Assessment of Field Compaction of Aggregate Base Materials for Permeable Pavements Based on Plate Load Tests

sustainability

Article Assessment of Field Compaction of Aggregate Base Materials for Permeable Pavements Based on Plate Load Tests

Yong-Jin Choi, Donghyun Ahn, Tan Hung Nguyen and Jaehun Ahn * Department of Civil and Environmental Engineering, Pusan National University, Busan 46241, Korea; [email protected] (Y.-J.C.); [email protected] (D.A.); [email protected] (T.H.N.) * Correspondence: [email protected]; Tel.: +82-10-510-7650

Received: 27 September 2018; Accepted: 16 October 2018; Published: 22 October 2018

Abstract: The permeable pavement is one of Low Impact Development technics that allows stormwater to infiltrate through the pavement surface and the underlying base layer, thereby reducing surface runoff and preventing water contamination. For permeable base layers of permeable pavements, open-graded aggregates are often used to infiltrate and store stormwater in the pore of aggregate base layers. The mechanical behavior of open-graded aggregates has not been a major interest of pavement industry and society, and therefore there is much less information known for behavior of compacted open-graded aggregates comparing to dense-graded materials. This study aims at investigating the mechanical behavior of compacted permeable or open-graded aggregate base materials based on field experiments. Five different open-graded aggregates were selected, and they were compacted in the field up to 12 passes with a 10-ton vibratory compaction roller. The mechanical behaviors of aggregates were evaluated by conducting plate load tests at 2, 4, 8, and 12 passes of roller. For the test conditions considered herein, the strain modulus at the first loading seems to provide more consistent results with respect to aggregate types and level of compaction than other stiffness measures from plate load tests.

Keywords: permeable pavement; low impact development; open-graded aggregate; ﬁeld test; plate load test

1. Introduction Increase in impervious areas due to urbanization has adversely affected the hydrologic cycle by reducing stormwater infiltration, increasing river peak flows, reducing groundwater baseflow, and increasing urban pollutant inflows into rivers [1,2]. The permeable pavements, one of most widely used Low Impact Development (LID) technics, is considered as an approach to solving this problem [3]. Unlike conventional impervious pavements, permeable pavements have pores that allow stormwater to infiltrate through the pavement surface and the underlying base layer, thereby reducing surface runoff and preventing water contamination [2]. For road pavements, compaction is a key process during construction because it directly affects the stability and durability of the pavement aggregate bases [4]. Therefore, proper compaction quality control (QC) is essential to achieving adequate engineering performance. In permeable pavements, for stormwater infiltration and retention, the base layer is composed of open-graded aggregates with even particle size distributions and large voids between particles unlike conventional (dense-graded) aggregate base materials. Despite the wide application of permeable pavements, reasonable specifications or standards for compaction QC of open-graded aggregate materials do not seem well established. Generally, the compaction quality of pavement base layers is assessed by density in terms of relative compaction, which is defined as the ratio of the field dry density to

