Foam concrete is a type of lightweight concrete that is manufactured from cement, sand or fly ash, water, and the foam. Foam concrete is in the form of foamed grout or foamed mortar.
Foam concrete can be defined as a cementitious material that consists of minimum 20 percent of foam, that is mechanically entrained into the plastic mortar. The dry density of foamed concrete may vary from 300 to 1600 kg/m3. The compressive strength of foam concrete determined at 28 days, ranges from 0.2 to 10N/mm2 or can go higher.
Foam concrete is differentiated from air entrained concrete in terms of the volume of air that is entrained. The air entrained concrete takes in the air of 3 to 8 percent. It also differs from the retarded mortar and aerated concrete for the same reason of percentage of air entrained.
In the case of retarded mortar systems, it is 15 to 22 percent. The bubbles are chemically formed in the case of an aerated concrete.
History of Foam Concrete
The foamed concrete has a long history and it was first made into use in the year 1923. It was initially used as an insulating material. Improvements throughout the past 20 years in the areas of production equipment and better quality foam making agents make the use of foam concrete in large scale.
Production of Foam Concrete
The production of foam concrete involves the dilution of surfactant in water, which is passed through a foam generator that will produce foam of stable form. The foam produced in mixed with the cementitious mortar or the grout, so that foamed quantity of required density is produced.
These surfactants are also used in the manufacture of low density fills. These are also called as controlled low strength Material (CLSM). Here, to obtain an air content of 15 to 25 percent, the foam is added directly to a mix of low cement content and rich sand.
It must be kept in mind that low density fillers are supplied as foamed concrete by some manufacturers, so misleading must be taken care.
Two main methods are used for production of foamed concrete:
- Inline Method and
- Pre-foam Method
Inline Method of Foam Concrete Production
The base mix of cement and sand is added to a unit. In this unit, the mix is blended with foam thoroughly. The process of mixing is carried out with proper control. This will help in mixing of larger quantities. The inline method comprises two processes;
- Wet Method- Inline System
- Dry Method -Inline System
Wet Method of Inline System: The materials used in the wet method will be wetter in nature. With the help of a series of static inline mixers, the base material and the foam are fed and mixed together. The continual on board density monitor is used to check the blending of the whole mix.
The output volume is dependent on the density of the foamed concrete and not on the ready mixed truck. That is one 8m3 base material delivery would produce 35m3 of a foamed concrete of 500kg/m3 density.
Dry Method of Inline System: Here the dry materials are used. They are taken into the onboard silos. From here they are weighed properly and mixed with the help of on- board mixers. The mixed base materials are then pumped to a mixing chamber.
In wet method of foam concrete production, foam is added and mixed. This method employs a large amount of water for mixing. 130 cubic meters of foamed concrete can be produced from a single delivery of cement or fly ash blend.
Pre- Foam Method of Foam Concrete Production
Here, the ready-mix truck brings the base material to the site. Through the other end of the truck, the pre- formed foam is injected into the truck, while the mixer is rotating. So, small quantities of foam concrete can be produced for small works, like for grouting or trench fill works.
This method would provide foam concrete with densities ranging from 300 to 1200 kg/m3. The foam input will be from 20 to 60 percentage air. The final volume of the foam can be calculated by reducing the amount of other base material. As this is carried out in the truck.
Control of stable air and density is difficult for this method. So, a degree of under and over yield must be specified and allowed.
When the foam is formed, it is combined with a cement mortar mix having water cement ratio of 0.4 to 0.6. If the mortar is wet, the foam becomes unstable. If it is too dry, the pre-foam is difficult to blend.
Composition of Foamed Concrete
Foamed concrete composition varies with the density that is demand. Generally, the foamed concrete that has densities lesser than 600kg/m3 will have cement, foam, water also some addition of fly ash or limestone dust.
To achieve higher densities for foamed concrete, sand can be employed. The base mix is 1:1 to 1:3 for heavier foamed concrete, which is filler to portland cement ratio (CEM I).
For more densities, say greater than 1500kg/m3 more filler and medium sand is employed. To reduce the density, the filler amount should be reduced. It is recommended to eliminate the foam concrete with density lesser than 600kg/m3.
Materials for Foamed Concrete
Cement for Foam Concrete
Ordinary Portland cement is commonly used, but rapid hardening cement can also be used if necessary. Foam concrete can incorporate a wide range of cement and other combination, for example, 30 percent of cement, 60 percent of fly ash and limestone in 10 percent. The content of cement range from 300 to 400 kg/m3.
Sand for Foam Concrete
The maximum size of sand used can be 5mm. Use of finer sands up to 2mm with amount passing through 600 micron sieve range from 60 to 95%.
