Study of Mechanical Properties of Geopolymer Concrete Reinforced with Steel Fiber

Study of Mechanical Properties of Geopolymer Concrete Reinforced with Steel Fiber

Arya Aravind1

1P.G Scholar, Civil Department, Mar Athanasius College of Engineering,

Kothamangalam, Kerala, India

Mathews M Paul2

2Professor, Civil Department,

Mar Athanasius College of Engineering, Kothamangalam, Kerala,

India

Abstract- Concrete industry is the largest user of natural resources in the world and ordinary Portland cement production is the second only to the automobile as the major generator of carbon dioxide, which polluted the atmosphere. Also climate change due to global warming has become a major issue. Geopolymer concrete is an innovative construction material which shall be produced by the chemical action of inorganic molecules. In terms of reducing the global warming, the geopolymer could reduce the CO2 emission to the atmosphere caused by cement and aggregate industries by about 80%. Fly ash is rich in silica and alumina reacts with alkaline solution produced aluminosilicate gel that acted as the binding material for the concrete. It is an excellent alternative construction material to the existing plain cement concrete. Geopolymer concrete is a type of amorphous alumino-silicate cementitious material. Geopolymer can be polymerized by polycondensation reaction of geopolymeric precursor and alkali polysilicates. This study focuses on the compressive strength and split tensile strength of geopolymer concrete reinforced with steel fiber.

Key words- Geopolymer concrete; Fly ash concrete; Alkaline activator

I. INTRODUCTION

The term geopolymer was first used by Davidovits to alkali aluminosilicate binders formed by the alkali silicate activation of aluminosilicate materials as an alternative binder to the portland cement. Geopolymer concrete is a type of amorphous alumino-silicate cementitious material. Geopolymer can be polymerized by polycondensation reaction of geopolymeric precursor and alkali polysilicates. Geopolymer results from the reaction of a source material that is rich in silica and alumina with alkaline liquid. The polymerization process involves a substantially fast chemical reaction under alkaline condition on Si-Al minerals that result in a three dimensional polymeric chain and ring structure consisting of Si-O-Al-O bonds. Alumino-silicates oxides react with alkali polysilicates yielding polymeric Si O Al bonds. Polysilicates are generally sodium or potassium silicate supplied by chemical industry.

ash to chemical ratio (0.3, 0.35, 0.4), percentage of steel fiber (0, 0.5, 1) and temperature of curing: 28°C, 60°C, 90°C.

1. MATERIALS AND METHODOLOGY

   Low-calcium (ASTM Class F) fly-ash is preferred as a source material than high calcium (ASTM Class C) fly-ash. The presence of calcium in high amount may interfere with the polymerization process. Fine aggregate used is M sand. Coarse aggregates of size 20mm and 12.5mm are used in the study in the ratio 60:40 and combinations of sodium hydroxide and sodium silicate is used as alkali activator. The physical and chemical properties of each ingredient has considerable role in the desirable properties of concrete like strength and workability. Fly ash used in the study is having specific gravity 2.1. Steel fiber used in the study has a length of 60mm.

   A. Box Behnken Design

   A Box-Behnken experiment is an example of Response Surface Methodology (RSM). Response surface methodology (RSM), which is based on factorial design, is a mathematical and statistical technique for designing experiments, fitting models, and determining the optimal operating conditions for a target response. BoxBehnken designs are experimental designs for response surface methodology, devised by George E.P.Box and Donald Behnken in 1960. Generally, a large number of specimens are required to study the properties of geopolymer concrete with three variables i.e., fly ash to alkaline liquids ratio, steel fiber percentage and temperature of curing. For that purpose, Box-Behnken design with three variable and three-level factors to reduce the numbers of specimen is adopted. Three control factors used in this experimental work are fly ash to alkaline liquids ratio, percentage of steel fiber and temperature of curing. In this design the treatment combinations are at the midpoints of the edges of the process space and at the center. FIGURE I show the Box Behnken for three factors.

