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Effects of parent concrete and mixing method on the resistance to freezing and thawing of air-entrained recycled aggregate concrete
Construction and Building Materials 106 (2016) 264–273
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Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
Effects of parent concrete and mixing method on the resistance to freezing and thawing of air-entrained recycled aggregate concrete Kaihua Liu a,b, Jiachuan Yan a,b, Qiong Hu a,b, Yao Sun c, Chaoying Zou a,b,⇑ a
Key Lab of Structures Dynamic Behavior and Control of the Ministry of Education (Harbin Institute of Technology), Harbin 150090, China School of Civil Engineering, Harbin Institute of Technology, Heilongjiang, Harbin 150090, China c Tianjin University Research Institute of Architectural Design and Urban Planning, Tianjin 300073, China b
h i g h l i g h t s We study the frost resistance of air-entrained recycled aggregate concrete (ARAC). The effects of four parent concretes and three mixing approaches are investigated. The failure mechanism of ARAC after freezing and thawing is established. This is done on the basis of a mesostructural analysis of the test samples.
a r t i c l e
i n f o
Article history: Received 6 July 2015 Received in revised form 5 October 2015 Accepted 10 December 2015
Keywords: Parent concrete Recycled aggregate concrete Mixing method Freezing and thawing Mesostructure
a b s t r a c t We elucidated the effects of the parent concrete and mixing approach used on the freezing/thawing resistance of air-entrained recycled aggregate concrete (ARAC). Three non-air-entrained concretes and one air-entrained concrete were used to prepare recycled coarse aggregate (RCA) samples. Three mixing approaches were also investigated. The frost resistances of the ARAC samples produced using an RCA obtained from the non-air-entrained concrete with high strength as well as the air-entrained one were close to that of conventional concrete. The mixing approach used had no effect on the frost resistance. A possible mechanism for the failure of ARAC is proposed. Ó 2015 Elsevier Ltd. All rights reserved.
1. Introduction With the rapid developments taking place in the construction industry, environmental problems, including the excessive use of natural resources and the increase in the amount of construction and demolition (C&D) waste, are becoming pressing issues. Approximately 2 billion tons of C&D waste, which amounted to 10% of the gross municipal waste, was generated in China in
Abbreviations: ARAC, air-entrained recycled aggregate concrete; RCA, recycled coarse aggregate; C&D, construction and demolition; RAC, recycled aggregate concrete; FT, freezing and thawing; RDME, relative dynamic modulus of elasticity; URDME, ultrasonic RDME; WRA, water-reducing agent; WSSD, water-saturated surface dry density; ITZ, interfacial transition zone. ⇑ Corresponding author at: School of Civil Engineering, Harbin Institute of Technology, Heilongjiang, Harbin 150090, China. E-mail addresses: [email protected] (K. Liu), [email protected] (J. Yan), [email protected] (Q. Hu), [email protected] (Y. Sun), [email protected] (C. Zou). http://dx.doi.org/10.1016/j.conbuildmat.2015.12.074 0950-0618/Ó 2015 Elsevier Ltd. All rights reserved.
2013 [1]. Recycled aggregate concrete (RAC) is attracting significant attention all over the world, as it is a very promising solution for dealing with C&D waste and saving construction materials [2]. In cold regions, hydraulic structures, such as bridges, dams, and ports, are susceptible to freeze/thaw (FT) cycling, which can lower their durability and result in structural deterioration. To expand the range of applications of RAC and explore the feasibility of using it in cold areas, it is essential to determine the frost resistance of RAC. Salem and Burdette [3] reported that the resistance of RAC to FT cycling of RAC can be improved by adding an air-entraining agent and higher amounts of fly ash to it. Salem et al. [4], Zaharieva et al. [5], and de Oliveira and Vazquez [6] found that the degree of water saturation is the critical factor determining the frost resistance of RAC. Ajdukiewicz and Alina [7] reported that a high-performance concrete prepared using RCA obtained from a high-performance parent concrete exhibited frost durability similar to or better than that of conventional concrete. Gokce et al. [8] found that the frost resistance of RAC produced using RCA
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analyses. The added iron oxide red had little effect on the parent mortar, which exhibited stabilities high enough to satisfy the test requirements (Table 1). Four parent concretes were designed for this study; their compositions are listed in Table 2. The properties of the freshly prepared and hardened parent concretes are listed in Table 3.
derived from an air-entrained concrete was similar to that of conventional concrete, while RAC containing RCA derived from non-air-entrained concrete exhibited poor frost resistance. Kawamura and Torii [9] reported that the FT resistance of RAC can be improved by reducing the amount of adhering mortar in the RCA used. Abbas et al. [10] reported that the equivalent mortar volume method can help increase the frost resistance of RAC to the extent that it complies with the requirements of ASTM C666-97 [11]. However, there are few systematic studies on the effects of the parent concrete used on the frost resistance of RAC [8]. The mesostructure of RAC is far more complex than that of conventional concrete, owing to the presence of the adhering mortar on the surfaces of the RCA particles [12,13]. Several approaches have been proposed for enhancing the mesostructure of RAC, including handling the RCA directly [9,14–16] and modifying the mixing process used [13,17–18], while little research has been performed on the effects of the mixing method used on the FT resistance of RAC [19]. In addition, currently, macroscopic indices such as the mass loss and the relative dynamic modulus of elasticity (RDME) are used to assess the frost resistance of RAC [3–6,9], and analyses of its mesostructure after FT cycling are relatively rare [8]. The aim of this article is to study the effects of parent concrete and mixing method on the frost resistance of air-entrained recycled aggregate concrete (ARAC) on the macroscale and reveal the failure mechanism of ARAC during freezing and thawing cycles based on mesostructural analyses. First, a systematical study was carried out to elucidate the effects of the parent concrete used on the frost resistance of ARAC on the basis of four macroscale indices, namely, the mass loss, the RDME, the ultrasonic RDME (URDME), and the strength loss. Three mixing approaches were explored in this study. Finally, a possible mechanism for the failure of ARAC after FT cycling is proposed on the basis of the results of mesostructural analyses.
2.1.2. Concrete mixes ingredients The parent concrete samples were cured under standard conditions (20 ± 2 °C and 95% relative humidity) for 90 days before being crushed. They were subjected to a primary crushing process, using a jaw crusher. RCA samples with particles with diameters of 4.75–26.5 mm were selected after screening. The physical properties of the coarse aggregates are shown in Table 4 [20]. With an increase in the strength of the parent concrete used, the crushing index and water absorption rate of the RCA decreased, in agreement with the results of Padmini et al. [22]. However, the values of these parameters were greater than those of natural aggregate. In addition, the water-saturated surface dry density (WSSD) and residual mortar content of the RCA increased with the increase in the strength of the parent concrete, consistent with the study of Etxeberria et al. [23]. The higher crushing index in the case of the AC type were probably owing to the decreases in strength caused by the entrainment of air in the parent concrete. The particle size distribution for all the aggregates met the grading requirements, according to standard GB/T 25177-2010 [24]. The cementitious material used in this study was ordinary Table 3 Properties of freshly prepared and hardened parent concretes. Type
Slump (mm)
Air content (%)
Compressive strength on 28th day (MPa)
L M MR H AR
110 100 90 120 130
1.2 1.1 1.2 1.1 5.5
34.9 47.2 45.9 57.3 36.7
Table 4 Physical properties of coarse aggregates.
