Influence of parent concrete on the properties of recycled aggregate concrete

Construction and Building Materials 23 (2009) 829–836

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Influence of parent concrete on the properties of recycled aggregate concrete A.K. Padmini, K. Ramamurthy *, M.S. Mathews Building Technology and Construction Management Division, Department of Civil Engineering, Indian Institute of Technology Madras, Chennai 600 036, India

a r t i c l e

i n f o

Article history: Received 12 March 2008 Accepted 25 March 2008 Available online 12 May 2008 Keywords: Concrete Recycled aggregate Mix proportioning Compressive strength Water–cement ratio

a b s t r a c t This paper discusses first the properties of recycled aggregates derived from parent concrete (PC) of three strengths, each of them made with three maximum sizes of aggregates. The relative physical and mechanical properties of fresh granite aggregate are discussed. Using these nine recycled aggregates, three strengths of recycled aggregate concrete (RAC) were made and studied. Typical relationship between water–cement ratio, compressive strength, aggregate-cement ratio and cement content have been formulated for RAC and compared with those of PC. RAC requires relatively lower water–cement ratio as compared to PC to achieve a particular compressive strength. The difference in strength between PC and RAC increases with strength of concrete. The relative evaluation of tensile and flexural strengths and modulus of elasticity has also been made. Ó 2008 Elsevier Ltd. All rights reserved.

1. Introduction As the rate of demolition is increasing day-by-day, it is essential to effectively reuse demolition waste in order to conserve the nonrenewable natural resources. Recycling of concrete demolition waste as coarse aggregate for new concrete would facilitate, its large-scale utilisation. A classification of earlier studies on recycled aggregate concrete (RAC) is presented in Table 1. Salient observations drawn from these studies [1–18,23–25] are (i) method of crushing of parent concrete has significant effect on recycled aggregate, (ii) particle shape of recycled aggregate is more irregular than natural aggregate and presents a coarser surface, (iii) RAC requires more water for the same workability than normal concrete [11,16], (iv) density, compressive strength and modulus of elasticity of RAC are relatively lower than that of parent concrete, and (v) for a given water–cement ratio, permeability, rate of carbonation and risk of reinforcement corrosion are higher [2]. Most of the studies on recycled aggregate concrete have adopted nominal mixes while some have given importance to partial replacement of recycled aggregate [8]. Direct use of recycled aggregates by many researchers lead to the adoption of different maximum size of aggregate in RAC as compared to the parent concrete. For a rational comparison, maximum size of aggregate in parent and RAC should be kept the same. The review further indicates that the characteristics of recycled aggregates produced from different strengths of parent concrete require a systematic investigation. This paper reports the properties of recycled aggregate con-

crete made with different maximum sizes of aggregates derived from different strengths of parent concrete. 2. Experimental programme 2.1. Parent concrete As a first step the physical and mechanical properties of three maximum sizes (10 mm = type-1, 20 mm = type-2 and 40 mm = type-3) of fresh crushed granite aggregates were determined (Table 2). Ordinary portland cement (OPC) having a 28day compressive strength of 45 MPa and river sand from a single source were used. Three strengths of parent concrete mixes were designed adopting Bureau of Indian Standards (BIS) method of mix proportioning [19] with each maximum size of fresh crushed granite aggregate. For each design strength of concrete, three workabilities (compacting factors 0.75, 0.85, and 0.95; equivalent slump of 25, 50, and 150 mm, respectively) were adopted (Table 3a). As expected, the strength results of parent concrete obtained for the same target mean strength with different workability by keeping the water–cement ratio constant are almost the same, and hence the average values are only reported. These results are used as benchmark for comparison with those of recycled aggregate concrete. The number and size of test specimen used for different tests are presented in Table 3b. 2.2. Recycled aggregate concrete

* Corresponding author. Tel.: +91 44 22574265; fax: +91 44 22574252. E-mail address: [email protected] (K. Ramamurthy). 0950-0618/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.conbuildmat.2008.03.006

Tested specimens of parent concrete of each mix were crushed in a Jaw crusher by suitably adjusting its opening size to match the maximum size of aggregate used in parent concrete and were

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A.K. Padmini et al. / Construction and Building Materials 23 (2009) 829–836

Table 1 Classification of studies on recycled aggregate concrete Author, Year (Reference no.)

