Statistical Prediction of Strength Characteristics of Fly Ash Concrete

Statistical Prediction of Strength Characteristics of Fly Ash Concrete

Abstract

Cement is a fundamental raw material used in construction. Production of cement releases large amount of CO2 which is a major cause for the global warming issue.Hence calculated percentage of cement can be replaced by other materials such as fly ash, rice husk ash, Meta kaolin, silica fume, geo- polymer etc. without cutting down much the strengths of concrete. Fly ash is one of the most normally preferred substitutes for cement because concrete workability and durability are enhanced by fly ash by their small size and round shape. A mathematical model was constructed using SPSS software for the strength characteristics like compressive strength, flexural strength and split tensile strength when cement is replaced with fly ash at a range of 0% to 50 %. As the percentage of replacement of fly ash in the concrete increases, the strength characteristics decrease. Water content and cement/aggregate ratio are the main parameters which influence the strength characteristics.A comparative study between the results obtained from the laboratory and SPSS has been made and it is found that the differences between these two values are minimum.

Key words: Cement, Fine Aggregate, Coarse Aggregate, Fly Ash, Concrete cubes, cylinders, beams, mathematical modeling, SPSS software.

1. Introduction

   Fly ash is the mineral residue produced during combustion of coal, and comprises the fine particles that rise with the flue gases. Fly ash comes primarily from coal-fired electricity generating power plants. These plants grind coal to powder fineness before it

   is burned. Effective utilization of wastes should be carried out in a profitable manner thereby reducing the volume of waste. The disposal of fly ash has become mandatory as it poses a serious environmental issue. In India, out of the more than 110 million tons of fly ash produced annually, the unused fly ash occupies vast track of valuable land as ash ponds. It is being used in production of concrete by partial (only 20-25 %) replacement of cement. As per the recommendation of IRC: SP : 62-2004, the fly ash content as partial replacement of ordinary Portland cement can be up to 35 %.

2. Literature review

   Some important observations of the review are presented.Charles Berryman, Jingyi Zhu, Wayne Jense, Maher Tadros (2005) concluded that Test cylinders with varying percentages of Class C (25- 65%) and Class F (25-75%) fly ash and a water- reducing admixture (WRA) were created under field manufacturing conditions and tested for 7-day compressive

   strength. Seven-day compressive strength for the concrete/fly ash/WRA was found to be highest when the concrete mix included approximately 35%'Class C or 25% Class F fly ash.

   Binod Kumar, Tike G.K, Nanda P.K (2007) in theirlaboratory test results showed that fly ash concrete mixtures containing 50-60% fly ash can be designed to fulfil the requirement of strength and workability suitable for cement concrete pavement construction. At all w-cm ratios, the concrete mixture containing 60% OPC and 40% fly ash developed maximum strength at the age of 90 days and beyond

3. Materials and methods

   Fly ash is comprised of the non-combustible mineral portion of coal. When coal is consumed in a power plant, it is first ground to the fineness of powder. Blown into the power plants boiler, the carbon is consumed leaving molten particles rich in silica, alumina and calcium. These particles solidify as microscopic, glassy spheres that are collected from the power plants exhaust before they can fly away. Thus fly ash is a residue generated in combustion that rises with the flue gases and is captured by electrostatic precipitators or other particle filtration equipment.

   Using the different mixes the concrete moulds are prepared and are tested for strengths after curing in 7, 28, 56, 90 days. The split tensile and compressive strengths are determined using the Compression Testing Machine ( CTM ).

4. Experimental results and discussion

   1. Concrete mix design

      Cement is partially replaced by fly ash in the range of 0% to 50%. Detailed mix proportions as per IRC: 44-2008are given below. Results of compression strength test, split tensile strength and flexural strength on these mixes test are listed.

   2. Mix Proportions as per IRC: 44-2008

      The mix proportions are designed as per IRC 44- 2008 and cement is replaced by fly ash in increasing steps of 10% for the same quantity of fine aggregate, coarse aggregate and water(Table1).

      M ix	% of fly ash con tent	Cement (kg)	Fly ash (kg)	Fine aggr egat e (kg)	Coar se aggr egat e (kg)	Wat er (lit)
      M 1	0	412.00	0	679	1216	159
      M 2	10	359.23	42.97	679	1216	159
      M 3	20	334.56	89.65	679	1216	159
      M 4	30	298.87	129.1 2	679	1216	159
      M 5	40	258.25	168.3 6	679	1216	159
      M 6	50	221.32	207.9 6	679	1216	159

