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Fracture behaviour of heat cured fly ash based geopolymer concrete
Materials and Design 44 (2013) 580–586
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Fracture behaviour of heat cured fly ash based geopolymer concrete Prabir K. Sarker a,⇑, Rashedul Haque b, Karamchand V. Ramgolam c a
Department of Civil Engineering, Curtin University, Western Australia, Australia SGS Australia, Western Australia, Australia c Shire of Kalamunda, Western Australia, Australia b
a r t i c l e
i n f o
Article history: Received 3 July 2012 Accepted 3 August 2012 Available online 11 August 2012 Keywords: Fly ash Fracture Geopolymer concrete
a b s t r a c t Use of fly ash based geopolymer as an alternative binder can help reduce CO2 emission of concrete. The binder of geopolymer concrete (GPC) is different from that of ordinary Portland cement (OPC) concrete. Thus, it is necessary to study the effects of the geopolymer binder on the behaviour of concrete. In this study, the effect of the geopolymer binder on fracture characteristics of concrete has been investigated by three point bending test of RILEM TC 50 – FMC type notched beam specimens. The peak load was generally higher in the GPC specimens than the OPC concrete specimens of similar compressive strength. The failure modes of the GPC specimens were found to be more brittle with relatively smooth fracture planes as compared to the OPC concrete specimens. The post-peak parts of the load–deflection curves of GPC specimens were steeper than that of OPC concrete specimens. Fracture energy calculated by the work of fracture method was found to be similar in both types of concrete. Available equations for fracture energy of OPC concrete yielded conservative estimations of fracture energy of GPC. The critical stress intensity factor of GPC was found to be higher than that of OPC concrete. The different fracture behaviour of GPC is mainly because of its higher tensile strength and bond strength than OPC concrete of the same compressive strength. Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction The global demand of cement for construction of infrastructures is continuously increasing in order to maintain the ongoing growth and accommodate the needs of the increasing population. OPC has been traditionally used as the binder in concrete. About 1 tonne of carbon dioxide is emitted into the atmosphere in the production process of 1 tonne of cement. This makes a significant contribution to the global greenhouse gas emission. Therefore, development of alternative binders utilising industrial by-products is necessary to reduce the carbon footprint of the construction industry. Geopolymer is an emerging alternative binder for concrete that uses by-product materials. A base material that is rich in Silicon (Si) and Aluminum (Al) is reacted by an alkaline solution to produce the geopolymer binder. Source materials such as fly ash [1,2], metakaolin [3] and blast furnace slag [4] can be used to make geopolymer. Fly ash blended with blast furnace slag [5] and rice husk ash [6] has also been used as the base material for geopolymer. The product of the reaction is an inorganic polymer which binds the aggregates together to make geopolymer concrete. The coal-fired power stations worldwide generate substantial amount of fly ash
⇑ Corresponding author. Tel.: +61 8 9266 7568; fax: +61 8 9266 2681. E-mail addresses: [email protected] (P.K. Sarker), Rashedul.Haque@sgs. com (R. Haque), [email protected] (K.V. Ramgolam). 0261-3069/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.matdes.2012.08.005
as a by-product that can be efficiently used in geopolymer concrete to help reduce the carbon footprint of concrete production. The results of recent studies have shown the potential use of heat-cured fly ash based geopolymer concrete as a construction material. As a relatively new material, it is necessary to study the various properties of GPC as compared to the traditional OPC concrete in order to determine its suitability for structural applications. The ongoing research on fly ash-based geopolymer concrete studied several short-term and long-term properties. It was shown that heat-cured geopolymer concrete possesses the properties of high compressive strength, low drying shrinkage and creep, and good resistance to sulfate and acid [1,7]. Geopolymer concrete was found to have higher bond strength with reinforcing steel and relatively higher splitting tensile strength than OPC concrete [2,8,9]. Geopolymer concrete beams and columns were tested to failure and they showed similar or better performance as compared to OPC concrete members [10–12]. Heat-cured geopolymer concrete showed higher residual strength than OPC concrete cylinders after exposure to high temperature heat of up to 800 °C [13]. Therefore, heat-cured geopolymer concrete is considered as an ideal material for precast concrete structural members. Development of the constitutive model for a material requires its fracture parameters. The fracture characteristics of a material are used to describe the formation and propagation of cracks in the material. The crack path through a composite material such
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P.K. Sarker et al. / Materials and Design 44 (2013) 580–586 Table 1 Chemical composition and loss on ignition of cement and fly ash (mass%). Parameter
SiO2
Al2O3
Fe2O3
CaO
MgO
SO3
K2O
Na2O
P2 O5
Loss on ignition
Cement Fly ash
21.10 50.50
4.70 26.57
2.70 13.77
63.60 2.13
2.60 1.54
2.50 0.41
– 0.77
0.50 0.45
– 1.00
2.00 0.60
Table 2 Mixture proportions of concrete (kg/m3). Mixture
Cement
Fly ash
Water
Sodium hydroxide
Sodium silicate
Sand
Coarse aggregate 7 mm
10 mm
OPC1 OPC2 OPC3 OPC4
310 345 375 420
– – – –
152 163 180 190
– – – –
– – – –
870 900 815 830
341 320 355 360
682 620 711 648
GPC1 GPC2 GPC3
– – –
408 408 408
4 – –
62 62 68
93 93 103
647 647 647
647 647 647
554 554 554