   1. SCOPE

      Study of literatures reveals that limited studies have been done in the study of geopolymer using steel fibers. The study involves finding the compressive strength and split tensile strength of geopolymer concrete by varying the Fly

      FIGURE I. Box Behnken Design for Three Factors

      The model is designed as given in the equation 3.1

      2 2

      C. Curing

      The specimens were kept at room temperature for 3 days (rest period) and after that demoulded and placed in the hot air oven for 24 hours. The specimens were cured in hot air oven for 24 hours at 60°C and 90°C according to the study. Heat-curing substantially assists the chemical reaction that occurs in the geopolymer paste. Both curing time and curing temperature influence the strength of geopolymer concrete. The ambient curing specimens were kept at room temperature and demoulded after 3 days.

      C. Quantity of Materials Required in 1 m3 of Concrete

      12

      = 0 + 1 x1 + 2×2 + 3×3 + 11 x1 + 22 x2 +

      33

      x32 +

      x1x2 + 13

      x1x3 + 23

      x2x3 3.1

      Several mix designs were proposed for geopolymer concrete. The mix design for the investigation is adopted

      Where y is the predicted response, 0 is model

      constant; x1, x2 and x3 independent variables; 1, 2 and 3 are linear coefficients; 12, 13 and 23 are cross product coefficients and 11, 22 and 33 are the quadratic coefficients.

      B. Casting

      The molarity of NaOH solution used in this study is 10 molar. The sodium silicate solution and the sodium hydroxide solution were mixed together at least one day prior casting. In preparation of NaOH solution, 400gm of NaOH pellets were dissolved in one litre of water in a volumetric flask for getting 10 molar solution. Pan mixer was used for mixing. The compressive strength and the workability of geopolymer concrete are influenced by the proportions and properties of the constituent materials that make the geopolymer paste.

      from the journal optimum mix for geopolymer concrete [4]. Quantity of materials required in 1 m3 of concrete is shown in the TABLE I.

      TABLE I

      QUANTITY OF MATERIAL REQUIRED IN 1M3OF CONCRETE

      Percentage of steel fiber (%)	Ratio of fly ash to alkaline liquids	Fly ash (kg)	Fine aggregate (kg)	Coarse aggregate (kg)	Alkaline liquids (kg)		Steel fiber (kg)	Designation
      					NaOH	Na2SiO3
      0	0.3	467	612	1346	40	100	0	R0.3F0
      	.35	400	612	1346	40	100	0	R0.35F0
      	0.4	350	612	1346	40	100	0	R0.4F0
      0.5	0.3 /td>	464	608	1339	39	99	39.25	R0.3F0.5
      	0.35	398	608	1339	39	99	39.25	R0.35F0.5
      	0.4	348	608	1339	39	99	39.25	R0.35F0.5
      1	0.3	462	605	1332	38	98	78.5	R0.3F1
      	0.35	396	605	1332	38	98	78.5	R0.35F1
      	0.4	346	605	1332	38	98	78.5	R0.4F1

      1. RESULTS AND DISCUSSIONS

         Compressive strength and Split tensile Strength of specimen are tabulated in the TABLE II. TABLE II.

         COMPRESSIVE STRENGTH AND SPLIT TENSILE STRENGTH OF SPECIMEN

         Designation	Temperature °C	Compressive strength N/mm2	Split tensile strength N/mm2
         R0.3F0	60	24	2.0
         R0.35F0	27	15.5	1.7
         R0.35F0	90	26	2.4
         R0.4F0	60	25	2.3
         R0.3F0.5	27	15.7	2.1
         R0.3F0.5	90	25	3.0
         R0.35F0.5	60	26.2	3.1
         R0.4F0.5	27	17.8	2.3
         R0.4F0.5	90	27	3.4
         R0.3F1	60	24.6	3.3
         R0.35F1	27	18	2.9
         R0.35F1	90	28	3.7
         R0.4F1	60	26.8	3.5

