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Recycling of partially hydrated concrete
Waste Materials in Construction G.R. Woolley, J.J.J.M. Goumans and P.J. Wainwright (Editors) 9 2000 Elsevier Science Ltd. All rights reserved.
383
Recycling o f partially hydrated concrete Amnon Katz National Building Research Institute, Department of Civil Engineering, Technion, Israel Institute of Technology, Haifa 32000, Israel
Concrete having a 28 days compressive strength of 28 MPa was crushed at 1, 3 and 28 days to serve as a source for aggregates for a new concrete, simulating the situation exists in pre-cast concrete plants. The properties of the recycled aggregate and of the new concrete made from it were tested. Significant changes were observed when comparing the properties of the aggregates based on the various size groups of the aggregates, but not when comparing the effect of crushing age. The properties of the concrete made with the recycled aggregates were only slightly affected by the crushing age when the cement matrix of the new concrete was relatively weak, but some effects were seen for a stronger cement matrix. 1. I N T R O D U C T I O N The use of waste from building materials as aggregates for the production of new concrete has became more common in the recent decade. The increasing charge for landfill on the one hand, and deficiency with natural recourses for aggregates on the other hand, encourages the use of waste from construction sites as a source for aggregates. Some successful projects were reported lately, in which waste from the demolition of old structure was recycled into a new one (Collins, 1996, Tavakoli and Soroushian, 1996). RILEM committee 121-DRG (1994) published recommendation for the use of recycled aggregates. They classified the aggregates into three groups: Group I: aggregates that are mainly from masonry rubble; Group II: aggregates that are mainly from concrete rubble; and Group III: mixture of natural aggregates (>80%) together with rubble form the other two groups (with up to 10% of Group I). Group III can be used for the production of all type of concrete and restrictions exist on the other groups. This definition highlights the one of the major difficulties in the recycling demolished waste: the variability of the quality of the recycled aggregates. As long as the aggregates come from a single source, as in the case of highway replacement (Tavakoli and Soroushian, 1996), uniform aggregates source is guaranteed. However, when the aggregates source is a center for recycling aggregates, the rubble arrives from various sources and the properties of the aggregates are not tmiform leading to difficulties in using them to produce new concrete (BRE, 1993; Hansen and Narud, 1983). Apparently, no problem should exist in recycling concrete in pre-cast concrete plants. The various products are routinely made from the same type of concrete and therefore the problem
384 of variability in the properties of the rubble should not exist. However, products are often rejected during the manufacturing process and the concrete of these elements is not properly cured. As a result the properties of the recycled aggregates may vary as well. The effect of partially hydrated concrete on the properties of the aggregates made of it and the resulting properties of new concrete made from these aggregates is reported in this study.
2. EXPERIMENTAL PROGRAM The experimental program consisted of in two stages: 1) comprehensive study of the properties of concrete made of partially hydrated concrete; 2) study the effect of fines on the properties of the new concrete. This paper presents the results of stage 1 of the study. Old concrete is made of concrete elements used for standard tests in the process of quality assurance of cement manufacturing. Ordinary Portland Cement (OPC) from the cement plant is produced continuously in almost the same mineralogical composition and the compressive strength is tested routinely by mixing the cement with standard aggregates, in a standard process. Table 1 lists the mix composition for the old concrete. At ages l, 3 and 28 days cubes of 100x100x100 mm made from the old concrete were tested for compressive strength. Immediately after the compression tests the cubes were crushed by a mini jaw crusher and dried in an oven at 105~ to cease any further hydration. Good manufacturing in the plant enabled the acceptance of uniform concretes despite being cast in different ages. Table 1 lists the average compressive strength and standard deviation of the old concrete. The new concrete was prepared from 100% of the crushed old concrete with the addition of some natural sand that was needed to maintain good workability. Two types of cements were used for the new concrete, white Portland cement (WPC) and ordinary Portland cement. It was expected that it would be possible to distinguish between the new and old cements by using the white cement. However it appeared lately that the fine aggregate from the crushed concrete that was made mainly of old cement dispersed well in the mix and eliminated the distinction between new and old cement. It should be noted however that the OPC was weaker than the white cement (a compressive strength of 34.6 MPa compare with 42.1 MPa for the white cement, at 28 days). The Table 1" Composition and properties of the old concrete. composition of the new Component Quantity (gr.) concrete is listed in Table 2. Coarse aggregate (12-25 ram) 5415 In the followings the term old Midsize aggregate (2.36-9.5 mm) 1415 concrete will be referred to the waste concrete and the new Fine aggregate (1.2-0.15 mm) 3120 concrete will be referred to the Portland cement 1800 new concrete prepared from water 1080 the crushed old concrete. Compressive strength (MPa) at*" The crushed concrete was sieved over 9.5 and 2.36 mm 7.4 (0.9) 1 day meshes and were divided to 14.4 (1.2) 3 days the following size fractions: 28.3(3.1) 28 days coarse (larger than 9.5 mm), *Number in parenthesis - standard deviation medium (smaller than 9.5 mm _
