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Variation in mechanical properties of MSW incinerator bottom ash
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.
567
V a r i a t i o n in m e c h a n i c a l p r o p e r t i e s o f M S W i n c i n e r a t o r b o t t o m ash Results from triaxial tests Maria Arm Swedish National Road and Transport Research Institute (VTI), Link6ping, Sweden
ABSTRACT This study deals with laboratory testing of municipal solid waste (MSW) incinerator bottom ash. The aim was to investigate the mechanical properties such as stiffness and stability of the ash for future use in unbound road layers. Especially the effect of the material variation on the mechanical properties was analysed. Specimens of bottom ash from four different plants and four seasons were tested by repeated load triaxial tests. Results so far suggest that there is a significant variation in the mechanical properties, both seasonal fluctuations and differences between incinerator plants. However, the variation is not greater than for studied natural aggregates. It is also shown that the organic content has a limiting effect on the resilient modulus, as expected. If MSWI bottom ash is utilised instead of sand in a capping layer, the same design modulus could be used as for the sand. All tested materials are stable and give reasonable permanent deformations at the stress level that is relevant for a Swedish capping layer.
1. I N T R O D U C T I O N Despite Sweden's relative richness in natural aggregate reserves, there is a governmental ambition to facilitate and increase the use of alternative materials in Swedish road constructions. Several research projects are going on in purpose to establish both design and environmental guidelines. One material that has not been used in Sweden in large-scale projects yet, is bottom ash from incineration of municipal solid waste, here called MSWI bottom ash. This study deals with laboratory testing of unbound MSWI bottom ash. The aim was to investigate the mechanical properties of the ash when used in unbound road layers, at first hand in capping layers. Great variations in the ash properties were expected, due to seasonal variations in the waste. Different incineration processes could also be expected to have an influence on the mechanical properties of the bottom ash. Therefore, ashes from different seasons and different incineration plants were studied. MSWI bottom ash from four plants was chosen, namely from Stockholm, Gothenburg, Malm6 and Link6ping. These plants incinerate both municipal and industrial wastes. They are among the largest in Sweden and produce both heat and electricity. The production of bottom ash for each of the stations is between 35 000 and 80 000 metric tons per year, as can be seen in table 1. This can be compared to the total amount for Sweden, which was about 340 000 metric tons in 1997.
568
2. M E T H O D S The Swedish Geotechnical Institute (SGI) made sampling in the plants as part of another study. The sampling was carried out during different times of the year, approximately every third month. Then, the 16 materials were stored outdoors for at least six months in wooden boxes without roofs. From these boxes representative samples were taken for this study. The mechanical properties were studied using repeated load triaxial tests. Complementary properties such as composition, particle size distribution, organic content, optimum water content and maximum dry density were also investigated. Comparison with corresponding properties of conventional materials, such as different sandy materials was made. SGI has done chemical and environmental characterisation on the same material and those results will be reported separately. In the repeated load triaxial tests, the material deformation under simulated traffic conditions is investigated. The resilient strain is used to calculate the stiffness expressed as resilient modulus and the accumulated permanent deformation can be used tbr classification purpose. Since the specimen exposed to loading consists of the whole composite material up to a certain grain size, it is in fact the function of the material that is tested. The method is well known both for fine-grained and coarse-grained materials and a CEN standard is under development. The tests were carried out on undrained specimens with the dimensions of 150 mm diameter and 300 mm height in VTI:s servo-hydraulic material testing system (VMS). This allowed ashes with maximum particle size up to 30 mm to be tested. The specimens were undrained since this is most similar to reality. The traffic load is quick compared to the long time loading that is used in soil mechanics. The specimens were manufactured in o n e layer by means of vibrating compacting equipment called Vibrocompresseur in a special cylinder. This is a "friendly" method that was chosen because earlier experiments showed crushing tendency for these materials. After compaction the specimens were pushed out of the cylinder and equipped with platens in both ends and a thin rubber membrane around. The aim was to test all materials at the same relative water content and the same relative density. The conditions that were chosen were optimum water content and 90 % of maximum dry density from modified proctor. However, problems in compacting some of the specimens resulted in differences in the attained compaction degree. The impact this fact has had on the test results is discussed later. For each material investigated, three specimens were compacted and tested. The results that are discussed here are averages for these three specimens. The tests are still carried on and will be finished during this year.
569 3. RESULTS
The stations produce bottom ash with grain size distributions according to figure 1.
Figure 1.
Grain size distribution for MSWI bottom ash from four different stations and three different seasons.
