Effect of Fly Ash on Recycled Aggregate Concrete
Dr. C. D. Modhera, Professor, Applied Mechanics Department; SVNIT; Surat.
Introduction
Review of Literature
Fly ash is known to be a good pozzolanic material and has been used to increase the ultimate compressive strength and workability of fresh concrete (Mehta 1985) [13]. Naik and Ramme (1989) [14] produced concrete mixes containing large quantities of fly ash which achieved compressive strengths of 21 and 28 MPa within 28 days.
Experimental Programme
All materials, i.e. fly ash, cement, aggregates etc were tested as per respective IS. Both the normal and recycled aggregates used in this study were in an air-dried state. A constant effective water/binder ratio (w/b) was maintained at 0.5 (or effective water of 266.7 kg/m3) for all concrete mixtures. Since the recycled aggregates had greater water absorption than normal aggregates, they required more water to maintain the slump of fresh concrete from 50 to 100 mm.
Now, Mix type i. e. Mix A prepared with constant proportions of 0.5:1:1:2. Cement content in this mix has been replaced by fly ash in 10%, 20%, 30% and 40%. Natural aggregates (NA) were replaced by Recycled Aggregate (RA) of 0%, 10%, 20%, 30%, 50%, and 100%. Detail proportions with mix designations are given below in Table 1.
Table 1 Mix Proportions A with fly ash | ||||||
Designation | Water | Cement | Fly Ash | Fine Aggregate | Coarse Aggregate | |
Recycled | Natural | |||||
AF00R00 | 0.5 | 1 | 0 | 1 | 0 | 2 |
AF00R10 | 0.5 | 1 | 0 | 1 | 0.2 | 1.8 |
AF00R20 | 0.5 | 1 | 0 | 1 | 0.4 | 1.6 |
AF00R30 | 0.5 | 1 | 0 | 1 | 0.6 | 1.4 |
AF00R50 | 0.5 | 1 | 0 | 1 | 1 | 1 |
AF00R100 | 0.5 | 1 | 0 | 1 | 2 | 0 |
AF10R00 | 0.5 | 0.9 | 0.1 | 1 | 0 | 2 |
AF10R10 | 0.5 | 0.9 | 0.1 | 1 | 0.2 | 1.8 |
AF10R20 | 0.5 | 0.9 | 0.1 | 1 | 0.4 | 1.6 |
AF10R30 | 0.5 | 0.9 | 0.1 | 1 | 0.6 | 1.4 |
AF10R50 | 0.5 | 0.9 | 0.1 | 1 | 1 | 1 |
AF10R100 | 0.5 | 0.9 | 0.1 | 1 | 2 | 0 |
AF20R00 | 0.5 | 0.8 | 0.2 | 1 | 0 | 2 |
AF20R10 | 0.5 | 0.8 | 0.2 | 1 | 0.2 | 1.8 |
AF20R20 | 0.5 | 0.8 | 0.2 | 1 | 0.4 | 1.6 |
AF20R30 | 0.5 | 0.8 | 0.2 | 1 | 0.6 | 1.4 |
AF20R50 | 0.5 | 0.8 | 0.2 | 1 | 1 | 1 |
AF20R100 | 0.5 | 0.8 | 0.2 | 1 | 2 | 0 |
AF30R00 | 0.5 | 0.7 | 0.3 | 1 | 0 | 2 |
AF30R10 | 0.5 | 0.7 | 0.3 | 1 | 0.2 | 1.8 |
AF30R20 | 0.5 | 0.7 | 0.3 | 1 | 0.4 | 1.6 |
AF30R30 | 0.5 | 0.7 | 0.3 | 1 | 0.6 | 1.4 |
AF30R50 | 0.5 | 0.7 | 0.3 | 1 | 1 | 1 |
AF30R100 | 0.5 | 0.7 | 0.3 | 1 | 2 | 0 |
AF40R00 | 0.5 | 0.6 | 0.4 | 1 | 0 | 2 |
AF40R10 | 0.5 | 0.6 | 0.4 | 1 | 0.2 | 1.8 |
AF40R20 | 0.5 | 0.6 | 0.4 | 1 | 0.4 | 1.6 |
AF40R30 | 0.5 | 0.6 | 0.4 | 1 | 0.6 | 1.4 |
AF40R50 | 0.5 | 0.6 | 0.4 | 1 | 1 | 1 |
AF40R100 | 0.5 | 0.6 | 0.4 | 1 | 2 | 0 |
These mixes were prepared tested in laboratory for fresh and hardended properties of RAC. Comparisons for the different results are prepared with graphs.
Materials Testing
The fly ash used here was in accordance with its IS 3812 (Part 1): 2003 [15]. Chemical requirem- ents and physical requirements of fly ash were in accordance with table 1 and table 2 of IS 3812 (Part 1): 2003 [15] respectively.
