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Properties of self-compacting-concrete containing fly ash subjected to elevated temperatures
Construction and Building Materials 30 (2012) 274–280
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Properties of self-compacting-concrete containing fly ash subjected to elevated temperatures Neelam Pathak, Rafat Siddique ⇑ Department of Civil Engineering, Thapar University, Patiala, Punjab, India
a r t i c l e
i n f o
Article history: Received 27 June 2011 Received in revised form 21 October 2011 Accepted 24 November 2011 Available online 30 December 2011 Keywords: Self-compacting-concrete Temperature effects Fly ash Mechanical and durability properties
a b s t r a c t This paper aimed to study the properties of Self-Compacting-Concrete (SCC) such as compressive strength, splitting tensile strength, rapid chloride permeability, porosity, and mass loss when exposed to elevated temperatures. In this research, mixes were prepared with three percentages of class F fly ash ranging from 30% to 50% and for comparison; one controlled mixture without fly ash was also produced. The variables included were the temperature effects (20 °C, 100 °C, 200 °C, and 300 °C) using Ordinary Portland Cement. SCC mixes developed 28 days compressive strength ranging from 21.43 to 40.68 MPa and splitting tensile strength ranging from 1.35 to 3.60 MPa. Test results clearly show that there is little improvement in compressive strength within temperature range of 200–300 °C as compared to 20–200 °C but there is little reduction in splitting tensile strength ranging from 20 to 300 °C and with the increase in percentage of fly ash. Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction Concrete is the most widely used construction material in the world. The term ‘Self-Compacting-Concrete’ (SCC) was introduced by Professor Ozowa in Japan [1] and then developed by Bartos and Grauers [2] and Okamura and Ouchi [3]. SCC fills all sections of forms without the need of mechanical vibration and has reasonable flow-ability, homogeneity, resistance against segregation and mechanical strength [4]. Use of SCC in the construction industry has been widely growing in almost the entire world and a good growth of use is predicted for it in the near future [5]. Although the use of SCC has many technical, social, and overall economical advantages, however its supply cost could be two to three times higher then that of normal concrete. Hence to reduce the cost of SCC use of mineral admixtures such as fly ash, limestone filler, and ground granulated blast furnace slag could be used to increase the slump of the concrete mix. Among these materials fly ash, a byproduct of thermal power plants, has been reported to improve the mechanical properties and durability of concrete when used as a cement replacement material. The incorporation of fly ash also reduces the need for viscosity modifying chemical admixtures [6]. Degradation of mechanical behaviour of concrete due to exposure to high temperature has been studied since 1950s in western countries [7,8]. At early stages of heating the evaporable water from concrete is lost over in the range of 20–110 °C. Above 110 °C the cement hydrates decompose, calcium hydroxide is ⇑ Corresponding author. E-mail address: [email protected] (R. Siddique). 0950-0618/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.conbuildmat.2011.11.010
broken down and the calcium carbonate suffers decarbonation. The aggregate also suffers changes, which contributes to the general loss of structure safety [9]. Within the normal temperature, the thermal properties of concrete can be considered constant. However at elevated temperature these properties will be changed because of the change of moisture content of the concrete components [10]. Some of the studies on SCC subjected to high temperature showed both decrease in strength and increase in the risk of spalling or a similar behaviour to that of vibrated concrete. Noumowe and Aggoun [11] presented the experimental work on the high temperature behaviour of conventional vibrated high strength concrete and self-compacting high-strength concrete by using polypropylene fibre, and concluded that residual mechanical properties in reference to the initial mechanical properties of self-compacting high-strength concretes were similar to that of conventional high strength concrete. Castillo presented the experimental results of hot compressive strength in the temperature range of 100–300 °C and observed the decrease in compressive strength of high strength concrete by 15–20% [12]. Jin et al. [13] presented the experimental results on compressive strength for SCC subjected to high temperature. They concluded that after heating to 100 °C and subsequent cooling, the compressive strength of all mixtures decreased as compared to the room-temperature strength. Hanaa et al. [14] studied the mechanical and microstructural properties of self-consolidating concrete using limestone filler subjected to normal and high temperature and reported that after a moderate decrease in compressive strength between 20 and 150 °C, an important increase was observed between 150 and 300 °C. In other studies Phan and Carino [15] and Khoury [16]
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reported the decrease in strength between 20 and 150 °C on vibrated concretes. Bakhtiyari et al. [17] evaluated the mechanical properties and changes in the phase composition of the paste of SCC containing different powders at elevated temperatures. They found that, if fine quartzite powder is used as a filler in the SCC, it will accelerate the strength development at high temperatures up to 500 °C, because of its partial pozzolanic activity, enhanced at such temperatures. They also concluded that it can be said as a general rule that 500 °C is a critical temperature for concretes exposed to fire. Ghandehari et al. [18] evaluated the residual mechanical properties of high strength concretes after exposure to elevated temperatures by using silica fume, and reported that after heating to 200 °C the strength of all of the concretes slightly improved when compared to strength at 100 °C. Siddique [19] studied the properties of SCC mixes, incorporating fly ash content up to 35% and reported that, SCC mixes developed compressive strength ranging from 29 to 35 MPa at 28 days. Patel et al. [20] reported that the use of fly ash decreased the rapid chloride penetration. Also, Nehdi et al. [21] reported that the presence of fly ash reduced the penetrability from approximately 3000 Coulomb to less than 1000 Coulomb. Koulombi et al. [22] observed that the chloride binding capacity of concrete tends to increase with fly ash addition. Zhu et al. [23] reported the significant reduction of chloride diffusivity of SCC with fly ash. Shi and Yanzhong [24] states the use of supplementary cementing material such as fly ash may have a significant effect on the chloride migration of concrete as measured by the RCPT effect. Mustafa et al. [25] also showed that the rapid chloride permeability of most SCC decreases with increase in fly ash and foundry sand content. The objective of the present research was to evaluate the behaviour of SCC subjected to elevated temperature, by using locally available materials. The effect of elevated temperature ranging from 100 to 300 °C on compressive strength, splitting tensile strength and rapid chloride permeability, porosity, and mass loss was investigated and developed 28 days compressive strength varied between 20 and 35 MPa. Water-to-cementitious material ratio for various fly ash SCC mixes, ranging from 0.38 to 0.42, total mass of cementitious materials of 500 kg/m3 with 30–50% replacement of cement with fly ash of total powder content. The superplasticizer content was below 2% of the total powder content (cement + fly ash) for all the mixes. In the present research, behaviour of SCC subjected to elevated temperature with varying fly ash content has been evaluated. 2. Experimental programme
Table 1 Physical properties of Portland cement (IS: 8112-1989) [26]. Physical property
Results obtained
Specifications
Fineness (retained on 90-lm sieve) Standard consistency Initial setting time (min) Final setting time (min) Compressive strength 3-days (MPa) Compressive strength 7-days (MPa) Compressive strength 28 days (MPa) Specific gravity Soundness (mm)
1.0% 30% 65 215 23.6 35.4 43.0 3.12 2.50
<10% – 30 minimum 600 maximum 22.0 minimum 33.0 minimum 44.3 3.15 10 maximum
Table 2 Physical properties of fly ash. Sr. No.
Physical properties
Test results
1 2 3
Colour Specific gravity Lime reactivity – average compressive strength after 28 days of mixture
Blackish grey 2.13 4.90 MPa
Table 3 Chemical properties of fly ash (IS: 3812-2003) [27]. Sr. No
Constituents
Percent by weight
1 2 3 4 5 6 7 8 9
Loss on ignition Silica (SiO2) Iron Oxide (Fe2O3) Alumina (Al2O3) Calcium Oxide (CaO) Magnesium Oxide (MgO) Total Sulphur (SO3) Insoluble residue Alkalies: (a) Sodium Oxide (Na2O) (b) Potassium Oxide (K2O)
4.17 58.55 3.44 28.20 2.23 0.32 0.07 – 0.58 1.26
Table 4 Physical properties of coarse and fine aggregates. Properties
Coarse aggregates
Fine aggregates
Specific gravity Fineness modulus Bulk density (kg/m3) Water absorption (%)
2.66 6.46 1545 0.90
2.65 2.507 1781 0.98
2.1. Materials 2.1.1. Cement Ordinary Portland Cement (Grade 43) was used in this research. Its physical properties are as given in Table 1.