Sustainability 2018, 10, 3817; doi:10.3390/su10103817 www.mdpi.com/journal/sustainability SustainabilitySustainability 2018,, 10,, x 3817 FOR PEER REVIEW 22 of of 13 13 terms of relative compaction, which is defined as the ratio of the field dry density to the maximum drythe density maximum [5]. dryThe densitycharacteristics [5]. The of characteristicsthese aggregates of are these different aggregates from arethose different of conventional from those base of materials,conventional andbase it is difficult materials, to anddefine it is the difficult maximum to define dry density the maximum and field dry density density using and conventional field density testusing methods conventional [6–10]. test methods [6–10]. WithWith the recent recent adoption adoption of of the the mechanistic mechanistic-empirical-empirical pavement pavement design design (MEPDG) (MEPDG) method, method, there there havehave been increasing effortsefforts toto monitor monitor compaction compaction quality quality by by evaluating evaluating the the stiffness stiffness or strengthor strength of the of thecompacted compacted soil soil [4,11 [4,11,12],12]. Modulus-based. Modulus-based compaction compaction QC directlyQC directly measures measures the modulus,the modulus rather, rather than thanthe density, the den ofsity the, of on-site the on compacted-site compacted fill. The fill measured. The measured modulus modulus has the has advantage the advantage of being of directly being directlyrelated to related structural to structural performance performance and design and parameters. design parameters. Various tests Various and devices tests and for devices measuring for measuringmodulus have modulus been developedhave been anddeveloped are currently and are in currently use, including in use, the including plate load the test plate (PLT) load [13 test,14 ],(PLT) light [13,14],weight deflectometerlight weight deflectometer (LWD) [15], soil (LWD) stiffness [15], gauge soil (SSG) stiffness [16 ], gauge and dynamic (SSG) [16], cone and penetrometer dynamic cone [17]. penetrometerSeveral related [17]. studies Several using related these tests studies and devicesusing these have tests also beenand conducteddevices have [4,5 ,also12,18 been–27]. conducted With these [4,5,12,18trends, the–27]. Interlocking With these Concrete trends, the Pavement Interlocking Institution Concrete (ICPI) Pavement [1] and theInstitution American (ICPI) Society [1] and of Civil the AmericanEngineers (ASCE) Society [ of28 ] Civil have Engineersrecommended (ASCE) assessing [28] have the compaction recommended quality assessing of permeable the compaction base layers qualityusing devices of permeable such as base the layers LWD andusing SSG. devices However, such as since the thereLWD are and no SSG. standards However, or target since modulusthere are novalues standards for assuring or target proper modulus compaction, values itfor is assuring difficult inproper practice compaction, to evaluate it theis difficult quality ofin compactionpractice to evaluateeven when the these quality devices of compaction are used. Seoul even Metropolitan when these Citydevices [29 ],are ICPI used. [1], andSeoul ASCE Metropolitan [28] have suggestedCity [29], ICPIspecifications [1], and ASCE based [28] on the have number suggested of passes specifications of a vibratory based compaction on the number roller, of but passes it is hard of a tovibratory find the compactibasis publishedon roller, for but selecting it is hard such to afind number. the basis published for selecting such a number. GivenGiven the challenges ofof usingusing density-baseddensity-based compaction compaction QC, QC, assessing assessing the the compaction compaction quality quality of ofpavement pavement base base layers layers by by modulus-based modulus-based methods methods can can provide provide a valid a valid alternative. alternative. However, However, there there have havebeen fewbeen studies few studies attempting attempting to elucidate to elucidate the compaction the compaction characteristics characteristics of permeable of permeable base materials base materialsusing the using modulus the valuesmodulus for values proper for quality proper control. quality The control. main objectiveThe main of objective this study of isthis to investigatestudy is to investigateand understand and understand the mechanical the mechanical behavior of b open-gradedehavior of open aggregates-graded inaggregates the field basedin the field on PLT, based which on PLTcan,provide which can a valuable provide inputa valuable in establishing input in establishing the methods the and methods specifications and specifications for QC of permeablefor QC of permeableaggregate bases.aggregate bases.

