Strength Properties Of Class 'C' Fly Ash Concrete With Variable Aggregate Binder Ratio

Transcription

1 Strength Properties Of Class 'C' Fly Ash Concrete With Variable Aggregate Binder Ratio 39 Strength Properties Of Class 'C' Fly Ash Concrete With Variable Aggregate Binder Ratio Rama Mohan Rao P. and H. Sudarsana Rao Abstract : In this paper the experimental work has been carried out to study the strength characteristics of class 'C' fly ash concrete with cement replacement levels by 30%, 40% and 50%. The aggregate to binder (a/b) ratio of.50 and.00 with water binder (w/b) ratios of 0.35, 0.40, 0.45 and 0.50 for each concrete mix. In total twenty two fly ash concrete mixes each of different mix proportions of fly ash, cement, water binder ratios were cast and their characteristics were studied. For each fly ash concrete mix consisting of standard sizes of cubes, cylindrical and prisms specimens were cast and tested for compressive strength at 8 and 90 days and split tensile strength, flexural strength. at the ages of 8 days. The test results were analysed and compared among the two aggregate binder ratios. The water/binder ratio increases, the compressive strength at 8 days for 30%, 40% and 50% fly ash replacement levels were reduced. The concrete mix having 40% fly ash replacement and water/ binder ratio 0.40 shows better split tensile strength of aggregate binder ratio of.00. Keywords : Fly Ash, Aggregate Binder Ratio, Compressive Strength, Split Tensile Strength, Flexural Strength INTRODUCTION Environmental and economic conditions require the revision of present concrete making methods for constructional activities. Mineral admixtures such as fly ash, slag and silica fume, as a replacement for Portland cement in concrete presents one viable solution with maximum benefits for the sustainable development of the construction industry. The benefit of replacing the Portland cement with mineral admixtures is the alteration of the mechanical properties of the fresh and hardened state of concrete. This influences faster or slower rates of setting and hardening, less heat of hydration, enhanced durability in aggressive environments and gain of high strength. Fly ash is one of the industrial by product from thermal power plants. The availability of this mineral admixture is plenty. The standard categories of fly ash are two classesclass 'F' and class 'C'. While the fly ash containing equal to or more than 0% lime is classified as class 'C', the one containing less than 0% lime is classified as class 'F'. In this study, Class 'C' Fly ash with different percentage replacement levels with different water binder ratios were used to find out the strength properties of concrete. LITERATURE REVIEW Ozkan Sengul et. al studied the effect of partial replacement [0% to 70%] of cement by fly ash in concrete on its compressive strength, brittleness index and chloride penetration. He reported that high volume fly ash concrete has decreased compressive strength at 8 days, strength has Assistant Professor (SG), CDMM, VIT University, Vellore, India. . : rao_pannem@vit.ac.in Professor of Civil Engineering, JNTU College of Engineering, Anantapur, India

2 40 NICMAR-Journal of Construction Management, Vol. XXVII, No. 4, Oct.-Dec. 0 improved at later ages i.e. 56 and 0 days, and better resistance to chloride ion penetration. Serdar Ayd?n et. al investigated the effects of ASTM Class 'C' fly ash incorporation on mechanical properties and sulfuric acid resistance of concrete with fly ash up to 70% replacement of cement. According to their study, long-term strength values decreased significantly for concrete mixtures over 30% of fly ash replacement levels and sulfuric acid resistance of steam-cured concrete could be improved significantly by incorporation of fly ash. Oner et. al reported that the strength of fly ash concrete increases with increase upto 40% optimum level of replacement. Naik T.R. et. al showed the effect of class 'C' fly ash on the abrasion resistance of concrete. They noticed that at 40% fly ash replacement level, the abrasion resistance was not influenced, but at 50% fly ash replacement level, the abrasion resistance decreased with reference mixture without fly ash. Jerath S. and Hanson N showed that the increase of fly ash content from 30 to 45% increased the durability of concrete mixtures without the loss of compressive and flexural strengths. The concrete mixtures containing dense graded aggregates, and higher percentage of fly ash required less water content indicating the beneficial effects of fly ash and dense graded aggregates. When the fly ash content was increased from 30 to 45% in the concrete mixtures, the permeable pore space decreased in the specific gravity, absorption, and voids in hardened concrete tests. Naik Tarun R et. al developed mix proportions for paving roadway concrete with 0 and 50% Class C fly ash and 40% Class 'F' fly ash replacements for Portland cement. These mixes were tested for compressive strength, flexural strength, tensile strength, and freezing and thawing durability data. They observed that high volumes of Class 'C' and Class 'F' fly ash concrete mixtures can be used to produce good quality pavements in concrete with excellent durability properties. MATERIALS USED Cement Ordinary Portland cement of 53 grade available in local market was used. The specific gravity and fineness of cement were 3.4 and 97% respectively. Coarse Aggregate Crushed angular granite of mm size from a local source was used as coarse aggregate. The values of specific gravity and fineness modulus of coarse aggregate were.68 and 7.66 respectively. Fine Aggregate River sand available in local source comprising of zone-iii was used as fine aggregate. The fine aggregate had specific gravity of.6 and fineness modulus.5 respectively. Fly Ash Fly ash was procured from Neyveli Lignite Corporation Limited, Neyveli Tamilnadu. The fly ash was collected from this plant fall under Class C category. The fly ash was used to replace in various percentages of weight portion of cement content. The specific gravity of fly ash is.5 Water Potable water was utilized for casting of all concrete specimens. CONCRETE MIX DESIGN Absolute volume method was used to find out the concrete mix proportions. Absolute volume proportioning method is an accurate method and involves use of specific gravity values for all the ingredients of mix to calculate the absolute volume and each will occupy in a unit volume of concrete. In this study the aggregate to binder ratio of.50 and.00 were used. For each concrete mix the water binder ratios of 0.35, 0.40, 0.45 and 0.50 were used. The replacement levels of class C fly