SGI has carried out compaction tests on the materials by means of modified proctor. The same institute has also reported organic content measured as loss on ignition (LOI) for the studied materials. Table 2 and 3 summarises the results so far. Table 2.
Season Autumn Winter Spring Table 3. Autumn Winter Spring
Maximum dry density and optimum water content, from modified Proctor tests (from SGI). Stockholm Gothenburg Malm6 Link6ping Max dens Optw Max dens Optw Max dens Opt w Max dens Opt w (%) (%) (t/m3) (t/m3) (%) (t/m3) (%) (t/m3) 1,61 13,5 1,73 17 1,65 15-18 1,58 18,0 1,66 9,0 1,72 16,1 1,63 16,5 1,48 19,5 1,58 11-19 1,66 17-19 1,64 16,4 1,43 20,5 Loss on ignition (LOI) in %, sample A/sample B. (from SGI) Stockholm Gothenburg Maim/3 Link6ping 3,9/4,1 2,8/4,0 3,8/3,8 6,5/7,0 3,2/4,0 2,7/2,7 4,4/4,4 9,1/9,2 4,1/4,3 3,7/3,7 4,9/5,0 8,9/9,0
570
Furthermore, SGI has investigated the environmental properties by leaching tests on the same materials. Results from these investigations will be reported separately by Dr AnnMarie F&illman.
3.1. Stiffness and stability Data from triaxial tests on bottom ash from four stations and three seasons are presented here. In figure 2 the stiffness expressed as resilient modulus for different stresses is plotted for one of the stations. Results from the other stations showed the same pattern.
Figure 2.
Stiffness expressed as resilient modulus for MSWI bottom ash from Stockholm.
Two things can be noticed. First, the curves are rather flat, which indicates a weak stress dependency and is typical for sandy materials. Second, the differences between "spring", "winter" and "autumn" ashes are significant. However, the relative position of curves, with the "spring" modulus being higher than "winter" and "autumn" modulus, is not general. This is illustrated in figure 3, where the results from one stress condition are compared for all four stations. The scatter among the three specimens tested for each material is also illustrated. Some of this scatter and also the variation between materials can be explained by unintended differences in density of the specimens. This is discussed in a subsequent chapter. The stability expressed as permanent compressive deformation will also be evaluated within this project. Preliminary results suggest that the permanent deformations are very small as long as the mean normal stress is less than 60 kPa. In figure 4 the deformations at a mean normal stress of 50 kPa are compared.
571
Station and season Sthlm autumn
m
Sthlm winter
m
Sthlm spring G bg autumn _
Gbg winter
m m
Gbg spring
mm
Maim6 autumn
m
Maim6 winter Maim6 spring
m
Lkpg autumn
m
Lkpg winter
m
m
m .
.
.
.
Lkpg spring
t
m
0
25
50
75
100
125
150
175
Resilient modulus (MPa)
Figure 3.
Variation in resilient modulus at one stress condition. 5 % significance. Mean normal stress = 50 kPa.
Station and season Sthlm autumn
m
Sthlm winter
m _
Sthlm spring
m
Gbg autumn
m
Gbg winter
m
Gbg spring
m
Maim6 autumn Maim6 winter
m
Maim5 spring
m
m
Lkpg autumn
m) _
Lkpg winter
m _
Lkpg spring
m _
0.0
0.5
1.0
1.5
2.0
Permanent strain (%)
Figure 4.
Variation in accumulated axial permanent compression. 5 % significance. Mean normal stress = 50 kPa.
572
Comparison with conventional materials The test results have been compared with the corresponding results for natural aggregates that the bottom ash could possibly replace, such as sand with different grading. Those sandy materials were all taken from a construction site, from different sections of the new E6 at the west coast of Sweden. They have been tested in the same way within another research project at VTI (2). In all, it was twelve sandy materials with a sand content between 45 and 9 1 % and with a water content of ca 0,6 x opt w. The M S W I bottom ash showed about the same resilient modulus as the sand, but much smaller permanent deformations, see figure 5 and 6.
Figure 6.
Variation in acc. perm. compression for bottom ash from Stockholm and different sandy materials. 5 % sign. Mean normal stress = 50 kPa.
573 The fact that the sandy materials showed greater permanent deformations was not surprising since many of the sands were rather even-grained, see figure 7. Nevertheless, they were all "real" material that is being used in road constructions.
100% 0. 06
fine
Sand 0 2 medium 0 6
coarse
Gravel medium21
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coarse 6(
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~
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Particle size, mm
Figure 7.
