

Where, C – CaO / S- SiO2 / A – Al2O3 /

F – Fe2O3 / H – H2O / CH- Ca(OH)2

C-S-H gel – C3S2H3 – 3Cao.SiO2.3H2O

C-S-H gel

C-S-H gel


Hydration of C3A in presence of gypsum:

C3A+32H + 3CaSO4 = C6AS3H32 (tri-sulphate hydrate - “ettringite”)

C3A+ 18H + CaSO4 = C4ASH18 (mono-sulphate hydrate)

Addition of gypsum in cement control the flash set reaction of C3A. In presence of gypsum it

reacts with 32 molecule of water resulting large volume change. As this reaction takes place

in green stage of concrete, volume change will not be the problem. But if this takes place in

harden stage of concrete due to external sulphate environment or presence of excess of

gypsum in cement, the large volume change cause crack & detoriation in concrete called

sulphate attack.

Calcium Silicate Hydrate (C-S-H gel) : C-S-H gel is the most important & major product

obtained from hydration. It covers around 50-60% solid volume in completely hydrated paste.

It shows poorly crystalline fibrous mass. It exhibits the strength & binding property to

concrete.

Calcium Hydroxide (Ca(OH)2): Ca(OH)2 is not a desirable product in concrete mass but it

also covers around 20-25% solid volume. It shows distinctive hexagonal prism morphology.

It reacts with sulphate present in environment/water to form calcium sulphate which reacts

with C3A causing detoriation in concrete (also called Sulphate Attack). Ca(OH)2 is alkaline

in nature which maintain PH value of concrete around 13 & resists corrosion of

reinforcement.

Sulphate attack chemistry:

Ca(OH)2+ Sulphur compound = CaSO4

C3A+32H + 3CaSO4 = C6AS3H32 (tri-sulphate hydrate - “ettringite” – cause large volume

change & detoriation in harden concrete)

C-S-H gel

Ca(OH)2

C-S-H gel


Fig: Various microscopic view of Hydrated Cement Paste

Structure of Hydrated Cement / Three phase system of Concrete:

Fresh paste acts like a plastic network of cement partical in water while harden paste at any

stage of hydration contains various compound called C-S-H gel , Ca(OH)2, ettringite & other

compounds. Harden concrete structure is normally considered as a 3-Phase system.

Paste phase: Paste phase is the most important phase of concrete as it influences the

overall behavior of harden concrete. Strength, durability, creep, shrinkage & elastic

property of concrete is greatly affected by paste structure of concrete. In microscopic

view it contains mainly dense network of fibrous C-S-H gel & hexagonal Ca(OH)2.

Aggregate phase: Aggregate phase is also one of the important phase of the concrete &

influence the overall strength of concrete but has less influence than paste phase in

property of concrete.

Transition zone phase: In micro level, between aggregate & paste phase another phase

can be visualized called transition zone phase. This is also called the plane of weakness &

greatly affects the mechanical behavior of concrete. Because of larger concentration of

crystalline compound like Ca(OH)2 & ettringite and also due to bleeding & water

accumulation along aggregate particles, these acts like a weak link in concrete. Due to

shrinkage & temperature variation transition zone develops micro crack and upon loading

the structure these cracks propagate causing failure of bond in concrete.

Water requirement for hydration:

It has been found that the C3S requires 24% of water for its hydration & C2S takes 21% of

water by weight of cement. In average cement requires around 23% of water by its weight.

This 23% of water required for the chemical reaction with cement is also called bound water.

Certain quantity of water also can be found within gel pore called gel water. About 15% of

water requires to fill up gel pore. It has been found that if the water require to fillup the gel-

pore is inadequate formation of gel itself will stop so bound-water & gel-water are also called

Concrete as

3- Phase System


complementary. Total 38% (23%+15%) of water is required to complete the full hydration of

cement. If this quantity is not available complete hydration will not be possible. And also the

addition of water greater than this will cause unnecessary capillary pores in concrete.

Heat of hydration:

As we know the reaction of cement with water is

exothermic. It produce heat during its reaction, the rate

of hydration of various compound in cement is

different thus producing different amount of heat

during the way of reaction. The rapid initial rate of

heat liberation (Peak A in fig.) is due to the rapid

hydration of C3A/C3S while the reduction in heat

liberation occurs due to retardation of C3A cause by

gypsum. The next peak B is due to the reaction of C3S

/ formation of ettringite. While this rate of heat

evolution also decrese withtime.

Verbec & Foster had given the heat of hydration of

various compound after their extensive research &

tests as given below:

3.4 TYPES OF CEMENT:

ASTM (American Society of Testing Material) Classification:

Type I: Normal Cement Type: Also called general purpose cement Used in general

construction type where corrosive environment/ sulphur is not present & the special property

of cement is not required. C3A shall not exceed 15%. This is the most used type of cement in

all type of general construction.

The typical compound compositions: 55% (C3S), 19% (C2S), 10% (C3A), 7% (C4AF), 2.8%

MgO, 2.9% (SO3), 1.0% Ignition loss, and 1.0% free CaO.

Type II: Sulphate Resistance Cement: This type of cement is manufactured to have

moderate sulphate resistance. Normally used where moderate sulphate attack / moderate

corrosive environment is present. Used in underground concrete work & in presence of

ground water where sulphate may present. C3A content is limited to < 8% to control the

sulphate attack.

The typical compound compositions: 55% (C3S), 19% (C2S), 10% (C3A), 7% (C4AF), 2.8%


http://en.wikipedia.org/wiki/Loss_on_ignition

http://en.wikipedia.org/wiki/Loss_on_ignition


TypeIII: High early strength: This cement is similar to Type I, but ground finer. In some

case C3S and C3A content is also increased to achive early strength. This gives the concrete

using this type of cement a three day compressive strength equal to the seven day

compressive strength types I. It may be used in emergency construction and repairs and

precast concrete job.

Its typical compound composition is: 57% (C3S), 19% (C2S), 10% (C3A), 7% (C4AF), 3.0%


TypeIV: Low heat of hydration: The percentages of (C2S) and (C4AF) are kept relatively

high and (C3S) and (C3A) are relatively low. This type of cement gives low heat of hydration

& rate of strength gain is also low. Suitable for mass concreting such as dam where low heat

of hydration is desirable.

Typical compound composition: 28% (C3S), 49% (C2S), 4% (C3A), 12% (C4AF), 1.8% MgO,

1.9% (SO3), 0.9% Ignition loss, and 0.8% free CaO.

TypeV: High Sulphate Resistance: This cement is manufactured with very low (C3A). The

maximum content of (C3A) allowed is 5%. Normally used where sulphate & alkali content is

high which react with (C3A) causing detoriation of concrete.

Its typical compound composition is: 38% (C3S), 43% (C2S), 4% (C3A), 9% (C4AF), 1.9%


TypeIS: (Type I + Slag): Manufactured with (Type I cement + blast furnance slag). Slag

content may varies from 25-70%. This type of cement gives low heat of hydration & better

corrosion resistance. But its strength gain is also slower.

TypeIP: (Type I + Pozzolona): Manufactured with intimate blending of Type I cement &

fine pozzolona. Pozzolona content may varies from 15-40%. This type of cement gives low

heat of hydration & better corrosion resistance. But its strength gain is also slower. Flyash,

Silica fume is common pozzolona material used to produce this type of cement.

TypeIA/IIA/IIIA: (Types I or II or III + air-entraining agent): Have the same

composition as types I, II, and III + air-entraining agent is ground into the mix. They

introduce the fine air bubble in concrete & increase workability / improve resistance to

freezing under low temperatures.

Types II(MH) and II(MH)a: have recently been added with a similar composition as types

II and IIa but with a mild heat.


BIS (Bureau of Indian Standard) Classification:

a) Ordinary Portland Cement (OPC):

Extensively being used in general construction where corrosive environment/ sulphur

is not present & the special property of cement is not required.

Normally available in three different grade – 33grade, 43grade & 53grade.

b) Sulphate Resisting Cement:

Manufactured with low C3A content (<5%) & comparatively low C4AF to prevent

the sulphte attack reaction.