as concrete is dependent on the mechanical interaction between the aggregates and the binder matrix. Fracture energy of a composite material depends on the deviation of the crack path from an idealized crack plane [14,15]. Since the binder in geopolymer concrete is different from that in OPC concrete, the effect of the interaction between the aggregates and the geopolymer binder needs to be investigated. Thus, it is necessary to study the fracture parameters of geopolymer concrete to understand its failure behaviour. In this study, the fracture properties of heat cured fly ash based geopolymer concrete specimens were determined from three-point bending test of notched beams. Fracture energy and the critical stress intensity factor were also determined for OPC concrete specimens to compare with those of geopolymer concrete specimens of similar compressive strengths and containing the same aggregates. The fracture behaviours of both types of concrete were compared using the test results. 2. Experimental work Geopolymer and OPC concrete notched beam specimens were cast and tested for three-point bending. The fracture energy values were calculated from the load–deflection curves of the test specimens by using the work of fracture method. The critical stress intensity factors of the specimens were calculated by using the maximum load and the geometry of the specimen. The load– deflection curves and fracture planes of GPC and OPC concrete specimens were compared. 2.1. Materials General purpose Portland cement was used for the OPC concrete specimens. Commercially available fine grade class F [16] fly ash was used to make geopolymer concrete. The percentage of the fly ash passing through a 45 l sieve was 75%. The chemical compositions of cement and fly ash are given in Table 1. The alkaline liquids for geopolymer concrete were sodium hydroxide and sodium silicate solutions. Sodium hydroxide pellets were dissolved in water to make 14 M solution. A 14 M sodium hydroxide solution contains 560 g of sodium hydroxide solids in 1 l of the solution. In other words, 1 kg of the 14 M sodium hydroxide solution is found to have 404 g of solid sodium hydroxide and 596 g of water. The sodium silicate solution had a chemical composition of 9.1% Na2O, 28.9% SiO2, and 62% water by mass. The coarse aggregates were 7 and 10 mm nominal size crushed stone. Locally available
river sand was used as the fine aggregate and potable tap water was used in mixing of the concretes. Four OPC and three GPC mixtures were used to cast the test specimens. The mixture proportions of GPC and OPC concrete are given in Table 2. These mixture proportions were obtained by carrying out trial mixes before the actual mixing. No extra water was used in the GPC mixtures GPC2 and GPC3. Slump of the OPC concrete varied between 75 and 120 mm and that of the geopolymer concrete varied from 185 to 220 mm. The test specimens were cast in moulds made of form-ply and compacted by using a vibrating table. The OPC concrete specimens were cured by immersing in water for 28 days and the GPC specimens were heat cured at 60 °C for 24 h and then left in ambient condition until testing. 2.2. Test specimens and testing Standard cylinder specimens of 100 mm 200 mm size were tested for compressive strength of each batch of concrete. The fracture test specimens were 100 mm 100 mm section and 600 mm long beams with a 25 mm deep notch in the middle of the beam. Different ratios of the notch depth to beam depth were used in fracture test specimens available in literature [17–20], though a ratio of 0.5 is recommended by RILEM [21]. Elices and Planas [22] conducted an analysis of test results for different notch to beam’s depth ratios and showed that a ratio of 0.2–0.5 can be considered as a practical experimental range. A ratio of 0.25 was used in the specimens of this study to make the ligament area sizable in order to enable the observation of the crack propagation in the concrete. The notch was generated during casting of the specimens. Cutting of the notch after drying of the specimen was not used in order to avoid formation of fine cracks in the ligament during the cutting process. The specimens were painted with white lime water to facilitate observation of the propagation of cracks during the test. Three point bending tests were performed in deflection controlled mode by using a very stiff closed loop Instron Servo Control machine. The ends of the test specimen were placed on the supporting rollers at a span of 500 mm with the notch on tension side, as shown in Fig. 1. The Instron machine had a built in digital data acquisition system. It was incorporated with a load cell to record the load with an accuracy of 0.001 kN and a digital strain gauge measuring the vertical displacement with an accuracy of 0.001 mm. The data acquisition system had the ability to record up to 1000 data per second. Three identical specimens were tested for each mixture except for mixture GPC2, for which two specimens were tested.
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ing test of the notched beam and the dimensions of the specimen [25,26]. Eq. (2), [25,27] has been used in this study to calculate the critical stress intensity factor from the specimen geometry and the maximum value of load recorded in the test.
K ic ¼
3Pl pffiffiffi að1:93 3:07A þ 14:53A2 25:11A3 þ 25:8A4 Þ 2
2bd
ð2Þ
where A = (a/d), a is the depth of the notch (mm), d is the depth of the beam (mm), b is the width of the beam (mm), P is the maximum load (N) and l is the span length of the beam (mm). 3. Test results and discussions The test results are given in Table 3. The 28-day compressive strengths of the Mixtures OPC1, 2, 3 and 4 were 31, 36, 43 and 51 MPa respectively. The compressive strengths of the mixes GPC1, 2 and 3 were 32, 36 and 48 MPa respectively. Generally, the modulus of elasticity increased with the increase of compressive strength in both types of concrete. These test results are the mean values for three specimens of each mixture.
Fig. 1. Notched beam specimen in the test machine.
The loading rate used in the fracture tests of concrete specimens by previous researchers varied in a considerable range, such as 0.05 mm/min [19], 0.18 mm/min [23] and 0.5 mm/min [24]. A loading rate of 0.18 mm/min was used in the tests of this study. A high rate of data scanning per second was used to capture the post-peak part of the load–deflection curve. The load–deflection data were plotted to calculate the area under the curve. The fracture energy (GF) was then calculated from the work of fracture by using Eq. (1) [21].
