PHYSICAL, MECHANICAL AND CHEMICAL EVALUATION …agremax.com/Downloads/Final Report - TTI.pdf ·...
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PHYSICAL, MECHANICAL AND CHEMICAL EVALUATION OF MANUFACTURED AGGREGATE BY Syam Kochyil and Dallas N. Little, Ph.D, P.E. Texas Transportation Institute Texas A&M University College Station, Texas
Transcript of PHYSICAL, MECHANICAL AND CHEMICAL EVALUATION …agremax.com/Downloads/Final Report - TTI.pdf ·...
PHYSICAL, MECHANICAL AND CHEMICAL EVALUATION OF MANUFACTURED AGGREGATE
BY
Syam Kochyil and
Dallas N. Little, Ph.D, P.E.
Texas Transportation Institute
Texas A&M University
College Station, Texas
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TABLE OF CONTENTS Page
List of Figures......................................................................................................... iii List of Tables ........................................................................................................................ iv Objective ................................................................................................................ 1 Approach................................................................................................................ 1 Results and Analysis .............................................................................................. 2
Gradation and Sieve Analysis............................................................................. 2 Specific Gravity................................................................................................... 3 Fine Aggregate Angularity .................................................................................. 4 Optimum Moisture Content................................................................................. 4 Modified Compaction Energy.............................................................................. 5 pH....................................................................................................................... 6 Permeability........................................................................................................ 7 Durability and Abrasion Resistance.................................................................... 8 Resilient Modulus ............................................................................................... 8 Compressive Strength ........................................................................................ 11 Shear Strength ................................................................................................... 16 California Bearing Ratio...................................................................................... 19 Swell Testing ...................................................................................................... 23 X-Ray Diffraction ................................................................................................ 24 ESEM ................................................................................................................. 31 Stability Models .............................................................................................................. 34
CONCLUSIONS..................................................................................................... 37 Applications............................................................................................................ 38 Use as a Pavement Base or Sublease ................................................................ 38 Use as a Fill Material ........................................................................................... 39 Use as a Landfill Cover ....................................................................................... 39 Potential for Expansion Due to Formation of Ettringite ........................................ 41 Impact of Stabilization ......................................................................................... 41 Use as asphalt aggregate and/or Filler ................................................................ 41 RECOMMENDATIONS.......................................................................................... 42 APPENDIX A.......................................................................................................... 43
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LIST OF FIGURES Figure Page 1. Typical gradation curve of RA sample blended from three 55-gallon drums ............................................................................................. 2 2. Moisture-density relationship of MA following ASTM D-698 protocol ............. 5 3. Moisture-density relationship following modiifed proctor compaction protocol,
ASTM D-1557 ................................................................................................ 6 4. Resilient modulus versus bulk stress for MA sample (no cure) ...................... 9 5. Resilient modulus (14-days cure)................................................................... 10 6. Resilient modulus (14-days cure)................................................................... 11 7. Unconfined compressive strength (typical, representative gradation) ............ 12 8. Unconfined compressive strength (maximum particle size 2.36 mm) ............ 13 9. Unconfined compressive strength (Type 2 cement 7-day cure) ..................... 14 10. Unconfined compressive strength (Type-1 cement 7-day cure) ..................... 15 11. Texas triaxial test results (without cure) ......................................................... 17 12. Texas triaxial test results (14-days cure)........................................................ 18 13. Texas triaxial classification (14-days cure)..................................................... 18 14. CBR plot (without cure) .................................................................................. 19 15. Variation in CBR as a function of dry density ................................................ 22 16. Impact of compaction on gradation ................................................................ 23 17. Swell test results (samples compacted at optimum moisture content following
ASTM D-698) ................................................................................................. 24 18. XRD-stockpiled MA specimen without curing................................................. 25 19. XRD-stockpiled MA specimen (14 days cure) ................................................ 26 20. XRD-stockpiled MA specimen (14 days cure with 6% Type-1 cement).......... 27 21. XRD-stockpiled MA specimen (14 days cure with 6% Type-II/V cement) ...... 28 22. XRD-stockpiled MA specimen (28 days cure without cement) ....................... 29 23. XRD-stockpiled MA specimen (28 days cure with 6% Type-II/V cement) ...... 30 24. XRD-stockpiled MA specimen (28 days cure with 6% Type-1 cement).......... 31 25. ESEM 0-day cure ........................................................................................... 32 26. ESEM (7-day cure 6% cement Type II/V cement).......................................... 32 27. ESEM (28-day cure 6% Type 1 cement) ........................................................ 33 28. ESEM (28-days cure 6% Type-2 cement) ...................................................... 34 29. Stockpiled MA stability model-1 ..................................................................... 35 30. Stockpiled MA stability model-2 ..................................................................... 36 31. Stockpiled MA stability model-3 ..................................................................... 36
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LIST OF TABLES
Table Page 1. Typical gradation in tabular form.................................................................... 3 2: Specific gravity results ................................................................................... 3 3. pH of MA-water paste (1:5 ratio of MA-to-water)............................................ 6 4. Permeability or MA sample ............................................................................ 7 5. Compressive strength (typical representative gradation) ............................... 12 6. Unconfined compressive strength (fine gradation) ......................................... 13 7. 7- Days unconfined compressive strength with Portland cement ................... 16 8. CBR results without cure................................................................................ 20 9. Change in CBR values with cure and addition of cement............................... 20 10. CBR-dry density relationship.......................................................................... 21 11. Effect of compaction on gradation.................................................................. 22
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Objective The objective of the study was to analyze the potential of Manufactured
Aggregate (MA) to be used as a civil engineering construction material. Like other coal combustion by- products, MA consists primarily of agglomerated ash particles, which gain strength with time due to cementitious reactions. All coal combustions by-products possess unique properties based on the composition of the ash and the production processes. It is necessary to evaluate the mechanical properties of the ash and the ability of the ash to withstand environmental fluctuations. Specifically this study focused on mechanical and durability properties of MA when used as a base or subbase pavement layer, as a structural fill, or as an embankment material.
Approach First, a typical gradation or the MA was established based on three 55-gallon
drum samples received from AES, Puerto Rico. Material from the drums was combined and quartered according to American Association of State Highway and Transportation Officials (AASHTO) sampling protocol to achieve a representative sample for gradation analysis. The same approach was used to obtain representative samples for the second step, which was to measure physical, mechanical, and chemical properties. Physical and mechanical testing revealed properties required to define engineering applications of the MA. Chemical and mineralogical evaluation was performed in order to provide the necessary background information to determine the propensity of the MA to form deleterious, expansive minerals during hydration reactions in the presence of water of construction and water introduced by the environment. Finally, the impact of adding Portland cement to improve mechanical and engineering properties was evaluated.
Historically, coal combustion by-products containing sulfates have demonstrated the propensity to swell if calcium, aluminum and sulfate from the ash material combine in the presence of water to form an expansive mineral, typically ettringite. The potential for ettringite to form was evaluated based on chemical and thermodynamic modeling and by performing swell testing on samples prepared to mimic a typical construction process and considering typical environmental effects. Laboratory tests for determining the engineering properties of the material were based on American Society for Testing and Materials (ASTM), American Association of State Highway and Transportation Officials (AASHTO) and Texas Department of Transportation (TxDOT) standard test methods. A new test protocol developed by the Texas A&M University for the Texas Department of Transportation was used to perform swell testing where MA samples were prepared using a gyratory compactor. Specimens are compacted to the required dry density as a single layer using a gyratory compactor. Specimens were then exposed to air for 72 hours and then subjected to controlled soaking. The results and analysis of the tests performed are presented in the following sections.
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Results and Analysis
Gradation and Sieve Analysis
A representative MA sample was obtained by mixing sub-samples obtained following AASHTO T-248. The composite sample was then sieved following ASTM C 136/AASHTO T 27 specifications. A gradation of the material, which is deemed to be typical based on the sampling technique, is presented in Figure 1 and in Table 1.
