Laboratory validation of a gradation design concept for sustainable applications of unbound granular materials in pavement construction

Construction and Building Materials xxx (2016) xxx–xxx

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Laboratory validation of a gradation design concept for sustainable applications of unbound granular materials in pavement construction Yuanjie Xiao a,⇑, Liuxin Chen a,b, Zhen Zhang a,b, Daiqi Lyu c, Erol Tutumluer d, Jiasheng Zhang a,b a National Engineering Laboratory for High-speed Railway Construction, Ministry of Education Key Laboratory for Heavy-haul Railway Engineering Structures, Department of Geotechnical Engineering, School of Civil Engineering, Central South University, 22 South Shaoshan Rd., Changsha, Hunan 410075, China b Department of Geotechnical Engineering, School of Civil Engineering, Central South University, 22 South Shaoshan Rd., Changsha, Hunan 410075, China c School of Civil Engineering, Central South University, 22 South Shaoshan Rd., Changsha, Hunan 410075, China d Department of Civil and Environmental Engineering, University of Illinois at Urbana-Champaign, 205 North Mathews Ave., Urbana, IL 61801, USA

h i g h l i g h t s
A newly introduced gradation design concept is validated through laboratory testing.
The mechanical stability and drainability of varying gradations are investigated.
Gradation optimizations are applied to achieve cleaner production and sustainability.

a r t i c l e

i n f o

Article history: Received 2 August 2016 Received in revised form 24 October 2016 Accepted 25 October 2016 Available online xxxx Keywords: Gradation Unbound permeable aggregate base Quarry byproducts Direct shear est Triaxial compression test Shear strength Particle breakage Permeability

a b s t r a c t Unbound aggregates are becoming increasingly scarce and expensive due to the loss of rock quarries and gravel mines to other land uses; therefore, it is important to engineer and optimize aggregate gradations for the targeted end-performances. This paper is aimed at validating in the laboratory a newly introduced gradation design concept intended for controlling structural assembly strength (stability) and drainage characteristics (field drainability). Aggregate gradation optimizations were studied for two applications: (1) unbound permeable aggregate base (UPAB) and (2) mixing proportions of blending fine granite tailings (FGT), a typical crushed granite mining by-product that has long been considered ‘‘waste” materials, with coarse crushed granite aggregates (CCGA). To this goal, five representative gradations were first selected according to the current Minnesota DOT UPAB gradation band, and the effects of different UPAB gradation designs on the shear strength properties and particle breakage potential were investigated using a large-scale direct shear test device. In the second application, one of the common quarry byproduct wastes (i.e., FGT), were mixed with CCGA in varying percentages to explore their potential use for building pavement foundations. Both laboratory permeability and large-scale monotonic triaxial compression tests were performed to investigate the effects of blending proportions on the stress–strain behavior. Based on the test results, the optimum aggregate gradations recommended by the new gradation design concept provided enhanced stability without compromising drainability. The new gradation design concept, hence validated in this study with produced optimum gradations, is expected to achieve sustainable and beneficial unbound aggregate applications for cost-effective long-life pavements. Ó 2016 Elsevier Ltd. All rights reserved.

1. Introduction Construction of pavement foundation layers, i.e., unbound aggregate base and subbase, requires the use of large quantities of annually produced aggregate materials (crushed stone, sand, ⇑ Corresponding author. E-mail addresses: [email protected] (Y. Xiao), [email protected] (L. Chen), [email protected] (Z. Zhang), [email protected] (D. Lyu), tutumlue@ illinois.edu (E. Tutumluer), [email protected] (J. Zhang).

and gravel). The primary function of base and subbase layers in flexible pavements is to distribute the repeated wheel load over weaker subgrade soils, whereas those unbound layers in rigid pavements are built mainly for providing uniform support, adequate drainage, and long-term durability for Portland cement concrete (PCC) slabs [1,2]. As demands for heavier and greater number of loads and environmental awareness (e.g., conserving natural resources) are increasingly placed, unbound aggregate layers need to not only be accurately characterized for structural behavior but also be adequately designed for cost-effectiveness.

http://dx.doi.org/10.1016/j.conbuildmat.2016.10.108 0950-0618/Ó 2016 Elsevier Ltd. All rights reserved.

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Especially when unbound aggregates are becoming increasingly scarce and expensive in many parts of the U.S. and around the world due to the loss of rock quarries and gravel mines to other land uses, how to make sustainable and beneficial applications of unbound aggregates in pavements becomes a challenge. Since gradation (or particle size distribution) is among the most important aggregate physical properties affecting the end-performances [3], it is important to engineer and optimize aggregate gradations for the targeted end-performances. It is worth mentioning that herein ‘‘the targeted end-performances” (or quality) refer exclusively to structural assembly strength (stability) and drainage characteristics (field drainability), not to other important aspects of quality such as modulus and deformation behavior, freeze–thaw or wet-dry durability, etc. To date, in terms of unbound aggregate gradation, few performance-based guidelines, except traditional ‘‘recipe-based” gradation bands specified by state highway agencies, are currently available for structural analysis and design of unbound pavement layers. Such gradation bands, either several decades old or intended for ‘‘standard” materials with proven field performance, are likely to lack mechanistic linkage with the targeted end-performances [3]. Naturally occurring or locally available (‘‘non-standard”) unbound aggregate materials could also be excluded for use due to undesired grading lying outside the specification requirements. To promote sustainability, more adaptable gradation design methods and performance-based gradation specifications need to be developed to engineer the targeted end-performances, as well as to accommodate locally available marginal quality aggregates (e.g., quarry by-product wastes). Nonetheless, adjusting gradations appears to be one of the most straightforward, effective, and economical alternatives to improve quality of unbound layers. Accordingly, this research study is aimed at exploring the feasibility and potential benefits of optimizing gradations to achieve desired layer performance. To be specific, two different engineering applications of unbound aggregate gradation optimization were studied in this paper: (1) optimizing gradation of unbound permeable (opengraded) aggregate base (UPAB) for field drainability and stability and (2) optimizing the mixing proportions of fine granite tailings (a crushed granite mining by-product waste) and coarse crushed granite aggregates for stability and field drainability.