GF ¼ ðW 0 þ mgd0 Þ=Alig
3.1. Load–deflection behaviour As the applied load on the notched beam was increasing, no cracks were observed until the load reached its peak value. A crack appeared from the end of the notch and started to propagate fast in the ligament when the load reached its peak value. The crack opened faster in the GPC specimens than in the OPC concrete specimens. Failure occurred by opening of a single crack in the ligament in both types of concrete specimens. For calculating the fracture energy by the work of fracture method, the load–deflection curves were corrected for the initial non-linearity due to deformation of the specimen at the supports, as recommended in the RILEM guidelines [21]. The typical load– deflection diagrams of GPC and OPC concrete specimens are given in Figs. 2–4. It is seen from these figures that the peak load of geopolymer concrete specimen was generally higher than that of the OPC concrete specimen of the similar compressive strength. The maximum load for each test specimen is given in Table 3. The post-peak parts of the load–deflection curves of geopolymer con-
ð1Þ
where W0 is the area under the load–deflection curve (N–m), m is the mass of the beam between the supports (kg), g is the acceleration due to gravity (m2/s), d0 is the deflection at final failure of the beam (m) and Alig is the area of the ligament (m2). The term fracture toughness or critical stress intensity factor (Kic) is used to indicate the magnitude of the stress concentration that exists in front of the crack tip when the crack starts to propagate. The most common method of calculation of the critical stress intensity factor is to use the peak load from the three point bend-
Table 3 Test and calculated results of the OPC and GPC specimens. Batch
Compressive strength, fc (MPa)
Specimen
Peak load, P (kN)
Fracture energy, GF (N/m)
Mean GF (N/m)
GF Eq. (3) (N/m)
31
1 2 3
2.10 1.98 2.28
113 109 154
125
56.7
36
1 2 3
2.68 1.96 2.71
134 97 140
124
43
1 2 3
3.42 3.33 3.65
141 123 170
51
1 2 3
3.06 3.58 3.07
32
1 2 3
OPC1
OPC2
OPC3
OPC4
GPC1
GPC2
36
GPC3 48
Critical stress intensity factor, Kic (MPa-mm0.5)
Mean Kic
85.8
14.1 13.3 15.3
14.2
63.0
91.8
18.0 13.2 18.2
16.4
144
71.3
98.2
22.9 22.3 24.4
23.2
162 210 167
180
80.4
106.3
20.5 24.0 20.5
21.7
3.11 2.68 3.02
102 107 92
100
58.0
105.5
20.9 17.9 20.3
19.7
1 2
3.91 3.36
142 140
141
63.0
114.0
1 2 3
4.62 5.03 4.52
170 213 209
198
77.0
GF Eq. (4) (N/m)
125.3
26.2 22.5 30.9 33.6 30.3
24.4 31.6
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Fracture energy (N/m)
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300 Test OPC
250 200
Test GPC
150 CEB -FIP
100 50 0 30.0
35.0
40.0
45.0
50.0
Bazant and BecqGiraudon: OPC Bazant and BecqGiraudon: GPC
55.0
Compressive strength (MPa)
Fig. 6. Variation of fracture energy of GPC and OPC concrete with compressive strength.
Fig. 2. Load–deflection diagrams of specimens of batches OPC1 and GPC1.
crete specimens were generally steeper than those of the similar OPC concrete specimens. The post-peak load usually dropped faster in the GPC specimens than in the OPC concrete specimens, as shown by the load–deflection diagrams of Figs. 2–4. As expected, the post-peak part of the curve was generally steeper for higher compressive strength in both types of concrete. 3.2. Fracture energy
Fig. 3. Load–deflection diagrams of specimens of batches OPC2 and GPC2.
Fig. 4. Load–deflection diagrams of specimens of batches OPC3, OPC4 and GPC3.
The area under the load–deflection curve for each notched beam specimen was calculated and used in Eq. (1) to obtain the fracture energy of each specimen. These values are given in Table 3. The mean value of the fracture energy for the specimens of each batch is also given in the table. The mean fracture energy values of the GPC and OPC concrete batches are plotted in Fig. 5. It can be seen that the fracture energy of batch GPC1 is slightly smaller than that of batch OPC1, while the fracture energy of batch GPC2 is slightly higher than that of batch OPC2. The fracture energy of batch GPC3 is higher than those of OPC3 and OPC4. Generally, it can be seen that the fracture energy of GPC is comparable to that of OPC concrete mixtures of similar compressive strength. The fracture energy tended to be higher for GPC than OPC concrete as the compressive strength increased. The fracture energy values of the specimens of each batch are plotted against compressive strength in Fig. 6. Considerable scatter is observed in the fracture energy values of specimens of each batch for both geopolymer and OPC concrete. The coefficient of variation of the fracture energy values of OPC concrete varied from 15% to 20% and that of the GPC varied from 8% to 15%. It can be seen from the plots that fracture energy tends to increase with the compressive strength in both types of concrete. It is also seen from the trend of the data that the fracture energy of GPC tends to increase with compressive strength at a higher rate than OPC concrete. CEB-FIP [28] proposed Eq. (3) to estimate the fracture energy of OPC concrete in terms of the compressive strength and the maxi-
35 30
200 150
OPC
100
GPC 50 0
Kic (MPa-mm 0.5)
Fracture energy (N/m)
250
25 20 15
OPC
10
GPC
5 1
2
3
4
Mix Id Fig. 5. Mean fracture energy of the GPC and OPC concrete batches.
0
1
2
3
4
Mix Id Fig. 7. Mean critical stress intensity factors of the OPC and GPC batches.
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mum aggregate size. This equation was used to calculate the fracture energy of the GPC and OPC concrete test specimens. The values calculated by Eq. (3) are given in Table 3 and plotted in Fig. 6. It is seen from Fig. 6 that the test values of fracture energy of both GPC and OPC concrete specimens obtained by using the work of fracture method are higher than those predicted by the CEB-FIP equation.