Figure 1. Typical gradation curve of MA sample blended from three 55-gallon
drums.
0.00%
10.00%
20.00%
30.00%
40.00%
50.00%
60.00%
70.00%
80.00%
90.00%
100.00%
0.0001 0.001 0.01 0.1 1 10 100
sieve size (mm)
Per
cent
Pas
sing
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Table 1. Typical gradation in tabular form.
Sieve size Sieve size (mm)
Wt. retained (gm)
% Retained on sieve
Total % passing
2 in 50 0 0.00 100.00 1 ½ in 37.5 9990 6.73 93.27
1 in 25 12932 8.71 84.55 ¾ in 19 11036 7.44 77.12 ½ in 12.5 15027 10.13 66.99
3/8 in 9.5 10814 7.29 59.70 #4 4.75 22061 14.87 44.84 #8 2.36 15847 10.68 34.16
#16 1.18 12525 8.44 25.72 #30 0.6 8997 6.06 19.65 #50 0.3 7414 5.00 14.66 #100 0.15 13973 9.42 5.24 #200 0.075 4934 3.32 1.92 P-200 0 2844 1.92 0.00
Specific Gravity
Specific gravities of different size fractions of coarse aggregate in the MA specimen were determined following AASHTO T-85. The average values of specific gravity for three replicates of coarse aggregate (larger than 2.32 mm) are presented in Table 2. Specific gravities of the fine fraction (smaller than 2.32 mm) were also determined following ASTM D 854-02, and the results are included in Table 2.
Table 2. Specific gravity results.
Type of Specific Gravity Aggregate Type Value Bulk specific Gravity Coarse Aggregate 1.164 Bulk SSD specific Gravity Coarse Aggregate 1.733 Apparent specific Gravity Coarse Aggregate 2.701 Bulk specific gravity Fines 2.698
The difference between bulk specific gravities of the fine (smaller than 2.36 mm) and coarse (larger than 2.36 mm) fraction reflects the high void content of the coarse aggregate particles that are actually agglomerations of fine particles. Such agglomerations include a high air void content. The high void content (low specific gravity) of the agglomerated coarse aggregate particles is consistent with their sensitivity to abrasion as reflected by the Los Angeles abrasion test and the general overall weakness of individual particles. The average bulk specific gravity of the coarse aggregate fraction was found to be approximately 1.16, while the average bulk specific
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gravity of fine fractions (passing #4 sieve size) was found to be 2.69. While the fine aggregate fraction specific gravity is typical of natural aggregates, the specific gravity value of the coarse aggregate is less than one-half of the specific gravity of traditional natural aggregates, i.e., between about 2.5 and about 2.7. The low specific gravities of the coarse aggregate are consistent with the low abrasion test values, which predict a poor level of durability during handling and compaction. The ultimate concern is that gradation specifications for the MA will be difficult to define and achieve due to the high level of deterioration of the coarse MA. The degree of breakdown and degradation will likely be exacerbated in the presence of a high level of moisture as the high void content of the coarse aggregate promotes absoption of the water leading to mechanical degradation.
Fine Aggregate Angularity
The fine aggregate angularity test was performed following ASTM C 1252. Angularity of the fines is an indicator of the workability of the composite aggregate (fines and coarse fraction). It is also an indicator of the resistance to shear developed due to fine aggregate particle interaction. The void content of the MA fine fraction is approximately 60 percent. Based on this parameter alone the fine aggregate would appear to enhance the stability of hot mix asphalt if used in the blended aggregate. However, abrasion resistance must also be considered. Based on the considerably high specific gravity values for the fine aggregate compared to the coarse fraction, one can reasonably assume that the fine aggregate will have better abrasion resistance than the coarse aggregate when used as an aggregate in asphalt. However, the performance of the fine aggregate in hot mix asphalt concrete must be more thoroughly evaluated. Furthermore, the interaction between bitumen and the aggregate surface defines the adhesive bond between these two constituents. The composition of the fly ash will define the strength of the bond, and this requires analysis beyond the scope of this project.
The combined effect of a high void content based on the fine aggregate angularity test and the typical specific gravity of the fine aggregate, indicates that the fine MA is a good material with strong inter-particle interaction. One would expect the fines (smaller than about 2.36-mm) to compare favorably with sandy to silty sand fill or structural fill (ranging from AASHTO classifications of A-4 to A-2-4) with good resistance to shear and deformation.
Optimum Moisture Content
Optimum moisture content at maximum density of the representative composite sample, Table 1, was determined following ASTM D 698. Samples of a representative, typical gradation were compacted at different moisture contents using a standard Proctor compaction effort of about 12,400 ft-lb/ft3, and the relation between water content and maximum dry density was determined. The MA was found to have a maximum dry density of 67 pounds per cubic foot (pcf) and optimum moisture content of approximately 50 percent. These values are relatively typical of compacted fly ash. Maximum dry densities of soils are typically between about 100 and 120 pcf and optimum moisture contents for soils seldom exceed 30 percent. In fact soils of low plasticity, sands and silts, typically have optimum moisture contents of less than about
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18 percent. The low unit weight of the MA may be viewed as a beneficial attribute when considered as a fill.
Figure 2. Moisture-density relationship of MA following ASTM D-698 protocol.
The ability of the MA to accommodate so much water is probably due in part to
the presence of fine particles and the agglomerated fine particles, which provide voids that can hold water by capillary tension. Apparently a considerable amount of water is required to break the capillary tension and accommodate densification. Furthermore, some of the water is likely to be physico-chemically bound to the MA particles as part of the pozzolanic strength development process.
Modified Compaction Energy
When a modified Proctor level of compaction energy is applied during compaction, following ASTM D 1557, the compaction effort is slightly over 56,000 ft-lb/ft3. This compaction energy simulates a much more aggressive field compaction effort. It is important to assess the impact of compaction effort, as increased densification of stabilized materials generally results in considerable increase in strength. In fact an increase in compactive effort from 12,000 ft-lb/ft3 to 56,000 ft-lb/ft3 can increase the unconfined compressive strength of stabilized soils by from about 100 to 400 percent. Under increased compactive effort, the MA attained a moderately higher dry density of 69 pcf at a slightly lower moisture content of 48.50 percent.
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62.5
63
63.5
64
64.5
65
65.5
66
66.5
67
40 45 50 55 60 65Moisture Content (%)
Dry
Den
sity
(pcf
)
6
Figure 3. Moisture-density relationship following modified proctor compaction
protocol, ASTM D-1557.
The impact of the higher compactive effort is not substantial for the MA, and it can be concluded that a standard compactive effort is sufficient to produce an acceptable MA material. It can also be concluded that the specification target maximum density following an ASTM D-698 compactive effort is approximately 69 pcf at a moisture content of between about 45 and 50 percent. It is also noteworthy that the optimum moisture content does not change much between standard and modified compaction energies. The void content in the MA matrix is very high, approaching 50 percent voids. The light unit weight of the MA is due in large part to the high voids content as the apparent specific gravity of the MA is approximately the same as most mineral aggregates. The low unit weight of the MA is advantageous as a fill material as lighter material is easier to transport and manipulate. However, as a base material the low unit weight is not necessary advantageous, and it may well be appropriate to consider blending of hard, angular coarse aggregate with the MA.
pH
Manufactured Aggregate slurries were tested for pH, and the results are listed in Table 3. A paste was formed with the MA prior to pH testing. The solution ratio for the paste was one part MA to five parts water. The pH of the water used was also tested.
Table 3. pH of MA-water paste (1:5 ratio of MA-to-water).
Trial pH Type of material
1 10.66 Paste
63
64
65
66
67
68
69
40 50 60
Moisture Content(%)
Dry
den
sity
(pcf
)
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2 10.79 Paste
3 10.80 Paste
4 10.48 Solution
5 10.47 Solution
6 10.50 Solution
7 9.16 Water
The relatively high pH levels of the MA specimens indicates that these MA samples, which were collected from stockpiles approximately 2-months old, still possess some potential for pozzolanic reactions and may still possess some moderate risk of ettringite formation. When samples were made into a solution with water and allowed to remain in solution for about two hours and then tested, the pH of the solution increased to an average value of 10.5.This pH increase could be due to the dissolution of calcium and hydroxide ions from free lime in the MA.
