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Light Weight Concrete (LWC) - An overview

The use of Light Weight Concrete (LWC) can be traced to some of the historical structures including the building of ‘The Pantheon’ which is still standing in Rome since last 18 centuries. The concept of Light Weight Concrete (LWC) was first introduced by Romans in the second century and The Pantheon was constructed using Pumice, the most common type of aggregate used during that period. Subsequently, the use of LWC has spread across other countries such as USA, UK and Sweden.

Volcanic Pumice Stone	Expanded Aggregate

Concrete that is made with conventional hard rock natural aggregates has a narrow range of density because the specific gravity of most rocks varies a little. In practice, Structural LWC has a density (unit weight) in the order of 1440 to 1840 kg/cum compared to normal weight concrete a density in the range of 2200 to 2600 kg/cum. For structural applications the concrete strength should be greater than 17.0 MPa. Use of LWC with lower density permits construction on ground with a low load-bearing capacity and results in significant benefits in terms of load-bearing elements of smaller cross section and a corresponding reduction in the size of foundations. Furthermore, with lighter concrete, the formwork need withstand a lower pressure than would be the case with normal weight concrete, and also the total mass of materials to be handled is reduced with a consequent increase in productivity.

Classification of LWC

The reduction in density of a normal weight concrete can be achieved by replacing some of the solid materials in the mix by air voids. As per Neville, there are three possible locations of the air; in the aggregate particles, which are known as light weight aggregate; in the cement paste, the resulting concrete being known as Cellular Concrete; and between the coarse aggregate particles, the fine aggregate being omitted. Concrete made in this manner is known as No-Fines Concrete. Similarly, concrete made of light weight aggregates is known as Light Weight Aggregate Concrete. Thus, we have a classification based on method of production. Typical properties of common light weight concretes are shown in Table 1.

• Light Weight Aggregate Concrete-by using porous lightweight aggregate of low apparent specific gravity, i.e. lower than 2.6. This type of concrete is known as lightweight aggregate concrete.
• Aerated or Cellular or Foamed  Concrete- by introducing large voids within the concrete or mortar mass; these voids should be clearly distinguished from the extremely fine voids produced by air entrainment. This types of concrete is variously knows as aerated, cellular, foamed or gas concrete.
• No-Fines Concrete- By omitting the fine aggregate from the mix so that a large number of interstitial voids are present; normal weight coarse aggregate is generally used. This concrete as no-fines concrete.

 

No-fines concrete used to backfill

LWC can also be classified based on the density and the purpose for which it is used and applied. It can distinguish between structural lightweight concrete (ASTM C 330-82a), concrete used in masonry units (ASTM C 331-81), and insulating concrete (ASTM C 332-83). Based on density, LWC can be classified as:

• Structural Light Weight Concrete-having a density range between 1350 and 1900 kg/cum and as the name implies, used for structural purpose. It has a minimum compressive strength of 17 MPa.
• Low density concrete-having a density range between 300 and 800 kg/cum. This concrete is not used for structural purposes.
• Moderate Strength Concrete-actually falls between the above two categories with a density range 800 and 1350 kg/cum with a compressive strength range of 7-17 MPa.

Types of LWC

 Light weight concrete can be produced using one of the following three methods: 

 

• By injecting air/gases in its composition using an appropriate equipment or admixture. In this case it is known as – Aerated Concrete
• By omitting the finer aggregates. In this case it is known as No-Fines Concrete and/or
• By replacing normal aggregates with highly porous natural or manufactured aggregates, known as Light Weight Aggregate Concrete.

Aerated Concrete

Aerated concrete is a lightweight, cellular material consisting of cement and/or lime and sand or other silicious material. It is made by either a physical or a chemical process during which either air or gas is introduced into a slurry, which generally contains no coarse material. Aerated concrete used as a structural material is usually high-pressure steam-cured. It is thus factory-made and available to the user in precast units only, for floors, walls and roofs. Blocks for laying in mortar or glue are manufactured without any reinforcement. Larger units are reinforced with steel bars to resist damage through transport, handling and superimposed loads. Autoclaved aerated concrete, which was originally developed in Sweden in 1929, is now manufactured all over the world.