Used where environment (groung water/soil ) is corrosive & sulphur attack is present.

Normally used in foundation, buried pipeline & marine environment.

c) Rapid Hardening Cement:

Similar to OPC, but ground finer.

High C3S and low C2S content is maintained to achieve early strength.

Produce more heat of hydration, creates problem in mass concreting.

Can be used where high early strength is desirable Eg, Prefabrication, road repair,

cold weather.

d) Extra Rapid hardening Cement:

Manufactured with intergrinding Rapid hardening cement with calcium chloride

(CaCl2) < 2%.

Produce very high heat of hydration.

Can be used where very fast hardening is desirable.

In concreting mixing, transportation, placing & finishing needs to be done within

20min.

Storage of this type of cement shall not be greater than 1 month.

e) Low heat Cement:

Manufactured with low C3A & C3S and increased C2S.

Ideal for mass concreting where low heat is desirable.

Heat of hydration for 7days<65Cal/gm and for 28days< 75Cal/gm.

f) Super-sulphate Cement:

Manufactured with granulated slag (80-85%) + hard burnt gypsum (10-15%) + OPC

clinker (5%)

Used where high sulphate resistance is desirable.

Produce low heat of hydration.

Used where environment (groung water/soil ) is highly corrosive & sever sulphur

attack is present.


g) Portland Slag Cement:

Manufacture intergrinding OPC clinker + gypsum + blast furnance slag (25-65%).

Slag need to be the ground granulated blast furnance slag (GGBS), presence

crystalline slag cause low quality cement.

Produce low heat of hydration, making ideal for mass concreting.

Better corrosion resistance.

But have slow strength development.

h) Portland Pozzolana Cement.

Manufactured with intimate blending of OPC cement & fine pozzolona.

Pozzolona content may varies from 15-35%.

This type of cement gives low heat of hydration & better corrosion resistance.

But strength gain is also slower & requires more curing. Ideal for hydraulic structure.

Flyash, Silica fume is common pozzolona material used.

i) Coloured Cement

Manufactured with high quality limestone (96% CaCO3 & Fe2O3<0.07%) with very

low iron oxide content.

5-10% pigment is groung together with OPC clinker to produce colour cement.

To preserve colour due to environmental change pigment are selected accordingly.

j) Hydrophobic Cement:

Manufactured with OPC Clinker + water repellent film forming substance (Oleic acid

/ Stearic acid / Calcium Oleate).

Prevent the cement to react with environmental moisture thus can be used with long

storage & poor storage condition.

The preventive film broken out in mixing process of concreting, allowing cement

particle for hydration.

k) Mortar Cement

Manufactured to overcome the drawback of OPC interms of workability & water

retainability for masonry construction.

Air entraining agent and/or admixtures are mixed with cement to achive better

workability & water retainability.

Ideal for all type of mortar work.

l) Oil Well Cement

Used to fill gap between steel strainer & rock strata in oil well.

Manufactured to withstand the temp. of 175 deg C & pressure of 1300 Kg/cm2 inside

oil-well.


m) High Alumina Cement:

Manufactured with Limestone (CaCO3) + Bauxite (High alumina) heating at 1550-

1600 deg C. & casing in mould (pig).

Upon hydration produce AHn gel.

Characterized by very high alumina content & rapid strength gain.

Produce very high heat of hydration.

Used in high temperature application: Furnance, combustion chamber, boiler etc.

OTHER TYPES AVAILABLE BUT NOT COVERED IN BIS:

a) Air Entraining Cement:

OPC Cement + Air entraining agent mixed together.

Increase workability & resistance to freezing & thawing.

b) Expansive Cement:

Loss of water from concrete due to various reson cause shrinke in concrete causing

shrinkage crack to prevent the same non shrink expansive agents are mixed.

Manufacture with 1 Part cement + 0.08 - 0.2 part sulphoaluminate + 0.15 part stabilizer.

Used where shrinkage might be the problem.

c) Quick Setting Cement:

Manufactured mixing less gypsum in cement & sometime high C3A thus causing fastest.

Used where fast setting is desirable Eg underwater concreting.

d) Rediset Cement:

Initially manufactured in USA as Regset, later in india from research.

Manufactured with high alumina & mixing CaCl2 /lignosulphonate/ Cellulose.

Very rapid hardening, can be used in rapid repair.

High heat & poor sulphate resistance.

Typical mix of 1:3 cement mortar shows 20Mpa in 4hrs & 42Mpa in 24hrs.

e) Very High Strength Cement:

To achive very high strength various new technologies is being introduces & some under

research also.

Warm Press: Applying heat & pressure, upto 650 Mpa had been achived with warmpress

technology.

Silica fume: With the densely packed system upto 270 Mpa had been achived using silica

fume.

Polymer: With various polymer 150-300Mpa had been reported to achived.

Lithium salt: Upto 27Mpa in 3hr & 49Mpa in 1day had been successfully achived with

the use of lithium salt.


4.0 WATER

4.1 Quality of Water used in concrete for various purpose (IS 456:2000):

As given by IS456:2000

Water used for mixing & curing shall be free from any reactive substances that can harm the

concrete in any way. Normally potable water is considered satisfactory with maximum

permissible value as given:

(d) PH value value of water shall not be less than 6.

Sea Water:

Mixing & curing of concrete with sea water is not recommended. But under unavoidable

condition sea water may be used in plain concrete without embedded steel, and taking due

consideration of negative effect of water & using suitable cement system.

4.2 W/C ratio, workability, segregation, bleeding & Other properties of fresh concrete:

Abram’s rule W/C ratio:

Abram’s W/C rule is taken as one of the most popular rule.

This explains the natural consequence of progressive weakening of concrete matrix by

porosity & W/C ratio.

𝒇𝒄 = 𝑲𝟏

𝑲𝟐𝒘/𝒄


fc = Strength of concrete.

K1 / K2 = Constants

W/C = Water Cement ratio

Although this rule explain the weakening of paste phase due to increase in W/C ratio thus

porosity but effect of W/C ratio in transition zone is not explained by this rule.

Workability:

Workability can be defined as the, ease by which concrete can be mixed, placed, compacted

& finished.

There is mainly two force acting against workability,

a) Internal friction between individual particles.

b) Friction between concrete & formwork / reinforcement surface.

Factor affecting workability,

a) W/C ratio – W/C ratio greatly affects the workability of concrete. Higher the W/C

ratio higher will be the workability. But excessive W/C ratio induce unnecessary

voids/porosity in concrete thus reducing strength & durability of concrete.

b) Grading of aggregate – With the proper grading good consistency / workability of

concrete mix can be achieved.

c) Fine Agg. to Coarse Agg. Ratio (FA/CA) – Higher the fine aggregate, higher will be

the water requirement to moisten the surface area of aggregate, which ultimately

reduce the workability of concrete. So workability is inversely proportional to FA to

CA ratio.

d) Aggregate shape – Round aggregate will have less surface area / friction also thus

producing more workability than crushed aggregate.

e) Aggregate texture – Smooth texture aggregate will give less friction thus producing

more workability than rough aggregate.

f) Admixture / Additives – Now a days various chemical & mineral admixtures are

available which can greatly enhance the workability of concrete by its physical

mechanisms.

Measurement of Workability:

There are various method of measurement of workability developed depending upon the mix

consistency. Some popular methods are:

a) Slump Cone test.

b) Compaction factor test.

c) Vee-bee test.

d) Flow table test.

e) Slump flow test.

f) Kelly ball test.

g) K – slump tester.


Various Measurement of Workability:

1) Slump Cone test:

Among the various method of measurement of workability slump cone is the most popular & one of

the easy way to measure the workability. Normally slump cone method is only consider suitable for

plastic mix, for stiff mix this test is not very sensitive. (Other method like Compaction Factor needs to

be used for stiff concrete mix).

Apparatus: i) Slump cone as shown below. Ii) Tamping rod 16mm dia x 600mm length.

Clean the internal surface of apparatus. Place the mould in uniform horizontal surface. Then fill the

mould with 4 layer (1/4 each layer). Tamp the each layer @25 times by tamping rod uniformly. Level

the upper surface with trowel & remove mould slowly & vertically. The vertical settlement from its

initial position gives the Slump value.


