FLEXURAL BEHAVIOUR OF
ACTIVATED FLY ASH CONCRETE
SUNILAA GEORGE 1, *
Assistant Professor, Civil Department, VLBJCET Coimbatore 641042, Tamilnadu, South.India
http://www.vlbjcet.ac.in Dr.R.THENMOZHI2
Associate Professor, Civil Department, TPGIT Vellore 632 002, Tamilnadu, South India
www.tpgit.edu.in
Abstract
Cement concrete is the most widely used construction material in many infrastructure projects. The development
and use of mineral admixture for cement replacement is growing in construction industry mainly due to the
consideration of cost saving, energy saving, environmental production and conservation of resources. Present
study is aimed at replacing cement in concrete with activated fly ash. The paper highlights the chemical
activation of low calcium fly ash using CaO and Na2SiO3 in the ratio 1:8 for improving the pozzolanic
properties of fly ash .The investigation deals with the flexural behavior of beams using chemically activated fly
ash at various cement replacement levels of 10%, 20%, 30%, 40%, 50%, and 60% with water binder ratio
0.45.The results are compared with OPC and Activated Fly ash at the same replacement levels.
Keywords: Fly Ash; Activated fly ash; Calcium oxide; Sodium silicate; Flexural behaviour; ductility 1. Introduction
The development and use of mineral admixture for cement replacement is growing in construction industry mainly due to the consideration of fast construction, cost savings, environmental protection, and low consumption of resources and utilizing the bye products that are available. Mineral admixture generally used is raw fly ash, rice husk ash, metakaoline, silica fume, geopolymers etc. Addition of such materials improves the concrete and mortar property. The fly ash is among the commonly used mineral admixtures which is available in large quantities in many developing countries. As per the estimation of Government of India, power plants are going to use 1800 million tons of coal that may result 600 million tons of fly ash by 2031-2032 [1]. Ujjwal Bhattacharjee et al.,(2002),[2] has enlightened the areas in which fly ash usage has potential in India. He pointed out that despite quite optimistic levels of utilization of fly ash in India; only less than 25% of the total fly ash produced is being utilized.
Another important issue is reduction of the CO2 emissions generated during the Portland cement manufacture
The fly ash reactivity vs. Ca (OH)2, released during the cement or lime hydration process, is influenced by its
oxide, mineralogical and phase composition as well as its grain size distribution[5,6]. Several studies[ 7,8,9] provide information regarding the chemical activation of the fly ash – Ca(OH)2 reaction, also called “pozzolanic
reaction”, in order to permit the increase of the fly ash content in the blended Portland cements without a major negative impact on the early strength or even more in the attempt to produce clinker free binders[10].
The use of Na2SO4, CaCl2.2H2O or CaSO4.0.5H2O admixtures to fly ash-lime (80-20) systems, associated with
the hydrothermal curing at 50oC, lead to important increases of the mechanical strengths. Still the mechanical strengths of these binders after 28 days of hardening were small – 13-15 MPa[10,11] The chemical activation of class F fly ashes (with less than 10% CaO content) with sodium or potassium silicates with/without hydroxides (NaOH or KOH) admixtures, can lead to the hardening of this system even at the normal temperature.[12-14]. A new method of chemical activation with addition of Ca (OH) 2 (calcium hydroxide) and a small quantity of
Na2SiO3 was reported by Yueming F et al., (1990. Alkali activation showed improved accelerated setting and
hardening [10- 16]. Activated fly ash with CaO and Na2SiO3 improves the properties of RCC ,Sunilaa et
al.,(2011)[17].Although many studies are available with different activation methods , studies of reinforced concrete with activated fly ash(AFA) using CaO and Na2SiO3 are not available .Therefore a realistic assessment
of activated fly ash concrete is essentially needed that can be adopted easily . The objectives of present investigation are
to activate fly ash using CaO and Na2SiO3 in the ratio of 1:8 and
to study the mechanical properties as well as
to study flexural behaviour and ductility properties with various replacements of cement
To find maximum possible dosage of AFA in concrete. 2. Experimental Programme:
2.1 Materials Used
Ordinary Portland cement (OPC-43grade) confirming IS 8112:1989 was used for the entire investigation. Local river sand conforming to Zone III was used as Fine aggregate. Coarse aggregate used was hard broken granite stones drawn from an approved quarry near Coimbatore Tamilnadu, India. Aggregates used are conforming to IS: 383 (1970). Potable water was used in all mixes. The specific gravity of the materials used is given in Table.1.