Buck (1977) [9] Yannas (1977) [10] Nixon (1978) [1] Akhtaruzzaman and Hansat (1983) [24] Hansen (1985) [2] Sri Ravindrarajah et al. (1985, 1987) [13,14] Kumar et al. (1988) [4] Tadayoshi and Fujii (1988) [17] Topcu (1997) [15] O’Mahony, 1997 [23] Pimienta (1998) [5] Barra and Vazquez (1998) [16] Kikuchi et al. (1998) [6] Di Niro et al. (1998) [27] Knights (1998) [7] Stuart Park (1999)[3] Devenny and Khalaf (1999) [25]

Source

Type

C

CA

FA

p p

p

B

p p p p

Workability y

Compressive strength

Tensile strength

p p p

p p p p

p p p

p

p p

p p

p

p p

p p p

p

p

p p

p p p p

p p p p

p p p p

p p

p p

p p

p p

p p p

p

p

p p

Flexural strength

Shear strength

p p p

Bond strength Aggregatemortar

Reinforceconcrete

Drying shrinkage

Durability

p p p p

p p p

p

p

p

p p

p p

p p p

Modulus of elasticity

p

p

p p

p

p

p

p p

p

p

p

Note: C – concrete, B – brick, CA – coarse aggregate, FA – fine aggregate.

Table 2 Properties of fresh granite and recycled concrete aggregate Property

Fresh crushed granite aggregate

Physical properties Specific gravity Water absorption Bulk density kg/m3 Percentage

Loose Rodded Loose Rodded

Mechanical properties Crushing value (%) Impact value (%) Abrasion value (%)

Recycled concrete aggregate of maximum size

Maximum size of aggregate

10 mm Derived from parent concrete of compressive strength

20 mm Derived from parent concrete of compressive strength

40 mm Derived from parent concrete of compressive strength

10 mm

20 mm

40 mm

35 MPa

49 MPa

56 MPa

37 MPa

50 MPa

58 MPa

31 MPa

45 MPa

52 MPa

2.8 0.3 1408 1561 50 44

2.8 0.3 1462 1625 48 42

2.8 0.3 1406 1590 49 43

2.46 4.60 1338 1468 46 40

2.4 4.8 1327 1438 45 40

2.38 5.0 1324 1427 44 40

2.52 3.65 1432 1568 43 38

2.51 4.1 1421 1536 43 39

2.48 4.86 1394 1498 44 40

2.56 2.2 1341 1480 48 42

2.53 2.5 1334 1474 47 42

2.52 2.8 1329 1470 47 42

25 18 29

22 17 26

– – 26

32 38 48

30 32 46

30 31 46

26 25 38

25 24 35

23 21 33

– – 30

– – 29

– – 29

Table 3a Details of mixes used and the corresponding compressive strengths Target mean strength (MPa)

Water–cement ratio

PC-1 21

0.58

PC-2 34

0.43

PC-3 45

0.34

Workability (CF)

0.75 0.85 0.95 0.75 0.85 0.95 0.75 0.85 0.95

Mix proportion by weight and compressive strength with crushed granite aggregate of maximum size 10 mm

20 mm

40 mm

Mix

Compressive strength (MPa)

Mix

Compressive strength (MPa)

Mix

Compressive strength (MPa)

1:1.9:3.1 1:1.8:3.0 1:1.8:2.9 1:1.2:2.3 1:1.2:2.2 1:1.1:2.1 1:0.9:1.7 1:0.8:1.7 1:0.8:1.8

35

1:2.0:4.1 1:1.9:3.9 1:1.8:3.8 1:1.3:3.0 1:1.2:2.9 1:1.2:2.8 1:0.9:2.3 1:0.9:2.2 1:0.8:2.2

37

– 1:1.9:4.8 – – 1:1.3:3.7 – – 1:0.9:2.9 –

31

49

56

sieved. As the strength of parent concrete with same design strength with different workabilities were almost the same, recycled aggregates derived from a particular strength of parent con-