385 Table 2: Composition of the new concrete (kg/m3). Ordinary Portland cement
White cement Reference
1 day* 3 days*
28 days*
Reference
Crushed aggregate (9.5-25 mm)
896
907
Crushed aggregate (2.36-9.5 mm)
448
454
Crushed sand
212
Natural Sand
421
Recycled aggregate
1 day* 3 days*
28 days*
215 254
219
238
1440
1484
1457
427
259
217
240
1453
1460
1456
Water
161
160
165
162
163
166
168
163
Cement
294
293
302
296
298
298
300
298
*Age of recycled concrete and larger that 2.36 mm) and fine (smaller hat 2.36 mm). This was done in order to study the effect of aggregate' s size on its properties, and to be able to prepare the new concrete with the same size distribution of the aggregates. Each size group was tested as follows: size distribution, bulk density, unit weight, water absorption, crushing value (British Standard 812) and cement content. The new concrete was tested for compressive strength on 100 mm cubes at ages of 7, 28 and 90 days. Four points bending strength, splitting and modulus of elasticity (beams of 70x70x280 mm), rate of capillary absorption of water and total water absorption were tested at 28 days. 3. RESULTS 3.1 Properties of the aggregates made from the crushed concrete
Size distribution curves of the aggregates prepared from the crushed concretes at various ages are presented in Figure 1. The three curves shown in the figure, representing the aggregates prepared from concrete crushed in different ages, show the same size distribution. It appears that as long as the jaw crusher is set to a specific opening there is no significant change in the aggregates grading despite the differences in the concrete strengths they were made from, as seen in Table 1. The 28 days compressive strength of the old concrete was not very high (28.3 MPa at 28 days) and it is possible that the cement matrix between the aggregates was the first to break during the crushing operation. The crack path of normal strength concrete in this range of strengths is known to pass through the cement matrix, and therefore the crushing action led to almost the same size distribution of the aggregates shown in Figure 1.
386
Figure 1: Grading of the recycled aggregates crushed at various ages. The aggregates were sieved over meshes size 9.5 and 2.36 mm and were divided to three size fractions in order to distinguish between properties that might be related to their size. ~I'able 3 presents the unit weight, bulk density, absorption, crushing value and cement content in the recycled aggregates in the different size groups and crushing ages. Normalized values in which they are compared relative to the value of the coarse fraction crushed at 28 days are presented in Figure 2. Comparison of the three size groups shows significant changes between the groups. However, the changes with the same size group crushed at different ages, are minor. These changes represent the change in the composition within each size fraction, which seems to be a result of the relative amount of cement paste in the crushed material. As seen in Table 3 the amount of cement (hydrated and not hydrated) significantly increases from approximately 6.5% in the coarse fraction to approximately 25% in the fine fraction. The cement phase is relatively porous and therefore any increase in its relative content leads to a significant increase in the total absorption of the recycled aggregate as their size becomes smaller. The unit weight and bulk density, however, are only slightly changed with the aggregate size. The natural aggregate composes the major phase in the recycled aggregate in all fractions, and the density of the cement phase is only slightly lower than the aggregates. Therefore, the changes in the cement content in the recycled aggregates have only a small effect on their density. The bulk density of the mid-size aggregates was lower than the one of the fine aggregates (approximately 1240 kg/m 3 compare with 1330 kg/m 3, respectively) despite the lower unit weight of the latter. This phenomenon is probably a result of the better grading of the fine aggregates creating denser packing of the particles in this range.
387 Table 3: Properties of the recycled aggregates Crushing age
1 day
3 days
28 days
Absorp -tion (%)
Crushing value
Cement content
Unit weight (kg/m 3)
Bulk density (kg/m 3)
coarse
2590
1462
medium
2350
1220
9.7
N/A
6.9 15.8
fine
2230
1324
11.2
N/A
26.6
3.2
(%) 25.4
25.3
(%)
6.1
coarse
2600
1433
3.4
medium
2380
1234
8.1
N/A
15.2
fine
2250
1342
11.4
N/A
25.4
coarse
2550
1433
3.3
medium
2320
1278
8.0
N/A
13.2
fine
2230
1321
12.7
N/A
24.5
24.3
6.8
Figure 2: Properties of the recycled aggregates normalized to the coarse fraction crushed at 28 days. The variation in the properties of the aggregates crushed at different ages is not significant. This suggests that the amount of the cement paste that was still adhered to the natural aggregates in each size fraction was uniform, regardless of the crushing age. The strength of the cement matrix, at the level of strengths studied here, has probably only a minor effect on the mode of crushing of the old concrete.