2. Experimental 2.1. Materials 2.1.1. Parent concrete Iron oxide red was added to air-entrained type concrete and partial non-airentrained type concrete as colorant, in order to be able to distinguish between the new mortar and the residual mortar during the subsequent mesostructural
a
ID
Source
WSSD (kg/m3)
Water absorption (%)
Crushing index (%)
RMCa (%)
CG LC MC NC HC AC
Natural gravel L M MR H AR
2772 2476 2515 2480 2539 2415
0.6 6.4 5.6 6.8 4.6 5.9
2.6 15.1 11.7 11.8 7.8 12.6
– 35.4 37.2 42.5 42.1 41.7
Residual mortar content, determined by a previously reported method [21].
Table 1 Effects of colorant on properties of parent mortar. Type
a b
Composition of parent mortar Cement (kg)
Water (kg)
Sand (kg)
AEa (‰)
Cb (%)
Air content (%)
Compressive strength on 28th day (MPa)
Non-air-entrained
Noncolored Colored
1 1
0.45 0.45
1.314 1.314
– –
– 5
1.5 1.7
31.1 30.0
Air-entrained
Noncolored Colored
1 1
0.45 0.45
1.314 1.314
0.05 0.05
– 5
7.9 8.4
23.5 24.0
Air entraining agent (alpha olefin sulfonate, permillage for cement). Colorant (percentage of cement weight, added together with cement).
Table 2 Approaches used to mix the parent concrete (1 m3). ID
w/c
Cement (kg)
Water (kg)
Sand (kg)
Natural coarse aggregate (kg)
WRAa (%)
Low strength Moderate strength Moderate strength, colored High strength
L M MR H
0.60 0.45 0.45 0.30
342 456 456 520
205 205 205 156
685 599 599 629
1118 1112 1112 1169
7.8
Moderate strength, colored
AR
0.45
456
205
599
1112
Type Non-air-entrained
Air-entrained a
Water-reducing agent (naphthalene sulfonate, percentage of cement weight).
C (%)
AE (‰)
5 5
0.05
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2.2. Freezing and thawing test
100 mm 100 mm 400 mm were prepared. The cubic specimens were used to test the compressive strength and splitting tensile strength on the 28th day. Three of the prisms were used for the FT cycling tests, while the others were used to determine the strength loss. Three additional prisms of the RC-NC-N and RC-AC-N samples were produced for the mesostructural analyses. Table 6 lists the properties of the freshly prepared and hardened concrete samples.
2.2.1. Mixture proportion The RCA was presoaked for 24 h to achieve the saturated-surface-dry condition to take into account its high water absorption before being mixed [25]. Details of the mixing schemes used are listed in Table 5. The ID for each set of specimens consisted of three parts: the type of concrete used (NC refers to conventional concrete while RC refers to ARAC), the type of coarse aggregate used (CG, LC, MC, HC, NC, AC and NAC; NAC refers to a mixture of NC and AC in a blending ratio of 1:1), and the mixing approach used (N, E, and T represent the normal mixing method, the cement paste encapsulating aggregate method, and the two-stage mixing method, respectively). The three mixing methods are also described in Fig. 1. Twelve cubes with dimensions of 100 mm 100 mm 100 mm and six prisms with dimensions of
2.2.2. Testing procedure The testing procedure used in this study for evaluating the FT resistances of the various samples, which involved rapid freezing and thawing for 250 cycles, was in accordance with the GB/T 50082-2009 standard [26]. The temperature of the concrete specimens was controlled with a thermocouple embedded in the centers of the specimens. During each FT cycle, the temperature was decreased from 5 to 20 °C and then increased back to 5 °C within 3 h. After 25 cycles, the specimens were taken out of the testing device, and their masses, transverse frequencies, and ultrasonic wave propagation times were measured. Three prisms of each type were tested, and the average values were used for all the parameters. The axial
Portland cement with a standard compressive strength of 42.5 MPa after 28 days. The fine aggregate used was natural sand with a fitness modulus of 2.68 and a silt content of 3.5%. Tap water was used for mixing.
Table 5 Approaches used for mixing the concrete mixtures. ID
Type of coarse aggregate
Cement (kg)
Water (kg)
Sand (kg)
Coarse aggregate (kg)
AE (‰)
CC-CG-N RC-LC-N RC-MC-N RC-MC-E RC-MC-T RC-HC-N RC-NC-N RC-AC-N RC-NAC-N
CG LC MC MC MC HC NC AC NC + AC
456 456 456 456 456 456 456 456 456
205 205 205 205 205 205 205 205 205
599 599 599 599 599 599 599 599 599
1112 1112 1112 1112 1112 1112 1112 1112 556 + 556
0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05
(a) Normal mixing method
(b) Cement paste encapsulating aggregate method
(c) Two-stage mixing method Fig. 1. The three mixing methods used.
Table 6 Properties of the freshly prepared and hardened concrete samples. ID
Slump (mm)
Air content (%)
Compressive strength on 28th day (MPa)
Splitting tensile strength on 28th day (MPa)
CC-CG-N RC-LC-N RC-MC-N RC-MC-E RC-MC-T RC-HC-N RC-NC-N RC-AC-N RC-NAC-N
130 100 110 100 100 110 100 120 100
5.4 5.3 5.3 5.0 5.5 5.3 5.5 5.2 5.4
34.9 28.4 30.9 32.9 33.3 32.3 29.3 28.6 28.3
2.56 2.04 2.08 2.14 2.29 2.79 2.21 2.19 2.14
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Fig. 3. Typical test slice before the start of FT cycling.
Fig. 2. Locations at which mesostructures of the test slice were made (unit: mm).
Table 7 Procedure used for polishing the test slices. Stage
Abrasive material used
1 2
Water Emery with a mesh 120 (124 lm) Emery with a mesh 240 (59 lm) Emery with a mesh 600 (24 lm) Emery with a mesh 1100 (13 lm)
3 4 5
Time (min)
Pressure (N)
Rotation rate (r/min)
size of
5 5
10 10
120 120
size of
10
10
120
size of
15
10
120
size of
30
10
120
compressive strengths of the specimens were determined after 250 FT cycles, and the results were compared with those for control samples cured under standard conditions. Fig. 4. Observation fields (unit: mm). 2.3. Mesostructural analysis 2.3.1. Slice preparation To analyze the extent of damage at the mesostructural level, slices of the RCNC-N and RC-AC-N samples were taken before FT cycling and after 150 and 250 FT cycles (Fig. 2). The slices were polished in order to eliminate the adverse effects of surface scratches and the overlapping mortar. The details of the polishing procedure are listed in Table 7. The slices and the abrasive wheel used for polishing were cleaned ultrasonically, in order to remove the debris and the residual emery, at the end of each stage. After being polished, the slices were placed in an oven at 40 °C for 24 h. A photograph of a typical polished slice taken before the start of FT cycling is shown in Fig. 3. 2.3.2. Testing process The concrete slices were observed under a digital microscope with a visual field of 1.96 mm 1.96 mm. An annular section with a width of 1.96 mm and at a distance of approximately 20 mm from the edge of the slice (Fig. 4) was divided into 116 observation fields. The typical mesostructure of ARAC is shown in Fig. 5; the original natural aggregate, the new mortar, the adhering mortar, and three different interfacial transition zones (ITZs) [8] can be seen in the image. To assess the extent of damage incurred at the mesostructure level after FT cycling, the areas of the new and adhering mortars, the lengths of the ITZs, and the locations and lengths of the cracks formed were determined; this was done using the image processing software bundled with the microscope. The crack density and the interface cracking ratio [8] were the two indices used to quantify the extent of damage of the mortars and ITZs, respectively, after FT cycling. The crack density can be calculated as follows:
DM ¼ ðAcr;M =Atot;M Þ 100
ð1Þ
where DM (%) is the crack density, Acr,M (mm2) is the total area of the cracks within the new or adhering mortar, and Atot,M (mm2) is the total area of the constituent corresponding to 116 observation fields. The width of a crack is much smaller than its length. Thus, measurements of the width are susceptible to statistical errors. Hence, the crack length was used instead of Acr,M and a newly defined parameter crack density, D0 M (%), was calculated:
D0M ¼ Lcr;M =Atot;M
ð2Þ
where Lcr,M (lm) is the total crack length across the new or adhering mortar within 116 observation fields. The interface cracking ratio can be defined as follows:
DITZ ¼ ðLcr;ITZ =Ltot;ITZ Þ 100
ð3Þ
where DITZ (%) is the interface cracking ratio, Lcr,ITZ (mm) is the total crack length across an ITZ, and Ltot,ITZ (mm) is the total length of each ITZ within 116 observation fields.