Table 1.SPECIFIC GRAVITY
Material Specific Gravity
Cement 3.15
Fly Ash 2.2
Activated Fly Ash 2.3 Coarse Aggregate 2.86 Fine Aggregate 2.6
Fly ash : Fly ash(FA) in dry powder form conforming to IS: 3812 Part 1 – 2003 was obtained from Mettur Thermal power plant in India .The properties of fly ash is as given in Table.2
Fly ash Properties min% by mass
Chemical properties IS:3812-1981 Fly ash MTPP
SiO2+Al2O3+Fe2O3 70 90.5
SiO2 35 58
CaO 5 3.6
SO3 2.75 1.8
Na2O 1.5 2
L.O.I 12 2
MgO 5 1.91
2.2 Chemical Activation
Fly ash in dry powder form obtained from Mettur Thermal power plant in India was used for the entire study .Activation of fly ash was carried out using Calcium Oxide and Sodium Silicate in the ratio 1:8(this was arrived by trial study using various ratios of chemicals i.e. 1:2,1:4,1:6,1:8,1:10) ,[18]. The required quantity of sodium silicate in gel form and calcium oxide in paste form are mixed in a vessel and heated at a temperature of 103°C to ensure proper mixing. Further fly ash was added and mixed well and used for the studies. The reason for lower activity of FA arise mainly due to the dense glass layer which make fly ash chemically stable [15]
2.3. Mechanical Properties
Mechanical properties of AFA concrete and FA were studied using varying percentage replacement (0%, 10%, 20%, 30%, 40%, 50% and 60%) of cement by mass. The mix for concrete was designed as per IS: 10262:2009. The mix was designed for target strength of 27 MPa .The mix used for study is 1:1.18:3.02 and water to binder ratio chosen is 0.45 .All concrete specimens for testing were done in accordance with IS 516:1959. After 24 Hrs of casting, the specimens were de-molded and kept in water for curing. The cubes, cylinders and prisms are cast in replicates of three from the same batch of concrete.
2.3.1 Compressive Strength
The compressive strength of concrete was determined at 7, 14, 28, 56 and 90 days of curing. Tests were carried out on 150mm x 150mm x 150mm size cubes. A 2000 kNcapacity standard compression testing machine was used to conduct the test. The results of AFA concrete are compared with that of Fly ash concrete without activation
2.3.2. Split Tensile Strength
Split tensile strength was determined for 28, 56 and 90 days. The test was carried out on cylindrical specimens of 150mmdiameter and length 300 mm using 2000kN capacity compression testing machine.
2.3.3. Flexure Strength
The test was carried out on 100mm Х100mm Х500 mm size prisms using two point loading method. Maximum load applied to break the specimen and appearances of the fractured faces of concrete are noted.
2.4 FLEXURE STUDY ON AFA BEAMS
Two legged stirrups of 6mm diameter bar are provided at a spacing of 125mm c/c. the test variables include: Table 3. Gives the test program details
(a) Water /binder ratio
(b) Percentage replacement of FA (c) Percentage replacement of AFA
Beams are tested for 28 days curing. Beam set up details are given in Fig. 1
Table .3 Test Programme Details
series
Cross section of Beams: 100mm x 200mm, Length: 2.0m, Effective span: 1.5m
Beam Id Percentages of ash w/b
Ratio
No. of Specimens
B1
C1 0 0.45 1x3 sets
F1,F2,F3,F4,F5,F6 10,20,30,40,50,60 0.45 6x3 sets
AF1,AF2,AF3,AF4,AF5,AF6 10,20,30,40,50,60 0.45
6x3 sets
Fig.1 Test set up Fig.2 Beam after testing with flexure cracks
All the beams were tested for flexure under a loading frame of capacity 1000kN.These beams were tested on a span of 1500mm with simply supported conditions under two point loading. The companion cube specimens were also tested in the compression testing machine capacity 2000kN.
Fig.3 the crack patterns and failure of FA beams
Fig.4 the crack patterns and failure of AFA beams
3. RESULTS AND DISCUSSIONS 3.1 CHEMICAL ACTIVATION
The dense glass layer of fly ash protects the inside constituents which are porous, spongy and amorphous having higher chemical activity. The silica –alumina glassy chain of high Si, Al and low Ca is firm. This chain must be disintegrated to accelerate the pozzolanic activity. If concentration of hydroxyl ion (OH-) is high enough, the silica-alumina glassy chain will rapidly disintegrate and produce a large number of active groups [19,20] with a small addition of CaO paste and Na2SiO3, fly ash hydrolyses and forms NaOH. This
increases the pH of the medium thus greatly facilitating silica –alumina glassy chain corrosion [15].