388 As a result of the similarity among the size distribution of the aggregates crushed at different ages, it was decided later on that separation of the recycled aggregates to the different size fractions is not needed for the production of the new concrete. Thus the recycled concrete was added as a whole to the mix (see Table 2). 3.2 Properties of new concrete made from recycled aggregates. Fresh concrete
Properties of the new concrete are listed in Table 4. The bulk density of the fresh concrete made from natural aggregates as reference was in the range known for normal concrete (approximately 2400 kg/m3). However, the concrete made from recycled aggregates was significantly lighter and was approximately 2150 kg/m 3 regardless of the cement type or crushing age. The lower density is the result of the lower density of the aggregates discussed before (2.6, 2.3 and 2.2 for the coarse, medium and fine crushed aggregates, while the density of the natural aggregates is 2.63-2.74 kg/m3). In addition, an increased air content was observed leading also to additional reduction in the density of the fresh concrete. Air content was calculated by the gravimetric method (ASTM C138). The results indicated normal air content for the reference concrete and increased air content of 4-5.5% for the concrete made from recycled aggregates. The reason for the increased air content is not clear. The air in the aggregate' s voids is taken into account through the unit weight of the aggregate, Table 4: Fresh and hardened properties of the new concrete White cement
Ordinary Portland cement
Refer- 1 day" 3 days" 28 ence days* Bulk density (kg/m 3)
Reference
1 day" 3 days"
28 days*
2462
2146
2170
2153
2463
2175
2145
2156
170
170
155
185
81
178
175
134
1.3%
5.4%
4.1%
5.0%
0.0%
4.8%
5.4%
5.6%
36.8
19.0
23.4
20.0
21.6
18.3
17.0
17.1
42.1
24.1
30.5
29.1
34.6
26.6
25.8
26.8
58.9
28.9
38.7
35.2
33.0
28.7
30.6
Flexural strength (MPa)
6.7
4.7
5.3
4.6
6.1
6.1
5.4
5.4
Splitting strength (MPa)
5.0
3.1
3.6
2.7
3.3
3.4
2.9
3.1
23.1
11.4
13.7
11.5
22.7
13.6
12.6
12.8
Slump (mm) Calculated air content (%) 7 Compressive days lstrength 28 (MPa) days 90 days
Modulus of elasticity (GPa)
389 thus the values above represent additional air that was entrapped in the concrete. Additional study is needed in order to better understand this phenomenon. It should be noticed that determination of the air content by the gravimetric method is very sensitive as minor changes in the unit weight of the aggregates may lead to large changes in the air content. Accurate determination of the unit weight is impossible due to difficulties in the determination of the saturated surface dry (SSD) state of the porous aggregates. Therefore the general trends of increased air content should be considered and not the exact values which may include an error of approximately +1%. An increased air content is known also to occur in lightweight aggregates concrete (Wischers and Manns, 1974) that shows some similarities with the recycled aggregates from crushed concrete. The slump of almost all the mixes was in the range of 135-185 mm (mostly-175 mm) except the OPC reference mix that had an unexplained slump of 81 mm. The similar slump was achieved with a similar quantity of free water (see Table 2) indicating that water requirement for a given slump is not changed by the effect of aggregates type and crushing age. It should be noted however, that some quantities of natural sand were still needed for proper workability and cohesivity due to the insufficient amount of fines in the crushed aggregates. Hardened concrete
Compressive strength: the compressive strength of the various mixes is shown in Table 4 for ages 7, 28 and 90 days. The differences between the two cements is shown when comparing the 28 compressive strength of the reference concretes; the one made from OPC was weaker by 18% compare to the white cement concrete. The effect of using the recycled aggregates was a reduction in the compressive strength of the concrete both when white cement or OPC was used. The reduction in strength was 30-40% when white cement was used, the maximum reduction was observed for the concrete made from aggregates crushed at 1 day. The reduction in strength was more moderate for the OPC (approximately 24%), regardless of the crushing age of the recycled aggregates. No significant change was seen when comparing the effect of crushing age on the OPC concrete. However, significant changes were seen for the white cement concrete (see Figure 3). The white cement concrete with aggregates crushed at 1 day exhibited lower strength compare with the other ages of crushing. The aggregates crushed at 3 days exhibited the highest strength at all testing ages. The differences were moderate at 7 days and more pronounced at later ages (5% and 18% of differences between crushing age of 1 day and 28 days, for new concretes tested at 7 and 90 days, respectively). 4. DISCUSSION The difference between the quality of the new cement matrix and the recycled aggregates seems to have an effect on the properties of the new concrete. Two opposing mechanisms seem to control the properties of the new concrete: (1) The mechanical properties of the recycled aggregates crushed at different ages and (2) The residual cementing ability of the unhydrated cement which remained in the recycled aggregates. The mechanical properties of the aggregates crushed at different ages were not uniform as indicated by the compressive strength of the old concrete (see Table 1). Additional hydration