         TABLE III

         EXPERIMENTAL AND PREDICTED STRENGTH

         Designation	Temperature °C	Experimental fcu N/mm2	Predicted fcu N/mm2	Experimental fct N/mm2	Predicted fct N/mm2
         R0.3F0	60	24	23.62899	2	1.98573
         R0.35F0	27	15.5	15.99689	1.7	1.638066
         R0.35F0	90	26	26.02157	2.4	2.463123
         R0.4F0	60	25	24.85255	2.3	2.31308
         R0.3F0.5	27	15.7	15.55646	2.1	2.175524
         R0.3F0.5	90	25	25.3671	3	2.951826
         R0.35F0.5	60	26.2	26.2000	3.1	3.1000
         R0.4F0.5	27	17.8	17.44354	2.3	2.349476
         R0.4F0.5	90	27	26.4829	3.4	3.323174
         R0.3F1	60	24.6	24.74745	3.3	3.28692
         R0.35F1	27	18	18.00311	2.9	2.836934
         R0.35F1	90	28	27.47843	3.7	3.761877
         R0.4F1	60	26.8	27.17101	3.5	3.51427

         1. Regression analysis

            Experiments were performed using the BoxBehnken experimental design. BoxBehnken experimental design is a type of response surface methodology. Response surface

            methodology is an empirical optimization technique for evaluating the relationship between the experimental outputs and factors called x1, x2, and x3. For obtaining the results for Box-Behnken design, analysis of variance has been calculated to analyze the accessibility of the model and was carried in Microsoft Office Excel 2007.

            The significance of second-order polynomial for the tensile strength was assessed by carrying out analysis of variance. ANOVA is an analytical technique that is used to identify the importance of the model. The coefficient of determination (R2) is defined as the ratio of the explained variation to the total variation, and is a measure of the degree of fit. A good model fit should yield an R2 of at least 0.8. A p-value lower than 0.05 indicates that the model is statistically significant, whereas a higher value indicates that the model is not significant [1]. The R value obtained in the regression analysis for compression test is 0.9971 and split tensile test is 0.9967This means that the response model evaluated in this study can explain the reaction very well. The experimental and predicted values of compressive and split tensile strength are shown in the TABLE III.

            . The model accuracy was checked by comparing the predicted and experimental strength. From the FIGURE II it is clear that majority of the model values falls on the line which indicates that predicted values and experimental values are the same.

            Figure II Comparison of Predicted and Experimental Strength

      2. CONCLUSIONS The conclusions obtained are

         • As the concentration of sodium hydroxide increased

           compressive strength of geopolymer increased.

         • Higher the ratio of sodium silicate-to-sodium hydroxide liquid ratio by mass, showed higher compressive strength of geopolymer concrete.

         • The compressive strength of geopolymer concrete is gradually increased with prolonged curing period and significantly improved at curing period of 28 days.

         • Increase in curing temperature in the range of 30°C to 90

           °C increased the compressive strength of geopolymer concrete and longer curing time also increased the compressive strength.

         • Geopolymer concrete up to120 minutes will not show any sign of setting and without any degradation in the compressive strength, resulted very little drying shrinkage and low creep.

         • Water to geopolymer solids ratio has negative effect on the strength of geopolymer concrete.

           • Another important observation was that curing under normal sunlight yielded strength of 16 N/mm2..

           • Split tensile strength of geopolymer concrete increased as percentage of steel fiber increased.

           • Box Behnken design was successfully adopted. The model fitted the experimental data since R2 approximates to 1 and also large F value.

         ACKNOWLEDGMENT

         I wish to express my deep sense of gratitude to all faculties of Department of Civil Engineering, M A College of Engineering, Kothamangalam. I thank God, the almighty for his blessings without which nothing would have been possible.

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