2.2. Permeable Permeable Base Base and and Compaction Compaction Quality Quality Control

22.1..1. Permeable Permeable Base Base and and Ope Open-Gradedn-Graded Aggregates Aggregates TheThe base base of of a a road road pavement, pavement, which which is is the the layer layer installed installed directly directly below below the the surface surface layer, layer, plays plays thethe im importantportant roles of receiving vehicular loads from the surface layer layer and and supporting supporting the the surface surface layer layer itselfitself [30]. [30]. In In addition to to these key functions, a permeable base should ensure adequate stormwater infiltrationinfiltration into into the the underlying underlying pavement pavement layers. layers. Thus, Thus, u unlikenlike general general base base layers layers that that are are constructed with dense dense-graded-graded aggregates, permeable bases use open open-graded-graded aggregates (Figure 11).). InIn thethe UnitedUnited States,States, ASTM ASTM No. No. 57 57 aggregates aggregates are are widely widely adopted adopted for for permeable permeable base base (choker (choker course) course) [1,28]. [1,28]. In In Korea, Korea, forfor exa example,mple, Seoul Metropolitan City City [29] [29] proposed its own specificationspecification of open-gradedopen-graded aggregates basesbases for the permeable block pavements. Since open open-graded-graded aggregates have have very uniform particle sizesize distributions, distributions, and and the the fraction fraction of of fine fine particles particles is is negligibl negligiblyy small, small, the the base base layers layers constructed constructed with with thesethese aggregates aggregates contain contain large large voids voids among particles. Stormwater Stormwater infiltrates infiltrates through through or or is is retained in thesethese voids, voids, thus thus satisfying satisfying the the hydrologic hydrologic requirements requirements of of the the permeable permeable base. base.

(a) (b)

FigureFigure 1. 1. ConceptualConceptual illustration illustration of pavement of pavement base materials; base materials; (a) Open- (gradeda) Open-graded aggregates; aggregates;(b) Dense- graded(b) Dense-graded aggregates. aggregates. Sustainability 2018, 10, 3817 3 of 13

2.2. Compaction Quality Control and Permeable Base Compaction of the base layer is a significant process in the construction of permeable pavements and a key step towards achieving the desired engineering performance. Inadequate compaction of the base layer can undermine the stability and durability of the entire pavement [11]. Thus, compaction quality must be properly assessed during the construction of permeable pavements. The most common criterion for ensuring adequate road base compaction quality is the relative compaction, the ratio of the field dry density to the maximum dry density. However, conventional density-based compaction control is hard to implement for open-graded aggregate bases. During the construction of the base layer, the relative compaction of the compacted lift should meet certain target values. Typically, the maximum dry density is obtained by the Proctor compaction test [6,7] and the field dry density is measured by the techniques such as sand-cone [8], rubber balloon [9], and nuclear density gauge [10] methods. However, because the characteristics of permeable base materials, which is open-graded aggregates, are different from those of typical soils, it is difficult to measure the maximum dry density and the field dry density with these conventional test methods. First, because the particles of open-graded aggregates are very large and uniform, it is difficult to meet the specifications of the Proctor test [6,7]. Moreover, the impact hammer used in the Proctor compaction test often breaks the large particles, and thus changes the specimens [31]. Second, in the sand-cone method, the shape of the test hole cannot be maintained due to the collapse of the aggregate, and sand replacement is not valid because the sand used for filling the test hole enters the voids and pore openings [8]. The rubber balloon test is also highly challenging due to problems with maintaining the shape of the hole, and the rupturing of the rubber due to the sharp edges of the aggregate particles [5,9]. The nuclear density gauge method may also be affected by the gravel and large particles or the large voids in the permeable base, and has not been calibrated for such condition. The safety of radioactive wave is also arguable resulting in implementation and management challenges [5,10,11]. As mentioned in the introduction, modulus-based compaction QC has received increasing attention due to its merits and validity. Because the measurement of field dry density and maximum dry density of open-graded aggregates is challenging, modulus-based compaction QC can make an effective alternative method for monitoring the compaction quality of permeable base layers. However, very few studies have been conducted on the modulus-based compaction QC of permeable base layers. Although some permeable pavement specifications such as those of the ICPI [1] and ASCE [28] have suggested the use of the LWD and SSG to assess compaction quality, there are no specific target modulus values in these specifications to ensure proper compaction. In addition, the ICPI and ASCE [1,28] have proposed at least four passes of a 10-ton vibratory compaction roller as a standard for achieving proper compaction, but, the proposed standards do not elaborate the basis for this specification. There are several studies that used the PLT to evaluate the modulus of dense-graded ground materials. The results from literatures are summarized in Table1. Examples of compaction QC specifications with target modulus values that can be assessed with the PLT are presented in Table2.