The effectiveness of fly ash activation by CaO and Na2SiO3 is verified by scanned electron microscope
Fig 5(a) FA Fig.5 (b) AFA
Fig 5 Scanning electron microscopic observations of FA (a) and AFA (b)
a. The setting structure formation has close relation with the numbers of connecting points among cement hydration particles. Y. Fan et al., (1999). The AFA particle is smaller than that of cement particles, which can increase the degree of connection Fig. 6 and form inhomogeneous coagulation among cement particles, S.P. Jiang et al., (1993) promoting cement setting. For FA, which is inert in the early period, its connection with cement particles is weaker.
b. The strong capability of CaO absorption by AFA reduces the super saturation degree in liquid resulting from early hydration. This can speed up hydration, so that hydrates of AFA can behave as “crystal seeds,” promoting the growth of C-S-H and Ca (OH)2, which is advantageous to coagulative structure formation and gives more confinement leading to more strength.
Fig.6 FA and AFA effects on cement particles connection
(n - Number of connecting points).
3.2 MATERIAL PROPERTY
Table 4 gives the results of material property for control specimens FA specimens and AFA specimens. In the table fck fct, fcr shows the compressive strength, split tensile strength and modulus of rupture respectively.
Optimum value in case of FA is obtained in 30% replacement of ash where as in case of AFA it is found that 50% replacement is possible without adding any super plasticizers. FA concrete gains strength after 28 days and on reaching 90 days the strength for FA and AFA are comparable.
From the test results of cubes, cylinders and prisms it is found that due to the activation of fly ash by chemicals CaO and Na2SiO3 , there is significant improvement in the strength enhancement .The chemical activators
for FA are studied by many researchers Joseph F. Lamond 1983, Saraswathy et al [4,23]and found that only 30% replacement is only possible to give a comparable strength hence validates the present study result for FA.
T
Taabbllee 44.. MMaatteerriiaall ssttrreennggtthh rreessuullttss
ID
Average Compressive strength MPa
7 Days 14 Days 28 Days 56 Days 90 Days
fck fct fcr fck fct fcr fck fct fcr fck fct fcr fck fct fcr
CM 18.1 2.25 2.9 21.6 2.35 3.2 27.3 4.22 5.2 28.3 4.88 5.45 28.9 4.95 5.85
FA 10 15 0.91 2.62 16.1 1.65 2.92 19 4.08 4.56 28 4.68 4.85 29 4.74 4.9
FA 20 16.25 1.15 2.6 15.8 1.75 2.9 19.6 4.26 4.12 28.2 4.73 4.43 29.3 4.79 4.65
FA 30 16.5 1.25 2.35 15.4 1.85 2.84 19.8 4.38 3.83 29 4.75 4.1 29.8 4.8 4.45
FA 40 15.23 1.23 2.45 14 1.68 2.83 18.5 4.02 3.75 26.2 4.4 3.85 27 4.5 4.25
FA 50 14 1.21 2.21 13.6 1.56 2.71 18 3.76 3.65 25 4.2 3.67 25.5 4.3 4
FA 60 12 1.12 2.19 12.5 1.25 2.63 17.7 3.51 3.55 23.6 3.96 3.65 24 4.22 3.85
AFA 10 18 2.1 2.85 20.2 2.4 3.23 28 4.75 5.25 29.3 5.3 5.65 30.5 5.92 5.75
AFA 20 18.2 2.25 2.92 21.6 3.1 3.25 28.5 4.96 5.4 30.1 5.57 5.85 31 6.1 5.98
AFA 30 19 2.45 2.96 22.5 3.85 3.3 29.4 5.16 5.5 31 5.66 6.05 32 6.18 6.15
AFA 40 19.4 2.85 3.01 22.7 4.25 3.45 29.6 5.23 5.65 31.3 5.78 6.15 32.4 6.29 6.23
AFA 50 19.8 3.15 2.98 23 4.32 3.35 29.8 5.35 5.68 31.5 5.86 6.2 32.6 6.52 6.25
AFA 60 16.9 2.95 2.93 22.1 4.12 3.25 28.25 5 5.55 30.75 4.81 6.13 28.7 5.08 6.2
3.2LOAD DEFLECTION FOR B1 SERIES BEAMS
On application of incremental loads control specimens showed cracks at 25 kN and the ultimate load observed was 30kN with maximum deflection of 15.67 mm .The failure of the beam was due to diagonal tension. In the case of FA beams 30 % replacement gave higher first crack load (22kN) and ultimate load (24kN) with maximum deflection 13.7 mm.
