STUDY OF USAGE OF BOTTOM ASH AS PART REPLACEMENT OF SAND FOR MAKING CONCRETE BLOCKS

Transcription

1 STUDY OF USAGE OF BOTTOM ASH AS PART REPLACEMENT OF SAND FOR MAKING CONCRETE BLOCKS Satish Sharma*, V.V. Arora *, Adarsh Kumar N S * Centre for Construction Development & Research * National Council for Cement and Building Materials ABSTRACT In coal based thermal power stations large quantity of bottom ash is generated as by product. Bottom ash is collected at the bottom of boiler furnace characterized by better geotechnical properties and good material for fill, embankment construction etc. Bottom Ash consists of about 20% of total ash produced in a thermal power station. There is need to develop alternative for sand for use in concrete works in India. Studies have been carried out (1-6) to evolve bottom ash as alternative to sand for use in concrete. With this objective, bottom ash containing fines of flyash have been tried for manufacturing concrete blocks. In the present investigation, laboratory studies have been carried out at NCB laboratory to utilize bottom ash as part replacement of sand in concrete. This study covers manufacturing of concrete blocks without flyash & with bottom ash for making solid blocks as per specification laid down in IS: 2185 using vibro compaction machine available in NCB. Three different sources of bottom ash were used in concrete mix 30%, 40% & 50% replacement by weight of sand for making concrete blocks. Comparative study of compressive strength of concrete at different age of curing, wet density, drying shrinkage is reported in this study. Wet density is found to be lower in blocks containing bottom ash & dry shrinkage values are found well within the limits of specifications. Concrete Blocks having bottom 30% by weight of sand are found suitable for use in the manufacture of concrete blocks. 1.0 BACKGROUND The paper shows the experimental investigation about bottom ash replacement in different percentage of fine aggregate. P. Aggarwal, et al have investigated the replacement of bottom ash upto 50% replacement by using the superplasticizer. The study of replacement of bottom ash is done by using high range water reducing admixture because as the replacement of fine aggregate increases the water demand rapidly increases which also enhances due to high specific surface area bottom ash due to high fine content. Hence by this, all mechanical properties get affected due to less cementitious material available in per cubic meter content in early age of concrete mix. Detailed studies done are reported in references. Bottom Ash (BA) is a combination of heavier particulate matter and molten slag, which forms on the walls and the bottom of the combustion chamber of power station boiler fired with pulverized fuel. In appearance it usually ranges from a highly verified, glossy and heavy material to a lightweight, open textured and more friable type. Sometimes it is found mixed with fly ash in stockpiles. Its precise nature depends on the boiler plant and the coal type but higher fines of fly ash in bottom ash are generally a part of bottom ash (BA).

2 This study utilized bottom ash of Unchahar Super Thermal Power Plant (NTPC) currently there are two boiler systems wet and dry. Each system produces a uniquely different BA wet BA and dry BA. Wet BA has a relatively low melting point and coalesces into large molten masses known as slag. Its particles usually are angular to sub angular in shape, have a smooth surface texture and looked much like crushed glass. Dry BA has quite angular particles and a highly porous surface texture, usually has the appearance similar to fine sand and lighter in weight per unit volume and colour than bottom ash (2, 3). Compared with fly ash, BA usually has no cementitious properties, very little pozzolonic activity, coarse particle size, higher carbon content and higher water demand. Therefore, previous studies are mainly focused on using it in blocks, highway sub-base materials, structural filling materials etc. This has led to little recycling and large dumping of BA in landfill sites, which has gradually become a serious environmental problem in both developed and developing countries. Furthermore, owing to the construction boom in both developed and developing countries, there is a critical shortage of good quality sand in many areas throughout the world. Therefore, there is a need for reducing the use of natural aggregate by recycling industrial by products. This study was carried out by using BA from a thermal power plant, to replace part of sand in concrete. The main purpose was to study the possibility of using BA to make a lightweight, high performance and environmentally friendly concrete. The effect of BA on the workability, compressive strength and drying shrinkage of concrete are reported in this paper. 2.0 TRIALS CONDUCTED 2.1 Selection of Mix Proportion With Different W/C Ratio Trial mixes were prepared by using bottom ash obtained from three different boilers of NTPC s Unchahar Thermal Power Plant (U.P) to optimize the mixes at different water cement ratio. Direct replacement of bottom ash (by weight of coarse sand in percentage) was adopted to determine the suitability of the mix for manufacture of concrete blocks. Details of trials are given below. Selection of W/C ratio & Ratio of concrete mix W/C Ratio of Cement : Sand : Aggregate by Weight Ratio 1:3:6 1:3.125:6.25 1:2.5: Seems more water required for casting to block in vibro Blocks cast but did not give clear dimensions, so the mix machine was rejected option 0.50 Less sand & less water content observed Block cast successfully Block cast successfully but did not give cost-effective Block cast successfully 0.55 Less sand content observed Block cast successfully Block cast successfully Ratio of 1:3.125:6.25 for mix proportion corresponding to W/C ratio 0.50 was selected on the basis of these initial trials on mouldability in Vibro compaction machine. Method of mix design by