2. Background Tutumluer et al. [4] conducted a research study with MnDOT to investigate possible improvements in pavement base layers in terms of stability, stiffness, and permeability relative to the current MnDOT gradation specifications. As part of the study, they undertook three different analyses to estimate gradation and other parameters to optimize stiffness and stability while maintaining drainability. These included an optimum particle packing analysis [5], a coordination number analysis (the average number of contacts each particle makes with other particles) [6,7], and an analysis of gradations using the MEPDG [8]. They introduced a new gradation parameter termed gravel to sand (G/S) ratio (or equivalently coarse to fine ratio) and found that this parameter controls the shear strength behavior of both ‘‘standard” and reclaimed materials. Using this new gradation concept, the optimal gradation for a drainable aggregate base including a significant amount of recycled Portland cement concrete (PCC) was proposed [2,8]. Later, such a concept was further validated by the discrete element method (DEM) simulations using three-dimensional polyhedral particles with realistic aggregate shape properties [6–8]. The DEM simulation results confirmed that the G/S concept presented appears to work well for typical Minnesota aggregate gradations studied. The optimum range of 1.4–1.6 was developed for the G/

S ratio using the two different analyses. More recently, utilizing the MEPDG software and a model for predicting resilient modulus of an unbound aggregate layer, Wilde et al. [9] developed several distributions of resilient modulus and analyzed the resulting effect on MEPDG-predicted pavement performance. The optimum gravel to sand ratio obtained by this method ranges from 0.8 to 1.4 – similar to those obtained using the previous two methods, if a little lower [9]. Yideti et al. [10] developed a gradation model based on packing theory to evaluate the effect of the aggregate size distribution on the strength and permanent deformation performance of unbound granular materials. Relating such gradation analysis parameters to field rutting performance of several asphalt mixtures, Lira et al. [11] found that those mixtures with a more balanced combination between coarse and fine material (around 60/40) showed a low rut depth. Interestingly, this 60/40 combination coincides with the optimal value of around 1.5 proposed from the G/S concept [5]. The first engineering application of unbound aggregate gradation optimization was motived by the following facts. The quality of the base (or subbase) layer beneath concrete slabs is vital for the long-term performance of PCC pavements. Low stiffness and shear strength of the base layer can result in a loss of support and increased tensile stresses in concrete slabs under loads. To maintain uniform support under concrete pavements and ensure satisfactory performance, a stable, non-erodible, and drainable base layer is necessary [12,13]. Despite considerable research efforts dedicated to develop permeable bases (both unbound and stabilized) for facilitating subsurface drainage (e.g. [14–16]), few studies have focused on the structural stability of such permeable bases. In fact, the unstable base could be possibly responsible for the early extensive distresses found in PCC pavements; therefore, the development of a reliable method of predicting in situ stability of drainable bases is needed. On the other hand, the second engineering application of unbound aggregate gradation optimization was motived by the fact that byproduct mineral fine materials commonly known as quarry waste fine or quarry dust can be produced during the routine aggregate quarry processes such as blasting, crushing, and screening of coarser grade aggregates. Such quarry by-products typically consists of coarse, medium, and fine sand particles, and a clay–silt fraction that is less than No. 200 sieve (0.075 mm) in size. While only low amounts of such quarry by-products are currently used in limited applications, excess amounts of such quarry by-products might either remain in the stockpiles or be disposed as waste materials. Tutumluer et al. [4] recently pioneered a research study to explore the feasibility of using increased quantities of quarry by-products in sustainable and beneficial pavement applications. They studied chemical admixture stabilization alternatives using cement and fly ash as a means to improve the strength and durability characteristics of quarry by-products. Alternatively, this paper explored the feasibility of adjusting the undesired gradations of quarry by-products by blending them with coarse-grained aggregates in different proportions to meet the targeted end-performances. As such, making greater use of materials that have long been considered ‘‘waste” byproducts would make both environmental and economical senses. In order to substantiate this promising alternative, it is noteworthy to study the effects of blending proportions on the stability and field drainability of mixtures of quarry by-products and coarse aggregates under actual field conditions.

3. Objective and scope The primary objective of this paper is to further validate a newly introduced gradation design concept intended for controlling stability and field drainability. The scope first covers five

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Fig. 1. Different gradations used in the laboratory direct shear tests [7].

representative gradations selected according to the current UPAB gradation band as specified by Minnesota DOT (MnDOT), and the effects of different UPAB gradation designs on the shear strength properties and particle breakage potential were investigated using a large-scale direct shear test device. As a second application, four different mixing proportions of blending fine granite tailing (FGT) with coarse crushed granite aggregates (CCGA) were designed using the newly introduced gradation design concept and subsequently tested for their respective shear strength properties using a large-scale triaxial apparatus. Note that without compromising the drainability, the gradations are adjusted (or optimized) within the specified gradation bands that are generally considered to satisfy the drainability requirements.

4. Test materials and experimental program

accordance with the Unified Soil Classification System (USCS) described in ASTM D 2487 [19]. In the second application, the coarse crushed granite aggregates (CCGA) and fine granite tailings used in this research were illustrated in Fig. 2. The CCGA having angular particle shapes are widely used in unbound layer applications ranging from highway pavements to railway track substructures. In this study, the coarse crushed granite aggregates (CCGA) were mixed intentionally with the fine granite tailings (FGT) in percentages of 20% CCGA and 80% FGT (20C-80F), 40% CCGA and 60% FGT (40C-60F), 60% CCGA and 40% FGT (60C-40F), and 80% CCGA and 20% FGT (80C-20F), respectively, all on a dry mass basis. In fact, this yields four different gradations with varying G/S ratios of 1.0, 1.6, 2.0, and 2.5, respectively. These combinations, as well as the original CCGA and FGT gradations, are shown in Fig. 3. Note that the G/S ratio values for the original CCGA and FGT gradations are about 0.7 and 3.0, respectively.