Permeability
Permeability of MA is an important parameter in the design of pavement subbases and fill material. Table 4 shows the change in permeability versus time and flow. The permeability (using a constant head permeameter, ASTM D 2434-68) stabilized at a representative value of approximately 4.07E-05 cm/sec. This value is similar to that of other fly ash materials and lower than bottom ash. This value of also similar to permeabilities of unstabilized fine sands and is considerably higher (by an order of magnitude) than fine silts. These permeability values for the MA are also similar to those of cement stabilized sands.
Table 4. Permeability or MA samples.
Quantity of flow (ml) Elapsed time (sec) Permeability k (cm/sec)
100 2001 3.96528E-05 200 3840 4.13257E-05 300 5848 4.07038E-05
The permeability values reported are representative of a typical sample prepared following ASTM D-698 compaction with an energy of approximately 12,300 foot-pounds per cubic foot. The permeability can be affected by allowing the MA to cure in situ before compaction. The impact of curing is the development of cementitious
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flocculation. The impact of curing on particle size and its effect on gradation is probably minor for thin lifts but is probably more significant for thicker lifts.
Durability and Abrasion Resistance
The Los Angeles (LA) abrasion test was performed following ASTM C-131 in order to assess the potential of MA to degrade during production and construction. Normally for aggregates used in Portland cement concrete, in asphalt mixtures, or in unbound bases an LA abrasion loss of less than about 35 percent is recommended. The average LA abrasion of MA samples was 60 percent, which makes it highly susceptible to degradation during construction operations. The low abrasion resistance of MA may be due to the high voids content of the coarse fraction where individual particles are comprised of agglomerates. This is verified by the low specific gravity values of the coarse particles. Although the material cannot be considered as a traditional aggregate, because of cementitious development over time, it can be considered for use as a specialty aggregate base that derives some of its favorable mechanical properties from inter-particle shear resistance and some from a low to moderate level of pozzolanic interaction.
The void structure in the fine aggregate, defined as aggregate smaller than 2.36-mm, is apparently different from the void structure in the coarse aggregate, larger than 2.36-mm. This is evinced by the considerably higher specific gravity of the fine aggregate compared to the coarse aggregate, i.e. 2.69 versus 1.16 for the fine and coarse aggregate, respectively. This difference in void structure between fine and coarse aggregate is most probably due to the fact that the fine aggregate is comprised primarily of individual particles in most cases while the coarse aggregate is a complex conglomerate of particles. This is a complex issue and affects the use of the MA as a fine aggregate in hot mix asphalt. Because of the high specific gravity of the fine particles, it can reasonably be assumed that the abrasion resistance of the finer materials will be greater than that of the coarser fraction. Moreover the angularity of the fine aggregates, as seen from the FAA test, will also provide interlocking properties making it possible to use the fine aggregate in hot mix asphalt. Image analysis of the finer fraction will give an exact shape property of the fine materials, and this testing is underway. The suitability of MA as mineral filler (smaller than 75µm) depends mainly on the chemical reactivity of the material with asphalt if any. This is also being evaluated.
Resilient Modulus
The purpose of a base layer is to protect the natural subgrade soil from being overstressed by traffic and to properly spread wheel loads. In order to accomplish this, the base must possess an acceptable resilient modulus, which is defined as the ratio of applied load stress to recoverable strain under that load. A higher modulus means less deformation under load and less transmittal of stress from the load to the underlying soil. Figure 4 presents the results of modulus values of uncured specimens compacted at optimum moisture content following ASTM D 698.
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Figure 4. Resilient modulus versus bulk stress for MA sample (no cure).
Resilient modulus testing was performed on samples following AASHTO 307-99 without curing. According to AASHTO 307-99 modulus values are recorded over a range of stress states from low to high. Modulus values at low stress states (bulk stress) are much lower than acceptable values for a good or even moderate quality base course. However, modulus values at high stress states (bulk stress) while well below acceptable modulus values for good quality aggregates are comparable lower quality subbases. Based on these results, one would expect MA to have a strong tendency to permanently deform at low bulk stress states. These low stress states are typically associated with poor subgrade support and poor lateral support. In order to investigate the effect of curing on modulus values, samples were tested after a 14-day curing period with and without the addition of cement. The results are presented in Figures 5 and 6.
Resilient Modulus w/o cement
1
10
100
1000
10000
100000
1 10 100
Bulk stress, θ (psi)
Res
ilien
t Mod
ulus
, Er (
psi)
10
Figure 5. Resilient modulus (14-day cure).
The effect of a 14-day moist cure period was considerable as shown in Figure 5. The effect of the curing period was obviously to develop some “cementitious” activity that resulted in sufficient cohesion to provide a considerably reduced sensitivity to stress state and modulus of between about 25,000 to about 35,000 psi between bulk stress states of about 10 to 30 psi. This is in the general range of moderate quality unbound aggregate base material when the base is supported by a moderate to good quality subgrade so that the bulk stress will be above about 10 psi. The significant impact of the 14-day curing period may be somewhat offset by the sensitivity of the MA to moisture. If the MA remains saturated for long periods of time, strength tests show that the strength will degrade. It is therefore logical that the resilient modulus will degrade also. However, if the MA layer can be effectively drained so that periods of near saturation are short, the MA has the propensity to function as a moderate quality subbase or low to moderate quality base.
1
10
100
1000
10000
100000
1 10 100
Bulk stress, θ (psi)
Res
ilien
t Mod
ulus
, Er (
psi)
11
Figure 6. Resilient modulus (14-day cure).
Addition of 4 percent Portland cement to the typical gradation of MA increases
the strength properties of the material but does not have much influence on the modulus properties of the material at low bulk stresses. The addition of crushed fractions of coarse ash material might improve the modulus properties by providing particle interlocking.
Compressive Strength
Compressive strengths of the MA were measured following ASTM 1633. The specimens were compacted at optimum moisture content as defined by ASTM 698. Specimens were subjected to accelerated curing at 1000
F for 7, 14 and 28 days in order to (approximately) simulate 30 to 60 days of field curing at temperatures of approximately 70oF. The results of the compression tests and stress values are presented in Figures 7 through 10.
1
10
100
1000
10000
100000
1 10 100
Bulk stress, θ (psi)
Res
ilien
t mod
ulus
, Er (
psi)
12
Figure 7. Unconfined compressive strength (typical representative gradation).
Table 5. Compressive strength (typical representative gradation). Typical, Representative Gradation
Deformation (in) Stress (psi) 0.091 61.04 0.137 59.14 14 days cure 0.121 43.82 0.169 48.54 0.138 38.11 7 days 0.159 37.42 0.121 11.46 0.134 10.19 0 days 0.133 8.41
Manufactured aggregate specimens gain strength with curing time due to a
modest level of pozzolanic and/or cementitious reaction. The impact of gradation on compressive strength was assessed by altering the gradation so that all material passed the No. 8 (2.36-mm) sieve size. From that point the gradation was proportionally adjusted to match the representative gradation. When particle size decreases, total
0
10
20
30
40
50
60
70
0 0.05 0.1 0.15 0.2 0.25Deformation (in)
Stre
ngth
(psi
)
0 days-1
0 days-2
0 days-3
7 days-4
7 days-57 days-6
14 days-7
14 days-8
14 days-9
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surface area of the sample increases, which provides more surface area for development of cementitious bonds among particles. Tthe result is that a finer gradation produces an increased level of cementitious reaction and higher compressive strengths. These results are summarized in Figure 8.
Figure 8. Unconfined compressive strength (maximum particle size 2.36 mm).