390
Figure 3: The compressive strength of recycled concrete relative to the reference concretes at different ages. of the old cement in the recycled aggregates may somewhat improve the properties of the aggregates, mainly of those crushed at 1 day. This effect is seen in the case where the cement matrix of the new concrete was stronger than the aggregates, as in the case of the new concretes prepared with the white cement. At the age of 7 days, when the cement matrix was relatively weak, there was no significant difference between the strength of aggregates crushed at 1 day or 28 days. At later ages (28 and 90 days) the new cement matrix became much stronger and the changes between aggregates properties were more significant; at 90 days the new concrete made from 1 day aggregates was weaker by 18% than the concrete made from aggregates crushed at 28 days. For the recycled concrete crushed at 3 days, the strength of the old concrete at the crushing age together with the contribution of some unhydrated old cement lead to stronger concretes compared with the other concrete made from aggregates crushed at 1 or 28 days (see Figure 3). The properties of the cement matrix of the new OPC concrete was relatively weak, thus no significant change could be seen between the various crushing ages, similar to the results from the white cement at 7 days. 5. SUMMARY AND CONCLUSIONS 1. The properties of the aggregates crushed at different ages were quite similar. Size distribution of the aggregates was the same for the three ages of crushing, as well as other properties such as absorption, unit weight, bulk density, cement content and crushing value of the coarse fraction. These observations indicate that at these strength levels and structure of the old concrete the aggregates that are made of it have quite similar
391 properties. Other properties, however, such as additional cementing properties or the mechanical properties of the fine fractions were not tested at this stage of the study. 2. Concrete made from 100% of the recycled aggregates was weaker than other concrete made from virgin aggregates at the same water to cement ratio. When the new concrete was made from the same type of OPC and the same water to cement ratio as of the old concrete, the strength reduction was o f - 2 0 % regardless of the crushing age of the old concrete. When white cement was used, the reduction was of 30-40% depending on the crushing age of the old concrete (the white cement provides with 20% higher compressive strength compare with the OPC concrete at the same water to cement ratio). 3. The properties of the aggregates made from the crushed concrete and the effect of the aggregates on the new concrete (strength, modulus of elasticity etc.) resemble those of lightweight aggregate concrete and similar attention should be given when dealing with this type of aggregates. 4. Two opposing mechanisms seem to affect the properties of the new concrete: the physical properties of the old concrete and the presence of unhydrated cement in the crushed concrete. These effects are seen when the new cement matrix is significantly stronger than the old concrete. Additional study in the next stages will clarify this assumption as well as determine the role of air entrapment and its effect on concrete properties. ACKNOWLEDGMENTS This research was supported by the fund for the promotion of research at the Technion.
REFERENCES Building Research Establishment, "Effective Use of Aggregates and Bulk Construction Materials: The Role of Specifications", Building Research Establishment, Watford, 1993. Collins, R.J., "Recycled Aggregates in Ready-Mixed Concrete", in Proceedings of Sustainable use of Materials, Llewellyn, J.W. and Davis H. (Editors), September 1996, Building Research Establishment, UK. Hansen T.C. (editor), "Recycling of Demolished Concrete and Masonry", E&FN Spon, London, 1992, 316p. Mansur, M.A., Wee, T.H. and Cheran, L.S., 1999, "Crushed bricks as coarse aggregate for concrete", ACI Materials Journal, Vol. 96, No. 4, pp. 478-484. RILEM 121-DRG (1994), "Specification for Concrete with Recycled Aggregates", Materials and Structures, Vol. 27, pp. 557-559. Tavakoli M. and Soroushian P., "Strengths of Recycled Aggregate Concrete Made Using Field-Demolishd Concrete as Aggregate", ACI Materials Journal, Vol. 93, N. 2, 1996, pp. 182-190. Wischers, G. and Manns, W., 1974, "Technology of structural lightweight concrete", in Lightweight Aggregate Concrete Technology and World Application, Bologna G. (editor), CEMBUREAU Paris, pp.23-35.