4.1. Test materials In order to further validate the aforementioned concept of the G/S ratio gradation parameter, in the first application, five different gradations tested in the laboratory were chosen to be consistent with those previously modeled in the DEM packing simulations [6,7]. As shown in Fig. 1, these five different gradations correspond to the G/S ratio values of 1.0, 1.6, 2.0, and 2.5, respectively. Note that the inclusion of the gradation with Dmax = 16 mm and G/ S = 1.6 was to study the effect of the maximum aggregate size, while the maximum aggregate size for all other gradations was chosen as 25 mm. Also shown in Fig. 1 is the current UPAB gradation band specified by MnDOT. The gradation with a G/S ratio of 1.0 lies outside of the gradation band, because the minimum G/S ratio calculated for such a gradation band is 1.4. Crushed limestone aggregates commonly used in China highways were selected for laboratory testing in this study. The sieving and size separation of the aggregates were undertaken first, followed by blending test samples according to the chosen gradations. To remove the fines (material passing No. 200 or 0.075-mm sieve) sticking to the surfaces of larger particles during dry sieving, the washed (or wet) sieve analysis was performed as well. Grain-size distributions of the soils were determined following ASTM D 422 [17]. Specific gravities of the soils were measured following the procedure described in ASTM D 854 [18]. The soils were classified in

4.2. Moisture-density tests The moisture-density characteristics of the aggregate samples in both applications were subsequently studied from compaction tests following the modified Proctor method [19]. A minimum of five points were used for each gradation design to establish the optimum moisture content (OMC) and the maximum dry density (MDD) values. Two duplicate aggregate specimens were also prepared and tested for each gradation design to ensure repeatability and reduce testing errors, and the reported values were averaged from those of the two specimens. The compaction curve for each of the five different gradations was obtained with the corresponding maximum dry density and optimum moisture content values identified. Both OMC and MDD values established for each gradation were later used to prepare aggregate specimens for largescale direct shear and monotonic triaxial compression testing. It is worth mentioning that the gradations of the UPAB aggregate specimens (in the first application) after compaction and direct shear tests were also obtained to assess particle breakage potential, respectively. The degree of compaction (i.e., the ratio of achieved dry density to maximum dry density) for specimens in each of the two applications was then controlled to be the same for all the corresponding tests in order to justify the trends of changes

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20

10

40mm

20mm

(a) 5

10mm

(b) Fig. 2. Photographs of (a) coarse crushed granite aggregates (CCGA) and (b) fine granite tailings (FGT).

Fig. 3. Different G/S ratio curves of mixtures blended from coarse crushed granite aggregates (CCGA) and fine granite tailings (FGT) in varying percentages.

in the mechanical and hydraulic properties of specimens with different gradations. 4.3. Laboratory permeability tests In the first application, the hydraulic conductivity values for UPAB aggregate samples of varying gradations were not measured in the laboratory; instead, they were estimated from gradation parameters using empirical equations, such as the Hazen model [20] and the MEPDG EICM model [21]. On the other hand, in the second application, the saturated permeabilities (ksat) of the CCGA– FGT specimens blended in percentages of 0% CCGA and 100% FGT (100F), 20% CCGA and 80% FGT (20C-80F), and 40% CCGA and 60% FGT (40C-60F) were determined using the falling head method in accordance with ASTM D 5856 [22], while the saturated permeabilities of the CCGA–FGT specimens blended in percentages of 60% CCGA and 40% FGT (60C-40F), 80% CCGA and 20% FGT (80C-20F),

and 100% CCGA and 0% FGT (100C) were measured using the constant head method as described in ASTM D 2434 [23]. Note that the permeability of relatively open-graded materials (i.e., coarse aggregates with large pores) was tested with the constant head aggregate permeameter at low hydraulic gradients to maintain laminar flow for obtaining appropriate Ksat values. The initial drainage prior to the falling head test for relatively fine-graded materials was performed to ensure the saturated conditions (i.e., the completely saturated pore space). Due to the not well-graded nature of the aggregate specimens, the finer particles in the pore space of coarser particles may be flushed out with the water flow and then deposited at the bottom of the permeameter during the test, which would cover the only possible flow path of the water in the permeameter. This could certainly cause differences among laboratory-measured, field-measured, and realistic permeability values, because the water in the field might flow along other alternative paths that are less likely to get clogged by the finer materials.

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Fig. 4. Illustration of the large-scale direct shear test device: (a) configuration sketch and (b) close-up view [24].

4.4. Large-scale direct shear tests The UPAB aggregate samples of five different gradations were tested at optimum moisture contents for shear strength properties using a large-scale direct shear test device. As illustrated in Fig. 4, this test setup for conducting direct shear tests consists of a measurement and control unit (I), a closed-loop servo hydraulic unit (II), and a loading actuator in the loading unit (III). It can simulate the monotonic or cyclic loading at the given frequency range of 0– 5.0 Hz. The main technical features of the test setup are as follows: a maximum load of 800 kN, a load range of 1–100% full capacity, a precision level of ±0.5%, a load increment step of 1/180,000 full capacity, a displace range of 0–600 mm, and a displace increment of 0.001 mm [24]. The variation in each load during the test is less than 3%, which satisfies the requirements of ASTM D 5321 [24]. The dimensions of both upper and lower shear boxes are 500 mm  500 mm  150 mm, while the dimensions of the lower shear box are 500 mm  670 mm  150 mm. During the testing, the normal stress was applied on the steel plate placed on the top of the aggregate specimens, and the vertical deformation was recorded by a linear variable differential transformer (LVDT). A shear deformation rate of 1.0 mm/min was used in this study. Shear stress was applied on the upper shear box until the vertical deformation became stable. The shearing process was terminated when the tangential displacement reached 75 mm (around 15% tangential strain). For each UPAB gradation design, three different aggregate specimens prepared at the optimum moisture content were tested at normal stresses of 100, 250, and 350 kPa (14.5, 36.2, and 50.7 psi) to determine the cohesion (c) and friction angle (/) values as shear strength properties of the classic Mohr-Coulomb failure criterion. The peak deviator stress value or maximum deviator stress at failure, was used as an indicator of aggregate shear strength. The aggregate specimens for direct shear tests were compacted using an impact hammer (similar to the one used in the compact tests), instead of a pneumatic vibratory compactor. This is to avoid excessive water loss due to splashing and its subsequent effects on aggregate behavior.

