In this article we will discuss about:- 1. Meaning and Classification of Light Weight Concrete 2. Characteristics of Light-Weight Concrete 3. Properties 4. Advantages 5. Applications 6. Mix Proportion.
Meaning and Classification of Light Weight Concrete:
The self-weight of normal cement concrete varies from 2200 to 2600 kg/m3. This is one of the main disadvantages of conventional cement concrete as this heavy weight of concrete makes it uneconomical structural material. To increase the efficiency of concrete as a structural material attempts have been made to reduce the weight of normal cement concrete. A cement concrete having self-weight ranging from 300 kg/m3 to 1850 kg/m3 is called light weight concrete.
Classification:
The light weight concrete can be classified into the following groups, depending on the method of production:
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1. Light Weight Aggregate Concrete:
This type of concrete is produced by using porous light weight aggregate of low specific gravity usually less than 2.6.
2. Aerated, Cellular, Foamed or Gas Concrete:
This type of concrete is produced by introducing larger voids with in the concrete or mortar. These voids should be clearly distinguished from the fine voids produced by air entrainment.
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3. No Fines Concrete:
This type of concrete is produced by omitting the fine aggregate from the mix, which results in the large number of interstitial voids. In this concrete normal weight coarse aggregate is used.
In essence, the decrease in density of the concrete in each method is obtained by the presence of voids either in the mortar or in the aggregate or in the interstices between the particles of coarse aggregate. The presence of voids reduces the strength of light weight concrete in comparison to normal weight concrete. As stated above, in many applications high strength is not essential.
The light weight concrete provides good thermal insulation and has a satisfactory durability, but is not highly resistant to abrasion. Light weight concrete in general is more expensive than ordinary concrete. It requires more care in mixing, handling and placing than ordinary concrete.
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The light weight concrete may also be classified according to its use as follows:
1. Structural light weight concrete.
2. Masonry light weight concrete or non-load bearing concrete.
3. Insulating concrete.
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This classification of structural light weight concrete is based on a minimum strength:
1. Structural Light Weight Concrete:
The 28 day cylinder compressive strength of this concrete should not be less than 17 MPa. Its density should not exceed 1840 kg/m3. Usually it should be between 1400 1800 kg/m3.
2. Masonry Concrete:
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The 28 day cylinder compressive strength of this concrete should be between 7 to 14 MPa. Its density should be between 500 to 800 kg/m3.
3. Insulating Concrete:
Its coefficient of thermal conductivity should be below 0.3 J/m2S°C/m. Its strength should be between 0.7 and 7 MPa and density generally lower than 800 kg/m3.
In recent years light weight concrete has become more popular due to the manifold advantages it offers over the conventional concrete. A better understanding and development of modern technologies have also helped in the promotion and use of light weight concrete. The light weight structural concrete, is lighter than conventional concrete, but at the same time strong enough to be used for structural purposes.
It combines the advantages of normal weight concrete and discards the disadvantages of normal concrete This type of light weight concrete has great future. Out of the main groups of light weight concrete the light weight aggregate concrete and aerated concrete are more often used than no fines concrete. Previously aerated concrete mainly was used for insulating purposes. Nowadays it is also used for structural purposes along with steel reinforcement.
The aerated concrete is more widely manufactured and used in Scandivian countries. In U.K. France, Germany and U.S. light weight aggregate concrete is widely manufactured and used due to the production of large scale artificial industrial light weight aggregate Now a days considerable varieties of industrial light weight aggregates of varying properties are available by different trade names.
Some of them are as follows:
(a) Aglite and Hadite (Expanded shale)
(b) Leca (Expanded clay)
(c) Lytag (Sintered pulverised fuel ash) etc.
The groups of light weight concrete are shown in Table 22.1.
Types of Light Weight Aggregate:
Light weight aggregates can be classified into the following two groups:
1. Natural light weight aggregate.
2. Artificial light weight aggregate.
These aggregate are further classified as follows:
Natural Light Weight Aggregate:
1. Pumice
2. Diatomite
3. Scoria
4. Volcanic cinders
5. Saw dust, and
6. Rice husk.
Artificial Light Weight Aggregate:
1. Artificial cinders
2. Coke breeze
3. Foamed slag
4. Bloated clay
5. Expanded shale’s and slate
6. Sintered fly ash
7. Expanded perlite
8. Thermocole beeds, and
9. Exfoliated vermiculite.
Light Weight Aggregate Concrete:
Often the light weight concrete is made by using the light weight aggregate. As already discussed, the different light weight aggregates have different densities. Hence light weight concrete made using different types of light weight aggregate will have different densities.
Concrete produced by using expanded perlite or vermiculite will have a low density of the order of 300 kg/m3, whereas by the use of expanded slag, Sintered fly ash or bloated clay etc. a concrete having density as high as 1900 kg/m3 can be obtained. The strength of light weight concrete varies from 0.3 N/mm2 to 40 N/mm2. In the production of such concrete, cement content varying from 200 kg/m3 to about 500 kg/m3 may be used. Typical ranges of densities of different light weight concrete are shown in Fig.22.1.