5.6
8
11.2
16
31.5 45
Grain size distribution for twelve bottom ash types and six sandy materials. Dotted lines represent bottom ash.
4. DISCUSSION Mechanical properties of a pavement layer is very much depending on how well the compaction work has succeeded and this in turn is depending on the grain size distribution and the grain shape of the compacted material. Compaction tool and water supply are other important parameters, but they are no material properties. Furthermore, if the mechanical properties should be kept during the lifetime of the road, the grains must be resistant to both mechanical and climate wear, but that is not discussed in this paper. A property that is seldom problematic for Swedish conventional road materials, but could be crucial for an incineration bottom ash, is the organic content. Beside the raw material, it is the manufacturing process that controls the organic content and other properties of the ash. Variations in mechanical behaviour are therefore depending on variation in some or several of the properties mentioned above. These properties are discussed further here below.
4.1. Raw material and organic content As earlier mentioned, the four incineration stations incinerate both municipal and industrial wastes. The proportions are listed in table 4.
574
Table 4. Station Stockholm Gothenburg Malm6 Link6ping
(RVF 1997), (from SGI) Proportion municipal waste 81% 60 % 59 % 76 %
Organic content (LOI) 3,6-4,2 % 2,7-3,7 % 2,6-4,9 % 5,9-9,1%
According to the table the plants can be grouped into two, namely Gothenburg and Malm6 with about 60 % municipal waste and Stockholm and Link6ping with ca 80 ~ Bottom ash from Link6ping has the highest organic content. The ash from this station also turned out to have the lowest stiffness expressed as resilient modulus. An attempt to relate the resilient modulus and the permanent deformation to the organic content has been made in figure 8.
Figure 8.
Organic content measured as LOI as a function of resilient modulus and accumulated permanent compression respectively.
It is generally agreed that organic matter has a bad impact on the stiffness. Most country has limited the organic content in road materials. An early Swedish study (3) shows that already a small proportion of organic matter, 6 % of the material with grain size less than 2 mm, deteriorates the E-modulus greatly. The study in this paper suggests that even the permanent deformation is influenced in a negative way. Also, the material composition has been defined through visual observation. The results are shown in figure 9.
575
Figure 9.
Main constituents of bottom ash from Stockholm, Gothenburg, Malm6 and Link6ping
From the diagrams it can be seen that there are no great differences between the ashes except for one of the plants, Stockholm, whose ash contains a lot more glass than the other, about 50 % compared to ca 35 %. 4.2. Grain size d i s t r i b u t i o n
In figure 10 the grain size distribution for the ash from each of the studied stations is plotted. The three curves represent material sampled at different seasons. Stockholm Sand Gravel 900/0006 fine 02medium06 coarse:~! fine 6....medium 21)coarse l~="~"~-6(
Gothenburg
Sand
Gr
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.
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).0750.125 0.25 0.5
1 2 4 Particle size, mm
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Gravel
lOO%o
80% .=_ 70% (/) ca.
I
'
60~
9
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I
1.0750.1250.25 0.5
1 2 4 5.6 8 11.2 16 31.545 Particle size, mm
" r
0%
0% 10% .:,....~.;. I" 0%
Figure 10. Grain size distribution for studied materials.
/,
winter ~ . j ~ ~-'7~
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' 0%
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576
According to the grain size distribution all materials can be classified as sandy gravel. It can also be seen that the time dependent variation is small. Even between the stations the differences are small. Table 5 contains some of the parameters describing the grain size distribution for studied materials.
Nevertheless, from figure 10 and table 5 a few things can be pointed out. First, the "winter" ash from Malm6 contains more coarse particles (> 6 mm) than the other ashes. This should be favourable for the stiffness. Second, the "winter" ash from Malm6 and the "spring" ash from Link6ping have the flattest curve shape. The curve shape can be characterised by a uniformity coefficient, Cu, that is the ratio of d60 to d~0. D60 means the mesh of the sieve through which 60 % of the material pass. For conventional materials, it has been shown that the less steep the curve is, the more stable is the material (2). Third, the fines content is between 5 and 8 % for all materials with two exceptions, Link6ping "winter" with 2 % and Link6ping "spring" with 10 %. The fines content allowed in road materials is usually limited because of the risk of frost heave. In Sweden the limit is 8 %. However, a certain amount of fines is essential for a good compaction result.