2.1.2. Fly ash Class F Fly ash procured from ‘‘Ropar Thermal Power Plant’’, Punjab, India was used. The physical and chemical properties of fly ash are given in the Tables 2 and 3, respectively.
2.1.3. Aggregates Locally available natural sand with 4.75 mm maximum size was used as fine aggregate. Crushed stone with 10 mm maximum size was used as coarse aggregate. Both fine aggregate and coarse aggregate conformed to Indian Standard Specifications IS: 383-1970 [28]. Table 4 gives the physical properties of the coarse and fine aggregates.
2.1.4. Admixtures Polycarboxylic ether based super plasticizer complying with IS: 9103:1999, ASTMC-494 type F, BS 5057 part III [29] with density approximately 1.10 and pH approximately 5.0 was used in this research.
2.2. Mix proportions Four concrete mixes were made, which had total powder content of 500 kg/m3 (cement + fly ash). Coarse aggregate content was maintained at 51% by volume of concrete and fine aggregate content at 49% by volume of mortar in concrete, the w/p ratio was varied from 0.38 to 0.42 by weight with air content being assumed to be 2%. In this work, one control mix (SCC1) was designed with Ordinary Portland Cement and the other three concrete mixes (SCC2, SCC3, and SCC4) were made by replacing cement with 30%, 40%, and 50% of Class F fly ash by weight of total powder content. Their mix proportions are given in Table 5. 2.3. Mixing and casting For these mix proportions, required quantities of materials were weighed. Cement and fly ash were mixed in dry state, and that of coarse and fine aggregates were mixed dry separately. After adding water, all materials were mixed together to obtain the homogeneous mix. After carrying out the tests for fresh properties, final casting of the mixes was done immediately. After casting, test specimens were left in the casting room for 24 h at a temperature of about 20 °C. The specimens were removed from mould after 24 h and were put into a water-curing tank until the time of the test or as per requirement of the test. The cubes of size 150 mm were cast for determination of compressive strength and 150 300 mm cylinders for
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Table 5 Mix proportions and fresh concrete properties of SCC mixes. Mix SCC SCC SCC SCC
1 2 3 4
Cement (kg/m3)
Fly Ash (%)
FA (kg/m3)
CA (kg/m3)
W/b
SP (%)
Slump (mm)
U-box (mm)
Room temp.
Conc. temp.
500 500 500 500
– 30 40 50
830 830 845 856
876 876 860 856
0.38 0.38 0.40 0.42
2.00 1.82 1.80 1.72
620 634 652 678
12 10 8.64 7.92
34 35 37 35
33 32 35 34
FA stands for fine aggregates; CA for coarse aggregates; W/b for water-binder ratio; and SP for Superplasticizer.
Table 6 Chloride ion penetrability based on charge passed (ASTM 1202-97). Charge passed (Coulomb)
Chloride ion penetrability
>4000 2000–4000 1000–2000 100–1000 <100
High Moderate Low Very low Negligible
Table 7 Mass loss for SCC mixes. Mix
SCC SCC SCC SCC SCC SCC SCC SCC SCC SCC SCC SCC
Temperature (°C)
1 1 1 2 2 2 3 3 3 4 4 4
100 200 300 100 200 300 100 200 300 100 200 300
Fly ash (%)
0 0 0 30 30 30 40 40 40 50 50 50
Mass loss (gm) 28 days
91 days
225 242 285 173 196 230 182 216 234 190 225 245
124 130 139 120 135 142 154 175 205 156 178 195
Table 8 Porosity for SCC mixes. Mix
SCC SCC SCC SCC SCC SCC SCC SCC SCC SCC SCC SCC
Temperature (°C)
1 1 1 2 2 2 3 3 3 4 4 4
100 200 300 100 200 300 100 200 300 100 200 300
Fly ash (%)
0 0 0 30 30 30 40 40 40 50 50 50
Porosity (%)
700 mm the concrete might segregate and if it is less than 500 mm the concrete is considered to have insufficient flow to pass through highly congested reinforcement. U-box test gives us the filling and passing ability of SCC [32]. 2.4.2. Strength and durability properties After the required curing period, the specimens of each concrete mixture were heated at a rate of 1 °C/min up to different temperatures (100, 200 and 300 °C). In order to ensure a uniform temperature throughout the specimens, the temperature was held constant at the maximum temperature for 1 h before cooling. All heated specimens were cooled slowly and tested at room temperature. The rate of heating refers to the recommendations of the RILEM Technical Committee TC-129 [33]. The changes in the mechanical properties of SCC due to high temperature were examined. Compressive strength and splitting tensile strength test was determined at the ages of 28 and 91 days, as per Bureau of Indian Standards, IS: 516:1959 [34]. Rapid chloride penetrability test (RCPT) for durability was conducted as per ASTM C 1202-97 [30] at the ages of 28 and 91 days. 2.4.3. Concrete mass loss The mass of each specimen was measured before and after each temperature cycle. Weight change of concrete was mainly due to dehydration of cement paste. This allows us to quantify the dehydration of concrete after each heating. Results of mass loss are presented in Table 7. 2.4.4. Porosity Total porosity of the different mixes was studied. These properties were measured before and after each temperature cycle. Three samples were tested for each concrete and each temperature cycle. Results of porosity are presented in Table 8. ASTM vacuum saturation technique was used to measure the porosity of concrete [35]. ASTM standard procedures were employed using 100 50 mm cylinder specimens. The specimens were dried in the oven at 100 °C for more than 48 h and then specimens were removed from the oven, cooled in dry air to a room temperature of 20 °C and then weighted to obtain the oven dry mass. After that the specimens were vacuum saturated. Porosity of concrete was calculated based on the concept of weight gain due to water absorption and weight loss because of buoyancy. The porosity is determined according to the following:
28 days
91 days
Porosity ¼ ðW s W d Þ=ðW s W b Þ 100%:
0.77 0.86 0.95 1.20 1.46 1.57 1.53 1.68 1.75 1.90 2.25 2.42
0.55 0.64 0.79 1.00 1.35 1.39 1.42 1.55 1.64 1.68 1.75 1.95
where W s and Wb are the saturated mass of a sample measured in the air and in water respectively. Wd is the mass of oven dried specimen, weighted in air.
splitting tensile strength. Cylindrical specimens of size 100 50 mm size were also cast for rapid chloride penetration test as per ASTM C 1202-97 [30]. The top surface of the specimens was scraped to remove excess material and to achieve smooth finish. All test measurements were taken as the average of five readings. 2.4. Testing 2.4.1. Fresh properties For determining the self-compactibility properties, slump flow test and U-box (difference in height) tests were performed. The slump flow represents the mean diameter of the mass of concrete after release of a standard slump cone; the diameter is measured in two perpendicular directions. According to Nagataki and Fujiwara [31], a slump flow ranging from 500 to 700 mm is considered as the slump required for a concrete to be self-compacted. If the slump flow is greater than
2.4.5. Rapid chloride permeability test The chloride permeability test was conducted to assess the concrete quality as per ASTM C 1202-97. To measure the permeability cylindrical specimens of size 100 mm dia and 50 mm length were cut in cylinders. Only the central part of the cylinders was kept for the measures. A potential difference of 60 V DC was maintained across the specimen. One of the surface was in contact with a sodium chloride solution (NaCl) and the other with a sodium hydroxide solution (NaOH). The total charge passing through in 6 h was measured, indicating the degree of resistance of the specimen to chloride ion penetration. The residual permeability obtained after each temperature cycle was determined. Heated specimens were tested 24 h after cooling in order to maintain the same hydric conditions for all the specimens.
