By
Kaushal Kishore, Materials Engineer, Roorkee
Foamed concrete, also called cellular light weight concrete is produced by the mixing of Portland cement, sand including or alone fly ash, water and preformed stable foam. The foam is produced with the help of a foam generator by using foaming agent. The air content is typically between 40 to 80 percent of the total volume. The bubbles vary in size from around 0.1 to 1.5 mm in diameter. Foamed concrete differentiates from (a) gas or aerated concrete, where the bubbles are chemically formed through the reaction of aluminium powder with calcium hydro oxide and other alkalies released by cement hydration and (b) air entrained concrete, which has a much lower volume of entrained air is used in concrete for durability. Curing of foamed concrete unit may be done as per IS: 456-2000. Curing can be speeded up by steam.
Foamed concrete may be produce by mixing the above mentioned ingredients in ready mix plant or ordinary concrete mixer. Foamed concrete is self compacting concrete requires no compaction, and will flow readily from a pump outlet to fill mould, form, restricted and irregular cavities. It can be pumped successfully over significant height and distances. The 28 days strength and dry density of the material vary according to its composition, largely its air voids content, but usually they range from 1.0 to 25.00 N/mm2 and 400 to 1800 kg/m3. The plastic density of the material is about 150 to 200 kg/m3 higher than its dry density.
USES:
1. Foamed light weight concrete in the form of bricks, blocks or poured in-situ is used for thermal insulation over flat roofs or for cold storage walls or as non-load bearing walls in RCC/Steel framed buildings or for load bearing walls for low-rise buildings.
2. Fire rating of foamed concrete is far superior to that of brick work or dense concrete.
3. Bulk filling, using relatively low strength material, for redundant sewerage pipes, wells, disused cellars and basements, storage tanks, tunnels and subways etc.
4. Infill to the spandrel walls of arch bridges.
5. Backfill to retaining walls and bridge abutment.
6. Stabilizing soil, for example in the construction of embankment slopes.
7. Grouting for tunnel work.
BATCHING AND MIXING :
The dry ingredients like cement, sand, sand + fly ash or fly ash alone shall be fed into the mixer first and thoroughly mixed to ensure even distribution of cement. The appropriate amount of water shall be added thereafter continuing the mixing. The preformed foam, which is made by blending the foam concentrate, water and compressed air in predetermine proportion in a foam generator, calibrated for a specific discharge rate, shall be added in measured amount to the slurry of cement, sand, fly ash and water in the batch mixer. After an additional mixing to get uniform consistency, the slurry form of foamed cellular concrete of desired wet unit weight shall be ready to be poured out into forms/moulds etc. When truck mixing equipment is used for foamed concrete, the preformed foam shall be added at the job site just prior to pumping or otherwise conveying the concrete into forms.
The building blocks may be de-moulded after 24 hours from pouring of foamed concrete. Curing shall be done s per IS: 456-2000. To speed up the production, the blocks shall be cured by saturated steam at an average temperature of 460C ± 150C for a period of 24 hours or more to attained the required strength. After curing, the blocks shall be allowed to dry under shade for a period 2 to 3 weeks, so as to complete their initial shrinkage before being used in the work.
MIX DESIGN:
There is at present, no guidance or standard method for proportioning foamed concrete, because the hardened density of foamed concrete depends on the saturation level in its pores. Sample mix proportions of foamed concrete are given in table- 1, 2 & 3. However, conclusive mix proportions by actual trials may be worked out with the given set of site materials for required workability, plastic density and compressive strength.
Usually the OPC cement of foamed concrete lie between 300 to 500 kg/m3 and W/C or W/C+FA ratio including the water in foam value will be between 0.4 to 0.8. The higher values are required with finer grained binders such as fly ash.
Table- 1. Sample mix proportion for cement, fly ash foamed concrete for first trial.
|
Required density (kg/m3) |
Required Compressive Strength at 28-day (N/mm2) |
W/C+FA ratio |
OPC 53 grade (kg) |
Fly ash (kg) |
Water (kg) |
|
800 |
2.5 |
0.50 |
350 |
183 |
267 |
|
1000 |
3.5 |
0.45 |
400 |
290 |
310 |
|
1200 |
6.5 |
0.40 |
450 |
407 |
343 |
|
1400 |
12.0 |
0.35 |
500 |
537 |
363 |
Table- 2 : Sample mix proportion for cement, sand foamed concrete for first trial.