383
Recycling o f partially hydrated concrete Amnon Katz National Building Research Institute, Department of Civil Engineering, Technion, Israel Institute of Technology, Haifa 32000, Israel
Concrete having a 28 days compressive strength of 28 MPa was crushed at 1, 3 and 28 days to serve as a source for aggregates for a new concrete, simulating the situation exists in pre-cast concrete plants. The properties of the recycled aggregate and of the new concrete made from it were tested. Significant changes were observed when comparing the properties of the aggregates based on the various size groups of the aggregates, but not when comparing the effect of crushing age. The properties of the concrete made with the recycled aggregates were only slightly affected by the crushing age when the cement matrix of the new concrete was relatively weak, but some effects were seen for a stronger cement matrix. 1. I N T R O D U C T I O N The use of waste from building materials as aggregates for the production of new concrete has became more common in the recent decade. The increasing charge for landfill on the one hand, and deficiency with natural recourses for aggregates on the other hand, encourages the use of waste from construction sites as a source for aggregates. Some successful projects were reported lately, in which waste from the demolition of old structure was recycled into a new one (Collins, 1996, Tavakoli and Soroushian, 1996). RILEM committee 121-DRG (1994) published recommendation for the use of recycled aggregates. They classified the aggregates into three groups: Group I: aggregates that are mainly from masonry rubble; Group II: aggregates that are mainly from concrete rubble; and Group III: mixture of natural aggregates (>80%) together with rubble form the other two groups (with up to 10% of Group I). Group III can be used for the production of all type of concrete and restrictions exist on the other groups. This definition highlights the one of the major difficulties in the recycling demolished waste: the variability of the quality of the recycled aggregates. As long as the aggregates come from a single source, as in the case of highway replacement (Tavakoli and Soroushian, 1996), uniform aggregates source is guaranteed. However, when the aggregates source is a center for recycling aggregates, the rubble arrives from various sources and the properties of the aggregates are not tmiform leading to difficulties in using them to produce new concrete (BRE, 1993; Hansen and Narud, 1983). Apparently, no problem should exist in recycling concrete in pre-cast concrete plants. The various products are routinely made from the same type of concrete and therefore the problem
384 of variability in the properties of the rubble should not exist. However, products are often rejected during the manufacturing process and the concrete of these elements is not properly cured. As a result the properties of the recycled aggregates may vary as well. The effect of partially hydrated concrete on the properties of the aggregates made of it and the resulting properties of new concrete made from these aggregates is reported in this study.
2. EXPERIMENTAL PROGRAM The experimental program consisted of in two stages: 1) comprehensive study of the properties of concrete made of partially hydrated concrete; 2) study the effect of fines on the properties of the new concrete. This paper presents the results of stage 1 of the study. Old concrete is made of concrete elements used for standard tests in the process of quality assurance of cement manufacturing. Ordinary Portland Cement (OPC) from the cement plant is produced continuously in almost the same mineralogical composition and the compressive strength is tested routinely by mixing the cement with standard aggregates, in a standard process. Table 1 lists the mix composition for the old concrete. At ages l, 3 and 28 days cubes of 100x100x100 mm made from the old concrete were tested for compressive strength. Immediately after the compression tests the cubes were crushed by a mini jaw crusher and dried in an oven at 105~ to cease any further hydration. Good manufacturing in the plant enabled the acceptance of uniform concretes despite being cast in different ages. Table 1 lists the average compressive strength and standard deviation of the old concrete. The new concrete was prepared from 100% of the crushed old concrete with the addition of some natural sand that was needed to maintain good workability. Two types of cements were used for the new concrete, white Portland cement (WPC) and ordinary Portland cement. It was expected that it would be possible to distinguish between the new and old cements by using the white cement. However it appeared lately that the fine aggregate from the crushed concrete that was made mainly of old cement dispersed well in the mix and eliminated the distinction between new and old cement. It should be noted however that the OPC was weaker than the white cement (a compressive strength of 34.6 MPa compare with 42.1 MPa for the white cement, at 28 days). The Table 1" Composition and properties of the old concrete. composition of the new Component Quantity (gr.) concrete is listed in Table 2. Coarse aggregate (12-25 ram) 5415 In the followings the term old Midsize aggregate (2.36-9.5 mm) 1415 concrete will be referred to the waste concrete and the new Fine aggregate (1.2-0.15 mm) 3120 concrete will be referred to the Portland cement 1800 new concrete prepared from water 1080 the crushed old concrete. Compressive strength (MPa) at*" The crushed concrete was sieved over 9.5 and 2.36 mm 7.4 (0.9) 1 day meshes and were divided to 14.4 (1.2) 3 days the following size fractions: 28.3(3.1) 28 days coarse (larger than 9.5 mm), *Number in parenthesis - standard deviation medium (smaller than 9.5 mm _