3. Results and discussion 3.1. Freezing and thawing test The parameters mass loss, RDME, URDME, and strength loss were employed to assess the frost resistances of the concrete samples on the macroscale.
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1—Original natural aggregate
2—Adhering mortar
3—New mortar
4—ITZ between the adhering
5—ITZ between the new mortar
6—ITZ between the adhering
mortar and the ONA (A&ONA)
and the ONA (N&ONA)
mortar and the new mortar
(ONA)
(A&N) Fig. 5. The mesostructure of ARAC.
Fig. 6. Mass loss versus number of FT cycles.
3.1.1. Mass loss The surface mortar went from being compact to porous after FT cycling. This was because of a decrease in its mass. This loss in mass is indicative of the spalling of the surface mortar and was calculated as follows:
Fig. 7. RDME versus the number of FT cycles.
DW n ¼ ð1 W n =W 0 Þ 100
ð4Þ
where DWn (%) is the mass loss after n FT cycles, W0 (kg) is the mass of the specimen at the beginning of the test, and Wn (kg) is the mass of the specimen after n FT cycles.
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Fig. 8. URDME versus the number of FT cycles.
The mass loss values for all the samples are shown in Fig. 6. The mass loss in the case of the partial types was negative in the early stage of the test; this was because water penetrated the inner cracks of these concrete samples after FT cycling. As the number of cycles was increased, the weights of all the specimens increased gradually, owing to the spalling of the surface paste. The mass loss values for all the samples were lower than 1% and met the requirements of GB/T 50082-2009 [26]. Finally, it was found that the properties of the parent concrete and the mixing approach used had no effects on the mass loss values of the ARAC specimens.
Fig. 10. Crack density versus number of FT cycles.
3.1.2. Relative dynamic modulus of elasticity The RDME is an indicator of the extent of deterioration of concrete after FT cycling, as determined by the change in its fundamental transverse frequency, and can be expressed as follows: 2
2
Pn ¼ ðf n =f 0 Þ 100
ð5Þ
where Pn (%) is the RDME after n FT cycles, f0 (Hz) is the fundamental transverse frequency before the start of FT cycling, and fn (Hz) is the fundamental transverse frequency after n FT cycles. The changes in the RDME values of all the samples are shown in Fig. 7. With an increase in the number of FT cycles, the RDME values at the end of test for the RC-LC-N and RC-MC-N samples decreased rapidly, while that of RC-HC-N demonstrated a small
Fig. 11. Interface cracking ratio versus the number of FT cycles.
Fig. 9. Strength losses of the various samples.
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decline comparable to the CC-CG-N. It was thus concluded that the frost resistance of ARAC can be improved significantly by using RCA derived from a high-strength parent concrete. The RDME of RC-AC-N was very close to that of CC-CG-N, indicating that the former sample underwent only minor damage. The RDME of RC-NAC-N was slightly lower than of RC-AC-N, but apparently higher than that of RC-NC-N. Thus, RCA obtained from air-entrained concrete or by blending air-entrained and non-air-entrained concretes (the proportion of RCA obtained from air-entrained concrete was not less than 50% in this study) can be used to prepare ARAC with good frost resistance. The RDME curves corresponding to the three mixing methods were very similar; this indicated that the mixing approach used had little effect on the frost resistance of ARAC.
mortar, owing to the crushing process. In the case of RC-AC-N, the initial crack density of the adhering mortar was larger than that in the case of RC-NC-N; this was probably because the lower strength of the air-entrained adhering mortar facilitated the development of cracks during the crushing process, in contrast to the case for the non-air-entrained mortar. The crack density of the adhering mortar in the case of RC-NC-N after 150 FT cycles was more than five times of the initial value and subsequently increased to six times the initial value, indicating that this sample underwent severe damage. The adhering mortar of RC-AC-N exhibited good resistance during FT cycling, with the final crack density being almost similar to the initial one. The crack density of the new mortar after the 250 FT cycles for RC-NC-N was slightly higher than that of the new mortar for RC-AC-N.
3.1.3. Ultrasonic relative dynamic modulus of elasticity The URDME is indicative of the damage incurred by concrete after FT cycling and is determined by measuring the propagation time of an ultrasonic wave along the long side of the concrete test specimen. The URDME value was calculated as follows:
3.2.2. Interface cracking ratio The interface cracking ratios of RC-NC-N and RC-AC-N are shown in Fig. 11. As was the case with the mortar, a few cracks existed across the ITZs before the FT cycling test. The interface cracking ratio of the ITZ between A and N for both samples remained less than 5%; this indicated that this ITZ was not the weak link during the FT cycles. The interface cracking ratio of the ITZ between A and ONA for RC-NC-N was four times higher than
sn ¼ ðt20 =t2n Þ 100
ð6Þ
where sn (%) is the URDME after n FT cycles, t0 (ls) is the initial propagation time of an ultrasonic wave, and tn (ls) is its propagation time after n FT cycles. Fig. 8 shows the change in the URDME value for all the samples. The RDME and URDME curves essentially led to the same conclusion regarding the frost resistance of RAC and further confirmed each other’s validity. 3.1.4. Strength loss With an increase in the number of FT cycles, the evolution of the cracks existing in the paste and the ITZs of the concrete samples led to the paste becoming loose and decreased the strength of the bond between the paste and the aggregate. This caused the strength of the concrete samples to decrease. This loss in strength (or strength loss), which is a measure of the extent of the damage caused by FT cycling, was calculated by comparing the axial compressive strength of the test specimen after the FT test with that of a control specimen cured under standard conditions.
Df c ¼ ð1 f cn =f c0 Þ 100
ð7Þ
where Dfc (%) is the strength loss, fcn (MPa) is the axial compressive strength of the test specimen after the completion of the FT test, and fc0 (MPa) is the axial compressive strength of a control sample cured under standard conditions. The strength losses of all the specimens are shown in Fig. 9. The values for RC-LC-N and RC-MC-N were similar, while the decrease in strength of RC-HC-N was much smaller than the first two and similar to that of CC-CG-N. The strength loss of RC-NAC-N was slightly higher than RC-AC-N, but apparently lower than RC-NC-N. These results suggested that the decrease in the strength of ARAC can be reduced by using RCA obtained from high-strength or air-entrained parent concrete. Finally, it was found that the choice of mixing method had no effect on the strength loss of ARAC after FT cycling.