|
Required density (kg/m3) |
Required Compressive Strength at 28-day (N/mm2) |
W/C ratio |
OPC 53 grade (kg) |
Fine sand passing 4 mm IS sieve (kg) |
Water (kg) |
|
1200 |
6.5 |
0.55 |
350 |
657 |
193 |
|
1400 |
12.0 |
0.50 |
400 |
800 |
200 |
|
1600 |
17.5 |
0.45 |
450 |
947 |
203 |
|
1800 |
25.0 |
0.40 |
500 |
1100 |
200 |
Table- 3: Sample mix proportion for cement, sand, fly ash foamed concrete for first trial:
|
Required density (kg/m3) |
Required Compressive Strength at 28-day (N/mm2) |
W/C+FA ratio |
OPC 53 grade (kg) |
Fly ash (kg) |
Fine sand passing 4 mm IS sieve (kg) |
Water (kg) |
|
1200 |
6.5 |
0.55 |
294 |
126 |
549 |
231 |
|
1400 |
12.0 |
0.50 |
336 |
144 |
680 |
240 |
|
1600 |
17.5 |
0.45 |
378 |
162 |
817 |
243 |
|
1800 |
25.0 |
0.40 |
420 |
180 |
960 |
240 |
Note:
1. If superplasticizer is being used its dosage should not be more than 0.2 bwc.
2. Ignore the amount of water contained in the foam in the mix design calculation.
3. Determine the amount of air (kg/m3) in the mix from consideration of a unit volume, and from the target density of the foam, estimate the required quantity of foam. Worked out final mix proportion for trials.
4. Usually the total cement content lie between 300 to 500 kg/m3. The gain in strength is small above cement content of 500 kg/m3.
5. Fly ash is added, at level of upto 100% of the OPC content, to enhance workability and increase long-term strength of foamed concrete. Because of greater surface area of OPC/FA mixes have a greater water demand than OPC/sand mixes. The addition of fly ash to a mix leads to a more uniform bubble structure in the paste, which in turn improve some of the engineering properties of the concrete.
6. Fly ash can be used as a total replacement for sand to produce foamed concrete with a dry density of upto 1400 kg/m3.
7. In all cases trial mixes should be done with proposed materials to determine workability, plastic density, if need be the mix should be modified. Specimens shall be cast and tested for the compliance of required specifications.
8. To minimize shrinkage the W/C or W/C+FA ratio should be kept as low as possible.
9. Total fly ash based foamed concrete products are eco friendly as no sand is used.
REFERENCES :
1. IS: 383-1970 Specification for coarse and fine aggregates from natural sources for concrete (second revision), BIS, New Delhi.
2. IS : 456-2000 Plain and reinforced concrete- Code of practice (fourth revision), BIS, New Delhi.
3. IS : 2185 (Part 4) 2008 Concrete masonry units- Specification preformed foam cellular concrete blocks, BIS, New Delhi.
4. IS: 3346-1980 Method for determination of thermal conductivity of thermal insulation materials (two slab guarded hot plate method) (first revision), BIS, New Delhi.
5. IS: 3812 (Part-1) 2003 Pulverized fuel ash- specification for use as pozzolana in cement, cement mortar and concrete (second revision), BIS, New Delhi..
6. IS: 12269-1987 Specification for 53 grade ordinary port land cement, BIS, New Delhi.
7. IS: 6598-1972 Cellular concrete for thermal insulation, BIS, New Delhi.
8. ASTM C 869-91 Standard specification for foaming agents used in making preformed foam for cellular concrete.
9. Dhir R.K., Jones M.R and L.A Nicol (1991) Development of structural grade foamed concrete, DETR Research Project, University of Dundee, Scotland.
10. Van Deijk S (1991) Foam concrete, Concrete July/August pp 49-54.
We at engineeringcivil.com are thankful to Sir Kaushal Kishore for submitting his research paper on “Foamed Cellular Light Weight Concrete” to us. This will be of great help to all civil engineers seeking information on Light Weight Concrete.
If you have a query, you can ask a question here.
The paper is outstanding and the author sir. Kaushal Kishore has done a depth research. it would be great if they provided more details on the foam based addmixer
I am waiting for the feed back. Further details will be published in my next paper.
what kind of super plasticizers can be added …. awaiting your reply
The paper is outstanding ,i thank the author . Can u enlight on porous concrete.
Thanks for Very Useful Info. I need some help .. I have installed plant for Foamed Concrete Bricks & now the supplier company of plant is underground… need help on mixture ratios & strength. Thanks
For trial mixes you can follow the mix ratios given in this paper. Cast the specimens and test them,if the test results found not according to your requirements then the mix may be altered as per your specifications.
Sir, i want to get know more about the clc plant. So ,i will be very thanful to you if you can send something by which i can be in contact with you.