3 replacing the ratio by weight was found suitable. Water demand was found higher due to higher water absorption by bottom ash. The mix proportions thus optimized are given as under. Optimised Mix Proportion for Trials without using Bottom Ash: Ingredient in Mix Cement Coarse Sand (Natural) 10 mm Aggregate (Crushed Stone) Free W/C Mix proportion Replacement of Sand with Bottom Ash Optimized mix proportion for three types of Bottom Ash collected and identified as BA I, BA II and BA III were selected for the trials. Type of Mix using BA % BA Replacement Cement, Kg FA, Kg BA, Kg CA, Kg Free Water Control Mix Nil 50 4 BA I (Bottom Ash) BA II (Bottom Ash) BA III (Bottom Ash) (Note: FA Fine Aggregates, BA Bottom Ash & CA Coarse Aggregate) 2.2 Procedure for Making Concrete Blocks without Bottom Ash & also using three sources of Bottom Ash using Vibro Compaction Technique Step I Step II Step III Step IV Step V Step VI Step VII Take all the material as per batch given above. Make a dummy trial batch (around 10% quantity with same proportion) and remove from concrete mixture so that a thin layer of mix can adhere with surface of mixture for simulating the continuous batch mix. Now add all the quantity (except aggregate) and mix it for 60 seconds. Now add aggregate and mix the total batch for 120 seconds. Now remove the batch to a non-water absorbing surface and fill the mould to cast the block in vibro compaction machine. When blocks are filled to total mould volume, vibrate for 60 seconds and remove from mould. Lift the cast block for drying (in shaded area) for 24 hours. Cure the block for 3 days with gunny bags/in moist environment and then cure for 7 days in submerged condition & there after cure the block stacks in air by sprinkling water.

4 2.3 Material Test Results Cement The results of cement are found complying with the Codal provision laid down in IS: Table 1 highlights physical and chemical properties of 43 grade Ordinary Portland Cement (OPC) used in this study. Table 1: Physical and Chemical Characteristics of 43 Grade Ordinary Portland Cement Sl No. Test Results Obtained 1 Blain s Fineness, m 2 /kg Setting Time, minutes: Initial Final 3 Soundness : Le Chatlier Exp., mm Autoclave Exp., % 4 Compressive Strength, N/mm 2. 3 days 7 days 28 days 5 Loss on Ignition Silica (SiO 2 ) Iron Oxide (Fe 2 O 3 ) Aluminium (Al 2 O 3 ) Calcium Oxide (CaO) Sulphate (SO 3 ) Magnesium Oxide (MgO) 6 Alkalies : Na 2 O K 2 O Coarse sand The coarse sand confirm to Zone II as per the provisions laid down in IS: Table 2 gives the physical properties of sand used in this study.

5 Table 2: Physical Characteristics of Fine Aggregates Sl No. Parameter/Tests Test Value 1 Silt content 2.33% 2 Clay lumps Nil 3 Organic impurities Nil 4 Sp. Gravity Water absorption 0.90% 6 Sieve analysis Sieve Size Percent Passing 10 mm mm mm mm micron micron micron Pan Nil Coarse aggregate The coarse aggregate is found to comply with the single size grading as per IS: Physical characteristics of coarse aggregate are shown in Table 3. Table 3: Physical Characteristics of Coarse Aggregates Sl No. Parameter/tests Test Results 1 Specific gravity Water absorption 0.40% 3 Crushing value 24.73% 4 Impact value Abrasion value Soundness 0.30% 7 Flakiness Index (< 10 mm) 29% 8 Elongation Index (<10 mm) 32.76% 9 Sieve analysis Sieve Size Percent Passing 10 mm 96.06% 4.75mm 16.40% 2.36mm Nil

6 2.3.4 Bottom Ash Physical and Chemical Characteristics of Bottom Ash I, II & III are shown in Table 4. Table 4: Physical and Chemical Characteristics of Bottom Ash Sl No. Test BA I BA II BA III Physical Test 1 Water absorption (%) Sp. Gravity Lime reactivity (Kg/cm 2 ) Sieve Analysis % Passing micron micron micron Pan Chemical Test 1 Loss on Ignition Silica (SiO2) Iron oxide Aluminum oxide Calcium oxide (CaO) Magnesium oxide (MgO) BA I=Bottom Ash I BA II=Bottom Ash II BA III=Bottom Ash III 3.0 TEST RESULTS AND DISCUSSION 3.1 Compressive Strength The compressive strength of concrete blocks made with bottom ash I, II and III is shown in Table 5. From these results, we conclude that 30% replacement of bottom ash I, II & III gives results of compressive strength almost similar to control concrete. We also see that the compressive strength of concrete blocks using bottom ash decreases at early age but slightly increases further at 56 day & 90 day. When we see the results of higher replacement of bottom ash (i.e. >30%), there is further no substantial increase in compressive strength.