4.3. Grain shape The grain shape can be rounded or more or less angular, which has influence on the deformation properties. A more angular material requires a bigger compaction effort, which in its turn can create crushing and an increase of fines. Conversely, a material with rounded particles is generally easy to compact but is also more unstable than an angular material. Results from another investigation (4) show that MSWI bottom ash has more angular particles than conventional sandy gravel and also more than crushed concrete. It is mostly the particles in the fraction 20-32 mm that are flaky. The content of angular particles could explain the difficulties in compacting the material to laboratory specimens. Field experiences, however, have reported that the bottom ash is very easy to compact (5). This is probably because of the heavy loads that have been used.
4.4. Compaction degree The fact that the specimens were not tested at the same compaction degree and the same relative water content must be analysed. The 300-mm high specimens with a diameter of 150 mm were compacted in one layer by simultaneous vibration and compression in a Vibrocompresseur. It is a method that has been used with success for conventional materials. Among the advantages can be mentioned that the material is homogeneous compacted and
577
that the water content and density for the specimen can be chosen in advance. Another advantage is that the vibrating compaction is more "friendly" This method was chosen because the modified proctor tests showed a certain crushing tendency for these materials. However, the target density of 90 % compaction degree was not reached and the water content was above the optimum in all materials except one. The reason for this was probably too angular material and too "careful" compaction. The actual values are given in table 6. Table 6.
Season Autumn Winter Spring
Compaction degree (actual dry density/max, dry density from modified proctor tests) and water content for tested materials. Average of three specimens. Stockholm Gothenburg Malm6 Link6ping Compact. w Compact. w Compact. w Compact. w degree degree degree degree (%) (%) (%) (%) (%) (%) (%) (%) 84 O p t - 0,5 89 Opt + 1,4 85 Opt + 1,8 86 Opt + 0,8 87 Opt + 1,0 89 Opt + 0,8 90 Opt + 0,4 88 Opt + 0,7 88 Opt - 0,1 89 Opt + 0,7 82 Opt + 0,6 80 Opt + 0,8
All the materials from Gothenburg, which had the smallest d90 and favourable water contents, were well compacted. They also had resilient modulus above 110 MPa (at a mean normal stress of 50 kPa). Stockholm's "autumn" ash had about the same grain size distribution but was too dry. This is the only material that was tested at a water content below optimum. The compaction degree was therefore considerably lower, 84 % instead of 89%. The resilient modulus for this ash was below 95 MPa. The different compaction degree and the resulting variation in stiffness show the importance of a good compaction.
5. CONCLUSIONS The experiments are still going on, but the results so far suggest that the mechanical properties expressed as measured permanent deformation and calculated resilient modulus from triaxial tests are reasonably uniform for each incineration station. There is a significant difference between the stations and even within a station. However, the scatter is not bigger than for the studied natural aggregates. The value of a relevant design modulus usually depends on where in the construction a material is utilised. Though, it seems that the stress level is not that important for MSWI bottom ash since the tests showed a weak stress dependency for the resilient moduli. From the comparison with some natural aggregates it may be said that if MSWI bottom ash is utilised instead of sand in a capping layer, the same design modulus could be used as for the sand. All tested materials are stable and give reasonable permanent deformations at the stress level that is relevant for a Swedish capping layer. Furthermore, a better stability expressed as a smaller permanent deformation could be expected if an even-grained sand is replaced by MSWI bottom ash. It is shown (as expected) that the organic content has a limiting effect on the resilient modulus. Finally, it can be said that it is possible to create a stiff and stable unbound capping layer of MSWI bottom ash if the organic content is low and the right compaction procedure is used.
578 ACKNOWLEDGEMENTS In this research several colleagues at the Swedish Road and Transport Research Institute have taken part. The author wish to express her appreciation for the co-operation, especially to Hhkan Arvidsson who did most of the laboratory tests. The Swedish Transport and Communications Research Board (KFB) provided funding of the study.