385 Table 2: Composition of the new concrete (kg/m3). Ordinary Portland cement
White cement Reference
1 day* 3 days*
28 days*
Reference
Crushed aggregate (9.5-25 mm)
896
907
Crushed aggregate (2.36-9.5 mm)
448
454
Crushed sand
212
Natural Sand
421
Recycled aggregate
1 day* 3 days*
28 days*
215 254
219
238
1440
1484
1457
427
259
217
240
1453
1460
1456
Water
161
160
165
162
163
166
168
163
Cement
294
293
302
296
298
298
300
298
*Age of recycled concrete and larger that 2.36 mm) and fine (smaller hat 2.36 mm). This was done in order to study the effect of aggregate' s size on its properties, and to be able to prepare the new concrete with the same size distribution of the aggregates. Each size group was tested as follows: size distribution, bulk density, unit weight, water absorption, crushing value (British Standard 812) and cement content. The new concrete was tested for compressive strength on 100 mm cubes at ages of 7, 28 and 90 days. Four points bending strength, splitting and modulus of elasticity (beams of 70x70x280 mm), rate of capillary absorption of water and total water absorption were tested at 28 days. 3. RESULTS 3.1 Properties of the aggregates made from the crushed concrete
Size distribution curves of the aggregates prepared from the crushed concretes at various ages are presented in Figure 1. The three curves shown in the figure, representing the aggregates prepared from concrete crushed in different ages, show the same size distribution. It appears that as long as the jaw crusher is set to a specific opening there is no significant change in the aggregates grading despite the differences in the concrete strengths they were made from, as seen in Table 1. The 28 days compressive strength of the old concrete was not very high (28.3 MPa at 28 days) and it is possible that the cement matrix between the aggregates was the first to break during the crushing operation. The crack path of normal strength concrete in this range of strengths is known to pass through the cement matrix, and therefore the crushing action led to almost the same size distribution of the aggregates shown in Figure 1.
386
Figure 1: Grading of the recycled aggregates crushed at various ages. The aggregates were sieved over meshes size 9.5 and 2.36 mm and were divided to three size fractions in order to distinguish between properties that might be related to their size. ~I'able 3 presents the unit weight, bulk density, absorption, crushing value and cement content in the recycled aggregates in the different size groups and crushing ages. Normalized values in which they are compared relative to the value of the coarse fraction crushed at 28 days are presented in Figure 2. Comparison of the three size groups shows significant changes between the groups. However, the changes with the same size group crushed at different ages, are minor. These changes represent the change in the composition within each size fraction, which seems to be a result of the relative amount of cement paste in the crushed material. As seen in Table 3 the amount of cement (hydrated and not hydrated) significantly increases from approximately 6.5% in the coarse fraction to approximately 25% in the fine fraction. The cement phase is relatively porous and therefore any increase in its relative content leads to a significant increase in the total absorption of the recycled aggregate as their size becomes smaller. The unit weight and bulk density, however, are only slightly changed with the aggregate size. The natural aggregate composes the major phase in the recycled aggregate in all fractions, and the density of the cement phase is only slightly lower than the aggregates. Therefore, the changes in the cement content in the recycled aggregates have only a small effect on their density. The bulk density of the mid-size aggregates was lower than the one of the fine aggregates (approximately 1240 kg/m 3 compare with 1330 kg/m 3, respectively) despite the lower unit weight of the latter. This phenomenon is probably a result of the better grading of the fine aggregates creating denser packing of the particles in this range.
387 Table 3: Properties of the recycled aggregates Crushing age
1 day
3 days
28 days
Absorp -tion (%)
Crushing value
Cement content
Unit weight (kg/m 3)
Bulk density (kg/m 3)
coarse
2590
1462
medium
2350
1220
9.7
N/A
6.9 15.8
fine
2230
1324
11.2
N/A
26.6
3.2
(%) 25.4
25.3
(%)
6.1
coarse
2600
1433
3.4
medium
2380
1234
8.1
N/A
15.2
fine
2250
1342
11.4
N/A
25.4
coarse
2550
1433
3.3
medium
2320
1278
8.0
N/A
13.2
fine
2230
1321
12.7
N/A
24.5
24.3
6.8
Figure 2: Properties of the recycled aggregates normalized to the coarse fraction crushed at 28 days. The variation in the properties of the aggregates crushed at different ages is not significant. This suggests that the amount of the cement paste that was still adhered to the natural aggregates in each size fraction was uniform, regardless of the crushing age. The strength of the cement matrix, at the level of strengths studied here, has probably only a minor effect on the mode of crushing of the old concrete.
