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The influence of high temperature curing on the compressive, tensile and flexural strength of pulverized fuel ash concrete
Building and Environment 35 (2000) 415±423
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The in¯uence of high temperature curing on the compressive, tensile and ¯exural strength of pulverized fuel ash concrete R.V. Balendran*, W.H. Martin-Buades Building and Construction Department, City University of Hong Kong, 83 Tat Chee Avenue, Kowloon Tong, Hong Kong Received 20 October 1998; received in revised form 20 January 1999; accepted 29 April 1999
Abstract This paper presents the results of a series of experiments conducted to investigate the eect of high temperature curing on the compressive strength, tensile splitting strength and ¯exural strength of concrete made with Hong Kong pulverized fuel ash (PFA). The curing temperatures adopted were 27, 34, 42 and 508C. The experimental results suggest that high temperature cured PFA concrete normally has a greater compressive strength, tensile splitting strength and ¯exural strength than similarly cured ordinary Portland cement (OPC) concrete. The results obtained further suggest that beyond 28 d age and at high curing temperatures, the dierence in strength properties between them is signi®cant. # 2000 Elsevier Science Ltd. All rights reserved.
1. Introduction In the temperate regions of the world, the eect on concrete of partially replacing its cement content by pulverized fuel ash (PFA) has been extensively researched and, in general, has led to the production of concrete having better strength properties in those regions. However, in the tropical regions of the world, the eect of this replacement on the strength properties of concrete has not been so thoroughly investigated. According to Balendran and Pang [1], the partial replacement of OPC by PFA has helped to enhance the quality of both the fresh and hardened concrete in terms of: . Lower heat of hydration. This extends the workability period, lowers the temperature rise during curing, and reduces thermal stresses; . Lower water demand. This lowers the possibility of bleeding and reduces drying shrinkage; . Greater medium term strength; . Reduction in creep; and . Increased long term elastic modulus; The vast majority of research on PFA concrete has * Corresponding author.
been conducted at temperatures below 258C Ð that is at what is considered as normal curing temperatures in the temperate regions of the world. Hence, the ®ndings of these research projects may not be completely relevant to PFA concrete subject to the much higher ambient and curing temperatures of the tropical regions of the world. This paper concentrates on the eect of high temperature curing on the compressive strength, tensile strength and ¯exural strength of PFA concrete and compares the results obtained with those for similarly cured OPC concrete. The curing temperatures adopted were 27, 34, 42 and 508C. Halstead [2] reported that if the heat of hydration generated by an OPC concrete with water:cement (W:C) ratio of 0.5 and an aggregate/cement (A/C) ratio of 6.0 during curing is wholly retained, it would raise the temperature of the OPC concrete by approximately 438C. Based on this ®nding, in the subtropical regions of the world where the ambient temperature is frequently above 308C, the temperature inside the cured concrete may exceed 508C.
2. Concrete cured at elevated temperatures Bouge [3] noticed that, for the normal range of Port-
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R.V. Balendran, W.H. Martin-Buades / Building and Environment 35 (2000) 415±423
land cements, one half of the heat of hydration is released between 1 and 3 d after casting, around three quarters of it is released during the ®rst 7 d after casting, and approximately 87% of it is released during the ®rst 6 months after casting. In fact, the heat of hydration released by a cement depends on its chemical composition and is approximately equal to the sum of the heats of hydration of the individual pure compounds when their respective proportions by mass are hydrated separately [4]. The compounds tricalcium aluminate (C3A) and tricalcium silicate (C3S) are known to contribute most of the heat of hydration with C3A contributing most of the early age heat of hydration. It hence follows that by reducing the proportion of C3A and C3S in the Portland cement, its heat of hydration can be reduced. A method of achieving this is by partially replacing the Portland cement with a pozzolana Ð for example, a PFA, with a low calcium oxide (CaO) content. Abbasi and Al-Tayyib [5] found that OPC concretes cured at high temperatures necessitate a higher initial volume of mixing water in order to achieve a required workability and this leads to a higher W:C ratio and consequently lower medium term compressive strength. They also found that, for OPC concretes cured in hotdry environments, the required ultimate compressive strength can be achieved but their modulus of rupture, splitting tensile strength and ¯exural strength were reduced to some extent. In addition, they also found that at high temperatures, the rate at which OPC concretes loose their workability is increased and, in order to overcome this, they require additional mixing water. Bamforth [6] observed that if ordinary Portland cement is partially replaced by PFA, the total mixing water demand of the resulting concrete is reduced. Hence, it can be said that PFA is a suitable replacement for ordinary Portland cement when the resulting concrete is to be cured at high temperature. There are diering viewpoints in relation to the eect of high curing temperature on the medium term compressive strength development of concrete [7]. Mustafa and Yusof [8] suggested that the medium term compressive strength of a concrete cured at high temperature will not be adversely aected, provided the ambient relative humidity during curing is high enough. A PFA concrete of similar compressive strength as an OPC concrete requires less water and is generally considered to have a better pumpability, and to exhibit a reduced tendency to segregate and bleed. However, the addition of PFA to concrete prolongs its workability period and delays its setting time. This delay in setting time increases the risk of the formation of plastic shrinkage cracks as suggested by Ravina [9], Harrison [10] and Concrete Working Party [11]. PFA concrete has a longer setting time than OPC
