Development Of Light Weight Concrete
This paper deals with the development of two types of lightweight concrete the one using lightweight aggregate (Pumice stone) and the other water floating type using Aluminium powder as an air entraining agent. This also shows the importance of water/cement ratio as in first type of concrete it relates to the smoothness of the surface and in second one it is a major factor which controls the expansion of concrete.
Lightweight concrete can be defined as a type of concrete which includes an expanding agent in it that increases the volume of the mixture while reducing the dead weight. It is lighter than the conventional concrete with a dry density of 300 kg/m3 up to 1840 kg/m3. The main specialties of lightweight concrete are its low density and low thermal conductivity.
There are many types of lightweight concrete which can be produced either by using lightweight aggregate or by using an air entraining agent. In this project I have worked on each of the above mentioned types. Both of them are non-structural concrete.
1) By using Pumice stone as a lightweight aggregate:
Pumice stone is a lightweight aggregate of low specific gravity. It is a highly porous material with a high water absorption percentage. In this we do not use the conventional aggregate and replace it by the pumice stone.
2) By using Aluminium powder as an air entraining agent:
Water floating aerated concrete is made by introducing air or gas into slurry composed of Portland cement and sand, so that when the mix sets and hardens, uniform cellular structure is formed. Thus it is a mixture of water, cement and finely crushed sand. We mix fine powder of Aluminium to the slurry and it reacts with the calcium hydroxide present in it thus producing hydrogen gas. This hydrogen gas when contained in the slurry mix gives the cellular structure and thus makes the concrete lighter than the conventional concrete.
Lightweight concrete is of utmost importance to the construction industry. The advantages of lightweight concrete are its reduced mass and improved thermal and sound insulation properties, while maintaining adequate strength. The marginally higher cost of the lightweight concrete is offset by size reduction of structural elements, less reinforcing steel and reduced volume of concrete, resulting in overall cost reduction. The reduced weight has numerous advantages; one of them is reduced demand of energy during construction.
TYPES OF LIGHTWEIGHT CONCRETE:
Using lightweight aggregates: This type is produced using lightweight aggregate such as volcanic rock or expanded clay. It can be produced with the use of naturally mined lightweight aggregates (bulk density in the range of 880 kg/m3) or manmade lightweight aggregates like “Aardelite” or “Lytag” (bulk density 800 kg/m3).
Using foaming agents: This one is produced through the addition of a foaming agent in cement mortar. This creates a fine cement matrix which has air voids throughout its structure. Aerated cement mortar is produced by the introduction of a gas into cementitious slurry so that after hardening a cellular structure is formed.
TYPES OF LIGHTWEIGHT AGGREGATE:
Lightweight aggregates used in structural lightweight concrete are typically expanded shale, clay or slate materials that have been fired in a rotary kiln to develop a porous structure. Other products such as air cooled blast furnace slag are also used. Also there are some non structural lightweight aggregates with lower density made with other aggregate materials and higher air voids in the cement paste matrix. These are typically used for their insulation properties.
Inorganic Natural Aggregates: Diatomite, pumice, scoria and volcanic cinders are natural, porous volcanic rocks with a bulk density of 500 – 800 kg/m3 which make a good insulating concrete
Organic Natural Aggregates: Wood chips and straw can be mixed with a binder to provide a lightweight natural aggregate. These are cellular materials which have air trapped within their structures once they have low moisture content.
1. Bloated clay, sintered fly ash and foamed blast furnace slag.
2. Lightweight expanded clay aggregate: This is produced by heating clay to a temperature of 1000 – 1200 oC, which causes it to expand due to the internal generation of gases that are trapped inside. The porous structure which forms is retained on cooling so that the specific gravity is much lower than what was before heating it.
There are some foaming agents which when added to the cement slurry forms air voids throughout its structure. Also there are some agents who react with the chemicals present in the cement slurry and evolve gases which results in the expansion of the slurry and when it hardens, leaves air voids in the concrete thus making it lighter than the normal concrete.