Table 1. Moduli of dense-graded ground materials evaluated by PLT 1 in literatures.

MDD 2 OMC 3 E 6 E 7 Literature Soil Type RC 4 (%) MC 5 (%) v1 v2 (g/cm3) (%) (MPa) (MPa) Fine sand subgrade 1.72 3 14.4 3 99–101 5.3–9.9 35–36 97–107 Natural gravel 2.18 3 3.7 3 99.7 2.4 65 190 Wiman [27] Crushed rock aggregate 2.17 3 4.7 3 99.8 2.7 61 185 Granular base 2.35 3 4.5 3 95.8 2.2 77.6 191 Kim and Park [18] Poorly graded gravel 2.27 - - 5.9 23–53 48–137 Silty sand - - - - 11~15 21~28 Kim et al. [32] Lean clay - - - - 12~23 18~37 1 Plate load test; 2 Maximum dry density from the standard Proctor test; 3 Optimum moisture content from the standard Proctor test; 4 Relative compaction; 5 Moisture content when PLT is conducted; 6 Strain modulus for ﬁrst loading cycle; 7 Strain modulus for second loading cycle. Sustainability 2018, 10, 3817 4 of 13 Sustainability 2018, 10, x FOR PEER REVIEW 4 of 13

TTableable 2.2. Modulus-basedModulus-based compaction speciﬁcations.specifications.

Requirement CorrespondingCorresponding LiteratureLiterature CountryCountry MaterialMaterial Ev2 (MPa)Ev2 (MPa) EEvv22//Evv1 k2.5 (kMPa/m)2.5 (MPa/m) RC (%)RC (%) GW, GI,GW, GU, GI, GT GU,1 GT 1 ≥70≥70 - - - ≥98 ≥98 ZTVE-StBZTVE 94-StB [33 ]94 [33] GermanyGermany --- - ≤≤22.5.5 - -- - StandardStandard specification specification for for KoreaKorea SubbaseSubbase -- - ≥294≥ 2294 2 ≥95 ≥95 road constructionroad construction [34] [34] 1 2 1 German soilsoil classification classification according according to DINto DIN 18196 18196 [35]; [35Modulus]; 2 Modulus of subgrade of subgrade reaction reaction defined defined at 2.5-mm at settlement using a 30-cm loading plate. 2.5-mm settlement using a 30-cm loading plate.

3. Field Test 3.1. Test Materials 3.1. Test Materials Three types of open-graded aggregates were used in the construction of the permeable base test Three types of open-graded aggregates were used in the construction of the permeable base test bed; the maximum particle sizes of these aggregates were 40 mm (D40), 25 mm (D25), and 13 mm bed; the maximum particle sizes of these aggregates were 40 mm (D40), 25 mm (D25), and 13 mm (D13), as they are presented in Figure2. These rhyolite aggregates were brought to the field in air-dried (D13), as they are presented in Figure 2. These rhyolite aggregates were brought to the field in air- condition two days before the test started. Two additional aggregates, “D40 + D25” and “D25 + D13”, dried condition two days before the test started. Two additional aggregates, “D40 + D25” and “D25 were prepared by mixing their component aggregates in equal proportions by volume. The mixing + D13”, were prepared by mixing their component aggregates in equal proportions by volume. The was conducted by a backhoe excavator, and the volumes of the aggregates were approximated by the mixing was conducted by a backhoe excavator, and the volumes of the aggregates were excavator bucket. The volumetric compositions, basic properties, and particle size distributions of the approximated by the excavator bucket. The volumetric compositions, basic properties, and particle five open-graded aggregate test materials are presented in Table3 and Figure3. Figure3 also presents size distributions of the five open-graded aggregate test materials are presented in Table 3 and Figure the lower and upper bounds of particle size distribution of two specifications, the ASTM No. 57 and 3. Figure 3 also presents the lower and upper bounds of particle size distribution of two specifications, Seoul Metropolitan City [29]. The particle size distribution of D40+D25 was comparable to ASTM No. the ASTM No. 57 and Seoul Metropolitan City [29]. The particle size distribution of D40+D25 was 57 (the material for choker layer in ICPI [1] and ASCE [28]) and specification of Seoul Metropolitan comparable to ASTM No. 57 (the material for choker layer in ICPI [1] and ASCE [28]) and specification City [29]. of Seoul Metropolitan City [29].

