“PRESENT & FUTURE OF LIGHTWEIGHT CONCRETE STRUTURES”

A Paper presented

By

Uttam Nangre-Patil & Mahadeo Nalawade

INTRODUCTION

Concrete design has evolved rapidly in the last 30 years. Construction

technology has seen the introduction of a variety of concrete products to

the market as well as an increased use of supplementary cementitious

materials and recently blended cements. Emphasis has been placed on

creating more durable concrete through changes to the mix constituents and

proportions, including the aggregates, admixtures and the water-cement

ratio. This evolution, along with improved reinforcing steel strength and the

use of l ightweight f iber reinforcement steel has lead to modifications in

design philosophy - most notably the use of thinner structural members.

Purpose of writing this article is not to go into the historical background or

evolution of the l ightweight concrete structures but to explore the present

day developments as well as look into future for l ightweight concrete

structures.

WHY LIGHTWEIGHT CONCRETE STRUTURES?

Basically one of the biggest disadvantages of normal conventional concrete

is its self weight of about 2200 to 2600 Kg/m3 which is so high and attempt

have been made in past to reduce the self weight of the concrete and to

increase the efficiency of the concrete as a structural material. Therefore

day by day the uti l ization of normal concrete in building across the globe is

going down due to its inflexibil ity, material cost and the associated cost of

labour for handling the materials. The weight of building on foundations is

important factor considered while designing the structures particularly in

case of weak soil and highrise structures. We know that a solid ordinary

concrete made of only f ly ash, Portland cement and aggregates can gives the

strength of 55to62 N/mm2. This strength is much more than the required

strength for most of the structural applications. So the need for going for

developing alternative ways to l ighten the strength of concrete as well as

make it l ightweight with keeping desired properties required for most of the

structural applications. But developing a viable l ightweight structural

concrete with least amount-of materials and manufacturing cost is a complex

science as it ’s not that easy to fulf i l l al l the desired parameters. The primary

use of the structural l ightweight concrete is to reduce the dead load of a

concrete structure, which then allows the structural designer to reduce the

size of columns, footings and other load bearing elements. Structural

l ightweight concrete provides a more efficient strength-to-weight ratio is

structural elements. In most cases, the marginally higher cost of the

l ightweight concrete is offset by size reduction of structural elements, less

reinforcement steel and reduced volume of concrete, resulting in lower

overall cost.

Currently the use of structural l ightweight concrete has l imited to large cast

structures where it is lower density is required such as bridges and high-

rises. Now days, you can find the use of l ightweight concretes with density

range from 300 Kg/m3 to 1800 Kg/m3. Considerable economy can be

achieved with use of l ightweight concretes with additional benefits of other

futures viz. faster construction due to l ightweight material handling, low

thermal conductivity helps conserving energy and the use of industrial

wastes l ike cl inker, f ly ash, slag & recycled plastic etc for manufacturing

l ightweight concrete. Apart from all these benefits, there are research shows

that if center of gravity doesn’t coincide with the center of r igidity of the

building, the higher amount of reinforcement steel required for normal

weight conventional concrete than l ightweight concrete for vertical

components l ike column or horizontal longitudinal components l ike beams.

There is no difference of qty of steel required for slabs but its phenomenal

savings on reinforcement cost in columns and beams in such cases.

INOVATIONS BEHIND DEVELOPMENT OF LIGHTWEIGHT CONCRETES

Lightweight concrete may be obtained through use of l ightweight

aggregates, or by special methods of production. These methods include the

use of foaming agents, such as aluminum powder, which produces concrete

of low unit weight through generation of gas while the concrete is sti l l

plastic. Lightweight concrete may weigh from 500 to 1800 Kg/M3, depending

on the type of l ightweight aggregate used or the method of production.

Natural l ightweight aggregates include pumice, scoria, volcanic cinders, tuff,

and diatomite. Lightweight aggregate can also be produced by heating clay,

shale, slate, diatomaceous shale, perl ite, obsidian, and vermiculite.

Industrial cinders and blast-furnace slag that has been special ly cooled can

also be used.

CELLULAR LIGHTWEIGHT CONCRETE

It is cementations paste of neat cement or cement and fine sand with a

multitude of micro/microscopic discrete air cells uniformly distributed

throughout the mixture to create a l ightweight concrete. One-way it’s

manufactured with creating permanent air bubbles in the concrete by using

pre-formed foam which is also called as surfactant and another way is

creating bubbles with mixing expansion agent in normal concrete and this

concrete called as Autoclaved Aerated Concrete (AAC). One more process

technology is being used recently is the use of aqueous gels. In this aqua gel

spheres, particles or pieces are formed from gelatinized starch and added to

a matrix. Starch modified or unmodified such as wheat, corn, r ice, potato or

of combination of modified or unmodified starches are examples of aqueous

gels. Agar is also used in the l ightweight concrete. During the curing process

the aqueous gels looses it moisture, it shrinks and then dried up to form a

l ightweight concrete bed. High carbon ash, recycled aluminum waste and

zeolite powders are additional mechanical structures suitable in the

production of cellular l ightweight concrete. High-Performance Cellular

Concrete has al l the properties of cellular concrete and can achieve

compressive strength of 55.37 N/mm2. Higher strengths can be produced

with mixing supplementary cementitious materials. Density and strengths

can be controlled to meet specific structural and nonstructural design

requirements.