50

57

45

52

crete for each maximum size of coarse aggregate used were stacked together (nine different stock piles). As a next step, an attempt was made to design the same three target mean strengths of

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A.K. Padmini et al. / Construction and Building Materials 23 (2009) 829–836 Table 3b Specimen details for each mix Test

Number of specimens

Dimensions of test specimens for concrete with max. size of aggregate of 10 and 20 mm

40 mm

Compressive strength Split tensile strength Flexural strength Modulus of elasticity

54 6 6 6

Cube – 100 mm Cylinder – 100 mm diameter and 150 mm height Beam – 100 mm  100 mm  500 mm 150 mm Diameter and 300 mm height

Cube – 150 mm Cylinder – 150 mm diameter and 300 mm height Beam – 150 mm  150 mm  700 mm 150 mm Diameter and 300 mm height

concrete using each of the above nine types of recycled aggregates and three workability, utilising the same type of cement and fine aggregate (Table 4). The relatively higher water absorption characteristics of recycled aggregate necessitate careful adjustment in water content in order to obtain the desired workability of RAC and to eliminate variation in water–cement ratio in different mixes. Hence, trial workability tests were conducted on RAC by varying the quantity of additional water added (with different short-term water absorption of recycled aggregates), and it was observed that 10 min water absorption value of recycled aggregate satisfied the workability requirements of RAC. 3. Discussion of results 3.1. Properties of recycled aggregates The physical and mechanical properties of recycled aggregates are compared with those of fresh granite aggregate (FGA) in Table 2, and the salient aspects are discussed below. 3.2. Physical properties Recycled aggregate contains crushed and uncrushed parent aggregate coated with mortar and small pieces of hardened mortar. Hence, the specific gravity and bulk density are relatively lower for recycled aggregates (RA). Specific gravity reduces marginally with an increase in strength of parent concrete from which the RA is obtained. The major difference between RA and FGA is the higher water absorption of RA. The mortar phase has higher porosity than that of aggregate phase hence RA absorbs more water than FGA. During the crushing of lower strength parent concrete, as the bond between mortar and aggregate being weaker, most of the mortar phase gets separated from the aggregate surface. At the same time the mortar phase is getting crushed to finer particles (lower than 4.75 mm size), which gets removed during sieving. Hence, the quantity of mortar in the form of attached mortar in granite aggregate and pieces of mortar in recycled aggregate is relatively lower for aggregates produced from lower strength

parent concrete. As the strength of parent concrete increases, quantity of mortar adhering to the aggregates increases due to comparatively higher bond between aggregate and the mortar phase. In this case, the size and quantity of mortar pieces present as recycled coarse aggregate also increases. The presence of higher quantity of comparatively lower density parent mortar in recycled aggregate results in higher reduction in specific gravity. In the case of smaller sized recycled aggregate, the presence of parent mortar is more due to the higher surface area available for equal volume of aggregate, which results in higher reduction in specific gravity. The reduced specific gravity of recycled aggregate results in reduced quantity of coarse aggregate to be used in recycled aggregate concrete. The water absorption of recycled aggregate [20] was found to be significantly higher, which is due to: (i) the type of parent aggregate, (ii) strength of parent concrete, and (iii) the maximum size of aggregate used in parent concrete. It is seen that the percentage water absorption increases with an increase in strength of parent concrete from which the recycled aggregate have been derived, which is due to the higher quantity of attached mortar in recycled aggregate obtained from higher strength parent concrete. This attached mortar as well as mortar pieces are more porous compared to that of fresh crushed granite aggregate, which increases the water absorption capacity of recycled aggregate. It is seen that for recycled aggregate, water absorption increases with decrease in size of aggregate used in parent concrete. This is due to the higher surface area available for such aggregates to adhere parent mortar for equal volume of aggregates. 3.3. Mechanical properties [20] As observed by several researchers, the resistance against crushing, impact and abrasion of RA are relatively lower than FGA, due to the separation and crushing of porous mortar coating from RA during testing. For a given strength of parent concrete from which the RA is derived, the resistance against mechanical actions decreases with the reduction in maximum size of aggregate. This can be attributed to the relatively large surface area of smaller sized aggregates facilitating higher mortar coating, as compared to larger size aggregate. As per British Standards BS 882-1992 [21],

