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Effect of steam curing cycles on strength and durability of SCC: A case study in precast concrete
Construction and Building Materials 49 (2013) 807–813
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Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
Effect of steam curing cycles on strength and durability of SCC: A case study in precast concrete A.A. Ramezanianpour, M.H. Khazali ⇑, P. Vosoughi Department of Civil and Environment Engineering, Concrete Technology and Durability Research Center, Amirkabir University of Technology, Tehran, Iran
h i g h l i g h t s Effects of 36 steam curing regimes on the compressive strength of SCC were studied. Permeability of steam-cured concrete was investigated. An optimum steam curing cycle was introduced.
a r t i c l e
i n f o
Article history: Received 25 July 2013 Received in revised form 20 August 2013 Accepted 27 August 2013
Keywords: Steam curing Self-Compacting Concrete (SCC) Compressive strength Surface resistivity Capillary absorption Energy consumption
a b s t r a c t Use of Self-Compacting Concrete (SCC) in pre-cast concrete plants is growing rapidly due to its benefits such as reduction in labor and equipment costs, increasing productivity, providing flexibility in filling highly reinforced sections and complex formworks, lowering the noise on job site and having superior surface quality. Also, considering the critical importance of ‘‘production time’’ in precast plants, accelerated curing is considered as an inevitable part of precast concrete elements production. In this study the effects of thirty-six different steam-curing regimes on the compressive strength and permeability of a self-compacting concrete mixture, used in precast concrete elements of Sadr elevated highway was investigated. Compressive strength measurements indicated that in a constant total time, increase in precuring period leads to lower immediate compressive strength. On the other hand, increase in treatment temperature and total cycle time (which means higher energy and time consumption) led to higher immediate compressive strength. Furthermore, durability tests results demonstrated that application of cycles with maximum temperature of 70 °C imposes negative effect on durability properties of reference SCC, such as surface resistivity and capillary absorption. Finally, on the basis of three criteria (compressive strength, permeability and energy consumption by steam curing cycle), an optimum steam curing cycle was introduced and utilized in the precast concrete plant. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction Self-Consolidating Concrete (SCC) has some advantages over conventional concrete making it suitable for use in pre-cast/prestressed concrete plants. These advantages include high workability (making it possible to omit vibration), reduction of labor costs, making feasible to develop more automated plants, and often possessing higher strength and durability properties. Three essential characteristics of fresh SCC are filling ability, passing ability, and segregation resistance which make it an ideal choice for use in elements with dense reinforcement or complex geometry [1–4]. There are some reasons such as limitation of formworks, facilities, storage area, and time which encourage precast concrete plants to obtain high early strength, to speed up the stripping of ⇑ Corresponding author. Tel./fax: +98 21 64543074. E-mail address: [email protected] (M.H. Khazali). 0950-0618/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.conbuildmat.2013.08.040
forms, and to shorten the curing period [5]. Special techniques are available to this aim including: (I) Using special cements with high early strength such as fine or high alumina ones (II) Utilizing suitable chemical additives (III) Use of accelerated curing (including increased temperature and humidity). According to economy, availability, and long-term performance of special materials, the most common method is accelerated curing by means of increased temperature and humidity. Various methods have been used including steam curing at atmospheric pressure (temperature less than 100 °C), steam curing at high pressure (autoclaving), electrical heating of reinforcement, imposing electrical current to concrete directly, and microwave heating. Among these, steam curing at low pressure is most common, especially for large precast units. A typical steam-curing cycle consists of a precuring (delay) period after surface finishing, a heating and cooling rate of 11–44 °C/h, and a treatment period with constant temperature for 6–18 h. Maximum treatment temperature in
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steam curing is usually limited to 60–90 °C [6–10]. The minimum early compressive strength of concrete, the most important factor for demolding of concrete elements, is suggested about 25 MPa for the most cases. Also, the ultimate strength of concrete in plants is commonly considered to be more than 50 MPa [9,10]. Some of common formworks of segments utilized in precast concrete planet of Sadr elevated highway are illustrated in Fig. 1. Generally, the maximum total duration of the steam curing cycle in a plant is limited to 18 h, since the production is carried out 24-h and enough time should be available to prepare the formwork and the arrangement of the reinforcements to continue the daily production procedure. Besides, it is already shown that continuing a definite steam curing regime for longer hours can lead to detrimental changes in porosity and pore size distribution of concrete [11]. Nowadays, durability of concrete structures is widely believed to be major concern and much research work is carried out on this issue. Mehta and Gerwick [12] investigated the San Mateo bridge over San Francisco bay after being exposed for 17 years in the environment; the bridge is comprised of both steam-cured and moistcured concrete beams with the same mixture proportions and materials. The study demonstrated that steam-cured beams had to be repaired according to corrosion damage, while moist-cured beams showed no signs of deterioration. Other studies [13,14] illustrated that accelerated curing by excessively increased temperature leads to porous concrete with coarse and continuous pore structure, and heterogeneous distribution of hydration products. Consequently, it increases the permeability of concrete against aggressive ions such as chlorides or sulfates, and decreases the strength of concrete. Moreover, it can cause initial decomposing of ettringite in fresh concrete, which can recreate Delayed Ettringite Formation (DEF) in hardened concrete and produce destructive expansion [15–18]. To the authors’ knowledge, limited research work is carried out on the effects of steam curing on the durability of SCC. Bingöl and Tohumcu [19] compared the compressive strength of SCC mixtures cured in the standard situation, by air, and by exposure to steam; the results demonstrated that concrete cured in air had the lowest strength, and the optimum temperature of steam curing is 70 °C. Moreover, Reinhardt and Stegmaier [20] studied the pore size distribution of steam cured SCC; it made clear that higher maximum temperature leads to coarser pores, and the changes are correlated to (w/c)eq. The object of this study is to investigate the effects of steam curing on the properties of limestone incorporated SCC, which is used in a precast plant in Tehran, Iran. For this purpose, compressive strength and permeability (in terms of surface resistivity and capillary absorption) of the reference SCC mixture were investigated after exposure to 36 different steam curing cycles. Three main parameters were selected as variables:
(1) Maximum temperature of treatment (50, 60, and 70 °C). (2) Total time of steam curing, which is desired to be at the minimum value, while achieving the required compressive strength (8, 10, 12, and 14 h). (3) Delay period before commencing the steam curing (1, 2, and 3 h). Also, it is to be mentioned that two main constraints were imposed by the design and project management team, and were considered in this experimental study: (1) A minimum of 24 MPa was required for demolding of precast concrete elements. (2) A maximum steam curing duration of 14 h was allowed. (3) Finally, on the basis of three criteria (compressive strength, permeability and energy consumption by steam curing cycle), an optimum steam curing cycle was introduced and utilized in the precast concrete plant. 2. Experimental program 2.1. Materials The cement used was ASTM C150/C150M-11 [21] Type II Portland cement. Limestone powder was also used as filler. The chemical composition and physical characteristics of Portland cement and limestone filler are listed in Table 1. Natural sand and crushed gravel were used as aggregates. The coarse aggregates had nominal maximum size of 19 mm, specific gravity of 2.6 gr/cm3, and water absorption of 1.5%. The fine aggregates had specific gravity of 2.3 gr/cm3, absorption of 2.7%, and fineness modulus of 3.6. The high fineness modulus indicates the low content of fines in the fine aggregates. Therefore, it was decided to add limestone filler (150 kg/m3) with a maximum particle size of 0.15 mm to compensate for the lack of fine particles in the local river sand and also to improve the mixture rheological characteristics [22,23]. The specific gravity of the limestone filler was 2.48 gr/cm3. The sieve analyses of fine, coarse and final mixture of aggregates used in the concrete are listed in Table 2. Moreover, a polycarboxylate-ether type High-Range Water Reducer Admixture (HRWRA) with a specific gravity of 1.1 gr/cm3 and solids content of 47% was consumed to achieve the required workability for the mixtures. 2.2. Mixture proportions The mix proportions were selected exactly the same as the reference SCC mixture which was used in the precast concrete plant. These proportions were already selected based on a comprehensive study of local materials and evaluation of a various number of mixtures regarding fresh and hardened properties. The mix proportions and properties of the reference SCC mixture are listed in Tables 3 and 4, respectively. In order to study the effects of different steam curing cycles, 36 batches were prepared according to the reference SCC mix proportions. In order to control the variations and provide the maximum similarity between mixes, ‘‘slump flow diameter’’ and ‘‘28-day compressive strength of water cured specimens’’ were measured each time. In order to consider a mixture as ‘‘acceptable’’, 28-day compressive strength of control specimens had to be in the range of ±5% of reference SCC mixture (59 MPa). Furthermore, slump flow diameter had to be in the range of 680– 700 mm while HRWRA dosage was maintained between 0.75 to 0.85 percentages of cement mass. If either condition was not satisfied for a mixture, it was considered as ‘‘rejected’’ and another batch was prepared and tested.
Fig. 1. Precast concrete element made with self-compacting concrete for Sadr elevated highway, Tehran, Iran.
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A.A. Ramezanianpour et al. / Construction and Building Materials 49 (2013) 807–813 Table 1 Materials properties.
Table 5 Steam curing regimes.
Chemical composition (%)
Cement
Limestone filler
CaO SiO2 Al2O3 Fe2O3 MgO SO3 K2O Na2O (Na2O)eq Loss on ignition (%)
62.08 21.10 4.18 3.34 3.79 2.84 0.69 0.14 0.59 3.12
50.17 3.12 1.19 0.53 3.46 0.20 0.25 –
Physical properties Specific gravity (gr/cm3) Blaine (cm2/g)
3.17 3519
2.48 –
40.31
Table 2 Sieve analysis of aggregates (cumulative percentage passing). Sieve size (mm)
River sand
Gravel
Aggregate mixture
19 12.5 9.5 4.75 2.38 1.19 0.6 0.3 0.15
100.0 100.0 100.0 92.8 63.6 40.6 26.3 13.3 4.0
100 62.6 38.3 1.1 0.4 0 0 0 0
100 89.3 82.4 67.1 47.9 32.8 22.9 13.6 6.5
Table 3 Mix proportions of reference SCC mixture.
a
Constituent
Quantity
Cement (kg/m3) Filler (kg/m3) Sand (kg/m3) Gravel (kg/m3) Water (kg/m3) w/c HRWRAa (%)
400 150 921 714 156 0.39 0.8
Superplasticizer percentage is presented by Portland cement mass.
The SCC mixtures were produced in a horizontal pan mixer with 100 l capacity. A specific mixing sequence was applied for all mixtures; which consisted of dry mixing the coarse and fine aggregates, limestone filler and Portland cement for 1 min. Then the whole water was added to the dry mixture and mixed for 2 min. Finally, the HRWRA was introduced to the mixture and mixed for another 2 min. After testing the fresh mixture for being self-compactible, cylinder and cube specimens were cast without using any vibration.
2.3. Steam curing regimes In order to study the effects of different steam curing cycles on the hardened properties of reference SCC mixture, three parameters were selected as variables:
No.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36
Curing regime ID
Precuring period (h)
Peak temp. (°C)
Total time of steam curing (h)
Energy index (min °C)
CTL T50-1-8 T50-1-10 T50-1-12 T50-1-14 T50-2-8 T50-2-10 T50-2-12 T50-2-14 T50-3-8 T50-3-10 T50-3-12 T50-3-14 T60-1-8 T60-1 -10 T60-1-12 T60-1-14 T60-2-8 T60-2-10 T60-2-12 T60-2-14 T60-3-8 T60-3-10 T60-3-12 T60-3-14 T70-1-8 T70-1-10 T70-1-12 T70-1-14 T70-2-8 T70-2-10 T70-2-12 T70-2-14 T70-3-8 T70-3-10 T70-3-12 T70-3-14
– 1
– 50 50 50 50 50 50 50 50 50 50 50 50 60 60 60 60 60 60 60 60 60 60 60 60 70 70 70 70 70 70 70 70 70 70 70 70
– 8 10 12 14 8 10 12 14 8 10 12 14 8 10 12 14 8 10 12 14 8 10 12 14 8 10 12 14 8 10 12 14 8 10 12 14
– 180 240 300 360 180 240 300 360 180 240 300 360 240 320 400 480 240 320 400 480 240 320 400 480 300 400 500 600 300 400 500 600 300 400 500 600
2
3
1
2
3
1
2
3
temperature was kept constant at 20 °C, while any temperature variation (heating and cooling) occurred during constant period of 2 h. It is common knowledge that decreasing the steam curing duration is highly desired in the large scale production of precast elements, since it leads to lower production cost and also higher productivity. In this study, a maximum allowable steam curing duration of 14 h was imposed by precast plant project management team. In addition, ‘‘Energy Index’’ was defined as the multiply of time by extra temperature (above ambient temperature which was considered 20 °C) during each steam curing regime. This was used as an indicator of the energy consumption by each cycle. High-accuracy automatic climate simulators were used to impose various steam curing cycles on SCC specimens (see Fig. 2). The devices are capable of controlling both ‘‘humidity’’ and ‘‘temperature’’ in the range of 20 °C to +120 °C with high accuracy. Also, the results ofactual humidity and temperature values during the steam curing cycle are reported in MS-Excel format.