388 As a result of the similarity among the size distribution of the aggregates crushed at different ages, it was decided later on that separation of the recycled aggregates to the different size fractions is not needed for the production of the new concrete. Thus the recycled concrete was added as a whole to the mix (see Table 2). 3.2 Properties of new concrete made from recycled aggregates. Fresh concrete
Properties of the new concrete are listed in Table 4. The bulk density of the fresh concrete made from natural aggregates as reference was in the range known for normal concrete (approximately 2400 kg/m3). However, the concrete made from recycled aggregates was significantly lighter and was approximately 2150 kg/m 3 regardless of the cement type or crushing age. The lower density is the result of the lower density of the aggregates discussed before (2.6, 2.3 and 2.2 for the coarse, medium and fine crushed aggregates, while the density of the natural aggregates is 2.63-2.74 kg/m3). In addition, an increased air content was observed leading also to additional reduction in the density of the fresh concrete. Air content was calculated by the gravimetric method (ASTM C138). The results indicated normal air content for the reference concrete and increased air content of 4-5.5% for the concrete made from recycled aggregates. The reason for the increased air content is not clear. The air in the aggregate' s voids is taken into account through the unit weight of the aggregate, Table 4: Fresh and hardened properties of the new concrete White cement
Ordinary Portland cement
Refer- 1 day" 3 days" 28 ence days* Bulk density (kg/m 3)
Reference
1 day" 3 days"
28 days*
2462
2146
2170
2153
2463
2175
2145
2156
170
170
155
185
81
178
175
134
1.3%
5.4%
4.1%
5.0%
0.0%
4.8%
5.4%
5.6%
36.8
19.0
23.4
20.0
21.6
18.3
17.0
17.1
42.1
24.1
30.5
29.1
34.6
26.6
25.8
26.8
58.9
28.9
38.7
35.2
33.0
28.7
30.6
Flexural strength (MPa)
6.7
4.7
5.3
4.6
6.1
6.1
5.4
5.4
Splitting strength (MPa)
5.0
3.1
3.6
2.7
3.3
3.4
2.9
3.1
23.1
11.4
13.7
11.5
22.7
13.6
12.6
12.8
Slump (mm) Calculated air content (%) 7 Compressive days lstrength 28 (MPa) days 90 days
Modulus of elasticity (GPa)
389 thus the values above represent additional air that was entrapped in the concrete. Additional study is needed in order to better understand this phenomenon. It should be noticed that determination of the air content by the gravimetric method is very sensitive as minor changes in the unit weight of the aggregates may lead to large changes in the air content. Accurate determination of the unit weight is impossible due to difficulties in the determination of the saturated surface dry (SSD) state of the porous aggregates. Therefore the general trends of increased air content should be considered and not the exact values which may include an error of approximately +1%. An increased air content is known also to occur in lightweight aggregates concrete (Wischers and Manns, 1974) that shows some similarities with the recycled aggregates from crushed concrete. The slump of almost all the mixes was in the range of 135-185 mm (mostly-175 mm) except the OPC reference mix that had an unexplained slump of 81 mm. The similar slump was achieved with a similar quantity of free water (see Table 2) indicating that water requirement for a given slump is not changed by the effect of aggregates type and crushing age. It should be noted however, that some quantities of natural sand were still needed for proper workability and cohesivity due to the insufficient amount of fines in the crushed aggregates. Hardened concrete
Compressive strength: the compressive strength of the various mixes is shown in Table 4 for ages 7, 28 and 90 days. The differences between the two cements is shown when comparing the 28 compressive strength of the reference concretes; the one made from OPC was weaker by 18% compare to the white cement concrete. The effect of using the recycled aggregates was a reduction in the compressive strength of the concrete both when white cement or OPC was used. The reduction in strength was 30-40% when white cement was used, the maximum reduction was observed for the concrete made from aggregates crushed at 1 day. The reduction in strength was more moderate for the OPC (approximately 24%), regardless of the crushing age of the recycled aggregates. No significant change was seen when comparing the effect of crushing age on the OPC concrete. However, significant changes were seen for the white cement concrete (see Figure 3). The white cement concrete with aggregates crushed at 1 day exhibited lower strength compare with the other ages of crushing. The aggregates crushed at 3 days exhibited the highest strength at all testing ages. The differences were moderate at 7 days and more pronounced at later ages (5% and 18% of differences between crushing age of 1 day and 28 days, for new concretes tested at 7 and 90 days, respectively). 4. DISCUSSION The difference between the quality of the new cement matrix and the recycled aggregates seems to have an effect on the properties of the new concrete. Two opposing mechanisms seem to control the properties of the new concrete: (1) The mechanical properties of the recycled aggregates crushed at different ages and (2) The residual cementing ability of the unhydrated cement which remained in the recycled aggregates. The mechanical properties of the aggregates crushed at different ages were not uniform as indicated by the compressive strength of the old concrete (see Table 1). Additional hydration
