The use of fly ash as replacement of sand is an economical solution for making green and denser concrete. The paper presents a concrete mix design procedure for partial replacement of sand with fly ash. Present method could produce additional compressive and flexural strength for concrete with partial replacement of sand with fly ash over control concrete, with higher slump. Addition of 0.5% super plasticizer could further improve compressive and flexural strength with higher slump over control concrete. Concrete with sand replaced by fly ash was also found to be economical without and with super plasticizer, when cost per N/mm 2 was compared. The beneficial effect may be attributed to better packing, pozzolanic activity of fly ash and internal curing by fly ash as partial replacement of sand. Based on experimental results, correlations are developed to predict compressive strength, flexural strength and cost per N/mm2 for percentage sand replacement with fly ash.
It is a pressing need today for the concrete industry to produce concrete with lower environmental impact, the so-called green concrete. This can be achieved in three ways. The first one is by reducing the quantity of cement as one tonne of cement saved will save equal amount of CO2 to be discharged into atmosphere. Secondly by reducing the use of natural aggregates whose resources are limited and are exhausting very fast. It is also achieved by utilizing maximum possible waste materials like fly ash in concrete. This will reduce the requirement of landfill area and make system more sustainable. The World Bank has reported that by 2015 disposal of fly ash will require 1000 square kilometre area or one square meter of land per person [
Fly ash is generally used as replacement of cement, as an admixture in concrete, and in manufacturing of cement. Concrete containing fly ash as partial replacement of cement poses problems of delayed early strength development. Concrete containing fly ash as partial replacement of fine aggregate will have no delayed early strength development, but rather will enhance its workability and strength. This higher workability and strength achieved gives scope for indirectly reducing the cement quantity in concrete. Earlier investigations in respect of development of strength of cement mortars with fly ash showed the 50% to 80% increase in 91 days strength. For better packing of concrete more quantity of particle size less than 75 micron is highly desirable. This addition of finer particles will also increase the water requirement of the concrete mix. Addition of fly ash as replacement of sand fulfils this requirement of additional finer particles and improves workability and strength at same water content. Considering present scope of work only workability, cost and strength properties of concrete were studied. In future durability studies are recommended.
Fine aggregate occupies about 25% to 40% of total volume of concrete and hence provides great opportunity to utilize about 150 kg per m3 waste materials like fly ash for replacement. The need of fly ash utilization also arises out of the fact that good quality Natural River sand required in concrete and in the cement mortar, is depleting day by day and scarcity of good quality sand is felt by all metro and mega cities in India. Hence this study explores the possibility of replacing part of fine aggregate with fly ash by maximum density method and minimum voids method as a means of incorporating fly ash and reducing the consumption of natural sand. Both methods used in present study for replacing sand with fly ash are the oldest and basic conceptual methods and will provide a simple but effective way of replacing sand with fly ash at site.
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Filho et al. [
Homnuttiwong [
Nambiar et al. [
Hwang et al. [
Arezoumandi et al. [
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From the literature reviewed it is clear that in India disposal of fly ash is a big problem. However as reported high volume fly ash concrete lowers compressive strength compared to cement concrete. In all referred literature replacement of sand with fly ash has produced higher strength than normal concrete with sand as fine aggregate. It is further observed that water absorbed by fly ash could also be used for internal curing of concrete, which may further reduce cracking.
