MATHEMATICAL MODELING FOR DURABILITY CHARACTERISTICS OF FLY ASH CONCRETE

MATHEMATICAL MODELING FOR DURABILITY CHARACTERISTICS OF FLY ASH CONCRETE JINO JOHN 1 1 Asst. Professor, Department of Civil Engineering, Sri Krishna College of Technology Coimbatore, Tamilnadu, India, 641042 Email:jinojohnvlb@gmail.com T.M.MAYA 2 2 PG Student, Department of Civil Engineering, Sri Krishna College of Technology Coimbatore, Tamilnadu, India, 641042 Email:tm.maya1@gmail.com T.MEENAMBAL 3 3 Professor (RD) Department of Civil Engineering, Government College of Technology Coimbatore, Tamilnadu, India, 641042 Abstract: This paper presents the results obtained from the mathematical modeling for the durability characteristics of fly ash concrete. A mathematical model is employed to predict the saturated water absorption, permeability, sorpitivity and acid resistance of the concrete containing fly ash as a replacement of cement at a range of 0%, 10%, 20%, 30%, 40% and 50 %. This model is valid for mixes with cement quantity 208 to 416 kg/m 3, water cement ratio 0.38 to 0.76, fly ash 0 to 208 kg/m 3 and cement/ total aggregate ratio varying from 0.11 to 0.22. Fly ash content and water cement ratio are the main parameters which influence the durability characteristics. The predicted mathematical model for saturated water absorption, permeability, sorpitivity and acid resistance produced accurate results for the respective ages when compared with the experimental results. KEYWORDS: Fly ash; saturated water absorption; permeability; sorpitivity; sulphate attack; chloride attack; mathematical modeling. 1. Introduction 1.1 Fly ash. Fly ash is an industrial by-product, generated from combustion of coal in the thermal power plants. The increasing scarcity of raw materials and an urgent need to protect the environment against pollution has accentuated the significance of developing new building materials based on industrial waste generated from coal fired thermal power station which is creating unmanageable disposal problems due to its potential to pollute the environment. Fly ash when used as a mineral admixture in concrete improves its strength and durability characteristics. Fly ash can be used either as an admixture or as a partial replacement of cement or as a partial replacement of fine aggregates or total replacement of fine aggregate and as supplementary addition to achieve different properties of concrete. 1.2 Mathematical modelling A mathematical model is a description of a system using mathematical concepts and language. The process of developing a mathematical model is termed mathematical modelling. Regression is the measure of the average relationship between two or more variables in terms of the original units of the data. Regression helps to estimate one variable or the dependent variable from the other variables. ISSN : 0975-5462 Vol. 4 No.01 January 2012 353

The main objective of the present investigation is to study the durability characteristics of fly ash concrete and to develop the mathematical model for the durability characteristics. The durability properties investigated in this study are sulphate attack, chloride attack, saturated water absorption, permeability, sorpitivity. 2. Experimental investigations 2.1 Materials used: Cement: Ordinary Portland Cement (OPC-43 grade) conforming to IS: 8112-1989 8. The specific gravity of cement is 3.15 Fly ash: Class F Fly ash procured from Mettur Thermal Power Plant. Fine aggregate: Locally available river sand conforming to Zone II of IS: 383-1970 6 was used as fine aggregate. The specific gravity of the fine aggregate is 2.67 Coarse aggregate: 20mm size crushed granite stone obtained from the local quarry. Water: Potable tap water available in laboratory with ph value of 7.0± 1 Super plasticizer: Sulphonated naphthalene formaldehyde condensate type, CONPLAST SP430. 2.2 Mix proportions: The control mix was proportioned by IRC-44:2008 5 to obtain compressive strength of 40 MPa. The identification, mix proportion and quantity of material taken for one meter cube of fly ash concrete mixes are given in table 1.The mixes FA 0,FA 10,FA 20,FA 30,FA 40 and FA 50 were obtained by replacing 0%,10%,20%,30%,40% and 50% of weight of cement by fly ash respectively. Super plasticizer CONPLAST SP430 is added 2% to the weight of binder. Table 1. mix proportions per m 3 Sl no % of fly ash content Cement (kg) Fly ash (kg) Fine aggregate (kg) Coarse aggregate (kg) Water (lit) FA 0 0 416.00 0 681 1219 158 FA 10 10 374.40 41.60 681 1219 158 FA 20 20 332.80 83.20 681 1219 158 FA 30 30 291.20 124.80 681 1219 158 FA 40 40 249.6 166.4 618 1219 158 FA 50 50 208 208 681 1219 158 2.3 Preparation, casting and testing of specimens The 150mm and 100mm cubes were cast for various mixes. The 150mm concrete cubes were tested for permeability and 100 mm cubes for other durability tests at 28 and 90 days. All the test specimens were stored at room temperature and were kept in curing tanks. 3. Experimental program Durability studies were conducted at 28 and 90 days for various mixes to find the resistance to acid attack, namely sulphuric and hydrochloric acid, saturated water absorption, permeability and sorpitivity. ISSN : 0975-5462 Vol. 4 No.01 January 2012 354