2.4. Tests procedures (1) Precuring (delay) period. (2) Peak temperature. (3) Total time of steam curing. Considering the precast factory limitations, values were assigned to the parameters and 36 different steam curing regimes were designed (see Table 5). Precuring
100 100 100 mm cubic specimens were tested for compressive strength immediately after steam curing cycle and at the ages of 7 and 28 days. Furthermore, maturity index (ASTM C1074 [24]) was utilized to study the effect of temperature– time history on compressive strength of steam cured specimens. This value was calculated using:
Table 4 Properties of reference SCC mixture. Fresh properties Wet density (kg/m3) Slump flow-Avg. of two diameters (mm) T50 (s) V-funnel (s) V-funnel at 15min (s)
Hardened properties (water cured specimens) 2370 690 2.5 9 14
Slump flow at 45 min (mm) U-box (mm) J-ring (h2–h1) (mm) L-box (%)
550 10 5 0.85
1-Day compressive strength (MPa) 3-Day compressive strength(MPa) 7-Day compressive strength(MPa) 28-Day compressive strength(MPa)
17 35 48 59
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Fig. 2. Automatic climate simulator devices used for steam curing of specimens.
3. Results and discussion
Table 6 Permeability classes based on surface resistivity by FM 5-578[27]. Chloride ion permeability
Surface resistivity (kX cm)
High Moderate Low Very low Negligible
<12 12–21 21–37 37–254 >254
MðtÞ ¼
t X ðT a T 0 ÞDt
ð1Þ
0
where M(t) is the temperature–time factor at age (t), degree-days or degree-hour; Dt is a time interval, days or hours; Ta is average concrete temperature during time interval, °C; T0 is datum temperature, °C. It is commonly assumed equal to 10 °C [24]. Capillary absorption was carried out at 28 days age. The capillary absorption rate of specimens were calculated using their weight after 48 h being in 110 °C as initial weight, and the measured weight after 72 h of being partly in contact with the water (on side exposed to water at 5 mm depth) [25]. Electrical resistivity is one of the intrinsic specifications of concrete which can be related to its permeability. In 1915, Wenner [26] presented a practical method to measure the earth electrical surface resistivity by means of a four probes apparatus which for the first time, has been standardized to use on concrete in 2004 [27]. Since it is a non-destructive, rapid, low-cost, and reliable method, it is a wide-accepted technique to investigate durability properties of concrete. Electrical resistivity refers to the resistance that any electrical charge experiences while passing through the concrete. The increased electrical resistivity of concrete impedes the movement of electrons from the anodic to the cathode regions, and consequently delays the propagation of the corrosion process[28]. As presented in Table 6, FM 5-578 test method [27] defines chloride ion permeability ratings according to the surface resistivity test results. The electrical resistivity meter (Fig. 3) was used to measure the surface resistivity of specimens at the ages of 1, 7, and 28 days age. Three saturated 100 200 mm cylinders were tested at each age. The test was carried out by the four-point Wenner array probe technique [29].
Fig. 3. Electrical resistivity meter.
3.1. Compressive strength and maturity index The compressive strength test was conducted on 100 mm cube specimens immediately after steam curing and at the ages of 7 and 28 days. The results are presented in Figs. 4–6. As observed in Fig. 4, increase in precuring period has led to lower immediate compressive strength values. For instance, considering the total cycle time of 8 h, 2 h increase in delay time has brought about 5 MPa decrease in immediate strength of reference SCC mixture. Furthermore, as expected, increase in temperature and total cycle time (which means higher energy and time consumption) led to higher immediate compressive strength. This is due to the accelerated hydration reactions and rapid formation of Calcium–Silica–Hydrate, C–S–H gel, the most important bind phase in hardened concrete [30] in the presence of moisture and high temperature. Also, the average ratio of initial compressive strength to 28-day compressive strength is calculated as 39%, 46% and 53% at the maximum temperatures of 50 °C, 60 °C and 70 °C, respectively. By taking 28-day results into account, it proves that increasing the maximum cycle temperature has negative effect on compressive strength of concrete at later ages, while it improves the immediate strength after curing. Figs. 5 and 6 illustrate the strength measurements for the specimens exposed to cycles with maximum temperatures of 60 °C and 70 °C, respectively. The same immediate strength pattern as for 50 °C cycles is observed for these cycles, too. Considering the strength development until the 7 days age, it is observed that increase in total cycle time reduces the strength development during
Fig. 4. Relative compressive strength values ofthe cycles with 50 °Cpeak temperature (Mean values are presented in MPa).
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Fig. 5. Relative compressive strength values ofthe cycles with 60 °Cpeak temperature (mean values are presented in MPa).
Fig. 7. Results of compressive strength versus maturity index after accomplishment of steam curing.
Table 7 Relationship between maturity index and compressive strength.