concrete, due to the slow initial reaction process of the PFA. Under normal curing conditions, the early stage reaction of the PFA is rather low and because of this normally cured OPC concretes of all types can be expected to have a better early compressive strength than comparable PFA concretes. The major drawback of using PFA as partial replacement of cement is the reduction it causes in the rate of early compressive strength development. High early compressive strength is important for concrete as it facilitates the early removal and re-use of formwork, thus helping to reduce the overall cost of concreting. 3. Strength properties OPC concrete cured at elevated temperatures (above 308C) has been shown to possess lower early and medium term strength properties than OPC concrete cured at 208C [7,12]. This reduction is probably due to cracking caused by thermal stresses. Mustafa [8], nonetheless, suggested that because of the reduction in the macro porosity of the concrete mass caused by maturity, a concrete subject to both a high curing temperature and ambient humidity should not experience any adverse eects on the medium term strength properties. Experimental work carried out by Alshami [13] showed that, at ambient temperatures of 20, 35 and 458C, the partial replacement of cement by PFA improved the compressive strength development of concrete. It also showed that, due to a reduction in the rate of heat of hydration, both the peak curing temperature and the time required to reach it decreased. These latter ®ndings means that the use of PFA concrete oers the possibility of quicker heat dissipation and hence reduced risk of thermal cracking. 4. Materials, mixing and testing conditions 4.1. Materials Ordinary Portland cement (OPC) [14], pulverized fuel ash (PFA) [15], 20 mm and 10 mm (maximum size) coarse granite aggregates, and crushed stone ®nes were used in the making of the test specimens. All materials used were from one single delivery and source. The chemical composition of the OPC and PFA has been provided by the suppliers concerned and is given in Table 1. 4.2. Mixing and testing conditions The concrete for the test specimens was mixed in an air-conditioned laboratory with an ambient tempera-
R.V. Balendran, W.H. Martin-Buades / Building and Environment 35 (2000) 415±423
417
Table 1 Chemical compositions of OPC and PFA
Fineness (m3/kg) Silicon dioxide (SiO2) (%) Iron (III) oxide (Fe2O3) (%) Aluminium oxide (Al2O3) (%) Calcium oxide (CaO) (%) Magnesium oxide (MgO) (%) Sulfur (SO3) (%) Sodium (as Na2O) (%) Titanium (TiO2) (%) Potassium (K2O) (%) Loss on ignition (%) Insoluble (%) Chloride content (%) Free lime (%) Lime Saturation Factor Tricalcium Aluminate (C3A) (%) Tricalcium Silicate (C3S) (%) a
OPC (China Cement Company, Hong Kong)
PFA (China Light & Power, Hong Kong)
320 21.1 3.1 5.9 64.6 1.0 2.6 0.6 ± ± 0.9 0.004 0.2 1.0 0.92 0.2 51.1
< 12.5a 54 4.8 29 4.8 1.0 0.6 0.4 1.4 0.9 5.7 ± ± ± ± ± ±
Retained on 40 mm sieve (% by weight).
ture of approximately 238C and a relative humidity of 80%. The mixing was carried out using a pan mixer. The OPC and PFA concretes were designed to have similar 28-day compressive strength and also similar workability. The W:C ratio of the OPC concrete was 0.48 and the W:(C+F) ratio of the PFA concrete was 0.45. Daratard 17, an initial setting retarder, was added to both mixes in order to simulate the production and delivery of ready-mix concrete Ð a very common method of supplying concrete in Hong Kong. The mix details of the OPC concrete and that of the PFA concrete are shown in Table 2. The test specimens were demoulded one day after casting and then water-cured at their designated high temperature until the age of 7 d. After 7 d, they were
transferred to curing tanks with water at 278C and left there until they reached their test age. The designated temperatures were 27, 34, 42 and 508C. The results of tests carried out on test specimens cured at 278C were used for control purposes as this is the standard curing temperature in Hong Kong [16] and in other tropical countries. Compressive strength, tensile splitting strength and ¯exural strength tests were performed in accordance with the appropriate standard speci®cations on the test specimens at dierent ages after casting.
5. Strength tests 5.1. Compressive strength
Table 2 Mix details
Cement PFA 20 mm aggregate 10 mm aggregate Fines Water Admixture (Daratard 17) Water cementitious ratioa
OPC Concrete (kg/m3)
PFA Concrete (kg/m3)
450 ± 650 515 520 215 900 ml 0.48
360 120 700 475 455 215 900 ml 0.45
a W:C and W:(C+F) ratios do not include absorption by aggregates.
100 mm concrete cubes were tested for compressive strength in accordance with BS 1881: Part 116: 1983 [17] at 2, 7, 14, 28, 56 and 91 d after casting. The cubes were subjected to water-curing at a temperature of either 27, 34, 42 or 508C for either 2 or 7 d as appropriate. In addition, all 100 mm concrete cubes tested at 14, 28, 56, and 91 d were water-cured at 278C after 7 d until tested. 5.2. Tensile splitting strength 100 mm ; 200 mm concrete cylinders were tested for tensile splitting strength in accordance with BS 1881: Part 117: 1983 [18] at 7, 28 and 56 d after casting. The cubes were subjected to water-curing at a
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temperature of either 27, 34, 42 or 508C for 7 d and thereafter at 278C until tested. 5.3. Flexural strength 100 100 500 mm concrete beams were tested for ¯exural strength in accordance with BS 1881: Part 118: 1983 [19] at 7, 28, 56 and 91 d after casting The cubes were subjected to water-curing at a temperature of either 27, 34, 42 or 508C for 7 d and thereafter at 278C until tested.