3. Results and discussion 3.1. Properties of fresh concrete The results of various fresh properties tested by slump flow test (slump flow diameter and T50 cm) and U-box test, for various mix compositions are given in Table 5. The slump flow test judges the capability of concrete to deform under its own weight against the friction of the surface with no restraint present. Slump flow for all the mixes in this research was with in the range of 600–700 mm
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Compresive Strength(MPa)
60 Control mix 30% Fly Ash 40% Fly Ash 50% Fly Ash
50
40
30
20
0
100
200
300
400
Temperature(0C) Fig. 3. Compressive strength versus temperature (91 days).
Fig. 1. Rapid chloride permeability test set up (ASTM C 1202).
Compressive Strength (MPa)
60
Control mix 30% flyash 40% flyash 50% flyash
50 40 30 20 10
0
100
200
300
400
Temperature(0C) Fig. 2. Compressive strength versus temperature (28 days).
and the time taken to reach the diameter of 500 mm was less than 5 s. The U-box difference in height of concrete in two chambers was in the range of 7–12 mm. 3.2. Residual compressive strength The behaviour of SCC and vibrated concrete differs significantly between 20 and 300 °C. The compressive strength of concretes with or without fly ash at normal temperature (20 °C) and after heating to 100, 200, 300 °C are shown in Figs. 2 and 3. At normal temperature (20 °C), with the increase in fly ash content from 30% to 50%, SCC mixes (SCC2–SCC4) developed compressive strengths between 30.67 and 21.43 MPa and, between 39.50 and 30.40 MPa at 28 and 91 days respectively. Strength values were 40.68 and 48.90 MPa at 28 and 91 days respectively, for control mix (SCC1). An increase of about 22.35% and 19.00% strength was observed at 28 and 91 days, with the decrease of fly ash content from 50% to 30% (SCC4–SCC2) as compared with control mix (SCC1). After heating to 100 °C and subsequent cooling, SCC mixes (SCC2–SCC4) developed compressive strength ranged between 29.56 and 19.96 MPa and, between 38.00 and 29.25 MPa at 28 and 91 days. An increase of about 24.30% and 18.40% strength was observed at 28 and 91 days, with the decrease of fly ash content from 50% to 30% (SCC4–SCC2) as compared with control mix (SCC1). With the increase in temperature from 100 to 200 °C, compressive strength varied between 29.00 and 19.25 MPa and, between
37.90 and 29.00 MPa at 28 and 91 days. An increase of about 24.00% and 19.00% strength was observed at 28 days and 91 days, with the decrease of fly ash content from 50% to 30% (SCC4–SCC2) as compared with control mix (SCC1). With the further increase in temperature from 200 to 300 °C, compressive strength ranged between 31.45 and 21.20 MPa and, between 39.00 and 30.20 MPa at 28 and 91 days. An increase of about 25.00% and 18.00% strength was observed at 28 days and 91 days, with the decrease of fly ash content from 50% to 30% (SCC4–SCC2) as compared with control mix (SCC1). The compressive strength increased with the decrease in fly ash content and the water-to-cementitious materials ratio, as compared with the SCC mix prepared without fly ash. Strength loss of SCC mixes (SCC2–SCC4) in the temperature range of 20–100 °C was about 1.95%. Between 100 and 200 °C, there was slight reduction in strength. With further increase in temperature from 200 to 300 °C, there was little improvement in strength (about 1%), as compared with SCC1. It has been well documented that the moisture content has a significant effect on the strength of concrete at temperatures ranging from 20 to 200 °C. For all of the mixtures, a small improvement occurred in the residual strength at 200 °C when compared to 100 °C. The increase in strength associated with this increase in temperature was attributed to the increase in surface forces between gel particles due to the removal of moisture content. This was most likely due to the compact microstructure, which results in a build up vapour pressure formed by the evaporation of physically and chemically bound water. The strength values of this research was comparable with those reported by Hanaa et al. [14] i.e. after a moderate decrease in compressive strength between 20 °C and 150 °C, noticeable increase was observed between 150 °C and 300 °C. Several hypotheses had been proposed in the literature to explain the increase in strength between 150 and 300 °C. Khoury [16] assumed that the silanol groups lose a part of their bonds with water, which induces the creation of shorter and stronger siloxane elements (Si–O–Si) with probably larger surface energies that contribute to the increase in strength. Dias et al. [36] attributed this to a rehydration of the paste due to the migration of water in the pores. This increase in strength was also observed by Xu et al. [37] who carried out microhardness tests on hardened cement paste and interfacial transition zone. These results clearly showed that the evolution of residual compressive strength between 200 and 300 °C was due to the hydration of anhydrous cement which leads to the formation of hydrates having better bonding properties. These results confirm the hypothesis of Dias et al. [36] i.e. the increase in strength was also due to an increase in the bonding properties of hydrates (a larger compressive strength was obtained for a larger porosity of the
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6
6
Control mix
30% flyash 40% flyash 50% flyash
5 4
Split tenslie strength(MPa)
Split tensile strength(MPa)
Control mix
3 2 1 0
0
100
200
300
400
40% Fly Ash 50% Fly Ash
4 3 2 1 0
Temperature(0C)
30% Fly Ash
5
0
100
200
300
400
Temperature(C)
Fig. 4. Split tensile strength versus temperature (28 days).
Fig. 5. Split tensile strength versus temperature (91 days).
3.3. Residual splitting tensile strength Splitting tensile strength test results of all SCC mixes at various temperatures are shown in Figs. 4 and 5. At normal temperature (20 °C), with the increase in fly ash content from 30% to 50%, SCC mixes (SCC2–SCC4) developed splitting tensile strength between 2.00 and 1.35 MPa and 2.24 and 1.54 MPa at 28 and 91 days. Strength values were 3.56 and, 3.78 MPa at 28 and 91 days, for control mix (SCC1). An increase of about 19.00% and 18.00% strength was observed at 28 and 91 days with the decrease of fly ash content from 50% to 30% (SCC4–SCC2) as compared with control mix (SCC1). After heating to 100 °C and subsequent cooling, SCC mixes (SCC2–SCC4) developed split tensile strength ranged between 1.84 and 1.26 MPa and, 2.00 and 1.48 MPa at 28 and 91 days. An increase of about 18.20% and 14.50% strength was observed at 28 and 91 days with the decrease of fly ash content from 50% to 30% (SCC4–SCC2) as compared with control mix (SCC1). With the increase in temperature from 100 to 200 °C, split tensile strength varied between 1.62 and 1.00 MPa, and 1.87 and 1.22 MPa at 28 and 91 days. An increase of about 20.00% and 14.50% strength was observed at 28 and 91 days, with the decrease of fly ash content from 50% to 30% (SCC4–SCC2) as compared with control mix (SCC1). With the further increase in temperature from 200 to 300 °C, splitting tensile strength ranged between 1.56 and 0.94 MPa, and 1.75 and 1.14 MPa at 28 and 91 days respectively. An increase of about 21.74% and 19.00% strength was observed at 28 days and 91 days, with the decrease of fly ash content from 50% to 30% (SCC4–SCC2) as compared with control mix (SCC1). Splitting tensile strength increased with the decrease in percentage of fly ash content and the water-to-cementitious materials ratio and decreased with increase in temperature, at all ages. Strength loss of SCC mixes (SCC2–SCC4) in the temperature range of 20–100 °C was about 0.2%. Between 100 and 200 °C, there was about 1.80%. With further increase in temperature from 200 to 300 °C; there was about 1.74% loss, as compared with SCC1. This strength loss was not only attributed to the decomposition of the hydration products but also due to the thermal incompatibility between aggregates and cement paste. The tensile strength results of this research was comparable with those reported by Noumowe and Aggoun [11] that split tensile strength decreased continuously with increase in temperature.
2500
Permeability (Coulombs)
material). Therefore these results are in line with the published literature. The strength loss in present research at elevated temperature was much less than that reported by previous researchers, and this is because of incorporation of fly ash.
Control mix 30%Fly Ash 40%FlyAsh 50%Fly Ash
2000 1500 1000 500 0
20
100
200
300
Temperature ( 0C) Fig. 6. Rapid chloride permeability of fly ash concrete (28 days).