3.1 Sulphate attack Concrete cubes of size 100mmx100mmx100mm, after 28 days of curing were dried. Initial dry weight of the cubes was found. Then, the cubes were immersed in 3% sulphuric acid for 28 and 90 days. After 28 and 90 days cubes were taken out from sulphuric acid solution. The surfaces of the cubes were cleaned. The final dry weight of the specimens was found. The difference in the initial dry weight and final dry weight of the specimen indicates the weight loss due to sulphate attack. 3.2 Chloride attack Concrete cubes of size 100mmx100mmx100mm, after 28 days of curing were dried. Initial dry weight of the cubes was found. Then, the cubes were immersed in 3% hydrochloric acid for 28 and 90 days. After 28 and 90 days cubes were taken out from hydrochloric acid solution. The surfaces of the cubes were cleaned. The final dry weight of the specimens was found. The differences in the initial dry weight and final dry weight of the specimen weight loss due to chloride attack. 3.3 Saturated water absorption Saturated water absorption test was conducted on 100mmx100mmx100mm cubes at the age of 28 and 90 days. The specimens were weighed before drying in hot air oven at 105 0 C. The drying process was continued, until the difference in mass between two successive measurements at 24 hours interval closely agreed. The dried specimens were cooled at room temperature and then immersed in water. The specimens were taken out at regular intervals of time, surface dried and weighed. The differences between the saturated mass and the oven dried mass expressed as the percentage of oven dried mass gives the saturated water absorption. 3.4. Permeability Permeability of concrete is an important one, lower the permeability of concrete better will be the resistance to chemical attack. Permeability test was carried out on 150mmx150mmx150mm cubes at the age of 28 and 90 days using the permeability test apparatus as per IS 3085-1965[7]. 3.5 Sorptivity Sorptivity measures the rate of penetration of water into the pores in concrete by capillary action. When the cumulative volume of water that has penetrated per unit surface area of exposure 'q' is plotted against the square root of time of exposure 'SQRT(t), the resulting graph could be approximated by a straight line passing through the origin. The slope of this straight line is considered as a measure of rate of movement of water through the capillary pores. This measurement is called sorptivity. In this present study, the test for sorptivity was conducted on 100mmx100mmx100mm cube specimens by drying the specimens in an oven at a temperature of 105 o C to constant mass and then immersing them in water after cooling the specimens to room temperature and measuring the gain in mass at regular intervals of 30 minutes duration, for a period of two hours. The sorptivity was computed by considering the slope of the plot 'p' versus 'SQRT (t). 4. Results and discussions Durability studies conducted on fly ash concrete namely sulphate attack, chloride attack, saturated water absorption, permeability and sorptivity and their results are discussed below. 4.1 Sulphate attack The results of sulphate attack on the various mixes FA 0, FA 10, FA 20, FA 30, FA 40, and FA 50 at 28 and 90 days are presented in table 2 and figure 1 ISSN : 0975-5462 Vol. 4 No.01 January 2012 355