2) Compaction Factor test: Measure the effect of standard amount of work on concrete

Suitable for very low to high workability mix (CF – 0.68 to 0.92) , one of the popular test in

lab.

CF is not suitable for very high workability flow type mix.

3) Vee-bee test: Measures the amount of work require for compaction in terms of time.

Suitable for very low to low workability mix. Mainly used in lab.

Fill the slump cone in 2 layers with each layer with 25 tamping of 16mmdia rod.

Slowly & vertically remove the cone & allow the concrete to settle by its self weight.

Start the vibrator & note the time taken for the concrete surface to be horizontal – the

time is the Vee-bee time.


4) Flow table test:


5) Slump Flow test:

Workability recommended for various jobs:


Segregation: Segregation is the separation of constituent material in concrete.

Segregation can also be define as the separation of constituents of homogeneous mix so that

their distribution will no longer be uniform.

Cause of segregation:

Due to presence of various size of constituents in concrete. (Cement : 1 - 50, Sand : 0.15

- 4.75mm, CA: 4.75 – 80mm)

Due to difference in sp gravity of constituent material.

Types of segregation:

a) Separation of Coarse particle from mix. – Normally seen in dry type of mix.

b) Separation of grout (Cement+Water) – Normally seen in wet type of mix.

Control method:

a) With the use of proper aggregate grading.

b) With correct handling, transportation & placing – care to be taken where placing from

height is involved.

c) With correct compaction method – Over / incorrect method of compaction also

increase the segregation.

d) Avoid large difference in Sp gravity of FA & CA.

e) Various air entrainment method in concrete also reduce the segregation greatly.

f) It is found that use of pozzolona material (Fly ash / Silica fume) in concrete also

reduce the segregation.

Bleeding: Bleeding is the phenomenon by which water in mix tends to rise to surface of

fresh concrete.

Bleeding can also be define as the total settlement per unit height of concrete. In slip-form &

multilayer concreting, accumulated water cause void thus reducing bond between adjacent

concrete. In pavement construction also bleeding cause the problem.Continuous travel of

water to the surface of concrete may cause the continuous porous channel in concrete which

is responsible for high permeability thus less durable & low strength concrete.

Cause of Bleeding:

Use of excessive water & Over / Wrong vibration method can be taken as main cause of

bleeding.

Control method:

a) With correct handling, transportation & placing method bleeding can be controlled a

lot.

b) With correct compaction method.

c) Use of air entraining agent.

d) Use of pozolona material also helps to reduce bleeding in concrete.


Measurement of Bleeding: Place & consolidate the sample in mould with 250mm dia &

280mm height. Remove the bleed water @ 10 minute interval in first 40min. then at 30min

interval. The amount of water accumulated by total mixed water in % gives the amount of

bleeding.

4.3 Codal Provision of W/C ratio for durability:

IS456:200 has given the various W/C ratio for durability as given below:

Exposure PCC RCC Minimum Grade of concrete

Min. Cement Kg/m3

Max. free W/C

Min. Cement Kg/m3

Max. free W/C

PCC RCC

Mild 220 0.6 300 0.55 - M20

Moderate 240 0.6 300 0.5 M15 M25

Severe 250 0.5 320 0.45 M20 M30

Very severe 260 0.4 340 0.45 M20 M35

Extreme 280 0.4 360 0.4 M25 M40


5.0 ADMIXTURES

Admixture can be define as the ingredient other than cement/aggregate/water added to

concrete in order to achieve desired property of concrete as required. Can also be define as

the additives added to concrete so as to obtained the specific requirement of concrete.

Admixtures are mainly classified into two types:

i) Chemical admixture – These are the admixture containing basic ingredient as various

chemicals. Normally available in liquid / powder form. Use relatively low dosage

normally 0.04% to 5% by wt. of cement. Normally used to increase the workability,

to retard or accelerate the setting time/hardening process, for air entraining & water

proofing etc.

ii) Mineral admixtures – These are the admixture basically obtained from various natural

or artificial minerals. Normally available in finely divided power form. Its dose is

relatively higher than chemical admixture ranging from 10 – 80% by wt of cement.

Eg: Fly ash, blast furnance slag, silica fume, brick dust, stone dust etc. In many case

used as cement replacement ingredient due to its pozzolana property.

Ca(OH)2 in hydrated paste + POZZOLANA = C-S-H gel

4.1 Classification Of Admixtures:

ASTM Classification: ASTM C494 has specifies the various types of Chemical admixtures

as given below:

a) Type A: Water Reducing Admixture (Plasticizer)

Admixture which increase the workability of fresh concrete/mortar without increasing

water content or maintain workability with reduced water.

Can reduce water requirement by 5-12%

Dosage based on cement per 100kg (Eg, 200ml per 100kg {or 2bag} cement)

High dose may cause excessive retardation in setting time.

Eg: (Various lignosulphonate normally derived from wood product), (synthetic raw

materials), (polyglycol esters) etc.

b) Type B: Retarding Admixture.

Retarding Admixture delay the setting time of concrete / mortar.

Keep concrete workable for long period giving additional time for mixing, placing,

compacting & finishing.

Normally used to overcome unwanted effect of high temperature & to reduce slump-

loss.

Facilitates finishing in hot weather.

Eg: Calcium sulphate (gypsum), starch/sugar, cellulose, lignosulphonic-acid etc.

c) Type C: Accelerating Admixture.

Accelerating Admixture when added increase rate of hydration of hydraulic cement,

shorten setting time & increase hardening process.


Normally used when fast setting & early strength gain is desirable (Eg, Urgent repair

work, road pavement construction etc)

Can be used in cold climate region for rapid strength gain.

Chloride is one of the economic/effective accelerating admixture but due to its action

on corrosion of steel its use is limited to 0.15% of Cement for RCC & 0.06% for

prestressed concrete.

Eg; Soluble carbonates, silicates & flurosilicates, Organic compound –

triethenolamine etc.

d) Type D: (A+B): Water Reducing & Retarding admixture.

Admixture which increase the workability & delay the setting time of concrete also.

Normally used when both water reducing & retarding action is desirable.

Widely used by “Ready mixed concrete” to facilitate long duration transportation,

mixing, placing & finishing & also to reduce slump-loss.

Eg; normally produced adding retarders to plasticizers, normally used retarders are

starch, gluconate etc.

e) Type E: (A+C): Water Reducing & Accelerating admixture.

Admixture which increase the workability & accelerate the setting time & hardening

of concrete also.

Normally used when both water reducing & accelerating action is desirable.

Suitable for urgent repairs.

Eg; triethenolamine-chloride, calcium nitrate, nitrites etc.

f) Type F: High Range Water Reducer (HRWR / Super-plasticizer).

Similar to plasticizing admixture with high water reducing capacity.

Normally used when high degree of water reduction is desirable.

Depending upon its type can reduce water content more than 30%.

Due to their powerful dispersing & fluidifying effect facilitates to work with very low

W/C ratio also.

HRWR normally available in market are: a) Sulphonated melanie-formaldehyde

(SMF), b) Sulphonated napthalene-formaldehyde (SNF), c) Carboxylate acrylic

easter, d) Other types.

Out of above listed HRWR Carboxylate polymer based superplasticisers are found

more effective & powerful.

g) Type G: (F+B): High Range Water Reducer & Retarding Admixture.

Admixture with both high range water reducing & retarding capability.

In addition to increase in workability, also prolong workability retention & retard

setting time.

Normally used when both high range water reducing & retarding action is desirable.

Widely used by “Ready mixed concrete” to facilitate long duration transportation,

mixing, placing & finishing & also to reduce slump-loss.

Eg; Produced by adding retarders to superplasticizers, normally used retarders are

starch, gluconate, cellulose, lignosulphonic acid etc.


BIS Classification:

IS9103:1999 has specifies the various types of Chemical admixtures as given below:

a) Accelerating admixture.

b) Retarding admixture.

c) Water reducing admixture.

d) Air-entraining admixture:

Induce micro-air bubble (5 to 80µ) to concrete.

Used to produce air entrained concrete.

Air entraining admixture induce millions of fine uniformly distributed air bubble to

concrete.