(a) RC-NC-N
3.2. Mesostructural analysis 3.2.1. Crack density The curves for the crack density versus the number of FT cycles for RC-NC-N and RC-AC-N are shown in Fig. 10. A few initial cracks existed within the mortar before the start of the FT test, owing to the self-shrinkage of the concrete samples. The initial crack density of the adhering mortar was slightly higher than that of the new
(b) RC-AC-N Fig. 12. Increases in the crack density and interface cracking ratio.
K. Liu et al. / Construction and Building Materials 106 (2016) 264–273
the initial value; in contrast, it was only slightly higher in the case of RC-AC-N. Finally, the interface cracking ratio of the ITZ between N and ONA for RC-NC-N was slightly higher than that for RC-AC-N.
3.2.3. Failure mechanism The increases in the crack density and interface cracking ratio of the ITZs with the number of cycles are shown in Fig. 12. The adhering mortar of RC-NC-N fractured severely in the early stage of the FT cycling test (between 0 and 150 cycles), accelerating the development of cracks in the ITZ between A and ONA late during the test (from 150 to 250 cycles). This is what led to the failure of the ARAC (Fig. 13). In the case of the ARAC prepared using RCA obtained from an air-entrained concrete, the FT damage of the adhering mortar was not as extensive; thus, the weak link was the new mortar (Fig. 12(b)). Compared to the adhering mortar in RC-NC-N, the extent of degradation of the new mortar in RC-AC-N was much lower, owing to the entrainment of air; this helped RAC exhibit good frost resistance. Furthermore, the pore structure of concrete, which is its primary feature at the mesoscale, was found to have a marked impact on its FT resistance [27]. The pore structure of the adhering mortar in the RCA obtained from an air-entrained concrete was similar to that of the air-entrained new mortar (Fig. 14 (a) and (b)). Its structure was dominated by tiny, closed holes, which helped prevent the formation of ice and relieve the ice pressure, thus decreasing the extent of FT damage of the mortar. The adhering mortar in the RCA obtained from a non-air-entrained concrete had fewer pores, which were mainly large-diameter one (i.e., harmful pores) [28]. As a result, this mortar became the weak link during FT cycling.
271
4. Conclusions We systematically investigated the effects of the parent concrete and mixing approach used on the frost resistance of ARAC. Based on the obtained results, the following conclusions can be drawn. (1) With an increase in the strength of the parent concrete from which the RCA was obtained, the water absorption and crushing index of the RCA decreased, while the WSSD and residual mortar content increased. Further, the WSSD of the RCA was lower and its crushing index was higher owing to the entrainment of air in the parent concrete. (2) The cement paste encapsulating aggregate method and the two-stage mixing method result in improvements in the mechanical properties of ARAC but have no significant effect on its FT resistance. (3) The frost resistance of ARAC is closely related to the RCA used, while the properties of RCA depend primarily on its parent concrete. The ARAC sample prepared using RCA derived from a parent concrete with a high frost resistance, such as a high-strength concrete or an air-entrained concrete, exhibited a high FT resistance as well as almost the same FT durability as that of conventional air-entrained concrete. On the other hand, the ARAC sample prepared using RCA obtained from a non-air-entrained concrete exhibited poor frost resistance. (4) The FT cycling process severely damaged the adhering mortar in the early stage of testing in the case of the ARAC sample produced using RCA obtained from a non-air-entrained
1—Crack in adhering mortar
2—Crack in ITZ between A and ONA
3—Crack in ITZ between N and ONA
4—Crack in new mortar
Fig. 13. The damage to the mesostructure of RC-NC-N after FT cycling.
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(b) Air-entrained new mortar.
(a) Air-entrained adhering mortar.
(c) Non-air-entrained adhering mortar Fig. 14. Typical pore structures of mortar.
concrete; this aggravated the cracks formed across the ITZ between A and ONA, eventually leading to the FT failure of the ARAC. Finally, in contrast to the air-entrained new mortar, the adhering mortar of the RCA obtained from an air-entrained concrete was no longer the weak link during FT cycling. As a result, the corresponding ARAC sample exhibited good frost resistance. The research presents a systematic study of the frost resistance of ARAC, but under the single action of FT cycles. Further work can be undertaken to study the damage and deterioration of ARAC subjected to the coupling effect of FT and other adverse factors, such as load, carbonation, sulfate attack, etc.
Acknowledgement The authors wish to acknowledge the support provided by the National Science Foundation of China (51278151).
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K. Liu et al. / Construction and Building Materials 106 (2016) 264–273 [3] R.M. Salem, E.G. Burdette, Role of chemical and mineral admixtures on the physical properties and frost-resistance of recycled aggregate concrete, ACI Mater. J. 95 (5) (1998) 558–563. [4] R.M. Salem, E.G. Burdette, N.M. Jackson, Resistance to freezing and thawing of recycled aggregate concrete, ACI Mater. J. 100 (3) (2003) 216–221. [5] R. Zaharieva, F. Buyle-Bodin, E. Wirquin, Frost resistance of recycled aggregate concrete, Cem. Concr. Res. 34 (10) (2004) 1927–1932. [6] M.B. de Oliveira, E. Vazquez, The influence of retained moisture in aggregates from recycling on the properties of new hardened concrete, Waste Manage. 16 (1) (1996) 113–117. [7] A. Ajdukiewicz, K. Alina, Influence of recycled aggregates on mechanical properties of HS/HPC, Cem. Concr. Compos. 24 (2) (2002) 269–279. [8] A. Gokce, S. Nagataki, T. Saeki, M. Hisada, Freezing and thawing resistance of air-entrained concrete incorporating recycled coarse aggregate: the role of air content in demolished concrete, Cem. Concr. Res. 34 (5) (2004) 799–806. [9] M. Kawamura, K. Torii, Reuse of recycled concrete aggregate for pavement, in: Proceedings of the 2nd International RILEM Symposium on Demolition and Reuse of Concrete and Masonry, Tokyo, Japan, 1988, 7–11p. [10] A. Abbas, G. Fathifazl, O.B. Isgor, A.G. Razaqpur, B. Fournier, S. Foo, Durability of recycled aggregate concrete designed with equivalent mortar volume method, Cem. Concr. Compos. 31 (8) (2009) 555–563. [11] ASTM, Concrete and Concrete Aggregates. Annual Book of American Society for Testing and Materials, vol. 4.02, ASTM, Philadelphia, 2003. [12] N. Otsuki, S. Miyazato, W. Yodsudjai, Influence of recycled aggregate on interfacial transition zone, strength, chloride penetration and carbonation of concrete, J. Mater. Civ. Eng. 15 (5) (2003) 443–451. [13] V.W.Y. Tam, X.F. Gao, C.M. Tam, Microstructural analysis of recycled aggregate concrete produced from two-stage mixing approach, Cem. Concr. Res. 35 (6) (2005) 1195–1203. [14] A. Katz, Treatments for the improvement of recycled aggregate, J. Mater. Civ. Eng. 16 (6) (2004) 597–603. [15] J. Li, H. Xiao, Y. Zhou, Influence of coating recycled aggregate surface with pozzolanic powder on properties of recycled aggregate concrete, Constr. Build. Mater. 23 (3) (2009) 1287–1291. [16] W.G. Chai, W.L. Li, W.X. Gao, Experimental study on frost resistance durability of concrete with intensified recycled aggregates, Adv. Mater. Res. 280 (2011) 152–158.