Fig. 5. Illustration of (a) sample preparation procedure, (b) the large-scale triaxial testing apparatus and (c) the sketch of test setup.

4.5. Monotonic triaxial compression tests In the second application, the shear strength values of the CCGA–FGT mixtures blended in varying percentages were investigated by conducting consolidated drained (CD) monotonic triaxial compression tests on mixture specimens prepared at their respective optimum moisture conditions for a target degree of compaction around 90%. The soil specimens prepared for the triaxial tests were 300 mm in diameter and 600 mm in height and encased

in a 3 mm thick rubber membrane. Triaxial testing standards, such as BS 1377-8 [25], specify that the maximum diameter of the largest particle (i.e., 40-mm size in this case) for consolidated drained triaxial test should not be greater than one-fifth of the specimen diameter. Therefore, the size effect is considered negligible. A close-up view of the large-scale triaxial apparatus (LSTA) used and the sample preparation procedure is presented in Fig. 5. The confining pressure was applied by the air over water pressure

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Table 1 Moisture-density properties from compaction tests for different applications. The first application (UPAB aggregate specimens) G/S ratio 1.0 Optimum moisture content xopt (%) 4.0 3 Maximum dry density cdmax (g/cm ) 2.36 The second application (CCGA–FGT mixture specimens) Mixture designation 100F G/S ratio 0.7 Optimum moisture content xopt (%) 10.0 Maximum dry density cdmax (g/cm3) 1.96

1.6 4.6 2.35

2.0 4.5 2.25

2.5 4.5 2.28

1.6 (Dmax = 16 mm) 4.6 2.26

20C-80F 1.0 8.2 2.05

40C-60F 1.6 10.1 2.19

60C-40F 2.0 10.0 2.16

80C-20F 2.5 8.1 2.11

system, with a maximum pressure of 3 MPa. The axial load was applied by the oil hydraulic system, with a maximum axial load of 1500 kN. The tests were performed at a constant axial displacement of 1 mm/min. The axial deviatoric load was measured by a load cell, and the axial deformation was measured by a digital dial gauge attached to the piston. The volumetric strain was derived from the expelled water. The triaxial compression test data for the CCGA–FGT mixtures were recorded automatically. The initial confining pressures used for testing the CCGA–FGT mixtures were 0.1, 0.2, 0.3, and 0.4 MPa. The samples were prepared with a measurement accuracy of 0.01 g using a split mold. A 3-mm-thick rubber membrane was placed inside the split mold to wrap the specimen. Filter stones were used at both ends, as shown in Fig. 5(a). The mixed material was divided into five equal parts for compaction inside the split mold. Each layer of the specimen was compacted using a vibrator with a frequency of 70 cycles per second. The specimen was sealed by latex rubber rings at each end, as shown in Fig. 5(a). The Large-scale Triaxial Testing Apparatus used is shown in Fig. 5(b). The specimen was subjected to the

required confining pressure and then sheared under the drained condition at a constant axial strain rate of 1 mm/min. The test was stopped when the accumulated axial strain reached 15%. The test results were presented and discussed in the following sections.

5. Test results and discussion 5.1. Moisture-density tests Important trends observed from the moisture-density testing of the aggregate specimens in both applications are presented. Table 1 summarizes the moisture-density parameters obtained from laboratory compaction tests for five different UPAB gradation designs in the first application, as well as for different CCGA–FGT mixtures in the second application. The laboratory-measured maximum dry density values are also plotted against the calculated G/S ratio values in Fig. 6. It can be seen from Fig. 6(a) that the densest packing was achieved with the G/S ratio value somewhere between 1.0 and

Fig. 6. Maximum dry density vs. gravel-to-sand ratio for: (a) UPAB aggregate specimens and (b) CCGA–FGT mixtures.

Fig. 7. Close-up views of aggregate specimens compacted for direct shear tests with: (a) G/S = 1.0 and (b) G/S = 2.5.

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Fig. 8. Shear stress vs. shear displacement obtained from direct shear tests for UPAB specimens with: (a) G/S = 1.0, (b) G/S = 1.6, (c) G/S = 2.0, (d) G/S = 2.5, (e) G/S = 1.6 but Dmax = 16 mm, and (f) G/S = 2.5 but 90% compaction level.