These micro-air bubble acts as flexible ball bearing thus increase workability, reduce

segregation & bleeding, also the harden concrete have better resistance to freezing &

thawing.

Eg; Natural wood resin. Water soluble soap of resin acid, hydrogen petroxide,

aluminium power, Animal & vegetable oil etc.

e) Super-plasticizing admixture.

IS2645 – Specifies one more chemical admixture as:

f) Waterproofing Admixture.

Normally used where water impermeability is desirable.

Mainly two types:

Pore-filler: Reduce permeability by its pore-filling action. Normally used materials

are Chalk, Talc, Silicates, Aluminium power.

Water repellent: Prevent water penetration by its water repellent action. Normally

used materials are: Resin, vegetable oil, waxes, calcium soap, soda etc.

Mineral admixture like silica-fume, fly-ash & air entraining admixture can also used

to improve impermeability of concrete.

4.2 Effect of Admixtures in Concrete:

Effect on green (plastic stage) concrete:

Depending upon the types of the admixture as seen above the admixtures modify the property

of concrete in its green stage. Eg,

Plasticizer/super-plasticizer increases the workability/fluidity of mix, & also some retarding

effect depending upon its amount in concrete.

Accelerating admixture accelerate the hardening process while retarders slowdown the setting

process of concrete.

Air entraining admixture entrains the minute air bubble in concrete, reducing the negative

effect of segregation & bleeding.

Waterproofing admixture reduce the penetration of water through concrete by water repellent

action & reducing the permeability of concrete.

Mineral admixtures normally reduce the segregation & bleeding of concrete. Also in some

case reduce the water requirement for given workability.


Effect on harden concrete:

Admixture shows both positive/negative effects on harden concrete.

Normally upto 3% no bad effect can be seen, but excessive addition shows retarding effect

(By plasticizer/super-plasticizer & air-entraining admixture) in both setting time & hardening

process of concrete.

Increase resistance to penetration thus durability to chemical deterioration is enhanced.

Higher creep with high W/C ratio but at low W/C ratio no effect is seen.

Drying shrinkage is not very much affected but in some case found improved.

Addition of mineral admixture shows improvement in durability & transition zone character

of concrete. But in some case delay in desired strength gain occurs due to slow hydration

process.

4.3 Local Materials as Admixtures in Nepal:

Present possibilities:

a) Rice husk ash

b) Calcined clay pozzolona (Brick dust / Burned clay dust)

c) Stone dust

Future possibilities with industrial development:

a) Silica fume

b) Fly ash

c) Blast furnace slag

d) Various chemical admixtures.

Mineral admixture: Fly ash, Blast furnace Slag, Rice husk ash, Silica fume, Brick /

stone dust. (Refer class note for detail)

Slump Loss & its prevention: (Refer class note)


6.0 Mix Design of Portland Cement Concrete

Mix design is the process of determining relative quantity of concrete ingredients with the objective:

To ensure the desire workability of concrete at plastic stage (green stage).

To obtain the required strength, durability and surface finish at its hardened stage.

To achieve the economic mix.

6.1 Nominal & Design Mix of Concrete:

Nominal Mix: A concrete mix in which the proportions are adopted in generalized form irrespective

of its ingredient properties.

Design Mix: A concrete mix in which the proportions are adopted depending upon the ingredient

properties to obtain the desired workability, strength & durability of concrete.

6.2 Introduction to Nominal Mix:

Nominal mixes are suitable only upto the M20 grade concrete and desired strength / workability

cannot be guaranteed. But still useful if no any mix design data is available in site.

Table 6.1: Nominal Mix given by IS456:2000

Concrete Grade

Total dry Aggregate( in Kg )per 50Kg Cement

Fine: Coarse (Proportion)

Water per 50Kg of Cement

M5 800 Generally 1:2 but

subjected to an upper limit of 1:1.5 & lower

limit of 1:2.5

60 M7.5 625 45 M10 480 34 M15 330 32 M20 250 30

Example: For M15 grade concrete if we adopt, Cement=50Kg, Total Aggregate=330Kg, Water=32Kg

Fine:Coarse =1:2, Fine=330/(1+2)=110Kg, Coarse=2*330/(1+2)=220Kg

Cement: Fine: Coarse = 50: 110: 220 = 1: 2.2: 4.4, Water/Cement = 32/50 = 0.64

Nominal Mix = 1: 2.2: 4.4 and W/C =0.64

6.3 Mix Design of Concrete by DOE, ACI and Indian Standard Methods:

DOE Method

Developed by “The Building Research Establishment Laboratory” (BRE), Department of Environment (DOE), U.K. DOE method outlines the procedure for design of concrete mix with 28 days cube strength as high as 75Mpa.

The proportion of fine aggregate is determined depending upon the max. size of aggregate, degree of workability, grading of fine aggregate & W/C ratio.

The degree of workability is indicated by specific value of slump & Vee-Bee time.


Design Steps:

Step1: Determine target mean strength (Fmean) from specified characteristic strength(Fck):

Fmean = Fck+ σ* K

σ = Standerd deviation (IS456:2000 page23 – For M10/15=3.5, M20/25=4.0 M30-50=5.0)

K= Himsworth constant (1.64 for 95% confidence level)

Figure 6.1: Normal Distribution curve.

Step2: Determine Minimum W/C ratio based on Target Strength and compare with W/C

requirement for Durability consideration.

Fig: 6.2: Compressive strength vs W/C ratio

Table 6.2: Approximate compressive strength of

concrete with free water cement ratio W/C 0.5

1 2


Table 6.3: Minimum Cement Content & Max. W/C for Durability:


Min. Cement Kg/m3

Max. free W/C

Min. Cement Kg/m3

Max. free W/C

PCC RCC

Mild 220 0.6 300 0.55 - M20

Moderate 240 0.6 300 0.5 M15 M25

Severe 250 0.5 320 0.45 M20 M30

Very severe 260 0.4 340 0.45 M20 M35

Extreme 280 0.4 360 0.4 M25 M40

Step3: Determine free water content depending upon max size & type of aggregate and degree of workability. Table 6.4: Approximate Water content (Kg/m3) for various degree of Workability:

Max. Size of Aggregate

mm

Slump 0-10 10-30 30-60 60-180

Vee-Bee >12 6-12 3-6 0-3

Aggregate Type

10 Uncrushed 150 180 205 225

Crushed 180 205 230 250

20 Uncrushed 135 160 180 195

Crushed 170 190 210 225

40 Uncrushed 115 140 160 175

Crushed 155 175 190 205

Step4: Calculate cement content from W/C ratio & water content of mix determined previously. Compare value with min. / max. value of cement content of durability requirement (Table 6.3) & modify if required. Step5: Determine wet density of concrete depending upon free water content & relative density of combine aggregate. Calculate Total aggregate content knowing the Wet density of concrete.

Fig: 6.3: Wet density of mix vs Free water content.


Calculate total aggregate content (Saturated surface dry): γo-γc-γw

γo= Wet density of concrete (Kg/m3) γc= Cement Content (Kg/m3) γw= Free Water Content (Kg/m3) Step6: Determine Proportion of fine aggregate depending upon the W/C ratio, Max size of aggregate, fine aggregate grading zone & workability level.

Fine Aggregate =Total Agg. * Proportion of Fine Agg. Coarse Aggregate= Total Agg. – Fine Agg.

Fig: 6.4 Recommended % of fine aggregate in total aggregate vs W/C ratio. (Nominal max. size 20mm)

Fig: 6.5 Recommended % of fine aggregate in total aggregate vs W/C ratio. (Nominal max. size

40mm)


EXAMPLE – DOE METHOD: Design concrete mix for RCC work for the moderate exposure

environment. The characteristic strength required is 30Mpa. Max aggregate size = 20mm crushed.

Sieve analysis of fine aggregate shows 50% passing through 600μ sieve. Average Sp. gr. of aggregate

is 2.65. Slump required 30-60 mm. OPC Cement (Type1) will be used.