Concrete of this type has the lowest density, thermal conductivity and strength. Like timber it can be sawn, screwed and nailed, but there are non-combustible. For works insitu the usual methods of aeration are by mixing in stabilized foam or by whipping air in with the aid of an air entraining agent. The precast products are usually made by the addition of about 0.2 percent aluminums powder to the mix which reacts with alkaline substances in the binder forming hydrogen bubbles. Air-cured aerated concrete is used where little strength is required e.g. roof screeds and pipe lagging. Full strength development depends upon the reaction of lime with the siliceous aggregates, and for the equal densities the strength of high pressure steam cured concrete is about twice that of air-cured concrete, and shrinkage is only one third or less.

No-Fines Concrete (NFC)

The term no-fines concrete generally means concrete composed of cement and a coarse (9-19mm) aggregate only (at least 95 percent should pass the 20mm BS sieve, not more than 10 percent should pass the 10mm BS sieve and nothing should pass the 5mm BS sieve), and the product so formed has many uniformly distributed voids throughout its mass. No-fines concrete is mainly used for load bearing, cast in situ external and internal wall, non load bearing wall and under floor filling for solid ground floors (CP III: 1970, BSI). No-fines concrete was introduced into the UK in 1923, when 50 houses were built in Edinburgh, followed a few years later by 800 in Liverpool, Manchester and London.

This description is applied to concrete which contain only a single size 10mm to 20mm coarse aggregate (either a dense aggregate or a light weight aggregate such as sintered PFA). The density is about two-third or three quarters that of dense concrete made with the same aggregates. No-fines concrete is almost always cast in situ mainly as load bearing and non load bearing walls including in filling walls, in framed structures, but sometimes as filling below solids ground floors and for roof screeds.

No-fines concrete is thus an agglomeration of coarse aggregate particles, each surrounded by a coating of cement paste up to about 1.3 mm thick. There exist, therefore, large pores within the body of the concrete which are responsible for its low strength, but their large size means that no capillary movement of water can take place. Although the strength of no-fines concrete is considerably lower than that of normal-weight concrete, this strength, coupled with the lower dead load of the structure, is sufficient in buildings up to about 20 storeys high and in many other applications.

Because of the large size of the pores, No-Fine Concrete is not subjected to capillary suction. In consequence, NFC is highly resistant to frost provided the pores are not saturated; if saturated, freezing would cause a rapid disintegration. High absorption of water makes NFC unsuitable for foundations and in situations where it may become saturated with water. A beneficial effect of the large pores in NFC is that it allows easy drainage under appropriate circumstances. Hence, NFC is most suitable in pavements around trees and domestic car parks.

Light Weight Aggregate Concrete (LWAC)

Loxo AAC lightweight concrete panels

Lightweight-aggregate concrete has a substantially lower bulk density than that of concrete made with gravel or crushed stone. This lower bulk density results from using lightweight aggregates, either natural or manufactured. Many types of aggregates are classified as lightweight, and are used to produce concretes with a wide range of densities and strengths. LWAC, based on application, is further categorized into two types:

• The partially compacted LWAC, mainly used for precast blocks, panels, cast in situ roofs, screeds etc.
• The Structural LWAC, fully compacted and reinforced similar to its normal weight conventional concrete.

Porous light weight aggregates of low specific gravity are used in producing this type of concrete in place of conventional aggregates. Lightweight aggregates can originate from natural resources, or they can be manmade. The major natural resource is the volcanic material, and manmade or synthetic aggregates are produced by a thermal process in factories. Synthetic aggregates are produced by thermal treatment of the materials, which have expansive properties. These materials can be divided in three groups—natural materials, such as perlite, vermiculite, clay, shale, and slate; industrial products, such as glass; and industrial by-products, such as fly ash, expanded slag cinder, and bed ash. The most common types of lightweight aggregates produced from expansive clays are known as Leca and Liapor and those made from fly ash are known as Lytag. The bulk density of the aggregates varies greatly depending upon the raw materials and the process used for their manufacture.