Portland Pozzolana cement fly ash based was used. About 25% cement is replaced with fly ash. It was tasted as per BIS 4031:1988. Properties are given in
S.N. | Property | Result |
---|---|---|
1 | Standard consistency | 24% |
2 | Initial setting time | 130 minutes |
3 | Final setting time | 200 minutes |
4 | Fineness by sieving | 3.5% |
5 | Soundness | 2 mm |
6 | Compressive strength 3 days 7 days 28 days | 33.5 N/mm2 41 N/mm2 58 N/mm2 |
PHYSICAL TESTS | ||
---|---|---|
Specific gravity | 2.227 | |
Fineness | Retained on 45 micron sieve | 9% |
Specific surface, blaine, m2/kg | 526 | |
CHEMICAL ANALYSIS (%) | ||
Silicon dioxide (SiO2) | 55.5 | |
Aluminium oxide (Al2O3) | 31.3 | |
Ferric oxide (Fe2O3) | 6.4 | |
Calcium oxide (CaO) | 1.02 | |
Magnesium oxide (MgO) | 0.21 | |
Titanium oxide (TiO2) | 2.7 | |
Sulphur trioxide (SO3) | 0.44 | |
Loss on ignition | 0.74 |
BIS sieve size | Sieves as per ASTM | Sand % passing | Coarse aggregate % passing | Combined grading % passing | Desirable grading IS 383% passing |
---|---|---|---|---|---|
40 mm | 1.5 in. | 100 | 100 | 100 | 100 |
20 mm | 3/4 in. | 100 | 98 | 99 | 95 - 100 |
10 mm | 3/8 in. (9.5 mm) | 100 | 46 | 58 | ---- |
4.75 mm | No.4 (4.45 mm) | 97 | 4 | 30 | 30 - 50 |
2.36 mm | No.8 (2.36 mm) | 92 | 0 | 26 | ---- |
1.18 mm | No.16 (1.18 mm) | 83 | 0 | 24 | ---- |
600 μ | No.30 (600 μm) | 50 | 0 | 14 | 10 - 35 |
300 μ | No.50 (300 μm) | 3 | 0 | 1 | ---- |
150 μ | No.100 (150 μm) | 0 | 0 | 0 |
M45 concrete mix (control mix) was designed as per Indian Standard concrete mix design method (IS 10262: 1982) and its comparative performance was verified with respect to minimum voids method and maximum density method for partial replacement of sand with fly ash along with the addition of super plasticizer dose to enhance the workability and strength. For the designed mix five alternative cases were studied. C1 was a M45 control concrete mix as per BIS code method. C2 was a M45 concrete mix as per BIS code method with partial replacement of sand so that voids in sand are fully filled by fly ash (minimum voids method). For the given sand minimum voids in vibrated case were found to be 27% hence to fill those voids 27% of sand was replaced by fly ash. Sand was replaced by weight by fly ash considering additional fly ash volume will also occupy the voids between cement particles. C3 was a M45 concrete mix as per BIS code method with partial replacement of sand to achieve maximum density of sand and fly ash combined mix (maximum density method). For given sand fly ash mix vibrated bulk density was highest for 12% sand replacement by fly ash hence 12% sand was replaced by fly ash. In order to study the effect of super plasticizer on concrete 0.5% super plasticizer was added to C4 and C5. C4 was a M45 concrete mix as per BIS code method with partial replacement of sand by fly ash with minimum voids method along with 0.5% super plasticizer. C5 was a M45 concrete mix as per BIS code method with partial replacement of sand by fly ash with maximum density method along with 0.5% super plasticizer. Mix proportions are given in
All the cubes and beams were vibrated on vibration table. After casting, all the test specimens were finished with a steel trowel. They were de molded after 24 hours and were put into a water-curing tank at standard temperature 27˚C ± 2˚C. Numbers of specimens cast for each mix are given in
Value of slump and compacting factor were found for fresh concrete. The results are presented in
Mix | W/C ratio | Cement (kg) | Sand (kg) | CA (kg) | FA (kg) | SP (kg) | Slump (mm) | CF | Density (kg/m3) | Remark |
---|---|---|---|---|---|---|---|---|---|---|
C1 | 0.32 | 450 | 531 | 1319 | 0 | 0 | 0 | 0.71 | 2584 | Without fly ash |
C2 | 0.32 | 450 | 388 | 1319 | 130 | 0 | 25 | 0.74 | 2482 | FA by MVM |
C3 | 0.32 | 450 | 467 | 1319 | 58 | 0 | 15 | 0.73 | 2516 | FA by MDM |
C4 | 0.32 | 450 | 388 | 1319 | 130 | 2.25 | 65 | 0.82 | 2507 | FA by MVM + SP |
C5 | 0.32 | 450 | 467 | 1319 | 58 | 2.25 | 50 | 0.79 | 2544 | FA by MDM + SP |
CA = Air Day Coarse Aggregate; FA = Fly Ash; Density = 24 Hr De Moulding Density; SP = Super Plasticizer; CF = Compacting Factor; MVM = Minimum Voids Method; MDM = Maximum Density Method.
S.N. | Mould dimensions | Number of specimens |
---|---|---|
1 | 100 mm cubes | 21 |
2 | 150 mm cubes | 06 |
3 | 100 mm × 100 mm × 500 mm beam specimens | 06 |
slump of concrete with 27% fly ash as replacement of sand.
