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Curing cement concrete by using shrinkage reducing admixture and curing compound
Construction and Building Materials 48 (2013) 992–997
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Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
Curing cement concrete by using shrinkage reducing admixture and curing compound Yudong Dang, Jueshi Qian ⇑, Yanzhao Qu, Lin Zhang, Zhi Wang, Dun Qiao, Xingwen Jia College of Materials Science and Engineering, Chongqing University, Chongqing 400045, China
h i g h l i g h t s Shrinkage reducing admixture and curing compound to cure concrete are used for concrete curing. The curing method significantly reduces drying shrinkage and moisture loss of cement mixtures. Concrete cured with the method has a high resistance to water and chloride-ion penetration.
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Article history: Received 28 September 2012 Received in revised form 19 April 2013 Accepted 24 July 2013 Available online 24 August 2013 Keywords: Curing Shrinkage reducing admixture (SRA) Curing compound Drying shrinkage Chloride-penetration resistance Water absorption
a b s t r a c t The performance of near surface concrete plays an important role in improving durability of concrete structures. The early-age drying shrinkage cracking of near surface concrete due to the moisture loss or poor curing is a main cause of durability deterioration. To minimize the drying shrinkage and moisture loss of concrete, a double-coating curing method was proposed in this study, which was coating the concrete with a shrinkage reducing admixture (SRA) and then a curing compound successively on the concrete surface after demolding. The effects of the curing method on drying shrinkage, moisture loss, strength, chloride-penetration resistance and water absorption of cement mortar and concrete were investigated. The results indicated that the curing method can obviously minimize the drying shrinkage and moisture loss of cement mortar. And the water absorption rate and chloride penetration of concrete cured under this method are significantly reduced. Crown Copyright Ó 2013 Published by Elsevier Ltd. All rights reserved.
1. Introduction The durability deterioration of cement concrete usually results from the movement of aggressive gases and/or liquids from the surrounding environment into concrete through connected pores or micro-cracks in near surface concrete. Due to the relative movement of cement paste and aggregates during the compaction of fresh concrete, the relative content of cement paste or mortar of the near surface concrete is much higher than that of deeper area [1]. As a consequence, the near surface concrete tends to more crack-sensitive. Meanwhile, because the water evaporation generally starts from the exposed surface of concrete, there exists a sharp moisture gradient in the near surface concrete at early ages [2,3]. Before ultimate moisture equilibrium has been reached, a shrinkage restraint occurs. If the shrinkage stresses develop and exceed the tensile strength of concrete, then followed by cracking or microcracking. Therefore, it is essential to properly and promptly cure the newly-placed concrete. ⇑ Corresponding author. Tel./fax: +86 (0)2365126109.
Several curing methods were used in field to control moisture content, which include water curing (water ponding or spraying, fog misting, saturated coverings), sealed curing (covered with water-proof paper, plastic sheeting, or curing compound), and internal curing [4]. Generally, except for internal curing, the basic principle and purpose of those curing methods is to reduce as much drying shrinkage as possible by preventing or retarding the occurrence of menisci in capillary pores, which partly depends on the moisture saturation in capillary pores of cement based materials. Certainly, it is optimum to continuously keep concrete under a moisture curing condition to improve the crack-resistance. However, it is generally impractical and difficult to implement. Thus, covering plastic sheeting or curing compound is another available curing method. Particularly, for some mass concrete construction, such as large concrete bridges, offshore drilling platforms, there have no other choices but using those covering methods for curing. Although the moisture loss can be minimized by using coving methods, their performance is barely satisfactory for preventing concrete cracking or microcracking.
E-mail addresses: [email protected] (Y. Dang), [email protected] (J. Qian). 0950-0618/$ - see front matter Crown Copyright Ó 2013 Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.conbuildmat.2013.07.092
Y. Dang et al. / Construction and Building Materials 48 (2013) 992–997
It is considered that capillary stress appears to be the predominant mechanism of concrete drying shrinkage in higher relative humidity [5]. When pore water evaporates from capillary pores in hardened concrete during drying, surface tension in the liquid is transferred to the capillary walls, resulting in shrinkage. For a given pore size distribution, the capillary stress resulted from water evaporation is proportional to the surface tension of the pore solution. The capillary stresses can be described by Kelvin–Laplace equation:
rcap ¼
2c cos h r
ð1Þ
where rcap is the capillary tension (Pa), c is the surface tension of the pore solution (N/m), h is the contact angle between the pore solution and the capillary pore walls, r is the meniscus radius (m). According to Eq. (1), the lower surface tension of pore solution, the less capillary stress formed in pore system of cement paste. A commercial available chemical admixture, shrinkage reducing admixture (SRA), can reduce short and long term drying shrinkage by reducing the surface tension of pore solution. In the past three decades, many researches have been performed regarding the application of shrinkage reducing admixture (SRA) for controlling shrinkage or shrinkage cracking of concrete [6–10]. Work reported in the literature [6,7,10] clearly demonstrate the reduction in the shrinkage and crack risk of concrete contributed to the presence of SRA. Nevertheless, as a liquid chemical admixture, SRA was generally mixed with the fresh concrete, consequently, some negative influences have also been attributed to the SRA, such as a delay in setting time [6,9], and a decrease in strength and modulus of elasticity [10,11]. In addition, the cost of concrete will slightly increase. As mentioned above, because of the non-uniform distribution of drying shrinkage stress across the cross-section of concrete element from the surface to the internal part, the shrinkage stress of concrete surface exposed to drying environment is extremely larger than that of internal part [2]. Thus, if the shrinkage of surface layer concrete could be reduced by coating a SRA solution onto the concrete surface, it will be beneficial to the formation of crack-free and denser microstructure of near surface concrete. The earlier studies have found that, when SRA solution was applied onto the concrete surface, the water evaporation rate obviously reduced [12]. Our recent study indicated that the application of a solution containing 50% SRA onto the surface of cement based materials can significantly reduce the drying shrinkage and moisture loss, and enhance the strength and crack-resistance of concrete [13]. However, when the SRA is applied onto the exposed surface of concrete, some constituents of SRA could volatilize into the atmosphere. Besides, SRA is readily water soluble and it can be leached or washed away by rain or flowing water [14]. This potential