The value of split tensile strength was reported as 4.8 MPa at ambient temperature and 2.8 MPa after heating to 400 °C. In another research by Ghandehari et al. [18] reported the reduction from 14.4% to 17.9% after being exposed from 100 to 600 °C. Reduction in split tensile strength observed in present study was much lower than that already studied in the previous researches. This is because of replacement of cement with fly ash. 3.4. Chloride penetration resistance Rapid chloride permeability test setup (ASTM C1202) is shown in Fig. 1. Chloride permeability test was conducted on all the concrete specimens. The total charge passed in 6 h as a measure of the chloride permeability is presented in Figs. 6 and 7. The chloride ion penetrability limits suggested by ASTM C1202 (Table 6) were compared with the results. Much like the findings of previous studies [19–25] these Figs. 6 and 7 showed that the use of fly ash significantly reduced the chloride permeability of hardened SCC mixtures when compared to the control concretes. On the other hand permeability increased with the increase in temperature, like previous research [14]. It is evident from the results that SCC mixes (SCC2– SCC4) made with fly ash reduced the rapid chloride ion penetrability to the low range (1000–2000 Coulomb) at the age of 28 days, between low range to very low range (<1000 Coulomb) at the age of 91 days. The incorporation of fly ash resulted in a reduction in Coulomb charges. At normal temperature (20 °C) the Coulomb charge of control mix (SCC1) was 1393, and 1186 Coulomb and fly ash mixes (SCC2–4) was between 1234 and 1143 Coulomb, and 1030 and 854 Coulomb at 28 and 91 days respectively.
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Permeability (Coulombs)
2000
Control mix 30%Fly Ash 40%Fly Ash 50%Fly Ash
1500
1000
500
0
20
100
200
300
Temperature(C) Fig. 7. Rapid chloride permeability of fly ash concrete (91 days).
The Coulomb charge of control mix (SCC1) at 300 °C was 1532 and 1432 Coulomb, and fly ash mixes (SCC2–4) was between 1367 and 1245 Coulomb, and 1245 and 923 Coulomb at 28 and 91 days respectively, indicating high chloride penetration resistance. In no case the value of the charges passed was greater than 1532 Coulomb implying dense SCC mix structure. The significant reduction in chloride ion penetration may be due to incorporation of fly ash whose spherical particles could improve particle-packing density in the matrix. In the present research, permeability was lower at 91 days, as compared with 28 days, as expected due to the hydration of Portland cement and pozzolanic reactions of fly ash. One of the most important factor affecting the permeability of concrete was the internal pore structure, which in turn was dependent on the extent of hydration of the cementitious materials. The curing conditions and the age of concrete thus largely determine the ease with which chloride ions can move into a concrete. From the test results it is observed that most concretes become significantly less permeable with increase in curing time. 3.5. Concrete mass loss Table 7 presents the evolutions of mass loss of cubical specimens of size 150 mm after heating. It was observed that the evolution of mass loss versus temperature was very close to five studied concretes. Between the room temperature and 200 °C, the variation of mass loss was less. This corresponds to the loss of the evaporable water and part of the physically bound water. Between 200 and 300 °C, noticeable increase in mass loss was observed for all concretes. This is because of loss of chemically bound water from the decomposition of the C–S–H, the carboaluminate hydrates and the dehydration of calcium silicate hydroxide. This was in good agreement with the observation by the Hanaa et al. [14] who observed that about 70% of all the water contained in the concretes had been evaporated at 300 °C. The results are in line with the published data. 3.6. Porosity Table 8 presents the results of porosity. For the initial stage of heating, the porosity was nearly stable up to 100 °C. By increasing the temperature from 100 to 200 °C or from 200 to 300 °C porosity increased. When the percentage of fly ash increased porosity also increased. The increase in porosity values was attributed to change SCC from impermeable to permeable material due to losses and pruned of fine, super fine and organic materials. These results are in line with the results reported by Hanaa et al. [14] and Noumowe
279
and Aggoun [11]. They also observed the increase in porosity with temperature. They attributed the increase in porosity with temperature to the departure of bound water and to the microcracks generated by differential expansion between the paste and aggregates. Noumowe and Aggoun [11] showed by mercury intrusion porosimetry an increase in the pores sizes beyond 120 °C. In other research by Gallé and Sercombe [38] attributed the evolution of porosity to the generation of large capillary pores. Their appearance was due to the release of adsorbed water of capillary pores and release of bound water in cement paste hydrates. If compare the evolution of compressive strength with that of porosity, observed both an increase in strength and an increase in porosity for the SCC’s between 200 and 300 °C. The increase in strength can therefore not be attributed to a decrease in porosity but rather to a modification of the bonding properties of the hydrates of cement paste. The experimental results are therefore in line with Hanna and Khoury’s hypothesis concerning the creation of shorter and stronger siloxane elements Si–O–Si (with probably larger surface energies) by the loss of a part of the bonds with water in silanol groups. 4. Conclusions Based on the results of this experimental study, the following conclusions can be drawn: 1. Although fly ash reduces the strength but still it is possible to produce SCC with adequate strength. 2. All the high volume fly ash concretes have satisfied the norms that were set to qualify them as SCC and the fresh properties like workability were in good agreement with European guidelines [39]. The developed SCC was highly segregation resistant and had good flowability and passing ability. 3. The SCC mixes developed compressive strengths ranging from 30.67 to 19.25 MPa, and from 39.50 to 29 MPa, at 28, and 91 days respectively. Splitting tensile strengths developed were from 0.94 to 2.00 MPa, and 1.14 to 2.24 MPa at 28, and 91 days respectively. The compressive strength increased with a decrease in the percentage of the fly ash and the water-to-cementitious materials ratio. 4. Between 20 and 200 °C, a small loss of strength was observed. Between 200 and 300 °C, the compressive strengths of all of the concrete specimens slightly improved as compared to strength at 100 °C. The increase in strength could be attributed to a modification of the bonding properties of the cement paste hydrates i.e. rehydration of the paste due to the migration of water in the pores. 5. At elevated temperatures, the rate of the splitting tensile strength loss is higher than the rate of the compressive strength loss. The splitting tensile strength continued to decrease in a similar way as was observed between 20 and 100 °C, due to the departure of bound water, corresponding to a large mass loss. 6. The high volume fly ash SCC mixes showed significantly lower chloride ion permeability than SCC without fly ash concretes. Most of the SCC mixes were assessed as ‘‘very low’’ chloride permeability concretes as per ASTM C1202– 94 assessment criteria, with less than 1000 coulombs of total charge passing.
