Advancement in Concrete Technnolog

7/27/2019 Advancement in Concrete Technnolog

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ADVANCEMENT IN CONCRETE

TECHNNOLOGY

HIGH PERFORMANCE LIGHT

WEIGHT CONCRETE

ABSTRACT:

Concrete is widely used construction material. Despite its versatility, it suffers

from certain setbacks like limited resistance to ductility, little resistance to cracking,

high density and so on. The purpose of this article is to introduce a new type of high

performance light weight concrete which is superior to conventional concrete in many

fronts. In other words, we can say high performance light weight concrete as a

structural light weight aggregate concrete with a minimized density at a desired

strength level and durability. Thus the term high performance light weight concrete is

not purely related to strength but also includes durability and density. This calls for a

new engineering approach in the field of concrete technology by adding silica fume

and fly ash and incorporating the desirable properties of the same. The materials used,

their mechanical properties and the mixproportion are presented in detail. The effect

of these components on fresh and hardened concrete has been discussed in detail.

INTRODUCTION:



High performance igh! "eigh! concre!e is a concrete with high strength and low

permeability with light weight aggregates. To attain low permeability the mix should contain

particles graded down to the finest size, this can be achieved by the use of silica fume which fills the

space between cement particle and the aggregates .The mix must be sufficiently workable and also

re!uires deflocculating of cement particles .This is achieved by the use of fly ash. The light weight

aggregates like pumice and blast furnace slag has low density which decreases the weight of

concrete.

MATERIALS USED:

". #or high strength

a$ %ilica fume

&. #or workability

a$ #ly ash

'. #or light weight

a$ (rtificial aggregate) #oamed blast furnace slag

b$ *atural aggregate) pumice

SILICA FUME:

SOURCE:

%ilica fume is a byproduct of the manufacture of silicon and ferrosilicon alloys from high

purity !uartz and coal in a submergedarc electric furnace. It cools condenses and is collected in

cloth bags. %ilica fume contains large amounts of silicon dioxide +between - and / $ and

consists of extremely fine particles. It is collected by filtering the escaping furnace gases.

PROPERTIES:

(verage size of these spherical particles is less than 0." µm.

1inimum specific surface area is "-,000 m&2kg

It contains - silicon dioxide.



3ulk density &00'00 kg2m'

It increases the strength of concrete upto 0 14a .

MECHANISM:

%ilica in the form of glass +amorphous$ is highly reactive, and smallness of the particles

speeds up the reaction with calcium hydroxide produced by the hydration of the 4ortland cement. This

improves the packing between the particles of cement .The size of particle is approximately one

hundred times finer than cement this lead to important benefits when silica fume is used in concrete.

5igher strength of concrete is attributed to the following actions)

POZZOLANIC ACTION:

%ilica reacts with calcium hydroxide which produced during the hydration of cement, improves

the strength and reduces the permeability. FILLER:

It fills the minute pores in the concrete and hence it improves the strength. In general, the

%mall voids present in the concrete reduce much strength which can be retained from using silica.

MIX PROPORTION :

%ilica can be used as

". 4artial replacement for cement from - to&0 of the weight of cement.

&. (ddition to cement or concrete not more than "0 of the weight of cement.

1ostly dry densified form of silica fume is used which is easy to handle during mixing.

FLY ASH:

SOURCE:



#ly ash is a byproduct of combustion coal transported by the flue gases and collected by the

electrostatic precipitator in the thermal power plants .It is removed by the dust collecting system as

fine particle residue from the combustion gases before they are discharged into atmosphere. The

slagged particles and unburnt carbon are collected as ash.

PROPERTIES:

4article size between "to "-0 micron.

%pecific surface is '-0 to-00 m&2kg.

it contains oxides of silica ,aluminium and calcium ,but the

amount of calcium oxide is considerably less.

It increases the workability.

MECHANISM:

#ly ash is a fine particle and amorphous in character .In finely divided form in the presence of

moisture chemically reacts with calcium hydroxide at ordinary temperature to form compounds

possessing cementitous properties.

#ly ash improves the workability of fresh concrete in two ways)

#ly ash disperses the cement floc there by freeing more paste to lubricate aggregates.

#ly ash acts as a ball bearings because of their spherical shape. It carries surface film of water

with it because of its hydrophilic nature. thus there is a water entrainment that disperse water.