GF ¼ ð0:0469 D2max 0:5Dmax þ 26Þ
GF ¼ 2:5a0
fc 0:051
0:7
fc 10
0:46 0:22 0:30 Dmax w 1þ c 11:27
ð3Þ
ð4Þ
where Dmax is the maximum aggregate size (mm), fc is compressive strength of concrete (MPa), a0 is aggregates shape factor (a0 = 1 for rounded aggregates, a0 = 1.44 for angular aggregates), w/c is the water–cement ratio of the concrete. Bazant and Becq-Giraduon [29] proposed Eq. (4) for fracture energy of OPC concrete in terms of the compressive strength, maximum aggregate size and the water to cement ratio of the concrete. The equation was proposed based on a statistical analysis of 238 test data on fracture energy obtained by the work of fracture test of specimens of size varying in a wide range. The equation has a term of water to cement ratio that is relevant to OPC concrete. Since GPC mixtures do not have cement and they use liquids different from water, the term water to cement ratio is not directly applicable to GPC. For this reason, the liquid to fly ash ratio was used for the GPC mixtures as an equivalent term of water to cement ratio. The liquid content was obtained by adding up the masses of sodium hydroxide solution, sodium silicate solution and extra water in calculation of the liquid to fly ash ratio. Thus, the fracture energy values of the GPC and OPC concrete specimens were also predicted by using Eq. (4). The predicted values are given in Table 3 and plotted in Fig. 6. It can be seen from Fig. 6 that the predictions by this equation are higher than those by the CEB-FIP equation (Eq. (3)). The predictions by Eq. (4) are also closer to the test results obtained by the work of fracture method for both the GPC and OPC concrete specimens. Smaller predictions of the fracture energy by the CEB-FIP equation than the test results of OPC concrete specimens were also observed by Camoes et al. [30].
Fig. 9. Typical fracture planes of OPC2 (top) and GPC2 (bottom) concrete specimens.
3.3. Critical stress intensity factor The value of stress intensity factor indicates the magnitude of the stress concentration in front of the crack tip when the crack starts to propagate. The critical stress intensity factors of the GPC and OPC concrete specimens were calculated by using Eq. (2). The value for each test specimen and the mean value for the specimens of each batch of concrete are given in Table 3. The mean val-
Fig. 10. Typical fracture planes of OPC4 (top) and GPC3 (bottom) concrete specimens.
Fig. 8. Variation of critical stress intensity factors of OPC and GPC with compressive strength.
ues of the critical stress intensity factors for each batch are plotted in Fig. 7. It can be seen from the figure that the critical stress intensity factor of geopolymer concrete is higher than that of OPC concrete of similar compressive strength. The critical stress intensity factors for all the specimens are plotted against compressive strength in Fig. 8. It can be seen that critical stress intensity factor tends to increase with compressive strength in both GPC and OPC concrete. Also, the critical stress
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It was also observed that geopolymer concrete has a higher tensile strength than OPC concrete of the same compressive strength [1,8]. The higher bond strength at the interface of the geopolymer binder and the aggregates makes the fracture plane pass more through the aggregates than through the interface. It was shown from morphological studies that the use of soluble silicates in geopolymers results in a denser interfacial transition zone (ITZ) between aggregates and geopolymer matrix as compared to that with OPC matrix [31]. Zhang et al. [32] showed that there is no obvious difference between microstructures of the ITZ and the bulk geopolymer matrix. Thus, the ITZ in GPC is considered to be stronger than that in OPC concrete. The stronger ITZ contributed to the higher splitting tensile strength and bond strength of geopolymer concrete. The higher tensile and bond strengths of geopolymer concrete increased its critical stress intensity factor which resulted in less tortuosity of the fracture plane and relatively more brittle type of failure than in the OPC concrete specimens of similar compressive strength. 4. Conclusions This study investigated the fracture behaviour of geopolymer concrete as compared to OPC concrete of similar compressive strength and containing the same size and type of aggregates. The experimental work consisted of three point bending test of notched beam specimens made from three GPC and four OPC concrete mixtures. The fracture energy was determined by the work of fracture method using the load–deflection curves and the critical stress intensity factor was determined from the peak load and geometry of the specimen. The following conclusions are drawn from the study:
Fig. 11. Typical fracture planes of 36 MPa OPC (top) and GPC concrete (bottom).
intensity factors for the GPC concrete specimens are higher than those of the OPC concrete specimens. This shows that the critical stress at which a cracking occurs is higher in GPC than in OPC concrete. Therefore, the crack resistance of GPC is higher than that of OPC of the same compressive strength. This behaviour is consistent with the previous findings that geopolymer concrete has higher tensile and bond strengths than OPC concrete of the same compressive strength. 3.4. Fracture planes Generally, the GPC specimens failed in a more brittle manner than the OPC concrete specimens. Typical fracture failure planes of GPC and OPC concrete specimens are shown in Figs. 9–11. It can be seen from these figures that the fracture planes in the OPC concrete specimens were more tortuous and those in the GPC specimens were relatively smooth for the concretes of similar compressive strength. The fracture planes in the OPC concrete specimens passed more around the aggregates and those in the GPC specimens generally passed more through the aggregates. This difference can be seen clearly in the fractured planes of OPC and GPC specimens shown in Fig. 11. The reason for the fracture plane passing more through the aggregates in GPC is believed to be the higher bond strength of geopolymer with aggregates than that of the OPC binder. It was shown in the previous studies [2,8,9] that GPC has higher bond strength with reinforcing steel than OPC concrete.
1. The failure modes of the heat cured GPC specimens were generally more brittle than those of the OPC concrete specimens. The fracture planes of the GPC specimens were less tortuous than those of OPC concrete specimens. 2. Fracture energy of geopolymer concrete was similar to that of OPC concrete. Fracture energy increased with compressive strength in both types of concrete. The test values of fracture energy of both types of concrete were higher than those calculated by the equations of CEB-FIP and Bazant and Becq-Giraudon. The predictions by the equation of Bazant and BecqGiraudon were closer to the test values. 3. The critical stress intensity factor of the GPC specimens was higher than that of the OPC concrete specimens for the same compressive strength. This indicates that GPC needs a higher stress than OPC concrete for the formation of cracks. This was also observed from the load–deflection diagrams as the peak load or the load at which cracks appeared were generally higher in the GPC specimens than in similar OPC concrete specimens. 4. The difference in the fracture behaviours of GPC and OPC concrete is because of the higher bond and tensile strengths of GPC. The denser interfacial transition zone of GPC resulted in higher critical stress intensity factor and more brittle type of failure with smoother fracture plane as compared to OPC concrete.
