What is Lightweight Concrete?
Most of the lightweight concrete mix is made with lightweight aggregates. Lightweight concretes strengths are generally in the range of 0.3 N/mm2(44 psi) to 40 N/mm2( 5800 psi) and cement content in the range of 13 lb/ft3 (200 kg/m3). The density of aggregate plays a vital role in the strength of lightweight concrete. Lightweight concrete is a special concrete whose density varies from 19 lb/ft3 (300 Kg/m3) to 115 lb/ft3 (1850 kg/m3). Structural lightweight concrete self-weight is comparatively lighter than conventional concrete also it has enough strength for structural propose.
From the thermal conductivity point of view, lightweight concrete is a supreme material.
For the aggressive climate condition where air-conditioning to be installed, then it is necessary of thermal comfort. It is achieved by using lightweight concrete it is also low power consumption for it.
The production of Lightweight concrete produces less industrial waste such as unused clinker, fly ash, slag, etc., therefore disposal cost is also low.
Methods making Concrete Light:
In general, practice making concrete light is achieved by inclusion of air in concrete. This is achieved in the following ways:
- We can use cellular porous or lightweight aggregate instead of usual mineral aggregates.
- By aerating concrete with gas or air bubbles in mineral makes aerated concrete.
- Concrete will be light weighted if the sand fraction is omitted. This is known as ‘no-fines’ concrete.
Lightweight concrete is increasing popularity as a structural element nowadays. Structural lightweight concrete self-weight is comparatively lighter than conventional concrete also it has enough strength for structural propose.
Classification of lightweight concrete:
According to use and application L.W.C. is classified as structural lightweight concrete (ASTM C 330-82a), concrete for Masonry Unit (ASTM C 331-81), insulating concrete (ASTM C 332-83).
According to ASTM structural lightweight concretes compressive strength should be greater than 2500 psi (17 MPa).
Lightweight concrete based on the method of production is classified as below:
- Lightweight aggregate concrete,
- Aerated concrete,
- No-fines concrete.
Out of this lightweight concrete and aerated concrete is more uses than no-fines concrete.
The aerated concrete is normally used for insulating purposes but, sometimes also used for structural purposes in conjunction with steel reinforcement. For the development of lightweight concrete following are the industrial lightweight aggregates with a various quality are Leca (expanded clay), Aglite (expanded shale), Lytag (Sintered pulverized fuel ash), Hydite (expanded shale).
Before going to details in LWC, here we discuss Lightweight aggregates.
Following tables show the group of lightweight concrete:
Table. 1. Different Category of Lightweight Concrete:
|No-fines concrete||Light weight aggregate concrete||Aerated Concrete|
|Chemical aerating||Foaming Mixture|
|Gravel||Clinker||Aluminum powder method||Performed foam|
|Crushed Stone||Foamed Slag||Hydrogen peroxide and bleaching powder method||Air-entrained foam|
|Coarse Clinker||Expanded clay||Performed foam and fly ash.|
|Sintered Pulverized fuel ash||Expanded Shale|
|Expanded Slate||Sintered Pulverized fuel ash|
|Foamed Slag||Exfoliated vermiculite|
Lightweight concrete is divided into groups: natural lightweight aggregate and artificial lightweight aggregate.
Natural lightweight aggregate is found in all places in different quality. All of these are not used for lightweight concrete. This pumice is most using. The followings are some lightweight aggregates that is suitable for structural and commercial LWC.
The acceptable property of pumice is it’s enough lightness and enough strangeness. As this rock is origin from a volcanic explosion, its lightness is due to explosion of gas from the hot molten lava at the time explosion from beneath the earth’s crest.
Its colour is light or nearly white with the texture of a meter connected cell. Pumice is using from older age even used in Roman structures. The physical proportion of pumice is: Bulk density of 30 lb/ft3 to 50 lb/ft3 (500 Kg/m3-800 kg/m3), dry density of concrete by 75 lb/ft3 tp 280lb/ft3 (1200Kg/m3-4500 kg/m3).
Diatomite is formed by remains of a microscopic aquatic plant called diatoms. It is hydrated amorphous silica. At the end of aquatic plants, there are deposited beneath the deep ocean bed. Subsequently where ocean bed is raised in a large period of time and diatomaceous earth becomes available on land. The average weight of pure diatomite is 450 kg/m3. Artificial lightweight aggregate can also be sintered in a rotary kiln by Diatomite.
Scoria is slightly weaker than pumice. It is a dark colour volcanic origin lightweight aggregate.
Sawdust is manufacture from softwood. By adding lime to the mix about 1/3 to ½ volume of cement with sawdust will counteract this. This is only for sawdust from softwood, but when sawdust is made from hardwood then, like boiling water and ferrous sulphate solutions were used to remove the effect of tannins. In the sawdust mix, the practical ratio cement to sawdust is 1:2 to 1:3. Uses of sawdust: nowadays sawdust concrete is using in the manufacture of precast concrete products, jointless flooring and roofing tiles, concrete blocks for holding the nail well.
For manufacture of the precast block, wood shavings are mixed with Portland cement or Gypsum for the product of wood wool concrete. This product is used for wall panels for acoustic purposes.