Table 6. Unconfined compressive strength (fine gradation). Deformation (in) stress (psi)
0.107 113.2 0.095 98.56 14 days cure 0.115 92.3 0.149 70.12 0.18 68.44 7 days
0.175 65.28 0.125 13.666 0.151 18.273 0 days 0.136 13.706
0
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40
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120
0 0.05 0.1 0.15 0.2 0.25
Deformation (in)
Stre
ngth
(psi
)
0 days-1
0 days-2
0 days-3
7 days-4
7 days-5
7 days-6
14 days-7
14 days-8
14 days-9
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With this level of strength development, it can be inferred that in a fully cured condition the MA can develop enough cohesive strength to function as a moderate quality base layer.
In order to verify the effect of cement on the strength properties of MA, specimens were cured for 7-days and then tested for compressive strength following accelerated curing. The test was performed following ASTM 1633. Three replicates of 4-in. high by 4.5-inches in diameter were prepared at different cement contents (2, 4 and 6 percent) using both Type 1 and Type 2 cement. Three specimens were prepared without the addition of cement in order to compare the effect of increases in strength due to addition of cement. The results of cement treated samples are summarized in Figures 9 and 10.
Figure 9. Unconfined compressive strength (Type 2 cement, 7-day cure).
0
100
200
300
400
500
600
0 0.05 0.1 0.15 0.2
Deformation (in)
stre
ngth
(psi
)
2%-1
2%-2
2%-3
4%-1
4%-2
4%-3
6%-1
6%-2
6%-3
0%
15
Figure 10. Unconfined compressive strength (Type-1 cement, 7-day cure).
As required by ASTM specifications the cured samples were subjected to
soaking for 4 hours before testing, however the 7-day cured samples without the addition of cement failed in soaking. The loss of strength upon soaking is quite significant. Samples prepared by compacting to standard Proctor compaction energy (ASTM D-698) and cured for 7-days without soaking achieved substantial compressive strength. The loss of this compressive strength upon soaking questions the efficacy of the utility of MA as an unbound base material in the climatic conditions of Puerto Rico. It is possible that the transient strength in unstabilized MA is due to a significant quantity of gypsum confirmed by x-ray diffraction (XRD) analysis. Hydrated forms of calcium sulfate can form in the presence of water, but these products can loose their strength in high moisture conditions.
The results of the 7-day compressive strength testing with cement are
summarized in Table 7. The fourth column of Table 7 reports the measured deformation during testing that consistently ranges between 0.10 and 0.15 inches. This equates to a strain at compressive failure of between about 0.2 and 0.33 percent.
100 150 200 250
300 350 400
450 500
0 0.05 0.1 0.15 0.2
Deformation (in)
Stre
ngth
(psi
)
2%-12%-22% -34% - 14% - 24% 36 %-16%-26%-30%
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Table 7. 7-day unconfined compressive strength with Portland cement.
No Cement Compressive strength Compression (in)
1 Type 2 (2%) 90.5 0.11
2 Type 2 (2%) 231.2 0.10
3 Type 2 (2%) 166.75 0.13
4 Type 2 (4%) 325.2 0.12
5 Type 2 (4%) 394.6 0.11
6 Type 2 (4%) 398.45 0.10
7 Type 2 (6%) 493.2 0.13
8 Type 2 (6%) 412.6 0.10
9 Type 2 (6%) 502.2 0.10
10 Type 1 (2%) 229.6 0.08
11 Type 1 (2%) 225.4 0.11
12 Type 1 (2%) 169.7 0.10
13 Type 1 (4%) 396.5 0.10
14 Type 1 (4%) 327.3 0.12
15 Type 1 (4%) 307.1 0.10
16 Type 1 (6%) 439.6 0.15
17 Type 1 (6%) 373.5 0.11
18 Type 1 (6%) 437.2 0.15
19 0 % cement 124.95 0.11
Shear Strength
Texas triaxial testing (Texas Department of Transportation (TxDOT) method Tex-117E)) was performed on the MA in order to determine the shearing resistance of the material. The Texas triaxial cell is unique in that confining pressure is applied in the radial direction of the cylindrical cell and not isotropically. Therefore, the results do not provide a “pure” cohesive intercept, C, nor angle of internal friction, φ. The Texas triaxial test was developed to assess the shear strength of an unbound soil or base material under the range of stress states typically encountered by base courses. The test is typically performed for base material following 24-hours of capillary soak. Therefore, the Texas triaxial test is a better indicator of performance than unconfined compressive strength on unbound materials as it considers both realistic states of moisture and stress for pavement bases. The results from the Texas triaxial test were compared with the standard Texas triaxial classification chart. This chart classifies the MA without curing as a class 2.6 base material. Samples cured for 14-days to verify the effect of
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pozzolanic or other forms of cementitious activity are summarized in Figure 12. These data demonstrate that the MA has reasonable internal friction and cohesive properties.
Figure 11. Texas triaxial test results (without cure).
A comparison of the 7-day cure and 14-day cure Texas triaxial test results
following a 24-hour period of soaking reveal a consistency with the unconfined compression strength data. The cohesive intercept and the angle of internal friction increase as the curing period increases. However, the difference is not large.
0
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20
30
40
50
60
70
0 20 40 60 80 100 120 140 160
Normal Stress (psi)
She
ar S
tress
(psi
)
0-psi3-psi5-psi10-psi15-psi
18
Figure 12.Texas triaxial test results (14-day cure).
Figure 13. Texas triaxial classification (14-day cure).
0
10
20
30
40
50
60
0 20 40 60 80 100 120 140
Normal Stress (psi)
Stre
ngth
(psi
) 0 psi 3 psi 5 psi 10 psi
0
10
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0 5 10 15 20 25 Normal Stress (psi)
Stre
ngth
(psi
)
0 psi 3 psi 5 psi 10 psi class 1 class 2 class 3 class 4 class 5
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California Bearing Ratio
California bearing ratio (CBR) tests were performed on MA samples because the CBR is widely accepted by many state highway agencies. In fact CBR values for unbound aggregate bases are allowed in the proposed 2002 update to the American Association of State Highway and Transportation Officials (AASHTO) Pavement Design Guide as a surrogate for more sophisticated resilient modulus testing. The results of the tests may be compared with the standard values for crushed rock to determine the CBR value of the specimen. A correction factor was applied to the stress penetration curve to account for the surface irregularities on the specimen. The results of the test are summarized in Figures and Tables that follow.
Figure 14. CBR plot without cure.
0
200
400
600
800
1000
1200
0 0.1 0.2 0.3 0.4 0.5 0.6Penetration (in)
Stre
ss (p
si)
20
Table 8. CBR results without cure. Stress (psi) Penetration
(in) Crushed Rock
Manufactured Aggregate
Corrected (Manufactured
Aggregate)
CBR %
0.1 1000 141.3 332 33.2 0.2 1500 458.5 628 41.9 0.3 1900 726.2 867 45.6 0.4 2300 952.2 1064 46.3 0.5 2600 1160.7 1254 48.2
According to ASTM D1883-99 that defines the methodology for CBR testing, the
CBR value is calculated based on the greater ratio of load between the material being tested and the standard at penetrations of 0.1 and 0.2 inches. The bearing ratio for 0.2 inch penetration was greater than the value for 0.1 inch and hence was used to calculate the CBR value for the MA specimens.
The effect of soaking time on CBR values was investigated. The soaking period
was either 24 or 96 hours. The MA showed little sensitivity to the soaking period. When a soaking period was 96 hours, following the ASTM protocol, the CBR dropped by less than 2 percent compared to the 24 hour soaking period. The variation of CBR values with curing was investigated and the results obtained are shown in Table 9. The CBR value of the material without Portland cement and after curing for 7-days was actually lower than that of the uncured specimen. This clearly points towards the variability/heterogeneity of the material. Basic material properties differ for different specimen.
Table 9. Change in CBR values with cure and addition of cement. Material CBR %
0-day cure; 0% cement 40 7-day cure; 0% cement 30 7-day cure; 4% cement 218
The impact of stabilization of the MA with Portland cement was also investigated.