Table 2 Direct shear test strength properties obtained from the UPAB gradations. G/S ratio Apparent cohesion c (kPa) Friction angle / (o)

1.0 31.05 36.90

1.6 145.70 44.68

1.6, as indicated by the greatest maximum dry density value obtained. In addition, the gradation with the G/S ratio of 1.6 and 16-mm maximum particle size yielded much lower maximum dry density value than the one with the same G/S ratio and 25mm maximum particle size. This indicates the effect of maximum particle size on the laboratory moisture-density characteristics, i.e., decreased maximum particle size may reduce the maximum dry density, given that other conditions remain unchanged. However, this finding is still subject to further verification through more laboratory compaction tests. As for the CCGA–FGT mixtures, Fig. 6(b) illustrates that the blending percentage of 40% CCGA and 60% FGT that corresponds to the G/S ratio of 1.6 yielded the peak value of the maximum dry density. The different trends shown from maximum dry density vs. G/S ratio curves in Fig. 6 for UPAB and CCGA–FGT mixtures may be possibly attributed to their differing aggregate structures,

2.0 264.20 22.40

2.5 40.52 45.73

1.6 (Dmax = 16 mm) 80.00 35.00

as UPAB gradations are more open-graded in nature and the composition of aggregate skeleton plays a more significant role. However, further research is needed to identify the underlying mechanisms causing such different trends. Regardless, the densest packing for both cases appears to occur in the vicinity of the G/S ratio of 1.6. Observing the inconsistent trends of maximum dry density vs. G/S ratio (see Fig. 6), one may realize that relying on maximum dry density alone to evaluate structural stability would be misleading. Besides maximum dry density, mechanical indicators such as shear strength needs to be examined especially, as elaborated later. Similarly, Tutumluer et al. [4] also reported that the quarry by-products they studied did not exhibit strong correlations between density and strength. In fact, this is similar as the motivation of the recent initiative undertaken by Nazarian et al. [26] to develop modulus-based construction specification for compaction of earthwork and unbound aggregate.

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5.2. Direct shear test results

Fig. 9. Alternative representations of shear strength properties determined from direct shear tests: (a) the peak shear stress and (b) the Mohr-Coulomb failure envelope.

As for the UPAB aggregate specimens tested in the first application, the two aggregate specimens compacted for direct shear tests with G/S ratio values of 1.0 and 2.5 are shown in Fig. 7(a) and (b), respectively. It can be clearly seen that the gradation with G/S ratio of 2.5 has much larger fraction of coarse aggregates. Fig. 8 shows shear stress and shear displacement results obtained from laboratory testing for each of the five different gradations. It appears that the G/S ratio of 1.6 almost yields the best shear stress-displacement relations. Unlike fine-grained soils, it appears that there are no noticeable peaks (signaling stresssoftening behavior in general) in the shear stress vs. horizontal shear displacement graphs for the aggregate specimens tested. This strain-hardening type behavior of UPAB materials follows a similar trend with other coarse-grained rockfill aggregates of comparable sizes (e.g. [27,28]). Meanwhile, further tests with greater compaction efforts may still be needed to confirm if increased compaction would result in softening behavior as indicated by a visible peak in the stress–strain curve. In addition to the gradation effects, Fig. 8 also demonstrates the effect of achieved density during compaction on the shear strength behavior of UPAB material. As the actual compaction level achieved through an impact hammer for all the aggregate specimens was around 85% of the maximum dry density before direct shear tests, one additional aggregate specimen with G/S ratio of 2.5 was prepared and then compacted to 90% of the maximum dry density before the direct shear test. Comparing Fig. 8(d) and (f), one can see that this relatively small (about 5%) increase in compaction level indeed improved the shear behavior significantly, especially at lower normal stress levels (e.g., 100 kPa). This clearly demonstrates the benefit of increased compaction efforts in improving shear strength performance. Table 2 lists the friction angle (/) and cohesion intercepts (c) determined from direct shear strength testing of aggregate

Fig. 10. Peak deviator stress at failure vs. gravel-to-sand ratio at different normal stress levels: (a) 100 kPa, (b) 250 kPa, and (c) 350 kPa.

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specimens with five different gradations. Note that the significant increase of apparent cohesion (c) and the significant decrease of friction angle (/) for the aggregate blend with G/S = 2, as shown in Table 2, may be possibly attributable to the linear curve fitting from which those two parameters (c and /) were determined, as well as to other factors such as the influence of packing with larger particles and moisture correction issues with larger particles that have less surface area and moisture demand than the same volume of finer particles. In fact, due to the open-graded and free-draining nature of the aggregate blends with relatively low amounts of fines, and associated variabilities induced in the shear strength properties, the cohesion intercepts (c) and the friction angle (/) determined from the three replicate specimens (tested at normal pressures of 100, 250, and 350 kPa, respectively) may not be always consistent. Alternatively, the use of either the average peak shear stress corresponding to each of the normal stress levels or the Mohr-Coulomb failure envelope constructed as a straight line for each G/S level may be more conclusive in this regard, as shown in Fig. 9. Fig. 10 shows the maximum shear stresses at failure obtained at each normal stress level against the calculated G/S ratio values, respectively. It can be seen from Fig. 9 that the gradation with the G/S ratio of 1.6 approximately exhibits the greatest maximum shear stress at each of the three different normal stress levels, as evidenced by the values of shear strength properties c and / (see

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Table 2). Therefore, the validity of the new concept of the G/S ratio gradation parameter for enhanced UPAB strength (or stability) is further confirmed by the direct shear test results. Despite the same G/S ratio value of 1.6, the gradation with Dmax = 16 exhibits much lower maximum shear stress at each normal stress level than the one with Dmax = 25. It may be concluded from the limited test data that decreasing the maximum particle size (Dmax) could adversely affect the shear strength behavior of UPAB materials. This is consistent with the findings by Bagherzadeh-Khalkhali and Mirghasemi [29] as well as the conventional wisdom that the load-carrying capacity of base or subbase materials increases with larger aggregate top sizes. 5.3. Monotonic triaxial compression test results As for the blended CCGA–FGT mixture specimens tested in the second application, the deviator stress (rd = r1  r3) vs. axial strain (e1) curves obtained from large-scale triaxial compression tests at four different confinement levels were depicted in Fig. 11. Based on the test results, it appears that no strainsoftening behavior was observed for the blended CCGA–FGT mixture tested. In all of the tests, high strain levels were reached by bulging without an obvious shear plane. Especially for the 80C20F blends with a G/S ratio of 2.5, the failure was indicated by obvious specimen disintegration because the bulging was much

Fig. 11. Deviator stress vs. axial strain curves obtained from large-scale triaxial compression tests for different CCGA–FGT mixture designations: (a) 100F, (b) 20C-80F, (c) 40C-60F, (d) 60C-40F, and (e) 80C-20F.