REFERENCES RVF Statistik 1997. Renhfillningsverksf6reningen, RVF Service AB, Malm6. Arm M. 2000. Mechanical properties of unbound road materials. Swedish Road and Transport Research Institute, Link6ping. (will be published during spring 2000, in Swedish with an English summary) B~ickman L. 1989. Best~imning av organisk halt i grovkorniga material (b~irlager). VTI notat V84. Swedish Road and Transport Research Institute, Link6ping. (in Swedish) Andersson H., Arm M., Carling M. & Schouenborg B. 1999. Provningsmetoder f6r alternativa v~igmaterial i underbyggnader. Swedish Road and Transport Research Institute, Link6ping. (in Swedish) Lundgren T. & Hartl6n J. 1991. Slagg frfin avfallsf6rbr~inning - Teknik och Milj6. REFORSK rapport FoU 61, Malm6. (in Swedish with an English summary)
567
V a r i a t i o n in m e c h a n i c a l p r o p e r t i e s o f M S W i n c i n e r a t o r b o t t o m ash Results from triaxial tests Maria Arm Swedish National Road and Transport Research Institute (VTI), Link6ping, Sweden
ABSTRACT This study deals with laboratory testing of municipal solid waste (MSW) incinerator bottom ash. The aim was to investigate the mechanical properties such as stiffness and stability of the ash for future use in unbound road layers. Especially the effect of the material variation on the mechanical properties was analysed. Specimens of bottom ash from four different plants and four seasons were tested by repeated load triaxial tests. Results so far suggest that there is a significant variation in the mechanical properties, both seasonal fluctuations and differences between incinerator plants. However, the variation is not greater than for studied natural aggregates. It is also shown that the organic content has a limiting effect on the resilient modulus, as expected. If MSWI bottom ash is utilised instead of sand in a capping layer, the same design modulus could be used as for the sand. All tested materials are stable and give reasonable permanent deformations at the stress level that is relevant for a Swedish capping layer.
1. I N T R O D U C T I O N Despite Sweden's relative richness in natural aggregate reserves, there is a governmental ambition to facilitate and increase the use of alternative materials in Swedish road constructions. Several research projects are going on in purpose to establish both design and environmental guidelines. One material that has not been used in Sweden in large-scale projects yet, is bottom ash from incineration of municipal solid waste, here called MSWI bottom ash. This study deals with laboratory testing of unbound MSWI bottom ash. The aim was to investigate the mechanical properties of the ash when used in unbound road layers, at first hand in capping layers. Great variations in the ash properties were expected, due to seasonal variations in the waste. Different incineration processes could also be expected to have an influence on the mechanical properties of the bottom ash. Therefore, ashes from different seasons and different incineration plants were studied. MSWI bottom ash from four plants was chosen, namely from Stockholm, Gothenburg, Malm6 and Link6ping. These plants incinerate both municipal and industrial wastes. They are among the largest in Sweden and produce both heat and electricity. The production of bottom ash for each of the stations is between 35 000 and 80 000 metric tons per year, as can be seen in table 1. This can be compared to the total amount for Sweden, which was about 340 000 metric tons in 1997.
568
2. M E T H O D S The Swedish Geotechnical Institute (SGI) made sampling in the plants as part of another study. The sampling was carried out during different times of the year, approximately every third month. Then, the 16 materials were stored outdoors for at least six months in wooden boxes without roofs. From these boxes representative samples were taken for this study. The mechanical properties were studied using repeated load triaxial tests. Complementary properties such as composition, particle size distribution, organic content, optimum water content and maximum dry density were also investigated. Comparison with corresponding properties of conventional materials, such as different sandy materials was made. SGI has done chemical and environmental characterisation on the same material and those results will be reported separately. In the repeated load triaxial tests, the material deformation under simulated traffic conditions is investigated. The resilient strain is used to calculate the stiffness expressed as resilient modulus and the accumulated permanent deformation can be used tbr classification purpose. Since the specimen exposed to loading consists of the whole composite material up to a certain grain size, it is in fact the function of the material that is tested. The method is well known both for fine-grained and coarse-grained materials and a CEN standard is under development. The tests were carried out on undrained specimens with the dimensions of 150 mm diameter and 300 mm height in VTI:s servo-hydraulic material testing system (VMS). This allowed ashes with maximum particle size up to 30 mm to be tested. The specimens were undrained since this is most similar to reality. The traffic load is quick compared to the long time loading that is used in soil mechanics. The specimens were manufactured in o n e layer by means of vibrating compacting equipment called Vibrocompresseur in a special cylinder. This is a "friendly" method that was chosen because earlier experiments showed crushing tendency for these materials. After compaction the specimens were pushed out of the cylinder and equipped with platens in both ends and a thin rubber membrane around. The aim was to test all materials at the same relative water content and the same relative density. The conditions that were chosen were optimum water content and 90 % of maximum dry density from modified proctor. However, problems in compacting some of the specimens resulted in differences in the attained compaction degree. The impact this fact has had on the test results is discussed later. For each material investigated, three specimens were compacted and tested. The results that are discussed here are averages for these three specimens. The tests are still carried on and will be finished during this year.
569 3. RESULTS
The stations produce bottom ash with grain size distributions according to figure 1.
Figure 1.
Grain size distribution for MSWI bottom ash from four different stations and three different seasons.