The bulk density of fine lightweight aggregates is around 1200 kg/m3.
The bulk density of coarse lightweight aggregates is around 960 kg/m3.
Light Weight: Density range from 650 Kg/m3 to 1850 Kg/m3 as compared to 1800
Kg/m3 to 2400 Kg/m3 for conventional brick and concrete respectively. Despite
millions of tiny air filled cells, it is strong and durable. There is Lightweight advantage for the structure design, leading to savings in supporting structures and foundation.
Compressive Strength: 2.0 to 7.0 N/mm2.
Excellent Acoustic Performance: It can be used as effective sound barrier and for acoustic solutions. Hence, highly suitable for partition walls, floor screens / roofing and panel material in auditoriums.
Earthquake Resistant: Since lighter than concrete & brick, the lightness of the material increases resistance against earthquake.
Insulation: Superior thermal insulation properties compared to that of conventional brick and concrete, so reduces the heating and cooling expenses. In buildings, light-weight concrete will produce a higher fire rated structure.
Workability: Products made from lightweight concrete are lightweight, making them easy to place using less skilled labour. The bricks can be sawed, drilled and shaped like wood using standard hand tools, regular screws and nails. It is simpler than brick or concrete.
Lifespan: Weather proof, termite resistant and fire proof.
Savings in Material: Reduces dead weight of filler walls in framed structures by more than 50% as compared to brickwork resulting in substantial savings. Due to the bigger and uniform shape of blocks, there is a saving in bed mortar and plaster thickness. In most cases the higher cost of the light-weight concrete is offset by a reduction of structural elements, less reinforcing steel and reduced volume of concrete.
Water Absorption: Closed cellular structures and hence have lower water absorption.
Skim Coating: Do not require plaster and water repellent paint suffices. Wallpapers and plasters can also be applied directly to the surface.
Modulus of Elasticity: The modulus of elasticity of the concrete with lightweight aggregates is lower, 0.5 – 0.75 to that of the normal concrete. Therefore more deflection is there in lightweight concrete.
It is produced by including large quantities of air in the aggregate, matrix or in between the aggregate particles or by a combination of these processes. Lightweight aggregates require wetting prior to use to achieve a high degree of saturation. If the aggregates aren’t fully saturated they have a tendency to float towards the surface of the mix after it has been placed.
Due to the higher moisture content of light-weight concrete, drying times are typically longer than regular concrete. Typically, a 0.5 water to cement ratio slurry is used as a base mixture for lightweight concrete. The water cement ratio varies according to specific project requirements.
Note that lightweight concrete obtains its natural fluidity from the air bubble structure, not from excess water content.
Effect of adding Fly ash: Fly ash added to the cement does not adversely affect the basic hardened state of lightweight concrete. Infusing and supporting the lightweight concrete with the air cell system is a mechanical action and is not problematic with fly ash or other additives. Note that some fly ash mixes may take longer to set than pure Portland cement applications.
The primary use of lightweight concrete is to reduce the dead load of a concrete structure which then allows the structural designer to reduce the size of columns, footing and other load bearing elements. So the marginally higher cost of the lightweight concrete is offset by the size reduction of structural elements, less
reinforcing steel and reduced volume of concrete, thus resulting in lower overall cost.
They can also be used for fire protection, where they can shield structural steel from fire. They are also used as an insulating block.
Lightweight concrete has been used to construct extremely large cantilevers, as the member can be narrower due to the decreased dead load. Using concrete of a lower density results in a lower dead load and can result in savings due to smaller member sizes. Occasionally this can allow construction on ground with a low load-bearing capacity.
The porosity of lightweight aggregate provides a source of water for internal curing of the concrete that provides continued enhancement of concrete strength and durability, but this does not prevent the need for external curing.
Structural light-weight concrete has been used for bridge decks, piers and beams, slabs and wall elements in concrete and steel buildings, parking structures, tilt-up walls, topping slabs and composite slabs on metal decking.