MIX PROPORTION:

The mix proportion described by (CI &"" is based on a simple replacement method .It

involves partial replacement of cement by fly ash. (ccording to this, the water2cement ratio changes

to water2cement 6 fly ash ratio.

The recommended amounts when used as a partial replacement of cement or sand in

concrete are &0 to '-.



LIGHT WEIGHT AGGREGATE :

NATURAL AGGREGATE – PUMICE

SOURCE:

4umice is a volcanic rock composed of bubbles or vesicles in a glass matrix formed by the

effervescence of gases and rapid cooling of molten granite during an eruption. Their lightness is due to

the escaping of gas from the molten lava when erupted from earth7s crest. Characteristically pumice is

frothy and lightweight, with a density low enough to float on water. The combination of its light

weight, hardness and abrasiveness makes pumice useful in numerous applications.

STRENGTH OF AGGREGATE:

4umice concrete meet all compressive strength

re!uirements while providing faster and easier

placement and reduced deadweight loads. 8ower

thermal conductivity results in less heat loss and

higher fire ratings than normal concrete. 4umice acts as

an acoustical insulator. Compressive strengths up to

9- 14a and unit weights as low as "000 kg2m' are attainable.

.

The properties and characteristics of pumice and foamed blast furnace slag are similar .They

are bulk density of aggregate ,drying shrinkage ,thermal conductivity and water absorption. The bond

between aggregate and hydrated cement paste is so strong that results in significant transfer of stress

across the transition zone.5ence,strength and shape of the aggregate plays a vital role in concrete.

#or the same crushing strength ,the density of concrete made with such an aggregate can be as

much as '- lower than the normal weight concrete. %o, in the high performance concrete ,the light

weight aggregate is used which results in high performance light weight concrete.



CHARACTERISTIC OF POZZOLANIC REACTION:

"$ The reaction is lime consuming instead of lime producing

&$ The reaction is pore refiner C%5 gel fills capillary space

'$ The reaction is grain size refiner : nucleation of Ca;5 around smaller pozzolan will replace

large Ca;5 crystals with numerous smaller crystals of Ca;5 .

9$ The reaction is transition zone refinerthe influence of pozzolan on transition zone can extend to

distances as for as -0micron from aggregate surface.

FOAMED BLAST FURNACE SLAG

SOURCE:

It is a byproduct of iron production. It is a nonmetallic product consisting essentially of

silicates and aluminates of calcium and other bases. The molten slag is rapidly chilled by !uenching in

water to form a glassy sand like granulated material.

PROPERTIES:

#oamed slag aggregates are available in two sizes

a. Coarse aggregate +'"-mm$

b. #ine aggregate+less than 'mm$.

The specific surface area is about 900 to <00 mm&2kg before foaming.



The specific gravity is about &.<.

The performance of slag depends on glass content and fineness of grinding.

The light weight concrete can be achieved by different ways.

3y replacing the usual mineral aggregate by light weight aggregate .

3y introducing gas or air bubbles in mortar known as aerated concrete.

3y omitting sand fraction from the aggregate called as =no fines =concrete.

STRENGTH OF AGGREGATE:

The strength of the light weight aggregate particles decreases with decreasing density .Crushed

and angular light weight aggregate re!uires high mortar content resulting in a higher density than that

with rounded. The strength of the light weight aggregate may also vary from ' *2mm& to 90 *2mm& .

GENERAL PROPERTIES OF HPLWAC

A)EFFECT OF PROPERTIES ON FRESH CONCRETE:

#ly ash is used to replace natural sand and basalt, so the water demand of the concrete

decreases. This is attributed to the spherical2round particle shape of #oamed blast furnace slag which,

compared to the angular particles of sand and basalt, have a =ballbearing effect> and thus reduce the

water demand of the fresh concrete.

#$ WOR%A&ILITY

To achieve the workability necessary for ease of placement, large cement paste volumes are

re!uired. The high strength concretes have low watercement ratios. In cementrich mixtures,

=stickiness> i.e. flocculation of cement particles can lead to placement and finishing problems . The

#ly ash contributes the reduction in water demand of concrete. This can be suitable for pumping slip

forming and finishing operation .The =ball bearing effect = of fly ash to provides the fluidity desired

to achieve acceptable levels of workability.