The strength of light weight concrete depends on the density of aggregate. Less porous aggregate, which is heavier in weight, produces stronger concrete particularly with higher cement content. The grading of aggregate, w/c ratio, and degree of compaction also affects the strength of concrete.
Structural Light Weight Concrete:
It is a concrete light in weight and sufficiently strong when used with steel reinforcement. Thus it is going to be more acceptable and economical material of construction than conventional concrete.
The 28 day compressive strength of structural light weight aggregate concrete is more than 17 MPa and air dried unit weight is not more than 1850 kg/m3. This concrete may consist either entirely of light weight aggregate or a combination of light weight and normal weight aggregate. For practical considerations, the common practice of producing structural light weight Concrete is to use normal sand as fine aggregate and light weight coarse aggregate of maximum size 19 mm. Concrete made with normal sand and light weight aggregate is known as sanded light weight concrete in contrast to “all light weight concrete.”
Workability:
Workability of light weight aggregate concrete needs special attention, as for equal workability light weight aggregate concrete gives a lower slump and a lower compacting factor than the normal weight aggregate concrete as work done by gravity is smaller in case of light weight aggregate concrete. On the other hand if a higher workability is kept, there will be a higher tendency of segregation.
In case of higher slump and over vibrations, the mortar goes down and aggregate tends to float, which is a reverse phenomenon that of normal weight aggregate concrete. In such conditions the finishing operations of deck slab and floors will be difficult. To overcome this difficulty, usually the maximum slump is restricted to 100 mm.
Due to the porous nature of light weight aggregates they have high and rapid water absorption, if the aggregate is dry at the time of mixing, it will absorb water rapidly and the workability decreases quickly. This problem can be overcome by mixing the aggregate with at least one half of the mixing water before adding cement to it However this procedure will increase the density and decrease its thermal insulation.
The light weight aggregate mixes tend to be harsh, which can be reduced by air entrainment. Air entrainment reduces water requirements and also the tendency of segregation and bleeding. The usual total amount of air content by volume is 4 to 8% for 20 mm maximum size of aggregate and 5 to 9% for 10 mm maximum size of aggregate.
Light weight aggregate concrete exhibits higher moisture movement than the normal weight concrete. The wet concrete swells more while dry concrete shrinks more. Due to the higher drying shrinkage and lower tensile strength, the light weight aggregate concrete develops shrinkage cracks. The coefficient of thermal expansion of light weight aggregate concrete is much lower than ordinary concrete. Typical values are shown in in Table 22.5.
Characteristics of Light-Weight Concrete:
Following are the important characteristics of light weight concrete:
1. Low Density:
The density of this concrete varies from 300 to 1200 kg/m3. The lightest variety is suitable for insulation purposes while the heavier variety is used for structural purposes. The low density of cellular concrete makes it suitable for precast roofing and floor units. These units being lighter are easy to handle and transport from factory to the site.
2. High Strength:
The compressive strength of cellular concrete is high in relation to its density. The compressive strength of such concrete has been found to increase with the increase in its density. The tensile strength of cellular concrete is about 15 to 20% of its compressive strength. The strength to mass ratio of cellular concrete is much higher than normal concrete. Thus the weight of roof slab and floor of the cellular concrete are about 25% of the normal reinforced concrete.
3. Durability:
Aerated concrete is slightly alkaline. Due to its porosity and low alkalinity it does not provide any protection to the steel reinforcement as provided by the dense compacted concrete. Thus the reinforcement used in cellular concrete needs special treatment for the protection against corrosion.
4. Thermal Insulation:
The insulation value of light weight concrete is about 3 to 4 times more than that of bricks and about 10 times that of concrete. The degree of insulation of 20 cm thick wall of aerated concrete of density of 800 kg/m3 is the same as that of 40 cm thick brick wall of 1600 kg/m3 density.
5. Fire Resistance:
The fire resistance properties of light weight concrete are excellent. Its low thermal conductivity makes it suitable for the protection of other structures from the effect of fire.
6. Sound Insulation:
The sound insulation of cellular concrete is not as good as that of dense concrete.
7. Shrinkage:
The shrinkage of light weight concrete is small. The autoclaving of cellular concrete reduces its dry shrinkage to l/5th i.e. 20% of that occurring during air curing.
8. Repairability:
The light weight concrete products can be easily cut, drilled, nailed and sawn. This property makes the construction easier. The local repair of the structure can be attended as and when required without affecting the rest of the structure.
9. Speed of Construction:
By adopting prefabrication of units, the structure can be designed on the concept of modular coordination, which ensures a faster rate of construction.
10. Economy:
Due to the high ratio of strength to mass and light weight of cellular concrete products, their use results in lesser consumption of steel. Composite floor construction using precast un-reinforced cellular concrete blocks and reinforced concrete grid beams results in appreciable saving in the consumption of cement and steel. This reduces the cost of construction of roofs and floors considerably. Using this type of construction a saving of about 15-20% can be effected in the construction of roofs and floors in comparison to conventional construction.