3 Strength Properties Of Class 'C' Fly Ash Concrete With Variable Aggregate Binder Ratio 4 ash 30, 40 and 50 percent by weight of the binder content were used for each concrete mix. MIXING AND CURING A concrete mixer of tilting type rotary drum was used for mixing of all the constituents of concrete. All the ingredients were placed in the mixer and the water was added during rotation. Initially 80% of the total water was added for mixing and after one and half minute, remaining water was added and mix was continued for another one minute. The mixed concrete was poured into the standard specimens conforming to IS: The specimens were placed on the vibration table for compaction. This procedure was repeated for all other batches of the concrete. The specimens were demoulded after 4 hours from the commencement of casting and placed into the curing tanks till the time of testing for their strength properties. TESTS ON HARDENED CONCRETE Compressive Strength Compression strength test was conducted on hardened concrete at the ages of 8 and 90 days of curing. The cubes were taken from the curing tank and air dried for half an hour before testing of specimens. The cubical specimens were placed under the compression testing machine of capacity 000 kn and tested in the available structural engineering lab. The compression strength of all combinations of water binder ratios with replacement levels of fly ash of variable aggregate binder ratios of.50 and.00 at the age of 8 and 90 days were tested. The test results are shown in the Fig. a, Fig. b, Fig. a and Fig. b respectively Split Tensile Strength The cylindrical specimens of size mm were cast and utilized for this test.. The test was carried out by placing a cylindrical specimen horizontally between the loading surfaces of the compression testing machine and the load was applied until failure of the cylinder, along the vertical cross section. The split tensile strength test results of all the concrete mixes of variable water binder ratio of 0.35 and 0.40 with aggregate binder ratios of.50 and.00 at the age of 8 days are shown in the Fig. 3a and Fig. 3b. Flexural Strength This experiment is to determine the maximum load carrying capacity of beam specimens. The prisms of size mm were cast and tested under digital universal testing machine of capacity 00kN. All the beams were loaded symmetrically under four point loading. The overall length of the test zone was 400 mm and the load was applied through two similar rollers mounted at the third point of the supporting span that is, spaced at 33mm center to center. The load was divided equally between the two loading rollers. The results obtained from the experimental test at the age of 8 days are shown in the Fig. 4a and Fig. 4b for different aggregate binder ratios of.50 and.00. RESULTS AND DISCUSSIONS Compressive Strength The compressive strength of cubical specimens at the age 8 days and 90 days were tested. Aggregate Binder Ratio.50 From Fig. a with w/b ratio of 0.35, the 30% fly ash as replacement material concrete mix shows better results than other replacements levels. From Fig. b, for w/b ratio of 0.40, the 40% fly ash as replacement material concrete mix shows better results than other replacements levels at the age of 90 days of curing. The improvement in the strength from 8 days to 90 days is varying from % to 3% for all the concrete mixes. This is

4 4 NICMAR-Journal of Construction Management, Vol. XXVII, No. 4, Oct.-Dec. 0 Figure a : Compressive Strength At 8 Days For A/B =.50 Figure b : Compressive Strength At 90 Days For A/B =.50 Figure a : Compressive Strength At 8 Days For A/B =.00 because of longer setting time of fly ash in the concrete matrix. For water binder ratio of 0.5 at 8 and 90 days, not much improvement in the strength of concrete with respect to fly ash replacement levels is seen. Aggregate Binder Ratio.00 From Fig a, for w/b ratio of 0.40, the 40% fly ash replacement concrete mix shows better strength results than other replacements levels as well as other water binder ratios. The fly ash 40% concrete mix shows drop in the strength for w/b ratio of 0.35 and gains the strength consistently for w/b ratio of 0.4, 0.45 and For 50% fly Figure b : Compressive Strength At 90 Days For A/B =.00 ash replacements, the mix shows strength decreases with increase of water binder ratio at the age of 8 days. The fly ash 30% concrete mix shows marginal gain strength at w/b ratio of 0.35 and drop in strength consistently for w/b ratio of 0.4, 0.45 and 0.50 compared to 40% fly ash concrete mix. From Fig. b, for fly ash 30% concrete mix at 90 days of curing shows improvement in the strength for all w/b ratios compared with other concrete mixes. The improvement in the strength from 8 days to 90 days is varying from % to 3% for all the concrete mixes. This is because of longer setting time of fly ash in the concrete matrix. For water binder ratio of 0.5, for all fly

