Basics of Concrete Science by Professor L. Dvorkin, Professor O. Dvorkin - HTML preview

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CHAPTER 1

CONCRETE. RAW MATERIALS

L. Dvorkin and O.Dvorkin

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1.1. Concrete. General

Concrete can be classified as composite material and that is a

combination of different components which improve their performance

properties.

In general case binder component which can be in hard crystalline or

amorphous state is considered as the matrix of composite material.

In concrete matrix phase the grains of aggregates (dispersed phase) are

uniformly distributed.

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Concrete classification

Classification

Types of concrete

indication

Cement, Gypsum, Lime, Slag-alkaline, Polymer, PolymerTypes of binders

cement

Density

Normal-weight, High-weight, Light-weight

Normal-weight, Heavy-weight, Light-weight, Inorganic,

Types of aggregates

Organic

Size of aggregates

Coarse, Fine

Workability of

Stiff and Plastic consistency

concrete mixtures

Porosity of concrete

High-density, Low-density, Cel ular

High-strength, Resistance to action of acids or alkalis, Sulfate

Typical properties

resistance, Rapid hardening, Decorativeness

Structural concrete, Concrete for road and hydrotechnical

Exploitation purpose

construction, Concrete for thermal isolation, Radiationprotective concrete, White and Coloured concrete

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1.2. Binders. Classification.

Nature of binding properties

Concrete can be produced on the basis of all types of glues which have

adhesion to the aggregates and ability for hardening and strength

development.

Organic glues

Organic –

Inorganic glues

mineral glues

Molten

Solutions,

materials,

pastes

Pastes

Solutions,

bond

Pastes

solders

Binding and production of composite materials

Fig.1.1. Types of adhesives

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Periodicity of chemical compounds binding properties

Oxide of

Oxide

chemical

Al2O3

SiO2

Fe2O3

Cr2O3

Mn2O3

GeO2

SnO2

element

BeO

----- - - - -

MgO

------- - - -

CaO

++ ++ ++ ++ ++ ++ ++

ZnO

--------- - -

SrO

++ ++ ++ + + + +

CdO

----- - - - -

BaO

++ ++ ++ ++ ++ ++ ++

Note: fixed (++) and predicted (+) existence of binding properties; fixed (--) and

foreseen (-) absence of binding properties.

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1.3. Portland cement and its types

Chemical composition of portland cement clinker is as a rule within following

range, %:

СаО-63...66

MgO-0.5...5

SiO2-22...24

SO3-0.3...1

Al2O3-4...8

Na2O+K2O-0.4...1

Fe2O3-2...4

TiO2+Cr2O3-0.2...0.5

Fig. 1.2. Crystals of alite

Fig. 1.3. Crystals of belite

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а P

а P

28 days

, М

М

thg

h,

nre

trengt

3 days

ve st

ssire

ssive s

p

re

m

p

o

m

C

oC

Age of hardening, days

Amount of alite, %

Fig. 1.4. Rate of cement paste hardening

under using cements with different grain

Fig. 1.5. Relationship between amount

sizes:

of alite and compressive strength of

1–<3 µm; 2 – 3…9 µm; 3 – 9…25 µm;

cement

4 – 25…50 µm

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1.4. Hydraulic non portland cement binders

Lime binders

Hydraulic lime binders contain materials produced by grinding or

blending of lime with active mineral admixtures (pozzolans) — natural

materials and industrial byproducts. At mixing of active mineral

admixtures in pulverized form with hydrated lime and water, a paste

which hardened can be obtained.

Typical hydraulic lime binders are lime-ash binders.

Slag binders

Slag binders are products of fine grinding blast-furnace slag which

contains activation hardening admixtures. Activation admixtures must

be blended with slag at their grinding (sulfate – slag and lime – slag

binders) or mixing with water solutions (slag - alkaline binders).

Activation admixtures are alkaline compounds or sulfates which contain

ions Са2+, (ОН)-and (SO )2-.

4

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Calcium - aluminate (high-alumina) cements

Calcium - aluminate (high-alumina) cements are quickly hardening hydraulic

binders. They are produced by pulverizing clinker consisting essentially of

calcium aluminates.

h

rcent

trengt

pe

s

h,

trengt

28 day

S

of

Age, days

Fig. 1.6. Typical curves of cement strength

increase:

1 - calcium - aluminate cement; 2 – high-early strength

portland cement; 3 – ordinary portland cement

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1.5. Concrete aggregates

Classification of aggregates for concrete

Classification

Characteristics

Kind of aggregates

indication

of classification indication

Fine aggregates

≤5 mm

Grain size

Coarse aggregates

>5 mm

Gravel

Smooth particles

Particle shape

Crushed stone

Angular particles

Heavy

ρ0>1100 kg/m3

Bulk density (ρ0)

Light

ρ0≤1100 kg/m3

Normal and high - density P≤10%

Porosity (P)

Low - density

P>10%

Normal, high and low –

density concrete,

Properties of aggregates

Exploitation purpose Concrete for

must conform to the

hydrotechnical, road and concrete properties

other kinds of construction

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Percentage retained

Percentage retained

(cumulative), by mass

(cumulative), by mass

Sieve sizes, mm

Sieve sizes, mm

Fig. 1.7. Curves indicate the limits

specified in Ukrainian Standard for fine

Fig. 1.8. Curves indicate the

aggregates:

recommended limits

specified in

1,2 - Minimum possible (Fineness

Ukrainian Standard

for coarse

modulus=1.5) and recommended

aggregates

(Fineness modulus=2) limits of aggregate

size;

3,4 - Maximum recommended (Fineness

modulus=2.25) and possible (Fineness

modulus=2.5) limits of aggregate size

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1.6. Admixtures

Chemical admixtures

European standard (EN934-2) suggested to classify chemical admixtures as follows.