(a) (b) (c)

FigureFigure 2. Three types of open open-graded-graded aggregates used to construct the test bed. The maximum particle sizes are: (a) 40 mm (D40);(D40); ((b)) 25 mm (D25);(D25); ((cc)) 13 mm (D13).(D13).

Table 3. Basic information of the test materials. Table 3. Basic information of the test materials. Material Composition by Volume Material Composition by Volume 1 2 TestTest Material Material LithologyLithology CCuu 1 Cc 2 Cc SpecificSpecific Gravity Gravity AbrasionAbrasion Rate Rate (%) (%) D40D40 D25D25 D13D13 D40D40 100%100% -- -- 2 2.88.88 1.19 1.19 12.812.8 D40D40 + D25 + D25 50%50% 50%50% -- 2 2.99.99 1.08 1.089.8 9.8 D25D25 RhyoliteRhyolite -- 100%100% -- 2 2.48.48 1.02 1.022.67–2.67–2.752.75 10.3 10.3 D25 + D13 - 50% 50% 2.84 1.16 11.2 D25 + D13 - 50% 50% 2.84 1.16 11.2 D13 - - 100% 2.79 1.16 12.3 D13 - - 100% 2.79 1.16 12.3 1 Coefficient of uniformity; 2 Coefficient of curvature. 1 Coefficient of uniformity; 2 Coefficient of curvature. Sustainability 2018, 10, 3817 5 of 13 Sustainability 2018, 10, x FOR PEER REVIEW 5 of 13

100 D40 90 D40+D25 80 D25 D25+D13 70 D13 ASTM No. 57 60 Seoul Metropolitan City [29] 50 40 30 Passing Passing by weight (%) 20 10 0 100 10 1 0.1 Particle size (mm)

Figure 3. Particle size distribution of test materials and specifications. Figure 3. Particle size distribution of test materials and specifications. 3.2. Test Program 3.2. Test Program The test program in this study (Table4) was set for each aggregate material. A 30-cm lift (first lift) wasThe laid test and program compacted in this by study 12 passes (Table of a4) 10-ton was set vibratory for each compaction aggregate material roller. After. A 30 2,- 4,cm 8, lift and (first 12 passes,lift) was two laid (duplicate) and compacted PLTs were by 12 performed passes of on a 10 each-ton compacted vibratory compaction aggregate. Another roller. After 30-cm 2, lift 4, 8, (second and 12 lift)passes, was laid two on(duplicate) the first lift, PLTs and were the previous performed compaction on each and compacted PLT sequences aggregate. were Another repeated. 30 Thus,-cm lift a total(second of 80 lift) cases was of laid PLT on (5 materials,the first lift, 2 lifts,and the 4 numbers previous of compaction passes, and and 2 tests) PLT were sequences performed. were Fromrepeated. the PLTThus, results, a total the of strain80 cases moduli of PLT for (5 thematerials, first and 2 lifts, second 4 number loadings of cycles passes, (Ev 1andand 2 Etests)v2, respectively) were performed. and theFrom modulus the PLT of subgrade results, reactionthe strain (defined moduli at for2.5 mm the of first settlement; and secondk2.5) wereloading obtained. cycles (Ev1 and Ev2, respectively) and the modulus of subgrade reaction (defined at 2.5 mm of settlement; k2.5) were obtained. Table 4. Test program.