Low weight concrete for special purposes can be manufactured by using the rice husk, groundnut husk, and bagasse.
Table. 2. Classification of Natural light weight aggregate and artificial light weight aggregate
|Natural light weight aggregate||artificial light weight aggregate|
|Volcanic Cinders||Bloated clay|
|Sawdust||Expanded shale and slate|
|Rice Husk||Sintered fly ash|
|Thermo Cole beads|
In the place of non-availability of natural aggregates or very costly available are Brickbats are used. The concrete made by brick bat aggregate is not exactly the light weight aggregate concrete, but its weight is slightly less than normal-weight concrete. Brickbat aggregate are made with slightly over burnt bricks. Sometimes, for the manufacture of heat resistance concrete, brick bat aggregate is used in conjunction with high alumina cement.
Cinder, clinker, and Breeze:
The particles obtained from the combustion of coal or partly fused or sintered are Clider, clinker, and Breeze. The main property of cinder is that it has high drying shrinkage and moisture movement.
The uses of cinder are:
- For building blocks for partition walls,
- Making screeding over flat roofs and plastering.
The presence of excessive unburnt coal particles makes clinker or cinder aggregates unsoundness. Actually, the unsoundness of concrete made with such aggregate is due to coals’ expansion on wetting and contraction on drying.
Foamed slag is such type of light weight aggregate which is a byproduct of quenching blast furnace slag as produce by the manufacturing of pig iron. The foam slag should have the following requirement:
- The heavy impurities should be removed from it.
- Volatile impurities such as coke or coal should not be contaminated in it.
- Sulphate should be removed from it.
Foamed slag is manufactured by iron and steel industries.
Uses of Foamed Slag:
- It is used in the manufacture of ready-made building blocks and partition walls panel.
- Foamed slag is used in the production of the small structural members and precast lightweight concrete by controlling density.
- Bloated clay:
It is a cellular structure formed by the cooling of certain material like glass or Shales, which is heated to the point of incipient fusion temperature. The industrial product of some of the bloated clay names are: “Hydrite”, “Rocklite”, “Gravelite”, “Leca”, “Agilite”, “Kermizite”.
Sintered Fly Ash (Pulverized Fuel Ash)
Sintered fly ash is nowadays vast used structural light-weight aggregates. Its trade name is “Lytag”. This material has a very high strength to density ratio and has low dry shrinkage. Fly ash is a residue from the combustion of powdered coal. Fly ash is mixed with a calculated amount of water to make pillets and then sintered at a temperature of 1000 ⁰C to 1200 ⁰C. This process is similar to the manufacture of Portland cement.
The raw vermiculite is a laminar saturated micaceous mineral. The concrete made with this aggregate has a very low density and low strength.
The use of vermiculite in concrete is for the following purpose: insulating purpose, manufacture of blocks are used for in-situ roof and floor screeds, slabs, and tiles for sound insulation, and for heat insulation. This product can be easily cut, sawn, nailed, or screwed. The encasing pipes carrying steam or hot water pipes can be made by using hollow concrete blocks of vermiculite.
Expanded perlite is a light cellular material having a density of 30 to 240 kg/m3. It is a type of natural volcanic glass-like pumice and is crushed and heated to the fusion temperature of 900 to 1100 ⁰C to make the desired product. This material is crushed in various shapes and used in light weight concrete. It is also used for insulation grade concrete.
Followings are the brief About three types of Light weight Concrete:
1. Lightweight aggregate concrete:
Most of the lightweight concrete is made with light weight aggregates. Lightweight concretes strengths is generally in the range of 44 psi (0.3 N/mm2) to 5800 psi ( 40 N/mm2 ) and cement content in the range of (13 lb/ft3) 200 kg/m3. The density of aggregate plays a vital role in the strength of lightweight concrete. Also, the porosity of aggregate, grading of aggregate, the water-cement ratio, degree of compaction carry the strength of concrete.
The workability of lightweight aggregate concrete can be improved by adding an excess of fine materials, pozzolanic material, or by mixing other plasticizer admixtures.
Sometimes natural sand is also used instead of crushed sand, to improve workability and reducing water requirement.
The normal mix design is as difficult to use me the lightweight aggregate design because it has a high and rapid absorption property. But, by using waterproof coating like bitumen coating improve its property.
The reinforcement in the reinforced concrete by light weight aggregate is coated with antiabsorptive component or the concrete should be plastered at the surface by normal mortar to reduce the penetration of moisture and air because light weight concrete is relatively porous.
Structural lightweight concrete:
Nowadays, structural lightweight concrete is a demanded material for construction, because a sufficient strength lightweight concrete used in conjunction with steel reinforcement is more economical than conventional concrete. A structural light weight concrete has strength in the range of: 28 days compressive strength greater than 17 MPa, and 28 days unit weight (air dry) less than 1850 Kg/m3. This concrete is made with completely lightweight aggregate or in combination with lightweight aggregate with normal-weight aggregates. In common practice, normal sand as fine grade and lightweight coarse grade aggregate of less than 19 mm are used to make a concrete called “Sanded lightweight concrete”.