      Table1. Mix Proportions

      Table2 Compressive strength result of fly ash concrete

      Mix	% of Fly Ash	Compressive strength (MPa)
      		7 days	28 days	56 days	90 days
      M1	0	29.26	42.39	44.51	45.65
      M2	10	28.65	40.54	43.06	45.31
      M3	20	25.32	38.35	41.26	42.21
      M4	30	22.45	37.28	40.31	41.62
      M5	40	18.85	32.28	37.07	40.76
      M6	50	14.24	28.47	34.54	39.52

      Table3Split tensile strength result of fly ash concrete

      Mix	% of Fly Ash	Split tensile strength (MPa)
      		7 days	28 days	56 days	90 days
      M1	0	2.31	3.29	4.47	4.97
      M2	10	2.19	3.03	4.29	4.81
      M3	20	2.01	2.85	4.11	4.62
      M4	30	1.79	2.61	3.63	4.42
      M5	40	1.72	2.32	3.47	4.31
      M6	50	1.59	2.13	3.14	4.24

   3. Comparative study of compressive strength of fly ash concrete

      compressive strength at 7 days (Mpa)

      The comparative study has been made between the experimental value and the theoretical values which are derived from the software.

      theoretical vs experimental (7 days)

      30

      20

      10

      0

      experimental

      backward forward stepwise

      0 10 20 30 40 50

      % of fly ash

      Figure1Comparative study of theoretical and experimental values of compressive

      strength at 7 days

      theoretical vs experimental (28 days)

      60

      40

      20

      0

      0 10 20 30 40 50

      % of fly ash

      experimental

      backward

      forward stepwise

      compressive strength at 28 days (Mpa)

      Figure2 Comparative study of theoretical and experimental values for compressive strength at 28 days

      40

      20

      0

      compressive strength at 90 days (Mpa)

      For compressive strength at 28 days the parameters influencing the strength in forward and stepwise regression are same.

      theoretical vs experimental (90 days)

      60

      experimental

      backward forward

      0 10 20 30 40

      % of fly ash

      50 stepwise

      Figure4 Comparative study of theoretical and experimental values for compressive strength at 90 days

      The replacement of cement with fly ash ranges from 0 % to 50 % and the compressive strength at 7, 28, 56 and 90 days are considered in this comparison Figures(1,2,3,4) . The experimental values and the results from three different regression methods are compared and it is clear that the differences in values between experimental results and the theoretical results from the software is minimum.

      From the result it has been noted that the forward regression and the stepwise regression is giving the same values and parameters affecting the strength are same. The backward regression is the method which gives more number variable which is affecting the strength properties hence the backward method can be adopted.

   4. Comparative Study of Split Tensile Strength of Fly Ash Concrete

      Experimental values for split tensile strength for 7, 28, 56, 90 days are compared with the theoretical

      results obtained from SPSS software as shown in figures5,6,7,8.

      theoretical vs experimental (7 days)

      3

      2

      1

      0

      0 10 20 30 40 50

      % of fly ash

      experimental

      backward

      forward stepwise

      backward

      forward stepwise

      2

      1

      0

      theoretical vs experimental (28 days)

      4

      3

      experimental

      split tensile strength at

      28 days (Mpa)

      split tensile strength at 7 days (Mpa)

      Figure 5 Comparative Study of theoretical and experimental values of split tensile strength at 7 days

      experimental

      4

      theoretical vs experimental (56 days)

      6

      0 10 20 30 40 50

      % of fly ash

      split tensile strength at

      56 days (Mpa)

      Figure6 Comparative study of theoretical and experimental values of split tensile strength at 28 days

      stepwise

      0 10 20 30 40 50

      % of fly ash

      forward

      0

      backward

      2

      Figure7 Comparative Study of theoretical and experimental values of split tensile strength at 56 days

      0 10 20 30 40 50

      % of fly ash

      4.5

      4

      3.5

      split tensile strength at 90 days (Mpa)

      Figure8 Comparative study of theoretical and experimental values of split tensile strength for 90 days

      In split tensile strength also the forward regression and stepwise regression is giving the same parameters which affects the split tensile strength. In the case of backward regression model more number of parameters were involved in strength characteristics and hence it can be adopted.

5. Conclusions

The following conclusions are arrived based on the results obtained from the regression model using SPSS software.

1. Water content and cement/ TA are the main parameters which are influencing the strength characteristics.

2. The difference in values of strength characteristics such as compression, flexural, tensile which were obtained from the laboratory and the predicted value from SPSS software is minimum.

3. As the percentage of fly ash increases, the strength characteristics of concrete containing fly ash slightly reduce.

4. Even though, strength characteristics decrease slightly, the nominal replacement of fly ash in cement reduces the wastage disposal problem.

Acknowledgement

We express our sincere gratitude and indebtedness to Dr.T.Meeenambal Ph.D., Professor, Department of Civil Engineering, Government College of Technology for her valuable suggestions and guidance.

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   experimental

   backward forward

   stepwise

   theoretical vs experimental (90 days)

   5

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