Test Materials Lift Number of Roller Passes Table 4. Test program. D40 2 D40 +T D25est Materials Lift Number of Roller Passes D40 First (30 cm) 4 D25 2 D40 + D25 Second (30 cm) 8 D25 + D13 First (30 cm) 4 D25 12 D13 Second (30 cm) 8 D25 + D13 12 D13 3.3. Test Bed Construction and Test Procedure 3.3. TheTest testBed Construction bed was constructed and Test Procedure at a site on the Yangsan campus of Pusan National University in SouthThe Korea. test First,bed was the siteconstructed was excavated at a site as on shown the Yangsan in Figure campus4. The overallof Pusan plan National dimensions University of the in excavatedSouth Korea. area First, were the 3.7 site× 20 was m withexcavated a trapezoidal as shown vertical in Figure cross-section, 4. The overall except plan the dimensions excavation of forthe rampexcavated needed area for were the entry 3.7 × of20 the m constructionwith a trapezoidal and testing vertical equipment. cross-section, After except excavation, the excavation the test bed for wasramp divided needed into for thethe fiveentry sections of the construction using rubber and partitions testing equipment. to isolate each After test excavation, material (Figurethe test5 a).bed Thewas height divided of into the the lift five (30 cm)sections was using guided rubber by the partitions rubber to partitions isolate each that test has material the same (Figure height 5a). with The theheight lift. of The the unloaded lift (30 cm) aggregates was guided were by evenly the rubber spread partitions with the tha buckett has ofthe the same excavator height with to level the thelift. surfaceThe unloaded (Figure5 b).aggregates Thereafter, were a 10-ton evenly vibratory spread rollerwith the (Dynapac) bucket wasof the used excavator to compact to level the aggregates the surface (Figure(Figure5 c). 5b). The Thereafter, roller operated a 10-ton along vibratory the longitudinal roller (Dynapac) direction was of the used test to bed. compact After the the aggregatesspecified number(Figure of5c). roller The passesroller op (Tableerated4), along the PLTs the werelongitudinal performed direction for each of testthe test material bed. After to determine the specified the modulusnumber of theroller compacted passes (Table lift (Figure 4), the5 d).PLTs After were the performed tests on the for first each lift test were material completed, to determine the second the liftmodulus was laid of on the top compacted of the first lift lift, (Figure and the 5d). compaction After the tests and on PLT the procedures first lift wer weree completed, repeated. the second lift was laid on top of the first lift, and the compaction and PLT procedures were repeated. Sustainability 2018, 10, 3817 6 of 13

SustainabilitySustainability 20182018,, 1010,, xx FORFOR PEERPEER REVIEWREVIEW 66 ofof 1313

((a))

((b))

FFigureigureigure 4.4. TestTest bed layout: ( (aa)) CrossCross Cross-sectional--sectionalsectional view; view; (b)) Plan Plan view. view.

((a)) ((b))

((cc)) ((d))

FFigureigureigure 5. 5. TestTest bed bed construction construction and and test test procedure: procedure: ( (aa))) TestTest Test bedbed excavationexcavation excavation andand testtest test sectionsection section separation;separation; separation; ((b)) Lift Lift construction;construction; ((cc)) Compaction;Compaction; ((dd)) PLT.PLT.