An OPC 53 grade conforming to IS 12269 – 1987 [16] was used throughout the study for concrete production. The samples of cement were taken in accordance with the requirements of standard IS: 3535-1986[17] and tested by using the relevant specification of IS [18-27]. All results were shown in Table 2 with their respective IS requirements for comparisons.
Table 2 : Results of Cement | |||
No | Type of Test | Results | IS - Limits |
1. | Type of Cement | OPC 53 Grade | - |
2. | Fineness Test | 8% | <10% |
3. | Normal Consistency | 29% | - |
4. | Soundness Test. | 4.9 mm | < 10 mm |
5. | Initial Setting time | 85 Minutes | >30 min |
6. | Final Setting time. |
198 Minutes |
|
7. |
Compressive Strength Test | 27.87MPa (3 – Days) | >27 MPa |
37.66MPa (7 – Days) | >37 MPa | ||
54.73MPa (28 – Days) | >53 MPa |
Fine and Coarse Aggregates
Figure 1: Grading Curve of Fine and Coarse Aggregate |
Elongation and flakiness index tests were carried out in accordance with IS: 2386 (PART I) – 1997[19], while specific gravity, apparent specific gravity, water absorption and bulk density were determined using IS: 2386 (PART III) – 1997[20]. Results were shown in Table 3.
Table 3 :Basic properties of aggregates | |||
NCA | RA | NFA | |
Specific Gravity | 2.84 | 2.35 | 2.70 |
Apparent Sp. Gravity | 2.77 | 2.31 | 2. 67 |
Water Absorption | 0.89% | 7.57% | 0.80% |
Bulk Density (kg/m3) | 1490 | 1290 | 1480 |
Apparent Density (kg/m3) | 2820 | 2510 | 2680 |
Flakiness Value (%) | 14.4 % | 25.33 % | - |
Elongation Value (%) | 16.45 % | 21.50 % | - |
Table 4 : Mechanical properties of Aggregates | ||
NCA | RA | |
Impact Value (%) | 8.88 % | 15.03 % |
Crushing Value (%) | 14.04 % | 25.52 % |
Abrasion Value (%) | 15.58 % | 26.70 % |
IS Limit (For Road) (%) |
30.00 % |
|
IS Limit (For Buildings) (%) |
|
Table 5 : Workability for Mix A with different % of FA and RCA (in slump value) | ||||||
Fly ash mix | Different % of RA | |||||
0 | 10 | 20 | 30 | 50 | 100 | |
AF00 | 135 | 125 | 110 | 85 | 75 | 60 |
AF10 | 135 | 130 | 115 | 90 | 85 | 75 |
AF20 | 140 | 135 | 120 | 95 | 85 | 80 |
AF30 | 145 | 135 | 120 | 100 | 90 | 80 |
AF40 | 155 | 145 | 125 | 105 | 100 | 90 |
Mechanical properties of aggregates were determined by using IS: 2386 (Part IV) – 1997[21] and shown in table 4 below. It shows that all aggregates results were within prescribed IS limits.
Concrete Testing
Sampling and analysis of concrete have been carried out as per IS: 1199 – 1959[18]. The quantities of cement, fly ash, silica fume, each size of aggregate, and water for each batch was determined by weight, to an accuracy of 0.1 percent of the total weight of the batch. Fresh properties of concrete (i.e. workability) was measured with help of slump cone test and test was carried out in accordance with IS:1199 – 1959[18]. The cubical moulds were used 150x150x150 mm while cylindrical mould was of 150 mm diameter and 300 mm height and both conforming to IS: 10086-1982[32]. All strength tests were carried out in accordance with IS: 516 – 1963 [33], while split tensile strength of concrete was measured in accordance with IS: 5816 – 1999[30].
Results and Discussions
Fly ash is spherical, and when used in concrete typically increases slump or reduces the required mixing water. However, results in figure 2 indicates that the slump of fresh concretes containing recycled aggregates and fly ash increased slightly with increased fly ash replacement. For example, AF00, AF10, AF20, AF30 and AF40 with 100% replacement of recycled aggregate had initial slumps of 60, 75, 80, 80 and 90 mm respectively. The slightly greater slump of fresh concrete containing greater fly ash replacement could have been caused by a greater volume of paste (ground fly ash had a much lower specific gravity than Portland cement), leading to reduced aggregate particle interference and enhanced concrete workability. In addition, perhaps not all of the fly ash particles were ground and sufficient spherical particles exist to help lubricate the mixture. This result was similar to Ravina (1984)[34] who found that fly ash can reduce slump loss when it is partially used to replace cement.