MICRO SILICA CONCRETE

Sil ica fume is a byproduct of the electric arc furnace production of si l icon

and ferro-si l icon al loys. This concrete produced with mixing condensed si l ica

fume with mixture of normal concrete materials. When condensed si l ica

fume is mixed with water, a chemical reaction occurs creating crystals which

physically f i l l any voids in the concrete containing pore water; thus, creating

both a water-resistant and a high-strength material.

LIGHWEIGHT AGREEGATES FOR LIGHTWEIGHT CONCRETE

This is continuation of what we have explained in the part “INOVATIONS

BEHIND……” regarding the use of l ightweight aggregates. Earl ier l ightweight

aggregates were natural origin and mostly volcanic l ike pumice, tuff etc. The

pumice is sti l l used in certain countries l ike Japan, Italy & Germany. In some

places l ike Malaysia, palm oil shells are used for making l ightweight

concrete. Today techniques have been developed to produce l ightweight

aggregates in the factories with using natural raw materials l ike expanded

clay, shale, slate etc as well as industrial by-products l ike f ly ash & blast

furnace slag etc. These artif icial aggregates can be produced with varying

densities from 50Kg/m3 to 1000 Kg/m3 which are much lesser than normally

used aggregates with having densities range from 1600Kg/m3 to 2000 Kg/m3.

The compressive strength unto 80 N/mm2 can be achieved with using

l ightweight aggregates. Lightweight aggregate is used not only for its l ighter

weight but also for its superior sound abatement, seismic performance, f ire

resistance, and insulation and geotechnical properties.

Natural Lightweight Aggregates:

i) Pumice: It forms from supper cooled l iquid of lava which

contains mainly SiO2 which is erupted from volcanoes and the

low density of it is due to presence of gas bubbles inside it.

i i) Palm Oil Shells : This is by-product of oi l industry as while

extracting oil , we get this palm oil shells which are very hard

and can produce l ightweight concrete with compressive strength

up to 19.5 N/mm2.

i i i) Perlite : In Japan new l ightweight aggregate have been

developed with using perl ite which is cal led as Asano Super

Light.

iv) Lightweight Aggregates from treatment of natural aggregates:

Heating clay or shale in a rotary ki ln to a temperature that

causes the material to expand or bloat makes conventional

l ightweight aggregate.

Lightweight Aggregates from Industrial by-products:

i) Fly Ash Aggregates: The fly Ash which is the by-product of

Thermal Power Stations in India available huge quantity. This Fly

ash with higher and variable carbon content is used for making

aggregates by adding extra pulverized coal to bring the carbon

content to about 12%. Cement and fly ash shall be mixed in

various proportions with 0.3 water-cement ratio in a concrete

mixture. The contents shall be thoroughly mixed in the drum

unti l the complete formation of f ly ash aggregates and this

method is cal led as pelletisation. These aggregates then dried

for one day in open and then cured in water tank for next 7

days. Finally with proper sieving process the aggregates shall be

separated as f ine and coarse aggregates.

ii) Lightweight Aggregate from Molten Blast Furnace Slag: Molten

slag from blast furnace shall be put under high pressure, high

volume & cold water sprays to rapidly cool down, resulting in

the formation of an amorphous and then with using Granulators

it can be converted into aggregate sized material. Lafarge, a well

known multinational company, known in India for Ready Mix

Concrete supplier is having a long 45 years experience in the

processing of slag. They supply low density aggregates under

various brands in North America mainly processed from slag.

Lafarge processes molten blast furnace slag using any of three

means: Expanding, pelletizing, and air-cooling. These yield raw

materials that can be used for many construction related

applications. Latex and True Light Weight Aggregate are the

some of the successful brands developed by Lafarge which are

almost 35% l ighter than normal aggregates.

STRUCTURE AROUND THE GLOBE WHERE LIGHTWEIGHT CONCRETE USED:

i) Under Mega brand Lafarge brought a l ightweight concrete in

India which is used in various projects India including Shoba

Lyfestyle Vil la at Bangalore, Kesar Solitaire Park at Navi Mumbai

& Technopolis IT Park at Kolkatta.

ii) Expanded shale aggregate supplied by TXI Pacific Custom Materials, Inc. in

California, was used to produce lightweight concrete for all the precast

components in the Wellington Stadium in New Zealand.

iii) The landmark 170 meter high tower “Australia Square” which was tallest

building in the world when it was constructed at Sydney in Australia in 1967

in which lightweight concrete was used.

iv)

LIGHTWEIGHT CONCRETE IN DREAM