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Y. Xiao et al. / Construction and Building Materials xxx (2016) xxx–xxx

less visible than others due to the highest concentration of largesize particles. Fig. 11 also depicts that the deviator stress at failure (i.e., corresponding to an axial strain level of 15%) in general increases with increasing G/S ratio. The 40C-60F blend with a G/S ratio of 1.6 exhibits relatively higher deviator stress at failure than other blends. It is expected that as the blending percentage of coarse crushed granite aggregates increases, the governing shear strength switches from a cohesion based mechanism to internal friction of fine fractions to the interlocking of coarse fractions, thus causing significant increase in shear strength. The shifting trends of curves in Fig. 11 demonstrate this point as well.

The stress paths from large-scale monotonic triaxial compression tests were depicted in the common p-q diagram, from which the shear failure envelopes could also be established. Note that p and q denote the mean principal stress and the deviatoric stress expressed in the triaxial space, respectively. The results for different FGT–CCGA mixtures are shown in Fig. 12. One can find from Fig. 12(c) that the mixture designated as 60C-40F has the failure envelope above those of other mixtures if they would have been plotted together, thus indicating the best shear strength behavior. This could be possibly related to the highest density achieved for the 60C-40F mixture [see Fig. 6(b)], as pointed out by Holtz and

Fig. 12. Stress paths in p-q diagrams obtained from large-scale triaxial compression tests for different CCGA–FGT mixture designations: (a) 100F, (b) 20C-80F, (c) 40C-60F, (d) 60C-40F, and (e) 80C-20F.

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Y. Xiao et al. / Construction and Building Materials xxx (2016) xxx–xxx

Kovacs [36] that higher density (or relative density) in drained triaxial tests generally corresponds to higher shear strength. Fig. 13 shows the trends of the volumetric strain (ev) changing with the axial strain (e1) for different CCGA–FGT blends. Note that the positive sign of axial strain (e1) indicates compression, whereas the positive sign of radial strain (e3) indicates expansion. Positive volumetric strain (ev) denotes volume contraction. It can be seen from Fig. 13 that the blended mixtures of 40C-60F, 60C-40F, and 80C-20F exhibited volume expansion during shearing at relatively low confinement levels. As the confinement level increases, they gradually changed from volume expansion to volume contraction. This can be explained that with high concentrations of coarse fractions, continuous aggregate skeleton is formed within the aggregate assembly by coarse particles, which tend to rotate and roll under low confinement, thus causing volume expansion. On the other hand, as the confinement increases, the rotating and rolling movements of coarse particles tend to be restricted with increasing particle breakage potential, thus leading to the transition from volume expansion to volume contraction. In addition, the volumetric strain and stress ratio changed only slightly at an axial strain of 15%, indicating that the soil parameters at this axial strain can be regarded as the critical state parameters [37].

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measured using mechanical sieve analysis, respectively. The results are presented in Fig. 14. The degree of particle breakage can be defined by the difference between the pre-test and post-test grain size distribution curves. Several methods are available for quantifying particle breakage (e.g. [30,31]). Among these, the method by Hardin [31] is widely accepted as a technique that adequately integrates particle breakage index (PBI) over a wide size fraction. The PBI is quantified by the parameter Br that is formulated by Eq. (1), where Bpi and Bpf are defined as the areas formed by the following three lines: the grain size distribution curve before or after direct shear tests, a horizontal line passing 100% finer, and a vertical line passing the No. 200 sieve (0.075 mm), respectively. Note that using this PBI concept to evaluate the crushing potential of recycled materials with the suggested blend ratios may still be subjected to further exploration and verification.

Br ¼

Bpi  Bpf Bpi

ð1Þ

5.4. Particle breakage potential results

where Br = relative breakage index (PBI); Bp = breakage potential; Bpi = pre-test breakage potential; Bpf = post-test breakage potential; and Bpi  Bpf = the total breakage.

The grain size distribution curves of the UPAB aggregate specimens before and after the compaction and direct shear tests were

The calculated Br values for different UPAB gradations were graphed in Fig. 15 against the G/S ratio values. It is evident that

Fig. 13. Volumetric strain vs. axial strain curves obtained from large-scale triaxial compression tests for different CCGA–FGT mixture designations: (a) 100F, (b) 20C-80F, (c) 40C-60F, (d) 60C-40F, and (e) 80C-20F.

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Y. Xiao et al. / Construction and Building Materials xxx (2016) xxx–xxx

U.S. Standard Sieve Sizes #4 #8 #16 #30

#60 #100

Hyd. Anal. #200 0

Original Gradation After Compaction After Direct Shear

80

20

60

40

40

60

20

80

0 50

30 20

10 8 7 6 5 4 3

2

1 0.7 0.5

0.3 0.2

3

/8"

G r a ve l Fine

40

40

60

20

80

0 50

Fine

Silt or Clay

30 20

Coarse

10 8 7 6 5 4 3

2

/8 "

U.S. Standard Sieve Sizes #4 #8 #16 #30

#60 #100

Hyd. Anal. #200 0

100

20

80

Original Gradation After Compaction After Direct Shear

80

60

40

40

60

20

80

0 50

30 20

10 8 7 6 5 4 3

2

1 0.7 0.5

0.3 0.2

Percent Finer by Weight

3

G r a ve l Fine

G r av e l Fine

Coarse

0.1

100 0.05

S an d Medium

Coarse

Fine

Silt or Clay

3

/8 "

U.S. Standard Sieve Sizes #4 #8 #16 #30

#50

#100

Hyd. Anal. #200 0

Original Gradation After Compaction After Direct Shear

20

40

40

60

20

80

0 50

30 20

10 8 7 6 5 4 3

2

1 0.7 0.5

0.3 0.2

0.1

100 0.05

Grain Size in Millimeters

S an d Medium

Fine

Silt or Clay

Coarse

Gravel Fine

Coarse

(c)