Note: The concrete cover to reinforcement using lightweight aggregates in concrete should be adequate. Usually it is 25mm more than that of normal concrete because of its increased permeability and also concrete carbonates rapidly by which the protection to the steel by the alkaline lime is lost.
AUTOCLAVED AERATED CONCRETE USING ALUMINIUM POWDER (AAC)
Autoclaved Aerated Concrete (AAC) or Autoclaved Lightweight Concrete (ALC) is a pre-cast construction material that is made from a variety of aggregate parts no larger than sand. At roughly one-fifth of the weight of normal concrete, it is an incredibly lightweight building material. It provides excellent thermal and acoustic resistance and also protects against household hazards as termites and fire. AAC is commonly referred to as autoclaved cellular concrete because hydrogen bubbles form during the production process, resulting in small pockets of air within the concrete that substantially increase the volume of the final concrete product. Though the precise composition of autoclaved aerated concrete may vary, it is generally made up of quartz sand or some other fine aggregate, cement and water or some other binding component and aluminium powder. The aluminium powder reacts with the cement and forms hydrogen bubbles to form within the mix, thereby increasing the volume-to-weight ratio of the concrete mix. After the mix is cast into the desired form and the volume-increasing chemical reactions occur, the concrete mix, which is still soft, is
The raw materials are batched by weight and delivered to the mixer. Measured amounts of water and expansive agent are added to the mixer and the cementitious slurry is mixed.
Steel moulds are prepared to receive the fresh AAC. If reinforced AAC panels are to be produced, steel reinforcing cages are secured within the moulds. After mixing, the slurry is poured into the moulds. The expansive agent creates small, finely dispersed voids in the fresh mixture, which increases the volume by about 50 percent in the moulds within three hours.
Within a few hours after casting, the initial hydration of cementitious compounds in the AAC gives it sufficient strength to hold its shape and support its own weight. After cutting, the aerated concrete product is transported to a large autoclave, where the curing process is completed. Autoclaving is required to achieve the desired structural properties and dimensional stability. The process takes about eight to 12
hours under a pressure of about 174 psi (12 Bars) and a temperature of about 180oC.
Density: 300 to 1600 kg per cum — this is light enough to float in water
Compressive strength: 300 to 900 psi
Allowable shear stress: 8 to 22 psi
Thermal resistance: 0.8 to 1.25 per in. of thickness
Sound Transmission Class (STC): 40 for 4 in. thickness; 45 for 8 in. thickness
Fire Resistance: Autoclaved aerated concrete provides the highest security against fire and meets the most stringent fire safety requirements. Due to its purely mineral composition, AAC is classified as a non-combustible building material. It is both resistant to fire up to 1200oC and heat resistant.
Structural Performance: Autoclaved aerated concrete is strong and durable despite its lightweight. AAC’s solidity comes from the calcium silicate that encloses its millions of air pores and from the process of curing in a pressurized steam chamber, an autoclave. Its excellent mechanical properties make it the construction material of choice for earthquake zones.
Sound Insulation: AAC has excellent sound insulation properties compared to other building materials with the same weight.
Durability: It retains its properties for the entire life of a building and can resist wind, earthquake, rain (also acid rain), storm and a wide range of external temperatures.
It has been refined into a highly thermally insulating concrete-based material used for both internal and external construction. Besides AAC’s insulating capability, one of its advantages in construction is its quick and easy installation, for the material can be routed, sanded and cut to size on site using standard carbon steel band saws, hand saws and drills.
The choice of the right building material is one of the key factors for sustainable buildings. AAC is a building material which offers considerable advantages over other construction materials. Its high resource efficiency gives low environmental impact in all phases of its life cycle, from the processing of raw materials to the disposal of AAC waste.
Resources: AAC is made from naturally occurring materials that are found in abundance – lime, fine sand, other siliceous materials, water and a small amount of aluminium powder (manufactured from a by-product of aluminium). Furthermore the production of AAC demands relatively small amounts of raw materials per m3 of product, and up to a fifth as much as other construction products.
