Statistical modeling of high and low volume of fly ash high compressive strength concrete.

Padmanaban, I. ; Kandasamy, S. ; Natesan, S.C. 等

Introduction

Fly ash is an inorganic, non-combustible by-product of coal-burning power plants. As coal is burnt at high temperatures, carbon is burnt off and most of the mineral impurities are carried away by the flue gas in the form of ash. Fly ash is a pozzolanic material possessing no cementitious value but which will, in finely divided form and in the presence of moisture, chemically react with calcium hydroxide at ordinary temperature to form compounds possessing cementitious properties (Basu Prabir, C. and Subhajit Saraswati 2006) [1]. In the presence of moisture, alumino-silicates within the fly ash react with calcium ions to form calcium silicate hydrates. Today, there is a general trend to replace higher levels of Portland cement with fly ash in concrete. The increased pressure to use higher levels of fly ash in concrete stems from three main aspects. The first aspect is economics. In most markets fly ash is less expensive than Portland cement. Therefore, as the replacement level of fly ash increases, the cost of concrete production decreases. The second aspect and arguably the most important is the environment. Fly ash is an industrial by-product, much of which is deposited in landfills if not used in concrete. Also from an environmental perspective, the more fly ash being utilized in concrete, the less demand for Portland cement, the less Portland cement production, and therefore lower C[O.sub.2] emissions. The third and final aspect influencing the use of higher replacement levels is the technical benefits of high volume fly ash concrete (HVFAC). HVFAC has improved performance over ordinary Portland cement concrete, especially in terms of durability when appropriately used. Although there are clearly economic and environmental benefits associated with the use of high levels of fly ash in concrete, there is relatively little information on the behavior of such concrete and almost no guidance on its production or use. The aim of the investigation is to utilize effectively low volume and high volume fly ash in concrete and to develop mathematical models using response surface methodology.

Experimental Investigations

Materials

Ordinary Portland Cement (OPC-53 grade) conforming to IS: 12269-1987 was used in the investigation. The required quantity was procured as single batch, stored in airtight bags and used for the experimental programme. Locally available river sand conforming to Zone II of IS: 383(1970) was used as fine aggregate. The coarse aggregate was 20mm size crushed granite stone obtained from the local quarry. Potable water was used for casting specimens and curing purposes. Usually superplasticizers are added as 2-4% of cement mass or 5 to 15 lts per [m.sup.3] of concrete. In this investigation, a sulphonated naphthalene polymer SUPAFLO- superplasticizer, 2% by weight of binder was added as admixture to enhance workability. Class F Fly ash procured from Mettur Thermal Power Plant was used as partial replacement of cement. Properties of Constituent materials are given in Table 1

Mix proportions

High Strength and performance mix can be produced by reducing water-cement ratio lower than that for normal concrete. This is possible because of the chemical admixtures .Mix proportioning of HVFAC is a more critical process than that for normal conventional concrete in view of high fines content and low w/b ratio. Jiang and Malhotra [2] suggested a mix proportioning method based on combination of empirical results and absolute volume method. Based on this method the mix proportion obtained was 1: 1.4: 2 & w/b 0.36. Concrete cubes, 150 mm in size were tested for compressive strength as per BIS IS 516-1959 [3]. A total of 105 Cube specimens were cast for testing. Details of various mixes used are given in Table 3.

Experimental Studies on HVFAC & LVFAC MIXES

Compressive strength studies were conducted on various mixes (M60) to study the effect of fly ash in Concrete. The test results are presented in the Table 4.

Results and Discussion

Compressive Strength

The results of compressive strength at the age of 3, 7, 28, 56, 90 days are reported in Table 4 and the variation is shown in Fig. 1. It has been observed that compressive strength decreased with increased addition of fly ash content. With low quantity of fly ash, characteristic behavior of fly ash mix is alike. The FA-10 of 10% replacement with fly ash showed compressive strength characteristics greater than control reference mix FA-0 at the age of 90 days. Similarly Higher volume of fly ash content, FA-40, FA-50, FA-60 are showing alike characteristic features. From Fig 2, it is clear that the age affects to a greater extent the strength of mix. With increase in age, the strength of mix also increases up to the age of 28, 56 and 90 days. With replacements of cement by fly ash up to 30 % shows similar proportions of strength at their ages; Whereas there is a vast variation in strength achievement in replacing cement by higher quantity of fly ash. The percentage decrease with reference to control mix is 6.23, 7.9, 9.1, 11.1, and 17.8 for the fly ash mixes FA-20, FA-30, FA-40, FA-50 and FA-60 respectively. At the age of 90 all the fly ash mixes are able to achieve strength near to 60MPa except FA 60 as suggested by Sivasundaram, V. and Malhotra V.M. (2004) [5]. The variation between compressive strength and the age is shown in Fig 3 and their relationship is given by y = 10.78 Ln(x) +13.837 where y is compressive strength in MPa and x is age in days.