Table 2. Test results for sulphuric acid resistance Mix % of fly ash Average reduction in weight (%) 28 days 90 days FA 0 0 2.12 3.10 FA 10 10 1.95 2.88 FA 20 20 1.82 2.76 FA 30 30 1.72 2.58 FA 40 40 1.65 2.42 FA50 50 1.59 2.33 Fig 1. Percentage reduction of weight when immersed in sulphuric acid for various mixes From the table 2 it was noted that with the increase in fly ash content the weight reduction gets decreased by 8%, 14.2%, 18.9%, 22.2% and 25% at 28 days and7.1%, 10.9%, 16.7%, 21.9% and 24.9% at 90 days respectively for the mixes FA 10, FA 20, FA 30, FA 40, and FA 50 when compared with control mix concrete. It is clear that when fly ash added as cement replacement in concrete enhances sulphate resistance in the concrete. 4.2 Chloride attack The results of chloride attack on the various mixes FA 0, FA 10, FA 20, FA 30, FA 40, and FA 50 at 28 and 90 days are presented in table 3 and figure 2 Table 3. Test results for hydrochloric acid resistance Mix % of fly ash Average reduction in weight (%) 28 days 90 days FA 0 0 0.64 1.05 FA 10 10 0.55 0.98 FA 20 20 0.52 0.91 FA 30 30 0.49 0.83 FA 40 40 0.45 0.75 FA50 50 0.39 0.64 Fig 2 Percentage reduction of weight when immersed in hydrochloric acid for various mixes ISSN : 0975-5462 Vol. 4 No.01 January 2012 356

It is observed from the table 3 that with the increase in fly ash content the weight reduction of the specimens gets reduced by 14%, 18.8%, 23.4%, 29.6% and 39% at 28 days and 6.7%, 13.3%, 20.9%, 28.5% and 39% at 90 days respectively for the mixes FA 10, FA 20, FA 30, FA 40, and FA 50 when compared with control mix concrete. It is clear that the fly ash addition as cement replacement in concrete improves chloride resistance. 4.3 Saturated water absorption The results of saturated water absorption on the various mixes FA 0, FA 10, FA 20, FA 30, FA 40, and FA 50 at 28 and 90 days are presented in table 4 and figure 3. Table 4. Test results for saturated water absorption Mix % of fly ash Saturated water absorption (%) 28 days 90 days FA 0 0 3.75 3.40 FA 10 10 3.50 3.20 FA 20 20 3.10 2.76 FA 30 30 2.70 2.50 FA 40 40 2.60 2.32 FA50 50 2.31 2.13 Fig 3 Saturated water absorption for various mixes From the table 4 it was noted that with the increase in fly ash content the saturated water absorption gets decreased by 6.7%, 17.3%, 28%, 30.6% and 38.6% at 28 days and 5.9%, 19.1%, 26.4%, 30.8% and 38.2% at 90 days respectively for the mixes FA 10, FA 20, FA 30, FA 40, and FA 50 when compared with control mix concrete. Fly ash acts as a filler material which fills the pores and there by reduces the water absorption. The reduction in percentage of water absorption at 90 days is less than that at 28 days by an average of 9.3 % when compared with the corresponding mixes at 28 days. With increase in age the pore space gets reduced, which reduces the water absorption. With increase in addition of fly ash the water absorption of concrete also reduces. 4.4 Permeability test The results of permeability on the various mixes FA 0, FA 10, FA 20, FA 30, FA 40, and FA 50 at 28 and 90 days are presented in table 5 and figure 4. ISSN : 0975-5462 Vol. 4 No.01 January 2012 357