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[17] D. Kong, T. Lei, J. Zheng, C. Ma, J. Jiang, J. Jiang, Effect and mechanism of surface-coating pozzalanics materials around aggregate on properties and ITZ microstructure of recycled aggregate concrete, Constr. Build. Mater. 24 (5) (2010) 701–708. [18] Y.W. Chen, W.H. Pan, X.Y. Sun, Performance study of pretreated concrete with recycled aggregate, Concrete 01 (2012) 81–83 (in Chinese). [19] E. Güneyisi, M. Gesog˘lu, Z. Algın, H. Yazıcı, Effect of surface treatment methods on the properties of self-compacting concrete with recycled aggregates, Constr. Build. Mater. 64 (2014) 172–183. [20] The Industry Standards Compilation Group of the People’s Republic of China, JGJ 52-2006 Standard for Technical Requirements and Test Method of Sand and Crushed Stone (or Gravel) for Ordinary Concrete, China Building Industry Press, Beijing, 2006 (in Chinese). [21] A. Abbas, G. Fathifazl, B. Fournier, O.B. Isgor, R. Zavadil, A.G. Razaqpur, et al., Quantification of the residual mortar content in recycled concrete aggregates by image analysis, Mater. Charact. 60 (7) (2009) 716–728. [22] A.K. Padmini, K. Ramamurthy, M.S. Mathews, Influence of parent concrete on the properties of recycled aggregate concrete, Constr. Build. Mater. 23 (2) (2009) 829–836. [23] M. Etxeberria, E. Vázquez, A. Marí, M. Barra, Influence of amount of recycled coarse aggregates and production process on properties of recycled aggregate concrete, Cem. Concr. Res. 37 (5) (2007) 735–742. [24] The National Standards Compilation Group of the People’s Republic of China, GB/T 25177-2010 Recycled Coarse Aggregate for Concrete, China Standards Press, Beijing, 2010 (in Chinese). [25] C.S. Poon, Z.H. Shui, L. Lam, H. Fok, S.C. Kou, Influence of moisture states of natural and recycled aggregates on the slump and compressive strength of concrete, Cem. Concr. Res. 34 (1) (2004) 31–36. [26] The National Standards Compilation Group of the People’s Republic of China, GB/T 50082-2009 Standard for Test Methods of Long-term Performance and Durability of Ordinary Concrete, China Standards Press, Beijing, 2009 (in Chinese). [27] Y.S. Deshpande, J.E. Hiller, Pore characterization of manufactured aggregates: recycled concrete aggregates and lightweight aggregates, Mater. Struct. 45 (1– 2) (2012) 67–79. [28] H.Z. Lian, L. Tong, E.Y. Chen, Research Foundation of Building Materials’ Physical Phase, Tsinghua University Press, Beijing, 1996 (in Chinese).
Contents lists available at ScienceDirect
Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
Effects of parent concrete and mixing method on the resistance to freezing and thawing of air-entrained recycled aggregate concrete Kaihua Liu a,b, Jiachuan Yan a,b, Qiong Hu a,b, Yao Sun c, Chaoying Zou a,b,⇑ a
Key Lab of Structures Dynamic Behavior and Control of the Ministry of Education (Harbin Institute of Technology), Harbin 150090, China School of Civil Engineering, Harbin Institute of Technology, Heilongjiang, Harbin 150090, China c Tianjin University Research Institute of Architectural Design and Urban Planning, Tianjin 300073, China b
h i g h l i g h t s We study the frost resistance of air-entrained recycled aggregate concrete (ARAC). The effects of four parent concretes and three mixing approaches are investigated. The failure mechanism of ARAC after freezing and thawing is established. This is done on the basis of a mesostructural analysis of the test samples.
a r t i c l e
i n f o
Article history: Received 6 July 2015 Received in revised form 5 October 2015 Accepted 10 December 2015
Keywords: Parent concrete Recycled aggregate concrete Mixing method Freezing and thawing Mesostructure
a b s t r a c t We elucidated the effects of the parent concrete and mixing approach used on the freezing/thawing resistance of air-entrained recycled aggregate concrete (ARAC). Three non-air-entrained concretes and one air-entrained concrete were used to prepare recycled coarse aggregate (RCA) samples. Three mixing approaches were also investigated. The frost resistances of the ARAC samples produced using an RCA obtained from the non-air-entrained concrete with high strength as well as the air-entrained one were close to that of conventional concrete. The mixing approach used had no effect on the frost resistance. A possible mechanism for the failure of ARAC is proposed. Ó 2015 Elsevier Ltd. All rights reserved.
1. Introduction With the rapid developments taking place in the construction industry, environmental problems, including the excessive use of natural resources and the increase in the amount of construction and demolition (C&D) waste, are becoming pressing issues. Approximately 2 billion tons of C&D waste, which amounted to 10% of the gross municipal waste, was generated in China in
Abbreviations: ARAC, air-entrained recycled aggregate concrete; RCA, recycled coarse aggregate; C&D, construction and demolition; RAC, recycled aggregate concrete; FT, freezing and thawing; RDME, relative dynamic modulus of elasticity; URDME, ultrasonic RDME; WRA, water-reducing agent; WSSD, water-saturated surface dry density; ITZ, interfacial transition zone. ⇑ Corresponding author at: School of Civil Engineering, Harbin Institute of Technology, Heilongjiang, Harbin 150090, China. E-mail addresses: [email protected] (K. Liu), [email protected] (J. Yan), [email protected] (Q. Hu), [email protected] (Y. Sun), [email protected] (C. Zou). http://dx.doi.org/10.1016/j.conbuildmat.2015.12.074 0950-0618/Ó 2015 Elsevier Ltd. All rights reserved.
2013 [1]. Recycled aggregate concrete (RAC) is attracting significant attention all over the world, as it is a very promising solution for dealing with C&D waste and saving construction materials [2]. In cold regions, hydraulic structures, such as bridges, dams, and ports, are susceptible to freeze/thaw (FT) cycling, which can lower their durability and result in structural deterioration. To expand the range of applications of RAC and explore the feasibility of using it in cold areas, it is essential to determine the frost resistance of RAC. Salem and Burdette [3] reported that the resistance of RAC to FT cycling of RAC can be improved by adding an air-entraining agent and higher amounts of fly ash to it. Salem et al. [4], Zaharieva et al. [5], and de Oliveira and Vazquez [6] found that the degree of water saturation is the critical factor determining the frost resistance of RAC. Ajdukiewicz and Alina [7] reported that a high-performance concrete prepared using RCA obtained from a high-performance parent concrete exhibited frost durability similar to or better than that of conventional concrete. Gokce et al. [8] found that the frost resistance of RAC produced using RCA
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analyses. The added iron oxide red had little effect on the parent mortar, which exhibited stabilities high enough to satisfy the test requirements (Table 1). Four parent concretes were designed for this study; their compositions are listed in Table 2. The properties of the freshly prepared and hardened parent concretes are listed in Table 3.