6. Results and discussion
.
.
6.1. Early compressive strength Fig. 1 shows plots of the compressive strength results obtained for the OPC and PFA concrete cubes tested at 2, 7, 14, and 28 d. According to these plots: . All PFA and OPC concrete cubes tested at 2 and 7 d and which had been cured at either 34, 42, or 508C had a higher early compressive strength than those cured for the same period of time at the standard curing temperature of 278C; . Two days after casting, the early compressive
.
.
strength of the PFA concrete cubes tested was always lower than that of the corresponding OPC ones. These results con®rm the already well established fact that the partial replacement of OPC by PFA slows down the early compressive strength development of the resulting concrete; After 7 d of curing at 428C, the early compressive strength of the PFA concrete cubes became higher than that of the corresponding OPC ones. Hence the results obtained indicate that the higher the curing temperature, the earlier PFA concrete can attain the compressive strength of OPC concrete cured at the same temperature; The rate of gain of early compressive strength of PFA concrete was always higher than that of the corresponding OPC concrete cubes. These results indicate that, in the case of high temperature curing, the partial replacement of OPC by PFA generally enhances the early compressive strength and its rate of gain of the resulting concrete; When cured at temperatures of either 42 or 508C, the OPC concrete cubes failed even to achieve the 28-day compressive strength at the standard curing temperature of 278C. According to these results, very high temperature
Fig. 1. Early compressive strength of OPC and PFA concrete cured at dierent elevated temperatures.
R.V. Balendran, W.H. Martin-Buades / Building and Environment 35 (2000) 415±423
419
Fig. 2. Medium term compressive strength of OPC and PFA concrete at dierent elevated temperatures.
curing adversely aects the early compressive strength of OPC concrete.
6.2. Medium term compressive strength Fig. 2 shows plots of the compressive strength results obtained for the OPC and PFA concrete cubes tested at 56 and 91 d. According to these plots, immaterial of curing temperature, the medium term compressive strength of the PFA concrete cubes tested at 56 and 91 d tested was always higher than that of the corresponding OPC concrete cubes Ð on average, approximately 15% higher. These results are similar to those obtained by Alshami [13]. A possible explanation for the increased medium term compressive strength of PFA concrete could be the reduction of C3A and the deceleration of rate of hydration permitting the formation of more dense hydrates of strength-contributing dicalcium silicate (C2S) to form [20]. 6.3. Early tensile splitting strength Fig. 3 shows plots of the tensile splitting strength results obtained for the concrete cylinders tested at 7 and 28 d. According to these plots: . Irrespective of curing temperature, all OPC concrete cylinders tested at 7 d had a higher early tensile splitting strength than the corresponding PFA concrete ones; . At 28 d, the early tensile splitting strength of the OPC concrete cylinders tested was either higher or equal to that of the corresponding PFA concrete
cylinders when the curing temperature was either 27, 42 or 508C. However, at 348C curing temperature, the early tensile splitting strength of the OPC concrete cylinders tested was lower than that of the corresponding PFA concrete ones; . At 7 d, the early tensile splitting strength of the OPC concrete cylinders tested increased noticeably with curing temperature. However, at 28 d, the early tensile splitting strength of the OPC concrete cylinders tested increased overall only slightly with temperature and, at 358C curing temperature, its value was actually lower than that for the standard curing temperature of 278C; . The early tensile splitting strength of the PFA concrete cylinders tested at 7 and 28 d increased to a peak value and then declined with increasing temperature. For those tested at 7 d, the peak value occurred when the curing temperature was 428C and the decline in value thereafter was rather slight. For those tested at 28 d, the peak value occurred when the curing temperature was 358C and the decline in value thereafter was noticeable. However, for both test ages, the PFA concrete cylinders cured at elevated temperatures had always higher early tensile splitting strengths than those cured at the standard curing temperature of 278C.
6.4. Medium term tensile splitting strength Fig. 4 shows plots of the tensile splitting strength results obtained for the concrete cylinders tested at 56 d. According to these plots:
420
R.V. Balendran, W.H. Martin-Buades / Building and Environment 35 (2000) 415±423
Fig. 3. Early tensile strength of OPC and PFA concrete cured at various elevated temperatures.
. The medium term tensile splitting strength of the OPC concrete cylinders tested did not increase as the curing temperature rose. In actual fact, the medium term tensile splitting strength of the OPC con-
crete cylinders tested which had been cured at either 34, 42 or 508C was always lower than the value for ones cured at the standard temperature of 278C; . At 348C curing temperature, the PFA concrete cylin-
Fig. 4. 56-day tensile strength of OPC and PFA concrete cured at dierent elevated temperatures.