When stabilized with 4 percent Portland cement, MA specimens developed good cementitious properties. The bearing capacity of MA increased by nearly five hundred percent with the addition of 4 percent (Type 1) cement. The CBR values of Manufactured Aggregate specimens after adding cement was approximately 2.2 times the standard values of a crushed rock.
The effect of compaction on the bearing ratio of the material was also
investigated. Manufactured Aggregate samples prepared at an equivalent moisture content were compacted at different energies and the change in CBR values as a function of compaction energy was measured. The effect of compaction energy on the
21
dry density was concomitantly determined and the results are presented in Table 10 and in Figure 15.
Table 10. CBR-dry density relationship.
Compaction Level (no. of blows per layer)
Dry Density (pcf) CBR
10 blows 61.76 18 25 blows 64.01 40 50 blows 65.55 36
As shown in Figure 15, as the compaction energy increases from 10 to 25 blows,
the density increases slightly from about 62 pcf to about 64 pcf. Normally, the peak density is achieved at lower moisture contents when compactive energies are increased. A notable result shown in Figure 15 is the substantial increase in CBR as the compaction energy is increased from 10 to 25 blows (i.e., increase in CBR from approximately 18 percent to approximately 40 percent). As the compaction effort was increased from 25 to 50 blows, the CBR actually decreased from about 40 to about 36 percent. We expect that the increase in compaction energy results in breakage and abrasion of conglomerates developed by weak cementation in the Manufactured Aggregate sample. The impact of compaction in the CBR study is consistent with that discussed previously. The consistent conclusion is that a standard compaction energy (ASTM D 698) is preferred for this material. Unlike other stabilized material, additional compactive effort, equating to about 25 to 30 blows per layer, does not provide substantial improvement in the strength properties of MA.
22
Figure 15. Variation in CBR as a function of dry density.
A plausible explanation of the change in density is related to breakdown of
weakly cemented conglomerates and the production of additional fines. These additional fines may fill voids and densify the mass up to a point when the fines matrix actually reduces density. Table 11 compares gradations after three different compaction efforts: 10, 25, and 50 blows.
Table 11. Effect of compaction on gradation.
Sieve size Total % passing Used original 10 blows 25 blows 50 blows ½ in 86.91 84.59 86.39 88.38
3/8 in 77.43 74.64 75.87 78.37 #4 58.19 56.29 56.13 60.55 #8 44.31 41.13 40.78 45.43
#16 33.36 29.11 29.09 33.16 #30 25.46 23.83 24.11 26.32 #50 18.96 18.91 19.50 20.65 #100 6.80 10.36 10.97 12.01 #200 2.51 6.63 7.11 8.05 P-200 0.00 0.00 0.00 0.00
(65.55, 36)
(64.01, 40)
(61.76, 18)
0 5
10 15 20 25 30 35 40 45
61.50 62.00 62.50 63.00 63.50 64.00 64.50 65.00 65.50 66.00
Dry Density (lb/cft)
CB
R (%
)
10 blows
25 blows
50 blows
23
It is evident that the coarse aggregate fraction breaks down due to compaction as there is an increase in the percentage of aggregate passing each sieve. The gradation curve of the MA specimen after compaction under different loading rate is presented in Figure 16.
Figure 16. Impact of compaction on gradation.
Swell Testing
Swell testing is being performed on the MA to determine the swelling potential of the material in presence of water. The MA tested was exposed to water absorption during the manufacturing process and does not show a spontaneous swelling potential. The swell testing has been performed for over 70 days, and the material shows little to moderate swelling potential. Less than 1.25 percent swell was observed during the test period. However, for many similar materials, it is not uncommon for swelling to begin after 30-45 days of water saturation. Swell testing is continuing. Figure 17 summarizes results to date. The x-ray diffraction (XRD) analysis results show the presence of ettringite in the specimen, but the quantity of ettringite does not increase significantly after 28 days of curing. Furthermore, stability models of the MA evaluated demonstrate that MA does not have energetic potential to form more ettringite. This evaluation is based on calculating the Gibbs free energy of the minerals that have the potential for form based on the chemistry of the MA. Of course the MA samples evaluated were at least two months old at the time of evaluation, and the propensity for ettringite formation
0.00%
10.00%
20.00%
30.00%
40.00%
50.00%
60.00%
70.00%
80.00%
90.00%
100.00%
0.0001 0.001 0.01 0.1 1 10 100
Sieve size (mm)
Per
cent
Pas
sing
10
25
50
original
24
could have subsided. It is also possible that ettringite could form randomly due to the heterogeneity of the material or to the migration of ions or reactive CaO.
It is important to note that since the constituents for ettringite development are present in the MA, the potential for ettringite development leading to swell must be considered on a case-by-case basis.
Figure 17. Swell test results (samples compacted at optimum moisture content following ASTM D-698).
X-Ray Diffraction
XRD analysis (Figure 18) was used to identify and quantify minerals in the raw specimen. The results indicate that ettringite has already formed in the material prior to curing, which is consistent with the preliminary chemical analysis and geochemical reaction models.
0
0.2
0.4
0.6
0.8
1
1.2
0 10 20 30 40 50 60 70 80 Time (days)
Vol
ume
chan
ge (%
)
sample 1sample 2
25
Figure 18. XRD-stockpiled MA specimen without curing.
XRD data for both 7-day and 14-days cured samples indicates the
presence of ettringite. Results from samples cured for 14-days are given in Figure 19. Samples with additives, Type 1 and Type 2 (low C3A content) cement, were also tested, and similar results were obtained. Cement additives are used to determine the feasibility of their usages in stabilization of the material, which could be an option in enhancing the engineering properties of the material. Figures 20 through 21 summarize the XRD results in the presence of added cement. We hoped to use a Type V Portland cement in this testing, but were unable to obtain Type V cement in time for this research. Type V cement containing 0 percent tricalcium aluminate (C3A) could substantially reduce, if not eliminate the potential for ettringite development and heaving. Even though Type V cement contains some tetra-calcium alumino ferrite (C4AF), the amount is limited and C4AF is generally of much less concern that C3A.
26
Figure 19. XRD- stockpiled MA specimen (14-day cure).
27
Figure 20. XRD-stockpiled MA specimen (14-day cure with 6% Type-1 cement).
Gypsum was identified as the source of sulfates in the XRD scans. The constituents for further ettringite formation are present in all specimens. However, a comparison of peak intensities among samples cured for 0, 7, and 14-days revealed no significant differences among the samples and therefore, no significant additional ettringite growth.
28
Figure 21. XRD-stockpiled MA specimen (14-day cure with 6% Type-II/V cement). Manufactured aggregate samples were analyzed using XRD with and without the
addition of cement and after being subjected to accelerated curing for 28-days (Figures 22 and 23). The XRD peak intensity and area under the peak did not change significantly during the curing period indicating that if ettringite did form during the curing period, the amount was not significant. The phase diagram is consistent with the XRD data as it indicates that further thermodynamic potential for ettringite growth is very low.
29
Figure 22. XRD-stockpiled MA specimen (28-day cure without cement).
Cement used as a stabilizer in the MA could be trigger ettringite reaction if the pH increased induced during cement hydration releases soluble alumina from the ash and/or if the alumina in the cement provides a sufficient source for ettringite growth.
30
Figure 23. XRD-stockpiled MA specimen (28-day cure with 6% Type-II/V cement).
From the results of XRD analysis it is evident that the addition of Portland cement
has little impact of expansive potential of the specimens tested as the change in the ettringite peak intensity and in the area under the peak following the addition of cement and the period of curing is inconsequential.
31
Figure 24. XRD-stockpiled MA specimen (28-day cure with 6% Type-1 cement).
One other possible reason for not seeing additional ettringite crystal formation may be the presence of significant concentrations of silicates in the specimen, which can interrupt the formation of ettringite by allowing other minerals to form instead, thereby suppressing the potential for ettringite to form.