Table 5. Test results for permeability Mix % of fly ash Permeability (cm/sec) 28 days 90 days FA 0 0 8.65X10-7 3.70X10-7 FA 10 10 8.20X10-7 3.50X10-7 FA 20 20 7.90X10-7 3.10X10-7 FA 30 30 7.60X10-7 2.90X10-7 FA 40 40 7.40X10-7 2.70X10-7 FA50 50 6.80X10-7 2.60X10-7 Fig 4. Permeability for various mixes From figure 4 and table 5 it is found that with the increase in fly ash content permeability gets reduced by 5.2%, 8.6%, 13.3%, 17.9% and 21.4% at 28 days and 5.4%, 16.2%, 21.6%, 27% and 29.7% at 90 days respectively for the mixes FA 10, FA 20, FA 30, FA 40, and FA 50 when compared with control mix concrete. The reduction of permeability with increase in fly ash content is due to reduction in pores since fly ash acts as a filler material. The reduction in percentage of water absorption at 90 days is less than that at 28 days by an average of 60.4 % when compared with the corresponding mixes at 28 days. With the increase in age C-S-H gel constitute the reduction of the pores which helps in reducing the permeability. It is clear that the fly ash added as cement replacement in concrete reduces the permeability with increase in replacement level as well as with the age. 4.5 Sorptivity test The results of sorptivity on the various mixes FA 0, FA 10, FA 20, FA 30, FA 40, and FA 50 at 28 and 90 days are presented in table 6 and figure 5. Table 6. Test results for Sorptivity Mix % of fly Sorptivity in m/sec 0.5 ash 28 days 90 days FA 0 0 1.61X10-4 1.43X10-4 FA 10 10 1.39X10-4 1.18X10-4 FA 20 20 1.29X10-4 1.11X10-4 FA 30 30 1.18X10-4 1.03X10-4 FA 40 40 1.13X10-4 0.95X10-4 FA50 50 1.09X10-4 0.91X10-4 ISSN : 0975-5462 Vol. 4 No.01 January 2012 358

Fig 5. Sorptivity for various mixes From the table 6 it was noted that with the increase in fly ash content sorptivity gets decreased by 13.6%, 19.8%, 26.7%, 29.8% and 32.2% at 28 days and 17.4%, 22.3%, 27.9%, 32.5% and 36.3% at 90 days respectively for the mixes FA 10, FA 20, FA 30, FA 40, and FA 50 when compared with control mix concrete. It is clear that the fly ash added as cement replacement in concrete enhances sorptivity in the concrete. Fly ash acts as a filler material which fills the pores and there by reduces the sorptivity. The reduction in percentage of water absorption at 90 days is less than that at 28 days by an average of 13.5 % when compared with the corresponding mixes at 28 days. Sorpitivity of the concrete reduces with the increase in age as well as the increase in fly ash cotent. The capillary pores get reduced by the formation of secondary C-S-H gel due to pazzolonic action, which leads to the reduction in sorpitivity of concrete. 5. Statistical approach Based on statistical approach given by Padmanaban.I 11, the following models were developed for the sulphate attack, chloride attack, saturated water absorption, permeability and sorpitivity. The following parameters are taken for developing the model 1. Cement (kg) C 2. Fly ash (kg) F 3. Water cement ratio W/C 4. Cement/Total Aggregate C/TA This model is valid for mixes with cement quantity 208 to 416 kg/m 3, water cement ratio 0.38 to 0.76, fly ash 0 to 208 kg/m 3 and cement/ total aggregate ratio varying from 0.11 to 0.22. The sample coding is given in the table 7. Table 7. Sample Coding Mix C F W/C C/TA FA 0 1 1-1 -1 FA 10 0.6 0.6-0.77-0.6 FA 20 0.2 0.2-0.521-0.2 FA 30-0.2-0.2-0.152 0.2 FA 40-0.6-0.6 0.321 0.6 FA 50-1 -1 1 1 Based on the four parameters cement, fly ash, water cement ratio and cement/total aggregate, the following equations can be used to predict the durability characteristics of fly ash concrete at 28 and 90 days. Loss of weight when immersed in sulphuric acid solution, at 28 days = 1.85-0.489FA+0.233W/C R 2 =0.99 (1) 90 days = 2.71-0.534 FA+0.152W/C R 2 =0.99 (2) ISSN : 0975-5462 Vol. 4 No.01 January 2012 359