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[22] Koulombi N, Batis G, Malami, CH. Progress in the understanding and prevention of corrosion. In: Costa JM, Mercer AD, editors. The institute of materials. UK; 1993. p. 619. [23] Zhu W, Gibbs JC, Bartos PJM. Uniformity of in situ properties of selfcompacting concrete in full scale structural elements. Cem Concr Compos 2001;23:57–64. [24] Shi C, Yanzhong W. Mixture proportioning and properties of self consolidating light weight concrete containing glass powder. ACI Mater J 2005:355–63. [25] Mustafa S, Mohamed L, Tahir KE, Hasan EY. Use of spent foundry sand and fly ash for the development of green self –consolidating concrete. Mater Struct 2010:1–9. [26] IS: 8112-1989. Specifications for 43 grade Portland cement. Bureau of Indian Standards, New Delhi, India. [27] IS: 3812-2003. Specifications for Pulverized fuel ash. Bureau of Indian Standards, New Delhi, India. [28] IS: 383-1970. Specifications for Coarse and Fine aggregates from Natural sources for Concrete. Bureau of Indian Standards, New Delhi, India. [29] IS: 9103-1999. ASTM C-494 Type F, BS5057 part III, New Delhi, India. [30] Standard test method for electrical indication of concrete’s ability to resist chloride ion penetration, ASTMC 1202-97. Annual book of ASTM standards, vol.04; 2006. p. 639–44. [31] Nagataki S, Fujiwara S. In: Malhotra VM, editor. Self-compacting property of highly-flowable concrete, vol. 154. American Concrete Institute SP; 1995. p. 301–14. [32] EFNARP (European Federation of national trade associations representing producers and applicators of specialist building products). Specification and Guidelines for self-compacting concrete. Hampshire, UK; 2002. [33] Rilem Technical Committees 129-MHT. Test methods for mechanical properties of concrete at high temperatures, part 1: introduction, Part 2: Stress–strain relation, part 3: compressive strength for service and accident conditions, Mater Struct vol. 28 no. 181;1995. p. 410–4. [34] IS: 516-1959. Indian Standard Code of Practice-Method of Strength of concrete. Bureau of Indian Standards (BIS), New Delhi, India. [35] Mohammad S, Natalita H. Comparison of ASTM saturation technique for measuring the permeable porosity of concrete. Cem Concr Res 2005;35(5):1008–113. [36] Dias WPS, Khoury GA, Sullivan PJE. Mechanical properties of hardened cement paste exposed to temperatures up to 700 °C. ACI Mater J 1990;87(2):160–6. [37] Xu Y, Wong YL, Poon CS, Anson M. Influence of PFA on cracking of concrete and cement paste after exposure to high temperatures. Cem Concr Res 2003;33:2009–16. [38] Gallé C, Sercombe J. Permeability and pore structure evolution of silicocalcareous and hematite high-strength concretes submitted to high temperatures. Mater Struct 2001;34:619–28. [39] European guidelines for self compacting concrete specifications productions and use; 2005. p. 1–60.
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Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
Properties of self-compacting-concrete containing fly ash subjected to elevated temperatures Neelam Pathak, Rafat Siddique ⇑ Department of Civil Engineering, Thapar University, Patiala, Punjab, India
a r t i c l e
i n f o
Article history: Received 27 June 2011 Received in revised form 21 October 2011 Accepted 24 November 2011 Available online 30 December 2011 Keywords: Self-compacting-concrete Temperature effects Fly ash Mechanical and durability properties
a b s t r a c t This paper aimed to study the properties of Self-Compacting-Concrete (SCC) such as compressive strength, splitting tensile strength, rapid chloride permeability, porosity, and mass loss when exposed to elevated temperatures. In this research, mixes were prepared with three percentages of class F fly ash ranging from 30% to 50% and for comparison; one controlled mixture without fly ash was also produced. The variables included were the temperature effects (20 °C, 100 °C, 200 °C, and 300 °C) using Ordinary Portland Cement. SCC mixes developed 28 days compressive strength ranging from 21.43 to 40.68 MPa and splitting tensile strength ranging from 1.35 to 3.60 MPa. Test results clearly show that there is little improvement in compressive strength within temperature range of 200–300 °C as compared to 20–200 °C but there is little reduction in splitting tensile strength ranging from 20 to 300 °C and with the increase in percentage of fly ash. Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction Concrete is the most widely used construction material in the world. The term ‘Self-Compacting-Concrete’ (SCC) was introduced by Professor Ozowa in Japan [1] and then developed by Bartos and Grauers [2] and Okamura and Ouchi [3]. SCC fills all sections of forms without the need of mechanical vibration and has reasonable flow-ability, homogeneity, resistance against segregation and mechanical strength [4]. Use of SCC in the construction industry has been widely growing in almost the entire world and a good growth of use is predicted for it in the near future [5]. Although the use of SCC has many technical, social, and overall economical advantages, however its supply cost could be two to three times higher then that of normal concrete. Hence to reduce the cost of SCC use of mineral admixtures such as fly ash, limestone filler, and ground granulated blast furnace slag could be used to increase the slump of the concrete mix. Among these materials fly ash, a byproduct of thermal power plants, has been reported to improve the mechanical properties and durability of concrete when used as a cement replacement material. The incorporation of fly ash also reduces the need for viscosity modifying chemical admixtures [6]. Degradation of mechanical behaviour of concrete due to exposure to high temperature has been studied since 1950s in western countries [7,8]. At early stages of heating the evaporable water from concrete is lost over in the range of 20–110 °C. Above 110 °C the cement hydrates decompose, calcium hydroxide is ⇑ Corresponding author. E-mail address: [email protected] (R. Siddique). 0950-0618/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.conbuildmat.2011.11.010
broken down and the calcium carbonate suffers decarbonation. The aggregate also suffers changes, which contributes to the general loss of structure safety [9]. Within the normal temperature, the thermal properties of concrete can be considered constant. However at elevated temperature these properties will be changed because of the change of moisture content of the concrete components [10]. Some of the studies on SCC subjected to high temperature showed both decrease in strength and increase in the risk of spalling or a similar behaviour to that of vibrated concrete. Noumowe and Aggoun [11] presented the experimental work on the high temperature behaviour of conventional vibrated high strength concrete and self-compacting high-strength concrete by using polypropylene fibre, and concluded that residual mechanical properties in reference to the initial mechanical properties of self-compacting high-strength concretes were similar to that of conventional high strength concrete. Castillo presented the experimental results of hot compressive strength in the temperature range of 100–300 °C and observed the decrease in compressive strength of high strength concrete by 15–20% [12]. Jin et al. [13] presented the experimental results on compressive strength for SCC subjected to high temperature. They concluded that after heating to 100 °C and subsequent cooling, the compressive strength of all mixtures decreased as compared to the room-temperature strength. Hanaa et al. [14] studied the mechanical and microstructural properties of self-consolidating concrete using limestone filler subjected to normal and high temperature and reported that after a moderate decrease in compressive strength between 20 and 150 °C, an important increase was observed between 150 and 300 °C. In other studies Phan and Carino [15] and Khoury [16]
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reported the decrease in strength between 20 and 150 °C on vibrated concretes. Bakhtiyari et al. [17] evaluated the mechanical properties and changes in the phase composition of the paste of SCC containing different powders at elevated temperatures. They found that, if fine quartzite powder is used as a filler in the SCC, it will accelerate the strength development at high temperatures up to 500 °C, because of its partial pozzolanic activity, enhanced at such temperatures. They also concluded that it can be said as a general rule that 500 °C is a critical temperature for concretes exposed to fire. Ghandehari et al. [18] evaluated the residual mechanical properties of high strength concretes after exposure to elevated temperatures by using silica fume, and reported that after heating to 200 °C the strength of all of the concretes slightly improved when compared to strength at 100 °C. Siddique [19] studied the properties of SCC mixes, incorporating fly ash content up to 35% and reported that, SCC mixes developed compressive strength ranging from 29 to 35 MPa at 28 days. Patel et al. [20] reported that the use of fly ash decreased the rapid chloride penetration. Also, Nehdi et al. [21] reported that the presence of fly ash reduced the penetrability from approximately 3000 Coulomb to less than 1000 Coulomb. Koulombi et al. [22] observed that the chloride binding capacity of concrete tends to increase with fly ash addition. Zhu et al. [23] reported the significant reduction of chloride diffusivity of SCC with fly ash. Shi and Yanzhong [24] states the use of supplementary cementing material such as fly ash may have a significant effect on the chloride migration of concrete as measured by the RCPT effect. Mustafa et al. [25] also showed that the rapid chloride permeability of most SCC decreases with increase in fly ash and foundry sand content. The objective of the present research was to evaluate the behaviour of SCC subjected to elevated temperature, by using locally available materials. The effect of elevated temperature ranging from 100 to 300 °C on compressive strength, splitting tensile strength and rapid chloride permeability, porosity, and mass loss was investigated and developed 28 days compressive strength varied between 20 and 35 MPa. Water-to-cementitious material ratio for various fly ash SCC mixes, ranging from 0.38 to 0.42, total mass of cementitious materials of 500 kg/m3 with 30–50% replacement of cement with fly ash of total powder content. The superplasticizer content was below 2% of the total powder content (cement + fly ash) for all the mixes. In the present research, behaviour of SCC subjected to elevated temperature with varying fly ash content has been evaluated. 2. Experimental programme
Table 1 Physical properties of Portland cement (IS: 8112-1989) [26]. Physical property
Results obtained
Specifications
Fineness (retained on 90-lm sieve) Standard consistency Initial setting time (min) Final setting time (min) Compressive strength 3-days (MPa) Compressive strength 7-days (MPa) Compressive strength 28 days (MPa) Specific gravity Soundness (mm)
1.0% 30% 65 215 23.6 35.4 43.0 3.12 2.50
<10% – 30 minimum 600 maximum 22.0 minimum 33.0 minimum 44.3 3.15 10 maximum
Table 2 Physical properties of fly ash. Sr. No.