R.V. Balendran, W.H. Martin-Buades / Building and Environment 35 (2000) 415±423
421
Fig. 5. Early ¯exural strength of OPC and PFA concrete cured at various elevated temperatures.
ders tested experienced a signi®cant increase in their medium term tensile splitting strength. However, at either 42 and 508C curing temperature, the medium term tensile splitting strength of the PFA concrete cylinders tested was lower than that of the ones tested at the standard curing temperature of 278C. From the results obtained it is not possible to see any obvious trend regarding the medium term tensile splitting strength of PFA concrete cured at elevated temperatures. . If the results shown in Figs. 3 and 4 are compared, it can be concluded that the tensile splitting strength
at 56 d is not signi®cantly higher than that at 28 d for either the OPC or PFA concrete. 6.5. Early ¯exural strength Fig. 5 shows plots of the ¯exural strength results obtained for the concrete beams tested at 7 and 28 d. According to these plots: . The ¯exural strength of the PFA concrete beams tested at 7 d increased uniformly with curing temperature and, at both 42 and 568C curing temperature, was higher than that of the corresponding
Fig. 6. Medium term ¯exural strength of OPC and PFA concrete cured at dierent elevated temperatures.
422
R.V. Balendran, W.H. Martin-Buades / Building and Environment 35 (2000) 415±423
OPC ones. On the other hand, the ¯exural strength of the OPC concrete beams tested at the same age remained substantially the same irrespective of curing temperature except for those cured at 348C. The beams cured at 348C experienced an increase of approximately 10% in ¯exural strength; . The ¯exural strength of the PFA concrete beams tested at 28 d increased with curing temperature and, at the elevated curing temperatures, was always greater than that of the corresponding OPC ones. In actual fact, at the elevated curing temperatures, the ¯exural strength of the latter at 28 d was always lower than that at the standard curing temperature of 278C.
6.6. Medium term ¯exural strengths Fig. 6 shows plots of the ¯exural strength results obtained for the concrete beams tested at 56 and 91 d. According to these plots: . The medium term ¯exural strength of the OPC concrete beams tested remained substantially the same immaterial of curing temperature and test age. This ®nding indicates that curing temperature has little eect on the medium term ¯exural strength of an OPC concrete; . At the standard curing temperature of 278C, the medium term ¯exural strength of the PFA concrete beams tested at 91 d was markedly higher than that of the ones tested at 56 d. At 348C curing temperature, however, the position was reversed, with the 91-day medium term ¯exural strength not only lower than the 56-day one but also lower than its own value at the standard curing temperature of 278C. Beyond 348C curing temperature, the medium term ¯exural strength of the PFA concrete beams tested became similar immaterial of test age. It is, however, not possible to detect from the results obtained, any obvious overall trend as to how the ¯exural strength of a PFA concrete is aected by high curing temperatures; . At all the curing temperatures and test ages, the ¯exural strength of the PFA concrete beams tested was always greater than that of the corresponding OPC concrete ones. This ®nding indicates that a PFA concrete cured at high temperatures generally possess a greater ¯exural strength than a comparable OPC concrete cured under similar conditions.
7. Concluding remarks Curing OPC concrete at elevated temperatures can
help to improve the early strength development of the concrete. The partial replacement of the OPC by PFA further improves its early compressive and tensile splitting strength development and also has a bene®cial eect on its ¯exural strength. Generally, the higher the curing temperature, the better the medium term compressive strength, tensile splitting strength and ¯exural strength of PFA concrete. OPC concrete did not behave in a similar manner when cured at high temperatures. Its reduced performance under high temperature curing may be due to thermal stresses generated by its high heat of hydration. The PFA concrete, due to its reduced heat of hydration, has a lower peak temperature and rate of temperature rise and therefore avoids such thermal stresses. The improved performance of the PFA concrete at the later ages under high temperature curing may be due to delayed pozzolanic reaction.
References [1] Balendran RV, Pang HW. Strength development, deformation properties and mix design of pulverized fuel ash concrete. In: Structural survey journal, vol. 13. England: MCB Press, 1995. p. 7±11. [2] Halstead P. Concreting in hot weathers. In: Our world in concrete and structures, vol. I. Singapore: Ready Mixed Concrete Association of Singapore, 1980. p. 425±8. [3] Bogue RH. Chemistry of portland cement. New York: Reinhold, 1955. [4] Neville AM. In: Properties of concrete, 4th ed. England: Addison Wesley Longman, 1995. p. 37±40. [5] Abbasi AF, Al-Tayyib AJ. Eect of hot weather on shear strength of concrete. In: Transportation research record 924. Washington DC: Transportation Research Board, 1983. p. 27± 32. [6] Bamforth PB. Alternative cements for hot climates. Concrete 1986;20(2):18±20. [7] Berhane Z. The behaviour of concrete in hot climates. Materials and Structures 1992;25:157±62. [8] Mustafa MA, Yusof KM. Mechanical properties of hardened concrete in hot-humid climate. Cement and Concrete Research 1991;21:601±13. [9] Ravina D, Shalon R. Plastic shrinkage cracking. ACI Journal 1968;65:282±92. [10] Harrison TA, Spooner DC. The properties and use of concrete made with composite cements. In: Cement & Concrete Association Interim Technical Report 10, 1986. p. 1±28. [11] Concrete Society Working Party. In: Non-structural cracks in concrete. London: The Concrete Society, 1986. p. 1±38. [12] Thomas MDA, Matthews JD, Haynes CA. The eect of curing on the strength and permeability of PFA concrete. In: American Concrete Institute SP 114. Detroit: American Concrete Institute, 1989. p. 91±217. [13] Alshamsi AM. Temperature rise inside paste during hydration in hot climates. Cement and Concrete Research 1994;24(2):353± 60. [14] British Standards Institution. BS 12: 1978 ordinary and rapidhardening Portland cement. London: BSI, 1983. [15] British Standards Institution. BS 3892: Part 1: 1982 speci®cation
R.V. Balendran, W.H. Martin-Buades / Building and Environment 35 (2000) 415±423 for pulverized fuel ash for use as cementitious component in structural concrete. London: BSI, 1982. [16] Hong Kong Government. In: Construction standard: testing concrete, 1. Hong Kong: Hong Kong Government Publishers, 1990. [17] British Standards Institution. BS 1881: Part 116: 1983 method for determination of compressive strength of concrete cubes. London: BSI, 1983.