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volatilization or dissolution of SRA might result in failure to shrinkage reduction. So if any material or method could avoid these shortages, coating a solution containing SRA onto the concrete surface would be a good curing method for reducing drying shrinkage and moisture loss. The present paper aims at proposing and evaluating a doublecoating curing method for cement concrete by coating SRA and curing compound successively onto the concrete surface. The drying shrinkage, water-retention, strength, chloride-penetration resistance and water absorption of the cement mortar or concrete cured with the new method was investigated, compared with conventional curing methods. 2. Experimental procedure 2.1. Materials and procedures of mixing cement mortar and concrete The cement mortar or concrete used P.O.42.5R with a Blaine fineness of 380 m2/ kg, according to Chinese Standard GB175-2007 Ordinary Portland Cement, and its chemical compositions were: 21.20% SiO2, 3.46% Fe2O3, 5.55% Al2O3, 63.68% CaO, 0.89% MgO, and 3.01% SO3. A commercial shrinkage reducing admixture (SRA), with the density of 960 kg/m3, was used for preparing the SRA curing solution at 50% concentration. The concrete curing compound was a styrene–acrylate based emulsion with 30% solid content. Sand with fineness modulus of 3.10 and coarse aggregate with continuous grading ranged from 5 mm to 20 mm were used. Mortar specimens were used for drying shrinkage and compressive strength tests. The water–cement ratio was 0.3, 0.5 and the sand–cement ratio 3.0, respectively. Concrete specimens were used for chloride-permeability and water absorption tests. The proportion of concrete was presented as follows: cement: sand: coarse aggregate: water = 400 kg/m3: 750 kg/m3: 1080 kg/m3: 180 kg/m3. The mixing procedure of mortar and concrete was according to ASTM C305 and ASTM C31 respectively. 2.2. Curing methods The specimens for all tests were made with same materials and mix proportion, and prepared under the same environmental conditions. Thus, the experimental results of drying shrinkage, moisture loss, chloride-permeability and water absorption were predominantly governed by curing methods. The details of curing methods used in this study were described in Table 1. 2.3. Test methods 2.3.1. Drying shrinkage and moisture loss According to ASTM C40, Free drying shrinkage and moisture (or water) loss tests were performed on mortar prisms with size of 25 mm 25 mm 285 mm, and 3 prisms were used for each curing method. The initial length and weight of prisms was immediately measured by a digital micrometer (precision = 0.001 mm) and electronic balance (precision = 0.1 g) after the demolding, and then transferred to four different conditions of DA, SRA, CC and SRA + CC, as listed in Table 1. The weight of prisms cured by SRA, CC and SRA + CC was tested again after the application of SRA and CC. The lengths and weights at 1, 3, 7, 14, 21 and 28 d age were recorded. Finally, the average ratio of drying shrinkage and moisture loss of 3 specimens was calculated, the standard deviation of measured shrinkage and moisture loss was ±25 106 and ±0.20%, respectively.
Table 1 Curing methods used in this study. Abbreviations of curing method
Before demolding
After demolding
MO DA MO5
After casting the samples, keep it in moisture room (RH > 95%, Temp. = 20 ± 2 °C) for 24 h
A. Continuous moisture curing (RH > 95%, Temp. = 20 ± 2 °C) for 27 d B. Continuous dry-air curing in laboratory (RH = 60 ± 5%, Temp. = 20 ± 2 °C) for 27 d C. Continuous moisture curing (as described in A) for 4d then dry-air curing (as described in B) for 22 d D. Let the specimens drying for 30 min after demolding in laboratory (RH = 60 ± 5%, Temp. = 20 ± 2 °C), then Coat a SRA solution (containing 50% of a SRA by mass fraction) onto the all exposed surface of cement mortar or concrete. Typically, the dosage of SRA applied is 100 g/ m2. Finally, the specimens were cured at dry condition (as described in B) for 27 d E. Coat a curing compound onto the all exposed surface of cement mortar or concrete. Then the specimens were cured at dry condition (as described above in B) for 27 d F. Let the specimens drying for 30 min, then Coat with the SRA (the same method and dosage as described in D) and then coating a curing compound (the same as E) successively. The intervallic time between two applications was between 30 and 60 min. Finally, the specimens were cured at dry condition as described in B for 27 d
SRA
CC SRA + CC
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In order to exam the curing efficiency of SRA, CC and SRA + CC after suffering from washing by running water, a simple simulated experiment was conducted on mortar prism with same size like drying shrinkage test: 10 and 30 min after finishing the coating of SRA or curing compound, immediately immerse the specimens into a tank containing circulated flowing water for 10 min. After that, the specimens were transferred to the dry-air condition (RH = 60 ± 5%, temp. = 20 ± 2 °C). The length change and moisture loss of the specimens are measured and compared to those of without washing. 2.3.2. Compressive strength Compressive strength test specimens consisted of nine 40 40 160 mm cement mortar prisms according to ASTM C349. The curing methods of MO, DA, SRA, CC, SRA + CC for all specimens were same as shown in Table 1. 2.3.3. Chloride-penetration resistance The chloride-penetration resistance test was performed on 3 concrete cylinders with 100 mm in diameter and 50 mm in height. The ASTM 1202 rapid chloride permeability test method was employed to evaluate the chloride penetration resistance of the concrete by applying a 60 V DC to monitor the charge passed through the concrete specimen. The compressive strength of reference concrete under the moisture curing was 52 MPa at 28 d age. The preparation procedure of specimens was presented as below: (1) Casted the fresh concrete into a cubic steel mold (150 mm 150 mm 150 mm), and cured in standard curing room for 24 h; (2) then cured in 6 different methods for 27d: MO, DA, MO5, SRA, CC, SRA + CC as described in Table 1; and (3) drilled and cut two cylinder samples (with 100 mm in diameter and 100 mm in height) from the central portion of each cubic specimen. The preparation procedures are schematically illustrated in Fig. 1. It should be noted that all cubic specimens were placed on table, and the performance of the upper samples may be different from that of the lower samples. The test result of chloride-penetration resistance is the average value of 6 samples. 2.3.4. Rate of water absorption The preparation procedure of samples for water absorption test is the same as those of chloride-penetration resistance test described in Section 2.3.3. The specimen was pre-conditioned and water absorption tested according to ASTM C1585. The mass increment (Dm) at time t, exposition area of the specimen (a), and density of water (d), was used to calculate the water absorption (I) as per the equation [15]:
I¼
Dm ad
ð2Þ
For each test, two slices were tested and the results were averaged.