'$ PERMEA&ILITY(



?educed permeability attributed to filler effect and pozzolanic reaction of silica fume, there is

a significant reduction of about one order of magnitude in the volume of pores larger than "000,when

silica fume is added

B)EFFECT ON HARDENED CONCRETE:

#$COMPRE))IVE )TRENGTH(

The compressive strength of 548@(C can be achieved more than -014a. In the contribution

of silica fume to strength at &days, increase with an increase in content of silica fume in the mix.

The effect of replacing ;4C with fly ash increases the compressive strength when its fineness fraction

increases.

The stress2strain behaviour of 548@(C in compression is compared to normal density

concrete of the same compressive strength, generally characterized by a linear ascending branch, a

lower Amodulus and less ductility in the postfailure region. These characteristics are usually more pronounced with increasing compressive strength and decreasing dry density. It comprises of high

strength with lower density.

'$C*RING(

Affect from the use of the low watercement ratio concretes is selfdesiccation, %elfdesiccation

refers to the process by which concrete dries itself from the inside. Internal moisture is consumed from

within the paste by the hydration reactions. The result is that the hydration and maturity of the

concrete will terminate at an early age if additional moisture is not provided. #or this reason,

proper curing of low watercement ratio concrete at early ages is essential if the concrete is to attain its

potential properties. The detrimental effects of selfdesiccation can be largely controlled by careful

attention to curing, especially during the initial B days after placement.

a$ 5ydration has not yet occurred and pores are filled with water

b$ 3eginning of hydration.



c$ 5ydration continues although empty spaces still exist, they are filled with water and calcium

hydroxide.

d$ 5ardened cement paste.

Pores in calcium silicate through different stages of hydration.

+$ CREEP AND )HRIN%AGE

It was found that shrinkage strains are inversely proportional to the moist curing time longer

the curing time, the lower the amount of shrinkage. There were no significant effects on shrinkage

and creep from the use of #ly ash and foamed blast furnace slag. The initial rate of shrinkage is high

because a large surface area of the cement paste is exposed to the air.

,$ )*LPHATE ATTAC%(

The influence of concrete to sulphate attack is variable and depend on fly ash .It depends on

Ca; and #e&;' on fly ash. In general, fly ash, by virtue of its higher resistance value results in higher

concrete expansion. 3oth fly ash and foamed blast furnace slag offers more sulphate resistance.



?elationship between the water2cement ratio and porosity.

-$AL%ALI AGGREGATE REACTION (

#ly ash have been found to reduce expansion due to alkali silica reaction. Its effectiveness

depends on alkali content,fineness,pozzolanic activity .%mall addition of fly ash may increase

expansion whereas large amounts decreases the expansion .#ly ash ,silica fume and foamed blast

furnace slag can be used to counteract alkaliaggregate reaction.

#oamed blast furnace slag reduces the expansion due to alkali :silica reaction which is due

to number of facts such as

a$ The dilution effect related to addition of slag and reduction of

calcium hydroxide

b$ 4resence of more C%5 gel.c$ 8ower permeability and diffusivity of foamed blast furnace slag

.

.$CORRO)ION(

#ly ash improves the concrete resistance to corrosion in three ways.

"$ It provides alkali medium to steel surface .

&$ It offers physical and chemical barrier to ingress of moisture,;& ,Co&, Cl.'$ It provides electrically resistive medium around steel.

/$ CHEMICAL ATTAC%(

%ilica fume concrete had very high resistance to all chemicals except (mmonium %ulphate .It

is due to lowered permeability then silica fume was exposed to ammonium sulphate C%5 gel

decomposed .

0$ THERMAL E1PAN)ION(

The thermal expansion mainly depends on thermal property of aggregate .In slag there is a

considerable reduction in thermal coefficient with a B- replacement of aggregate.

FIELD APPLICATION:

The 548@(C can be used widely in many places like



?epair and rehabilitation.

Construction of tunnel.

Construction of high rise structure.

Construction of bridge deck overlays.

nder water castings.

CONCLUSION:

The 548@(C offers good compressive strength and by using light weight aggregate like

pumice, foamed blast furnace slag the self weight is reduced. In high rise structure the self weight

can be reduced by using light weight aggregate. The advantages of 548@(C like cost and energy

savings by replacing some part of ;4C with these materials and Anvironmental protection by

providing beneficial way to use by products like #ly ash and 3last furnace slag especially relative to

disposing them in the air or water or in landfills.

Advancement in Concrete Technnolog

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