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[18] British Standards Institution. BS 1881: Part 117: 1983 method for determination of tensile splitting strength. London: BSI, 1983. [19] British Standards Institution. BS 1881: Part 118: 1983 method for determination of ¯exural strength. London: BSI, 1983. [20] Mani AC, Tam CT, Lee SL. In¯uence of high early temperatures on properties of PFA concrete. Cement and Concrete Composites 1990;12:109±15.
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The in¯uence of high temperature curing on the compressive, tensile and ¯exural strength of pulverized fuel ash concrete R.V. Balendran*, W.H. Martin-Buades Building and Construction Department, City University of Hong Kong, 83 Tat Chee Avenue, Kowloon Tong, Hong Kong Received 20 October 1998; received in revised form 20 January 1999; accepted 29 April 1999
Abstract This paper presents the results of a series of experiments conducted to investigate the eect of high temperature curing on the compressive strength, tensile splitting strength and ¯exural strength of concrete made with Hong Kong pulverized fuel ash (PFA). The curing temperatures adopted were 27, 34, 42 and 508C. The experimental results suggest that high temperature cured PFA concrete normally has a greater compressive strength, tensile splitting strength and ¯exural strength than similarly cured ordinary Portland cement (OPC) concrete. The results obtained further suggest that beyond 28 d age and at high curing temperatures, the dierence in strength properties between them is signi®cant. # 2000 Elsevier Science Ltd. All rights reserved.
1. Introduction In the temperate regions of the world, the eect on concrete of partially replacing its cement content by pulverized fuel ash (PFA) has been extensively researched and, in general, has led to the production of concrete having better strength properties in those regions. However, in the tropical regions of the world, the eect of this replacement on the strength properties of concrete has not been so thoroughly investigated. According to Balendran and Pang [1], the partial replacement of OPC by PFA has helped to enhance the quality of both the fresh and hardened concrete in terms of: . Lower heat of hydration. This extends the workability period, lowers the temperature rise during curing, and reduces thermal stresses; . Lower water demand. This lowers the possibility of bleeding and reduces drying shrinkage; . Greater medium term strength; . Reduction in creep; and . Increased long term elastic modulus; The vast majority of research on PFA concrete has * Corresponding author.
been conducted at temperatures below 258C Ð that is at what is considered as normal curing temperatures in the temperate regions of the world. Hence, the ®ndings of these research projects may not be completely relevant to PFA concrete subject to the much higher ambient and curing temperatures of the tropical regions of the world. This paper concentrates on the eect of high temperature curing on the compressive strength, tensile strength and ¯exural strength of PFA concrete and compares the results obtained with those for similarly cured OPC concrete. The curing temperatures adopted were 27, 34, 42 and 508C. Halstead [2] reported that if the heat of hydration generated by an OPC concrete with water:cement (W:C) ratio of 0.5 and an aggregate/cement (A/C) ratio of 6.0 during curing is wholly retained, it would raise the temperature of the OPC concrete by approximately 438C. Based on this ®nding, in the subtropical regions of the world where the ambient temperature is frequently above 308C, the temperature inside the cured concrete may exceed 508C.