Environmental impact during production: No raw materials are wasted in the production process and all production off cuts is fed back into the production circuit. The manufacture of AAC requires less energy than for all other masonry products, thereby reducing use of fossil fuels and associated emissions of carbon dioxide (CO2). Industrial-quality water is used and neither water nor steam is released into the environment. No toxic gases are created in the production process.
Environmental impact during use: AAC’s excellent thermal efficiency makes a major contribution to environmental protection by sharply reducing the need for space heating and cooling in buildings.
In addition, AAC’s easy workability allows accurate cutting that minimizes the generation of solid waste during use. The fact that AAC is up to five times lighter than concrete leads to significant reductions in CO2 emissions during transportation.
Reuse, recovery and disposal: Throughout the life cycle of AAC, potential waste is reused or recycled wherever possible to minimize final disposal in landfill. Where AAC waste is sent to landfill, its environmental impact is minor since it contains no toxic substances.
Autoclaved aerated concrete is not without its disadvantages. For example, it is not as strong as less porous varieties of concrete, so it must often be reinforced if it is to be used for intense load-bearing work. Though it can be shipped just about anywhere with relative ease because of its light weight, autoclaved aerated concrete is not widely produced, so it may be difficult for many to obtain it locally. It must also be coated with some form of protective material, as it tends to degrade over time because of its porous nature.
It is a lightweight, precast building material that simultaneously provides structure, insulation, and fire and mould resistance. AAC products include blocks, wall panels, floor and roof panels and lintels.
USE OF ALUMINIUM POWDER:
The reactants in aerated concrete are lime (which is present in cement) and aluminium powder. When the aluminium powder is added to slurry of lime, hydrogen is produced in the form of bubbles. Thick slurry is made with lime/cement along with aggregates. Aluminium powder is added in the final stage of mixing. The mix is poured into moulds. The moulds are autoclaved which imparts strength. AAC is produced using no aggregate larger than sand.
Quartz sand, lime and/or cement and water are used as a binding agent. Aluminium powder is used at a rate of 0.05% – 0.08% by volume of cement.
The hydrogen gas foams and doubles the volume of the raw mix (creating gas bubbles up to 1/8 inch in diameter). At the end of the foaming process, the hydrogen escapes into the atmosphere and is replaced by air. Depending on its density, up to 80% of the volume of an AAC block is air. AAC’s low density also accounts for its low structural compression strength. It can carry loads up to 1,200 Psi, approximately only about 10% of the compressive strength of regular concrete.
AAC material can be coated with a stucco compound or plaster against the elements. Siding materials such as brick or vinyl siding can also be used to cover the outside of AAC materials.
EXPERIMENTS OF AAC:
Since the “autoclave” facility was unavailable at the place where I was working, I did not autoclave my samples and thus was unable to find its actual strength.
The mix design for the first sample was decided based on studies. Then further samples were made by changing some proportions in the previous ones.
Sample no. 1: In this the cement/sand ratio taken is 1:1. Also the w/cm ratio taken is 0.4. Aluminium powder is 0.4 – 0.5% by weight of cement.
Cement (OPC): 1.08 kg
Sand: 1.08 kg
Water: 440 gm
Aluminium powder: 4 – 5 gm
The mixture was hot just after mixing which confirmed the chemical reaction in that. Also hissing sound was coming which confirmed the evolution of gas. Since this is aerated concrete, it should expand. But it did not. The reason was less amount of water since it did not form slurry and there were gaps between the particles through which all the evolved gases escaped out from the concrete. These gases should remain there only so that the concrete expands but it did not happen.
So for the next sample, I increased the w/cm ratio to make the slurry.
Sample no. 2: With w/cm ratio = 0.45
Cement (OPC): 540 gm
Sand: 540 gm
Water: 243 gm
Aluminium powder: 3 gm
This mixture made slurry which was easily flowing. In this just after filling the cube the initial depth of the top surface of the slurry was 11.5 cm. After just 5 minutes, the depth was 10 cm which showed that it expanded as we predicted.