3. Results and discussion 3.1. Drying shrinkage and moisture loss Figs. 2 and 3 illustrate the drying shrinkage and moisture loss of the mortar cured under the different conditions. Fig. 2 shows that the drying shrinkage of the specimens cured with SRA, CC and SRA + CC are much lower than that of the continuous dry-air cured (DA). The specimens cured with SRA have a much smaller drying shrinkage than those of cured with DA and CC. The reason for these phenomena may be as follows: In addition to reduction on drying shrinkage by decreasing the surface tension of the pore solution in cement paste, SRA can lead to a concurrent reduction on evaporation rates of cement mortar or concrete, regardless of mixing SRA into fresh concrete or applying it onto the surface of cement based materials [4,12,13,16]. A hypothesis [12,15] for explaining the reduction of evaporation rate considered that the surface layer of cement paste specimens, which is rich in SRA if initial drying occurs, can prevent the inner
Fig. 2. Effect of curing methods on drying shrinkage of cement mortar.
layer from drying due to its lower surface tension. And also, some researchers have indicated that, after the water evaporated, SRA still remained in the liquid state in capillary pores and prevented much moisture loss [9]. In this study, the water retention of the specimens coated with SRA is even as good as that of the specimens cured with curing compound (CC), especially for 0.5 w/c mortar specimens, as shown in Fig. 3. As expected, the specimens cured with SRA + CC show a minimum drying shrinkage. At early age (0–9 d), SRA + CC curing method reduces more than 30% and 50% drying shrinkage of 0.3 w/c and 0.5 w/c mortar specimens, respectively, and the shrinkage reduction can reach approximately 50% and 70% for 0.3 and 0.5 w/c before 3 d, respectively. It is well known that, if a concrete is cured with a curing compound, the moisture loss can be prevented significantly (as shown in Fig. 3). This is achieved by blocking water transmission due to the relatively impermeable film formed by the curing compound. When the pore water is locked, a higher RH can be maintained in the majority of water-filled pores; as a result, the existing meniscus hardly results in volumetric shrinkage. Therefore, by the synergistic effect of the reduction of surface tension and the lower rate of moisture loss, the specimens cured with SRA + CC show a minimum moisture loss, which will be very helpful to the further hydration of cement. According to Fig. 2, it is obvious that the shrinkage reduction due to SRA + CC is much effective on 0.5 w/c mortar specimens.
Fig. 1. Preparation procedure of specimens for chloride-penetration resistance test.
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change and moisture loss of the specimens cured with coating SRA and SRA + CC suffering from the washing by running water, respectively. It should be noted that the control specimens were directly immersed into water for 10 min, and then transferred to the drying condition. Fig. 4 shows the influence of flowing water on the shrinkage reduction of the curing methods of SRA and SRA + CC. The specimen cured with SRA suffering from washout for 10 min has the largest shrinkage than SRA without water migration. Besides, the longer interval between coating with SRA and starting to washout, the less influence on shrinkage. Although Fig. 5 shows that the specimens coated SRA + CC undergoing water dissolution has a larger moisture loss than those of coated with SRA without washout, the shrinkage reduction is less sensitive to the influence of water migration, which indicates that the presence of curing compound can protect the SRA from dissolution during washout. 3.3. Compressive strength
Fig. 3. Effect of curing methods on moisture loss of cement mortar.
This might be due to the formation of denser microstructure in the cement system with a lower w/c ratio. Hence, the SRA solution hardly penetrated into deeper area of mortar. Nonetheless, at 28 d age, 28% drying shrinkage is reduced by using SRA + CC for 0.3 w/c mortar, which is very close to those of 0.5 w/c. Of course, for a mass concrete or high-strength concrete with a much lower w/c ratio, the SRA + CC curing method might have no significant reduction for the overall shrinkage. But an earlier test has verified that the shrinkage stress of concrete surface exposed to a dry ambient is extremely larger than its internal part [2], so it is predictable that a marked decrease of shrinkage stress of concrete surface layer due to the presence of SRA and CC will be beneficial to improving the crack-resistance and leading to denser microstructure. Besides, at much earlier time (0–2 d), the length change of the 0.5 w/c mortar specimens has a slight expansion of 20 106, as shown in Fig. 2. The phenomenon is also observed by other researchers when the SRA was mixed into fresh concrete [9]. This expansion played a significant role in improving the performance of crack-resistance of cement based materials, especially at early ages [8]. In this paper, when the solution containing 50% SRA was coated onto the surface of small specimens (the cross-section is only 25 mm 25 mm), it could penetrate into the internal part of the specimens, therefore, the SRA mixing into or coating onto concrete have similar mechanism in shrinkage reduction. However, it does not mean that the effect of the SRA curing method was confined to small-sized specimens, on the contrary, it has been reported that the application of SRA as a curing solution can reduce the drying shrinkage and enhance the crack-resistance of larger concrete specimens as well [13].
For continuous moisture curing, the strength of concrete at 28 and 90 d are 62 MPa and 75 MPa respectively, compared with 50–55 MPa (28 d) and 52–57 MPa (90 d) of the specimen cured under the other curing methods, as shown in Fig. 6. Indeed, the moisture curing significantly affects the strength development. However, for DA, SRA, CC, SRA + CC, because no water was added, the strength development of the cement mortar is obviously retarded. Compared with DA and CC, SRA and SRA + CC have less negative effect on the strength development, which maybe contributed to the positive effects of SRA and CC on the stress reduction and moisture maintenance. Our recent study has also observed that mortar strength was enhanced by coating a SRA
Fig. 4. Effect of washout on drying shrinkage of cement mortar coated with SRA.
