Foam Concrete and its Applications
Dr. R N Krishna, Concrete Technologist shares with Built Expressions about the applications and production process of foamed concrete.
Introduction
|
|
Light weight concrete has found niche applications in several structures and pavements. Although not a substitute for normal grade concrete, the Light weight concrete offers several advantages in Tall buildings, pavement sub-grades and side filling in large construction sites. Light weight Concrete is produced either in a factory using finely divided aluminium powder and controlled autoclave curing. However this process is limited to production of light weight blocks/panels of a uniform chosen density. The other method of Light weight concrete production is with Foam concrete wherein a chemical passed through a foam generator gives out thick foam which is mixed with the cement slurry to have a foam concrete of different densities.
|
Materials
The use of liquid foam for production of light weight concrete is an economical and elegant method that is ideal for on-site use for a variety of applications including blocks, light weight panels, insulating concrete and light weight sub-grade concrete in pavements.
Foam concrete admixtures are protein-based admixtures or blends of surfactants that are diluted with water and used as a foaming admixture. For foam generation protein-based admixtures are used. This foam has a shaving cream consistency and mixed with cement sand mortar or concrete containing light weight aggregates or 6 mm stone chip aggregates to obtain concrete with ranging densities for different applications.
Methodology of Production
The production of foamed concrete is a fairly simple process wherein the foam chemical is added to a foam generator and the foam output thus obtained is added to a regular concrete mixer containing pre-determined ratio of cement, sand and mortar. Though the output foam is stable for some time depending on the ambient temperature it is advisable to mix the foam at the earliest to achieve a cohesive flowing foam concrete. This can be straightaway cast in precast moulds for blocks and into formwork for in-situ walls, floors, etc. The usual output capacity of a normal size foam generator-mixer–pump would be around 8-10 cu.m/hr. Normal RMC pumps are not recommended for use as they disturb and break the foam bubble and cracks on the wet concrete surface will occur. Foam concrete pumps can easily pump up to a distance of around 100 m linear distance normally. Foam concrete is recommended for both site precast and in-situ mixes as the foam generators are small and size and portable.
Concrete Mix Design
There is no specific code for design of light weight foam concrete but DIN-EN 4232 Design and Construction of Lt. Weight Concrete can be used. Foam generation mixer manufacturers give specific guidelines for mix design for various densities. The Federal Lab of tests Materials in Zurich have also published test data on light weight concrete.
Normally the target density – depending upon the application is assumed first. For example, in light weight walls for structural applications a density of a mix of 1000 kg/m3 can be adopted. It is also to be noted that there is at least 75kg /m3 – 85kg/m3 reduction in the wet density; i.e. when a target density of 1000 kg/m3 to be achieved the wet density to be considered for the design mix shall not be less than 1075 kg/cu. m. In this process, a variation in the target density of around 3% - 5% is normally expected.
A typical foam concrete mix design is given below:
|
Dry density
kg/m3
|
Sand
kg/cu. m
|
6 mm aggregate
kg/cu. m.
|
Cement kg/cu. m.
|
Foam liquid
lts. *
|
Water
lts.
|
Com. Strength @ 28 days
|
|
1000
|
920
|
-
|
315
|
0.72
|
145
|
10 N/m2
|
|
1600
|
800
|
310
|
360
|
0.55
|
155
|
15 N/m2
|
|
OPC - 43 Grade - IS : 8110
|
*Active ingredients - Approx. 30%
|
|
Sand - Zone 3
|
pH - 6 to 7.5
|
|
Aggregate - 6mm granite stone chips
|
Dark Brown Liquid
|
Curing
Water curing is essential to prevent early age cracking especially on exposed surfaces since the volume of the cement paste is high. Fly ash can also be used but it does result in delay in the removal of forms in precast moulds and cast-insitu wall panels. Shrinkage crack mitigation is a challenge in all the wet foam concrete exposed areas and apart from curing, sand grading and use of lt. weight aggregates for higher densities (≥ 1000 kg/m3) helps to avoid shrinkage cracks.
Applications
Light weight foam concrete can be advantageously used in internal walls of multi-storied buildings to reduce the dead load on foundations. Thermal comfort of a building with blocks/walls of foam concrete is very high. Sound insulation and fire-resistance properties are far superior to normal concrete. The distinct advantage is that a range of densities from 600 – 800 kg/m3to around 1400-1600 kg/m3 can be made at site for various applications ranging from foundation back fills and basement insulation terrace filling to wall blocks/precast or cast-insitu walls with a smooth finish. However, additional care should be taken for dampness through external walls exposed to heavy rainfall.
Author is a Concrete Technologist, Former Secretary General, Indian Concrete Institution, Deputy Secretary General, Asian Concrete Construction Institution, Singapore