Total data Max. temp. 50 Max. temp. 60 Max. temp. 70
A
B
R2
174 144 155 172
866 699 740 823
0.77 0.88 0.88 0.91
Fig. 6. Relative compressive strength values ofthe cycles with 70 °C peak temperature (Mean values are presented in MPa).
this period. On the other hand, strength development from 7 to 28 days of age is approximately the same for different cycles. Among all 36 steam curing cycles, ‘‘T70-2-14’’ led to the highest immediate compressive strength (36 MPa), while maximum 28day compressive strength was measured for SCC specimens which were exposed to ‘‘T60–3-10’’ cycle (approximately 64 MPa). Considering the obtained results and the fact that a minimum of 24 MPa was required for demolding of precast concrete elements, it was decided not to carry out the durability tests on specimens which were exposed to cycles with total duration of 8 h. In fact, the strength measurements demonstrated that none of these cycles developed the requiredminimum strength after exposure to steam curing. Maturity index was proposed earlier as a technique to account for combined effect of time and temperature on the compressive strength of concrete [31,32]. Although some modified methods are presented to increase accuracy and reliability of this method for various types of concrete [33,34], using the basic equation explained in ASTM C1074 [24]seems to be more reliable. Based on the data presented in Fig. 7, it can be observed that logarithmic relations with high R2 values rule the data. However, constant parameters (A and B) based on Eq. (2) presented in Table 7 show that using a specified equation for each maximum temperature of curing is more appropriate with higher correlation coefficient (R2).
Compressive Strength ¼ A lnðMaturity IndexÞ B
ð2Þ
Fig. 8. Influence of temperature = 50 °C).
steam
curing
regimes
on
surface
resistivity
(peak
3.2. Surface resistivity The effect of different steam curing cycles on surface resistivity is illustrated in Figs. 8–10. Considering the values measured shortly after steam curing (1-day results) at 50 °C, it seems that total cycle period has minor effect on the resistivity value, since a maximum 7% variation is measured between cycles of the same delay time. At higher temperatures, immediate resistivity is more sensitive to total cycle time variations. For example, as observed in Fig. 10, at the constant delay time of 3 h and peak temperature of 70 °C, a 4-h increase in total cycle period has improved the resistivity value by 33%. It is also observed that the highest immediate resistivity value is achieved by exposure to T70-2-14 cycle, which has the highest energy index (600 min °C) among 36 designed steam curing cycles. Considering the fact that all the specimens are exposed to steam curing cycles at early ages, the 28-day resistivity measurements could be considered as the appropriate criteria for durability assessment of reference SCC. Accordingly, it can be inferred that ‘‘T60-3-10’’ is the optimum cycle regarding electrical resistivity
812
Fig. 9. Influence of temperature = 60 °C).
A.A. Ramezanianpour et al. / Construction and Building Materials 49 (2013) 807–813
steam
curing
regimes
on
surface
resistivity
(peak Fig. 12. Capillary absorption percentages of steam curing cycles with peak temperature of 60 °C.
Fig. 10. Influence of steam curing regimes on surface resistivity (peak temperature = 70 °C).
of reference SCC. The test result for SCC specimens which were exposed to this curing cycle is measured 15.5 kX cm, which is rated as ‘‘moderate’’ based on the chloride ion permeability classes defined by FM 5-578 [27]. Also, an average28-day electrical resistivity of 12.2 kX cm is calculated for 50 °C cycles (Fig. 8), while the average values of 11.9 kX cm and 11.6 kX cm are measured for 60 °C and 70 °C cycles, respectively. Therefore, it is concluded that increasing the maximum steam curing temperature does not have any positive effect on surface resistivity of reference SCC; even a minor decrease should be expected.
Fig. 13. Capillary absorption percentages of steam curing cycles with peak temperature of 70 °C.
3.3. Capillary absorption Capillary absorption test was carried out at the age of 28 days. The test results are presented in Figs. 11–13. It is observed that at a constant peak temperature and curing period, highest absorption values were measured for specimens exposed to cycles with 1hour delay time. Furthermore, at the three peak temperatures the highest absorption values were recorded for 14-h long cycles, which prove the negative effect of exposure to these cycles on capillary absorption of reference SCC. Also, an average absorption percentage of 6.4 is calculated for 50 °C cycles, while the average value of 5.5% is measured for both 60 °C and 70 °C steam curing cycles. It proves that increase in maximum cycle temperature from 60 °C to 70 °C does not improve the impermeability of reference SCC regarding capillary pores. This finding is of high importance regarding the high energy cost, which plays a crucial role in decision making in large scale precast plants. Accordingly, T60-3-10 could be introduced as an optimum steam curing regime, regarding both capillary absorption (4.3%) and energy consumption.
4. Conclusions
Fig. 11. Capillary absorption percentages of steam curing cycles with peak temperature of 50 °C.
In this study the effects of steam curing on compressive strength and permeability of filler-type SCC were investigated. Also, energy consumption was considered as a selection basis among various steam curing cycles. The major conclusions are as follows:
A.A. Ramezanianpour et al. / Construction and Building Materials 49 (2013) 807–813
1. Compressive strength measurements indicated that increase in precuring period leads to lower immediate compressive strength values. For instance, considering the total curing cycle of 8 h, 2 h increase in delay time has brought about 5 MPa decrease in immediate compressive strength of reference SCC mixture. 2. Increase in temperature and total cycle time (which means higher energy and time consumption) led to higher immediate compressive strength. This could be due to the accelerated hydration reactions and rapid formation of Calcium–Silica– Hydrate(C–S–H gel). 3. An average electrical resistivity of 12.2 kX cm is calculated for 50 °C cycles, while the average values of 11.9 kX cm and 11.6 kX cm are measured for 60 °C and 70 °C cycles, respectively. It proves that increasing the maximum steam curing temperature does not have any positive effect on surface resistivity of reference SCC; even a minor decrease should be expected. 4. Capillary absorption test results indicated that at the three peak temperatures the highest absorption values were recorded for 14-h long cycles, which prove the negative effect of exposure to these cycles on capillary absorption of reference SCC. 5. Durability tests results demonstrated that application of cycles with maximum temperature of 70 °C could have negative effect on durability of reference SCC. This could be due to rapid formation of C–S–H gel and CH crystalline which results in a heterogeneous and coarser pores structure. 6. Considering the three factors of compressive strength, permeability (in terms of surface resistivity and capillary absorption) and energy consumption by steam curing regime, ‘‘T60-3-10’’ was introduced as an optimum steam curing cycle and was applied to precast concrete elements for Sadr elevated highway.
