PHYSICAL, MECHANICAL AND CHEMICAL EVALUATION OF MANUFACTURED AGGREGATE

Transcription

1 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

2 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 Shear Strength California Bearing Ratio Swell Testing X-Ray Diffraction ESEM Stability Models CONCLUSIONS Applications Use as a Pavement Base or Sublease Use as a Fill Material Use as a Landfill Cover Potential for Expansion Due to Formation of Ettringite Impact of Stabilization Use as asphalt aggregate and/or Filler RECOMMENDATIONS APPENDIX A ii

3 Figure LIST OF FIGURES Page 1. Typical gradation curve of RA sample blended from three 55-gallon drums Moisture-density relationship of MA following ASTM D-698 protocol Moisture-density relationship following modiifed proctor compaction protocol, ASTM D Resilient modulus versus bulk stress for MA sample (no cure) Resilient modulus (14-days cure) Resilient modulus (14-days cure) Unconfined compressive strength (typical, representative gradation) Unconfined compressive strength (maximum particle size 2.36 mm) Unconfined compressive strength (Type 2 cement 7-day cure) Unconfined compressive strength (Type-1 cement 7-day cure) Texas triaxial test results (without cure) Texas triaxial test results (14-days cure) Texas triaxial classification (14-days cure) CBR plot (without cure) Variation in CBR as a function of dry density Impact of compaction on gradation Swell test results (samples compacted at optimum moisture content following ASTM D-698) XRD-stockpiled MA specimen without curing XRD-stockpiled MA specimen (14 days cure) XRD-stockpiled MA specimen (14 days cure with 6% Type-1 cement) XRD-stockpiled MA specimen (14 days cure with 6% Type-II/V cement) XRD-stockpiled MA specimen (28 days cure without cement) XRD-stockpiled MA specimen (28 days cure with 6% Type-II/V cement) XRD-stockpiled MA specimen (28 days cure with 6% Type-1 cement) ESEM 0-day cure ESEM (7-day cure 6% cement Type II/V cement) ESEM (28-day cure 6% Type 1 cement) ESEM (28-days cure 6% Type-2 cement) Stockpiled MA stability model Stockpiled MA stability model Stockpiled MA stability model iii

4 LIST OF TABLES Table Page 1. Typical gradation in tabular form : Specific gravity results ph of MA-water paste (1:5 ratio of MA-to-water) Permeability or MA sample Compressive strength (typical representative gradation) Unconfined compressive strength (fine gradation) Days unconfined compressive strength with Portland cement CBR results without cure Change in CBR values with cure and addition of cement CBR-dry density relationship Effect of compaction on gradation iv

5 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. 1

6 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 % 90.00% 80.00% 70.00% Percent Passing 60.00% 50.00% 40.00% 30.00% 20.00% 10.00% 0.00% sieve size (mm) Figure 1. Typical gradation curve of MA sample blended from three 55-gallon drums. 2

7 Table 1. Typical gradation in tabular form. Sieve size Sieve size Wt. % Retained Total % (mm) retained (gm) on sieve passing 2 in ½ in in ¾ in ½ in /8 in # # # # # # # P 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 , 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 Bulk SSD specific Gravity Coarse Aggregate Apparent specific Gravity Coarse Aggregate Bulk specific gravity Fines 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 3

8 gravity of fine fractions (passing #4 sieve size) was found to be 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 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/ft 3, 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 4

9 18 percent. The low unit weight of the MA may be viewed as a beneficial attribute when considered as a fill Dry Density (pcf) Moisture Content (%) 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 ftlb/ft 3. 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/ft 3 to 56,000 ft-lb/ft 3 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 percent. 5

10 69 68 Dry density (pcf) Moisture Content(%) Figure 3. Moisture-density relationship following modified proctor compaction protocol, ASTM D 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 Paste 6

11 Paste Paste Solution Solution Solution 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 ) 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) E E E-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 7

12 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 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 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

13 Resilient Modulus w/o cement Resilient Modulus, Er (psi) Bulk stress, θ (psi) Figure 4. Resilient modulus versus bulk stress for MA sample (no cure). Resilient modulus testing was performed on samples following AASHTO without curing. According to AASHTO 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. 9

14 Resilient Modulus, Er (psi) Bulk stress, θ (psi) 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. 10

15 Resilient modulus, Er (psi) Bulk stress, θ (psi) 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 The specimens were compacted at optimum moisture content as defined by ASTM 698. Specimens were subjected to accelerated curing at F for 7, 14 and 28 days in order to (approximately) simulate 30 to 60 days of field curing at temperatures of approximately 70 o F. The results of the compression tests and stress values are presented in Figures 7 through