S and Medium

Fine

Silt or Clay

(d) 3

100

Percent Finer by Weight

0.3 0.2

60

100 0.05

0.1

1" 3/4"

Grain Size in Millimeters Coarse

1 0.7 0.5

(b)

Percent Coarser by Weight

Percent Finer by Weight

1" 3/4"

20

60

(a)

100

Hyd. Anal. #200 0

Grain Size in Millimeters

Sa nd Medium

Coarse

#60 #100

Original Gradation After Compaction After Direct Shear

Grain Size in Millimeters Coarse

U.S. Standard Sieve Sizes #4 #8 #16 #30

80

100 0.05

0.1

1" 3/4"

100

Percent Coarser by Weight

/8 "

Percent Coarser by Weight

3

/4"

3

/8"

U.S. Standard Sieve Sizes #8 #16 #30

#4

#60 #100

Hyd. Anal. #200 0

Original Gradation After Compaction After Direct Shear

80

20

60

40

40

60

20

80

0 20

10 8 7 6 5 4

3

2

1

0.7 0.5

0.3 0.2

0.1

Percent Coarser by Weight

1" 3/4"

Percent Finer by Weight

Percent Finer by Weight

100

Percent Coarser by Weight

12

100 0.05

Grain Size in Millimeters C

Gr av e l Fine

Coarse

S a nd Medium

Fine

Silt or Clay

(e) Fig. 14. Comparisons of original gradation, gradation after compaction tests, and gradation after direct shear tests for UPAB specimens with: (a) G/S = 1.0, (b) G/S = 1.6, (c) G/ S = 2.0, (d) G/S = 2.5, and (e) G/S = 1.6 but Dmax = 16 mm.

the gradation with G/S ratio of 1.6 and Dmax = 25 mm experienced the least particle breakage during the laboratory large-scale direct shear tests. It could be attributed to the stable composition of the particle assembly, which not only forms a stable aggregate skeleton

for transferring external loads but also achieves a balance between coarse and fine fractions that minimizes the particle breakage potential. When such a balance is broken by increasing the relative concentration of either coarse or fine fractions, greater particle

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not measured in the laboratory; instead, they were estimated from gradation parameters using empirical equations, such as the Hazen model [20] and the MEPDG EICM model [21], as formulated in Eqs. (2) and (3), respectively. On the other hand, in the second application, the saturated permeabilities (ksat) of different CCGA–FGT blends were measured from three replicates using either falling head or constant head method. The estimated and measured permeability values are plotted in Fig. 16 against corresponding G/S ratios values for different UPAB and CCGA–FGT blend gradations, respectively.

K sat ðcm=sÞ ¼ C H  D210 Fig. 15. Particle breakage index (Br) vs. gravel-to-sand ratio for different UPAB gradations after direct shear tests.

breakage would occur, as demonstrated in Fig. 15. This actually agrees well with the findings by Hall and Gordon [32] that wellgraded sands are much less prone to particle breakage than poorly-graded ones when tested by large-scale high pressure triaxial equipment. Note that for the different CCGA–FGT mixtures blended in the second application, such particle breakage potential analysis was not performed for now due to the considerable material quantities of the large-scale triaxial compression test specimens.

5.5. Permeability results As presented previously, the hydraulic conductivity (or saturated permeability ksat) values for different UPAB gradations were

ð2Þ 

K sat ðcm=sÞ ¼ 10

6

 10



D

5:3D10 þ0:049D60 þ0:0092D60 0:1P 200 þ1:5 10

ð3Þ

where Ksat = saturated permeability; CH = a coefficient varying from 1 to 1.5; D10, D60 = particle sizes corresponding to 10 and 60 percent passing, respectively; and P200 = Percentage of Material passing No. 200 or 0.075-mm sieve. As shown in Fig. 16, the estimated permeability values for UPAB gradations were approximately within the MnDOT specified range of 0.35–1.06 cm/s except the one with the G/S ratio of 1.0, whereas the measured permeability values for the CCGA–FGT blends were found to increase with increasing percentage of CCGA, especially when beyond 40% CCGA. This result is similar to the findings observed in other research studies (e.g. [33–35]). It can be seen from

Fig. 16. Permeability values for UPAB gradations estimated from (a) the Hazen model and (b) the MEPDG EICM model and measured for (c) CCGA–FGT mixtures.

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Fig. 16(a) and (b) that the Hazen model [20] and the MEPDG EICM model [21] predicted different permeability values for the same G/ S ratio, thus indicating the necessity to further improve both models for better accuracy and consistency. However, measuring the permeability values of UPAB specimens with varying G/S ratios and then developing proper predictive equations would be the scope of the next step. For blended mixtures containing up to 40% CCGA, it was likely that all voids developed between the large particles of the CCGA were filled by the fine-grained granite tailing. With the addition of more than 40% CCGA, macro pores developed between the large particles of the CCGA and these macro pores were only filled partially by the much reduced content of fine granite tailing. Note that the red line in Fig. 16(c) represents the line segments connecting the average values of permeability measurements of each blend, whereas the blue line in Fig. 16(c) represents the curve fitted from all the permeability measurements combined. As a result, the saturated permeability of the CCGA–FGT mixtures started to increase significantly beyond 40% CCGA content. In addition, a power-law type statistical relation appears to exist between measured permeability and G/S ratio for the CCGA–FGT blends studied in this research. The variation in the laboratory measured permeability values among the replicate specimens, as shown in Fig. 16 (c), actually reflects the nature of the constant head or falling head permeability tests of which the results are highly variable (with as much as an order of magnitude or more) even on specimens taken from the same source and prepared with the same procedure followed. This variability is generally attributable to variations in material properties including particle-size distribution, porosity, shape and orientation of particles, degree of saturation (presence of air), and viscosity of the soil water.