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Materials and Design journal homepage: www.elsevier.com/locate/matdes
Fracture behaviour of heat cured fly ash based geopolymer concrete Prabir K. Sarker a,⇑, Rashedul Haque b, Karamchand V. Ramgolam c a
Department of Civil Engineering, Curtin University, Western Australia, Australia SGS Australia, Western Australia, Australia c Shire of Kalamunda, Western Australia, Australia b
a r t i c l e
i n f o
Article history: Received 3 July 2012 Accepted 3 August 2012 Available online 11 August 2012 Keywords: Fly ash Fracture Geopolymer concrete
a b s t r a c t Use of fly ash based geopolymer as an alternative binder can help reduce CO2 emission of concrete. The binder of geopolymer concrete (GPC) is different from that of ordinary Portland cement (OPC) concrete. Thus, it is necessary to study the effects of the geopolymer binder on the behaviour of concrete. In this study, the effect of the geopolymer binder on fracture characteristics of concrete has been investigated by three point bending test of RILEM TC 50 – FMC type notched beam specimens. The peak load was generally higher in the GPC specimens than the OPC concrete specimens of similar compressive strength. The failure modes of the GPC specimens were found to be more brittle with relatively smooth fracture planes as compared to the OPC concrete specimens. The post-peak parts of the load–deflection curves of GPC specimens were steeper than that of OPC concrete specimens. Fracture energy calculated by the work of fracture method was found to be similar in both types of concrete. Available equations for fracture energy of OPC concrete yielded conservative estimations of fracture energy of GPC. The critical stress intensity factor of GPC was found to be higher than that of OPC concrete. The different fracture behaviour of GPC is mainly because of its higher tensile strength and bond strength than OPC concrete of the same compressive strength. Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction The global demand of cement for construction of infrastructures is continuously increasing in order to maintain the ongoing growth and accommodate the needs of the increasing population. OPC has been traditionally used as the binder in concrete. About 1 tonne of carbon dioxide is emitted into the atmosphere in the production process of 1 tonne of cement. This makes a significant contribution to the global greenhouse gas emission. Therefore, development of alternative binders utilising industrial by-products is necessary to reduce the carbon footprint of the construction industry. Geopolymer is an emerging alternative binder for concrete that uses by-product materials. A base material that is rich in Silicon (Si) and Aluminum (Al) is reacted by an alkaline solution to produce the geopolymer binder. Source materials such as fly ash [1,2], metakaolin [3] and blast furnace slag [4] can be used to make geopolymer. Fly ash blended with blast furnace slag [5] and rice husk ash [6] has also been used as the base material for geopolymer. The product of the reaction is an inorganic polymer which binds the aggregates together to make geopolymer concrete. The coal-fired power stations worldwide generate substantial amount of fly ash
⇑ Corresponding author. Tel.: +61 8 9266 7568; fax: +61 8 9266 2681. E-mail addresses: [email protected] (P.K. Sarker), Rashedul.Haque@sgs. com (R. Haque), [email protected] (K.V. Ramgolam). 0261-3069/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.matdes.2012.08.005
as a by-product that can be efficiently used in geopolymer concrete to help reduce the carbon footprint of concrete production. The results of recent studies have shown the potential use of heat-cured fly ash based geopolymer concrete as a construction material. As a relatively new material, it is necessary to study the various properties of GPC as compared to the traditional OPC concrete in order to determine its suitability for structural applications. The ongoing research on fly ash-based geopolymer concrete studied several short-term and long-term properties. It was shown that heat-cured geopolymer concrete possesses the properties of high compressive strength, low drying shrinkage and creep, and good resistance to sulfate and acid [1,7]. Geopolymer concrete was found to have higher bond strength with reinforcing steel and relatively higher splitting tensile strength than OPC concrete [2,8,9]. Geopolymer concrete beams and columns were tested to failure and they showed similar or better performance as compared to OPC concrete members [10–12]. Heat-cured geopolymer concrete showed higher residual strength than OPC concrete cylinders after exposure to high temperature heat of up to 800 °C [13]. Therefore, heat-cured geopolymer concrete is considered as an ideal material for precast concrete structural members. Development of the constitutive model for a material requires its fracture parameters. The fracture characteristics of a material are used to describe the formation and propagation of cracks in the material. The crack path through a composite material such
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P.K. Sarker et al. / Materials and Design 44 (2013) 580–586 Table 1 Chemical composition and loss on ignition of cement and fly ash (mass%). Parameter
SiO2
Al2O3
Fe2O3
CaO
MgO
SO3
K2O
Na2O
P2 O5
Loss on ignition
Cement Fly ash
21.10 50.50
4.70 26.57
2.70 13.77
63.60 2.13
2.60 1.54
2.50 0.41
– 0.77
0.50 0.45
– 1.00
2.00 0.60
Table 2 Mixture proportions of concrete (kg/m3). Mixture
Cement
Fly ash
Water
Sodium hydroxide
Sodium silicate
Sand
Coarse aggregate 7 mm
10 mm
OPC1 OPC2 OPC3 OPC4
310 345 375 420
– – – –
152 163 180 190
– – – –
– – – –
870 900 815 830
341 320 355 360
682 620 711 648
GPC1 GPC2 GPC3
– – –
408 408 408
4 – –
62 62 68
93 93 103
647 647 647
647 647 647
554 554 554