ESEM
Environmental scanning electron microscopy was performed on 7-day and 28- days cured samples with and without the addition of cement. Ettringite could not be identified in the ESEM evaluation. Some of the images obtained for the cement treated material shows cementitious products binding the particles together. The results are summarized in Figures 25 through 28. It is, however, important to realize that expansion can occur due to ettringite growth even if ettringite cannot be identified via ESEM analysis. The ettringite crystals can grow in the dense matrix without being detected.
32
Figure 25. ESEM, 0-day cure.
Figure 26. ESEM (7-day cure, 6% Type-II/V cement).
33
The nonhomogeneous nature of the material and the low extent of ettringite present in the material makes it more difficult to find visual trace of ettringite inside the specimen. Some traces of ettringite are visible in the 28-days cured sample with Type 1 cement but the quantity is inconsequential.
Figure 27. ESEM (28-day cure, 6% Type 1 cement).
34
Figure 28. ESEM (28-day cure, 6% Type-2 cement).
Stability Models
Stability models were used to analyze the potential formation of ettringite in the MA specimens. The analysis results are presented in Figures 29, 30, and 31. None of the models show potential for ettringite formation in the future during service.
35
5 6 7 8 9 10 11 12 13 14–5
–4
–3
–2
–1
0
1
2
3
pH
log
a SO
4--
Ash Rock FG–1
¢
Al(SO4)2-
Clinoptil-Ca
Ettringite
PrehnitePyrophyllite
25°C
Sachin Kunagalli Mon Sep 13 2004
Dia
gram
Al(O
H) 4- ,
T =
25
°C
, P
=
1.0
13 b
ars,
a [
mai
n]
=
10–3
.596
, a
[H2O
] =
1,
a [
Ca++
] =
10
–1.9
1 , a
[SiO
2(aq)
] =
10
–3.1
46; S
uppr
esse
d: G
ross
ular
Figure 29. Stockpiled MA stability model-1.
36
5 6 7 8 9 10 11 12 13 14–5
–4
–3
–2
–1
0
1
2
3
pH
log
a SO
4--
Ash Rock FG–2
¢
Al(SO4)2-
Ettringite
Kaolinite
Prehnite
25°C
Sachin Kunagalli Mon Sep 13 2004
Diag
ram
Al(O
H)4- , T
= 2
5 °C
, P
= 1
.013
bar
s, a
[m
ain]
= 1
0–3.5
97, a
[H2O
] =
1,
a [C
a++]
= 1
0–1.9
03, a
[Si
O2(a
q)]
= 1
0–4.1
65; S
uppr
esse
d: G
ross
ular
Figure 30. Stockpiled MA stability model-2.
5 6 7 8 9 10 11 12 13 14–5
–4
–3
–2
–1
0
1
2
3
pH
log
a SO
4--
Ash Rock FG–3
¢Al(OH)4-
Al(SO4)2- Ettringite
Gibbsite
Prehnite
25°C
Sachin Kunagalli Mon Sep 13 2004
Diag
ram
Al(O
H)4- , T
= 2
5 °C
, P
= 1
.013
bar
s, a
[main
] =
10–4
.597
, a [H
2O]
=
1, a
[Ca++
] =
10–3
.083
, a [S
iO2(a
q)]
= 1
0–5.4
08; S
uppr
esse
d: G
ross
ular
Figure 31. Stockpiled MA stability model-3.
37
CONCLUSIONS
1. Manufactured aggregate has excellent properties for use as a fill or structural fill. The material is relatively light, approximately 60 to 70 percent of the weight of traditional, natural soils; can be compacted with a modest effort; has good interpartical shear strength as validated by both the Texas Triaxial test and the CBR test; and develops a high stiffness or modulus when sufficiently confined.
2. Standard compaction energy, ASTM D 698, is the preferred compactive effort for MA. The optimum moisture content for such a compactive effort is approximately 50 percent and the maximum density is approximately 70 pcf. Additional compactive effort has no significant impact on the MA. This can be perceived as an advantage as most contractors find it onerous to achieve densities higher than standard Proctor energy.
3. Manufactured aggregate is an abrasion sensitive material. Therefore, it is difficult
to use gradation as a specification parameter to control effectiveness. Coarse MA particles, larger than about 2.36–mm, have a low specific gravity due to the fact that they are agglomerations of moderately to weakly cemented smaller ash particles with high porosity. Compaction energy is sufficient to break these agglomerates down into smaller particles, especially when the large particles are wet.
4. The shear strength of MA was measured or approximated by three methods:
unconfined compression, CBR, and the Texas Triaxial method. The unconfined compressive strength increases significantly with curing time, and compressive strengths are substantially greater than those of soils with comparable classifications due to a modest level of pozzolanic and/or cementitious activity that occurs within MA. However, in an unconfined state, the MA will lose the majority of the strength upon capillary soak. When confinement is provided, the shear strength of MA, is in the range of a moderate quality base according to the Texas Triaxial classification (classification value of 2.6 following 24-hours of capillary soak) and with a CBR of approximately 30 percent following he traditional 96-hour period of soak. For this one may conclude that MA will perform as a low to moderate quality subbase or base if it is supported by a consistent subgrade, and if it is not subjected to extended periods of saturation. One should expect deterioration at the edge of the pavement, where confining stresses are low.
5. The resilient modulus of MA when sufficiently confined is comparable to a
moderate quality base. However, when the bulk stress is less than about 10 psi, the modulus drops rapidly.
6. Due to the composition of MA, the potential to form the expansive mineral,
ettringite, is always a threat. Ettringite forms from available calcium, aluminum, sulfates, and water. However, the thermodynamic potential for ettringite growth was not deemed to be present on 55-gallon drum samples that were
38
approximately 60 days old at the time of evaluation. This was verified by one-dimensional swell testing, XRD, and ESEM analyses. The phase diagrams and the XRD analysis clearly showed the potential for ettringite formation on unaged MA. Therefore, the phase diagram offers an effective quality control tool by which to assess the potential for further chemically induced expansion. A quick chemical analysis and phase diagram analysis could define the potential for chemically induced expansion for selected materials. This approach could be used to asses the risk of using the MA for various applications (with regard to swell).
Applications
Use as a Pavement Base or Subbase
Manufactured Aggregate may serve successfully as a subbase or base layer in pavements subjected to low levels of traffic. A low traffic level in this case is defined as less than about 500,000 equivalent (18,000 pound) single axle load applications during the life of the pavement. The Manufactured Aggregate evaluated during this study proved to be a unique material in several ways. First, even though an initial gradation can be selected that may be consistent with an accepted specification, the MA cannot maintain such a gradation during handling, processing, and compaction. This was demonstrated by subjecting various initial gradations to standard Proctor compaction (ASTM D 698) and comparing initial and final gradations. However, MA possesses unique properties including a moderate propensity to develop strength through pozzolanic and/or cementitious reactions between free lime (CaO) in the MA and glassy (amorphous) forms of alumina and silica in the ash. Practically, this means that compacted MA does not depend on interparticle friction to the level that a traditional unbound aggregate base does. Hence, gradation is not as critical in the MA as in a traditional base. Instead, the primary mode of strength development within the MA is a modest level of pozzolanically/cementitious derived strength. This strength level can approach about 70 psi following a curing period of approximately 14-days. This is approximately four times the compressive strength of traditional unbound moderate quality subbases, which derive most of their strength from inter particle interaction. The unconfined compressive strength of MA is severely diminished when the MA is saturated. However, if the MA is used in areas where drainage is sufficient to prevent saturation and/or where subgrade soil suction is enough to pull water into the subgrade rather than to release water to the MA, the MA should be able to function in a moderate fashion. This is further substantiated by CBR testing where an average CBR of approximately 30 following 96-hours of soaking was recorded. This value is consistent with moderate quality subbases and lower quality bases. The Texas Triaxial test, which was performed following a 24-hour period of capillary soak, demonstrated that the MA performs as a moderate to lower quality base (Texas Triaxial classification of 2.6). Furthermore, resilient modulus testing following AASHTO T-307 demonstrates that MA can achieve a respectable resilient modulus of over about 15,000 psi when the subgrade offers reasonably consistent support. Practically, this means that the MA may not be able to develop an acceptable resilient modulus when placed over a soft, wet
39
subgrade. However, if placed over a moderate quality subgrade, i.e., silty sand or better or over a stabilized subgrade, the MA should be able to develop an acceptable response modulus.