Admixtures by classification (Standard EN934-2)

Type of admixture

Technological effect

Reduce water required for given consistency or

Water reducer – plasticizer*

improve workability for a given water content

Essentially reduce water required for given

High water reducer –

consistency or high improve workability for a

superplasticizer**

given water content

Increase bond of water in

Prevention of losses of water caused by

concrete mixture

bleeding (water gain)

Entrainment of required amount of air in

concrete during mixing and obtaining of uniform

Air-entraining

distribution of entrained-air voids in concrete

structure

Accelerator of setting time

Shorten the time of setting

Note:

Increase the rate of hardening of concrete with

Accelerator of hardening

change of setting time or without it.

* Plasticizer reduces the

Retarder

Retard setting time

quantity of mixing water

Dampproofing and

Decrease permeability

required to produce concrete of

permeability-reducing

Water reducer/

Combination of reduce water and retard set

a given slump at 5-12%.;

retarder

effects

** Superplasticizer reduces the

High water reducer/

Combination of superplasticizer (high water

retarder

reduce) and retard set effects

quantity of mixing water at 12Water reducer/ Accelerator

Combination of reduce water and shorten the

30 % and more.

of setting time

time of setting effects

Influence on a few properties

Complex effect

of concrete mixture and concrete

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Classification of plasticizers

Plasticizer effect

Reduce the quantity of

Category

Type of plasticizer

(increase the slump

mixing water

from 2...4 sm)

for a given slump

І

Superplasticizer

to 20 sm and more

no less than 20 %

ІІ

Plasticizer

14-19 sm

no less than 10 %

ІІІ

Plasticizer

9-13 sm

no less than 5 %

ІV

Plasticizer

8 and less

less than 5 %

Air-entrained admixtures are divided into six groups (depending on

chemical composition):

1) Salts of wood resin;

2) Synthetic detergents;

3) Salts of lignosulphonated acids;

4) Salts of petroleum acids;

5) Salts from proteins;

6) Salts of organic sulphonated acids.

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As gas former admixtures silicon-organic compounds and also aluminum

powder are used basically. As a result of reaction between these admixtures

and calcium hydroxide, the hydrogen is produced as smallest gas bubbles.

Calcium chloride is the most explored accelerating admixture. Adding this

accelerator in the concrete, however, is limited due to acceleration of

corrosion of steel reinforcement and decrease resistance of cement paste in

a sulfate environment.

As accelerators are also used sodium and potassium sulfates, sodium and

calcium nitrates, iron chlorides, aluminum chloride and sulfate and other

salts-electrolytes.

Some accelerating admixtures are also anti-freeze agents which providing

hardening of concrete at low temperatures.

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In technological practice in some cases there is a necessity in retarding

admixtures.

4

Forsen has divided retarders into

em

four groups according to their

tign

influence on the initial setting

tti

2

e

time:

1

l s

itiaIn

1. CaSO ·2H O, Ca(ClO ) ,

4

2

3 2

CaS .

2

2. CaCl , Ca(NO ) , CaBr ,

2

3 2

2

3

CaSO ·0.5H O.

4

2

3. Na CO , Na SiO .

2

3

2

3

Amount of retarder

4. Na PO , Na S O , Na AsO ,

3

4

2 4

7

3

4

Ca(CH COO) .

3

2

Fig.1.9. Effect of retarding admixrures

on initial setting time (from Forsen)

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Mineral admixtures

Mineral admixtures are finely divided mineral materials added into concrete

mixes in quantity usually more than 5 % for improvement or achievement

certain properties of concrete.

As a basis of classification of the mineral admixtures accepted in the

European countries and USA are their hydraulic (pozzolanic) activity and

chemical composition.

Fly ash is widely used in concrete mixes as an active mineral admixture.

Average diameter of a typical fly ash particle is 5 to 100 µm. Chemical

composition of fly ash corresponds to composition of a mineral phase of

burning fuel (coal).

Silica fume is an highly active mineral admixture for concrete which is widely

used in recent years. Silica fume is an ultrafine byproduct of production of

ferrosilicon or silicon metal and contains particles of the spherical form with

average diameter 0,1µm. The specific surface is from 15 to 25 m2/kg and

above; bulk density is from 150 to 250 kg/m3.

The chemical composition contains basically amorphous silica which quantity

usually exceeds 85 and reaches 98 %.

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