150 mm cubes were tested on seventh and twenty eighth day. The results are given in
Compressive strength of 150 mm cubes at 7 and 28 days is given in
Mix | Description | Compressive strength in N/mm2 | |
---|---|---|---|
7 days | 28 days | ||
C1 | Without fly ash | 38.7 | 54.7 |
C2 | FA by MVM | 44.2 | 63.1 |
C3 | FA by MDM | 41.3 | 59.7 |
C4 | FA by MVM + SP | 48.6 | 69.2 |
C5 | FA by MDM + SP | 46.3 | 66.7 |
FA = Fly Ash; SP = Super Plasticizer. Rounding off is done up to 1st place of decimal.
Mix | Description | Flexural strength in N/mm2 | Compressive strength in N/mm2 | ||
---|---|---|---|---|---|
7 days | 28 days | 7 days | 28 days | ||
C1 | Without fly ash | 4.4 | 4.4 | 45.8 | 66.2 |
C2 | FA by MVM | 4.8 | 4.8 | 51.4 | 75.6 |
C3 | FA by MDM | 4.6 | 4.6 | 48.2 | 70.7 |
C4 | FA by MVM + SP | 5.0 | 6.4 | 56.9 | 83.4 |
C5 | FA by MDM + SP | 5.0 | 6.2 | 54.5 | 78.7 |
ing the voids between the cement particles. Better packing and voids filling was also confirmed by ultrasonic pulse velocity results. The compressive strength is increasing up to 147 days due to pozzolanic activity of the fly ash. The compressive strength with 28 days of air curing from 119 days to 147 had also increased. 100 × 100 × 500 mm beam results given in
Mix | Description | 7 days | 28 days |
---|---|---|---|
C1 | Without fly ash | Control concrete | |
C2 | FA by MVM | 14.3 | 15.4 |
C3 | FA by MDM | 6.8 | 9.1 |
C4 | FA by MVM + SP | 25.7 | 26.6 |
C5 | FA by MDM + SP | 19.7 | 22.1 |
Mix | Description | 3 days | 7 days | 28 days | 56 days | 91 days | 119 days | 147 days |
---|---|---|---|---|---|---|---|---|
C1 | Without fly ash | Control concrete | ||||||
C2 | FA by MVM | 13.26 | 12.15 | 14.25 | 14.88 | 15.13 | 15.41 | 15.61 |
C3 | FA by MDM | 7.08 | 5.17 | 6.85 | 8.74 | 8.53 | 8.88 | 9.05 |
C4 | FA by MVM + SP | 22.68 | 24.15 | 26.04 | 22.36 | 24.42 | 26.65 | 28.04 |
C5 | FA by MDM + SP | 20.53 | 18.92 | 18.94 | 19.21 | 18.24 | 20.86 | 21.81 |
Mix | Description | 7 days | 28 days |
---|---|---|---|
C1 | Without fly ash | Control concrete | |
C2 | FA by MVM | 9.1 | 11.1 |
C3 | FA by MDM | 4.5 | 7.4 |
C4 | FA by MVM + SP | 13.6 | 18.5 |
C5 | FA by MDM + SP | 13.6 | 14.8 |
ash without and with the super plasticizer, respectively. This increase is clearly due to reduction of voids or better packing of materials as pozzolanic action of fly ash starts after 7 days. Also about 11% and 18% increase in the flexural strength was achieved over the control concrete with the partial replacement of the sand with the fly ash without and with the super plasticizer, respectively due to pozzolanic action and reduction of voids.
With super plasticizer
The compliance of above equation is justified since they were found to have R2 = 1.
Where fck119 = compressive strength at 119 days; ft28 = flexural strength at 28 days; x = percentage replacement of sand with fly ash.
Mix | Description | Ratio of compressive strength % fck 150 mm/fck 100 mm | |
---|---|---|---|
7 days | 28 days | ||
C1 | Without fly ash | 84.4 | 82.6 |
C2 | FA by MVM | 86.0 | 83.5 |
C3 | FA by MDM | 85.7 | 84.4 |
C4 | FA by MVM + SP | 85.4 | 83.0 |
C5 | FA by MDM + SP | 85.0 | 84.8 |
Mix | Description | Ratio of flexural strength to compressive strength in % | |
---|---|---|---|
7 days | 28 days | ||
C1 | Without fly ash | 9.6 | 8.2 |
C2 | FA by MVM | 9.3 | 7.9 |
C3 | FA by MDM | 9.5 | 8.2 |
C4 | FA by MVM + SP | 8.8 | 7.7 |
C5 | FA by MDM + SP | 9.2 | 7.9 |
The slump and compacting factor were determined for fresh concrete to understand effect on workability. The results are given in
Density for different concrete mixes was determined. The results are given in
To compare the cost effectiveness of the partial replacement of the sand with the fly ash cost per N/mm2 strength is calculated. These values are given in
Cost per N/mm2 without super plasticizer
Cost per N/mm2 with super plasticizer
x = percentage replacement of sand with fly ash.