For additional information regarding this application see Appendix A.
Use as a Fill Material
Manufactured Aggregate possesses attractive properties for use as a fill material provided it is allowed to drain reasonably well and does not become saturated for extended periods of time. Effective drainage should be able to be achieved since MA has a moderate permeability, similar to fine sand. The MA has a low unit weight and when compacted to maximum density its unit weight is about 60 percent of the unit weight of traditional soil fill material. Although the optimum moisture content for compaction of the MA is very high compared to traditional fill material, some of the water used in compaction interacts in a physico-chemical manner with the MA and, therefore, it is not necessarily “free water”. Nevertheless, the moisture required for maximum density for the MA is several times the moisture required to achieve maximum density for traditional fill material. The testing performed in this study demonstrates that if MA can be compacted to within about 10 percent of maximum density at a standard Proctor compactive effort (ASTM D 698), the MA will achieve an internal friction angle of approximately 25 degrees or better and a cohesive intercept of approximately 10 psi or better, following approximately 14-days of curing. This is superior to traditional fill soils such as AASHTO classifications of A-4, A-2-4, A-6, and such.
Use as a Landfill Cover
Requirements for Use as Landfill Cover Three general types of cover are typically used in landfills: daily cover, intermediate cover, and final cover. Daily cover is the soil, or other material, that is placed on top of the landfill at the end of each day. Sandy soil is often used, but other materials have also been used including clay. However, when clay is used it must be treated by some means to increase its permeability. This usually requires breaking-up the clay so that it is permeable enough to allow fluids placed on top to easily pass through. Other landfill daily cover materials include textile cover, chemical foams, tire chips, bark and wood chips. The purpose of daily cover is typically to: (1) improve access to the landfill (be able to accommodate traffic), (2) cover trash reducing its ability to blow away, (3) reduce the risk of disease by reducing the potential of birds, insects, and animals that feed on the waste or trash, (4) reduce odors, and (5) reduce the risk for fire. The use of Manufactured aggregate (MA) as a daily fill offers no particular advantages to those
40
listed above except to provide a better load-carrying layer, i.e., improve access. Daily cover is typically placed in 6-inch thicknesses. Intermediate cover is used on the top areas of a landfill that are not planned for use for a long period of time. This cover reduces the amount of liquid that will accumulate in the landfill, since any water that accumulates on the surface of intermediate cover is allowed to run off the site and into the surrounding land. Intermediate cover is typically 12-inches thick. The final cover for landfill is comprised of topsoil, a protective layer, a drainage layer, a barrier layer, and a grading layer. Manufactured aggregate is a candidate for use as a protective layer or barrier layer. The protective layer is designed to protect the barrier layer from damage. Potential for Use of Manufactured aggregate as Landfill Cover The best potential application of MA is as a daily cover, where each application is typically 6-inches thick. MA possesses sufficient shear strength and modulus to provide protection for the underlying layers from traffic. Manufactured Aggregate can be easily compacted with minimal energy in approximately 9-inch lifts which will consolidate to the required 6-inches of cover generally required. The required compaction energy can be achieved using a typical tracked vehicle such as a bulldozer. We are currently performing laboratory permeability testing to define the relationship between degree of compaction and permeability. However, we expect that the permeability of the MA layers will be appropriate for landfill cover material.
Advantages of Using MA for Daily Cover There are several advantages for using MA for daily cover. These include:
1. The shear strength of MA due to pozzolanic activity in the MA and some cementitious reactions is higher than competing materials. This translates into more resistance of a MA layer to deformation under traffic and to a better resilient modulus in the MA leading to a better “bridging” action over the trash and waste below.
2. The MA can be easily compacted into 6-inch layers. If needed, rolling after compaction can be used to adjust permeability to a target level by producing cracks in the MA mat.
Potential for Use of MA as Intermediate and Final Cover Manufactured Aggregate should function well as intermediate and final cover. It can be easily compacted into a 12-inch thick layer with standard equipment such as bulldozers. It can be compacted to permeability levels that will make the mat resistant to permeation of rainwater into the fill. The layer will be sufficiently strong to resist damage
41
due to traffic loads of the type expected on the fill. A topsoil layer will have to be added over the intermediate cover if vegetation is expected to grow.
42
Potential for Expansion Due to Formation of Ettringite
The MA evaluated in this study was approximately 60 days old at the time of testing. A thermodynamic phase diagram was used to assess the statistical potential of the material to form the expansive mineral ettringite, which is composed of calcium, alumina, sulfate, and water. The thermodynamic phase diagrams demonstrated that the MA no longer has the potential to form expansive ettringite. However, X-ray diffraction (XRD) showed that ettringite had already formed. Scanning electron microscopy (SEM) coupled with XRD showed that a very modest amount of ettringite was formed. This ettringite forms rapidly in the life of the product, and one can expect that the formation can be controlled with the assistance of phase diagrams as a quality control tool. The possibility of ettringite formation must be considered in every application using the phase diagram approach. An associated approach based on stoichiometrics can be used in conjunction with the thermodynamic phase diagram analysis to assess the quantity of ettringite developed and hence the potential quantity of volume change.
Impact of Stabilization
The addition of as little as two percent Portland cement by weight of MA makes a very substantial difference in compressive strength, shear strength, and resilient modulus. Each of these properties can be improved by at least 100 percent through the small addition of Portland cement. The addition of two percent Portland cement is recommended to produce a good quality base that will provide a strong, durable platform for a pavement of structure without becoming too stiff and hence without incurring excessive shrinkage. We recommend that a Type V Portland cement with little or no tri-calcium aluminate be used as the additive of choice. Type V cement was not available at the time of this study. Such an additive should help reduce the potential for ettringite formation.
Use as Asphalt Fine Aggregate and/or Filler Potential to Use Manufactured Aggregate as a Fine Aggregate Manufactured Aggregate probably does not have acceptable properties to be used as a fine aggregate in asphalt. A fine aggregate is defined as aggregate that is smaller than 2.36 mm but is larger than 0.75 mm or the number 200 sieve. Because the MA has a high abrasion potential based on ASTM C-131, it will not likely maintain particle size consistency during production. This will make it difficult to use MA as a fine aggregate and remain within tolerance limits. Potential to Use Manufactured aggregate as a Mineral Filler
43
Manufactured Aggregate should perform well as a mineral filler in asphalt. Mineral filler is defined by size as material that passes the number 200 sieve. Such material comprises from 2 to 7 percent of hot mix asphalt. However, stone matrix asphalt mixtures require approximately twice as much as a mineral filler, in the range of 6 to 10 percent. Mineral filler is suspended in the asphalt cement or binder and serves the purpose of stiffening the binder by its filler action. Texas A&M University has developed a dynamic mechanical analysis (DMA) technique that has proven to be an excellent tool by which to assess the impact of mineral filler. The DMA has been used recently to differentiate among the impact of a variety of fillers and has shown great sensitivity in its ability to differentiate. A mineral filler with the chemical composition of fly ash has excellent potential to produce a good quality mastic in asphalt mixtures that will resist permanent deformation and fatigue cracking. Free calcium or calcium oxide in the ash has the potential to interact with acids in the asphalt cement to produce an interactive filler and a high quality mastic. Dynamic mechanical analysis testing is currently being performed using MA. Recommendations Based on the findings from this study, several further research activities are recommended:
1. Evaluate fresh samples from the production cycle. These fresh samples will be used to evaluate the potential for early swell due to ettringite formation.
2. Continue the thermodynamic evaluation of the potential for ettringite
development with the fresh samples. 3. Develop the phase diagram approach together with the stoichiometric approach
as a quality control tool to determine the potential and thus risk for ettringite development in the product.