Structural lightweight concretes have densities ranging from 1360 to 1920 kg/m3 and minimum compressive strengths of 17.0 MPa. Their insulating efficiency is lower than that of low-density concretes, but substantially higher than that of normal weight concretes. The most common aggregates used in this type of concrete are expanded slags; sintering-grate expanded shale, clay, or fly ash; and rotary-kiln expanded shale, clay, or slate.

Sintering can produce either crushed or pelletized aggregates. Crushed aggregates are produced using raw materials that either contains organic matter that can serve as fuel, or have been mixed with fuel such as finely ground coal or coke. The raw materials are pre moistened and burned so that gases are formed causing expansion. The resulting clinker is then cooled, crushed, and screened to the required gradation. The finished product tends to be generally sharp and angular with a porous surface texture. Pelletized aggregates are produced by mixing clay, pulverized shale, or fly ash with water and fuel; pelletizing or extruding that mixture; and then burning it. The resultant aggregate particles are generally spherical or cylindrical.

In the rotary kiln process, raw material such as shale, clay, or slate is introduced in a continuous stream at the upper end of a rotary kiln. As the material slowly moves toward the burner at the lower end, the heat, slope, and slow rotation of the kiln cause the material to soften and to trap internally forming gases into an internal cellular structure. In one variation of this process, the expanded (bloated) material is discharged, cooled, and then crushed and screened to the required aggregate gradations. The resultant particles tend to be cubical or angular in shape and have a porous surface texture. Alternatively, before being introduced into the kiln, the raw material is pre sized by crushing and screening, or by pelletizing. The individual particles then bloat without sticking together. They tend to have a smooth shell over a cellular interior. These two variants can be combined to produce coarse aggregate consisting mostly of uncrushed particles, obtained by screening, and fine particles obtained by crushing the fired product.

Listed below are several types of LWA suitable for structural reinforced concrete:

• Pumice – is used for reinforced concrete roof slab, mainly for industrial roofs in Germany.
• Foamed Slag – was the first LWA suitable for reinforced concrete that was produced in large quantity in the UK.
• Expanded Clays and Shales – capable of achieving sufficiently high strength for prestressed concrete. Well established under the trade names of Aglite and Leca (UK), Haydite, Rocklite, Gravelite and Aglite (USA).
• Sintered Pulverised – fuel ash aggregate – is being used in the UK for a variety of structural purposes and is being marketed under the trade name Lytag.

Water in Lightweight-Concrete

Lightweight concrete is often implicated in moisture-related failures because it often has a significantly higher water content than normal-weight concrete. Unlike natural aggregates, which tend to become saturated with water only on their surfaces, lightweight aggregate pore networks absorb and store water within the aggregate particles, releasing it gradually over time. To understand how water content affects concrete, we need to consider how the water reacts in the mix.

ACI 304.2, Placing Concrete by Pumping Methods, considers two types of water in lightweight concrete namely; Free water and Absorbed water. Free water influences the volume of the mix, the slump and workability of the mix, and the amount of water available for cement hydration reaction. Absorbed water is held in the pores of the lightweight aggregate. During mixing, some free water is converted to absorbed water, reducing the slump and the amount of water available for hydration. In addition, the pumping pressure drives additional free water into the porous lightweight aggregate, further reducing slump between the pump hopper and the point of discharge. To reduce the amount of mixing water absorbed by the lightweight aggregate, concrete suppliers pre-saturate the lightweight aggregates to fill the pore spaces prior to mixing. Concrete suppliers frequently use water-reducing admixtures to help reduce the total amount of mix water and, consequently, the amount of water that will potentially leave the slab over time.

In both normal-weight concrete and lightweight concrete, water that is not consumed in the hydration of the cement particles slowly evaporates through the exposed surfaces of the concrete. Almost all concrete mixes contain more water than necessary for the cement hydration reaction, but the excess water facilitates placement and finishing. After the cement paste has hardened, the hydration reaction continues, albeit at a slower pace, throughout the life of the concrete as the excess water evaporates. In lightweight aggregate, some absorbed pore water will be drawn out and contribute to more complete hydration of the cement in a layer around the aggregates, but there will still be significant amounts of absorbed water remaining in the pores which, will escape over time.