as concrete is dependent on the mechanical interaction between the aggregates and the binder matrix. Fracture energy of a composite material depends on the deviation of the crack path from an idealized crack plane [14,15]. Since the binder in geopolymer concrete is different from that in OPC concrete, the effect of the interaction between the aggregates and the geopolymer binder needs to be investigated. Thus, it is necessary to study the fracture parameters of geopolymer concrete to understand its failure behaviour. In this study, the fracture properties of heat cured fly ash based geopolymer concrete specimens were determined from three-point bending test of notched beams. Fracture energy and the critical stress intensity factor were also determined for OPC concrete specimens to compare with those of geopolymer concrete specimens of similar compressive strengths and containing the same aggregates. The fracture behaviours of both types of concrete were compared using the test results. 2. Experimental work Geopolymer and OPC concrete notched beam specimens were cast and tested for three-point bending. The fracture energy values were calculated from the load–deflection curves of the test specimens by using the work of fracture method. The critical stress intensity factors of the specimens were calculated by using the maximum load and the geometry of the specimen. The load– deflection curves and fracture planes of GPC and OPC concrete specimens were compared. 2.1. Materials General purpose Portland cement was used for the OPC concrete specimens. Commercially available fine grade class F [16] fly ash was used to make geopolymer concrete. The percentage of the fly ash passing through a 45 l sieve was 75%. The chemical compositions of cement and fly ash are given in Table 1. The alkaline liquids for geopolymer concrete were sodium hydroxide and sodium silicate solutions. Sodium hydroxide pellets were dissolved in water to make 14 M solution. A 14 M sodium hydroxide solution contains 560 g of sodium hydroxide solids in 1 l of the solution. In other words, 1 kg of the 14 M sodium hydroxide solution is found to have 404 g of solid sodium hydroxide and 596 g of water. The sodium silicate solution had a chemical composition of 9.1% Na2O, 28.9% SiO2, and 62% water by mass. The coarse aggregates were 7 and 10 mm nominal size crushed stone. Locally available
river sand was used as the fine aggregate and potable tap water was used in mixing of the concretes. Four OPC and three GPC mixtures were used to cast the test specimens. The mixture proportions of GPC and OPC concrete are given in Table 2. These mixture proportions were obtained by carrying out trial mixes before the actual mixing. No extra water was used in the GPC mixtures GPC2 and GPC3. Slump of the OPC concrete varied between 75 and 120 mm and that of the geopolymer concrete varied from 185 to 220 mm. The test specimens were cast in moulds made of form-ply and compacted by using a vibrating table. The OPC concrete specimens were cured by immersing in water for 28 days and the GPC specimens were heat cured at 60 °C for 24 h and then left in ambient condition until testing. 2.2. Test specimens and testing Standard cylinder specimens of 100 mm 200 mm size were tested for compressive strength of each batch of concrete. The fracture test specimens were 100 mm 100 mm section and 600 mm long beams with a 25 mm deep notch in the middle of the beam. Different ratios of the notch depth to beam depth were used in fracture test specimens available in literature [17–20], though a ratio of 0.5 is recommended by RILEM [21]. Elices and Planas [22] conducted an analysis of test results for different notch to beam’s depth ratios and showed that a ratio of 0.2–0.5 can be considered as a practical experimental range. A ratio of 0.25 was used in the specimens of this study to make the ligament area sizable in order to enable the observation of the crack propagation in the concrete. The notch was generated during casting of the specimens. Cutting of the notch after drying of the specimen was not used in order to avoid formation of fine cracks in the ligament during the cutting process. The specimens were painted with white lime water to facilitate observation of the propagation of cracks during the test. Three point bending tests were performed in deflection controlled mode by using a very stiff closed loop Instron Servo Control machine. The ends of the test specimen were placed on the supporting rollers at a span of 500 mm with the notch on tension side, as shown in Fig. 1. The Instron machine had a built in digital data acquisition system. It was incorporated with a load cell to record the load with an accuracy of 0.001 kN and a digital strain gauge measuring the vertical displacement with an accuracy of 0.001 mm. The data acquisition system had the ability to record up to 1000 data per second. Three identical specimens were tested for each mixture except for mixture GPC2, for which two specimens were tested.
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ing test of the notched beam and the dimensions of the specimen [25,26]. Eq. (2), [25,27] has been used in this study to calculate the critical stress intensity factor from the specimen geometry and the maximum value of load recorded in the test.
K ic ¼
3Pl pffiffiffi að1:93 3:07A þ 14:53A2 25:11A3 þ 25:8A4 Þ 2
2bd
ð2Þ
where A = (a/d), a is the depth of the notch (mm), d is the depth of the beam (mm), b is the width of the beam (mm), P is the maximum load (N) and l is the span length of the beam (mm). 3. Test results and discussions The test results are given in Table 3. The 28-day compressive strengths of the Mixtures OPC1, 2, 3 and 4 were 31, 36, 43 and 51 MPa respectively. The compressive strengths of the mixes GPC1, 2 and 3 were 32, 36 and 48 MPa respectively. Generally, the modulus of elasticity increased with the increase of compressive strength in both types of concrete. These test results are the mean values for three specimens of each mixture.
Fig. 1. Notched beam specimen in the test machine.
The loading rate used in the fracture tests of concrete specimens by previous researchers varied in a considerable range, such as 0.05 mm/min [19], 0.18 mm/min [23] and 0.5 mm/min [24]. A loading rate of 0.18 mm/min was used in the tests of this study. A high rate of data scanning per second was used to capture the post-peak part of the load–deflection curve. The load–deflection data were plotted to calculate the area under the curve. The fracture energy (GF) was then calculated from the work of fracture by using Eq. (1) [21].
