“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 lightweight fiber 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 lightweight concrete structures but to explore the present day developments as well as look into future for lightweight 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 utilization of normal concrete in building across the globe is going down due to its inflexibility, 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 fly 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 lighten the strength of concrete as well as make it lightweight with keeping desired properties required for most of the structural applications. But developing a viable lightweight structural concrete with least amount-of materials and manufacturing cost is a complex science as it’s not that easy to fulfill all the desired parameters. The primary use of the structural lightweight 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 lightweight concrete provides a more efficient strength-to-weight ratio is structural elements. In most cases, the marginally higher cost of the lightweight 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 lightweight concrete has limited 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 lightweight concretes with density range from 300 Kg/m3 to 1800 Kg/m3. Considerable economy can be achieved with use of lightweight concretes with additional benefits of other futures viz. faster construction due to lightweight material handling, low thermal conductivity helps conserving energy and the use of industrial wastes like clinker, fly ash, slag & recycled plastic etc for manufacturing lightweight concrete. Apart from all these benefits, there are research shows that if center of gravity doesn’t coincide with the center of rigidity of the building, the higher amount of reinforcement steel required for normal weight conventional concrete than lightweight concrete for vertical components like column or horizontal longitudinal components like 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 oflightweight 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 still plastic. Lightweight concrete may weigh from 500 to 1800 Kg/M3, depending on the type oflightweight aggregateused or the method of production. Naturallightweight aggregatesinclude pumice, scoria, volcanic cinders, tuff, and diatomite.Lightweight aggregatecan also be produced by heating clay, shale, slate, diatomaceous shale, perlite, obsidian, and vermiculite. Industrial cinders and blast-furnace slag that has been specially 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 lightweight 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, rice, potato or of combination of modified or unmodified starches are examples of aqueous gels. Agar is also used in the lightweight concrete. During the curing process the aqueous gels looses it moisture, it shrinks and then dried up to form a lightweight concrete bed. High carbon ash, recycled aluminum waste and zeolite powders are additional mechanical structures suitable in the production of cellular lightweight concrete. High-Performance Cellular Concrete has all 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

Silica fume is a byproduct of the electric arc furnace production of silicon and ferro-silicon alloys. This concrete produced with mixing condensed silica fume with mixture of normal concrete materials. When condensed silica fume is mixed with water, a chemical reaction occurs creating crystals which physically fill 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 lightweight aggregates. Earlier lightweight aggregates were natural origin and mostly volcanic like pumice, tuff etc. The pumice is still used in certain countries like Japan, Italy & Germany. In some places like Malaysia, palm oil shells are used for making lightweight concrete. Today techniques have been developed to produce lightweight aggregates in the factories with using natural raw materials like expanded clay, shale, slate etc as well as industrial by-products like fly ash & blast furnace slag etc. These artificial 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 lightweight aggregates. Lightweight aggregate is used not only for its lighter weight but also for its superior sound abatement, seismic performance, fire resistance, and insulation and geotechnical properties.

Natural Lightweight Aggregates:

i)  Pumice: It forms from supper cooled liquid 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.

ii)  Palm Oil Shells : This is by-product of oil industry as while extracting oil, we get this palm oil shells which are very hard and can produce lightweight concrete with compressive strength up to 19.5 N/mm2.

iii)  Perlite : In Japan new lightweight aggregate have been developed with using perlite which is called as Asano Super Light.

iv)  Lightweight Aggregates from treatment of natural aggregates: Heating clay or shale in a rotary kiln to a temperature that causes the material to expand or bloat makes conventional lightweight 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 until the complete formation of fly ash aggregates and this method is called 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 fine 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% lighter than normal aggregates.

STRUCTURE AROUND THE GLOBE WHERE LIGHTWEIGHT CONCRETE USED:

i)  Under Mega brand Lafarge brought a lightweight concrete in India which is used in various projects India including Shoba Lyfestyle Villa 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