5 Strength Properties Of Class 'C' Fly Ash Concrete With Variable Aggregate Binder Ratio 43 ash replacement at 90 days, there is not much improvement in the strength of concrete. Split Tensile Strength From Fig. 3a, for w/b ratio of 0.35 at 8 days of curing, the 40% fly ash replacement concrete mix shows better strength compared to other fly ash concrete mixes. The strength decreases with increases of fly ash content. From Fig. 3b, for w/ b ratio of 0.40, the 40% fly ash replacement concrete mix at 8 days of curing shows better strength compared to other fly ash mixes. Overall the aggregate binder ratio of.00 for concrete mixes shows better strength compared to aggregate binder ratio of.50 (Refer Fig. 4a & 4b). Flexural Strength From Fig.4a, the flexural strength of 30% fly ash concrete mix with w/b ratio 0.35 shows improvement strength for aggregate binder ratio of.50. The flexural strength decreases with increases in water binder ratio and percentage of fly ash replacement. From Fig 4b, the flexural strength of 40% fly ash concrete mix with w/b ratio 0.40 is better for aggregate binder ratio of.00.the improvement in increase in strength of Split tensile strength, MPa % 40% 50% Fly ash replacement P90d Q90d P8d Q8d Figure 3a : Split Tensile Strength For W/b = 0.35 Figure 3b : Split Tensile Strength For W/b = 0.40 Flexural str ength,mpa FA 30 % FA4 0% FA50% Flexural strength, Mpa FA 30% FA40% FA50% Water binder ratio Water binder ratio Figure 4a : Flexural Strength For Mixes Of A/B =.50 Figure 4b : Flexural Strength For Mixes Of A/B =.00

6 44 NICMAR-Journal of Construction Management, Vol. XXVII, No. 4, Oct.-Dec. 0 A/B ratio.00 is % to 7% compared with A/B ratio of.50. CONCLUSION From the experimental work carried out on the fly ash concrete mixes of aggregate binder ratio.50 and.00, the following conclusions are drawn. () The compressive strength values from 8 days to 90 days increase from % to 3% for aggregate binder ratio of.50 and.00. () The variation of compressive strength of concrete mixes of A/B =.50 shows marginally increases when compared with A/B =.00. (3) As the water binder ratio increases, the strength of concrete decreases for respective fly ash replacement levels. (4) For water binder ratio of 0.35, with A/B=.50, the 40% fly ash replacement mix shows better split tensile strength than other fly ash replacement levels. The split tensile values of A/B ratio.00 show higher improvement than at A/B ratio of.50. (5) The flexural strength increase for A/B ratio.00 is % to 7% compared with A/B ratio of.50. REFERENCES Indian standard code of practice for specification for fly ash use as a Pozzolona and admixture, IS:38 98, BIS, New Delhi. Indian standard method of test for splitting tensile strength of concrete, IS 586: 999, BIS, New Delhi. Indian standard methods of tests for strength of concrete, IS 56: 959 (Reaffirmed 999), BIS, New Delhi. Indian standard specification for 53 grade ordinary Portland cement, IS:69, BIS, New Delhi. Jerath S. and Hanson N., Effect of fly ash content and aggregate gradation on the durability on concrete pavements, Journal of Materials in Civil Engineering, Vol. 9, No. 5, pp , May 007. Naik, T.R., Singh S.S. and Ramme B.W. Effect of source of fly ash on abrasion resistance of concrete, Journal of Materials in Civil Engineering, Vol.4, No.5, pp 47-46, Sept. 00. Naik Tarun R., Ramme Bruce W. and Tews John H., Use of high volumes of Class C and Class F fly ash in concrete, Cement, Concrete and Aggregates, Vol. 6, No., pp. -0, June 994. Oner A., Akyuz S. and Yildiz R., An experimental study on strength development of concrete containing fly ash and optimum usage of fly ash in concrete, Cement and Concrete Research, Vol. 35, No. 6, pp. 65-7, June 005. Ozkan Sengul and Mehmet Ali Tasdemir, Compressive strength and rapaid chloride permeability of concretes with ground fly ash and slag, Journal of Materials in Civil Engineering, Vol., No. 9, pp , Sept Serdar Aydin, Halit Yazici, Hu' Seyin Yig' iter and Bu' Lent Baradan, Sulfuric acid resistance of high-volume fly ash concrete, Building and Environment, Vol. 4, pp. 77-7, 007.