SGI has carried out compaction tests on the materials by means of modified proctor. The same institute has also reported organic content measured as loss on ignition (LOI) for the studied materials. Table 2 and 3 summarises the results so far. Table 2.
Season Autumn Winter Spring Table 3. Autumn Winter Spring
Maximum dry density and optimum water content, from modified Proctor tests (from SGI). Stockholm Gothenburg Malm6 Link6ping Max dens Optw Max dens Optw Max dens Opt w Max dens Opt w (%) (%) (t/m3) (t/m3) (%) (t/m3) (%) (t/m3) 1,61 13,5 1,73 17 1,65 15-18 1,58 18,0 1,66 9,0 1,72 16,1 1,63 16,5 1,48 19,5 1,58 11-19 1,66 17-19 1,64 16,4 1,43 20,5 Loss on ignition (LOI) in %, sample A/sample B. (from SGI) Stockholm Gothenburg Maim/3 Link6ping 3,9/4,1 2,8/4,0 3,8/3,8 6,5/7,0 3,2/4,0 2,7/2,7 4,4/4,4 9,1/9,2 4,1/4,3 3,7/3,7 4,9/5,0 8,9/9,0
570
Furthermore, SGI has investigated the environmental properties by leaching tests on the same materials. Results from these investigations will be reported separately by Dr AnnMarie F&illman.
3.1. Stiffness and stability Data from triaxial tests on bottom ash from four stations and three seasons are presented here. In figure 2 the stiffness expressed as resilient modulus for different stresses is plotted for one of the stations. Results from the other stations showed the same pattern.
Figure 2.
Stiffness expressed as resilient modulus for MSWI bottom ash from Stockholm.
Two things can be noticed. First, the curves are rather flat, which indicates a weak stress dependency and is typical for sandy materials. Second, the differences between "spring", "winter" and "autumn" ashes are significant. However, the relative position of curves, with the "spring" modulus being higher than "winter" and "autumn" modulus, is not general. This is illustrated in figure 3, where the results from one stress condition are compared for all four stations. The scatter among the three specimens tested for each material is also illustrated. Some of this scatter and also the variation between materials can be explained by unintended differences in density of the specimens. This is discussed in a subsequent chapter. The stability expressed as permanent compressive deformation will also be evaluated within this project. Preliminary results suggest that the permanent deformations are very small as long as the mean normal stress is less than 60 kPa. In figure 4 the deformations at a mean normal stress of 50 kPa are compared.
571
Station and season Sthlm autumn
m
Sthlm winter
m
Sthlm spring G bg autumn _
Gbg winter
m m
Gbg spring
mm
Maim6 autumn
m
Maim6 winter Maim6 spring
m
Lkpg autumn
m
Lkpg winter
m
m
m .
.
.
.
Lkpg spring
t
m
0
25
50
75
100
125
150
175
Resilient modulus (MPa)
Figure 3.
Variation in resilient modulus at one stress condition. 5 % significance. Mean normal stress = 50 kPa.
Station and season Sthlm autumn
m
Sthlm winter
m _
Sthlm spring
m
Gbg autumn
m
Gbg winter
m
Gbg spring
m
Maim6 autumn Maim6 winter
m
Maim5 spring
m
m
Lkpg autumn
m) _
Lkpg winter
m _
Lkpg spring
m _
0.0
0.5
1.0
1.5
2.0
Permanent strain (%)
Figure 4.
Variation in accumulated axial permanent compression. 5 % significance. Mean normal stress = 50 kPa.
572
Comparison with conventional materials The test results have been compared with the corresponding results for natural aggregates that the bottom ash could possibly replace, such as sand with different grading. Those sandy materials were all taken from a construction site, from different sections of the new E6 at the west coast of Sweden. They have been tested in the same way within another research project at VTI (2). In all, it was twelve sandy materials with a sand content between 45 and 9 1 % and with a water content of ca 0,6 x opt w. The M S W I bottom ash showed about the same resilient modulus as the sand, but much smaller permanent deformations, see figure 5 and 6.
Figure 6.
Variation in acc. perm. compression for bottom ash from Stockholm and different sandy materials. 5 % sign. Mean normal stress = 50 kPa.
573 The fact that the sandy materials showed greater permanent deformations was not surprising since many of the sands were rather even-grained, see figure 7. Nevertheless, they were all "real" material that is being used in road constructions.
100% 0. 06
fine
Sand 0 2 medium 0 6
coarse
Gravel medium21
fine
coarse 6(
90%
"~,kI'
80% ._,- 70%
/
O9
Q.