derived from an air-entrained concrete was similar to that of conventional concrete, while RAC containing RCA derived from non-air-entrained concrete exhibited poor frost resistance. Kawamura and Torii [9] reported that the FT resistance of RAC can be improved by reducing the amount of adhering mortar in the RCA used. Abbas et al. [10] reported that the equivalent mortar volume method can help increase the frost resistance of RAC to the extent that it complies with the requirements of ASTM C666-97 [11]. However, there are few systematic studies on the effects of the parent concrete used on the frost resistance of RAC [8]. The mesostructure of RAC is far more complex than that of conventional concrete, owing to the presence of the adhering mortar on the surfaces of the RCA particles [12,13]. Several approaches have been proposed for enhancing the mesostructure of RAC, including handling the RCA directly [9,14–16] and modifying the mixing process used [13,17–18], while little research has been performed on the effects of the mixing method used on the FT resistance of RAC [19]. In addition, currently, macroscopic indices such as the mass loss and the relative dynamic modulus of elasticity (RDME) are used to assess the frost resistance of RAC [3–6,9], and analyses of its mesostructure after FT cycling are relatively rare [8]. The aim of this article is to study the effects of parent concrete and mixing method on the frost resistance of air-entrained recycled aggregate concrete (ARAC) on the macroscale and reveal the failure mechanism of ARAC during freezing and thawing cycles based on mesostructural analyses. First, a systematical study was carried out to elucidate the effects of the parent concrete used on the frost resistance of ARAC on the basis of four macroscale indices, namely, the mass loss, the RDME, the ultrasonic RDME (URDME), and the strength loss. Three mixing approaches were explored in this study. Finally, a possible mechanism for the failure of ARAC after FT cycling is proposed on the basis of the results of mesostructural analyses.
2.1.2. Concrete mixes ingredients The parent concrete samples were cured under standard conditions (20 ± 2 °C and 95% relative humidity) for 90 days before being crushed. They were subjected to a primary crushing process, using a jaw crusher. RCA samples with particles with diameters of 4.75–26.5 mm were selected after screening. The physical properties of the coarse aggregates are shown in Table 4 [20]. With an increase in the strength of the parent concrete used, the crushing index and water absorption rate of the RCA decreased, in agreement with the results of Padmini et al. [22]. However, the values of these parameters were greater than those of natural aggregate. In addition, the water-saturated surface dry density (WSSD) and residual mortar content of the RCA increased with the increase in the strength of the parent concrete, consistent with the study of Etxeberria et al. [23]. The higher crushing index in the case of the AC type were probably owing to the decreases in strength caused by the entrainment of air in the parent concrete. The particle size distribution for all the aggregates met the grading requirements, according to standard GB/T 25177-2010 [24]. The cementitious material used in this study was ordinary Table 3 Properties of freshly prepared and hardened parent concretes. Type
Slump (mm)
Air content (%)
Compressive strength on 28th day (MPa)
L M MR H AR
110 100 90 120 130
1.2 1.1 1.2 1.1 5.5
34.9 47.2 45.9 57.3 36.7
Table 4 Physical properties of coarse aggregates.
2. Experimental 2.1. Materials 2.1.1. Parent concrete Iron oxide red was added to air-entrained type concrete and partial non-airentrained type concrete as colorant, in order to be able to distinguish between the new mortar and the residual mortar during the subsequent mesostructural
a
ID
Source
WSSD (kg/m3)
Water absorption (%)
Crushing index (%)
RMCa (%)
CG LC MC NC HC AC
Natural gravel L M MR H AR
2772 2476 2515 2480 2539 2415
0.6 6.4 5.6 6.8 4.6 5.9
2.6 15.1 11.7 11.8 7.8 12.6
– 35.4 37.2 42.5 42.1 41.7
Residual mortar content, determined by a previously reported method [21].
Table 1 Effects of colorant on properties of parent mortar. Type
a b
Composition of parent mortar Cement (kg)
Water (kg)
Sand (kg)
AEa (‰)
Cb (%)
Air content (%)
Compressive strength on 28th day (MPa)
Non-air-entrained
Noncolored Colored
1 1
0.45 0.45
1.314 1.314
– –
– 5
1.5 1.7
31.1 30.0
Air-entrained
Noncolored Colored
1 1
0.45 0.45
1.314 1.314
0.05 0.05
– 5
7.9 8.4
23.5 24.0
Air entraining agent (alpha olefin sulfonate, permillage for cement). Colorant (percentage of cement weight, added together with cement).
Table 2 Approaches used to mix the parent concrete (1 m3). ID
w/c
Cement (kg)
Water (kg)
Sand (kg)
Natural coarse aggregate (kg)
WRAa (%)
Low strength Moderate strength Moderate strength, colored High strength
L M MR H
0.60 0.45 0.45 0.30
342 456 456 520
205 205 205 156
685 599 599 629
1118 1112 1112 1169
7.8
Moderate strength, colored
AR
0.45
456
205
599
1112
Type Non-air-entrained
Air-entrained a
Water-reducing agent (naphthalene sulfonate, percentage of cement weight).
C (%)
AE (‰)
5 5
0.05
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2.2. Freezing and thawing test
100 mm 100 mm 400 mm were prepared. The cubic specimens were used to test the compressive strength and splitting tensile strength on the 28th day. Three of the prisms were used for the FT cycling tests, while the others were used to determine the strength loss. Three additional prisms of the RC-NC-N and RC-AC-N samples were produced for the mesostructural analyses. Table 6 lists the properties of the freshly prepared and hardened concrete samples.
2.2.1. Mixture proportion The RCA was presoaked for 24 h to achieve the saturated-surface-dry condition to take into account its high water absorption before being mixed [25]. Details of the mixing schemes used are listed in Table 5. The ID for each set of specimens consisted of three parts: the type of concrete used (NC refers to conventional concrete while RC refers to ARAC), the type of coarse aggregate used (CG, LC, MC, HC, NC, AC and NAC; NAC refers to a mixture of NC and AC in a blending ratio of 1:1), and the mixing approach used (N, E, and T represent the normal mixing method, the cement paste encapsulating aggregate method, and the two-stage mixing method, respectively). The three mixing methods are also described in Fig. 1. Twelve cubes with dimensions of 100 mm 100 mm 100 mm and six prisms with dimensions of
2.2.2. Testing procedure The testing procedure used in this study for evaluating the FT resistances of the various samples, which involved rapid freezing and thawing for 250 cycles, was in accordance with the GB/T 50082-2009 standard [26]. The temperature of the concrete specimens was controlled with a thermocouple embedded in the centers of the specimens. During each FT cycle, the temperature was decreased from 5 to 20 °C and then increased back to 5 °C within 3 h. After 25 cycles, the specimens were taken out of the testing device, and their masses, transverse frequencies, and ultrasonic wave propagation times were measured. Three prisms of each type were tested, and the average values were used for all the parameters. The axial
Portland cement with a standard compressive strength of 42.5 MPa after 28 days. The fine aggregate used was natural sand with a fitness modulus of 2.68 and a silt content of 3.5%. Tap water was used for mixing.
Table 5 Approaches used for mixing the concrete mixtures. ID
Type of coarse aggregate
Cement (kg)
Water (kg)
Sand (kg)
Coarse aggregate (kg)
AE (‰)
CC-CG-N RC-LC-N RC-MC-N RC-MC-E RC-MC-T RC-HC-N RC-NC-N RC-AC-N RC-NAC-N
CG LC MC MC MC HC NC AC NC + AC
456 456 456 456 456 456 456 456 456
205 205 205 205 205 205 205 205 205
599 599 599 599 599 599 599 599 599
1112 1112 1112 1112 1112 1112 1112 1112 556 + 556
0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05
(a) Normal mixing method
(b) Cement paste encapsulating aggregate method
(c) Two-stage mixing method Fig. 1. The three mixing methods used.