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Effect of steam curing cycles on strength and durability of SCC: A case study in precast concrete A.A. Ramezanianpour, M.H. Khazali ⇑, P. Vosoughi Department of Civil and Environment Engineering, Concrete Technology and Durability Research Center, Amirkabir University of Technology, Tehran, Iran
h i g h l i g h t s Effects of 36 steam curing regimes on the compressive strength of SCC were studied. Permeability of steam-cured concrete was investigated. An optimum steam curing cycle was introduced.
a r t i c l e
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Article history: Received 25 July 2013 Received in revised form 20 August 2013 Accepted 27 August 2013
Keywords: Steam curing Self-Compacting Concrete (SCC) Compressive strength Surface resistivity Capillary absorption Energy consumption
a b s t r a c t Use of Self-Compacting Concrete (SCC) in pre-cast concrete plants is growing rapidly due to its benefits such as reduction in labor and equipment costs, increasing productivity, providing flexibility in filling highly reinforced sections and complex formworks, lowering the noise on job site and having superior surface quality. Also, considering the critical importance of ‘‘production time’’ in precast plants, accelerated curing is considered as an inevitable part of precast concrete elements production. In this study the effects of thirty-six different steam-curing regimes on the compressive strength and permeability of a self-compacting concrete mixture, used in precast concrete elements of Sadr elevated highway was investigated. Compressive strength measurements indicated that in a constant total time, increase in precuring period leads to lower immediate compressive strength. On the other hand, increase in treatment temperature and total cycle time (which means higher energy and time consumption) led to higher immediate compressive strength. Furthermore, durability tests results demonstrated that application of cycles with maximum temperature of 70 °C imposes negative effect on durability properties of reference SCC, such as surface resistivity and capillary absorption. Finally, on the basis of three criteria (compressive strength, permeability and energy consumption by steam curing cycle), an optimum steam curing cycle was introduced and utilized in the precast concrete plant. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction Self-Consolidating Concrete (SCC) has some advantages over conventional concrete making it suitable for use in pre-cast/prestressed concrete plants. These advantages include high workability (making it possible to omit vibration), reduction of labor costs, making feasible to develop more automated plants, and often possessing higher strength and durability properties. Three essential characteristics of fresh SCC are filling ability, passing ability, and segregation resistance which make it an ideal choice for use in elements with dense reinforcement or complex geometry [1–4]. There are some reasons such as limitation of formworks, facilities, storage area, and time which encourage precast concrete plants to obtain high early strength, to speed up the stripping of ⇑ Corresponding author. Tel./fax: +98 21 64543074. E-mail address: [email protected] (M.H. Khazali). 0950-0618/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.conbuildmat.2013.08.040
forms, and to shorten the curing period [5]. Special techniques are available to this aim including: (I) Using special cements with high early strength such as fine or high alumina ones (II) Utilizing suitable chemical additives (III) Use of accelerated curing (including increased temperature and humidity). According to economy, availability, and long-term performance of special materials, the most common method is accelerated curing by means of increased temperature and humidity. Various methods have been used including steam curing at atmospheric pressure (temperature less than 100 °C), steam curing at high pressure (autoclaving), electrical heating of reinforcement, imposing electrical current to concrete directly, and microwave heating. Among these, steam curing at low pressure is most common, especially for large precast units. A typical steam-curing cycle consists of a precuring (delay) period after surface finishing, a heating and cooling rate of 11–44 °C/h, and a treatment period with constant temperature for 6–18 h. Maximum treatment temperature in
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steam curing is usually limited to 60–90 °C [6–10]. The minimum early compressive strength of concrete, the most important factor for demolding of concrete elements, is suggested about 25 MPa for the most cases. Also, the ultimate strength of concrete in plants is commonly considered to be more than 50 MPa [9,10]. Some of common formworks of segments utilized in precast concrete planet of Sadr elevated highway are illustrated in Fig. 1. Generally, the maximum total duration of the steam curing cycle in a plant is limited to 18 h, since the production is carried out 24-h and enough time should be available to prepare the formwork and the arrangement of the reinforcements to continue the daily production procedure. Besides, it is already shown that continuing a definite steam curing regime for longer hours can lead to detrimental changes in porosity and pore size distribution of concrete [11]. Nowadays, durability of concrete structures is widely believed to be major concern and much research work is carried out on this issue. Mehta and Gerwick [12] investigated the San Mateo bridge over San Francisco bay after being exposed for 17 years in the environment; the bridge is comprised of both steam-cured and moistcured concrete beams with the same mixture proportions and materials. The study demonstrated that steam-cured beams had to be repaired according to corrosion damage, while moist-cured beams showed no signs of deterioration. Other studies [13,14] illustrated that accelerated curing by excessively increased temperature leads to porous concrete with coarse and continuous pore structure, and heterogeneous distribution of hydration products. Consequently, it increases the permeability of concrete against aggressive ions such as chlorides or sulfates, and decreases the strength of concrete. Moreover, it can cause initial decomposing of ettringite in fresh concrete, which can recreate Delayed Ettringite Formation (DEF) in hardened concrete and produce destructive expansion [15–18]. To the authors’ knowledge, limited research work is carried out on the effects of steam curing on the durability of SCC. Bingöl and Tohumcu [19] compared the compressive strength of SCC mixtures cured in the standard situation, by air, and by exposure to steam; the results demonstrated that concrete cured in air had the lowest strength, and the optimum temperature of steam curing is 70 °C. Moreover, Reinhardt and Stegmaier [20] studied the pore size distribution of steam cured SCC; it made clear that higher maximum temperature leads to coarser pores, and the changes are correlated to (w/c)eq. The object of this study is to investigate the effects of steam curing on the properties of limestone incorporated SCC, which is used in a precast plant in Tehran, Iran. For this purpose, compressive strength and permeability (in terms of surface resistivity and capillary absorption) of the reference SCC mixture were investigated after exposure to 36 different steam curing cycles. Three main parameters were selected as variables:
(1) Maximum temperature of treatment (50, 60, and 70 °C). (2) Total time of steam curing, which is desired to be at the minimum value, while achieving the required compressive strength (8, 10, 12, and 14 h). (3) Delay period before commencing the steam curing (1, 2, and 3 h). Also, it is to be mentioned that two main constraints were imposed by the design and project management team, and were considered in this experimental study: (1) A minimum of 24 MPa was required for demolding of precast concrete elements. (2) A maximum steam curing duration of 14 h was allowed. (3) Finally, on the basis of three criteria (compressive strength, permeability and energy consumption by steam curing cycle), an optimum steam curing cycle was introduced and utilized in the precast concrete plant. 2. Experimental program 2.1. Materials The cement used was ASTM C150/C150M-11 [21] Type II Portland cement. Limestone powder was also used as filler. The chemical composition and physical characteristics of Portland cement and limestone filler are listed in Table 1. Natural sand and crushed gravel were used as aggregates. The coarse aggregates had nominal maximum size of 19 mm, specific gravity of 2.6 gr/cm3, and water absorption of 1.5%. The fine aggregates had specific gravity of 2.3 gr/cm3, absorption of 2.7%, and fineness modulus of 3.6. The high fineness modulus indicates the low content of fines in the fine aggregates. Therefore, it was decided to add limestone filler (150 kg/m3) with a maximum particle size of 0.15 mm to compensate for the lack of fine particles in the local river sand and also to improve the mixture rheological characteristics [22,23]. The specific gravity of the limestone filler was 2.48 gr/cm3. The sieve analyses of fine, coarse and final mixture of aggregates used in the concrete are listed in Table 2. Moreover, a polycarboxylate-ether type High-Range Water Reducer Admixture (HRWRA) with a specific gravity of 1.1 gr/cm3 and solids content of 47% was consumed to achieve the required workability for the mixtures. 2.2. Mixture proportions The mix proportions were selected exactly the same as the reference SCC mixture which was used in the precast concrete plant. These proportions were already selected based on a comprehensive study of local materials and evaluation of a various number of mixtures regarding fresh and hardened properties. The mix proportions and properties of the reference SCC mixture are listed in Tables 3 and 4, respectively. In order to study the effects of different steam curing cycles, 36 batches were prepared according to the reference SCC mix proportions. In order to control the variations and provide the maximum similarity between mixes, ‘‘slump flow diameter’’ and ‘‘28-day compressive strength of water cured specimens’’ were measured each time. In order to consider a mixture as ‘‘acceptable’’, 28-day compressive strength of control specimens had to be in the range of ±5% of reference SCC mixture (59 MPa). Furthermore, slump flow diameter had to be in the range of 680– 700 mm while HRWRA dosage was maintained between 0.75 to 0.85 percentages of cement mass. If either condition was not satisfied for a mixture, it was considered as ‘‘rejected’’ and another batch was prepared and tested.
Fig. 1. Precast concrete element made with self-compacting concrete for Sadr elevated highway, Tehran, Iran.
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Table 5 Steam curing regimes.
Chemical composition (%)
Cement
Limestone filler
CaO SiO2 Al2O3 Fe2O3 MgO SO3 K2O Na2O (Na2O)eq Loss on ignition (%)
62.08 21.10 4.18 3.34 3.79 2.84 0.69 0.14 0.59 3.12
50.17 3.12 1.19 0.53 3.46 0.20 0.25 –
Physical properties Specific gravity (gr/cm3) Blaine (cm2/g)
3.17 3519
2.48 –
40.31
Table 2 Sieve analysis of aggregates (cumulative percentage passing). Sieve size (mm)
River sand
Gravel
Aggregate mixture
19 12.5 9.5 4.75 2.38 1.19 0.6 0.3 0.15
100.0 100.0 100.0 92.8 63.6 40.6 26.3 13.3 4.0
100 62.6 38.3 1.1 0.4 0 0 0 0
100 89.3 82.4 67.1 47.9 32.8 22.9 13.6 6.5
Table 3 Mix proportions of reference SCC mixture.
a
Constituent
Quantity
Cement (kg/m3) Filler (kg/m3) Sand (kg/m3) Gravel (kg/m3) Water (kg/m3) w/c HRWRAa (%)
400 150 921 714 156 0.39 0.8
Superplasticizer percentage is presented by Portland cement mass.
The SCC mixtures were produced in a horizontal pan mixer with 100 l capacity. A specific mixing sequence was applied for all mixtures; which consisted of dry mixing the coarse and fine aggregates, limestone filler and Portland cement for 1 min. Then the whole water was added to the dry mixture and mixed for 2 min. Finally, the HRWRA was introduced to the mixture and mixed for another 2 min. After testing the fresh mixture for being self-compactible, cylinder and cube specimens were cast without using any vibration.
2.3. Steam curing regimes In order to study the effects of different steam curing cycles on the hardened properties of reference SCC mixture, three parameters were selected as variables:
No.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36
Curing regime ID
Precuring period (h)
Peak temp. (°C)
Total time of steam curing (h)
Energy index (min °C)
CTL T50-1-8 T50-1-10 T50-1-12 T50-1-14 T50-2-8 T50-2-10 T50-2-12 T50-2-14 T50-3-8 T50-3-10 T50-3-12 T50-3-14 T60-1-8 T60-1 -10 T60-1-12 T60-1-14 T60-2-8 T60-2-10 T60-2-12 T60-2-14 T60-3-8 T60-3-10 T60-3-12 T60-3-14 T70-1-8 T70-1-10 T70-1-12 T70-1-14 T70-2-8 T70-2-10 T70-2-12 T70-2-14 T70-3-8 T70-3-10 T70-3-12 T70-3-14
– 1
– 50 50 50 50 50 50 50 50 50 50 50 50 60 60 60 60 60 60 60 60 60 60 60 60 70 70 70 70 70 70 70 70 70 70 70 70
– 8 10 12 14 8 10 12 14 8 10 12 14 8 10 12 14 8 10 12 14 8 10 12 14 8 10 12 14 8 10 12 14 8 10 12 14
– 180 240 300 360 180 240 300 360 180 240 300 360 240 320 400 480 240 320 400 480 240 320 400 480 300 400 500 600 300 400 500 600 300 400 500 600
2
3
1
2
3
1
2
3
temperature was kept constant at 20 °C, while any temperature variation (heating and cooling) occurred during constant period of 2 h. It is common knowledge that decreasing the steam curing duration is highly desired in the large scale production of precast elements, since it leads to lower production cost and also higher productivity. In this study, a maximum allowable steam curing duration of 14 h was imposed by precast plant project management team. In addition, ‘‘Energy Index’’ was defined as the multiply of time by extra temperature (above ambient temperature which was considered 20 °C) during each steam curing regime. This was used as an indicator of the energy consumption by each cycle. High-accuracy automatic climate simulators were used to impose various steam curing cycles on SCC specimens (see Fig. 2). The devices are capable of controlling both ‘‘humidity’’ and ‘‘temperature’’ in the range of 20 °C to +120 °C with high accuracy. Also, the results ofactual humidity and temperature values during the steam curing cycle are reported in MS-Excel format.