3.2. Potential of washout Generally, because SRA will not chemically combine with cement hydration products, thus, it might be potentially available been washout by flowing water, especially when the SRA was existed on the surface of concrete. Figs. 4 and 5 illustrate the length
Fig. 5. Effect of washout on moisture loss of cement mortar coated with SRA.
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Fig. 6. Compressive strength of cement mortar cured under different methods.
solution [13]. CC presents a slight negative influence on early strength, which might resulted from the delayed hydration due to the penetration of curing compound into cement mortar. 3.4. Chloride-penetration resistance The results shown in Fig. 7 indicates that the curing methods remarkably affect the resistance of the concrete to the chlorideion penetration. The effectiveness of different curing methods is in the following descending order: MO, MO5, SRA + CC, SRA, CC and DA as expected. In addition, the results indicate that the chloride-penetration resistance of bottom side concrete is superior to that of the top side, which is due to less moisture loss in the bottom side. Compared to DA, SRA reduces about 15% of charge passed. Some researches have also reported similar values of the rapid chloride permeability in concrete with and without SRA [17]. The promotion of chloride-penetration resistance may contribute to the porosity reduction or microcracking minimization of cement paste due to the presence of SRA [13], or the increase of viscosity of pore solution by SRA [18]. As shown in Fig. 7, compared with DA, CC reduces 10% of charge. But it has been reported that curing compound did not significantly enhance the chloride-penetration resistance of concrete [19,20]. For SRA + CC, the specimens have higher chloride-penetration resistance, which are close to those of the concrete cured with MO5.
sion analysis. Table 2 illustrates the initial and secondary rate of water absorption, which is equal to the calculated slope of the line in Fig. 8. It can be seen in Fig. 8 and Table 2, for curing methods of DA and SRA, the rate of initial water absorption of the top layer samples are greatly different from the bottom ones, which is consistent with the earlier viewpoint that a better curing can reduce the difference between the top and middle/bottom layers of concrete [21]. There is a significant decrease in the rate of initial absorption for poor curing of DA (the average rate of top and bottom layer is 86 104 mm/min0.5), in contrast to the curing method of MO (the average rate is 49 104 mm/min0.5). For the curing method of SRA + CC, the average rate of initial absorption (57 104 mm/ min0.5) is even lower than that of MO5 (62 104 mm/min0.5), while for the curing methods of SRA and CC, the average rates of initial water absorption (the average rate is 79 104 mm/min0.5 and 75 104 mm/min0.5, respectively) are much higher than that of MO5. As illustrated in Table 2, the average rate of secondary absorption of DA, SRA and CC are much higher than MO, MO5 while SRA + CC has 18 104 mm/min0.5, which is very close to that of MO (15.5 104 mm/min0.5). Obviously, the results of water absorption rate and chloridepenetration resistance substantiate that the SRA + CC can significantly enhance the anti-penetration of concrete. There are two potential reasons: firstly, the presence of SRA decreased the penetration of water or chloride [18]. However, the results shown in Fig. 7 and Table 2 indicate that only coating SRA cannot clearly enhance the permeability of concrete, on the contrary, the rate of water absorption of SRA is very close to that of DA, which may result from the limited penetration depth of SRA on the surface of concrete. Secondly, a denser microstructure may be formed resulted from a higher internal relative humidity in SRA + CC cured concrete. It has been proved that the degradation of relative
3.5. Rate of water absorption The initial and secondary water absorption of concrete cured with six different curing methods for 28 d are presented in Fig. 8. The lines in Fig. 8 are the plots of I versus t0.5 using linear regres-
Fig. 7. Effect of curing methods on concrete permeability.
Fig. 8. Water absorption of concrete specimens under different curing methods (Top-Initial absorption; Bottom-Secondary absorption).
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Y. Dang et al. / Construction and Building Materials 48 (2013) 992–997 Table 2 Slope of initial and secondary water absorption line under different curing methods (104 mm/min0.5). Curing methods
MO
Location of sampling Slope of Initial sorptivity line Slope of Secondary sorptivity line
T 46 15
DA B 52 16
T 93 33
MO5 B 79 34
T 58 23
SRA B 66 23
T 92 37
CC B 66 25
T 76 28
SRA + CC B 74 26
T 58 18
B 55 18
Note: T and B indicate the top and bottom layer concrete of sampling location, respectively.
humidity in cement mixture containing SRA is much slower than those of without SRA [16], meanwhile the presence of CC can further mitigate the moisture loss of concrete. Thus, the cement hydration of concrete cured under SRA + CC would be higher than that of DA, SRA and CC, which would lead to lower the connectivity of capillary pores in concrete, the slight higher strength of SRA + CC shown in Fig. 6 also demonstrated this hypothesis to a certain extent. However, further studies will be needed to clarify it. 4. Conclusions This paper has described a double-coating curing method of coating a shrinkage reducing admixture and a curing compound successively (SRA + CC) onto concrete surface. The volume change, moisture loss, permeability, water absorption of mortar and concrete cured with SRA + CC were examined comparing with other conventional curing methods. Based on the results presented and discussed, following conclusions are drawn. Compared with continuous dry-air curing, only coating SRA solution or curing compound, the mortar specimens cured with SRA + CC had a minimum drying shrinkage and moisture loss. It is demonstrated that the use of curing compound (CC) accompanied with SRA can not only protect SRA solution from dissolution, but also has a superposition effect on shrinkage reduction and moisture maintenance. Thus, it could significantly improve the crack-resistance of cement concrete. The curing method of SRA + CC can significantly improve the chloride-penetration resistance and reduce water absorption of concrete. It could be explained that SRA + CC leads to a higher internal relative humidity in concrete then resulting to a higher cement hydration and lower conductivity of capillary pores. It is suggested that SRA + CC can be used as a potential curing method for a supplement or replacement of conventional curing methods without water added during curing. Acknowledgments The research reported herein was financially supported by Chongqing Science & Technology Project under Grant No. CSTC2009AC6204 and National Key Technology R&D Program under Grant No. 2006BAJ04A04. The authors also gratefully acknowledge Pengkun Hou, Yanfei Yue, Yiying Zhang for their suggestions and review of this paper. The authors would like to extend their thanks to Dr. Yun Bai of Queen’s University of Belfast for his helpful advices as well.