Table 6 Properties of the freshly prepared and hardened concrete samples. ID
Slump (mm)
Air content (%)
Compressive strength on 28th day (MPa)
Splitting tensile strength on 28th day (MPa)
CC-CG-N RC-LC-N RC-MC-N RC-MC-E RC-MC-T RC-HC-N RC-NC-N RC-AC-N RC-NAC-N
130 100 110 100 100 110 100 120 100
5.4 5.3 5.3 5.0 5.5 5.3 5.5 5.2 5.4
34.9 28.4 30.9 32.9 33.3 32.3 29.3 28.6 28.3
2.56 2.04 2.08 2.14 2.29 2.79 2.21 2.19 2.14
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267
Fig. 3. Typical test slice before the start of FT cycling.
Fig. 2. Locations at which mesostructures of the test slice were made (unit: mm).
Table 7 Procedure used for polishing the test slices. Stage
Abrasive material used
1 2
Water Emery with a mesh 120 (124 lm) Emery with a mesh 240 (59 lm) Emery with a mesh 600 (24 lm) Emery with a mesh 1100 (13 lm)
3 4 5
Time (min)
Pressure (N)
Rotation rate (r/min)
size of
5 5
10 10
120 120
size of
10
10
120
size of
15
10
120
size of
30
10
120
compressive strengths of the specimens were determined after 250 FT cycles, and the results were compared with those for control samples cured under standard conditions. Fig. 4. Observation fields (unit: mm). 2.3. Mesostructural analysis 2.3.1. Slice preparation To analyze the extent of damage at the mesostructural level, slices of the RCNC-N and RC-AC-N samples were taken before FT cycling and after 150 and 250 FT cycles (Fig. 2). The slices were polished in order to eliminate the adverse effects of surface scratches and the overlapping mortar. The details of the polishing procedure are listed in Table 7. The slices and the abrasive wheel used for polishing were cleaned ultrasonically, in order to remove the debris and the residual emery, at the end of each stage. After being polished, the slices were placed in an oven at 40 °C for 24 h. A photograph of a typical polished slice taken before the start of FT cycling is shown in Fig. 3. 2.3.2. Testing process The concrete slices were observed under a digital microscope with a visual field of 1.96 mm 1.96 mm. An annular section with a width of 1.96 mm and at a distance of approximately 20 mm from the edge of the slice (Fig. 4) was divided into 116 observation fields. The typical mesostructure of ARAC is shown in Fig. 5; the original natural aggregate, the new mortar, the adhering mortar, and three different interfacial transition zones (ITZs) [8] can be seen in the image. To assess the extent of damage incurred at the mesostructure level after FT cycling, the areas of the new and adhering mortars, the lengths of the ITZs, and the locations and lengths of the cracks formed were determined; this was done using the image processing software bundled with the microscope. The crack density and the interface cracking ratio [8] were the two indices used to quantify the extent of damage of the mortars and ITZs, respectively, after FT cycling. The crack density can be calculated as follows:
DM ¼ ðAcr;M =Atot;M Þ 100
ð1Þ
where DM (%) is the crack density, Acr,M (mm2) is the total area of the cracks within the new or adhering mortar, and Atot,M (mm2) is the total area of the constituent corresponding to 116 observation fields. The width of a crack is much smaller than its length. Thus, measurements of the width are susceptible to statistical errors. Hence, the crack length was used instead of Acr,M and a newly defined parameter crack density, D0 M (%), was calculated:
D0M ¼ Lcr;M =Atot;M
ð2Þ
where Lcr,M (lm) is the total crack length across the new or adhering mortar within 116 observation fields. The interface cracking ratio can be defined as follows:
DITZ ¼ ðLcr;ITZ =Ltot;ITZ Þ 100
ð3Þ
where DITZ (%) is the interface cracking ratio, Lcr,ITZ (mm) is the total crack length across an ITZ, and Ltot,ITZ (mm) is the total length of each ITZ within 116 observation fields.
3. Results and discussion 3.1. Freezing and thawing test The parameters mass loss, RDME, URDME, and strength loss were employed to assess the frost resistances of the concrete samples on the macroscale.
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1—Original natural aggregate
2—Adhering mortar
3—New mortar
4—ITZ between the adhering
5—ITZ between the new mortar
6—ITZ between the adhering
mortar and the ONA (A&ONA)
and the ONA (N&ONA)
mortar and the new mortar
(ONA)
(A&N) Fig. 5. The mesostructure of ARAC.
Fig. 6. Mass loss versus number of FT cycles.
3.1.1. Mass loss The surface mortar went from being compact to porous after FT cycling. This was because of a decrease in its mass. This loss in mass is indicative of the spalling of the surface mortar and was calculated as follows:
Fig. 7. RDME versus the number of FT cycles.
DW n ¼ ð1 W n =W 0 Þ 100
ð4Þ
where DWn (%) is the mass loss after n FT cycles, W0 (kg) is the mass of the specimen at the beginning of the test, and Wn (kg) is the mass of the specimen after n FT cycles.
K. Liu et al. / Construction and Building Materials 106 (2016) 264–273
Fig. 8. URDME versus the number of FT cycles.
The mass loss values for all the samples are shown in Fig. 6. The mass loss in the case of the partial types was negative in the early stage of the test; this was because water penetrated the inner cracks of these concrete samples after FT cycling. As the number of cycles was increased, the weights of all the specimens increased gradually, owing to the spalling of the surface paste. The mass loss values for all the samples were lower than 1% and met the requirements of GB/T 50082-2009 [26]. Finally, it was found that the properties of the parent concrete and the mixing approach used had no effects on the mass loss values of the ARAC specimens.
Fig. 10. Crack density versus number of FT cycles.
3.1.2. Relative dynamic modulus of elasticity The RDME is an indicator of the extent of deterioration of concrete after FT cycling, as determined by the change in its fundamental transverse frequency, and can be expressed as follows: 2
2
Pn ¼ ðf n =f 0 Þ 100
ð5Þ
where Pn (%) is the RDME after n FT cycles, f0 (Hz) is the fundamental transverse frequency before the start of FT cycling, and fn (Hz) is the fundamental transverse frequency after n FT cycles. The changes in the RDME values of all the samples are shown in Fig. 7. With an increase in the number of FT cycles, the RDME values at the end of test for the RC-LC-N and RC-MC-N samples decreased rapidly, while that of RC-HC-N demonstrated a small
Fig. 11. Interface cracking ratio versus the number of FT cycles.
Fig. 9. Strength losses of the various samples.
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decline comparable to the CC-CG-N. It was thus concluded that the frost resistance of ARAC can be improved significantly by using RCA derived from a high-strength parent concrete. The RDME of RC-AC-N was very close to that of CC-CG-N, indicating that the former sample underwent only minor damage. The RDME of RC-NAC-N was slightly lower than of RC-AC-N, but apparently higher than that of RC-NC-N. Thus, RCA obtained from air-entrained concrete or by blending air-entrained and non-air-entrained concretes (the proportion of RCA obtained from air-entrained concrete was not less than 50% in this study) can be used to prepare ARAC with good frost resistance. The RDME curves corresponding to the three mixing methods were very similar; this indicated that the mixing approach used had little effect on the frost resistance of ARAC.
mortar, owing to the crushing process. In the case of RC-AC-N, the initial crack density of the adhering mortar was larger than that in the case of RC-NC-N; this was probably because the lower strength of the air-entrained adhering mortar facilitated the development of cracks during the crushing process, in contrast to the case for the non-air-entrained mortar. The crack density of the adhering mortar in the case of RC-NC-N after 150 FT cycles was more than five times of the initial value and subsequently increased to six times the initial value, indicating that this sample underwent severe damage. The adhering mortar of RC-AC-N exhibited good resistance during FT cycling, with the final crack density being almost similar to the initial one. The crack density of the new mortar after the 250 FT cycles for RC-NC-N was slightly higher than that of the new mortar for RC-AC-N.