The supplementary cementitious materials like fly ash and ground granulated blast furnace slag have been used widely in the manufacture of foam concrete. The amount of fly ash used ranges from 30 to 70 percent. White GGBFS range from 10 to 50%. This reduces the amount of cement used and economical.
Silica fume can be added to increase the strength; at an amount of 10 percentage by mass.
The hydrolyzed proteins or the synthetic surfactants are the most common forms based on which foams are made. The synthetic based foam agents are easier to handle and are cheap. They can be stored for a longer period.
Lesser energy is required to produce these foams. The protein based foam are costly but have high strength and performance. The foam can be of two types: wet foam and dry foam.
Wet foams with densities lesser than 100 kg/m3 are not recommended for the manufacture of foam concrete. They have a very loosely place large bubble structure. To a fine mesh, the agent and the water are being sprayed. This process produces foam that has bubbles with size ranging from 2 to 5mm.
Dry foam is highly stable in nature. A solution of water and the foaming agent is forced by restrictions into a mixing chamber by compressor air. The produced foam have bubble size which is smaller than the wet foam. That is less than 1mm. These give a structure of bubbles, which are evenly arranged.
BS 8443:2005 covers the foaming admixtures.
Other Materials and Aggregates for Foam Concrete
The coarse aggregate or other replacement for coarse cannot be used. This is because these materials would sink in the lightweight foam.
Mix Details of Foam Concrete
The foam concrete properties depend upon the following factors:
- The volume of the foam
- The cement content in the mix
- The filler material
- The age
The influence of water cement ratio has a very little influence on the properties of the foam concrete, unlike foam and the cement content.
Properties of Foam Concrete
The foam concrete properties in its fresh and hardened state are explained below;
Visual Appearance of foam Concrete
The exact comparison for the foam that is manufactured to produce foam concrete resembles the shaving foam. When this is mixed with the mortar of standard specification, the final mix will resemble the consistency of yogurt or in the form of a milkshake.
Fresh Properties of Foam Concrete
The workability of foamed concrete is very high and have a slump value of 150mm to collapse. These have a strong plasticizing effect. This property of foam concrete makes it highly demanded in most of the applications. Once the flow of the mix has remained static for a longer period, it is very difficult to restart its original state. Foam concrete in the fresh state is thixotropic in nature.
The chances of bleeding in foamed concrete are reduced due to high air content. When the mix temperature increases, good filling, and contacts are carried out due to the expansion of air.
If the amount of sand used is higher or coarse aggregates is used other than the standard specifications, there are chances for segregation. This can also lead to the collapse of the bubble, which would reduce the total volume and the foam structure.
It is fine to carry out pumping of fresh foam concrete with care. Free fall of foam concrete at the end with turbulence, may result in the collapse of the bubble structure.
Hardened Properties of Foam Concrete
The physical properties of the foam concrete are clearly related to the dry density. The variation is seen in the tabulation given in the table below.
Table.1: Typical Properties of Foamed Concrete in its Hardened State
|Compressive Strength N/mm2||Tensile Strength|
|400||0.5 – 1||0.05-0.1||75|
The thermal conductivity of foam concrete ranges from 0.1W/mk to 0.7W/mk. The drying shrinkage ranges from 0.3 to 0.07% at 400 and 1600kg/m3 respectively.
The foamed concrete does not possess an equivalent strength similar to an autoclaved block with similar density. Under the action of load, there is internal hydraulic pressure created within the structure, which would cause the deformation of the foam concrete.
The hardened foam concrete has good resistance against freezing and thawing. It was observed that the application of foamed concrete in an area of temperature ranging from -18 degree Celsius to +25 degree Celsius showed no signs of damage. The density of foamed concrete employed here range from 400to 1400kg/m3.
Advantages of Foam Concrete
- The foam concrete mix does not settle. Hence it does not need any compaction
- The dead weight is reduced as it is light weight concrete
- The foamed concrete under its fresh state has freely flowing consistency. This property will help in completely filling the voids.
- The foam concrete structure has excellent load spreading and distributing capability
- Foamed Concrete Does not impose significant lateral loads
- The Water absorption property
- The foam concrete batches are easy to produce, so quality check and control are easily done
- The foam concrete has higher resistance to freezing and thawing
- Non-hazardous and faster work completion
- Cost effective, less maintenance
Disadvantages of Foam Concrete
- Presence of water in the mixed material make the foam concrete very sensitive
- Difficulty in finishing
- Time of mixing longer
- With the increase in density, the compressive strength and flexural strength decreases.
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