Weight of the sample: 1.14 kg
Volume: 15 x 15 x 5 cm3
Density: 1013.33 kg/m3
Sample no. 3: To reduce density further, I decreased the quantity of sand.
Cement (PPC): 1080 gm
Sand: 940 gm
Water: 490 gm
Aluminium powder: 6 gm
In this sample the initial depth of the top surface of the slurry was 6.8 cm and just after 5 minutes the depth was 3 cm.
Weight of the sample: 2.02 kg
Volume: 15 x 15 x 12 cm3
Density: 748.15 kg/m3
This was floating in water.
Sample no. 4: In this new sample I tried to use Pumice Stone powder and reduced the quantity of sand in the mixture. In this sample I had to use more quantity of water since pumice stone absorbs water.
Cement (PPC): 1080 gm
Sand: 840 gm
Pumice Stone Powder: 120 gm
Water: 660 gm
Aluminium powder: 6 gm
In this sample the initial depth of the top surface of the slurry was 8.6 cm and the final depth was 4.5 cm.
Weight of the sample: 2.04 kg
Volume of the sample: 15 x 15 x 10.5 cm3
Density: 863.49 kg/m3
Sample no. 5: Sample of 2 cubes.
Cement: 1620×2 = 3240 gm
Sand: 1260×2 = 2520 gm
Pumice stone powder: 180×2 = 360 gm
Water: 925×2 = 1850 gm
Aluminium powder: 9×2 = 18 gm
Sample no. 6: Sample of 2 cubes
Cement: 1296×2 = 2592 gm
Sand: 1008×2 = 2016 gm
Pumice stone powder: 144×2 = 288 gm
Water: 740×2 = 1480 gm
Aluminium powder: 7×2 = 14 gm
Here the initial depth of top surface of both the cubes was 6 cm and the final depth was 0 cm.
Weight of each cube: 2.45 kg
Volume of each cube: 15 x 15 x 15 cm3
Density: 725.92 kg/m3
So in all, sample numbers 3, 4, 5 and 6 were the successful ones. They all were floating in water.
LIGHTWEIGHT CONCRETE USING PUMICE STONE:
The word ‘Pumice’ is a general term used for a range of porous materials produced during volcanic eruptions. Pumice stone can be weak and porous or strong and less porous. Its water absorption is as high as 55% since it is a highly porous material. The major reason behind using pumice as an aggregate is its much light weight and comparatively high strength.
Pumice stone: light, spongy, highly porous kind of lava with a vitreous texture. Pumice has high silica & alkali and low calcium & magnesia content. Its spongy cellular texture is a result of the gases escaping from hot lava. It is having low strength and it is a good thermal insulator, sound insulator and fire insulator.
TESTS ON LIGHTWEIGHT CONCRETE USING PUMICE STONE AS A LIGHTWEIGHT AGGREGATE:
For this project, we got pumice stone as big as 50 mm size. So we crushed it to the size less than 20 mm.
The mix design for the first sample was decided based on studies. Then further samples were made by changing some proportions in the previous ones.
Sample no. 1: 1 cube
Cement: 1.18 kg
Sand: 2.63 kg
(> 10 mm): 590 gm
(4.75 – 10 mm): 910 gm
(< 4.75 mm): 155 gm
Water: 1230 gm
Next day when the cube was opened, its weight was 3.94 kg. So its density was 1167.40 kg/m3. It was light as desired but its finish was not good. The surfaces were not smooth. It happened because I did not consider the water absorption by pumice stone and also did not use the admixture.
3 days cube testing:
S.no, Weight of cube(Kg), Load (kN), Strength (MPa).
1, 3.94, 23.1, 1.03.
For calculating water absorption by pumice stone:
Take a sample of pumice stone in a bucket and note down its dry weight. Then fill the bucket with enough water and let it remain as it is for 5-6 hours. Then remove the excess water and note down the wet weight of the stones. The two weights will give us the % water absorption by the pumice stone.