GF ¼ ðW 0 þ mgd0 Þ=Alig
3.1. Load–deflection behaviour As the applied load on the notched beam was increasing, no cracks were observed until the load reached its peak value. A crack appeared from the end of the notch and started to propagate fast in the ligament when the load reached its peak value. The crack opened faster in the GPC specimens than in the OPC concrete specimens. Failure occurred by opening of a single crack in the ligament in both types of concrete specimens. For calculating the fracture energy by the work of fracture method, the load–deflection curves were corrected for the initial non-linearity due to deformation of the specimen at the supports, as recommended in the RILEM guidelines [21]. The typical load– deflection diagrams of GPC and OPC concrete specimens are given in Figs. 2–4. It is seen from these figures that the peak load of geopolymer concrete specimen was generally higher than that of the OPC concrete specimen of the similar compressive strength. The maximum load for each test specimen is given in Table 3. The post-peak parts of the load–deflection curves of geopolymer con-
ð1Þ
where W0 is the area under the load–deflection curve (N–m), m is the mass of the beam between the supports (kg), g is the acceleration due to gravity (m2/s), d0 is the deflection at final failure of the beam (m) and Alig is the area of the ligament (m2). The term fracture toughness or critical stress intensity factor (Kic) is used to indicate the magnitude of the stress concentration that exists in front of the crack tip when the crack starts to propagate. The most common method of calculation of the critical stress intensity factor is to use the peak load from the three point bend-
Table 3 Test and calculated results of the OPC and GPC specimens. Batch
Compressive strength, fc (MPa)
Specimen
Peak load, P (kN)
Fracture energy, GF (N/m)
Mean GF (N/m)
GF Eq. (3) (N/m)
31
1 2 3
2.10 1.98 2.28
113 109 154
125
56.7
36
1 2 3
2.68 1.96 2.71
134 97 140
124
43
1 2 3
3.42 3.33 3.65
141 123 170
51
1 2 3
3.06 3.58 3.07
32
1 2 3
OPC1
OPC2
OPC3
OPC4
GPC1
GPC2
36
GPC3 48
Critical stress intensity factor, Kic (MPa-mm0.5)
Mean Kic
85.8
14.1 13.3 15.3
14.2
63.0
91.8
18.0 13.2 18.2
16.4
144
71.3
98.2
22.9 22.3 24.4
23.2
162 210 167
180
80.4
106.3
20.5 24.0 20.5
21.7
3.11 2.68 3.02
102 107 92
100
58.0
105.5
20.9 17.9 20.3
19.7
1 2
3.91 3.36
142 140
141
63.0
114.0
1 2 3
4.62 5.03 4.52
170 213 209
198
77.0
GF Eq. (4) (N/m)
125.3
26.2 22.5 30.9 33.6 30.3
24.4 31.6
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Fracture energy (N/m)
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300 Test OPC
250 200
Test GPC
150 CEB -FIP
100 50 0 30.0
35.0
40.0
45.0
50.0
Bazant and BecqGiraudon: OPC Bazant and BecqGiraudon: GPC
55.0
Compressive strength (MPa)
Fig. 6. Variation of fracture energy of GPC and OPC concrete with compressive strength.
Fig. 2. Load–deflection diagrams of specimens of batches OPC1 and GPC1.
crete specimens were generally steeper than those of the similar OPC concrete specimens. The post-peak load usually dropped faster in the GPC specimens than in the OPC concrete specimens, as shown by the load–deflection diagrams of Figs. 2–4. As expected, the post-peak part of the curve was generally steeper for higher compressive strength in both types of concrete. 3.2. Fracture energy
Fig. 3. Load–deflection diagrams of specimens of batches OPC2 and GPC2.
Fig. 4. Load–deflection diagrams of specimens of batches OPC3, OPC4 and GPC3.
The area under the load–deflection curve for each notched beam specimen was calculated and used in Eq. (1) to obtain the fracture energy of each specimen. These values are given in Table 3. The mean value of the fracture energy for the specimens of each batch is also given in the table. The mean fracture energy values of the GPC and OPC concrete batches are plotted in Fig. 5. It can be seen that the fracture energy of batch GPC1 is slightly smaller than that of batch OPC1, while the fracture energy of batch GPC2 is slightly higher than that of batch OPC2. The fracture energy of batch GPC3 is higher than those of OPC3 and OPC4. Generally, it can be seen that the fracture energy of GPC is comparable to that of OPC concrete mixtures of similar compressive strength. The fracture energy tended to be higher for GPC than OPC concrete as the compressive strength increased. The fracture energy values of the specimens of each batch are plotted against compressive strength in Fig. 6. Considerable scatter is observed in the fracture energy values of specimens of each batch for both geopolymer and OPC concrete. The coefficient of variation of the fracture energy values of OPC concrete varied from 15% to 20% and that of the GPC varied from 8% to 15%. It can be seen from the plots that fracture energy tends to increase with the compressive strength in both types of concrete. It is also seen from the trend of the data that the fracture energy of GPC tends to increase with compressive strength at a higher rate than OPC concrete. CEB-FIP [28] proposed Eq. (3) to estimate the fracture energy of OPC concrete in terms of the compressive strength and the maxi-
35 30
200 150
OPC
100
GPC 50 0
Kic (MPa-mm 0.5)
Fracture energy (N/m)
250
25 20 15
OPC
10
GPC
5 1
2
3
4
Mix Id Fig. 5. Mean fracture energy of the GPC and OPC concrete batches.
0
1
2
3
4
Mix Id Fig. 7. Mean critical stress intensity factors of the OPC and GPC batches.
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mum aggregate size. This equation was used to calculate the fracture energy of the GPC and OPC concrete test specimens. The values calculated by Eq. (3) are given in Table 3 and plotted in Fig. 6. It is seen from Fig. 6 that the test values of fracture energy of both GPC and OPC concrete specimens obtained by using the work of fracture method are higher than those predicted by the CEB-FIP equation.