Loss of weight when immersed in hydrochloric acid solution, at 28 days = 0.51-0.131FA+0.019W/C R 2 =0.96 (3) 90 days = 0.845-0.126 FA-0.076W/C R 2 =0.99 (4) Water absorption, at 28 days = 2.78-1.01FA+0.354W/C R 2 =0.98 (5) 90 days = 3.05-1.04 FA+0.314W/C R 2 =0.98 (6) Permeability, at 28 days = 7.72x 10-7 -6.7x 10-8 FA-1.9x10-8 W/C R 2 =0.98 (7) 90 days = 3.17x 10-7 -9.9x10-8 FA+4.23x10-8 W/C R 2 =0.99 (8) Sorpitivity, at 28 days = 0.000134-5.6x 10-5 FA+3.3x 10-5 W/C R 2 =0.98 (9) 90 days = 0.00011-5.2x 10-5 FA+2.84x 10-5 W/C R 2 =0.96 (10) The variations between the predicted and experimental results for the sulphate test are -0.097% to 0.63% and -0.94% to 0.98% for 28 and 90 days respectively. The variations between the predicted and experimental results for the chloride test are -2.85% to 4.21% and -0.69% to 0.324% for 28 and 90 days respectively. The variations between the predicted and experimental results for permeability test are -2.82% to 3.40% and -1.46% to 2.77% for 28 and 90 days respectively. The variations between the predicted and experimental results for permeability test are -0.928% to 0.87% and -2.04% to 1.27% for 28 and 90 days respectively. The variations between the predicted and experimental results for the sorpitivity test are -1.80% to 2.79% and -2.87% to 4.84% for 28 and 90 days respectively. The main parameters affecting the durability characteristics are fly ash content and water cement ratio. 5. Conclusion 1. With increase in fly ash content the weight reduction when immersed in sulphuric acid gets reduced by 8% to 25% at 28 days and 7% to 25 % at 90 days when compared with control mix. 2. When immersed in hydrochloric acid the weight reduction is less than that for the control mix by 14% to 39 % at 28 days and 7% to 39 % at 90 days. 3. Addition of fly ash decreases the water absorption by 6% to 39% and 9% to 38 % at 28 and 90 days when compared with control mix. 4. Sorpitivity gets reduced by 13% to 32 % and 17% to 36% at 28 and 90 days with the increase in the fly ash content. 5. Addition of fly ash reduces the permeability by 5% to 21% and 5% to 30% at 28 and 90 days when compared with control mix. 6. Addition of fly ash to concrete improves the durability of concrete. 7. The mathematical model predicted accurate results when compared with experimental results. References [1] Basu Prabir, C. and Subhajit Saraswati 'High Volume fly ash concrete with Indian ingredients', The Indian Concrete Journal 80(3), (2006),pp. 37-48. [2] Brian B. Hope, Autoclaved concrete containing Fly Ash, Cement and Concrete research, Vol.11, pp. 227-233, 1981. [3] Charles Berryman, Jingyi Zhu, Wayne Jense, Maher Tadros High percentage replacement of cement with fly ash for reinforced concrete pipes Cement and Concrete research 35, 1088-1091, 2005. [4] Ghrici.M, Kenci.S,Said Mansour.M Mechanical properties and durability of mortar and concrete containing pozzolana and lime stone, blended cements Cement and Concrete Composites 29(2007) pp 542-549. [5] IRC 44 2008, 'Guidelines for Cement Concrete Mix Design for Pavements' Indian Road Congress, New Delhi, India. [6] IS 383-1970, 'Specification for coarse and fine aggregate from natural sources for concrete Bureau of Indian Standards, New Delhi, India. ISSN : 0975-5462 Vol. 4 No.01 January 2012 360

[7] IS 3085-1965 Method of test for permeability of cement mortar and concrete, Bureau of Indian Standards, New Delhi, 1965, India. [8] IS 8112-1989, 'Specifications for 43 grade Portland cement Bureau of Indian Standards, New Delhi, India. [9] Marina Alvarez, Julia'n Salas and Janer Veras, Properties of concrete made with Fly Ash, The International journal of Cement Composites and Lightweight Concrete, Vol 10, number 2, May 1988 [10] Maruthachalam.D, Gurunathan M, Padmanabhan.I and Vishnuram.B.G, Durability Properties Of Fibrillated Polypropolene Fibre Reinforced High Performance Concrete, Journal of Structural engineering, Vol 38 No.1, April may 2011 [11] Padmanaban.I., Kandasamy S. Natesan S.C Statistical Modelling of High and Low Volume of Fly ash in Concrete, International Journal of Applied Engineering Research, Vol. 4 No 7, (2009), pp.1161-1167. [12] Paul J. Tikalsky, Uses of fly ash in concrete ACI committee, ACI 232.2R-96, 2007. [13] Pawan Kalla, Anurag Misra and Gauray Sancheti (2011), Properties of Wollastonite and fly ash added concrete, Indian Highway Journal, December (2011), pp.25-31. ISSN : 0975-5462 Vol. 4 No.01 January 2012 361