Light Weight Aggregate Concrete

Chapter no. 1

For about half of a century concrete has been a material of choice for mankind. This popularity of concrete is due to some of its excellent properties in fresh and hardened state. Concrete is by far a cheaper material to produce when compared with its competitors (steel and Aluminum), it is easy to manufacture and does not require specialized techniques for its production, concrete has excellent resistance to weathering effects, it has high thermal mass and offers good insulating properties. In everyday life its use span from ornamental purposes to the construction of heavy structures including multistory buildings, bridges, water retaining structures and nuclear power plants.

Among above properties of concrete, one of its useful characteristic is its resistance to the transfer of heat. Thus concrete insulates steel when used in reinforced concrete construction and protects it against the effects of environmental changes. The insulating property of the concrete can further be improved by bringing about suitable changes in the ingredients of the concrete. Such concrete will have special application, mainly, in the form of an insulting layer and as structural concrete in selected cases to reduce the dead load of the RCC members. This purpose can be achieved if coarse aggregate which comprises of ~60% of the concrete volume can be made lightweight. Such aggregate is called as “Lightweight Aggregate” and the resulting concrete is called “Lightweight Aggregate Concrete (LWAC)”.

Lightweight Aggregate (LWA) describes a range of specially used aggregates that have an apparent specific gravity considerably below normal aggregate (sand and gravel) which were at one time used in almost all concrete. As mentioned earlier, main specialties of LWC are its low density and lower thermal conductivity. These lightweight aggregates will range from the extremely light materials used for insulating and non-structural concrete to the expanded clays and shales used for structural concrete. Since the lightness of these aggregates derives from the air trapped in each individual particle, the more air that is trapped per particle unit, the lighter the weight of aggregate and the better its insulation, but, conversely, strength will be lowered.

LWA of low specific gravity used in LWAC instead of ordinary aggregate can be aggregate such as pumice, scoria and all of those of volcanic origin and the artificial aggregate such as expanded blast-furnace slag, slate, shale, vermiculite and clinker aggregate. Figure shows view of typical LWA. The main characteristic of this lightweight aggregate is its high porosity which results in a low specific gravity.

                                                                                     Figure view of typical LWA

The LWAC can be divided into two types according to its application. One is partially compacted LWAC and the other is the Structural LWAC. The partially compacted lightweight aggregate concrete is mainly used for two purposes that is for precast concrete blocks or panels and cast in-situ roofs and walls. The main requirement for this type of concrete is that it should have adequate strength and a low density to obtain the best thermal insulation and a low drying shrinkage of resulting concrete to avoid cracking.

Structurally LWAC is fully compacted similar to that of the normal reinforced concrete of dense aggregate. It can be used with steel reinforced concrete construction. Of the various types of LWA, the one suitable for use in reinforced concrete is the one having comparatively denser characteristic, that is of higher density in the range of LWA.

LWA is expansive, but the cost is calculated just not on the basis of aggregates or LWAC. Other costs involved are taken into considerations also, like working cost, reinforcement cost, transport cost, etc. Being lightweight, it is easy for the workers to handle and they complain less of back pain. The biggest advantage, which is generally not raised, is the enormous expenditure involved in medical aid to workers. Consequently, the contractor has to find subtitle workers to avoid project delays. Another advantage is in demolition cost. It takes less energy to demolish LWAC compared to normal concrete, as smaller equipment can be used. Apart from this, since it contains air, the amount of use will be less than when using normal concrete.

The bond between the aggregate and the matrix is stronger in the case of LWAC than in normal weight concrete. Cement paste penetrates inside the aggregates due to their porous nature. Thus, there is very little or no interfacial transition zone between the aggregates and the matrix, the weakest zone. It is a very important feature from the durability aspect of LWAC. The use of LWAC is increasing and research and development are going on worldwide to develop high performance structural LWAC.