Density of lightweight concrete Mix:
Lightweight concrete mix design is generally doe by trial mixes. Because of the high value of absorption, varying specific gravity and moisture content varying in light weight aggregate. So, the mix design method follows in genera weight concrete mix are difficult to use in lightweight concrete mix.
The variation in water absorption is the main problem for design of mix proportion.
This type of aggregate sometimes becomes saturated before mixing then the water used for mixing becomes unused water. The use of aggregate with high absorption is difficult to make the workable and yet cohesive mix, also its resistance to frost is lower.
Mixing procedure of light weight concrete is different for different types of aggregates. In normal practice, aggregate is mixed with about 2/3 of the mixing water for time up to one minute after the addition of cement that is a balance structural lightweight concrete mix.
The process is done continuously up to required homogeneity, normally up to 2 or more minutes are required to get it. To minimize the degradation in insulating concrete, aggregates are added at the end.
2. Aerated Concrete:
An external agent like air or gas is inserted into slurry composed of Portland cement or lime are used to produce aerated Concrete. And then this mix is crushed with siliceous filler to get a uniform cellular structure after setting and hardening.
Another names of aerated concrete gas concrete, foam concrete or cellular concrete. Generally available aerated concrete in the market is Siporex.
- Followings are the process of manufacture of aerated concrete:
- By using certain chemical reaction gas is mixed in mass in the liquid or plastic state.
- Concrete slurry is mixed with a stable foam to make concrete aerated.
Slurry is mixed with powdered metal (e.g. Aluminium powder) that produce a huge amount of hydrogen gas during hydration. This hydrogen makes cellular structure. This process is used for the production of a large amount of aerated concrete in the factory.
In other method, cement fly ash or crushed sand slurry is mixed with foam that makes a cellular structure.
The foam concreting method is only used for a small amount of reduction or for in- situ work, where the tolerance for a small change in dimension can be accepted. But, we can make any density desire by this method.
Properties and Uses of Aerated Concrete:
- Aerated concrete has a low density and high thermal insulation.
- Its density is in the range of 300 Kg/m3 to 800 Kg/m3.
- For insulation purposes lower density grade is used.
- For, manufacture of building blocks or load-bearing walls medium density grade are used, these members are used as structural members in conjunction with steel reinforcement.
3. No-Fines Concrete:
The third method of making of lightweight concrete is to escape the fines aggregates fractions from conventional concrete. The main ingredients of non-fine concrete are coarse aggregates, cement, and water. In this process, a single sized aggregate passing through 20 mm and retained on 10 mm sizes are used.
Mix design of No-Fines Concrete:
The aggregates used in this concrete is basically passing through and retained on 10 mm and is mixed with the aggregate/cement ratio of 6:1 to 10:1. The strength controlling parameters in no-fines concrete are water-cement ratio, aggregate cement ratio, and density of concrete. Fig. below shows the relationship among these parameters.
The water-cement ratio for this concrete is adopted by our satisfactory need for consistency and is in the range of 0.38 to 0.52. A low water-cement ratio leads a non-adhesiveness among the particles.
If the water-cement ratio is greater than 0.52, at that time vibration the concrete slurry falls on the bottom and bottom portion voids are completely filled between aggregates and generate a high dense layer in the bottom.
In the practical field, an experienced visual examination and method of trial and error are generally adopted to judge the water-cement ratio.
The density of no-fines concrete is 360 Kg/m3 with lightweight aggregates but, 1600 to 1900 kg/m3 by using conventional aggregates.
For compaction during concrete casting simply rodding method gives the best result but, mechanical or vibration methods are not used.
Above simple compaction methods not act much side thrust on formworks. No-fines concrete 28 days compressive strength is in the range of 1.4 MPa to about 14 MPa.
It is better to use the deformed bar and apply a cement paste on the reinforced surface because in no-fines concrete bond strength is very low. In no-fines concrete, aggregates and aggregates bonds are bonded with a very thin layer of paste, so its drying shrinkage is low. The place where natural sand is not available, no-fines concrete is one of the best material to use.
Uses of No-Fines Concrete:
Followings are the uses of No-Fines Concrete:
- For single-story and multistory buildings cast external walls no-fines concrete are used for commercial production.
- For temporary construction elements it can be used because of low cost.
- No-fines concrete is used for aesthetic building parts.
- This concrete is used for external walls for thermal insulation.
Advantages of Lightweight Concrete:
- It reduces the dead load.
- Due to less weight it ingresses the progress of the building, and lower haulage and handling cost. In the case of weak soil and tall structure, the weight of the foundation is the main factor for safe design.
- Lightweight concrete gives less weight of wall and floors that places less weight on beam and coulomb in framed structure, this is an economical design.
- It reduces the dead load that is comfortable for work progress, hence reduce the haulage and handling cost.
- For weak soil and aggressive foundation condition the reduced weight of foundation leads to safe design.
- The construction of floor and walls by lightweight concrete is economical construction.
- It has low thermal conductivity. That makes low power consumption for air-conditioning and also a thermal comfort to us.
- The industrial waste during production is very less and easy for disposal.
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