6. Summary and conclusions As part of a comprehensive laboratory study undertaken to further validate the newly introduced gradation design concept for controlling stability and field drainability, this paper presents the results from large-scale direct shear and monotonic triaxial compression tests. Two different engineering applications of unbound aggregates in pavements were studied to demonstrate the promising potential and benefits of gradation optimization. In the first application, five representative UPAB gradations with varying G/S ratio values were selected from the current gradation band specified by MnDOT. The objective was to explore their potential use as ‘‘drainable and stable” base in long life PCC pavements. The effects of different UPAB gradation designs on the shear strength properties and particle breakage potential were investigated using large-scale direct shear tests accordingly. In the second application, one of the common quarry byproduct wastes, i.e., the fine granite tailings, were blended with coarse crushed granite aggregates in varying proportions to explore their potential for building pavement foundations. Both laboratory permeability tests and large-scale monotonic triaxial compression tests were carried out to investigate the effects of blending proportions on the stress–strain behavior. The major conclusions of this paper are summarized as follows:
The densest packing was achieved with the G/S ratio value in the vicinity of 1.6, as indicated by the greatest maximum dry density, maximum shear stress, and deviator stress at failure obtained from experiments. The gradation with the G/S ratio of 1.6 approximately exhibits the best structural stability and the least particle breakage potential, as evidenced by the results of both direct shear and monotonic triaxial compression tests.
Relying on maximum dry density alone would give misleading evaluation of unbound aggregate quality; instead, additional mechanical indicators such as shear strength behavior need to

be examined to give credible assessment of unbound aggregate quality. Decreased maximum particle size may reduce the maximum dry density and the shear stress at failure, given that other conditions remain unchanged.
The relatively small increase in compaction level improved the shear strength behavior significantly, especially at lower normal stress levels. This clearly indicates the benefit of increased compaction efforts in improving strength (or stability) of UPAB layers.
The validity of the new concept of the G/S ratio gradation parameter for enhanced strength (or stability) of both UPAB and quarry byproduct mixture blends is further confirmed by the large-scale direct shear and monotonic triaxial compression test results. In addition, optimizing gradations of unbound aggregates to promote their sustainable and beneficial use in pavement applications proves feasible at least in the laboratory.

Acknowledgements This material is based on work supported by the National Natural Science Foundation of China under Grant No. 51508577. The authors acknowledge this financial support, as well as that of the Graduate Student Autonomous Exploration Project of Central South University (Grant No. 2016ZZTS073). The authors also thank Mr. Qi Zhang and other undergraduate helpers, the staff and research engineers at both the National Engineering Laboratory for High-speed Railway Construction and the Ministry of Education Key Laboratory for Heavy-haul Railway Engineering Structures established at the Central South University for their assistance. The contents of this paper reflect the views of the authors who are responsible for the facts and the accuracy of the data presented herein. This paper does not constitute a standard, specification, or regulation. References [1] D.J. White, P. Vennapusa, C.T. Jahren, Determination of the Optimum Base Characteristics for Pavements, Final Report Iowa DOT Project TR-482, Iowa Department of Transportation, Ames, IA, 2004. [2] E. Tutumluer, Y. Xiao, W.J. Wilde, Cost-Effective Base Type and Thickness for Long Life Concrete Pavements, Minnesota Department of Transportation, St. Paul, MN, 2015. [3] J.P. Bilodeau, G. Dore, P. Pierre, Pavement base unbound granular materials gradation optimization, Proceedings of the 8th International Conference on the Bearing Capacity of Roads, Railways and Airfields, vol. 1, Taylor & Francis Group, London, UK, 2009, pp. 145–154. [4] E. Tutumluer, H. Ozer, W. Hou, V. Mwumvaneza, Sustainable Aggregates Production: Green Applications for Aggregate By-Products, Final Report FHWA-ICT-15-012, Illinois Department of Transportation, Springfield, IL, 2015. [5] Y. Xiao, E. Tutumluer, Y. Qian, J. Siekmeier, Gradation effects influencing mechanical properties of aggregate base and granular subbase materials in Minnesota, Transport. Res. Rec.: J. Transport. Res. Rec. 2267 (2012) 14–26. [6] Y. Xiao, Performance-based Evaluation of Unbound Aggregates Affecting Mechanistic Response and Performance of Flexible Pavements, Ph.D. Dissertation, University of Illinois at Urbana-Champaign, Urbana, Illinois, 2014. [7] Y. Xiao, E. Tutumluer, Gradation and packing characteristics affecting stability of granular materials: Aggregate imaging-based discrete element modeling approach, Int. J. Geomech. (2016), http://dx.doi.org/10.1061/(ASCE)GM.19435622.0000735, 04016064. [8] Y. Xiao, E. Tutumluer, Y. Qian, DEM Approach for Engineering Aggregate Gradation and Shape Properties Influencing Mechanical Behavior of Unbound Aggregate Materials, ASCE Geotechnical Special Publication No. 234 (2014). [9] W.J. Wilde, E. Tutumluer, Y. Xiao, T. Beaudry, J.A. Siekmeier, Optimizing stability and stiffness through aggregate base gradation, Transport. Res. Rec.: J. Transport. Res. Rec. No. 2578 (2015). [10] T.F. Yideti, B. Birgisson, D. Jelagin, A. Guarin, Packing-based theory framework to evaluate permanent deformation of unbound granular materials, Int. J. Pav. Eng. 14 (3) (2013) 309–320. [11] B. Lira, D. Jelagin, B. Birgisson, Gradation-based framework for asphalt mixture, Mater. Struct. 46 (2013) 1401–1414. [12] K.T. Hall, J.A. Crovetti, Effects of Subsurface Drainage on Pavement Performance: Analysis of the SPS-1 and SPS-2 Field Sections, NCHRP report 583, TRB, National Research Council, Washington, D.C., 2007.

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