Thank you for your paper, it gave me an extensive deep knowledge about foam concrete, Sir I we want to produce concrete blocks in Kolkata as red clay bricks are becoming more expensive by the day. We are confused between fly ash, clc and aac blocks. Investment in plants range from 25 lakhs to 22 crores, AAC being the most expensive, Sir in China there have been more than 1000 plants of AAC which have come up. Is AAC the superior product and does it justify such large investments, Kindly enlighten me on the same.
Thanks and awaiting your reply.
I want to thank the engineer and I would like to share experiencias.Mi company is located in the city of Mar del Plata, Argentina, and use very fine sand of the sea. Folders do monolithic subfloors and concrete cell, with very good results, currently we are working in another city, far from the sea and fine sand used. We have found better workability, therefore, use a lower w / c ratio. and very good measurements of resistance to compression at 28 days. Density 550 kg/m3 Dosage: Cement: 350 kg/m3, Arena: 350 Kg/m3, Water: 150 liters/m3 Foaming agent: 600cc, Water Foam: 40 liters/m3 28 day strength: 2.5 N/mm2. Construction engineer Jose Luis Masia
Estimado Ing. Masia: al igual que usted me estoy dedicando a la preparaciòn de mezclas livianas, en mi caso para construir casas en la selva. Le agradeceré mucho si pudiera enviarme alguna información sobre el espumígeno. En nuestro caso usamos una mezcla de ácido sulfónico más algunos otros químicos, pero creo que mejor sería comprar un buen producto en el mercado, mejor si no fuera muy caro. Si tuviera usted alguna referencia al respecto, mucho se lo agradeceré.
i am also a manufacturer of clc blocks , clc is better than AAC
outstanding information – many thanks.
my question is what admixtures are needed for increasing the curing time of foam concrete.
is it possible to add steel fibers in cellular lightweight concrete to increase its tensile strength?
Do you know of examples where foam concrete has been used to form free standing mass concrete pad foundations say 8m x 10m x 2.5m thick. The concrete blocks will not be contained on the sides by another structure. Load bearing applications found so far consist of foam concrete withing other supporting structure.
Dear sir,
Artical published by you is very much useful for CLC blocks.
I want to know,
Is there any chemical or binding agent available to be mixed in mixture of Cement, flyash, water and foaming agent?
Like some body says – add Gypsum or Calcium Chloride to above mixture, it will help to increase the strength.
What I mean to say, How to increase compressive strength of CLC block?
Pl guide me.
can i use the clc formula of lightwightbricks roof of a garage 3m wide by 4large with out using any steel.
I want to set a small clc product fasility for lightwight brick were i can bay the machines and the chemical for production.
dear sir,we have tried composition given by you for clc block but we are not getting the proper strengtg.we are getting only less than 3.
When you said that The building blocks may be de-moulded after 24 hours from pouring of foamed concrete. Curing shall be done s per IS: 456-2000. To speed up the production, the blocks shall be cured by saturated steam at an average temperature of 460C ± 150C for a period of 24 hours or more to attained the required strength. After curing, the blocks shall be allowed to dry under shade for a period 2 to 3 weeks, so as to complete their initial shrinkage before being used in the work. I understand that you are referring to Block production, but what if its poured into cast-in-place forms to build houses?, the foamed concrete would be exposed to direct sunlight and will not be drying under shades, will that affect the strenght and will that shrink the structure?
In place of steam curing, autogenous curing may be tried. Otherwise normal curing may be adopted including application of curing compound, and continued till the minimum required period. Test specimens shall be cast and cured in the same identical conditions.
Shrinkage and shrinkage cracking may be prevented/reduced by taking minimum possible W/C ratio, suitable
foaming agent, plasticizer, well graded clean sand, silica fume and suitable fibres. The evaporation of water of the mix must
be prevented by immediately just after casting covering the structural members with a layer of wet sacking and plastic .
sheet. For early gain in strength accelerators may be used. With the available site materials and facilities all the test trials
must be done well in advance before starting any construction work. KAUSHAL KISHORE
Dear sir,
I am facing problems in light weight block (can’t achieve proper strength & getting cracks).
So please sir can you guide me for exact proportion of block.
dear sir, what ratio to add in 1cubic meter plant cement, fly ash ,foam,
water,if brick size is 9*8*4 how much brick to be made in 1cubic meter plant
superplastisizer should not be more than 0.2 bwc. WHAT IS BWC
by weight of cement
fly ash is not available. can pond ash be used instead of fly ash
when u say to achieve a com. strength of 3.5 we have mix cement-400Kg, FA-290Kg where will the manufacturing cost go…..as we all know AAC is selling at Rs2900/Cu Mtr…..Mr Kaushal Please give mix ratio which can be sold in market