Physical properties
Test results
1 2 3
Colour Specific gravity Lime reactivity – average compressive strength after 28 days of mixture
Blackish grey 2.13 4.90 MPa
Table 3 Chemical properties of fly ash (IS: 3812-2003) [27]. Sr. No
Constituents
Percent by weight
1 2 3 4 5 6 7 8 9
Loss on ignition Silica (SiO2) Iron Oxide (Fe2O3) Alumina (Al2O3) Calcium Oxide (CaO) Magnesium Oxide (MgO) Total Sulphur (SO3) Insoluble residue Alkalies: (a) Sodium Oxide (Na2O) (b) Potassium Oxide (K2O)
4.17 58.55 3.44 28.20 2.23 0.32 0.07 – 0.58 1.26
Table 4 Physical properties of coarse and fine aggregates. Properties
Coarse aggregates
Fine aggregates
Specific gravity Fineness modulus Bulk density (kg/m3) Water absorption (%)
2.66 6.46 1545 0.90
2.65 2.507 1781 0.98
2.1. Materials 2.1.1. Cement Ordinary Portland Cement (Grade 43) was used in this research. Its physical properties are as given in Table 1.
2.1.2. Fly ash Class F Fly ash procured from ‘‘Ropar Thermal Power Plant’’, Punjab, India was used. The physical and chemical properties of fly ash are given in the Tables 2 and 3, respectively.
2.1.3. Aggregates Locally available natural sand with 4.75 mm maximum size was used as fine aggregate. Crushed stone with 10 mm maximum size was used as coarse aggregate. Both fine aggregate and coarse aggregate conformed to Indian Standard Specifications IS: 383-1970 [28]. Table 4 gives the physical properties of the coarse and fine aggregates.
2.1.4. Admixtures Polycarboxylic ether based super plasticizer complying with IS: 9103:1999, ASTMC-494 type F, BS 5057 part III [29] with density approximately 1.10 and pH approximately 5.0 was used in this research.
2.2. Mix proportions Four concrete mixes were made, which had total powder content of 500 kg/m3 (cement + fly ash). Coarse aggregate content was maintained at 51% by volume of concrete and fine aggregate content at 49% by volume of mortar in concrete, the w/p ratio was varied from 0.38 to 0.42 by weight with air content being assumed to be 2%. In this work, one control mix (SCC1) was designed with Ordinary Portland Cement and the other three concrete mixes (SCC2, SCC3, and SCC4) were made by replacing cement with 30%, 40%, and 50% of Class F fly ash by weight of total powder content. Their mix proportions are given in Table 5. 2.3. Mixing and casting For these mix proportions, required quantities of materials were weighed. Cement and fly ash were mixed in dry state, and that of coarse and fine aggregates were mixed dry separately. After adding water, all materials were mixed together to obtain the homogeneous mix. After carrying out the tests for fresh properties, final casting of the mixes was done immediately. After casting, test specimens were left in the casting room for 24 h at a temperature of about 20 °C. The specimens were removed from mould after 24 h and were put into a water-curing tank until the time of the test or as per requirement of the test. The cubes of size 150 mm were cast for determination of compressive strength and 150 300 mm cylinders for
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Table 5 Mix proportions and fresh concrete properties of SCC mixes. Mix SCC SCC SCC SCC
1 2 3 4
Cement (kg/m3)
Fly Ash (%)
FA (kg/m3)
CA (kg/m3)
W/b
SP (%)
Slump (mm)
U-box (mm)
Room temp.
Conc. temp.
500 500 500 500
– 30 40 50
830 830 845 856
876 876 860 856
0.38 0.38 0.40 0.42
2.00 1.82 1.80 1.72
620 634 652 678
12 10 8.64 7.92
34 35 37 35
33 32 35 34
FA stands for fine aggregates; CA for coarse aggregates; W/b for water-binder ratio; and SP for Superplasticizer.
Table 6 Chloride ion penetrability based on charge passed (ASTM 1202-97). Charge passed (Coulomb)
Chloride ion penetrability
>4000 2000–4000 1000–2000 100–1000 <100
High Moderate Low Very low Negligible
Table 7 Mass loss for SCC mixes. Mix
SCC SCC SCC SCC SCC SCC SCC SCC SCC SCC SCC SCC
Temperature (°C)
1 1 1 2 2 2 3 3 3 4 4 4
100 200 300 100 200 300 100 200 300 100 200 300
Fly ash (%)
0 0 0 30 30 30 40 40 40 50 50 50
Mass loss (gm) 28 days
91 days
225 242 285 173 196 230 182 216 234 190 225 245
124 130 139 120 135 142 154 175 205 156 178 195
Table 8 Porosity for SCC mixes. Mix
SCC SCC SCC SCC SCC SCC SCC SCC SCC SCC SCC SCC
Temperature (°C)
1 1 1 2 2 2 3 3 3 4 4 4
100 200 300 100 200 300 100 200 300 100 200 300
Fly ash (%)
0 0 0 30 30 30 40 40 40 50 50 50
Porosity (%)
700 mm the concrete might segregate and if it is less than 500 mm the concrete is considered to have insufficient flow to pass through highly congested reinforcement. U-box test gives us the filling and passing ability of SCC [32]. 2.4.2. Strength and durability properties After the required curing period, the specimens of each concrete mixture were heated at a rate of 1 °C/min up to different temperatures (100, 200 and 300 °C). In order to ensure a uniform temperature throughout the specimens, the temperature was held constant at the maximum temperature for 1 h before cooling. All heated specimens were cooled slowly and tested at room temperature. The rate of heating refers to the recommendations of the RILEM Technical Committee TC-129 [33]. The changes in the mechanical properties of SCC due to high temperature were examined. Compressive strength and splitting tensile strength test was determined at the ages of 28 and 91 days, as per Bureau of Indian Standards, IS: 516:1959 [34]. Rapid chloride penetrability test (RCPT) for durability was conducted as per ASTM C 1202-97 [30] at the ages of 28 and 91 days. 2.4.3. Concrete mass loss The mass of each specimen was measured before and after each temperature cycle. Weight change of concrete was mainly due to dehydration of cement paste. This allows us to quantify the dehydration of concrete after each heating. Results of mass loss are presented in Table 7. 2.4.4. Porosity Total porosity of the different mixes was studied. These properties were measured before and after each temperature cycle. Three samples were tested for each concrete and each temperature cycle. Results of porosity are presented in Table 8. ASTM vacuum saturation technique was used to measure the porosity of concrete [35]. ASTM standard procedures were employed using 100 50 mm cylinder specimens. The specimens were dried in the oven at 100 °C for more than 48 h and then specimens were removed from the oven, cooled in dry air to a room temperature of 20 °C and then weighted to obtain the oven dry mass. After that the specimens were vacuum saturated. Porosity of concrete was calculated based on the concept of weight gain due to water absorption and weight loss because of buoyancy. The porosity is determined according to the following:
28 days
91 days
Porosity ¼ ðW s W d Þ=ðW s W b Þ 100%:
0.77 0.86 0.95 1.20 1.46 1.57 1.53 1.68 1.75 1.90 2.25 2.42
0.55 0.64 0.79 1.00 1.35 1.39 1.42 1.55 1.64 1.68 1.75 1.95
where W s and Wb are the saturated mass of a sample measured in the air and in water respectively. Wd is the mass of oven dried specimen, weighted in air.