Fig7.a Comparison of F1 beams Fig7.b Comparison of F2 beams 0
5 10 15 20 25 30 35
0 2 4 6 8 10 12 14 16 18
LOAD (k
N)
DEFLECTION (mm)
B1F 1 vs B1AF1
BICI
B1F1
B1AF1
0 5 10 15 20 25 30
0 2 4 6 8 10 12 14 16 18 20
LOAD (k
N)
DEFLECTION (mm) B1F 2 vs B1AF2
B1C1
B1F2
Fig7.c Comparison of F3 beams Fig7.d Comparison of F4 beams
Fig7.e Comparison of F5 beams Fig7.f Comparison of F6 beams
Fig 7 Load deflection curve for B1 series Beams
AFA concrete beams showed less deflection up to cracking load and after the first crack load deflection increased non linearly. B1 AF1 , B1AF2 and B1AF6 beams showed slightly lower strength compared to B1C1 (S.Gopalakrishnan,N.Lakshmanan 2001) [24]where as all other beams showed better performance compared to B1C1.Optimum results are obtained in case of 50% replacement of AFA . Bending behaviour of AFA beams was found to be similar to B1C1. The crack width was measured using a travelling microscope having least count 0.01mm. The crack spacing was found to be slightly more in the case of the FA and AFA beam specimens leading to higher crack width compared to the Control beams. However, for all the series of beams investigated, the crack width under service load was within the permissible limit as per IS 456:2000. The crack width,
0 5 10 15 20 25 30 35
0 2 4 6 8 10 12 14 16 18 20
L
O
AD (k
N)
DEFLECTION (mm)
B1F 3 vs B1AF3
B1AF3 BIC1 B1F3 0 5 10 15 20 25 30 35 40
0 5 10 15 20 25 30 35
L
OAD (kN)
DEFLECTION (mm)
B1F 4 vs B1AF4
B1F4 B1C1 B1AF4 0 5 10 15 20 25 30 35 40 45 50
0 5 10 15 20 25 30 35 40
L
O
AD (k
N)
DEFLECTION (mm)
B1F 5 vs B1AF5
B1AF5 B1C1 B1F5 0 5 10 15 20 25 30 35
0 5 10 15 20 25 30
L
O
AD (k
N)
DEFLECTION (mm)
B1F 6 vs B1AF6
B1F6
B1C1
displacement ductility and service loads are given in Table .5.Service loads are determined by dividing ultimate load by factor 1.5.
Table 5 Crack width for Beam Series B1
Specimen Description Crack Width
mm
Displacement ductility Service loads
kN
C1 0.2 2.15 20
FA1 0.1 2.34 12
FA2 0.06 2.37 13.33
FA3 0.12 2.36 16
FA4 0.15 2.31 11.33
FA5 0.07 2.2 10.67
FA6 0.2 2.18 10.67
AFA1 0.19 2.28 18
AFA2 0.18 2.32 18.67
AFA3 0.2 2.46 21.33
AFA4 0.05 2.52 23.33
AFA5 0.11 2.77 31.33
AFA6 0.12 2.06 19.33
4. CONCLUSIONS
Activation of fly ash employed in this study is simple. The activated fly ash can be made available in dry form which is beneficial for transportation and storage
AFA concrete gave 50% more compressive strength compared to FA concrete at 28 days of curing. The reason for more material strength is due to the formation of C-S-H gel due to chemical activation of fly ash. Stable glass beads of fly ash particles are destroyed by alkaline chemicals resulting in closer link between particles resulting in confinement leading to more strength
At 90 days the results for FA are found to be comparable with AFA concrete. The reason for this is slow pozzolanic reaction of FA concrete which starts at a later stage than 28 days.
Split tensile strength of AFA 50% replacement showed an increase of 22% in case of FA concrete and 26.7% for Control specimen. This attributes for higher ultimate strength of beam.
Load deflection study gave similar post crack behaviour in comparison to Control beams. Ultimate strength for FA beams was comparatively less when compared to AFA beams and Control beams
The crack width under service load was within the permissible limit as per IS 456:2000
Displacement ductility vales are found to increase in case of AFA and FA concrete
Present study shows that an optimum replacement of 50% can be used in structures
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