Table 4 Design variables for recycled aggregate concrete Designation of recycled aggregate

Strength of parent concrete from which RA is derived (MPa)

Combinations of recycled aggregate concrete cast (three workability for each mix) M15

M25

M35

RA10-1 RA10-2 RA10-3

35 49 56

RAC10-1-1 RAC10-2-1 RAC10-3-1

RAC10-1-2 RAC10-2-2 RAC10-3-2

RAC10-1-3 RAC10-2-3 RAC10-3-3

RA20-1 RA20-2 RA20-3

37 50 58

RAC20-1-1 RAC20-2-1 RAC20-3-1

RAC20-1-2 RAC20-2-2 RAC20-3-2

RAC20-1-3 RAC20-2-3 RAC20-3-3

RA40-1 RA40-2 RA40-3

31 45 52

RAC40-1-1 RAC40-2-1 RAC40-3-1

RAC40-1-2 RAC40-2-2 RAC40-3-2

RAC40-1-3 RAC40-2-3 RAC40-3-3

Note: RA10-1 indicate that recycled aggregate of maximum size 10 mm derived from parent concrete of mix-1.

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the impact value should be less than 25% when the aggregate is to be used in heavy-duty floors, 30% in concrete for wearing surfaces, and 45% in other concrete surfaces. The impact value of 20 mm RA is 25–21%, and hence can be used for all purposes, while it is 38– 31% for 10 mm RA. 3.4. Properties of recycled aggregate concrete The design and actual values of the workability and compressive strength of RAC is presented in Table 5.

From Table 5, it is observed that, to achieve a particular compressive strength, RAC requires relatively lower water–cement ratio to be maintained as compared to concrete with fresh granite aggregate, and the difference in strength between parent concrete and recycled aggregate concrete increases with higher strength. This indicates that the presence of parent mortar in aggregates does not have much effect on lower strength recycled aggregate concrete. The reduction in compressive strength of RAC with corresponding parent concrete is 10–35% for different mixes. The reduction in compressive strength of RAC with PC is in between 20 and

Table 5 Design and actual values for recycled aggregate concrete Target mean strength of RAC (MPa)

Actual workability and mean compressive strength of recycled aggregate concrete for different design workability (compacting factor) (MPa)

Actual workability

Compressive strength (MPa)

Actual workability

Compressive strength (MPa)

Actual workability

Compresssive strength (MPa)

Actual workability

Compressive strength (MPa)

Actual workability

Compressive strength (MPa)

Actual workability

Comp. Strength (MPa)

Actual workability

Comp. Strength (MPa)

21 34 45 21 34 45 21 34 45

0.74 0.75 0.74 0.77 0.73 0.72 0.74 0.74 0.72

27 33 37 27 35 39 27 35 40

0.74 0.72 0.72 0.71 0.72 0.74 0.75 0.73 0.73

29 35 40 27 40 42 24 39 43

0.85 0.84 0.82 0.84 0.82 0.84 0.84 0.85 0.82

28 32 36 27 35 40 25 36 40

0.84 0.86 0.83 0.82 0.82 0.84 0.86 0.83 0.82

32 37 41 30 38 42 29 38 42

0.85 0.86 0.84 0.82 0.85 0.84 0.84 0.86 0.85

28 36 39 27 37 40 27 39 41

0.92 0.94 0.93 0.95 0.92 0.94 0.92 0.95 0.93

28 34 36 26 33 39 26 34 41

0.93 0.94 0.93 0.93 0.92 0.94 0.94 0.92 0.92

32 37 39 31 39 43 31 36 44

CF = 0.75

CF = 0.85

10 mm

20 mm

CF = 0.95

10 mm

20 mm

40 mm

10 mm

20 mm

60

Parent concrete

compressive strength (MPa)

55

RAC 50 45 40 35 30

700

600

500

400

0.4

Cement kg/cu.m

0.5

0.6

W/C

Parent concrete CF 0.75

A/C

3

4 Parent concrete CF 0.75

Parent concrete CF 0.85

Parent concrete CF 0.85

Parent concrete CF 0.95 RAC CF 0.75

Parent concrete CF 0.95

5

RAC CF 0.75

RAC CF 0.85

RAC CF 0.85

RAC CF 0.95

RAC CF 0.95

6

10mm aggregate

Fig. 1. Relative mix proportioning relationships for parent concrete recycled aggregate concrete (maximum size of aggregate – 10 mm).