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Contents lists available at ScienceDirect
Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
Curing cement concrete by using shrinkage reducing admixture and curing compound Yudong Dang, Jueshi Qian ⇑, Yanzhao Qu, Lin Zhang, Zhi Wang, Dun Qiao, Xingwen Jia College of Materials Science and Engineering, Chongqing University, Chongqing 400045, China
h i g h l i g h t s Shrinkage reducing admixture and curing compound to cure concrete are used for concrete curing. The curing method significantly reduces drying shrinkage and moisture loss of cement mixtures. Concrete cured with the method has a high resistance to water and chloride-ion penetration.
a r t i c l e
i n f o
Article history: Received 28 September 2012 Received in revised form 19 April 2013 Accepted 24 July 2013 Available online 24 August 2013 Keywords: Curing Shrinkage reducing admixture (SRA) Curing compound Drying shrinkage Chloride-penetration resistance Water absorption
a b s t r a c t The performance of near surface concrete plays an important role in improving durability of concrete structures. The early-age drying shrinkage cracking of near surface concrete due to the moisture loss or poor curing is a main cause of durability deterioration. To minimize the drying shrinkage and moisture loss of concrete, a double-coating curing method was proposed in this study, which was coating the concrete with a shrinkage reducing admixture (SRA) and then a curing compound successively on the concrete surface after demolding. The effects of the curing method on drying shrinkage, moisture loss, strength, chloride-penetration resistance and water absorption of cement mortar and concrete were investigated. The results indicated that the curing method can obviously minimize the drying shrinkage and moisture loss of cement mortar. And the water absorption rate and chloride penetration of concrete cured under this method are significantly reduced. Crown Copyright Ó 2013 Published by Elsevier Ltd. All rights reserved.
1. Introduction The durability deterioration of cement concrete usually results from the movement of aggressive gases and/or liquids from the surrounding environment into concrete through connected pores or micro-cracks in near surface concrete. Due to the relative movement of cement paste and aggregates during the compaction of fresh concrete, the relative content of cement paste or mortar of the near surface concrete is much higher than that of deeper area [1]. As a consequence, the near surface concrete tends to more crack-sensitive. Meanwhile, because the water evaporation generally starts from the exposed surface of concrete, there exists a sharp moisture gradient in the near surface concrete at early ages [2,3]. Before ultimate moisture equilibrium has been reached, a shrinkage restraint occurs. If the shrinkage stresses develop and exceed the tensile strength of concrete, then followed by cracking or microcracking. Therefore, it is essential to properly and promptly cure the newly-placed concrete. ⇑ Corresponding author. Tel./fax: +86 (0)2365126109.
Several curing methods were used in field to control moisture content, which include water curing (water ponding or spraying, fog misting, saturated coverings), sealed curing (covered with water-proof paper, plastic sheeting, or curing compound), and internal curing [4]. Generally, except for internal curing, the basic principle and purpose of those curing methods is to reduce as much drying shrinkage as possible by preventing or retarding the occurrence of menisci in capillary pores, which partly depends on the moisture saturation in capillary pores of cement based materials. Certainly, it is optimum to continuously keep concrete under a moisture curing condition to improve the crack-resistance. However, it is generally impractical and difficult to implement. Thus, covering plastic sheeting or curing compound is another available curing method. Particularly, for some mass concrete construction, such as large concrete bridges, offshore drilling platforms, there have no other choices but using those covering methods for curing. Although the moisture loss can be minimized by using coving methods, their performance is barely satisfactory for preventing concrete cracking or microcracking.
E-mail addresses: [email protected] (Y. Dang), [email protected] (J. Qian). 0950-0618/$ - see front matter Crown Copyright Ó 2013 Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.conbuildmat.2013.07.092
Y. Dang et al. / Construction and Building Materials 48 (2013) 992–997
It is considered that capillary stress appears to be the predominant mechanism of concrete drying shrinkage in higher relative humidity [5]. When pore water evaporates from capillary pores in hardened concrete during drying, surface tension in the liquid is transferred to the capillary walls, resulting in shrinkage. For a given pore size distribution, the capillary stress resulted from water evaporation is proportional to the surface tension of the pore solution. The capillary stresses can be described by Kelvin–Laplace equation:
rcap ¼
2c cos h r
ð1Þ
where rcap is the capillary tension (Pa), c is the surface tension of the pore solution (N/m), h is the contact angle between the pore solution and the capillary pore walls, r is the meniscus radius (m). According to Eq. (1), the lower surface tension of pore solution, the less capillary stress formed in pore system of cement paste. A commercial available chemical admixture, shrinkage reducing admixture (SRA), can reduce short and long term drying shrinkage by reducing the surface tension of pore solution. In the past three decades, many researches have been performed regarding the application of shrinkage reducing admixture (SRA) for controlling shrinkage or shrinkage cracking of concrete [6–10]. Work reported in the literature [6,7,10] clearly demonstrate the reduction in the shrinkage and crack risk of concrete contributed to the presence of SRA. Nevertheless, as a liquid chemical admixture, SRA was generally mixed with the fresh concrete, consequently, some negative influences have also been attributed to the SRA, such as a delay in setting time [6,9], and a decrease in strength and modulus of elasticity [10,11]. In addition, the cost of concrete will slightly increase. As mentioned above, because of the non-uniform distribution of drying shrinkage stress across the cross-section of concrete element from the surface to the internal part, the shrinkage stress of concrete surface exposed to drying environment is extremely larger than that of internal part [2]. Thus, if the shrinkage of surface layer concrete could be reduced by coating a SRA solution onto the concrete surface, it will be beneficial to the formation of crack-free and denser microstructure of near surface concrete. The earlier studies have found that, when SRA solution was applied onto the concrete surface, the water evaporation rate obviously reduced [12]. Our recent study indicated that the application of a solution containing 50% SRA onto the surface of cement based materials can significantly reduce the drying shrinkage and moisture loss, and enhance the strength and crack-resistance of concrete [13]. However, when the SRA is applied onto the exposed surface of concrete, some constituents of SRA could volatilize into the atmosphere. Besides, SRA is readily water soluble and it can be leached or washed away by rain or flowing water [14]. This potential