7 Compressive Strength at Different Age (Day) Using Bottom Ash I for NTPC, Unchahar (Source I) Using Bottom Ash II for NTPC, Unchahar (Source II) Using Bottom Ash III for NTPC, Unchahar (Source III) Table 5: Compressive Strength of Concrete Blocks Control Mix Percentage Replacement of Bottom Ash with Sand 30% 40% 50% Wet Density & Drying Shrinkage of Concrete Block Wet density of concrete blocks Wet density of concrete blocks obtained on control mix (without bottom ash) and those obtained on mixes by replacement of sand with bottom ash is shown in Table 6. The density of concrete block is found to decrease with increase in percentage of bottom ash. Table 6: Wet Density of Concrete Blocks (in Kg/m 3 ) Bottom Ash Replacement Percentage 30% 40% 50% Control Mix BA-I BA-II BA-III Drying shrinkage of concrete block Drying Shrinkage tests on control mix and mix using bottom ash was conducted after 28 days of curing as per IS 2185 (Part II) The test results are shown in Table 7. The drying shrinkage of all the three bottom ash samples using 30 %, 40 % & 50 % replacement by weight of sand is found to be higher than the control mix. The drying shrinkage of course is found within the permissible limits of IS code. Table 7: Drying Shrinkage of Concrete Blocks Bottom Ash Replacement Percentage 30% 40% 50% Control Mix BA-I % BA-II % BA-III %

8 4.0 CONCLUSION AND RECOMMENDATIONS i) Bottom Ash generally has higher fine particle content and higher water absorption. This is in line with the findings of the other authors as outlined in the literature survey carried out in NCB. ii) Test results of Compressive Strength of these concrete blocks shows that with 30% replacement of sand with bottom ash BA I, BA II & BA III gives results of 28 days compressive strength almost similar to control concrete. iii) Wet Density of concrete blocks using bottom ash BA I, BA II and BA III using 30%, 40% & 50% replacement of sand with bottom ash is found to be lower than the concrete blocks without bottom ash. iv) Drying shrinkage is found to be slightly higher in bottom ash samples prepared by cutting the concrete blocks samples using 30%, 40% & 50% of bottom ash as compared to control concrete blocks without bottom ash. v) For all the three sources of bottom ash, 30% replacement of sand with bottom ash is found meeting the strength requirement of IS: vi) Further R&D efforts are required on bottom ash after removal of fines to optimize the usage of bottom ash as alternative to sand for usage in different grades of concrete. 5.0 REFERENCES 1. P Aggarwal, Y Aggarwal, S M Gupta, Civil Engineering Department, National Institute of Technology, Kurukshetra, Effect of Bottom Ash as Replacement of Fine Aggregates in Concrete, Asian Journal of Civil Engineering (Building and Housing) Vol. 8, No. 1 (2007) Pages Bai Y and Barheer P A M Influence of Furnace Ash on Workability, Compressive Strength and Durability of Concrete Supplementary Papers, Seventh CANMET/ACI International Conference on Fly Ash, Silica Fume, Slug and Natural Pozzolana in Concrete, Chennai (Madras), India, July 22-27, Ghafoori, N Utilisation of Type C Bottom Ash in Cement Based Concrete Mixtures, Annual Report, Material Technology Center, Southern Illinois University at Carbondale, May 1992, pp Ghafoori, N and Cai &, Laboratory Made Roller Compacted Concretes Containing Dry Bottom Ash : Part I Mechnical Properties, ACI Materials Journal, V95, No. 2, March April 1998, pp Cheriaf M, Cavalcante R J and Pera J, Pozzolanic Properties of Pulverized Coal Combustion Bottom ash, Cement and Concrete Research 29, 1999, pp IS: Indian Standard Specification for coarse and fine aggregates from Natural sources for concrete. 7. IS: Indian Standard Specification for 43 grade Ordinary Portland Cement 8. IS:2185(Part II) 1983 Indian Standard Specification for Concrete Masonry Units Hollow and Solid Lightweight Concrete Blocks