Step1: Fmean = Fck+ σ* K = 30 + 1.64*5 = 38.2Mpa (For M30, σ =5 Ref; IS456:2000 page23)

Step2: Minimum W/C ratio based on Tar]\get Strength (Fig6.2 / Table 6.2) = 0.6

Minimum W/C ratio based on durability (Table 6.3) = 0.5

Adopt minimum W/C = 0.5

Step3: Free water content (Table 6.4) = 210kg/m3

Step4: Cement content (Step 2&3) = 210/0.5 = 420kg/m3 > min cement 300kg/m3 from Table 6.3

(OK)

Step5: Wet density of concrete for 210kg free water content & sp gr aggregate 2.65 (Fig 6.3) =

2375kg/m3

Total aggregate content = Total Concrete – Cement – Water = 2375 – 420 -210 = 1745kg/m3

Step6: % of fine aggregate (Fig 6.4 for slump 30-60 & W/C = 0.5 & FA 50% passing 600μ sieve.) = 35% of total aggregate

F A = 0.35 * 1745 = 610.75kg/m3

C A = 1745 – 610.75 = 1134.25kg/m3

Cement : Fine : Coarse = 420 : 610.75 : 1134.25 = 1 : 1.454 : 2.701


ACI METHOD

The American Concrete Institute mix design method is suggested by the ACI Committee 211.

One method is based on the estimated weight of the concrete per unit volume & the other method is based on calculation of the absolute volume occupied by concrete ingredient.

This method consider the requirement for workability, consistency, strength and durability of concrete.

Design Steps:

Collect the data required for mix design first:

- Fineness modulus of fine aggregate, Sp. Gravity of fine & coarse agg. , Unit wt. of dry rodded

coarse agg. & Sp. Gr. of cement.


Fmean = Fck+ σ* K

σ = Standerd deviation (IS456:2000 page23 – For M10/15=3.5, M20/25=4.0 M30-50=5.0)

K= Himsworth constant (1.64 for 95% confidence level)

Step2: Determine Minimum W/C ratio based on Target Strength and compare with W/C requirement for Durability consideration. Table 6.5: Relation between W/C ratio & Av. Compressive strength (ACI 211.1:91)

38.2 0.45


Table 6.6: Relation between W/C ratio & Exposure conditions (ACI 318-89)

Step3: Determine free water content depending upon max size & type of aggregate and degree of workability.

Table 6.7: Water content determination depending upon slump & max. agg. size

Step4: Calculate cement content from W/C ratio & water content of mix determined previously. Step5: Determine Bulk volume of dry rodded C A depending upon Max. Agg. size & Fineness Modulus of fine aggregate (Table 6.8). Calculate Wt. of C A = Bulk Volume * Bulk Density


Table: 6.8 Bulk Volume of Dry rodded Coarse Aggregate depending upon FM & Max Agg. size

Step6: Determine Wt. of Fresh concrete (Table 6.9) per m3: Table 6.9:

Wm = 10ρA(lOO-A)+ɣc(1-ρA/ ρC) -ɣw(ρA -1)

where

Wm = weight of fresh concrete, kg/m3 ρA = weighted average specific gravity of combined fine and coarse aggregate

ρC = specific gravity of cement (= 3:15)

ɣc = cement requirement, kg/m3

ɣw = mixing water requirement, kg/m3

A = air content, percent

Step7: Determine Wt of F A required:

F. A. (wt.) = Total wt of concrete (step6) – Cement (step4) – Water (step3) – C A (step5)

Now Mix proportion = Cement: FA: CA & W/C

OR


EXAMPLE – ACI METHOD: Design concrete mix for RCC underground water tank. The characteristic

strength required is 30Mpa at 28days. Max aggregate size = 20mm crushed. Sieve analysis of fine

aggregate shows Fineness modulus 2.6. Dry rodded bulk density of CA = 1600kg/m3. Average Sp. gr.

of aggregate is 2.65. Slump required 30-60 mm. OPC Cement (Type1) will be used.

Step1: Fmean = Fck+ σ * K = 30 + 1.64*5 = 38.2Mpa (For M30, σ =5 Ref; IS456:2000 page23)

Step2: Minimum W/C ratio based on Target Strength (Table 6.5) = 0.45



Step3: Free water content (Table 6.4) = 185kg/m3

Step4: Cement content (Step 2&3) = 185/0.45 = 411kg/m3

Step5: Bulk volume of dry rodded C A = 0.64 {depending upon Max. Agg. Size 20mm & Fineness

Modulus of fine aggregate 2.6 (Table 6.8)}

Calculate Wt. of C A = Bulk Volume * Bulk Density = 0.64m3 * 1600kg/m3 = 1024 kg/m3

Step6: Wet density of concrete for Max 20mm aggregate & Normal Concrete (Table 6.9) =

2355kg/m3

Step7: Determine Wt of F A required:

F A required: = F. A. (wt.) = Total wt of concrete (step6) – Cement (step4) – Water (step3) – C A

(step5)

= 2355 – 411 - 185 – 1024 = 735kg/m3

Cement : Fine : Coarse = 411 : 735 : 1024 = 1 : 1.788 : 2.491

W/C = 0.45


IS METHOD OF MIX DESIGN

This method is recommended by the Bureau of Indian standards covered in IS10262:1982. The method given can be applied from medium to high strength concrete. As the procedure developed in 1982 & since then lots of modification seen in concrete technology, code revision seems to be required. Design Steps:


Fmean = Fck+ 1.65 * S

S = Standerd deviation (IS456:2000 page23 – For M10/15=3.5, M20/25=4.0 M30-50=5.0)

Step2: Determine Minimum W/C ratio based on Target Strength & Cement type, and compare

with W/C requirement for Durability consideration.

Fig: 6.6: Compressive strength vs W/C ratio

IS10262:82

Fig 6.7: Compressive strength of concrete various

cement type IS10262:82


Fig 6.7: Compressive strength of concrete various cement type (Germany)

Table 6.10: Minimum Cement Content & Max. W/C for Durability (IS456:2000):


Min. Cement Kg/m3

Max. free W/C

Min. Cement Kg/m3

Max. free W/C

PCC RCC

Mild 220 0.6 300 0.55 - M20

Moderate 240 0.6 300 0.5 M15 M25

Severe 250 0.5 320 0.45 M20 M30

Very severe 260 0.4 340 0.45 M20 M35

Extreme 280 0.4 360 0.4 M25 M40

Step3: Determine free water content & Fine to total Agg. ratio depending upon max size of aggregate & concrete Strength for standard condition.

Make the adjustment depending upon Ingredient properties.

Table 6.11: Approximate Sand & Water content (Kg/m3) for Compaction Factor CF =0.8 (approximate slump 30mm), Angular coarse aggregate & Sand zone II as per IS383:1970

Upto M35 W/C - 0.6

CF =0.8

> M35 W/C – 0.35

CF =0.8


Table 6.12: Adjustment table for water content & % sand in total aggregate.

Step4: Calculate cement content from W/C ratio & water content of mix determined previously. Compare value with min. / max. value of cement content of durability requirement (Table 6.10) & modify if required. Step5: Calculate Aggregate content:

Step6: The mix proportion above obtained by the assumption that the aggregates are saturated & surface dry if any deviation, make adjustment in water & aggregate content.


EXAMPLE – IS METHOD: Design concrete mix for RCC Silo. The characteristic strength required is

30Mpa at 28days. Max aggregate size = 20mm crushed. Sieve analysis of fine aggregate shows Sand

zone III. Workability required compaction factor 0.9. Exposure condition moderate. Cement Type E -

53Mpa will be used. Sp. gr. of CA = 2.7 / FA = 2.6 / Cement = 3.15.

Step1: Fmean = Fck+ 1.65*S = 30 + 1.65*5 = 38.25Mpa (For M30, σ =5 Ref; IS456:2000 page23)

Step2: Minimum W/C ratio based on Target Strength (Fig 6.7) = 0.45



Step3: Free water content (Table 6.11) = 186kg/m3 & F A = 35% of Total Agg.