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volatilization or dissolution of SRA might result in failure to shrinkage reduction. So if any material or method could avoid these shortages, coating a solution containing SRA onto the concrete surface would be a good curing method for reducing drying shrinkage and moisture loss. The present paper aims at proposing and evaluating a doublecoating curing method for cement concrete by coating SRA and curing compound successively onto the concrete surface. The drying shrinkage, water-retention, strength, chloride-penetration resistance and water absorption of the cement mortar or concrete cured with the new method was investigated, compared with conventional curing methods. 2. Experimental procedure 2.1. Materials and procedures of mixing cement mortar and concrete The cement mortar or concrete used P.O.42.5R with a Blaine fineness of 380 m2/ kg, according to Chinese Standard GB175-2007 Ordinary Portland Cement, and its chemical compositions were: 21.20% SiO2, 3.46% Fe2O3, 5.55% Al2O3, 63.68% CaO, 0.89% MgO, and 3.01% SO3. A commercial shrinkage reducing admixture (SRA), with the density of 960 kg/m3, was used for preparing the SRA curing solution at 50% concentration. The concrete curing compound was a styrene–acrylate based emulsion with 30% solid content. Sand with fineness modulus of 3.10 and coarse aggregate with continuous grading ranged from 5 mm to 20 mm were used. Mortar specimens were used for drying shrinkage and compressive strength tests. The water–cement ratio was 0.3, 0.5 and the sand–cement ratio 3.0, respectively. Concrete specimens were used for chloride-permeability and water absorption tests. The proportion of concrete was presented as follows: cement: sand: coarse aggregate: water = 400 kg/m3: 750 kg/m3: 1080 kg/m3: 180 kg/m3. The mixing procedure of mortar and concrete was according to ASTM C305 and ASTM C31 respectively. 2.2. Curing methods The specimens for all tests were made with same materials and mix proportion, and prepared under the same environmental conditions. Thus, the experimental results of drying shrinkage, moisture loss, chloride-permeability and water absorption were predominantly governed by curing methods. The details of curing methods used in this study were described in Table 1. 2.3. Test methods 2.3.1. Drying shrinkage and moisture loss According to ASTM C40, Free drying shrinkage and moisture (or water) loss tests were performed on mortar prisms with size of 25 mm 25 mm 285 mm, and 3 prisms were used for each curing method. The initial length and weight of prisms was immediately measured by a digital micrometer (precision = 0.001 mm) and electronic balance (precision = 0.1 g) after the demolding, and then transferred to four different conditions of DA, SRA, CC and SRA + CC, as listed in Table 1. The weight of prisms cured by SRA, CC and SRA + CC was tested again after the application of SRA and CC. The lengths and weights at 1, 3, 7, 14, 21 and 28 d age were recorded. Finally, the average ratio of drying shrinkage and moisture loss of 3 specimens was calculated, the standard deviation of measured shrinkage and moisture loss was ±25 106 and ±0.20%, respectively.
Table 1 Curing methods used in this study. Abbreviations of curing method
Before demolding
After demolding
MO DA MO5
After casting the samples, keep it in moisture room (RH > 95%, Temp. = 20 ± 2 °C) for 24 h
A. Continuous moisture curing (RH > 95%, Temp. = 20 ± 2 °C) for 27 d B. Continuous dry-air curing in laboratory (RH = 60 ± 5%, Temp. = 20 ± 2 °C) for 27 d C. Continuous moisture curing (as described in A) for 4d then dry-air curing (as described in B) for 22 d D. Let the specimens drying for 30 min after demolding in laboratory (RH = 60 ± 5%, Temp. = 20 ± 2 °C), then Coat a SRA solution (containing 50% of a SRA by mass fraction) onto the all exposed surface of cement mortar or concrete. Typically, the dosage of SRA applied is 100 g/ m2. Finally, the specimens were cured at dry condition (as described in B) for 27 d E. Coat a curing compound onto the all exposed surface of cement mortar or concrete. Then the specimens were cured at dry condition (as described above in B) for 27 d F. Let the specimens drying for 30 min, then Coat with the SRA (the same method and dosage as described in D) and then coating a curing compound (the same as E) successively. The intervallic time between two applications was between 30 and 60 min. Finally, the specimens were cured at dry condition as described in B for 27 d
SRA
CC SRA + CC
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In order to exam the curing efficiency of SRA, CC and SRA + CC after suffering from washing by running water, a simple simulated experiment was conducted on mortar prism with same size like drying shrinkage test: 10 and 30 min after finishing the coating of SRA or curing compound, immediately immerse the specimens into a tank containing circulated flowing water for 10 min. After that, the specimens were transferred to the dry-air condition (RH = 60 ± 5%, temp. = 20 ± 2 °C). The length change and moisture loss of the specimens are measured and compared to those of without washing. 2.3.2. Compressive strength Compressive strength test specimens consisted of nine 40 40 160 mm cement mortar prisms according to ASTM C349. The curing methods of MO, DA, SRA, CC, SRA + CC for all specimens were same as shown in Table 1. 2.3.3. Chloride-penetration resistance The chloride-penetration resistance test was performed on 3 concrete cylinders with 100 mm in diameter and 50 mm in height. The ASTM 1202 rapid chloride permeability test method was employed to evaluate the chloride penetration resistance of the concrete by applying a 60 V DC to monitor the charge passed through the concrete specimen. The compressive strength of reference concrete under the moisture curing was 52 MPa at 28 d age. The preparation procedure of specimens was presented as below: (1) Casted the fresh concrete into a cubic steel mold (150 mm 150 mm 150 mm), and cured in standard curing room for 24 h; (2) then cured in 6 different methods for 27d: MO, DA, MO5, SRA, CC, SRA + CC as described in Table 1; and (3) drilled and cut two cylinder samples (with 100 mm in diameter and 100 mm in height) from the central portion of each cubic specimen. The preparation procedures are schematically illustrated in Fig. 1. It should be noted that all cubic specimens were placed on table, and the performance of the upper samples may be different from that of the lower samples. The test result of chloride-penetration resistance is the average value of 6 samples. 2.3.4. Rate of water absorption The preparation procedure of samples for water absorption test is the same as those of chloride-penetration resistance test described in Section 2.3.3. The specimen was pre-conditioned and water absorption tested according to ASTM C1585. The mass increment (Dm) at time t, exposition area of the specimen (a), and density of water (d), was used to calculate the water absorption (I) as per the equation [15]:
I¼
Dm ad
ð2Þ
For each test, two slices were tested and the results were averaged.