Fly ash used in present research work had water absorption of 16%. Hence available fly ash in concrete as replacement of sand could supply about 20 liters of water for internal curing. This should result in better hydration product, pore structure etc. along with reduced internal cracking. To check possible internal curing with partial replacement of sand with fly ash 100 mm cubes were air cured for 28 days after 119 days of pond curing. The results as indicated in
Fly ash with its spherical nature and glass like nature acts like ball bearing to improve the packing of the concrete. This reduces the voids in concrete. To know the effect of partial sand replacement by fly ash ultrasonic pulse velocity of various concretes was tested and reported in
Description | Cost of concrete (Rs.) | Cost per N/mm2 at 119 days (Rs.) | % decrease in cost per N/mm2 | |
---|---|---|---|---|
C1 | Without fly ash | 5710 | 71 | --- |
C2 | FA by MVM | 5580 | 60 | 15.3 |
C3 | FA by MDM | 5652 | 64 | 9.1 |
C4 | FA by MVM + SP | 5625 | 55 | 22.2 |
C5 | FA by MDM + SP | 5697 | 58 | 17.4 |
Mix | Description | Compressive strength in N/mm2 | |
---|---|---|---|
119 days | 147 days | ||
C1 | Without fly ash | 80.7 | 80.7 |
C2 | FA by MVM | 93.1 | 93.3 |
C3 | FA by MDM | 87.8 | 88.0 |
C4 | FA by MVM + SP | 102.2 | 103.3 |
C5 | FA by MDM + SP | 97.5 | 98.3 |
Mix | Description | Ultrasonic pulse velocity m/sec | % Increase in ultrasonic pulse velocity | ||
---|---|---|---|---|---|
7 days | 147 days | 7 days | 147 days | ||
C1 | Without fly ash | 2684 | 4198 | Control concrete | |
C2 | FA by MVM | 2830 | 4991 | 5.44 | 18.89 |
C3 | FA by MDM | 2773 | 4950 | 3.32 | 17.91 |
C4 | FA by MVM + SP | 2921 | 5291 | 8.83 | 26.04 |
C5 | FA by MDM + SP | 2885 | 5128 | 7.49 | 22.15 |
the scope of present study.
The following conclusions could be drawn from the present investigation.
1) Present mix design procedure clearly achieves lesser voids as indicated by higher pulse velocity, compressive and the flexural strength.
2) The compressive and the flexural strength of concrete mixes with partial replacement of sand by fly ash was found to be 15% higher without super plasticizer and 28% higher respectively with super plasticizer.
3) The compressive and the flexural strength of concrete mixes with the partial replacement of the sand by the fly ash by the minimum voids method could be higher than the replacement by the maximum density method. Hence the minimum voids method is preferable over the maximum density method for the partial replacement of the sand with the fly ash.
4) The maximum compressive and flexural strength could occur with the partial replacement of the sand with fly ash by the minimum voids method and the super plasticizer.
5) Slump was higher without and with super plasticizer in fly ash concrete over control concrete due to lubrication, ball bearing effect and better packing due to fly ash. This is also justified by lower Surface Index Factor by Murdock [
6) Density of concrete could decrease marginally though higher volume of material is packed in same volume by partial replacement of sand with fly ash. This reduction was higher for minimum voids method.
7) The cost per N/mm2 was lower without and with super plasticizer making the partial replacement of the sand with the fly ash economical.
8) Internal curing effect of fly ash as partial replacement of sand may reduce cracks in concrete.
9) Proposed concrete is environmental friendly green concrete as it will save about 150 kg per m3 scarcely available natural sand and utilize equal amount of fly ash per cubic meter of concrete.
10) The concept of replacement of natural fine aggregate by fly ash highlighted in the present investigation could be taken into consideration during mix design of the pumpable concrete with high workability which generally demands more fine aggregates for smooth and uninterrupted flow of the mix.
11) It could be finally concluded that fly ash could be very conveniently used as partial replacement of sand in structural concrete where its proportion and replacement of sand could be efficiently done by using minimum voids method for higher compressive strength, flexural strength and workability and lower voids at lower cost.