Concrete

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Introduction

Lightweight concretes can either be Lightweight Aggregate concrete, Foamed concrete, Autoclaved Aerated concrete (AAC) or No Fines concrete. Lightweight concretes are not new materials, they have been known since the early days of the Roman Empire and they were used to construct parts of the Colosseum so this shows that the characteristics of such a concrete were realised along time ago.

Lightweight Aggregate Concrete

Different types of lightweight aggregates are used for structural concrete including natural or artificial aggregates, with the most frequently used aggregates being expanded clay, It is produced by crushing vesicular volcanic rock and sedimentary rocks, some lightweight aggregate concretes are obtained by crushing ash, and a metallurgical slag. However most manufactures obtain lightweight aggregate concrete from expanded clay, sintered fly ash and expanded slag.

Before manufacturing a lightweight aggregate concrete, in a structure, its purpose must be known. The concrete could be used as a thermal insulator rather than a structural application. The reason that lightweight aggregate concrete is suitable for use as a form of thermal insulation is because it can be produced to have a low density, the concrete can also be used for its structural strength however a higher density aggregate would be required for this purpose. If a concrete uses fine aggregates from natural sources, the particular density of that coarse aggregate in the compacted concrete needs to be in the region of between 650kg/m3 to 1850 kg/m3 depending on the use of the concrete. It is the coarse aggregate that determines the density of concrete.

The advantages of using lightweight aggregate concrete

There are many benefits from using lightweight aggregates. Firstly the density can be reduced; this is an advantage because lightweight concrete can be made from a lower density which will therefore make the concrete lighter by up to 50%.

This reduction in density would not effect the strength of the concrete, if a lightweight aggregate concrete was used on a structure it will have a huge significant effect on the load on the foundation which is obviously an advantage because the foundation has a much lighter load by up to 50% with no loss of strength, this reduces initial cost when building a structure because the load wouldn’t be needing as much reinforcement to support a much heavier building, not only can money be saved from reinforcement but also by transporting cost and handling cost too, which all leads to reducing time of building a structure.

http://www.litebuilt.com/lightweight-concrete-handbook.html#D-1

Another advantage lightweight aggregate has is due to the shape of the aggregates being in a form of a sphere, this allows the concrete to settle in the shortest time, as more of the aggregates surface area is reacting with all the other elements involved. This reduces the time necessary to build a structure which is a key factor in construction as it reduces a company’s chance of facing late penalties.

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As the weather in the UK can reach freezing temperatures it is vital for concrete to have good properties that can cope with these extreme conditions, as quite often company’s face maintenance fees due to freeze thaw weathering occurring on concrete. However with lightweight aggregate concrete has a high void ratio which is around 40% therefore it gives lightweight aggregates good freeze thaw properties. This will reduce the cost of maintenance in the long run.

Lightweight aggregates concrete has excellent fire resistance properties, this is because of the aggregates being produced in a refractory process, and the cellular structure of the particles relieves any pressure from expanding gases. This is obviously an advantage.

Gaseous/ Aerated/ Foamed Concrete

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Gaseous/ Aerated/ Foamed Concrete

This concrete is not only highly workable, but also uses a low-density material which can incorporate up to 50% entrained air and therefore making it light. It is generally self-levelling, self-compacting and may be pumped increasing its workability. Foamed concrete is ideal for filling redundant voids such as disused fuel tanks, sewer systems, pipelines, and culverts - particularly where access is difficult. It is a recognised medium for the reinstatement of temporary road trenches. Good thermal insulation properties make foamed concrete also suitable for sub-screeds and filling under-floor voids.

The advantages of using foamed concrete is that it is highly reliable, it is almost an everlasting material which is not subjected to the impact of time. Due to the way foamed concrete is designed it will not decompose and it as resilient as a rock.

As this concrete can also withstand and resist high compression it allows the use of lightweight manufacture to be used within its construction which in turn increases the temperature insulation of a wall which is built with this type of concrete.

Due to the high temperature insulation property of foamed concrete it allows the building which has been constructed using foamed concrete to build up heat. This is advantageous to the occupier of the building because it can in fact reduce the heating costs by as much as 25% again another reason on why there has been a considerable increase in the use of this type of concrete.