2. Concrete cured at elevated temperatures Bouge [3] noticed that, for the normal range of Port-
0360-1323/00/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved. PII: S 0 3 6 0 - 1 3 2 3 ( 9 9 ) 0 0 0 3 1 - 1
416
R.V. Balendran, W.H. Martin-Buades / Building and Environment 35 (2000) 415±423
land cements, one half of the heat of hydration is released between 1 and 3 d after casting, around three quarters of it is released during the ®rst 7 d after casting, and approximately 87% of it is released during the ®rst 6 months after casting. In fact, the heat of hydration released by a cement depends on its chemical composition and is approximately equal to the sum of the heats of hydration of the individual pure compounds when their respective proportions by mass are hydrated separately [4]. The compounds tricalcium aluminate (C3A) and tricalcium silicate (C3S) are known to contribute most of the heat of hydration with C3A contributing most of the early age heat of hydration. It hence follows that by reducing the proportion of C3A and C3S in the Portland cement, its heat of hydration can be reduced. A method of achieving this is by partially replacing the Portland cement with a pozzolana Ð for example, a PFA, with a low calcium oxide (CaO) content. Abbasi and Al-Tayyib [5] found that OPC concretes cured at high temperatures necessitate a higher initial volume of mixing water in order to achieve a required workability and this leads to a higher W:C ratio and consequently lower medium term compressive strength. They also found that, for OPC concretes cured in hotdry environments, the required ultimate compressive strength can be achieved but their modulus of rupture, splitting tensile strength and ¯exural strength were reduced to some extent. In addition, they also found that at high temperatures, the rate at which OPC concretes loose their workability is increased and, in order to overcome this, they require additional mixing water. Bamforth [6] observed that if ordinary Portland cement is partially replaced by PFA, the total mixing water demand of the resulting concrete is reduced. Hence, it can be said that PFA is a suitable replacement for ordinary Portland cement when the resulting concrete is to be cured at high temperature. There are diering viewpoints in relation to the eect of high curing temperature on the medium term compressive strength development of concrete [7]. Mustafa and Yusof [8] suggested that the medium term compressive strength of a concrete cured at high temperature will not be adversely aected, provided the ambient relative humidity during curing is high enough. A PFA concrete of similar compressive strength as an OPC concrete requires less water and is generally considered to have a better pumpability, and to exhibit a reduced tendency to segregate and bleed. However, the addition of PFA to concrete prolongs its workability period and delays its setting time. This delay in setting time increases the risk of the formation of plastic shrinkage cracks as suggested by Ravina [9], Harrison [10] and Concrete Working Party [11]. PFA concrete has a longer setting time than OPC
concrete, due to the slow initial reaction process of the PFA. Under normal curing conditions, the early stage reaction of the PFA is rather low and because of this normally cured OPC concretes of all types can be expected to have a better early compressive strength than comparable PFA concretes. The major drawback of using PFA as partial replacement of cement is the reduction it causes in the rate of early compressive strength development. High early compressive strength is important for concrete as it facilitates the early removal and re-use of formwork, thus helping to reduce the overall cost of concreting. 3. Strength properties OPC concrete cured at elevated temperatures (above 308C) has been shown to possess lower early and medium term strength properties than OPC concrete cured at 208C [7,12]. This reduction is probably due to cracking caused by thermal stresses. Mustafa [8], nonetheless, suggested that because of the reduction in the macro porosity of the concrete mass caused by maturity, a concrete subject to both a high curing temperature and ambient humidity should not experience any adverse eects on the medium term strength properties. Experimental work carried out by Alshami [13] showed that, at ambient temperatures of 20, 35 and 458C, the partial replacement of cement by PFA improved the compressive strength development of concrete. It also showed that, due to a reduction in the rate of heat of hydration, both the peak curing temperature and the time required to reach it decreased. These latter ®ndings means that the use of PFA concrete oers the possibility of quicker heat dissipation and hence reduced risk of thermal cracking. 4. Materials, mixing and testing conditions 4.1. Materials Ordinary Portland cement (OPC) [14], pulverized fuel ash (PFA) [15], 20 mm and 10 mm (maximum size) coarse granite aggregates, and crushed stone ®nes were used in the making of the test specimens. All materials used were from one single delivery and source. The chemical composition of the OPC and PFA has been provided by the suppliers concerned and is given in Table 1. 4.2. Mixing and testing conditions The concrete for the test specimens was mixed in an air-conditioned laboratory with an ambient tempera-
R.V. Balendran, W.H. Martin-Buades / Building and Environment 35 (2000) 415±423
417
Table 1 Chemical compositions of OPC and PFA
Fineness (m3/kg) Silicon dioxide (SiO2) (%) Iron (III) oxide (Fe2O3) (%) Aluminium oxide (Al2O3) (%) Calcium oxide (CaO) (%) Magnesium oxide (MgO) (%) Sulfur (SO3) (%) Sodium (as Na2O) (%) Titanium (TiO2) (%) Potassium (K2O) (%) Loss on ignition (%) Insoluble (%) Chloride content (%) Free lime (%) Lime Saturation Factor Tricalcium Aluminate (C3A) (%) Tricalcium Silicate (C3S) (%) a
OPC (China Cement Company, Hong Kong)
PFA (China Light & Power, Hong Kong)
320 21.1 3.1 5.9 64.6 1.0 2.6 0.6 ± ± 0.9 0.004 0.2 1.0 0.92 0.2 51.1
< 12.5a 54 4.8 29 4.8 1.0 0.6 0.4 1.4 0.9 5.7 ± ± ± ± ± ±
Retained on 40 mm sieve (% by weight).
ture of approximately 238C and a relative humidity of 80%. The mixing was carried out using a pan mixer. The OPC and PFA concretes were designed to have similar 28-day compressive strength and also similar workability. The W:C ratio of the OPC concrete was 0.48 and the W:(C+F) ratio of the PFA concrete was 0.45. Daratard 17, an initial setting retarder, was added to both mixes in order to simulate the production and delivery of ready-mix concrete Ð a very common method of supplying concrete in Hong Kong. The mix details of the OPC concrete and that of the PFA concrete are shown in Table 2. The test specimens were demoulded one day after casting and then water-cured at their designated high temperature until the age of 7 d. After 7 d, they were
transferred to curing tanks with water at 278C and left there until they reached their test age. The designated temperatures were 27, 34, 42 and 508C. The results of tests carried out on test specimens cured at 278C were used for control purposes as this is the standard curing temperature in Hong Kong [16] and in other tropical countries. Compressive strength, tensile splitting strength and ¯exural strength tests were performed in accordance with the appropriate standard speci®cations on the test specimens at dierent ages after casting.