Adjustments (Table 6.12):

Condition Change Water Content % adjustment Sand content % Adjustment

Sand confirming Zone III 0 - 1.5%

Increase in Compaction Factor

(0.9-0.8) = 0.1 + 3% 0

Decrease in W/C ratio (0.45-0.6)

= -0.15 0

- (0.15/0.05)

= - 3%

Total +3% -4.5%

F A content = 35 -4.5 = 30.5%

Water content =186 +3% *186 = 191.6kg

Step4: Cement content (Step 2&3) = 191.6/0.45 = 425.8kg/m3 > 240kg ok

Step5: Calculate Aggregate content:

(1 − 0.02) = 191.6 +425 .8

3.15+

𝑓𝑎

.305∗2.6 ∗

1

1000

fa = 518kg/m3

𝐶𝑎 =1−0.305

0.305∗ 518 ∗

2.7

2.6

Ca= 1225.8kg/m3

Cement : Fine : Coarse = 425.8 : 518 : 1225.8 = 1 : 1.217 : 2.879 & W/C = 0.45

Air content


7.0 STRUCTURE & STRENGTH OF CONCRETE

7.1 Concrete as 3-Phase System & Structure Of Different Phases:

Refer “Structure of Hydrated Cement / Three phase system of Concrete” - page 17 of this note:

7.2 Strength Porosity Relationship Of Concrete:

Strength of concrete is primarily depends upon the strength of cement paste. Many research

has shown that strength of cement paste depend on the W/C ratio used. Abrams presented his

W/C rule in 1918 as:

S= A/B x Where, x = w/c ratio; A=14000lbs/sqin; B= 7 for 28 days compressive strength.

Abrams w/c law states that the strength of concrete depends on the w/c ratio only. This rule is

still taken as the fundamental tool for concrete mix design. This rule explains the weakening

of paste phase due to increase in W/C ratio thus porosity of concrete but effect of W/C ratio

in transition zone is not explained.

Also the effect of aggregate strength on concrete is not incorporated in the rule (assuming the

aggregates are stronger than paste & concrete failure takes place with bond failure only,

which may not be true always).

Feret has formulated the strength of concrete paste depending upon its constituents in 1897

as,

Strength S = K*[𝒄

(𝒄+𝒆+𝒂)]2

c, e & a – Volume of cement, water & air respectively.

Equation given by Feret explain the strength change not only due to the effect on w/c ratio

but also the degree of compaction (a-air content).

Gel-Space ratio:

Many research has shown that the strength parameter can be related more accurately to the

gel-space ratio than the w/c ratio. Power & Brownyard presented that the gel-space ratio

(Ratio of solid product of hydration of cement to space available for product formation).

The gel-space ratio can also be defined as the ratio of volume of hydrated cement paste to

sum of volume of hydrated paste & capillary pores.

Strength S = 240*x3

Where x= gel space ratio.

Calculation of gel-space ratio:

C= wt. of cement in gm

Vc=Sp. Volume of cement = 0.319ml/gm

Wo = Volume of water

It was presented by various researches that 1gm cement produce nearly 2.06ml of gel.


Assuming 1ml cement produce 2.06ml gel.

Volume of gel = Volume of Cement * 2.06 = (C* Vc) *2.06 = 2.06 C* Vc

Total Space available = Vol. of Cement + Vol. of Water = (C* Vc + Wo)

Gel Space ratio = Vol. of gel / Total Space available = 2.06 C Vc /( C Vc +Wo)

If cement hydrates by α% only;

Gel Space ratio (x) = (𝟐.𝟎𝟔 𝐂𝐕𝐜 𝛂%)

(𝐂𝐕𝐜 𝛂%+𝐖𝐨) 𝒐𝒓

(𝟐.𝟎𝟔 𝐕𝐜 𝛂%)

(𝐕𝐜 𝛂%+𝐖/𝐂)

If 1ml cement produce 2ml gel,

Gel Space ratio (x) =(𝟐.𝟎 𝐂𝐕𝐜 𝛂%)

(𝐂𝐕𝐜 𝛂%+𝐖𝐨) 𝒐𝒓

(𝟐.𝟎 𝐕𝐜 𝛂%)

(𝐕𝐜 𝛂%+𝐖/𝐂)

Strength S = 240*x3

(N/mm2)

Capillary porosity= Capillary void/ Total space =(𝑻𝒐𝒕𝒂𝒍 𝒔𝒑𝒂𝒄𝒆−𝒗𝒐𝒍𝒖𝒎𝒆 𝒐𝒇 𝒔𝒐𝒍𝒊𝒅)

(𝑻𝒐𝒕𝒂𝒍 𝒔𝒑𝒂𝒄𝒆)

Total space = (Volume of cement + volume of water) = (CVc+Wo)

Volume of solid = (volume of cement unhydrated + volume of gel from hydrated cement)

If cement hydrates by α% ;& 1ml cement produce 2.06ml gel.

Volume of solid = C Vc (1-α) + 2.06 CVc α

Capillary porosity = (𝑻𝒐𝒕𝒂𝒍 𝒔𝒑𝒂𝒄𝒆−𝒗𝒐𝒍𝒖𝒎𝒆 𝒐𝒇 𝒔𝒐𝒍𝒊𝒅)

(𝑻𝒐𝒕𝒂𝒍 𝒔𝒑𝒂𝒄𝒆) =

(𝐖/𝐂− 𝟏.𝟎𝟔 𝐕𝐜 𝛂%)

(𝐕𝐜+𝐖/𝐂)

If 1ml cement produce 2ml gel,

Capillary porosity = (𝐖/𝐂− 𝐕𝐜 𝛂%)

(𝐕𝐜+𝐖/𝐂)

7.3 Various Types Of Strength Of Concrete & Their Inter-Relationship:

i) Compressive Strength:

Compressive strength is taken as the one of the most important parameter of concrete. As

most of its other properties are also related to the compressive strength & also the concrete is

commonly employed to resist the compressive strength, its knowledge is primary importance

to all.

Normally compressive strength is determined by two method:

a) Cube compressive strength: Compressive strength of standard cube (normally taken as

150x150x150mm). It shows that the smaller size cube give relatively higher strength

value than bigger size. Typical test result is given below:


Size effect of cube test (cube other than standard):

Cube size (mm) 100 150 200 300

Relative Strength 1.05 1.00 0.95 0.87

b) Cylinder compressive strength: Compressive strength of standard cylinder (normally

150mm dia. X 300mm ht – ht. to dia. ratio = 2). It is found that the ratio of ht. to dia.

other than 2 (standard size) affect the cylinder strength as given below:

Effect of height to diameter ratio on cylinder strength:

As cylinder specimen seems to have more uniform result than cube specimen, is being

popular in research laboratory. But due to easiness in casing & transportation cube specimen

are popular in construction site in country like Nepal, India, UK etc.

It is found that the cylinder specimen gives lower strength value than cube strength. Various

relations between cube & cylinder strength are given below:

BS 1881:1970 ;

Cylinder strength = (4/5)* Cube Strength

L Hermite;

Cylinder strength = 0.76+0.2*log(fcm/2840) Here, fcm =cube strength lbs/sqin

Typical test result:

Cube Strength (Mpa) 9 20 30 45 49

Cylinder Strength (Mpa) 7 17 27 42 45

ii) Tensile Strength: Being weak in tension, tensile strength of concrete is normally

neglected in design. Various test method / tensile strengths are given below:

a) Direct tension: If tensile strength of concrete is determined applying direct (Pure) tension

to concrete specimen.

Tensile Strength (fct) = Tensile Force (P)/Area

P P


As perfect grip during loading is difficult & also due to induction of secondary

stress, the direct tensile test is not very popular. Normally tensile strength of

concrete can be taken as 10% of compressive strength (Fck).

IS456:2000 relates direct tensile strength to compressive strength as:

Tensile Strength (𝒇𝒄𝒕) = 𝟎.𝟑𝟓 ∗ 𝑭𝒄𝒌

b) Splitting tension: This is one of the popular indirect tensile test method also known as

Brazilian test. The test is carried out in cylinder specimen, placing it horizontally between

loading surface. Even the load applied is compressive; almost 5/6th of depth is subjected

to uniform tensile stress. Splitting tensile test gives slightly higher value than direct

tension.