3. Results and discussion 3.1. Drying shrinkage and moisture loss Figs. 2 and 3 illustrate the drying shrinkage and moisture loss of the mortar cured under the different conditions. Fig. 2 shows that the drying shrinkage of the specimens cured with SRA, CC and SRA + CC are much lower than that of the continuous dry-air cured (DA). The specimens cured with SRA have a much smaller drying shrinkage than those of cured with DA and CC. The reason for these phenomena may be as follows: In addition to reduction on drying shrinkage by decreasing the surface tension of the pore solution in cement paste, SRA can lead to a concurrent reduction on evaporation rates of cement mortar or concrete, regardless of mixing SRA into fresh concrete or applying it onto the surface of cement based materials [4,12,13,16]. A hypothesis [12,15] for explaining the reduction of evaporation rate considered that the surface layer of cement paste specimens, which is rich in SRA if initial drying occurs, can prevent the inner
Fig. 2. Effect of curing methods on drying shrinkage of cement mortar.
layer from drying due to its lower surface tension. And also, some researchers have indicated that, after the water evaporated, SRA still remained in the liquid state in capillary pores and prevented much moisture loss [9]. In this study, the water retention of the specimens coated with SRA is even as good as that of the specimens cured with curing compound (CC), especially for 0.5 w/c mortar specimens, as shown in Fig. 3. As expected, the specimens cured with SRA + CC show a minimum drying shrinkage. At early age (0–9 d), SRA + CC curing method reduces more than 30% and 50% drying shrinkage of 0.3 w/c and 0.5 w/c mortar specimens, respectively, and the shrinkage reduction can reach approximately 50% and 70% for 0.3 and 0.5 w/c before 3 d, respectively. It is well known that, if a concrete is cured with a curing compound, the moisture loss can be prevented significantly (as shown in Fig. 3). This is achieved by blocking water transmission due to the relatively impermeable film formed by the curing compound. When the pore water is locked, a higher RH can be maintained in the majority of water-filled pores; as a result, the existing meniscus hardly results in volumetric shrinkage. Therefore, by the synergistic effect of the reduction of surface tension and the lower rate of moisture loss, the specimens cured with SRA + CC show a minimum moisture loss, which will be very helpful to the further hydration of cement. According to Fig. 2, it is obvious that the shrinkage reduction due to SRA + CC is much effective on 0.5 w/c mortar specimens.
Fig. 1. Preparation procedure of specimens for chloride-penetration resistance test.
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change and moisture loss of the specimens cured with coating SRA and SRA + CC suffering from the washing by running water, respectively. It should be noted that the control specimens were directly immersed into water for 10 min, and then transferred to the drying condition. Fig. 4 shows the influence of flowing water on the shrinkage reduction of the curing methods of SRA and SRA + CC. The specimen cured with SRA suffering from washout for 10 min has the largest shrinkage than SRA without water migration. Besides, the longer interval between coating with SRA and starting to washout, the less influence on shrinkage. Although Fig. 5 shows that the specimens coated SRA + CC undergoing water dissolution has a larger moisture loss than those of coated with SRA without washout, the shrinkage reduction is less sensitive to the influence of water migration, which indicates that the presence of curing compound can protect the SRA from dissolution during washout. 3.3. Compressive strength
Fig. 3. Effect of curing methods on moisture loss of cement mortar.
This might be due to the formation of denser microstructure in the cement system with a lower w/c ratio. Hence, the SRA solution hardly penetrated into deeper area of mortar. Nonetheless, at 28 d age, 28% drying shrinkage is reduced by using SRA + CC for 0.3 w/c mortar, which is very close to those of 0.5 w/c. Of course, for a mass concrete or high-strength concrete with a much lower w/c ratio, the SRA + CC curing method might have no significant reduction for the overall shrinkage. But an earlier test has verified that the shrinkage stress of concrete surface exposed to a dry ambient is extremely larger than its internal part [2], so it is predictable that a marked decrease of shrinkage stress of concrete surface layer due to the presence of SRA and CC will be beneficial to improving the crack-resistance and leading to denser microstructure. Besides, at much earlier time (0–2 d), the length change of the 0.5 w/c mortar specimens has a slight expansion of 20 106, as shown in Fig. 2. The phenomenon is also observed by other researchers when the SRA was mixed into fresh concrete [9]. This expansion played a significant role in improving the performance of crack-resistance of cement based materials, especially at early ages [8]. In this paper, when the solution containing 50% SRA was coated onto the surface of small specimens (the cross-section is only 25 mm 25 mm), it could penetrate into the internal part of the specimens, therefore, the SRA mixing into or coating onto concrete have similar mechanism in shrinkage reduction. However, it does not mean that the effect of the SRA curing method was confined to small-sized specimens, on the contrary, it has been reported that the application of SRA as a curing solution can reduce the drying shrinkage and enhance the crack-resistance of larger concrete specimens as well [13].
For continuous moisture curing, the strength of concrete at 28 and 90 d are 62 MPa and 75 MPa respectively, compared with 50–55 MPa (28 d) and 52–57 MPa (90 d) of the specimen cured under the other curing methods, as shown in Fig. 6. Indeed, the moisture curing significantly affects the strength development. However, for DA, SRA, CC, SRA + CC, because no water was added, the strength development of the cement mortar is obviously retarded. Compared with DA and CC, SRA and SRA + CC have less negative effect on the strength development, which maybe contributed to the positive effects of SRA and CC on the stress reduction and moisture maintenance. Our recent study has also observed that mortar strength was enhanced by coating a SRA
Fig. 4. Effect of washout on drying shrinkage of cement mortar coated with SRA.