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35% for concrete made with 10 mm maximum size aggregate, 14– 35% for 20 mm and 10–25% for 40 mm. This shows that the higher reduction is for concrete made with smaller sized aggregate. It is due to the influence of higher content of comparatively weaker parent mortar in smaller sized aggregate. Using the mix proportioning data and the compressive strength of parent and recycled aggregate concrete the relationship between the water–cement ratio, compressive strength, aggregate-cement ratio and cement content have been plotted in Figs. 1–3 for 10, 20, and 40 mm maximum size of aggregates, respectively. It is observed that RAC requires relatively lower water–cement ratio and higher cement content as compared to parent concrete to achieve a particular compressive strength. The difference in strength between PC and RAC increases with strength of concrete. These graphs provide typical relationships for designing RAC mixes (i.e., for a given target mean strength, having determined the water, cement content and total aggregate content (from Figs. 1–3), the sand content can be calculated using any standard mix proportioning method [19,22]). The strength achieved by recycled aggregate concrete, as a percentage of strength of parent concrete is graphically represented in Fig. 4. The following observations are made from this figure: (i) for a given target mean strength, the strength achieved increases with an increase in the maximum size of aggregate obtained from almost same strength of parent concrete, (ii) for a given recycled aggregate, the strength achieved increases with an increase in the strength of recycled aggregate concrete, and (iii) for a given strength of RAC, the strength achieved reduces with an increase in the strength of parent concrete from which the recycled aggre-

gate is made. The reasons for such behaviour are due to the influence of recycled aggregate properties on concrete. As the maximum size of aggregate decreases, the quantity of old mortar present in recycled aggregate increases, which results in higher reduction in strength of RAC. As the strength of parent concrete increases, presence of parent mortar in recycled aggregates increases irrespective of the size of aggregate. This also has influence in determining the strength of RAC. The variation of split tensile and flexural strengths with respect to compressive strength is shown in Figs. 5 and 6. Visual examination of fractured surface showed that most failures in parent concrete occurred along the interface between cement mortar and aggregate. This shows that the weaker interfacial zone in parent concrete govern the failure. But in RAC, interfacial bond failure as well as aggregate failure has occurred. The failure surface showed that the crushed aggregate were mortar pieces of parent concrete. For a given compressive strength of concrete, the split tensile and flexural strengths are lower for RAC than parent concrete. This difference narrows down with a reduction in compressive strength. The modulus of elasticity of parent and recycled aggregate concrete is related to compressive strength in Figs. 7–9. For a given strength of concrete, the modulus of elasticity of RAC is lower than that of parent concrete. Higher percentage of reduction in modulus of elasticity was obtained for concrete made with smaller sized aggregates. Porosity of aggregate affects the modulus of elasticity of concrete, which controls the ability of aggregate to restrain matrix strain [26]. In RAC, the presence of relatively porous parent mortar reduces the ability to restrain matrix strains. Also higher porosity of smaller sized recycled aggregates causes further

60

compressive strength (MPa)

55

Parent concrete Recycled Agg concrete

50 45 40 35 30

600

500

400

0.4 3

Cement kg/cu.m

0.5

0.6

W/C

RAC CF 0.75 RAC CF 0.85

A/C

4

Parent concrete CF 0.75

5

RAC CF 0.95

Parent concrete CF 0.85 Parent concrete CF 0.95

6

RAC

CF 0.75

RAC

CF 0.85

RAC

CF 0.95

20mm aggregate Fig. 2. Relative mix proportioning relationships for parent concrete recycled aggregate concrete (maximum size of aggregate – 20 mm).