With the increasingly fast pace of construction, reducing the drying time -- the time between the end of curing and when floor finishes can be installed -- is often critical to the schedule. For example, Elevated slabs on metal decks are susceptible to longer drying times because the water can only escape through the top surface of the slab. ACI 302.2R-96 reports that "there is no reason to include or exclude any concrete materials with the exception of the addition of silica fume [in place of some Portland cement] in an attempt to reduce the needed drying time for a given water-to-cement ratio." The report notes that replacing 5% to 10% of the Portland cement with silica fume can decrease concrete drying time by several weeks. However, this is often not enough time savings for fast-track construction.

 

Table: Typical Properties of Common Light Weight Concrete
	Type of Concrete	Bulk density of Aggregates (Kg/cum)	Dry Density of Concrete ( Kg/cum)	Compressive Strength @ 28 days (Mpa)	Thermal Conductivity (Jm/m2 deg C)
1	Expanded Slag
	Fine	900	1850	21	0.69
	Coarse	650	2100	41	0.76
2	Rotary-Kiln Expanded Clay
	Fine	700	1200	17	0.38
	Coarse	400	1300	20	0.4
3	Rotary-Kiln Expanded Clay with natural Sand
	Coarse	400	1500	20	0.57
			1600	35	_
			1750	50	_
		1900 *	70#	_
4	Sinter-Strand Expanded Clay
	Fine	1050
	Coarse	650	1500	25	0.55
			1600	30	0.61
5	Rotary-Kiln Expanded Slate
	Fine	950	1700	28	0.61
	Coarse	700	1750	35	0.69
6	Sintered Flyash
	Fine	1050	1500	25	_
	Coarse	800	1540	30	_
7	Pumice	500-800	1200	15	_
			1250	20	0.14
			1450	30	_
8	Perlite	40-200	400-500	1.2-3.0	0.05
9	Vermiculite	60-200	300-700	0.3-3.0	0.1
10	Autoclaved Aerated		800	4	0.25
11	Cellular
	Fly Ash	950	750	3	0.19
	Sand	1600	900	6	0.22

Advantages of LWC

• Decreased Dead Load- Less mass is required to support additional weight, Structural reinforcement can be less demanding.
• Higher Seismic (Earthquake) Resistance- In lower densities concrete can actually absorb shock. LWC is often used in ballistic tests because of this ability. Hammer blows can be absorbed without fracturing the concrete.
• More Sound Absorption- The transmission of sound is inversely related to the number of air/solid interfaces. LWC has a high number of these interfaces, thus more sound is absorbed.
• Greater Insulation- Enhanced R-values, especially in the lower density range. Again, this is due to the increased number of air/solid interfaces.
• Increased Fire Resistance- Greatly improved due to lower thermal conductivity. Spalling (scaling or flake chipping from heat) is reduced or eliminated.
• Adaptability- Lighter weight increases options for onsite casting. Forming can be swifter and easier due to less supported weight.
• Simplicity-Ordinary tools can be used for alterations. It can be easily sawn and sculpted, and nailed or screwed without pre-drilling.
• Handling capabilities are vastly improved- Concrete does not need to be cold, damp, dense, and hard to work with.
• Pumping- Fewer failures at the pump from balling or compacting. Air bubbles from the foam act as miniature ball bearings in the mix. There is no settling out problems.
• Increased yield- Having a greater yield/ton equates to reduced fuel consumption and lower transit costs for the producer. Adding foam up to 10% volume (and subtracting aggregate and water) will not significantly reduce concrete strength - in some cases it may improve.
• No Surface Bleed water-Reduction of excess water in the mix allows for sooner finishing. Flowability is achieved through the foam. Water can be utilized strictly for cement hydration.
• Placement-Less handling weight makes all aspects of moving concrete easier, resulting in reduced labor costs and quicker turnover.
• Aesthetic improvements have a wide variety of implications as under:
  • Economic-The design of economic concrete structures is accomplished by increasing the strength/weight ratio. Structures can have much higher thermal efficiency resulting in lower heating or air conditioning costs.
  • Architectural- The beauty of concrete is its ability to adapt to any shape, whether angular or curvilinear. If desired, it can be cast in forms to make it look like wood.
  • Environmental-The problem of deforestation can be reduced by relinquishing the demand for timber and substituting LWC in residential construction. Lightweight concrete in conjunction with tasteful design can be the solution for this environmental dilemma. Deforestation is also a major contributor to global warming because vegetation removes a large quantity of carbon dioxide from the atmosphere.
  • LWC Applications-Screeds and thickening for general purposes especially when such screeds or thickening and weight to floors roofs and other structural members.
  • Screeds and walls where timber has to be attached by nailing.
  • Casting structural steel to protect it against fire and corrosion or as a covering for architectural purposes.
  • Heat insulation on roofs and screed
  • Insulating water pipes.
  • Construction of partition walls and panel walls in frame structures.
  • Fixing bricks to receive nails from joinery, principally in domestic or domestic type construction.
  • Elements with insulation properties-Thermal and sound.
  • Surface rendered for external walls of small houses.
  • It is also being used for reinforced concrete.
  • Pre-cast Exterior Walls.
  • Void Filling, Tank Fills, Mine fills, Tunnel fills.
  • Cast Insitu Applications.
  • Roadwork and Highway Substrates on Floating Soils.
  • Mine Shaft, Abandoned Pipe, and Tunnel Closures.
  • Underground Utilities Embedment (Easily Removed).
  • Floating Docks and Piers.
  • Lightweight Embankment Material, Replacing Soil.