16 70 Strength (psi) 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 Deformation (in) Figure 7. Unconfined compressive strength (typical representative gradation). Table 5. Compressive strength (typical representative gradation). Typical, Representative Gradation Deformation (in) Stress (psi) days cure days days 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 12

17 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. Strength (psi) 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 Deformation (in) Figure 8. Unconfined compressive strength (maximum particle size 2.36 mm). Table 6. Unconfined compressive strength (fine gradation). Deformation (in) stress (psi) days cure days days

18 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 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 strength (psi) %-1 2%-2 2%-3 4%-1 4%-2 4%-3 6%-1 6%-2 6%-3 0% Deformation (in) Figure 9. Unconfined compressive strength (Type 2 cement, 7-day cure). 14

19 Strength (psi) %-1 2%-2 2% -3 4% - 1 4% - 2 4% 3 6 %-1 6%-2 6%-3 0% Deformation (in) 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. 15

20 Table 7. 7-day unconfined compressive strength with Portland cement. No Cement Compressive strength Compression (in) 1 Type 2 (2%) Type 2 (2%) Type 2 (2%) Type 2 (4%) Type 2 (4%) Type 2 (4%) Type 2 (6%) Type 2 (6%) Type 2 (6%) Type 1 (2%) Type 1 (2%) Type 1 (2%) Type 1 (4%) Type 1 (4%) Type 1 (4%) Type 1 (6%) Type 1 (6%) Type 1 (6%) % cement 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 16

21 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 Shear Stress (psi) psi 3-psi 5-psi 10-psi 15-psi Normal Stress (psi) 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. 17

22 60 Strength (psi) psi 3 psi 5 psi 10 psi Normal Stress (psi) Figure 12.Texas triaxial test results (14-day cure). Strength (psi) psi 3 psi 5 psi 10 psi class 1 class 2 class 3 class 4 class Normal Stress (psi) Figure 13. Texas triaxial classification (14-day cure). 18

23 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 Stress (psi) Penetration (in) Figure 14. CBR plot without cure. 19

24 Penetration (in) Table 8. CBR results without cure. Stress (psi) Manufactured Aggregate Crushed Rock Corrected (Manufactured Aggregate) CBR % According to ASTM D 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 20

25 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 blows blows 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. 21

26 45 40 CBR (%) (61.76, 18) 10 blows (64.01, 40) 25 blows (65.55, 36) 50 blows Dry Density (lb/cft) 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 /8 in # # # # # # # P

27 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 % 90.00% 80.00% 70.00% Percent Passing 60.00% 50.00% 40.00% 30.00% 20.00% 10.00% original 0.00% Sieve size (mm) 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 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 23

28 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. Volume change (%) Time (days) sample 1 sample 2 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. 24

29 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 C 3 A 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 (C 3 A) could substantially reduce, if not eliminate the potential for ettringite development and heaving. Even though Type V cement contains some tetra-calcium alumino ferrite (C 4 AF), the amount is limited and C 4 AF is generally of much less concern that C 3 A. 25

30 Figure 19. XRD- stockpiled MA specimen (14-day cure). 26

31 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. 27

32 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. 28

33 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. 29

34 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. 30

35 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. 31

36 Figure 25. ESEM, 0-day cure. Figure 26. ESEM (7-day cure, 6% Type-II/V cement). 32

37 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). 33

38 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. 34

39 3 Ash Rock FG 1 log a SO Al(SO 4 ) 2 Clinoptil-Ca Pyrophyllite ph Ettringite Prehnite 25 C Sachin Kunagalli Mon Sep Figure 29. Stockpiled MA stability model-1. Diagram Al(OH) 4 -, T = 25 C, P = bars, a [main] = , a [H2 O] = 1, a [Ca ++ ] = , a [SiO 2 (aq)] = ; Suppressed: Grossular 35

40 3 Ash Rock FG 2 log a SO Al(SO 4 ) 2 Ettringite Kaolinite Prehnite ph 25 C Sachin Kunagalli Mon Sep Figure 30. Stockpiled MA stability model-2. Diagram Al(OH) 4 -, T = 25 C, P = bars, a [main] = , a [H2 O] = 1, a [Ca ++ ] = , a [SiO 2 (aq)] = ; Suppressed: Grossular 3 Ash Rock FG 3 log a SO Al(SO 4 ) 2 - Gibbsite Al(OH) ph Ettringite Prehnite 25 C Sachin Kunagalli Mon Sep Figure 31. Stockpiled MA stability model-3. Diagram Al(OH) 4 -, T = 25 C, P = bars, a [main] = , a [H2 O] = 1, a [Ca ++ ] = , a [SiO 2 (aq)] = ; Suppressed: Grossular 36

41 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 37

42 approximately 60 days old at the time of evaluation. This was verified by onedimensional 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 38