splitting tensile strength. Cylindrical specimens of size 100 50 mm size were also cast for rapid chloride penetration test as per ASTM C 1202-97 [30]. The top surface of the specimens was scraped to remove excess material and to achieve smooth finish. All test measurements were taken as the average of five readings. 2.4. Testing 2.4.1. Fresh properties For determining the self-compactibility properties, slump flow test and U-box (difference in height) tests were performed. The slump flow represents the mean diameter of the mass of concrete after release of a standard slump cone; the diameter is measured in two perpendicular directions. According to Nagataki and Fujiwara [31], a slump flow ranging from 500 to 700 mm is considered as the slump required for a concrete to be self-compacted. If the slump flow is greater than
2.4.5. Rapid chloride permeability test The chloride permeability test was conducted to assess the concrete quality as per ASTM C 1202-97. To measure the permeability cylindrical specimens of size 100 mm dia and 50 mm length were cut in cylinders. Only the central part of the cylinders was kept for the measures. A potential difference of 60 V DC was maintained across the specimen. One of the surface was in contact with a sodium chloride solution (NaCl) and the other with a sodium hydroxide solution (NaOH). The total charge passing through in 6 h was measured, indicating the degree of resistance of the specimen to chloride ion penetration. The residual permeability obtained after each temperature cycle was determined. Heated specimens were tested 24 h after cooling in order to maintain the same hydric conditions for all the specimens.
3. Results and discussion 3.1. Properties of fresh concrete The results of various fresh properties tested by slump flow test (slump flow diameter and T50 cm) and U-box test, for various mix compositions are given in Table 5. The slump flow test judges the capability of concrete to deform under its own weight against the friction of the surface with no restraint present. Slump flow for all the mixes in this research was with in the range of 600–700 mm
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Compresive Strength(MPa)
60 Control mix 30% Fly Ash 40% Fly Ash 50% Fly Ash
50
40
30
20
0
100
200
300
400
Temperature(0C) Fig. 3. Compressive strength versus temperature (91 days).
Fig. 1. Rapid chloride permeability test set up (ASTM C 1202).
Compressive Strength (MPa)
60
Control mix 30% flyash 40% flyash 50% flyash
50 40 30 20 10
0
100
200
300
400
Temperature(0C) Fig. 2. Compressive strength versus temperature (28 days).
and the time taken to reach the diameter of 500 mm was less than 5 s. The U-box difference in height of concrete in two chambers was in the range of 7–12 mm. 3.2. Residual compressive strength The behaviour of SCC and vibrated concrete differs significantly between 20 and 300 °C. The compressive strength of concretes with or without fly ash at normal temperature (20 °C) and after heating to 100, 200, 300 °C are shown in Figs. 2 and 3. At normal temperature (20 °C), with the increase in fly ash content from 30% to 50%, SCC mixes (SCC2–SCC4) developed compressive strengths between 30.67 and 21.43 MPa and, between 39.50 and 30.40 MPa at 28 and 91 days respectively. Strength values were 40.68 and 48.90 MPa at 28 and 91 days respectively, for control mix (SCC1). An increase of about 22.35% and 19.00% strength was observed at 28 and 91 days, with the decrease of fly ash content from 50% to 30% (SCC4–SCC2) as compared with control mix (SCC1). After heating to 100 °C and subsequent cooling, SCC mixes (SCC2–SCC4) developed compressive strength ranged between 29.56 and 19.96 MPa and, between 38.00 and 29.25 MPa at 28 and 91 days. An increase of about 24.30% and 18.40% strength was observed at 28 and 91 days, with the decrease of fly ash content from 50% to 30% (SCC4–SCC2) as compared with control mix (SCC1). With the increase in temperature from 100 to 200 °C, compressive strength varied between 29.00 and 19.25 MPa and, between
37.90 and 29.00 MPa at 28 and 91 days. An increase of about 24.00% and 19.00% strength was observed at 28 days and 91 days, with the decrease of fly ash content from 50% to 30% (SCC4–SCC2) as compared with control mix (SCC1). With the further increase in temperature from 200 to 300 °C, compressive strength ranged between 31.45 and 21.20 MPa and, between 39.00 and 30.20 MPa at 28 and 91 days. An increase of about 25.00% and 18.00% strength was observed at 28 days and 91 days, with the decrease of fly ash content from 50% to 30% (SCC4–SCC2) as compared with control mix (SCC1). The compressive strength increased with the decrease in fly ash content and the water-to-cementitious materials ratio, as compared with the SCC mix prepared without fly ash. Strength loss of SCC mixes (SCC2–SCC4) in the temperature range of 20–100 °C was about 1.95%. Between 100 and 200 °C, there was slight reduction in strength. With further increase in temperature from 200 to 300 °C, there was little improvement in strength (about 1%), as compared with SCC1. It has been well documented that the moisture content has a significant effect on the strength of concrete at temperatures ranging from 20 to 200 °C. For all of the mixtures, a small improvement occurred in the residual strength at 200 °C when compared to 100 °C. The increase in strength associated with this increase in temperature was attributed to the increase in surface forces between gel particles due to the removal of moisture content. This was most likely due to the compact microstructure, which results in a build up vapour pressure formed by the evaporation of physically and chemically bound water. The strength values of this research was comparable with those reported by Hanaa et al. [14] i.e. after a moderate decrease in compressive strength between 20 °C and 150 °C, noticeable increase was observed between 150 °C and 300 °C. Several hypotheses had been proposed in the literature to explain the increase in strength between 150 and 300 °C. Khoury [16] assumed that the silanol groups lose a part of their bonds with water, which induces the creation of shorter and stronger siloxane elements (Si–O–Si) with probably larger surface energies that contribute to the increase in strength. Dias et al. [36] attributed this to a rehydration of the paste due to the migration of water in the pores. This increase in strength was also observed by Xu et al. [37] who carried out microhardness tests on hardened cement paste and interfacial transition zone. These results clearly showed that the evolution of residual compressive strength between 200 and 300 °C was due to the hydration of anhydrous cement which leads to the formation of hydrates having better bonding properties. These results confirm the hypothesis of Dias et al. [36] i.e. the increase in strength was also due to an increase in the bonding properties of hydrates (a larger compressive strength was obtained for a larger porosity of the
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6
6
Control mix
30% flyash 40% flyash 50% flyash
5 4
Split tenslie strength(MPa)
Split tensile strength(MPa)
Control mix
3 2 1 0
0
100
200
300
400
40% Fly Ash 50% Fly Ash
4 3 2 1 0
Temperature(0C)
30% Fly Ash
5
0
100
200
300
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Temperature(C)
Fig. 4. Split tensile strength versus temperature (28 days).
Fig. 5. Split tensile strength versus temperature (91 days).
3.3. Residual splitting tensile strength Splitting tensile strength test results of all SCC mixes at various temperatures are shown in Figs. 4 and 5. At normal temperature (20 °C), with the increase in fly ash content from 30% to 50%, SCC mixes (SCC2–SCC4) developed splitting tensile strength between 2.00 and 1.35 MPa and 2.24 and 1.54 MPa at 28 and 91 days. Strength values were 3.56 and, 3.78 MPa at 28 and 91 days, for control mix (SCC1). An increase of about 19.00% and 18.00% strength was observed at 28 and 91 days with the decrease of fly ash content from 50% to 30% (SCC4–SCC2) as compared with control mix (SCC1). After heating to 100 °C and subsequent cooling, SCC mixes (SCC2–SCC4) developed split tensile strength ranged between 1.84 and 1.26 MPa and, 2.00 and 1.48 MPa at 28 and 91 days. An increase of about 18.20% and 14.50% strength was observed at 28 and 91 days with the decrease of fly ash content from 50% to 30% (SCC4–SCC2) as compared with control mix (SCC1). With the increase in temperature from 100 to 200 °C, split tensile strength varied between 1.62 and 1.00 MPa, and 1.87 and 1.22 MPa at 28 and 91 days. An increase of about 20.00% and 14.50% strength was observed at 28 and 91 days, with the decrease of fly ash content from 50% to 30% (SCC4–SCC2) as compared with control mix (SCC1). With the further increase in temperature from 200 to 300 °C, splitting tensile strength ranged between 1.56 and 0.94 MPa, and 1.75 and 1.14 MPa at 28 and 91 days respectively. An increase of about 21.74% and 19.00% strength was observed at 28 days and 91 days, with the decrease of fly ash content from 50% to 30% (SCC4–SCC2) as compared with control mix (SCC1). Splitting tensile strength increased with the decrease in percentage of fly ash content and the water-to-cementitious materials ratio and decreased with increase in temperature, at all ages. Strength loss of SCC mixes (SCC2–SCC4) in the temperature range of 20–100 °C was about 0.2%. Between 100 and 200 °C, there was about 1.80%. With further increase in temperature from 200 to 300 °C; there was about 1.74% loss, as compared with SCC1. This strength loss was not only attributed to the decomposition of the hydration products but also due to the thermal incompatibility between aggregates and cement paste. The tensile strength results of this research was comparable with those reported by Noumowe and Aggoun [11] that split tensile strength decreased continuously with increase in temperature.