390
Figure 3: The compressive strength of recycled concrete relative to the reference concretes at different ages. of the old cement in the recycled aggregates may somewhat improve the properties of the aggregates, mainly of those crushed at 1 day. This effect is seen in the case where the cement matrix of the new concrete was stronger than the aggregates, as in the case of the new concretes prepared with the white cement. At the age of 7 days, when the cement matrix was relatively weak, there was no significant difference between the strength of aggregates crushed at 1 day or 28 days. At later ages (28 and 90 days) the new cement matrix became much stronger and the changes between aggregates properties were more significant; at 90 days the new concrete made from 1 day aggregates was weaker by 18% than the concrete made from aggregates crushed at 28 days. For the recycled concrete crushed at 3 days, the strength of the old concrete at the crushing age together with the contribution of some unhydrated old cement lead to stronger concretes compared with the other concrete made from aggregates crushed at 1 or 28 days (see Figure 3). The properties of the cement matrix of the new OPC concrete was relatively weak, thus no significant change could be seen between the various crushing ages, similar to the results from the white cement at 7 days. 5. SUMMARY AND CONCLUSIONS 1. The properties of the aggregates crushed at different ages were quite similar. Size distribution of the aggregates was the same for the three ages of crushing, as well as other properties such as absorption, unit weight, bulk density, cement content and crushing value of the coarse fraction. These observations indicate that at these strength levels and structure of the old concrete the aggregates that are made of it have quite similar
391 properties. Other properties, however, such as additional cementing properties or the mechanical properties of the fine fractions were not tested at this stage of the study. 2. Concrete made from 100% of the recycled aggregates was weaker than other concrete made from virgin aggregates at the same water to cement ratio. When the new concrete was made from the same type of OPC and the same water to cement ratio as of the old concrete, the strength reduction was o f - 2 0 % regardless of the crushing age of the old concrete. When white cement was used, the reduction was of 30-40% depending on the crushing age of the old concrete (the white cement provides with 20% higher compressive strength compare with the OPC concrete at the same water to cement ratio). 3. The properties of the aggregates made from the crushed concrete and the effect of the aggregates on the new concrete (strength, modulus of elasticity etc.) resemble those of lightweight aggregate concrete and similar attention should be given when dealing with this type of aggregates. 4. Two opposing mechanisms seem to affect the properties of the new concrete: the physical properties of the old concrete and the presence of unhydrated cement in the crushed concrete. These effects are seen when the new cement matrix is significantly stronger than the old concrete. Additional study in the next stages will clarify this assumption as well as determine the role of air entrapment and its effect on concrete properties. ACKNOWLEDGMENTS This research was supported by the fund for the promotion of research at the Technion.
REFERENCES Building Research Establishment, "Effective Use of Aggregates and Bulk Construction Materials: The Role of Specifications", Building Research Establishment, Watford, 1993. Collins, R.J., "Recycled Aggregates in Ready-Mixed Concrete", in Proceedings of Sustainable use of Materials, Llewellyn, J.W. and Davis H. (Editors), September 1996, Building Research Establishment, UK. Hansen T.C. (editor), "Recycling of Demolished Concrete and Masonry", E&FN Spon, London, 1992, 316p. Mansur, M.A., Wee, T.H. and Cheran, L.S., 1999, "Crushed bricks as coarse aggregate for concrete", ACI Materials Journal, Vol. 96, No. 4, pp. 478-484. RILEM 121-DRG (1994), "Specification for Concrete with Recycled Aggregates", Materials and Structures, Vol. 27, pp. 557-559. Tavakoli M. and Soroushian P., "Strengths of Recycled Aggregate Concrete Made Using Field-Demolishd Concrete as Aggregate", ACI Materials Journal, Vol. 93, N. 2, 1996, pp. 182-190. Wischers, G. and Manns, W., 1974, "Technology of structural lightweight concrete", in Lightweight Aggregate Concrete Technology and World Application, Bologna G. (editor), CEMBUREAU Paris, pp.23-35.