Horizontal Tensile Stress = 2P/π LD

P = Compressive Load (N)

L= Length of Specimen (mm)

D= Diameter of Specimen (mm)

Vertical compressive Stress on cylinder = 2P/π LD ∗ [ 𝑫𝟐

𝒙 𝑫−𝒙 −𝟏]

c) Flexural tension: Flexural tension is the tensile force developed in concrete in bending.

As most of the concrete element has to resist bending tension instead of direct tension, the

flexural tension has also its own importance. Normally two types of flexural test is

performed to determine flexural tension:

i) Center point loading

ii) Three point loading

D-x

x


In given figure below, at any section:

б = M/I * y

Where,

б = Stress at distance y from neutral axis (NA)

M= Moment at given section of beam for given loading.

I= Moment of Inertia of section (For rectangular section = bd3/12)

y= Distance at which stress is required (For rect. section at extreme fiber =d/2)

The standard test specimen size for above test is 15x15x70cm (Span L =60cm) alternatively

10x10x50cm (Span L=40cm) specimen is also used in some case. The flexural strength of

specimen is determined as modulus of rupture fb (= бmax) at ultimate loading.

Test result shows that center pt. loading gives slightly high value than 3 point loading;

following empirical relation is given by center of road research,

fb (Centre pt. loading) = fb (3 pt. loading) + 0.72 fb= flexural strength (in Mpa)

IS456:2000 relates modulus of rupture (Flexural tension) to compressive strength as:

Flexural tensile strength (𝒇𝒃) = 𝟎.𝟕 ∗ 𝑭𝒄𝒌

iii) Shear Strength: Shear can be defined as the action of equal & opposite parallel force

acting in plane short distance apart. Direct determination of shear force is difficult, can be

taken as 12% of compressive strength if test data is not available. In beam shear failure

can be seen as combine bending & shear, normally acting at 450 to shear.

IS456:2000 relates Shear strength to compressive strength as:

Compressive strength M15 M20 M25 M30 M35 M40

Shear strength (N/mm2) 1.6 1.8 1.9 2.2 2.3 2.5

iv) Bond strength: In generalized form two types of bond strength can be discussed:

a. Bond between cement & aggregate: Bond between hydrated cement & aggregate affects

the overall strength of concrete. As for normal strength concrete aggregate is sufficiently

strong the strength of concrete depends entirely upon the bonding between cement &

aggregate. Lower W/C ratio & use of mineral admixture significantly enhance the bond

strength by reducing porosity & enhancing characteristics of transition zone.

T

C NA


b. Bond between steel & concrete: This bond is primarily due to the friction & adhesion

between steel & concrete surface. Mainly depends upon the strength of concrete &

surface and mechanical property of steel. Higher the compressive strength higher will be

the bond strength. Higher specific surface of gel also found to give higher bond strength

(C-S-H gel from C2S gives more sp. surface than from C3S compound). Also the

deformed bar has found to give higher bond strength than plain bar. The moisture content

(adversely affect) & stress condition (In tension low bond strength – in compression high)

also affect the bond strength.

IS456:200 has given the bond strength of plain bar in tension as:

Compressive strength M15 M20 M25 M30 M35 M40

Bond strength (N/mm2) 6 8 9 10 11 12

For deformed bar above value can be increased by 60%

Further increase in above value for bar in compression by 25%

v) Bearing strength: Concentrated loading on concrete surface tends to punch the surface

inside, resistance to which can be termed as bearing strength. Normally, bearing stress is

significant at the base of the steel/concrete column in foundation & in anchored area of

tendon in pre-stressed concrete. IS 456:2000 recommends,

Bearing Strength = 0.25*Fck for Working stress design method.

Bearing Strength = 0.45*Fck for Limit State design method.

7.5 Effect Of Time & Temperature On Concrete: As we know the hydration of cement

continues for long course of time, provided the moisture available for hydration process;

strength of concrete also increases in similar pattern with time. In general, strength of

concrete increases with time.

ACI -209 recommends the following

relationship to determine compressive strength

at any time ,

fcm (t) = fc28* 𝒕

(𝟒+𝟎.𝟖𝟓𝒕)

CEB-FIP Model Code (1990) recommends:

fcm (t)= Exp [S (1- (𝟐𝟖

𝒕/𝒕𝟏) )]* fc28

fcm (t)= Sterngth at any time t; fc28 = 28 days strength

S= 0.2 (Early strength cement); S= 0.25(Normal OPC) ; S= 0.38 (Slow hardening cement);

t=time in day; t1= 1day


Effect of Temperature: Depending upon the age & stage of concrete, temperature affects the

concrete in various ways. Let’s take the three different case to have the detail knowledge:

Case I: Concrete cast & cure at same temperature.

Case II: Concrete cast at different temp.& cure at normal (standard) temperature.

Case III: Concrete cast at normal (standard) temp. & cure at different temperature.

Case I: Concrete cast & cure at same

temperature.

Result shows that the increase in

temperature increases the rate of hydration

of cement thus rate of gain of strength also

increases with time.

This shows that the increase in temperature

has positive effect on strength gain.

Case II: Concrete cast at different temp.&

cure at normal (standard) temperature.

Result shows that decrease in initial (casting)

temperature increases the ultimate strength,

due to improved microstructure in low

temperature casting.

This indicates that the decrease in casting

temperature is beneficial to concrete

strength.

Case III: Concrete cast at normal (standard)

temp. & cure at different temperature.

Result shows that increase in temperature

after final setting time of cement; increases

the rate of strength gain of cement due to

increase in rate of hydration.

This indicates that the increase in curing

temperature has positive effect on concrete

strength.


7.6 Importance of Curing (Effect of humidity):

Various research/test data shows that the continuously moist cured concrete gives almost 2 to

3 times greater ultimate strength than entire air cured concrete. The decrease in strength of air

cured concrete is due to loss of moisture, which ultimately interrupt the hydration process of

cement. Loss of moisture thus strength gain mainly depends upon the exposed surface area &

temperature/humidity condition of surrounding & also some extent to thickness &

permeability of concrete. Normally minimum 7 days moist curing is recommended for

satisfactory result. For fly ash or slag cement long term curing is required than OPC. The

figure shows the effect of curing on strength gain.

7.7 Effects Of Cyclic & Dynamic Loads on Strength:

Cyclic loading: Any type of repeated loading on structure can be termed as cyclic loading.

Due to fatigue behavior of material repeated loading at high stress level affects negatively on

the strength. Many researchers had shown that at stress level greater than 50% of

compressive strength of concrete, cyclic loading has an adverse effect on concrete strength.

Typical test indicates that; at 5000 cycle of repeated loading concrete fails only at 70% of its

ultimate monotonic strength. Progressive micro-cracking at interfacial transition zone and

paste matrix are main cause of this strength degradation.

For stress level 50-75% of fc, gradual degradation in both characteristic strength (Fck) &

modulus of elasticity (E) occurs.

Gradual increase in cycle shows distinct nonlinear behavior of concrete in cyclic loading.


The relation between max stress level & number of cycle for failure can be given as:

fc = Strength at static loading.

fmax = Max. Stress in cycle.

John Hanson has given the following relation for 95% confidence level:

𝑳𝒐𝒈 𝑵 =(𝟏 −

𝒇𝒎𝒂𝒙𝒇𝒄 )

𝜷(𝟏 −𝒇𝒎𝒊𝒏𝒇𝒎𝒂𝒙)

− 𝟏.𝟗

fmin = Min. stress in cycle.

𝜷 = 0.0685

Another relation by Kakatuwa:

𝑳𝒐𝒈 𝑵 =𝟏𝟕 (𝟏 −

𝒇𝒎𝒂𝒙 − 𝒇𝒎𝒊𝒏𝒇𝒄 )

(𝟏 −𝒇𝒎𝒊𝒏𝒇𝒄 )

Dynamic/Impact Loading: If concrete is applied with very short term loading can be termed

as impact loading. In other word, if the rate of loading applied is very high can be termed as

impact loading and strength under such loading condition is termed as impact strength. It is

interesting that the impact strength of concrete increases with rate of loading (i.e. Higher the

impact rate higher the strength of concrete).