2.4. Tests procedures (1) Precuring (delay) period. (2) Peak temperature. (3) Total time of steam curing. Considering the precast factory limitations, values were assigned to the parameters and 36 different steam curing regimes were designed (see Table 5). Precuring
100 100 100 mm cubic specimens were tested for compressive strength immediately after steam curing cycle and at the ages of 7 and 28 days. Furthermore, maturity index (ASTM C1074 [24]) was utilized to study the effect of temperature– time history on compressive strength of steam cured specimens. This value was calculated using:
Table 4 Properties of reference SCC mixture. Fresh properties Wet density (kg/m3) Slump flow-Avg. of two diameters (mm) T50 (s) V-funnel (s) V-funnel at 15min (s)
Hardened properties (water cured specimens) 2370 690 2.5 9 14
Slump flow at 45 min (mm) U-box (mm) J-ring (h2–h1) (mm) L-box (%)
550 10 5 0.85
1-Day compressive strength (MPa) 3-Day compressive strength(MPa) 7-Day compressive strength(MPa) 28-Day compressive strength(MPa)
17 35 48 59
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Fig. 2. Automatic climate simulator devices used for steam curing of specimens.
3. Results and discussion
Table 6 Permeability classes based on surface resistivity by FM 5-578[27]. Chloride ion permeability
Surface resistivity (kX cm)
High Moderate Low Very low Negligible
<12 12–21 21–37 37–254 >254
MðtÞ ¼
t X ðT a T 0 ÞDt
ð1Þ
0
where M(t) is the temperature–time factor at age (t), degree-days or degree-hour; Dt is a time interval, days or hours; Ta is average concrete temperature during time interval, °C; T0 is datum temperature, °C. It is commonly assumed equal to 10 °C [24]. Capillary absorption was carried out at 28 days age. The capillary absorption rate of specimens were calculated using their weight after 48 h being in 110 °C as initial weight, and the measured weight after 72 h of being partly in contact with the water (on side exposed to water at 5 mm depth) [25]. Electrical resistivity is one of the intrinsic specifications of concrete which can be related to its permeability. In 1915, Wenner [26] presented a practical method to measure the earth electrical surface resistivity by means of a four probes apparatus which for the first time, has been standardized to use on concrete in 2004 [27]. Since it is a non-destructive, rapid, low-cost, and reliable method, it is a wide-accepted technique to investigate durability properties of concrete. Electrical resistivity refers to the resistance that any electrical charge experiences while passing through the concrete. The increased electrical resistivity of concrete impedes the movement of electrons from the anodic to the cathode regions, and consequently delays the propagation of the corrosion process[28]. As presented in Table 6, FM 5-578 test method [27] defines chloride ion permeability ratings according to the surface resistivity test results. The electrical resistivity meter (Fig. 3) was used to measure the surface resistivity of specimens at the ages of 1, 7, and 28 days age. Three saturated 100 200 mm cylinders were tested at each age. The test was carried out by the four-point Wenner array probe technique [29].
Fig. 3. Electrical resistivity meter.
3.1. Compressive strength and maturity index The compressive strength test was conducted on 100 mm cube specimens immediately after steam curing and at the ages of 7 and 28 days. The results are presented in Figs. 4–6. As observed in Fig. 4, increase in precuring period has led to lower immediate compressive strength values. For instance, considering the total cycle time of 8 h, 2 h increase in delay time has brought about 5 MPa decrease in immediate strength of reference SCC mixture. Furthermore, as expected, increase in temperature and total cycle time (which means higher energy and time consumption) led to higher immediate compressive strength. This is due to the accelerated hydration reactions and rapid formation of Calcium–Silica–Hydrate, C–S–H gel, the most important bind phase in hardened concrete [30] in the presence of moisture and high temperature. Also, the average ratio of initial compressive strength to 28-day compressive strength is calculated as 39%, 46% and 53% at the maximum temperatures of 50 °C, 60 °C and 70 °C, respectively. By taking 28-day results into account, it proves that increasing the maximum cycle temperature has negative effect on compressive strength of concrete at later ages, while it improves the immediate strength after curing. Figs. 5 and 6 illustrate the strength measurements for the specimens exposed to cycles with maximum temperatures of 60 °C and 70 °C, respectively. The same immediate strength pattern as for 50 °C cycles is observed for these cycles, too. Considering the strength development until the 7 days age, it is observed that increase in total cycle time reduces the strength development during
Fig. 4. Relative compressive strength values ofthe cycles with 50 °Cpeak temperature (Mean values are presented in MPa).
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Fig. 5. Relative compressive strength values ofthe cycles with 60 °Cpeak temperature (mean values are presented in MPa).
Fig. 7. Results of compressive strength versus maturity index after accomplishment of steam curing.
Table 7 Relationship between maturity index and compressive strength.