2500
Permeability (Coulombs)
material). Therefore these results are in line with the published literature. The strength loss in present research at elevated temperature was much less than that reported by previous researchers, and this is because of incorporation of fly ash.
Control mix 30%Fly Ash 40%FlyAsh 50%Fly Ash
2000 1500 1000 500 0
20
100
200
300
Temperature ( 0C) Fig. 6. Rapid chloride permeability of fly ash concrete (28 days).
The value of split tensile strength was reported as 4.8 MPa at ambient temperature and 2.8 MPa after heating to 400 °C. In another research by Ghandehari et al. [18] reported the reduction from 14.4% to 17.9% after being exposed from 100 to 600 °C. Reduction in split tensile strength observed in present study was much lower than that already studied in the previous researches. This is because of replacement of cement with fly ash. 3.4. Chloride penetration resistance Rapid chloride permeability test setup (ASTM C1202) is shown in Fig. 1. Chloride permeability test was conducted on all the concrete specimens. The total charge passed in 6 h as a measure of the chloride permeability is presented in Figs. 6 and 7. The chloride ion penetrability limits suggested by ASTM C1202 (Table 6) were compared with the results. Much like the findings of previous studies [19–25] these Figs. 6 and 7 showed that the use of fly ash significantly reduced the chloride permeability of hardened SCC mixtures when compared to the control concretes. On the other hand permeability increased with the increase in temperature, like previous research [14]. It is evident from the results that SCC mixes (SCC2– SCC4) made with fly ash reduced the rapid chloride ion penetrability to the low range (1000–2000 Coulomb) at the age of 28 days, between low range to very low range (<1000 Coulomb) at the age of 91 days. The incorporation of fly ash resulted in a reduction in Coulomb charges. At normal temperature (20 °C) the Coulomb charge of control mix (SCC1) was 1393, and 1186 Coulomb and fly ash mixes (SCC2–4) was between 1234 and 1143 Coulomb, and 1030 and 854 Coulomb at 28 and 91 days respectively.
N. Pathak, R. Siddique / Construction and Building Materials 30 (2012) 274–280
Permeability (Coulombs)
2000
Control mix 30%Fly Ash 40%Fly Ash 50%Fly Ash
1500
1000
500
0
20
100
200
300
Temperature(C) Fig. 7. Rapid chloride permeability of fly ash concrete (91 days).
The Coulomb charge of control mix (SCC1) at 300 °C was 1532 and 1432 Coulomb, and fly ash mixes (SCC2–4) was between 1367 and 1245 Coulomb, and 1245 and 923 Coulomb at 28 and 91 days respectively, indicating high chloride penetration resistance. In no case the value of the charges passed was greater than 1532 Coulomb implying dense SCC mix structure. The significant reduction in chloride ion penetration may be due to incorporation of fly ash whose spherical particles could improve particle-packing density in the matrix. In the present research, permeability was lower at 91 days, as compared with 28 days, as expected due to the hydration of Portland cement and pozzolanic reactions of fly ash. One of the most important factor affecting the permeability of concrete was the internal pore structure, which in turn was dependent on the extent of hydration of the cementitious materials. The curing conditions and the age of concrete thus largely determine the ease with which chloride ions can move into a concrete. From the test results it is observed that most concretes become significantly less permeable with increase in curing time. 3.5. Concrete mass loss Table 7 presents the evolutions of mass loss of cubical specimens of size 150 mm after heating. It was observed that the evolution of mass loss versus temperature was very close to five studied concretes. Between the room temperature and 200 °C, the variation of mass loss was less. This corresponds to the loss of the evaporable water and part of the physically bound water. Between 200 and 300 °C, noticeable increase in mass loss was observed for all concretes. This is because of loss of chemically bound water from the decomposition of the C–S–H, the carboaluminate hydrates and the dehydration of calcium silicate hydroxide. This was in good agreement with the observation by the Hanaa et al. [14] who observed that about 70% of all the water contained in the concretes had been evaporated at 300 °C. The results are in line with the published data. 3.6. Porosity Table 8 presents the results of porosity. For the initial stage of heating, the porosity was nearly stable up to 100 °C. By increasing the temperature from 100 to 200 °C or from 200 to 300 °C porosity increased. When the percentage of fly ash increased porosity also increased. The increase in porosity values was attributed to change SCC from impermeable to permeable material due to losses and pruned of fine, super fine and organic materials. These results are in line with the results reported by Hanaa et al. [14] and Noumowe
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and Aggoun [11]. They also observed the increase in porosity with temperature. They attributed the increase in porosity with temperature to the departure of bound water and to the microcracks generated by differential expansion between the paste and aggregates. Noumowe and Aggoun [11] showed by mercury intrusion porosimetry an increase in the pores sizes beyond 120 °C. In other research by Gallé and Sercombe [38] attributed the evolution of porosity to the generation of large capillary pores. Their appearance was due to the release of adsorbed water of capillary pores and release of bound water in cement paste hydrates. If compare the evolution of compressive strength with that of porosity, observed both an increase in strength and an increase in porosity for the SCC’s between 200 and 300 °C. The increase in strength can therefore not be attributed to a decrease in porosity but rather to a modification of the bonding properties of the hydrates of cement paste. The experimental results are therefore in line with Hanna and Khoury’s hypothesis concerning the creation of shorter and stronger siloxane elements Si–O–Si (with probably larger surface energies) by the loss of a part of the bonds with water in silanol groups. 4. Conclusions Based on the results of this experimental study, the following conclusions can be drawn: 1. Although fly ash reduces the strength but still it is possible to produce SCC with adequate strength. 2. All the high volume fly ash concretes have satisfied the norms that were set to qualify them as SCC and the fresh properties like workability were in good agreement with European guidelines [39]. The developed SCC was highly segregation resistant and had good flowability and passing ability. 3. The SCC mixes developed compressive strengths ranging from 30.67 to 19.25 MPa, and from 39.50 to 29 MPa, at 28, and 91 days respectively. Splitting tensile strengths developed were from 0.94 to 2.00 MPa, and 1.14 to 2.24 MPa at 28, and 91 days respectively. The compressive strength increased with a decrease in the percentage of the fly ash and the water-to-cementitious materials ratio. 4. Between 20 and 200 °C, a small loss of strength was observed. Between 200 and 300 °C, the compressive strengths of all of the concrete specimens slightly improved as compared to strength at 100 °C. The increase in strength could be attributed to a modification of the bonding properties of the cement paste hydrates i.e. rehydration of the paste due to the migration of water in the pores. 5. At elevated temperatures, the rate of the splitting tensile strength loss is higher than the rate of the compressive strength loss. The splitting tensile strength continued to decrease in a similar way as was observed between 20 and 100 °C, due to the departure of bound water, corresponding to a large mass loss. 6. The high volume fly ash SCC mixes showed significantly lower chloride ion permeability than SCC without fly ash concretes. Most of the SCC mixes were assessed as ‘‘very low’’ chloride permeability concretes as per ASTM C1202– 94 assessment criteria, with less than 1000 coulombs of total charge passing.
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