Compare to the 0.25Mpa/sec of standard loading condition, the lower rate (0.007Mpa/sec)

shows 12% reduction in fck, while increased loading rate (6.9Mpa/sec) shows 12% increase

in fck. Due to the availability of time for the propagation of micro-crack (in transition zone)

at low rate of loading, high loading rate shows improved strength value.

Impact strength found to be increased with:

Increase in rate of loading.

Increase in surface roughness of aggregate.

Decrease in aggregate size.

Improved transition zone.

Increase in tensile strength of concrete.

fmax / fc

Log N


The CEB-FIP Model Code (1990) recommends the following relation for compressive

strength due to impact:

𝑓𝑐 ,𝑖𝑚𝑝

𝑓𝑐= (𝜎𝑠 / σ0 )𝜶

Where,

fc,imp = impact compressive strength

fc = compressive strength of concrete,

σ0 = −1.0 MPa/s

σs = impact stress rate

α = 1/(5 + 9 fc/fo); fo = 10 MPa


8.0 Dimensional Stability of Concrete

8.1 Non-linear stress-strain relation & modulus of elasticity of concrete:

Stress-strain curve of concrete shows its distinct non linear behavior even at lower stress

level. Even the cement paste & aggregate shows linear behavior at its initial loading stage (as

shown in fig below), being heterogeneous & multiphase material stress strain curve of

concrete shows non-linear behavior even at the beginning. The fig below also indicates that

the properties of concrete is not equal to sum of properties of its components.

Experiment shows that stress strain curve of rich mix concrete shows less strain value for

particular stress level also the ultimate strain is less (ie low ductility behavior).

Behavior of concrete (in terms of Stress-strain curve & failure mechanism) at various stress

level can be expressed as given below.

At initial stage of loading upto 30% of ultimate stress level, concrete shows stable system of

micro-cracking. Micro crack exist in transition zone only & stress strain curve also shows its

linear behavior.


Upto 30-50% of ultimate stress level, increase in both length & width of micro-cracks occurs

in transition zone. Still the micro-crack system can be termed as stable & normally no crack

in paste phase but stress-strain curve begin to show its non linearity.

Upto 50-75% of ultimate loading crack start in paste phase also. Further propagation of crack

in both transition zone & paste matrix leads crack system to unstable stage. Stress strain

curve shows strong non-linearity.

Upto 75-100% of ultimate loading crack reach at its critical stage & with the increase in crack

spontaneous failure of specimen occur at its ultimate loading. Stress strain curve almost

become flat & moves down indicating failure.

Modulus of elasticity (E) :

Even for the non-linear material like concrete, determination of modulus of elasticity is really

important for any type of structural analysis & to know the load deflection behavior of any

structural system.

As shown above response of concrete to applied loading is quite complex & stress strain

curve also shows its non-linear behavior even at low stress level.

Due to its non-linear behavior there exists different E value at different stress level. Different

types of modulus of elasticity, depending upon its mode of determination can be given as:

a. Initial tangent modulus (Dynamic

modulus): If the E – value is determined

by drawing tangent at beginning of curve,

is termed as Initial tangent modulus. This

is only suitable for low stress level.

b. Tangent modulus: If modulus of elasticity

is determined by drawing tangent at any

point in curve, is termed as tangent

modulus. This is only suitable for stress

level near to that point.

c. Secant Modulus: If modulus of elasticity is

determined by joining any point in curve

to origin, is termed as secant modulus.

This is one of the widely used method for

determination of E.

d. Chord modulus: If modulus of elasticity is determined by joining any two point in

curve, is termed as chord modulus. This is mainly used for typical research purpose

only.

Normally modulus of elasticity can be determined in lab by:

Gradual loading to cylinder or cube specimen in uniaxial compression & noting series

of stress-strain value for given loading values.


Flexural modulus: Can also be determined by subjecting a concrete beam to bending

(loading can be the point load on middle or any other arrangement) & using the

defection relation derived in structural theory.

E= PL3/48IΔ (For pt. load at middle of span)

Dynamic modulus: Determined by subjecting concrete specimen to longitudinal

vibration at their its natural frequency & applying relation:

Ed = K n2 L

2 ρ ρ= Density of concrete

L= Specimen length mm

n= Resonating frequency HZ

K= 4x10-15

(Gpa)

Factor affecting E:

Strength of concrete (Directly proportional-

E=5000 𝑭𝒄𝒌 𝒂𝒔 𝒈𝒊𝒗𝒆𝒏 𝒃𝒚 𝑰𝑺𝟒𝟓𝟔:𝟐𝟎𝟎𝟎)

Moisture condition (Ewet > Edry)

Aggregate / Paste quality (1/E = Vol. of paste/E paste + Vol. of Agg/E Agg)

Poisson ratio µ:

Can be define as the ratio of lateral strain to corresponding longitudinal strain.

µ = 0.15 – 0.2 for most of the concrete.

Generally high for low strength concrete & low for high strength

For analysis & design of some typical structure its knowledge is required.

Ultrasonic pulse velocity method can be applied as alternative method to find out its value.

(𝑽𝟐/𝟐𝒏𝑳)𝟐 = (𝟏 − 𝝻)

(𝟏 + 𝝻 )(𝟏 − 𝟐𝝻 )

L= Specimen length mm

n= Resonating frequency HZ

V= Pulse velocity (mm/s)

8.2 Durability of concrete:

Ability of concrete to resist any type of deterioration (Eg. Weathering, abrasion, chemical

attack etc.) can be termed as durability.

Can also be defined as the ability to retain its original form, quality & serviceability when

exposed to environment.


For the durability of concrete permeability is treated as one of the key factor. Being small,

water molecules are capable of penetrating extremely fine pores / cavities. As a one of the

strong solvent it may dissolve acid/salt/alkalis/gas – ions which ultimately cause the

deterioration of concrete. For the same reason less the permeability of concrete less the effect

of chemical deterioration to it.

Cause of deterioration:

Physical / Mechanical Cause

Chemical Cause

Check the class note – in tabulated format for detail.

Characteristics of concrete influencing durability:

Permeability to water, O2, CO2, Chloride, Sulphate, & other potentially deteriorating

substances.

W/C ratio.

Cement type.

Compaction & Curing .

Rate of hydration & hydration process.

8.3 Creep & shrinkage phenomenon:

Creep is the time dependent strain phenomenon of concrete.

Creep can be defined as the gradual increase in strain with time even at sustained loading

condition. Cement paste plays important role in creep phenomenon. Even it is matter of

further research, creep is said to be due to the movement of gel & paste water in concrete by

various internal & external condition & at sustained loading.


Creep is partially reversible & partially irreversible phenomenon. The permanent deformation

due to creep is also called residual deformation.

Factors affecting creep:

Aggregate properties: Stronger the aggregate less the effect of creep. Aggregate with

higher E-value shows less creep.

Mix Proportion: Rich mix concrete with low W/C ratio shows low creep than poor

concrete mix.

Age/Time: Higher the age of concrete, more the formation of gel which ultimately

reduce creep phenomenon.

Shrinkage:

Change in volume of concrete by various phenomenon is called shrinkage. Shrinkage in

concrete is mainly due to the loss of moisture from concrete at its various stages. Various

types (as well as cause) of shrinkage can be listed as:

a) Plastic Shrinkage: Shrinkage due to the loss of moisture of concrete at its plastic stage

(green stage) is called plastic shrinkage. This type of shrinkage can be seen more in

large exposed surface concrete eg, road pavement, floor slab etc. Moisture loss due to

evaporation is main cause of this type of shrinkage.

b) Drying Shrinkage : Shrinkage due to the loss of moisture of harden concrete is called

drying shrinkage. Due to loss of moisture from concrete subjected to drying condition,

drying shrinkage occurs.

c) Autogeneous Shrinkage: Shrinkage due to without moisture movement from or to

concrete system can be termed as Autogeneous shrinkage. Very negligible compared

to other types of shrinkage.

d) Carbonation Shrinkage: CO2 present in environment react with Ca(OH)2 in concrete

producing CaCO3. This leads to the volume change in concrete, resulting carbonation

shrinkage.

8.4 Thermal properties of concrete: Check the class notes.

Class Note - Concrete (Moddified 10-01-2069)

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