GF ¼ ð0:0469 D2max 0:5Dmax þ 26Þ
GF ¼ 2:5a0
fc 0:051
0:7
fc 10
0:46 0:22 0:30 Dmax w 1þ c 11:27
ð3Þ
ð4Þ
where Dmax is the maximum aggregate size (mm), fc is compressive strength of concrete (MPa), a0 is aggregates shape factor (a0 = 1 for rounded aggregates, a0 = 1.44 for angular aggregates), w/c is the water–cement ratio of the concrete. Bazant and Becq-Giraduon [29] proposed Eq. (4) for fracture energy of OPC concrete in terms of the compressive strength, maximum aggregate size and the water to cement ratio of the concrete. The equation was proposed based on a statistical analysis of 238 test data on fracture energy obtained by the work of fracture test of specimens of size varying in a wide range. The equation has a term of water to cement ratio that is relevant to OPC concrete. Since GPC mixtures do not have cement and they use liquids different from water, the term water to cement ratio is not directly applicable to GPC. For this reason, the liquid to fly ash ratio was used for the GPC mixtures as an equivalent term of water to cement ratio. The liquid content was obtained by adding up the masses of sodium hydroxide solution, sodium silicate solution and extra water in calculation of the liquid to fly ash ratio. Thus, the fracture energy values of the GPC and OPC concrete specimens were also predicted by using Eq. (4). The predicted values are given in Table 3 and plotted in Fig. 6. It can be seen from Fig. 6 that the predictions by this equation are higher than those by the CEB-FIP equation (Eq. (3)). The predictions by Eq. (4) are also closer to the test results obtained by the work of fracture method for both the GPC and OPC concrete specimens. Smaller predictions of the fracture energy by the CEB-FIP equation than the test results of OPC concrete specimens were also observed by Camoes et al. [30].
Fig. 9. Typical fracture planes of OPC2 (top) and GPC2 (bottom) concrete specimens.
3.3. Critical stress intensity factor The value of stress intensity factor indicates the magnitude of the stress concentration in front of the crack tip when the crack starts to propagate. The critical stress intensity factors of the GPC and OPC concrete specimens were calculated by using Eq. (2). The value for each test specimen and the mean value for the specimens of each batch of concrete are given in Table 3. The mean val-
Fig. 10. Typical fracture planes of OPC4 (top) and GPC3 (bottom) concrete specimens.
Fig. 8. Variation of critical stress intensity factors of OPC and GPC with compressive strength.
ues of the critical stress intensity factors for each batch are plotted in Fig. 7. It can be seen from the figure that the critical stress intensity factor of geopolymer concrete is higher than that of OPC concrete of similar compressive strength. The critical stress intensity factors for all the specimens are plotted against compressive strength in Fig. 8. It can be seen that critical stress intensity factor tends to increase with compressive strength in both GPC and OPC concrete. Also, the critical stress
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It was also observed that geopolymer concrete has a higher tensile strength than OPC concrete of the same compressive strength [1,8]. The higher bond strength at the interface of the geopolymer binder and the aggregates makes the fracture plane pass more through the aggregates than through the interface. It was shown from morphological studies that the use of soluble silicates in geopolymers results in a denser interfacial transition zone (ITZ) between aggregates and geopolymer matrix as compared to that with OPC matrix [31]. Zhang et al. [32] showed that there is no obvious difference between microstructures of the ITZ and the bulk geopolymer matrix. Thus, the ITZ in GPC is considered to be stronger than that in OPC concrete. The stronger ITZ contributed to the higher splitting tensile strength and bond strength of geopolymer concrete. The higher tensile and bond strengths of geopolymer concrete increased its critical stress intensity factor which resulted in less tortuosity of the fracture plane and relatively more brittle type of failure than in the OPC concrete specimens of similar compressive strength. 4. Conclusions This study investigated the fracture behaviour of geopolymer concrete as compared to OPC concrete of similar compressive strength and containing the same size and type of aggregates. The experimental work consisted of three point bending test of notched beam specimens made from three GPC and four OPC concrete mixtures. The fracture energy was determined by the work of fracture method using the load–deflection curves and the critical stress intensity factor was determined from the peak load and geometry of the specimen. The following conclusions are drawn from the study:
Fig. 11. Typical fracture planes of 36 MPa OPC (top) and GPC concrete (bottom).
intensity factors for the GPC concrete specimens are higher than those of the OPC concrete specimens. This shows that the critical stress at which a cracking occurs is higher in GPC than in OPC concrete. Therefore, the crack resistance of GPC is higher than that of OPC of the same compressive strength. This behaviour is consistent with the previous findings that geopolymer concrete has higher tensile and bond strengths than OPC concrete of the same compressive strength. 3.4. Fracture planes Generally, the GPC specimens failed in a more brittle manner than the OPC concrete specimens. Typical fracture failure planes of GPC and OPC concrete specimens are shown in Figs. 9–11. It can be seen from these figures that the fracture planes in the OPC concrete specimens were more tortuous and those in the GPC specimens were relatively smooth for the concretes of similar compressive strength. The fracture planes in the OPC concrete specimens passed more around the aggregates and those in the GPC specimens generally passed more through the aggregates. This difference can be seen clearly in the fractured planes of OPC and GPC specimens shown in Fig. 11. The reason for the fracture plane passing more through the aggregates in GPC is believed to be the higher bond strength of geopolymer with aggregates than that of the OPC binder. It was shown in the previous studies [2,8,9] that GPC has higher bond strength with reinforcing steel than OPC concrete.
1. The failure modes of the heat cured GPC specimens were generally more brittle than those of the OPC concrete specimens. The fracture planes of the GPC specimens were less tortuous than those of OPC concrete specimens. 2. Fracture energy of geopolymer concrete was similar to that of OPC concrete. Fracture energy increased with compressive strength in both types of concrete. The test values of fracture energy of both types of concrete were higher than those calculated by the equations of CEB-FIP and Bazant and Becq-Giraudon. The predictions by the equation of Bazant and BecqGiraudon were closer to the test values. 3. The critical stress intensity factor of the GPC specimens was higher than that of the OPC concrete specimens for the same compressive strength. This indicates that GPC needs a higher stress than OPC concrete for the formation of cracks. This was also observed from the load–deflection diagrams as the peak load or the load at which cracks appeared were generally higher in the GPC specimens than in similar OPC concrete specimens. 4. The difference in the fracture behaviours of GPC and OPC concrete is because of the higher bond and tensile strengths of GPC. The denser interfacial transition zone of GPC resulted in higher critical stress intensity factor and more brittle type of failure with smoother fracture plane as compared to OPC concrete.
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