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This concrete also prevents the loss of heat in winter, the build up of heat and a humid temperature in summer by way of absorption and output of moisture. So the material gives good all round performance.

Due to its weight and therefore a resulting lightness in the concrete it allows faster laying when compared to the conventional brick, and because it comes in blocks more area can be covered in a shorter amount of time.

When compared to conventional concrete it’s a lot better to use because it carries the same properties but is lighter in weight which leads to increased efficiency when being laid. Foamed concrete is also easy to process a so when a structure is being built and the electric wires, water and gas piping needs to be added foamed concrete allows for easy trimming, cutting and the drilling of holes which makes the job a lot easier and quicker to do and therefore increasing the efficiency and speed of the work.

No Fines Concrete (Pervious Concrete)

No fines concrete, also known as pervious concrete is produced by adding controlled amounts of cement materials with controlled amounts of water to form a paste which forms a thick coating around aggregate particles giving it the strength of concrete. The fine materials are removed from the normal concrete mix, hence giving it the name of ‘no fines’ concrete. The use of sufficient paste to coat and attach the aggregate particles together creates a system of interconnected voids that drain quickly due to their high permeability.

Flow rates of water through pervious concrete are much higher than that of normal hardened concrete. The low mortar content of pervious concrete added to its high porosity means it has a lower strength than that of conventional concrete mixtures. However it does have the readily achieved strength for many applications.

Pervious concrete is very environmentally friendly and its use addresses key environmental issues and supports sustained growth. As rainwater flow’s over parking lots and other impervious surfaces it picks up many pollutants, namely bacteria and sediment oil and transports them towards downstream water bodies. However when rainwater soaks through pervious concrete and into the soil below most of these pollutant’s get filtered by the concrete.

By capturing the rainwater and allowing it to seep into the ground this type of concrete is instrumental in recharging groundwater and reducing the run off caused by storm water. Pavements made of this material create much more efficient and environmentally friendly land use by eliminating the need for storm water management devices, retention ponds and swales. On a first cost basis, using pervious concrete has the ability to lower overall project costs as otherwise these smaller structures would be required.

There are many advantages of this type of concrete. It is a material of low density. This means that it can be easily worked and transported. This saves costs and also allows for greater work site productivity as less manpower will be required to handle and use the concrete.

A lower level of thermal conductivity is another property of pervious concrete and this gives it the long term benefit of money saved on heating expenses within a building. Pervious concrete also has good insulating characteristics meaning that not only does it prevent heat from escaping; but it also keeps it within the structure.

Another advantage lies in the production of pervious concrete. Cement is of a high cost and is a vital ingredient to concrete. With no fines concrete the amount of cement required is lower than the amount required in normal concrete. Therefore there are saved costs due to a lower cement content being required.

The presence of large areas of empty space known as voids gives pervious concrete the ability to reduce the spread of structural load stresses through a building. These voids are also responsible for the insulating properties of this concrete.

Pervious concrete is made of particles that have very little tendency to gather together whenever moved or disturbed. In other words they have no segregation; the main advantage of this is that as the concrete particles are less likely to separate and the concrete is not prone to becoming weak. Pervious concrete has a low shrinkage level in comparison to normal concrete. In the drying process not as much surface area is lost as that of normal concrete and this means that less material covers a larger area, again saving money.

Pervious concrete has many uses as there are many advantages when choosing this material for a particular job. It is used for pavements, residential roads, alleys and driveways. As well as being used for larger structures such as parking areas, artificial reefs, hydraulic structures, seawalls and load bearing walls.

There are multiple other uses for this type of concrete both on a small and large scale. Its increased use over the past years is due to it being very economical. It is cheaper than normal weight concrete yet it provides better properties and is much cheaper to make or obtain. It is also environmentally friendly as it allows the absorption of water and prevents pollutants getting into the ground and damaging soil. Using pervious concrete also means time can be saved as it is easier to handle and use on a work site than normal concrete. For all these reasons no fines concrete has become much more widely used than in the past.

Comparing the properties of lightweight concrete to those of conventional, normal weight concrete.