11. Quality Control:
With the use of light weight concrete products a better quality control can be exercised as these units are factory made.
Properties of Light Weight Aggregate Concrete:
Followings are some of the other properties of light weight aggregate concrete as compared to normal weight concrete:
1. For the same strength, the modulus of elasticity of light weight concrete is lower by 25 to 50% than normal concrete. Hence its deflections are greater.
2. Its resistance to freezing and thawing is greater than normal weight Concrete due to the greater porosity of light weight aggregate, provided the aggregate is not saturated before mixing.
3. Its fire resistance is greater as light weight aggregate have a lesser tendency to spall. Thus concrete suffers a lesser loss of strength due to rise in temperature.
4. It is easy to cut to fix desired attachments.
5. For the same compressive strength its shear strength is lower by 15 to 25% and bond strength is lower by 20 to 50%. Thus in the design of reinforced concrete beams these differences have to be taken into account.
6. The tensile strain capacity of light weight aggregate is greater than normal weight aggregate. Thus the tensile strain capacity of light weight aggregate concrete is about 50% greater than normal weight concrete. Hence the ability to withstand restraint to movement i.e. due to internal temperature gradient is greater for light weight concrete.
7. For the same strength the creep of light weight aggregate concrete is about the same as that of normal weight concrete.
Advantages of Light Weight Concrete:
Following are the advantages of light-weight-concrete:
1. Light weight concrete reduces the dead load of the structure.
2. It increases the progress of construction of the structure.
3. It lowers the haulage and handling charges.
4. The weight of structure on the foundation is an important factor in design, specially in the case of multi-story buildings and in weak soils. Heavier the dead load, deeper and thicker the foundations involving higher cost.
5. In framed structures, columns and beams have to carry loads of walls and floors. If walls and floor are made of light weight concrete, the foundations also will be lighter, resulting in considerable economy in the construction.
6. The thermal conductivity of light weight concrete is relatively low, which dampens the heat transfer from roof and walls, resulting lower inside temperature of the building. This lower temperature provides comfort to the inhabitants. The thermal conductivity improves with decrease in density.
7. In case of buildings where air conditioning is to be installed, the use of light weight concrete has been found advantageous from the point of view of thermal comfort and lower consumption of power.
Applications of Light Weight Concrete:
Light weight concrete can be used as follows:
1. As Load bearing masonry walls using cellular concrete blocks.
2. As precast floor and roof panels in all types of buildings.
3. As partition walls in all types of buildings as residential, industrial and institutional buildings.
4. As insulating materials to exterior walls in all types of buildings, specially in office and industrial buildings.
5. As a filler in the form of precast reinforced wall panels in multistoryed buildings.
6. As precast composite floor or wall panels etc.
Mix Proportion of Light Weight Concrete:
The water/cement ratio has the same influence on the strength of light weight aggregate concrete as that on normal aggregate concrete. Hence theoretically same procedure may be adopted for mix design as in the case of normal weight aggregate concrete. But in the absence of accurate values of absorption, specific gravity and free moisture content in the aggregate makes it difficult to apply the water/cement ratio law accurately for the mix proportion of light weight aggregate concrete.
Light weight aggregate produced artificially usually is bone dry. If it is saturated before mixing, the strength of the concrete will be lower by 5 to 10% than, dry aggregate is used for the same cement content and workability. In case of bone dry aggregate, some of the mixing water is absorbed after mixing, but before setting, reducing the effective water/cement ratio.
The density of concrete made with saturated aggregate is higher, but the resistance to freezing and thawing is reduced. On the other hand if the aggregate with high absorption is used without presoaking, it will be difficult to obtain workable and cohesive mix. In general, aggregate with more than 10% absorption should be presoaked and air entrainment should be done.
Thus the light weight concrete mix design usually is established by trial mixes. The proportion of fine to coarse aggregate and water and cement requirements are estimated on the previous experience with a particular aggregate. The different degrees of absorption by different light weight aggregates are one of the main difficulties in the design of mix proportions.
There are several methods to determine the aggregate content. Here the use of effective water/cement ratio for the calculation of aggregate content has been discussed. The method described here is based on the well-known British mix design method.
The steps involved to obtain the mix proportion for the stipulated 28 day strength are as follows:
Step 1:
The target mean strength of the concrete is determined from the characteristic strength, coefficient of probability and standard deviation as ft = fck + K.S.
Step 2:
The required water/cement ratio for the required target strength is determined from, the curve of Fig. 22.2
Step 3:
For the water-cement ratio obtained in step 2 the aggregate-cement ratio (by volume), cement content in kg/m and optimum percentage of fine aggregate for desired workability are selected from table 22.6 given below.
Step 4:
To obtain the effective free water content for the mix, the water content is adjusted for water absorption and the moisture content of aggregates.
Step 5:
For the data, obtained as above, a trial mix is prepared and water content is adjusted to maintain the desired workability. The density of fresh wet compacted concrete is calculated and cement content checked. If the cement content and density are not found correct, minor adjustment may be made either by adding or subtracting some quantity of cement and adding or subtracting some volume of fine aggregate as follows-