60%
~
50%
~
40%
i pp-
,,
r
//Y!
E 30% 13_
/S
f
i p
,It
J
~
Fajp ~, F 4p~,Bp ,,,,, pqap ~ ~,'
Z) v
20% 10%
l
0% 0.125
0.25
0.5
1
2
4
Particle size, mm
Figure 7.
5.6
8
11.2
16
31.5 45
Grain size distribution for twelve bottom ash types and six sandy materials. Dotted lines represent bottom ash.
4. DISCUSSION Mechanical properties of a pavement layer is very much depending on how well the compaction work has succeeded and this in turn is depending on the grain size distribution and the grain shape of the compacted material. Compaction tool and water supply are other important parameters, but they are no material properties. Furthermore, if the mechanical properties should be kept during the lifetime of the road, the grains must be resistant to both mechanical and climate wear, but that is not discussed in this paper. A property that is seldom problematic for Swedish conventional road materials, but could be crucial for an incineration bottom ash, is the organic content. Beside the raw material, it is the manufacturing process that controls the organic content and other properties of the ash. Variations in mechanical behaviour are therefore depending on variation in some or several of the properties mentioned above. These properties are discussed further here below.
4.1. Raw material and organic content As earlier mentioned, the four incineration stations incinerate both municipal and industrial wastes. The proportions are listed in table 4.
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Table 4. Station Stockholm Gothenburg Malm6 Link6ping
(RVF 1997), (from SGI) Proportion municipal waste 81% 60 % 59 % 76 %
Organic content (LOI) 3,6-4,2 % 2,7-3,7 % 2,6-4,9 % 5,9-9,1%
According to the table the plants can be grouped into two, namely Gothenburg and Malm6 with about 60 % municipal waste and Stockholm and Link6ping with ca 80 ~ Bottom ash from Link6ping has the highest organic content. The ash from this station also turned out to have the lowest stiffness expressed as resilient modulus. An attempt to relate the resilient modulus and the permanent deformation to the organic content has been made in figure 8.
Figure 8.
Organic content measured as LOI as a function of resilient modulus and accumulated permanent compression respectively.
It is generally agreed that organic matter has a bad impact on the stiffness. Most country has limited the organic content in road materials. An early Swedish study (3) shows that already a small proportion of organic matter, 6 % of the material with grain size less than 2 mm, deteriorates the E-modulus greatly. The study in this paper suggests that even the permanent deformation is influenced in a negative way. Also, the material composition has been defined through visual observation. The results are shown in figure 9.
575
Figure 9.
Main constituents of bottom ash from Stockholm, Gothenburg, Malm6 and Link6ping
From the diagrams it can be seen that there are no great differences between the ashes except for one of the plants, Stockholm, whose ash contains a lot more glass than the other, about 50 % compared to ca 35 %. 4.2. Grain size d i s t r i b u t i o n
In figure 10 the grain size distribution for the ash from each of the studied stations is plotted. The three curves represent material sampled at different seasons. Stockholm Sand Gravel 900/0006 fine 02medium06 coarse:~! fine 6....medium 21)coarse l~="~"~-6(
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Figure 10. Grain size distribution for studied materials.
/,
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31.5 45
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576
According to the grain size distribution all materials can be classified as sandy gravel. It can also be seen that the time dependent variation is small. Even between the stations the differences are small. Table 5 contains some of the parameters describing the grain size distribution for studied materials.
Nevertheless, from figure 10 and table 5 a few things can be pointed out. First, the "winter" ash from Malm6 contains more coarse particles (> 6 mm) than the other ashes. This should be favourable for the stiffness. Second, the "winter" ash from Malm6 and the "spring" ash from Link6ping have the flattest curve shape. The curve shape can be characterised by a uniformity coefficient, Cu, that is the ratio of d60 to d~0. D60 means the mesh of the sieve through which 60 % of the material pass. For conventional materials, it has been shown that the less steep the curve is, the more stable is the material (2). Third, the fines content is between 5 and 8 % for all materials with two exceptions, Link6ping "winter" with 2 % and Link6ping "spring" with 10 %. The fines content allowed in road materials is usually limited because of the risk of frost heave. In Sweden the limit is 8 %. However, a certain amount of fines is essential for a good compaction result.