Conclusion

With the advancement of technology and manufacturing abilities, LWC has expanded its uses. Researches in respect of High Performance Cellular Concrete (HPCC) have shown to reach strengths up to 55 MPa. Higher strengths can be produced with the addition of supplementary cementitious materials. Density and strengths can be controlled to meet specific structural and nonstructural design requirements. There are several benefits for the use of lightweight concrete on roof applications. When provided with insulation, a thermal    R-value of R-30 can be easily achieved without insulation delamination, warping or attachment concerns. The lightweight concrete also provides a sound substrate for membrane application, and it can be formed to achieve proper slope without adding costly and complicated tapered insulation. It also provides a higher fire rating for the system.

Acknowledgements

This paper being is not based on any original research. The author has compiled information from various source material and industry technical literature besides his own experience during his long term association with the construction industry. The material compiled from various references is duly acknowledged.

References

1. Properties of Concrete by A M Neville
2. Advanced Concrete Technology by A K Jain, N V Nayak
3. Presentation on Fly Ash based Lightweight Aggregate (LWA)-Jindal Steel and Power
4. Structural Light weight aggregate and Concrete by jhon Ries
5. High Performance Cellular Concrete by LLC
6. Aggregates for Concrete-Developed by ACI Committee

BOX ITEM

• Today, lightweight aggregates are produced in a very wide range of densities varying from 50kg/Cum from expanded Perlite to 1000kg/Cum for clinkers. It is possible to make LWAC of 80Mpa compressive strength.
• Nearly all LWACs are fire resistant. In addition, depending upon the densities and strength, the concrete can be easily cut, nailed, drilled, and chased with ordinary wood working tools.
• Lightweight concrete is expensive, but the cost is calculated not just on the basis of aggregates or LWAC.
• The bond between the aggregate and the matrix is stronger in LWAC. Cement paste penetrate aggregate pores.
• LWC is now available from No-Fines Concrete of low density, mainly for block production, to structural concrete with densities from 1000 to 2000 kg/cum and compressive strength up to 80MPa.
• There have been difficulties pumping LWAC because the pump pressures forces water into the porous aggregates particles resulting in an increased stiffness in the concrete which blocks the pipes. So knowing the absorbency of the LWA before production is critical to the process.
• There are two types of pores in the LWA: open and closed pores. Open pores are the pores that are interconnected and take part in the permeation, whereas the closed pores are sealed and not interconnected.
• SEM (Scanning Electron Microscope) photographs of concrete from some mature bridge decks have shown that the LWAs were extremely well bonded to the cement paste matrix.
• Disadvantages of LWC include-Reduced resistance to locally concentrated loads, more brittle when compared to conventional concrete, requires additional care in Mix Design, special measures for pumping required.
• LWC is Eco?friendly as compared to normal concrete. LWA are factory manufactured under controlled conditions whereas Stone Crushers operation? which emits lot of dust and sound pollutions are used in Normal weight concrete.