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55

Parent concrete

Compressive strength (MPa)

50

Recycled Agg concrete

45

40

35

30

-500.00

-400.00

-300.00

0.4

0.5

Cement Kg/cu.m

0.6

w/c Parent concrete

4

A/C

RAC

5

6

Parent concrete Recycled Agg concrete

40mm aggregate

7

Fig. 3. Relative mix proportioning relationships for parent concrete recycled aggregate concrete (maximum size of aggregate – 40 mm) (compacting factor = 0.85).

6.0 10mm-Agg from PC35 MPa

20mm Agg from PC58 MPa

10mm-Agg from PC49 MPa

40mm-Agg from PC31 MPa

10mm-Agg from PC56 MPa

40mm-Agg from PC45 MPa

20mm-Agg from PC37 MPa

40mm-Agg from PC52 MPa

20mm Agg from PC 50 MPa

1.2

0.8

PC (10mm aggregate) RAC (10mm aggregate) PC (20mm aggregate)

5.5

RAC (20mmaggregate)

Split tensile strength (MPa)

Strength ratio of RAC to PC

1.6

PC (40mm aggregate)

5.0

RAC (40mm aggregate)

4.5

4.0

3.5

3.0

0.4

2.5 20

25

30

35

40

Target mean strength of RAC (MPa) Fig. 4. Influence of parent concrete strength on RAC.

45

25

30

35

40

45

50

55

Compressive strength (MPa) Fig. 5. Relation compressive strength and split tensile strength.

60

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A.K. Padmini et al. / Construction and Building Materials 23 (2009) 829–836

8.5

36

PC (10mm aggregate) RAC (10mm aggregate)

Parent concrete

PC (20mm aggregate)

RAC-Agg.from 37MPa PC

8.0

Modulus of elasticity (GPa)

Flexural strength (MPa)

RAC-Agg.from 50MPa PC

RAC (20mm aggregate)

7.5

PC (40mm aggregate)

7.0

RAC (40mm aggregate)

6.5 6.0 5.5 5.0

32

RAC-Agg.from 58MPa PC

28

24

4.5 4.0 25

30

35

40

45

50

55

20

60

20

Compressive strength (MPa)

30

40

50

60

Compressive strength (MPa)

Fig. 6. Relation between compressive strength and flexural strength.

Fig. 8. Relation between strength and modulus of elasticity (20 mm maximum size).

36 parent concrete

26

Parent concrete

RAC-Agg.from 35MPa PC RAC-Agg.from 31MPa PC

32

RAC-Agg.from 45MPa PC

RAC-Agg.from 56MPa PC

24

Modulus of elasticity (GPa)

Modulus of elasticity (GPa)

RAC-Agg.from 49MPa PC

28

24

RAC-Agg.from 52MPa PC

22

20

18

20 20

30

40

50

60

Compressive strength (MPa) Fig. 7. Relation between strength and modulus of elasticity of concrete (10 mm maximum size).

16 20

30

40

50

60

Compressive strength (MPa) Fig. 9. Relation between strength and modulus of elasticity (40 mm maximum size).

reduction in modulus of elasticity. For a given strength of RAC, the recycled aggregate derived from different strength of parent concrete does not cause variation in the modulus of elasticity. 4. Conclusions The conclusions drawn are applicable for the range of parameters investigated and the characteristics of materials used in the present study: (i) The water absorption of recycled aggregate increases with an increase in strength of parent concrete from which the recycled aggregate is derived, while it decreases with an

increase in maximum size of aggregate. Higher water absorption of recycled aggregate necessitates adjustment in mix water content to obtain the desired workability. (ii) Though the resistance of recycled aggregate to mechanical actions is lower than fresh crushed granite aggregate, the values are generally within acceptable limits. (iii) For achieving a design compressive strength, recycled aggregate concrete requires lower water–cement ratio and higher cement content to be maintained as compared to concrete with fresh granite aggregate. (iv) For a given target mean strength, the achieved strength increases with an increase in maximum size of recycled aggregate used.

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(v) For a given compressive strength of concrete, (i) the split tensile and flexural strengths are lower for RAC than parent concrete, and (ii) the modulus of elasticity of RAC is lower than that of parent concrete. Higher percentage of reduction in modulus of elasticity was obtained for concrete made with smaller sized aggregates.

[12]

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