Types of LWAC

LWAC can be prepared either by injecting air in its composition or it can be achieved by omitting the finer sizes of the aggregate or even replacing them by a hollow, cellular or porous aggregate. Particularly, LWAC can be categorized into three groups:

i) No-fines concrete

ii) Lightweight aggregate concrete

iii) Aerated/Foamed concrete

1. No-fines Concrete

No-fines concrete can be defined as a lightweight concrete composed of cement and fine aggregate. Uniformly distributed voids are formed throughout its mass. The main characteristics of this type of lightweight concrete is it maintains its large voids and not forming laitance layers or cement film when placed on the wall.

No-fines concrete is usually used for both load bearing and non-load bearing for external walls and partitions. The strength of no-fines concrete increases as the cement content is increased. However, it is sensitive to the water composition. Insufficient water can cause lack of cohesion between the particles and therefore, subsequent loss in strength of the concrete. Likewise too much water can cause cement film to run off the aggregate to form laitance layers, leaving the bulk of the concrete deficient in cement and thus weakens the strength.

2. Lightweight aggregate concrete

Porous LWA of low specific gravity is used in this lightweight concrete instead of ordinary aggregate. The lightweight aggregate can be natural lightweight aggregate such as pumice, scoria and all of those of volcanic origin and the artificial aggregate such as expanded blast-furnace slag, vermiculite and clinker aggregate. The main characteristic of this lightweight aggregate is its high porosity which results in a low specific gravity.

The LWA concrete can be categorized into two types according to its application. One is partially compacted LWAC and the other is the structural LWAC. Their specific applications have been describe in the preceding paragraphs.

3. Aerated Concrete

Aerated concrete does not contain coarse aggregate, and can be regarded as an aerated mortar. Typically, aerated concrete is made by introducing air or other gas into a cement slurry and fine sand. In commercial practice, the sand is replaced by pulverized fuel ash or other siliceous material, and lime maybe used instead of cement.

There are two methods to prepare the aerated concrete. The first method is to inject the gas into the mixing during its plastic condition by means of a chemical reaction. The second method, air is introduced either by mixing-in stable foam or by whipping-in air, using an air-entraining agent. The first method is usually used in precast concrete factories where the precast units are subsequently autoclaved in order to produce concrete with a reasonable high strength and low drying shrinkage. The second method is mainly used for in-situ concrete, suitable for insulation roof screeds or pipe lagging.

The differences between the types of lightweight concrete are very much related to its aggregate grading used in the mixes. Table 1.1 shows the types and grading of aggregate suitable for the different types of lightweight concretes while table 1.2 describes the advantages and disadvantages of LWAC.

Types and Grading of Lightweight Concrete 

Type of LWAC	Type Of Aggregate	Grading of Aggregate (Range of Particle Size)
No-fines concrete	Natural Aggregate Blast-furnace slag Clinker	Nominal single-sized material between 20mm and 10mm BS sieve
Partially compacted lightweight aggregate concrete	Clinker Foamed slag Expanded clay, shale, slate, vermiculite and perlite Sintered pulverized-fuel ash and pumice	May be of smaller nominal single sizes of combined coarse and fine (5mm and fines) material to produce a continues but harsh grading to make a porous concrete
Structural lightweight aggregate concrete	Foamed slag Expanded clay, shale or slate and sintered pulverized fuel ash	Continues grading from either 20mm or 14mm down to dust, with an increased fines content (5mm and fines) to produce a workable and dense concrete
Aerated concrete	Natural fine aggregate Fine lightweight aggregate Raw pulverized-fuel ash Ground slag and burnt shale	The aggregate are generally ground down to finer powder, passing a 75 μm BS sieves, but sometimes fine aggregate (5mm and fines) is also incorporated

  Advantages and disadvantages of LWAC 

Advantages	Disadvantages
i)                    Rapid and relatively simple construction ii)                  Economical in terms of transportation as well as reduction in manpower iii)               Significant reduction of overall weight results in saving structural frames, footing or piles	i)                   Very sensitive with water content in the mixtures. ii)                  Difficult to place and finish because of the porosity and angularity of the aggregate. In some mixes the cement mortar may separate the aggregate and float towards the surface.