5. Strength tests 5.1. Compressive strength
Table 2 Mix details
Cement PFA 20 mm aggregate 10 mm aggregate Fines Water Admixture (Daratard 17) Water cementitious ratioa
OPC Concrete (kg/m3)
PFA Concrete (kg/m3)
450 ± 650 515 520 215 900 ml 0.48
360 120 700 475 455 215 900 ml 0.45
a W:C and W:(C+F) ratios do not include absorption by aggregates.
100 mm concrete cubes were tested for compressive strength in accordance with BS 1881: Part 116: 1983 [17] at 2, 7, 14, 28, 56 and 91 d after casting. The cubes were subjected to water-curing at a temperature of either 27, 34, 42 or 508C for either 2 or 7 d as appropriate. In addition, all 100 mm concrete cubes tested at 14, 28, 56, and 91 d were water-cured at 278C after 7 d until tested. 5.2. Tensile splitting strength 100 mm ; 200 mm concrete cylinders were tested for tensile splitting strength in accordance with BS 1881: Part 117: 1983 [18] at 7, 28 and 56 d after casting. The cubes were subjected to water-curing at a
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temperature of either 27, 34, 42 or 508C for 7 d and thereafter at 278C until tested. 5.3. Flexural strength 100 100 500 mm concrete beams were tested for ¯exural strength in accordance with BS 1881: Part 118: 1983 [19] at 7, 28, 56 and 91 d after casting The cubes were subjected to water-curing at a temperature of either 27, 34, 42 or 508C for 7 d and thereafter at 278C until tested.
6. Results and discussion
.
.
6.1. Early compressive strength Fig. 1 shows plots of the compressive strength results obtained for the OPC and PFA concrete cubes tested at 2, 7, 14, and 28 d. According to these plots: . All PFA and OPC concrete cubes tested at 2 and 7 d and which had been cured at either 34, 42, or 508C had a higher early compressive strength than those cured for the same period of time at the standard curing temperature of 278C; . Two days after casting, the early compressive
.
.
strength of the PFA concrete cubes tested was always lower than that of the corresponding OPC ones. These results con®rm the already well established fact that the partial replacement of OPC by PFA slows down the early compressive strength development of the resulting concrete; After 7 d of curing at 428C, the early compressive strength of the PFA concrete cubes became higher than that of the corresponding OPC ones. Hence the results obtained indicate that the higher the curing temperature, the earlier PFA concrete can attain the compressive strength of OPC concrete cured at the same temperature; The rate of gain of early compressive strength of PFA concrete was always higher than that of the corresponding OPC concrete cubes. These results indicate that, in the case of high temperature curing, the partial replacement of OPC by PFA generally enhances the early compressive strength and its rate of gain of the resulting concrete; When cured at temperatures of either 42 or 508C, the OPC concrete cubes failed even to achieve the 28-day compressive strength at the standard curing temperature of 278C. According to these results, very high temperature
Fig. 1. Early compressive strength of OPC and PFA concrete cured at dierent elevated temperatures.
R.V. Balendran, W.H. Martin-Buades / Building and Environment 35 (2000) 415±423
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Fig. 2. Medium term compressive strength of OPC and PFA concrete at dierent elevated temperatures.
curing adversely aects the early compressive strength of OPC concrete.
6.2. Medium term compressive strength Fig. 2 shows plots of the compressive strength results obtained for the OPC and PFA concrete cubes tested at 56 and 91 d. According to these plots, immaterial of curing temperature, the medium term compressive strength of the PFA concrete cubes tested at 56 and 91 d tested was always higher than that of the corresponding OPC concrete cubes Ð on average, approximately 15% higher. These results are similar to those obtained by Alshami [13]. A possible explanation for the increased medium term compressive strength of PFA concrete could be the reduction of C3A and the deceleration of rate of hydration permitting the formation of more dense hydrates of strength-contributing dicalcium silicate (C2S) to form [20]. 6.3. Early tensile splitting strength Fig. 3 shows plots of the tensile splitting strength results obtained for the concrete cylinders tested at 7 and 28 d. According to these plots: . Irrespective of curing temperature, all OPC concrete cylinders tested at 7 d had a higher early tensile splitting strength than the corresponding PFA concrete ones; . At 28 d, the early tensile splitting strength of the OPC concrete cylinders tested was either higher or equal to that of the corresponding PFA concrete
cylinders when the curing temperature was either 27, 42 or 508C. However, at 348C curing temperature, the early tensile splitting strength of the OPC concrete cylinders tested was lower than that of the corresponding PFA concrete ones; . At 7 d, the early tensile splitting strength of the OPC concrete cylinders tested increased noticeably with curing temperature. However, at 28 d, the early tensile splitting strength of the OPC concrete cylinders tested increased overall only slightly with temperature and, at 358C curing temperature, its value was actually lower than that for the standard curing temperature of 278C; . The early tensile splitting strength of the PFA concrete cylinders tested at 7 and 28 d increased to a peak value and then declined with increasing temperature. For those tested at 7 d, the peak value occurred when the curing temperature was 428C and the decline in value thereafter was rather slight. For those tested at 28 d, the peak value occurred when the curing temperature was 358C and the decline in value thereafter was noticeable. However, for both test ages, the PFA concrete cylinders cured at elevated temperatures had always higher early tensile splitting strengths than those cured at the standard curing temperature of 278C.