Dry weight: 388 gm
Wet weight: 604 gm
% water absorption: (wet weight – dry weight) * 100 / (dry weight) = 55.67 %
Sample no. 2: 1 cube considering water absorption and using admixture. In this I reduced the quantity of sand to further reduce the density of the concrete and to compensate the effect of reduced fines, used more amount of pumice less than 4.75 mm.
Cement: 1 kg
Sand: 600 gm
(> 10 mm): 600 gm
(4.75 – 10 mm): 430 gm
(< 4.75 mm): 300 gm
Water: 1300 gm
Admixture: 6 gm
The admixture used was ‘Sika Viscocrete 5001’. This made water release from cement particles.
After opening it we found that its finish was not good. Some areas were smooth and some were not. Its reason came out to be the larger particles of pumice stone. So next time I did not use particles bigger than 10 mm.
Sample no. 3: for 3 cubes using aggregates less than 10 mm.
Cement: 3540 gm
Sand: 1800 gm
Pumice (smaller than 10 mm): 4100 gm
Water: 3400 gm
Admixture: 21 gm
These cubes had low density and also smooth surface.
7 days cube testing:
S.no, Weight of cube(Kg), Density(Kg/m3), Load (kN) Strength (MPa)
1, 4.2, 1244.44, 163.0, 7.24
2, 4.4, 1303.70, 148.4, 6.60
Sample no. 4: Sample for 2 cubes.
Cement: 3540 gm
Sand: 2100 gm
(4.75 – 10 mm): 2180 gm
(< 4.75 mm): 1930 gm
Water: 3400 gm
Admixture: 14 gm
Weight of each cube: 4882 gm
Volume of each cube: 15 x 15 x 15 cm3
Density: 1446.51 kg/m3
So in all, sample numbers 3 and 4 were the successful ones. Their finish was good and they were light also.
Based on the above experiments and samples made, following has been concluded:
1) The Aerated concrete is a much lighter concrete and can float on water. It does not contain coarse aggregates. It is composed of cement, sand, high water-cement ratio and aluminium powder. Just as we mix the aluminium powder in the cement-sand slurry, the expansion in the volume can be observed. Within 5 minutes it expands by 30%. It consists of many pores and thus is not structurally strong. It is a good insulator of heat and sound and thus can be used in place of conventional bricks or at the places which does not bear any load.
2) The lightweight concrete manufactured using Pumice stone as a lightweight aggregate is half the denser than the normal concrete. In this the normal coarse aggregates are replaced by pumice stone aggregate having size less than 10 mm. Its surface is flat and smooth and showing a good finish. Although it cannot be used as a structural concrete but its cube test results show considerable strength and can be used as an architectural concrete. It is a good insulator of heat and sound and thus has the same uses as of the above aerated concrete.
1) Ambuja Knowledge Centre Library. Ambuja Cements Ltd.
2) Samuel Green, Nicholas Brooke and Len McSaveney. Pumice aggregate for structural lightweight and internally cured concretes
3) Keertana. B, Sini Sara Mani and M. Thenmozhi. Utilization of ecosand and fly ash in aerated concrete for a richest mix design
4) Hjh Kamsiah Mohd.Ismail, Mohamad Shazli Fathi and Norpadzlihatun bte Manaf. Study of lightweight concrete behavior
5) Handbook on aerated concrete products by PTY Ltd
6) Giuseppe Campione and Lidia La Mendola. Behavior in compression of lightweight fibre reinforced concrete confined with transverse steel reinforcement (2002)
I, Dhawal Desai, 2nd year undergraduate student of Civil Engineering department, Indian Institute of Technology Bombay, gratefully acknowledge the support and lab facility provided by M/s Ambuja Cements Ltd. Mumbai, to carryout this project work on Lightweight Concrete during my internship in Dec, 2011 winter vacation
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