Total data Max. temp. 50 Max. temp. 60 Max. temp. 70
A
B
R2
174 144 155 172
866 699 740 823
0.77 0.88 0.88 0.91
Fig. 6. Relative compressive strength values ofthe cycles with 70 °C peak temperature (Mean values are presented in MPa).
this period. On the other hand, strength development from 7 to 28 days of age is approximately the same for different cycles. Among all 36 steam curing cycles, ‘‘T70-2-14’’ led to the highest immediate compressive strength (36 MPa), while maximum 28day compressive strength was measured for SCC specimens which were exposed to ‘‘T60–3-10’’ cycle (approximately 64 MPa). Considering the obtained results and the fact that a minimum of 24 MPa was required for demolding of precast concrete elements, it was decided not to carry out the durability tests on specimens which were exposed to cycles with total duration of 8 h. In fact, the strength measurements demonstrated that none of these cycles developed the requiredminimum strength after exposure to steam curing. Maturity index was proposed earlier as a technique to account for combined effect of time and temperature on the compressive strength of concrete [31,32]. Although some modified methods are presented to increase accuracy and reliability of this method for various types of concrete [33,34], using the basic equation explained in ASTM C1074 [24]seems to be more reliable. Based on the data presented in Fig. 7, it can be observed that logarithmic relations with high R2 values rule the data. However, constant parameters (A and B) based on Eq. (2) presented in Table 7 show that using a specified equation for each maximum temperature of curing is more appropriate with higher correlation coefficient (R2).
Compressive Strength ¼ A lnðMaturity IndexÞ B
ð2Þ
Fig. 8. Influence of temperature = 50 °C).
steam
curing
regimes
on
surface
resistivity
(peak
3.2. Surface resistivity The effect of different steam curing cycles on surface resistivity is illustrated in Figs. 8–10. Considering the values measured shortly after steam curing (1-day results) at 50 °C, it seems that total cycle period has minor effect on the resistivity value, since a maximum 7% variation is measured between cycles of the same delay time. At higher temperatures, immediate resistivity is more sensitive to total cycle time variations. For example, as observed in Fig. 10, at the constant delay time of 3 h and peak temperature of 70 °C, a 4-h increase in total cycle period has improved the resistivity value by 33%. It is also observed that the highest immediate resistivity value is achieved by exposure to T70-2-14 cycle, which has the highest energy index (600 min °C) among 36 designed steam curing cycles. Considering the fact that all the specimens are exposed to steam curing cycles at early ages, the 28-day resistivity measurements could be considered as the appropriate criteria for durability assessment of reference SCC. Accordingly, it can be inferred that ‘‘T60-3-10’’ is the optimum cycle regarding electrical resistivity
812
Fig. 9. Influence of temperature = 60 °C).
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steam
curing
regimes
on
surface
resistivity
(peak Fig. 12. Capillary absorption percentages of steam curing cycles with peak temperature of 60 °C.
Fig. 10. Influence of steam curing regimes on surface resistivity (peak temperature = 70 °C).
of reference SCC. The test result for SCC specimens which were exposed to this curing cycle is measured 15.5 kX cm, which is rated as ‘‘moderate’’ based on the chloride ion permeability classes defined by FM 5-578 [27]. Also, an average28-day electrical resistivity of 12.2 kX cm is calculated for 50 °C cycles (Fig. 8), while the average values of 11.9 kX cm and 11.6 kX cm are measured for 60 °C and 70 °C cycles, respectively. Therefore, it is concluded that increasing the maximum steam curing temperature does not have any positive effect on surface resistivity of reference SCC; even a minor decrease should be expected.
Fig. 13. Capillary absorption percentages of steam curing cycles with peak temperature of 70 °C.
3.3. Capillary absorption Capillary absorption test was carried out at the age of 28 days. The test results are presented in Figs. 11–13. It is observed that at a constant peak temperature and curing period, highest absorption values were measured for specimens exposed to cycles with 1hour delay time. Furthermore, at the three peak temperatures the highest absorption values were recorded for 14-h long cycles, which prove the negative effect of exposure to these cycles on capillary absorption of reference SCC. Also, an average absorption percentage of 6.4 is calculated for 50 °C cycles, while the average value of 5.5% is measured for both 60 °C and 70 °C steam curing cycles. It proves that increase in maximum cycle temperature from 60 °C to 70 °C does not improve the impermeability of reference SCC regarding capillary pores. This finding is of high importance regarding the high energy cost, which plays a crucial role in decision making in large scale precast plants. Accordingly, T60-3-10 could be introduced as an optimum steam curing regime, regarding both capillary absorption (4.3%) and energy consumption.
4. Conclusions
Fig. 11. Capillary absorption percentages of steam curing cycles with peak temperature of 50 °C.
In this study the effects of steam curing on compressive strength and permeability of filler-type SCC were investigated. Also, energy consumption was considered as a selection basis among various steam curing cycles. The major conclusions are as follows:
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1. Compressive strength measurements indicated that increase in precuring period leads to lower immediate compressive strength values. For instance, considering the total curing cycle of 8 h, 2 h increase in delay time has brought about 5 MPa decrease in immediate compressive strength of reference SCC mixture. 2. Increase in temperature and total cycle time (which means higher energy and time consumption) led to higher immediate compressive strength. This could be due to the accelerated hydration reactions and rapid formation of Calcium–Silica– Hydrate(C–S–H gel). 3. An average electrical resistivity of 12.2 kX cm is calculated for 50 °C cycles, while the average values of 11.9 kX cm and 11.6 kX cm are measured for 60 °C and 70 °C cycles, respectively. It proves that increasing the maximum steam curing temperature does not have any positive effect on surface resistivity of reference SCC; even a minor decrease should be expected. 4. Capillary absorption test results indicated that at the three peak temperatures the highest absorption values were recorded for 14-h long cycles, which prove the negative effect of exposure to these cycles on capillary absorption of reference SCC. 5. Durability tests results demonstrated that application of cycles with maximum temperature of 70 °C could have negative effect on durability of reference SCC. This could be due to rapid formation of C–S–H gel and CH crystalline which results in a heterogeneous and coarser pores structure. 6. Considering the three factors of compressive strength, permeability (in terms of surface resistivity and capillary absorption) and energy consumption by steam curing regime, ‘‘T60-3-10’’ was introduced as an optimum steam curing cycle and was applied to precast concrete elements for Sadr elevated highway.
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