3.1.3. Ultrasonic relative dynamic modulus of elasticity The URDME is indicative of the damage incurred by concrete after FT cycling and is determined by measuring the propagation time of an ultrasonic wave along the long side of the concrete test specimen. The URDME value was calculated as follows:
3.2.2. Interface cracking ratio The interface cracking ratios of RC-NC-N and RC-AC-N are shown in Fig. 11. As was the case with the mortar, a few cracks existed across the ITZs before the FT cycling test. The interface cracking ratio of the ITZ between A and N for both samples remained less than 5%; this indicated that this ITZ was not the weak link during the FT cycles. The interface cracking ratio of the ITZ between A and ONA for RC-NC-N was four times higher than
sn ¼ ðt20 =t2n Þ 100
ð6Þ
where sn (%) is the URDME after n FT cycles, t0 (ls) is the initial propagation time of an ultrasonic wave, and tn (ls) is its propagation time after n FT cycles. Fig. 8 shows the change in the URDME value for all the samples. The RDME and URDME curves essentially led to the same conclusion regarding the frost resistance of RAC and further confirmed each other’s validity. 3.1.4. Strength loss With an increase in the number of FT cycles, the evolution of the cracks existing in the paste and the ITZs of the concrete samples led to the paste becoming loose and decreased the strength of the bond between the paste and the aggregate. This caused the strength of the concrete samples to decrease. This loss in strength (or strength loss), which is a measure of the extent of the damage caused by FT cycling, was calculated by comparing the axial compressive strength of the test specimen after the FT test with that of a control specimen cured under standard conditions.
Df c ¼ ð1 f cn =f c0 Þ 100
ð7Þ
where Dfc (%) is the strength loss, fcn (MPa) is the axial compressive strength of the test specimen after the completion of the FT test, and fc0 (MPa) is the axial compressive strength of a control sample cured under standard conditions. The strength losses of all the specimens are shown in Fig. 9. The values for RC-LC-N and RC-MC-N were similar, while the decrease in strength of RC-HC-N was much smaller than the first two and similar to that of CC-CG-N. The strength loss of RC-NAC-N was slightly higher than RC-AC-N, but apparently lower than RC-NC-N. These results suggested that the decrease in the strength of ARAC can be reduced by using RCA obtained from high-strength or air-entrained parent concrete. Finally, it was found that the choice of mixing method had no effect on the strength loss of ARAC after FT cycling.
(a) RC-NC-N
3.2. Mesostructural analysis 3.2.1. Crack density The curves for the crack density versus the number of FT cycles for RC-NC-N and RC-AC-N are shown in Fig. 10. A few initial cracks existed within the mortar before the start of the FT test, owing to the self-shrinkage of the concrete samples. The initial crack density of the adhering mortar was slightly higher than that of the new
(b) RC-AC-N Fig. 12. Increases in the crack density and interface cracking ratio.
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the initial value; in contrast, it was only slightly higher in the case of RC-AC-N. Finally, the interface cracking ratio of the ITZ between N and ONA for RC-NC-N was slightly higher than that for RC-AC-N.
3.2.3. Failure mechanism The increases in the crack density and interface cracking ratio of the ITZs with the number of cycles are shown in Fig. 12. The adhering mortar of RC-NC-N fractured severely in the early stage of the FT cycling test (between 0 and 150 cycles), accelerating the development of cracks in the ITZ between A and ONA late during the test (from 150 to 250 cycles). This is what led to the failure of the ARAC (Fig. 13). In the case of the ARAC prepared using RCA obtained from an air-entrained concrete, the FT damage of the adhering mortar was not as extensive; thus, the weak link was the new mortar (Fig. 12(b)). Compared to the adhering mortar in RC-NC-N, the extent of degradation of the new mortar in RC-AC-N was much lower, owing to the entrainment of air; this helped RAC exhibit good frost resistance. Furthermore, the pore structure of concrete, which is its primary feature at the mesoscale, was found to have a marked impact on its FT resistance [27]. The pore structure of the adhering mortar in the RCA obtained from an air-entrained concrete was similar to that of the air-entrained new mortar (Fig. 14 (a) and (b)). Its structure was dominated by tiny, closed holes, which helped prevent the formation of ice and relieve the ice pressure, thus decreasing the extent of FT damage of the mortar. The adhering mortar in the RCA obtained from a non-air-entrained concrete had fewer pores, which were mainly large-diameter one (i.e., harmful pores) [28]. As a result, this mortar became the weak link during FT cycling.
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4. Conclusions We systematically investigated the effects of the parent concrete and mixing approach used on the frost resistance of ARAC. Based on the obtained results, the following conclusions can be drawn. (1) With an increase in the strength of the parent concrete from which the RCA was obtained, the water absorption and crushing index of the RCA decreased, while the WSSD and residual mortar content increased. Further, the WSSD of the RCA was lower and its crushing index was higher owing to the entrainment of air in the parent concrete. (2) The cement paste encapsulating aggregate method and the two-stage mixing method result in improvements in the mechanical properties of ARAC but have no significant effect on its FT resistance. (3) The frost resistance of ARAC is closely related to the RCA used, while the properties of RCA depend primarily on its parent concrete. The ARAC sample prepared using RCA derived from a parent concrete with a high frost resistance, such as a high-strength concrete or an air-entrained concrete, exhibited a high FT resistance as well as almost the same FT durability as that of conventional air-entrained concrete. On the other hand, the ARAC sample prepared using RCA obtained from a non-air-entrained concrete exhibited poor frost resistance. (4) The FT cycling process severely damaged the adhering mortar in the early stage of testing in the case of the ARAC sample produced using RCA obtained from a non-air-entrained
1—Crack in adhering mortar
2—Crack in ITZ between A and ONA
3—Crack in ITZ between N and ONA
4—Crack in new mortar
Fig. 13. The damage to the mesostructure of RC-NC-N after FT cycling.
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(b) Air-entrained new mortar.
(a) Air-entrained adhering mortar.
(c) Non-air-entrained adhering mortar Fig. 14. Typical pore structures of mortar.
concrete; this aggravated the cracks formed across the ITZ between A and ONA, eventually leading to the FT failure of the ARAC. Finally, in contrast to the air-entrained new mortar, the adhering mortar of the RCA obtained from an air-entrained concrete was no longer the weak link during FT cycling. As a result, the corresponding ARAC sample exhibited good frost resistance. The research presents a systematic study of the frost resistance of ARAC, but under the single action of FT cycles. Further work can be undertaken to study the damage and deterioration of ARAC subjected to the coupling effect of FT and other adverse factors, such as load, carbonation, sulfate attack, etc.
Acknowledgement The authors wish to acknowledge the support provided by the National Science Foundation of China (51278151).
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