4. Continue the study of blending MA with Portland cement to find a “combination”
that will provide both chemical and mechanical stability and moisture durability.
44
APPENDIX A
45
EXECUTIVE SUMMARY PREPARED FOR MR. NEIL WATLINGTON-ARMSTRONG VICE PRESIDENT, BUSINESS DEVELOPMENT AES PUERTO RICO, LP Prepared by Dallas N. Little, Ph.D., P.E. Based on testing performed at the Texas Transportation Institute, Texas A&M University September 14, 2004 Use As Unbound Aggregate Base Characteristics Necessary to Function as an Unbound Aggregate Base The primary purpose of an unbound base layer in a pavement is to distribute traffic wheel loads so as not to overstress the underlying natural subgrade soil. A second purpose of a base layer is to provide uniform and sufficient support for the surface layer so that it does not become overstressed and damaged. In order to accomplish this, the base material is required to meet several testing and characterization requirements. These include:
1. Durability and resistance to degradation during processing. 2. Shear strength. 3. Adequate modulus or load spreading efficiency. 4. Chemical stability.
Durability and Resistance to Degradation Several tests are normally required to assure that an aggregate can function durably as an unbound base layer. These include sulfate soundness tests that measure the resistance to the aggregate particles to the deleterious action of salt or ice crystals growing within the aggregate pores. A more recent and now very popular test used to assess the ability of a wet aggregate to resist degradation is the Micro Deval test. Neither the soundness test nor the Micro Deval test are appropriate tests for Manufactured aggregate (MA) because MA particles are conglomerates of smaller particles and are not discrete hard rock particles, for which the soundness and Micro Deval tests were designed. However, we did use the popular Los Angeles abrasion test (ASTM C-131) as an indicator of dry abrasion resistance in order to provide an indication of how the MA particles stand up to the rigors of production, mixing, and placing. Shear Strength In order for a base pavement layer to function acceptably under traffic loading, the layer must possess acceptable shear strength, which is normally (directly) measured as either unconfined compressive strength or triaxial shear strength. Adequate shear strength means that the material has the ability to carry load without
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deforming due to shear-induced distortion. Other, indirect measurements of shear strength in aggregate materials include tests such as the California Bearing Ratio (CBR). In the CBR approach the resistance of a certain material to penetration of a piston loaded to a certain stress level is compared to the amount of penetration what would occur in a standard material, where the standard material is a high quality, crushed stone base. Shear strength is perhaps the most important characteristic of and unbound aggregate base. Modulus and Load Spreading Efficiency If the base layer is to protect the natural soil from being overstressed by traffic load by properly spreading wheel loads, it must possess an acceptable resilient modulus, which is defined as the ratio of applied load stress to recoverable deformation under that load. Therefore, low modulus means high deformation under load and hence the transmittal of more stress to underlying layers due to that load. On the other hand, a higher modulus means less deformation under load and less transmittal of stress from the load to the underlying soil. The most widely used test to determine the resilient modulus of unbound aggregate base is the American Association of State Highway and Transportation Officials method T-307 (AASHTO T-307). Chemical Stability The composition of MA sometimes favors the formation of an expansive mineral called ettringite, which is the proximate cause of swelling of constructed layers of MA. In order for ettringite to form, sufficient amounts of alumina, calcium, sulfate, and water must be available. Manufactured Aggregate contains alumina and silica combined with calcium. During the hydration of calcium-silicate and calcium-aluminate, free calcium oxide is released, and this may be sufficient to trigger ettringite formation. Chemical composition tests show that MA is variable and phase diagrams developed based on these compositional analyses show that ettringite can form is some samples. X-ray diffraction (XRD) and differential scanning calorimetry (DSC) testing have verified the presence of ettringite in some samples. Further chemical testing is now underway in order to quantify the ettringite developed in certain samples. Assessment of Ability of Manufactured Aggregate to Function as an Unbound Base Ability to Resist Degradation The Los Angeles (LA) abrasion test (ASTM C-131) was used to assess the potential of the MA to degrade under production and construction. The LA abrasion test measures the amount of fine material produced after a coarse size fraction of the material is subjected to abrasion in a rotating drum charged with steel balls. Generally, hard durable aggregates used in asphalt, Portland cement concrete or as bases are required to have LA abrasion losses of less than about 30 percent. Three samples from each of three 55-gallon drums of MA were tested for LA abrasion. The average value was 59 percent with a coefficient of variation of only 5 percent. This means that the MA particles are highly susceptible to degradation under production. It essentially means that these particles cannot be viewed as traditional aggregate particles but, instead, a
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very unique mass of particles that although susceptible to degradation under production, have the potential to, at least partially, “re-cement” or re-bond with time. This produces as partially cemented mass with unique properties. Shear Strength The unconfined compressive strength (ASTM C-1633) of MA samples compacted at optimum moisture (about 50 percent moisture by weight or MA) was determined on samples that were tested one day after fabrication (curing at 75oF), and after curing at 100oF for 7, 14, and 28 days. Curing at a temperature of 100oF results in a more rapid rate of strength gain than would occur at 75oF. Therefore, curing at 7, 14, and 28 days at 100oF actually simulate the effects of curing at approximately 30 to 60 days at a 75oF. Three replicate samples were prepared and tested at each condition. The results show that the unconfined compressive strength increases from an average of about 20 psi for the low cure condition to an average value of about 80 psi for the fully cured condition. The strength for the fully cured condition (e.g., about 80 psi) means that, although the MA cannot mobilize shear strength due to particle-to-particle interaction like high quality aggregate particles do, it can develop enough cohesive strength to function as a moderate quality base. In addition to compressive strength testing, we performed Texas triaxial strength testing. In this approach samples of MA were subjected to 24-hours of capillary soak prior to compressive strength testing at various confining pressures. As the confining pressure increased, the compressive strength increased. A plot of compressive shear strength versus confining pressure defines a failure envelope that can be used to assess the strength of the samples based on the position of the failure envelope. We evaluated each failure envelope for the uncured conditions. The position of these envelopes can be used to classify the MA approximately as a class 2.1 material, which is a moderate quality base or subbase. Once the material is fully cured, the position of the envelope shifts higher in the class 2 range and approaches the class 1 envelope, which is used to differentiate good and high quality bases. AASHTO T-307, resilient modulus testing resulted in reasonable modulus values at higher total stress (bulk stress) states; however, the resilient moduli values are rather low at low total stress states. Total stress states are directly correlated with confining pressure. The resilient moduli values are consistent with the shear strength values and confirm that the MA can be used as a moderate quality base layer if properly compacted. Chemical Stability Based on preliminary chemical analyses and phase diagram development, it is clear that ettringite does have the potential to form in certain samples of MA. This is being more completely studied, as are techniques to minimize or altogether prevent the formation of ettringite. These methods include construction techniques that and adding small amounts of amendments that favor the formation minerals other than ettringite.
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We are currently working on a method based on simple chemical analysis and the use of phase diagrams that can be used to evaluate the MA product to determine the potential to form ettringite and therefore to assess the risk. This technique will also be used to assess the impact of the chemical amendments or construction techniques aimed at preventing ettringite development. This will be reported in the final technical report of September 30, 2004. The potential for MA to swell due to the formation of expansive minerals is dependent to a large degree on the age and pH of the material. It appears that the formation of ettringite may already have occurred in the older material; and therefore, in this case, the potential for continued formation is very low. Recommended Applications
1. Manufactured Aggregate can be used as a base or subbase in low traffic areas where chemical instability resulting in heaving can be repaired without disruption of significant areas of the pavement. The material when compacted to a reasonable target density near the target moisture content provides a base layer of sufficient strength and modulus to support light to moderate truck traffic. We can provide methods of quality control and quality assurance such as rapid chemical tests and phase diagrams that can be used to monitor the potential for expansion.
Manufactured Aggregate should not be used as a pavement base of structural foundation until the chemical stability potential is satisfactorily addressed.
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