4.3. Grain shape The grain shape can be rounded or more or less angular, which has influence on the deformation properties. A more angular material requires a bigger compaction effort, which in its turn can create crushing and an increase of fines. Conversely, a material with rounded particles is generally easy to compact but is also more unstable than an angular material. Results from another investigation (4) show that MSWI bottom ash has more angular particles than conventional sandy gravel and also more than crushed concrete. It is mostly the particles in the fraction 20-32 mm that are flaky. The content of angular particles could explain the difficulties in compacting the material to laboratory specimens. Field experiences, however, have reported that the bottom ash is very easy to compact (5). This is probably because of the heavy loads that have been used.
4.4. Compaction degree The fact that the specimens were not tested at the same compaction degree and the same relative water content must be analysed. The 300-mm high specimens with a diameter of 150 mm were compacted in one layer by simultaneous vibration and compression in a Vibrocompresseur. It is a method that has been used with success for conventional materials. Among the advantages can be mentioned that the material is homogeneous compacted and
577
that the water content and density for the specimen can be chosen in advance. Another advantage is that the vibrating compaction is more "friendly" This method was chosen because the modified proctor tests showed a certain crushing tendency for these materials. However, the target density of 90 % compaction degree was not reached and the water content was above the optimum in all materials except one. The reason for this was probably too angular material and too "careful" compaction. The actual values are given in table 6. Table 6.
Season Autumn Winter Spring
Compaction degree (actual dry density/max, dry density from modified proctor tests) and water content for tested materials. Average of three specimens. Stockholm Gothenburg Malm6 Link6ping Compact. w Compact. w Compact. w Compact. w degree degree degree degree (%) (%) (%) (%) (%) (%) (%) (%) 84 O p t - 0,5 89 Opt + 1,4 85 Opt + 1,8 86 Opt + 0,8 87 Opt + 1,0 89 Opt + 0,8 90 Opt + 0,4 88 Opt + 0,7 88 Opt - 0,1 89 Opt + 0,7 82 Opt + 0,6 80 Opt + 0,8
All the materials from Gothenburg, which had the smallest d90 and favourable water contents, were well compacted. They also had resilient modulus above 110 MPa (at a mean normal stress of 50 kPa). Stockholm's "autumn" ash had about the same grain size distribution but was too dry. This is the only material that was tested at a water content below optimum. The compaction degree was therefore considerably lower, 84 % instead of 89%. The resilient modulus for this ash was below 95 MPa. The different compaction degree and the resulting variation in stiffness show the importance of a good compaction.
5. CONCLUSIONS The experiments are still going on, but the results so far suggest that the mechanical properties expressed as measured permanent deformation and calculated resilient modulus from triaxial tests are reasonably uniform for each incineration station. There is a significant difference between the stations and even within a station. However, the scatter is not bigger than for the studied natural aggregates. The value of a relevant design modulus usually depends on where in the construction a material is utilised. Though, it seems that the stress level is not that important for MSWI bottom ash since the tests showed a weak stress dependency for the resilient moduli. From the comparison with some natural aggregates it may be said that if MSWI bottom ash is utilised instead of sand in a capping layer, the same design modulus could be used as for the sand. All tested materials are stable and give reasonable permanent deformations at the stress level that is relevant for a Swedish capping layer. Furthermore, a better stability expressed as a smaller permanent deformation could be expected if an even-grained sand is replaced by MSWI bottom ash. It is shown (as expected) that the organic content has a limiting effect on the resilient modulus. Finally, it can be said that it is possible to create a stiff and stable unbound capping layer of MSWI bottom ash if the organic content is low and the right compaction procedure is used.
578 ACKNOWLEDGEMENTS In this research several colleagues at the Swedish Road and Transport Research Institute have taken part. The author wish to express her appreciation for the co-operation, especially to Hhkan Arvidsson who did most of the laboratory tests. The Swedish Transport and Communications Research Board (KFB) provided funding of the study.
REFERENCES RVF Statistik 1997. Renhfillningsverksf6reningen, RVF Service AB, Malm6. Arm M. 2000. Mechanical properties of unbound road materials. Swedish Road and Transport Research Institute, Link6ping. (will be published during spring 2000, in Swedish with an English summary) B~ickman L. 1989. Best~imning av organisk halt i grovkorniga material (b~irlager). VTI notat V84. Swedish Road and Transport Research Institute, Link6ping. (in Swedish) Andersson H., Arm M., Carling M. & Schouenborg B. 1999. Provningsmetoder f6r alternativa v~igmaterial i underbyggnader. Swedish Road and Transport Research Institute, Link6ping. (in Swedish) Lundgren T. & Hartl6n J. 1991. Slagg frfin avfallsf6rbr~inning - Teknik och Milj6. REFORSK rapport FoU 61, Malm6. (in Swedish with an English summary)