6.4. Medium term tensile splitting strength Fig. 4 shows plots of the tensile splitting strength results obtained for the concrete cylinders tested at 56 d. According to these plots:
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Fig. 3. Early tensile strength of OPC and PFA concrete cured at various elevated temperatures.
. The medium term tensile splitting strength of the OPC concrete cylinders tested did not increase as the curing temperature rose. In actual fact, the medium term tensile splitting strength of the OPC con-
crete cylinders tested which had been cured at either 34, 42 or 508C was always lower than the value for ones cured at the standard temperature of 278C; . At 348C curing temperature, the PFA concrete cylin-
Fig. 4. 56-day tensile strength of OPC and PFA concrete cured at dierent elevated temperatures.
R.V. Balendran, W.H. Martin-Buades / Building and Environment 35 (2000) 415±423
421
Fig. 5. Early ¯exural strength of OPC and PFA concrete cured at various elevated temperatures.
ders tested experienced a signi®cant increase in their medium term tensile splitting strength. However, at either 42 and 508C curing temperature, the medium term tensile splitting strength of the PFA concrete cylinders tested was lower than that of the ones tested at the standard curing temperature of 278C. From the results obtained it is not possible to see any obvious trend regarding the medium term tensile splitting strength of PFA concrete cured at elevated temperatures. . If the results shown in Figs. 3 and 4 are compared, it can be concluded that the tensile splitting strength
at 56 d is not signi®cantly higher than that at 28 d for either the OPC or PFA concrete. 6.5. Early ¯exural strength Fig. 5 shows plots of the ¯exural strength results obtained for the concrete beams tested at 7 and 28 d. According to these plots: . The ¯exural strength of the PFA concrete beams tested at 7 d increased uniformly with curing temperature and, at both 42 and 568C curing temperature, was higher than that of the corresponding
Fig. 6. Medium term ¯exural strength of OPC and PFA concrete cured at dierent elevated temperatures.
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R.V. Balendran, W.H. Martin-Buades / Building and Environment 35 (2000) 415±423
OPC ones. On the other hand, the ¯exural strength of the OPC concrete beams tested at the same age remained substantially the same irrespective of curing temperature except for those cured at 348C. The beams cured at 348C experienced an increase of approximately 10% in ¯exural strength; . The ¯exural strength of the PFA concrete beams tested at 28 d increased with curing temperature and, at the elevated curing temperatures, was always greater than that of the corresponding OPC ones. In actual fact, at the elevated curing temperatures, the ¯exural strength of the latter at 28 d was always lower than that at the standard curing temperature of 278C.
6.6. Medium term ¯exural strengths Fig. 6 shows plots of the ¯exural strength results obtained for the concrete beams tested at 56 and 91 d. According to these plots: . The medium term ¯exural strength of the OPC concrete beams tested remained substantially the same immaterial of curing temperature and test age. This ®nding indicates that curing temperature has little eect on the medium term ¯exural strength of an OPC concrete; . At the standard curing temperature of 278C, the medium term ¯exural strength of the PFA concrete beams tested at 91 d was markedly higher than that of the ones tested at 56 d. At 348C curing temperature, however, the position was reversed, with the 91-day medium term ¯exural strength not only lower than the 56-day one but also lower than its own value at the standard curing temperature of 278C. Beyond 348C curing temperature, the medium term ¯exural strength of the PFA concrete beams tested became similar immaterial of test age. It is, however, not possible to detect from the results obtained, any obvious overall trend as to how the ¯exural strength of a PFA concrete is aected by high curing temperatures; . At all the curing temperatures and test ages, the ¯exural strength of the PFA concrete beams tested was always greater than that of the corresponding OPC concrete ones. This ®nding indicates that a PFA concrete cured at high temperatures generally possess a greater ¯exural strength than a comparable OPC concrete cured under similar conditions.
7. Concluding remarks Curing OPC concrete at elevated temperatures can
help to improve the early strength development of the concrete. The partial replacement of the OPC by PFA further improves its early compressive and tensile splitting strength development and also has a bene®cial eect on its ¯exural strength. Generally, the higher the curing temperature, the better the medium term compressive strength, tensile splitting strength and ¯exural strength of PFA concrete. OPC concrete did not behave in a similar manner when cured at high temperatures. Its reduced performance under high temperature curing may be due to thermal stresses generated by its high heat of hydration. The PFA concrete, due to its reduced heat of hydration, has a lower peak temperature and rate of temperature rise and therefore avoids such thermal stresses. The improved performance of the PFA concrete at the later ages under high temperature curing may be due to delayed pozzolanic reaction.
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