3.2. Potential of washout Generally, because SRA will not chemically combine with cement hydration products, thus, it might be potentially available been washout by flowing water, especially when the SRA was existed on the surface of concrete. Figs. 4 and 5 illustrate the length
Fig. 5. Effect of washout on moisture loss of cement mortar coated with SRA.
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Fig. 6. Compressive strength of cement mortar cured under different methods.
solution [13]. CC presents a slight negative influence on early strength, which might resulted from the delayed hydration due to the penetration of curing compound into cement mortar. 3.4. Chloride-penetration resistance The results shown in Fig. 7 indicates that the curing methods remarkably affect the resistance of the concrete to the chlorideion penetration. The effectiveness of different curing methods is in the following descending order: MO, MO5, SRA + CC, SRA, CC and DA as expected. In addition, the results indicate that the chloride-penetration resistance of bottom side concrete is superior to that of the top side, which is due to less moisture loss in the bottom side. Compared to DA, SRA reduces about 15% of charge passed. Some researches have also reported similar values of the rapid chloride permeability in concrete with and without SRA [17]. The promotion of chloride-penetration resistance may contribute to the porosity reduction or microcracking minimization of cement paste due to the presence of SRA [13], or the increase of viscosity of pore solution by SRA [18]. As shown in Fig. 7, compared with DA, CC reduces 10% of charge. But it has been reported that curing compound did not significantly enhance the chloride-penetration resistance of concrete [19,20]. For SRA + CC, the specimens have higher chloride-penetration resistance, which are close to those of the concrete cured with MO5.
sion analysis. Table 2 illustrates the initial and secondary rate of water absorption, which is equal to the calculated slope of the line in Fig. 8. It can be seen in Fig. 8 and Table 2, for curing methods of DA and SRA, the rate of initial water absorption of the top layer samples are greatly different from the bottom ones, which is consistent with the earlier viewpoint that a better curing can reduce the difference between the top and middle/bottom layers of concrete [21]. There is a significant decrease in the rate of initial absorption for poor curing of DA (the average rate of top and bottom layer is 86 104 mm/min0.5), in contrast to the curing method of MO (the average rate is 49 104 mm/min0.5). For the curing method of SRA + CC, the average rate of initial absorption (57 104 mm/ min0.5) is even lower than that of MO5 (62 104 mm/min0.5), while for the curing methods of SRA and CC, the average rates of initial water absorption (the average rate is 79 104 mm/min0.5 and 75 104 mm/min0.5, respectively) are much higher than that of MO5. As illustrated in Table 2, the average rate of secondary absorption of DA, SRA and CC are much higher than MO, MO5 while SRA + CC has 18 104 mm/min0.5, which is very close to that of MO (15.5 104 mm/min0.5). Obviously, the results of water absorption rate and chloridepenetration resistance substantiate that the SRA + CC can significantly enhance the anti-penetration of concrete. There are two potential reasons: firstly, the presence of SRA decreased the penetration of water or chloride [18]. However, the results shown in Fig. 7 and Table 2 indicate that only coating SRA cannot clearly enhance the permeability of concrete, on the contrary, the rate of water absorption of SRA is very close to that of DA, which may result from the limited penetration depth of SRA on the surface of concrete. Secondly, a denser microstructure may be formed resulted from a higher internal relative humidity in SRA + CC cured concrete. It has been proved that the degradation of relative
3.5. Rate of water absorption The initial and secondary water absorption of concrete cured with six different curing methods for 28 d are presented in Fig. 8. The lines in Fig. 8 are the plots of I versus t0.5 using linear regres-
Fig. 7. Effect of curing methods on concrete permeability.
Fig. 8. Water absorption of concrete specimens under different curing methods (Top-Initial absorption; Bottom-Secondary absorption).
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Y. Dang et al. / Construction and Building Materials 48 (2013) 992–997 Table 2 Slope of initial and secondary water absorption line under different curing methods (104 mm/min0.5). Curing methods
MO
Location of sampling Slope of Initial sorptivity line Slope of Secondary sorptivity line
T 46 15
DA B 52 16
T 93 33
MO5 B 79 34
T 58 23
SRA B 66 23
T 92 37
CC B 66 25
T 76 28
SRA + CC B 74 26
T 58 18
B 55 18
Note: T and B indicate the top and bottom layer concrete of sampling location, respectively.
humidity in cement mixture containing SRA is much slower than those of without SRA [16], meanwhile the presence of CC can further mitigate the moisture loss of concrete. Thus, the cement hydration of concrete cured under SRA + CC would be higher than that of DA, SRA and CC, which would lead to lower the connectivity of capillary pores in concrete, the slight higher strength of SRA + CC shown in Fig. 6 also demonstrated this hypothesis to a certain extent. However, further studies will be needed to clarify it. 4. Conclusions This paper has described a double-coating curing method of coating a shrinkage reducing admixture and a curing compound successively (SRA + CC) onto concrete surface. The volume change, moisture loss, permeability, water absorption of mortar and concrete cured with SRA + CC were examined comparing with other conventional curing methods. Based on the results presented and discussed, following conclusions are drawn. Compared with continuous dry-air curing, only coating SRA solution or curing compound, the mortar specimens cured with SRA + CC had a minimum drying shrinkage and moisture loss. It is demonstrated that the use of curing compound (CC) accompanied with SRA can not only protect SRA solution from dissolution, but also has a superposition effect on shrinkage reduction and moisture maintenance. Thus, it could significantly improve the crack-resistance of cement concrete. The curing method of SRA + CC can significantly improve the chloride-penetration resistance and reduce water absorption of concrete. It could be explained that SRA + CC leads to a higher internal relative humidity in concrete then resulting to a higher cement hydration and lower conductivity of capillary pores. It is suggested that SRA + CC can be used as a potential curing method for a supplement or replacement of conventional curing methods without water added during curing. Acknowledgments The research reported herein was financially supported by Chongqing Science & Technology Project under Grant No. CSTC2009AC6204 and National Key Technology R&D Program under Grant No. 2006BAJ04A04. The authors also gratefully acknowledge Pengkun Hou, Yanfei Yue, Yiying Zhang for their suggestions and review of this paper. The authors would like to extend their thanks to Dr. Yun Bai of Queen’s University of Belfast for his helpful advices as well.
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