Lightweight concrete has a high workability compared to conventional concrete and it is possible to easily produce various shapes, arches etc. This can also be carried out with normal concrete but it will take more work and it will not economically benefit you as it would if you used lightweight concrete. This is because it works out cheaper to use lightweight concrete because of the minimal use of expensive materials such as aggregate or cement and still getting the same results.

http://www.escsi.org

Foamed concrete will weigh from 10-85% less than the conventional average concrete; the advantage of this is that lighter concrete means easier handling and allows the process of construction to be a lot simpler.

http://www.litebuilt.com/lightweight-concrete-handbook.html#D-1

Conventional concrete may fail under the impact of intensive heat whereas this does not happen with the lightweight concrete, allowing it to be used in the construction of fire proof structures. That means that lightweight concrete will be used more increasingly because most of the structures built today should be fire proof although this is not the sole requirement for many structures.

The combination of weight and volume of lightweight concrete which is far less than your normal concrete allows it to be transported easily by roads or railway. For example normal concrete in its fresh state is quite heavy and reduces in weight when it hardens because the moisture content is reduced, imagine lightweight concrete, it is light in its fresh state and becomes even lighter when it hardens, this is because all the moisture has evaporated from within the concrete. This again makes the construction process easier in regards to handling the concrete. Therefore it is so much more convenient and increasingly used rather than normal weight concrete.

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Lightweight concrete is easy to produce in steady conditions and is easy to produce on a construction site. This is favourable for a construction company in terms of saving time, effort and money which would otherwise be spent producing the concrete at the factory and then transporting it to the site. The workability is also easier compared to that of conventional concrete which is harder.

The overall advantage always comes down to the prime cost of the concrete which is low compared to conventional concrete and therefore is a better alternative to use with the equivalent properties and more.

“Lightweight concrete can provide the competitive edge, since it is up to 40% cheaper compared with regular concrete.”

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Aerated concrete hardens quicker than normal concrete, because there is more air within this concrete and therefore reduces the moisture content of the concrete hence faster drying time. So this is used in construction where there is harsh cold weather showing that it has a property which allows it to hydrate faster than conventional concrete from its fresh state.

http://www.blackwell-synergy.com/doi/abs/10.1111/j.1151-2916.1994.tb07102.x

Conclusion

Over all it was found that the lower density and higher thermal insulation of lightweight concrete distinguish it from normal conventional concrete in its hardened state, and it is mostly the workability factor that distinguishes the two concretes in their fresh states, where the lightweight concrete has higher workability as tests have proved.

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As discussed in the above report there are many advantages that lightweight concretes have over normal concrete, and these reasons are why there is an increasing use of these types of concrete.

Where conventional concrete lacks in terms of its properties and uses; lightweight concrete surpasses and therefore making it more favourable to use. However in some cases the sheer mass of normal concrete resides over the lightness of the lightweight concretes e.g. where mass is needed to strengthen and hold down skyscrapers etc.

The conventional concrete will always have its uses but the lightweight concrete has many characteristics which justify the increasing attention and use of this construction material.

References

Internet Articles

Lightweight Concrete Mixes (online) Cited (20 December 2006) Available from:

Lightweight Concrete (online) Cited (19 December 2006) Available from: http://www.litebuilt.com/lightweight-concrete-handbook.html#D-1

Building and thermal blocks for reduced heat loss (online) Cited (19 December 2006) Available from: http://greenbusinesscentre.com/images/photos/4.pdf

Lightweight, Foamed or Cellular concrete technology(online) Cited (18 December 2006) Available from: http://www.litebuilt.com/

Structural Lightweight Concrete (online) Cited (16 December 2006) Available from:

http://www.escsi.org/New%20Web/Structural%20Lightweight%20Concrete.htm

Pervious, Porous Concrete Pavement Applications & Uses (online) Cited (15 December 2006) Available from:

http://www.perviouspavement.org/applications.htm

Journals

C.S. Poon, Ann T.W. Yu, L.H. Ng, (1999) “Comparison of low-waste building technologies adopted in public and private housing projects in Hong Kong” Engineering, Construction and Architectural Management, 10:2, 88-98.

R. Sadgrove, (1993) “Maintenance and restoration of concrete buildings” Structural Survey, 11:2.

R.V. Balendran, H.W. Pang, (1995) “Strength development, deformation properties and mix design of lightweight concrete” Structural Survey, 13:1, 7-11.