Ratio of different quantities i.e. fct/fck , fcr/fck or E/fck gives clear idea regarding relationship of both quantities with % of recycled coarse aggregates. Again similar pattern of results were observed for all graphs.
Figure 2: Workability for Mix A with different % of FA and RCA | Figure 3: fck7 for Mix A with different % of FA and RCA (MPa) |
Figure 4: fck for Mix A with different % of FA and RCA (MPa) | Figure 5: Ratio of fct/fck for Mix A with different % of FA and RCA |
Figure 6: Ratio of fcr/fck for Mix A with different % of FA and RCA | Figure 7: Ratio of E/fck for Mix A with different % of FA and RCA |
Table 6 : fck7 for Mix A with different % of FA and RCA (MPa) | ||||||
Fly ash mixes | Different % of RCA | |||||
0 | 10 | 20 | 30 | 50 | 100 | |
AF00 | 28.4 | 27.95 | 27.17 | 26.34 | 23.87 | 21.25 |
AF10 | 27.75 | 27.1 | 26.33 | 24.9 | 22.5 | 20.2 |
AF20 | 26.5 | 25.85 | 24.88 | 23.56 | 21.89 | 19.41 |
AF30 | 25.07 | 24.25 | 23.5 | 22.33 | 20.6 | 18.12 |
AF40 | 23.2 | 22.5 | 21.75 | 20.66 | 19.1 | 16.15 |
Table 7 : fck for Mix A with different % of FA and RCA (MPa) | ||||||
Fly ash mixes | Different % of RCA | |||||
0 | 10 | 20 | 30 | 50 | 100 | |
AF00 | 40.71 | 40.33 | 39.67 | 37.5 | 33.6 | 30 |
AF10 | 40.41 | 39.16 | 37.75 | 35.67 | 32.46 | 29.5 |
AF20 | 39.23 | 37.5 | 35.67 | 34.02 | 31.81 | 27.95 |
AF30 | 33.2 | 32.21 | 31.14 | 30.46 | 28.88 | 25.37 |
AF40 | 30.1 | 29.89 | 28.49 | 27.58 | 26.11 | 22.83 |
Table 8 : fct for Mix A with different % of FA and RCA (MPa) | ||||||
Fly ash mixes | Different % of RCA | |||||
0 | 10 | 20 | 30 | 50 | 100 | |
AF00 | 4.62 | 4.59 | 4.54 | 4.15 | 3.52 | 2.97 |
AF10 | 4.61 | 4.52 | 4.21 | 3.89 | 3.2 | 2.81 |
AF20 | 4.41 | 4.12 | 3.78 | 3.45 | 3.02 | 2.55 |
AF30 | 4.25 | 4.01 | 3.7 | 3.5 | 3.2 | 2.6 |
AF40 | 4.2 | 3.93 | 3.6 | 3.38 | 3.04 | 2.5 |
Table 9 : fcr for Mix A with different % of FA and RCA (MPa) | ||||||
Fly ash mixes | Different % of RCA | |||||
0 | 10 | 20 | 30 | 50 | 100 | |
AF00 | 4.62 | 4.59 | 4.54 | 4.15 | 3.52 | 2.97 |
AF10 | 4.61 | 4.52 | 4.21 | 3.89 | 3.2 | 2.81 |
AF20 | 4.41 | 4.12 | 3.78 | 3.45 | 3.02 | 2.55 |
AF30 | 4.25 | 4.01 | 3.7 | 3.5 | 3.2 | 2.6 |
AF40 | 4.2 | 3.93 | 3.6 | 3.38 | 3.04 | 2.5 |
Table 10 : E for Mix A with different % of FA and RCA (MPa) | ||||||
Fly ash mixes | Different % of RCA | |||||
0 | 10 | 20 | 30 | 50 | 100 | |
AF00 | 26729 | 26646 | 26500 | 26007 | 25073 | 24143 |
AF10 | 26664 | 26386 | 26065 | 25577 | 24786 | 24008 |
AF20 | 26401 | 26007 | 25577 | 25177 | 24619 | 23580 |
AF30 | 24973 | 24722 | 24445 | 24266 | 23839 | 22831 |
AF40 | 24170 | 24114 | 23731 | 23476 | 23051 | 22042 |
Conclusions
- 10% of RA and 10% of fly ash is giving the same results as that of 100% NA and 0% fly ash.
- Workability for fresh concrete was increased with increase of incorporation of fly ash. Though due to incorporation of RA workability is going on decreasing with increase of % of RA
- Compressive strength of mix A prepared with 100% RA and 40% fly ash are lower than 100% NA and 0% fly ash concrete, by nearly 44%.
- Ratio of fct/fck or fcr/fck is giving clear idea regarding change of values for 40% fly ash incorporation.
Acknowledgement
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