EFFECT OF BOTTOM ASH ON THE PROPERTIES OF CONCRETE
This study deals with the experimental investigation carried out to study the feasibility of utilizing bottom (a courser material, which falls into the bottom of furnace in thermal power plants) as a replacement for fine aggregates. The properties of bottom ash in particular make it a desirable admixture for concrete as a supplement material, considerably lowering the percentage of fine aggregates, in addition to reduced water demand. The compressive strength, split tensile strength and flexural strength of different modified concretes are compared for the possible replacement ratios (0%, 25%, 50%, 75% and 100% by bottom ash). Tests were conducted on the samples of various time periods (1 day, 7 days, 28 days and 90 days) cured by normal curing. The experimental details and results are discussed in the paper.
Bottom ash, Mix design, Compressive strength, Split tensile strength, Flexural strength
F OR many decades concrete has been using as a common building material and has become the backbone of Indian construction industry and the need for natural materials got increased day by day. Due to the increased use of natural materials, shortage occurs and cost of materials increased gradually. Because of this reason people started thinking about a material which can used as a replacement for natural materials and keeping this in mind more and more researches has been undergone in this field.
The recent boom in industrial sector demands large quantity of power generation. For the generation of electricity countries like India depends mainly on thermal power plant where mostly coal is used as the raw material, which is burned to generate electricity. As a result large quantity of waste materials is being generated each day, bottom ash (BA) is one among the waste material generated. These wastes are disposed either in open area available near the plant or by mixing with water and pumping into artificial lagoon and sometimes the mines itself. As the materials are in powdered format, it may be air bone and may be a reason for pollution. Recent studies on material properties of BA found out characteristics such as shape, size and porosity similar to that of fine aggregates (FA).
This paper deals with the experimental investigation carried out to study the effect of use of BA for FA, but the study on the use of BA has been very limited.
The use of fly ash in the production of concrete, both in the replacement of cement and in the substitution of the natural FA, have been widely presented as a suitable solution for the reduction of this waste. Though significant results have been reported on the use of fly ash in concrete, not much literature were available on the use of BA as a replacement of FA.
BA has been targeted in some publications for its use in concrete and mortar [1-3]. The most important properties of BA are the size and shape of the particles and the porosity. Such properties depend on the burning efficiency, the method in which the BA is obtained and the type of combustion. This in turn forms minute molten and well vitrified particles around 30 μm, this satisfies the basic requirements of a fine aggregate for concrete and mortar, even if grain size distribution varies. There is also pozzolanic potential, although this is low due to the grain size [4–9].
The utilization potential of BA is determined by its physical characteristics such as grain size. With adequate grinding, the pozzolanic activity of bottom ash can be improved and be used as a low-cost replacement for more expensive sands in concrete .
53 grade Ordinary Portland Cement confirming to IS: 12269-1987 reaffirmed in 2004 was used . Its properties are shown in Table 1.
Table 1 Cement characteristics
Consistency of Cement
Initial Setting Time
Final Setting Time
Fineness of Cement
Natural sand confirming to Zone II with a specific gravity of 2.65 was used. The testing was done as per IS: 383-1970 reaffirmed in 1997 . Figure 1 shows the grading curve for bottom ash.
Coarse aggregate used having 20 mm normal size with a specific gravity of 2.67. Figure 2 shows the grading curve coarse aggregates.
Bottom ash used for the experiment is obtained from Neyveli lignite thermal power plant at Tamil Nadu, in India, having a specific gravity of 2.51 and fineness modulus of 6.9. Figure 3 shows the grading curve for bottom ash.
Superplasticizer used is an Aqueous Solution of Sulpho Naphthalene Formaldehyde based Ceraplast 300 a high range water reducing admixture. The use of these materials benefits in larger reduction of water-cement ratio, early strength, early removal of formwork and good quality concrete of lower permeability and improved durability.
Table 2 shows the various mix design details. Five mix proportions where made for the investigation. The first one was reference mix without bottom ash, which is proportioned as per Indian Standard Specification IS: 10262-1982 reaffirmed in 2004 . Another four mixes contains bottom ash by weight as replacement for fine aggregates in various levels say 25%, 50%, 75% and 100%. The mix proportion of the specimen corresponds to 1:1.57:2.36:0.4. Machine mixing was done for the concrete mixes.
Table 2 Mix design
Mix proportion (Kg/m³)
CASTING OF TEST SPECIMEN
Specimen preparation for various strength tests was performed at room temperature. Machine mixing was done for concrete mixes and vibrating table is used for proper compaction. Per batch twelve numbers of 100 mm cubes where casted for compressive strength. Twelve cylinder of size 100×200 mm where casted for split tensile strength, in addition to that three cylinders of same size was casted for 28 days compressive test. Twelve number of `100×100×500 mm prisms where casted for flexural strength. After casting the specimens where stored in laboratory at room temperature for 24 hours and then demolded and placed in water for curing. One day strength is done soon after the demolding and the specimens kept for curing is taken out of water 15 min before testing and dried with cloth. The strength values were the average of three specimens and the values of concrete mix with additives were compared with reference mix.
The results show a gradual decrease in compressive strength of cube for all the test results. As the percentage of bottom ash is increasing the compressive strength is decreasing. There is been considerable increase comparing the 1day and 7 days strength and also with 7 days and 28 days strength. This shows it can be used as structural concrete. The replacement till B3 shows not much decrease as compared with B4 replacement. So the optimum replacement is till 75%.
The compressive strength of cube is higher compared to that of cylinder. The compressive strength of both R mix does not have much of a difference compared to that of B4 which has almost 75% higher value for cube than for the cylinder.
CYLINDER-Split Tensile Strength
Figure 6 shows the split tensile strength of cubes for 1 day, 7 days and 28 days having various levels of replacement. At 1 day time the split tensile strength value of reference mix is very much on higher side when compared to the other mixes containing various levels of replacements. On 7 days, when compared to the reference mix the split tensile strength of replacement mixes say B1, B2, B3 and B4 are gradually increased. The target mean strength to be achieved on the 28th day is kept at _ _ N/mm². On the 28th day testing of split tensile strength the test value of B1, B2, B3 and B4 shows favourable result on comparison with the reference mix.
Flexural strength values of prism having various levels of replacements are shown in figure 7. Flexural strength value is tested for 1 day, 7 day, 28 days and 90 days and is compared with the reference mix. On 1 day test except B4 the mixes B1, B2 and B3 shows a favourable result. On 7 days the trend is been changed, the value is been considerably reduced. But on 28 days the flexural strength values of B1, B2, B3 and B4 is considerably increased and achieved more than the required target mean strength.
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