Statistical approach

Based on statistical approach given by Ghezal A. and Kamal H. Khayat [4] the following models were developed for compressive strength at the ages of 3, 7, 28, 56, 90

The conversion between coded values and absolute values can be calculated as follows

Coded B = (absolute B-350)/150

Coded FA = (absolute FA-150)/150

Coded w/c = (absolute w/c-0.63)/0.27

Coded C/TA = (absolute C/TA-0.2058825)/0.0882355

The compressive strength models developed based on the coded values are as follows

90-day fc model, MPa = 60.75 -2.63 FA -3.25 W/C + 268.26 B.B- 6.03 W/C.W/C -266.57 C/TA.C/TA + 3.97 FA.W/C

56-day fc model, MPa = 79.37 -55.84 FA +47.62 W/C + 210.01 B.B- 8.92 W/C.W/C -223.37 C/TA.C/TA - 1.02 FA.W/C

28-day fc model, MPa = 88.48-104.84 FA + 93.75 W/C + 40.65 B.B + 26.39 W/C.W/C -29.59 C/TA.C/TA-75.96 FA.W/C

3-day fc model, MPa = 3.35+41.77 FA -49.57 W/C -102.62 B.B+16.44 W/C.W/C +11.33 C/TA.C/TA-2.54 FA.W/C

The main strength affecting parameters C, FA, W/C, and C/TA with their interrelationship are plotted in fig 4, 5, 6. The corresponding contour plot of the respective figure of 4, 5, 6 are shown in fig 7, 8, 9 respectively. It is clear from the contour graphs, with the increase in cement content and C/TA ratio, the compressive strength of fly ash mix increases. As the C/TA ratio increases cement quantity is more when compared to total Aggregate. Fifty percentages of Cement and fly ash each in a mix produce strength in range between 58-60 MPa as shown in the contour graph 8. Low w/c ratio and higher cement content produces high strength fly ash mix concrete, effective addition of fly ash and cement at optimum water content acquire the required strength. The above model produces accurate results at their respective ages.

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Conclusions

Based on the experimental investigations on the fly ash mixes, the following are concluded.

(1) Replacement of cement by fly ash in the range of 50-60% achieved required strength characteristics at the age of 90 days.

(2) Low volume fly ash concrete (FA-10, FA-20, FA-30) mix can be utilized for practical application at the age of 28 days. High Volume fly ash concrete (FA-50, FA-60) can be utilized at the age of 90 days.

(3) Predicted mathematical model is applicable for mixes with w/c ratio 0.36 to 0.9, Cement quantity 200-500 kg/[m.sup.3], fly ash in the range 0 to 300 kg/[m.sup.3] and C/TA varying from 0.11 to 0.29.

(4) Age, Cement content, fly ash content, aggregate content are the main parameters affecting the Compressive Strength.

(5) The predicted mathematical model for all ages for compressive strength produced accurate results for the respective ages.

Reference

[1] Basu Prabir, C. and Subhajit Saraswati (2006),'High Volume fly ash concrete with Indian ingredients', 'The Indian Concrete Journal 80(3),pp. 37-48

[2] Jiang L.H., and Malhotra, V.M. (2000), 'Reduction in water demand of non air-entrained concrete incorporating large volume of fly ash', 'Cement and Concrete Research.30, ,pp. 1785-1789.

[3] BIS 1959. IS 516-1959, 'Methods of Tests for Strength of concrete, Bureau of Indian Standards', New Delhi

[4] Ghezal A. and Kamal H.Khayat ,(2002), 'Optimizing Self-Consolidating Concrete with Limestone Filler by using Statistical Factorial Design Methods', 'ACI Materials Journal, pp. 264-272.

[5] Sivasundaram,V. and Malhotra V.M. (2004), 'High Performance high volume fly ash', 'The Indian Concrete Journal 78(11) , pp. 13-21

I. Padmanaban (1), S. Kandasamy (2) and S.C. Natesan (3)

(1) Sr. Lecturer, V.L.B Janakiammal College of Engineering and Technology, Coimbatore 641042, Tamil Nadu, India, E-mail: padu2kin@gmail.com

(2) Dean, Anna University Tiruchirappalli -Ariyalur Campus, India, E-mail: kandasks@yahoo.com

(3) Dean, V.L.B Janakiammal College of Engineering and Technology, Coimbatore 641042, Tamil Nadu, India,

Table 1: Properties of the Constituent material.

Sl.           Parameter             OPC    Fly      Fine       Coarse
No                                 used    Ash    Aggregate   Aggregate

1     Normal Consistency           26%     30%       --          -
2     Finess by Sieving             80      78       --          -(%
      45 micron)
3     Initial Setting Time (min)    30      85       --          -4
Final Setting Time(min)       360    400       --          -5
Specific Gravity             3.15    2.12     2.51        2.64
6     Bulk density(kg/[m.sup.3])    --      --      1700        1600
7     Finess Modulus                --      --      2.81        4.12
8     Water Absorption              --      --      1.0%        0.5%

Table 2: Mix Proportions of the concrete.

Mix designation     FA-0   FA-10   FA-20   Fa-30   FA40   FA-50   Fa-60

Fly ash in %         0      10      20      30      40     50      60
w-b ratio           0.36   0.36    0.36    0.36    0.36   0.36    0.36
Cement              500     450     400     350    300     250     200
(kg/[m.sup.3])
Fly ash              0      50      100     150    200     250     300
(kg/[m.sup.3])
Fine Aggregate      700     700     700     700    700     700     700
(kg/[m.sup.3])
Coarse              1000   1000    1000    1000    1000   1000    1000
Aggregate
(kg/[m.sup.3])
Water               180     180     180     180    180     180     180
(lit/[m.sup.3])
Super plasticizer    10     10      10      10      10     10      10
(kg/[m.sup.3])

Table 3: Compressive strength properties of concrete.

                        Cube Compressive Strength, MPa
  Days
           FA-0    FA-10   FA-20   FA-30   FA-40   FA-50   FA-60

3rd Day    33.76   30.37   30.56   26.45   23.83   20.12   18.15
7th Day    42.15   43.64   38.92   35.54   28.66   24.66   20.11
28th Day   61.12   60.68   56.23   54.53   48.4    40.12   38.45
56th Day   64.22   66.3    60.22   58.83   54.11   50.46   58.88
90th Day   66.25   67.48   62.12   61.01   60.21   58.88   54.42
The values reported in the table represent the average of three samples