The PLT was carried out in accordance accordance with DIN DIN 18134 18134 [14] [14] with with some some modifications modifications considering considering thethe prevailingprevailing fieldfield field conditions.conditions. conditions. InIn In thethe the loadingloading loading process,process, process, aa loadingloading a loading plateplate plate withwith with aa diameterdiameter a diameter ofof 3030 of cmcm 30 waswas cm usedwas used without without applying applying seating seating sand sand because because thethe surfacesurface the surface ofof aggregatesaggregates of aggregates rightright right afterafter after compactioncompaction compaction waswas verywas veryflat.. The flat.The excavatorexcavator The excavator waswas usedused was usedtoto deliverdeliver to deliver thethe reactionreaction the reaction force.force. force. TheThe normnorm The normalal stress stress acting acting on the on plate the wasplate measured was measured by reading by reading the thehydraulic hydraulic pressure pressure acting acting on on the the hydraulic hydraulic jack jack with with the the pressure measurement dial dial gauge. gauge. Before Before the the test test began, began, a preloading a preloading stress stress (~0.01 ((~ MPa)0.01 MPa) was applied was applied for proper for properseating seating of the plate, of the and plate, the and displacement the displacement measurement measurement dial gauge dial was gauge set towas zero. set to The zero. loading The loadingcycleloading (first cyclecycle loading (first(first cycle)loadingloading was cycle)cycle) applied waswas inappliedapplied eight approximatelyinin eighteight approximatelyapproximately equally spaced equalequallyly stages spacedspaced up stagesstages to the upup planned toto thethe plannedmaximum maximum stress (~0.5 stress MPa); (~0.5(~0.5 then, MPa)MPa) the;; thenthen load,, thethe was loadload reduced waswas reducedreduced to 50% toto and 50%50% 25% andand of 25%25% the of maximumof thethe maximummaximum stress stresuccessively,stressss successively,successively, and finally and backfinally to back the preloading to the preloading stress. The stress. reloading The reloading cycle (second cycle loading (second cycle) loading was cycle)cycle) waswas thethe samesame asas thethe firstfirst loadingloading cycle,cycle, butbut thethe lastlast loadingloading stagestage (the(the 8th8th stage)stage) was not applied followingfollowing thethe DINDIN 1813418134 [14][14].. The magnitude of preloading and maximum stresses varied slightly Sustainability 2018, 10, 3817 7 of 13 the same as the first loading cycle, but the last loading stage (the 8th stage) was not applied following Sustainability 2018, 10, x FOR PEER REVIEW 7 of 13 the DIN 18134 [14]. The magnitude of preloading and maximum stresses varied slightly among tests accommodatingamong tests accommodating equipment andequipment field conditions. and field conditions. The test locations The test werelocations offset were from offset each from other each to minimizeother to minimize interference interference among the among PLT measurements the PLT measurements (Figure6). (Figure The location 6). The of location PLT in each of PLT aggregate in each bedaggregate is illustrated bed is inillustrated Figure6; thein Figure numbers 6; 2,the 4, numbers 8, and 12 2, represent 4, 8, and the 12 number represent of rollerthe number passes, of and roller the symbolpasses, and “a” andthe symbol “b” represent “a” and the “b” first represent and second the tests.first and Additional second PLTstests. wereAdditional also conducted PLTs were on also the subgradeconducted by on the the test subgrade bed. by the test bed.

FigureFigure 6. Layout of test locations on a test bed section.

4.4. Results and Discussion

4.1.4.1. Modulus Calculation The values of the strain modulus at the first loading E and the strain modulus at the second The values of the strain modulus at the first loading Evv11 and the strain modulus at the second loading E were calculated based on the process specified in DIN 18134 [14]. The calculation method loading Evv2 were calculated based on the process specified in DIN 18134 [14]. The calculation method uses a second-ordersecond-order polynomialpolynomial regression analysisanalysis to determinedetermine the stress–settlementstress–settlement curve for thethe firstfirst andand secondsecond loadingloading cycles.cycles. TheThe determineddetermined curvecurve isis expressedexpressed byby thethe followingfollowing equation:equation: = + ∙ + ∙ 2 s = a0 + a1·σ0+ a2·σ0 (1(1)) where is the settlement of the plate (mm), is the normal stress below the plate (MPa), , , where s is the settlement of the plate (mm), σ is the normal stress below the plate (MPa), a , a , and a and is the constant determined from0 second-order polynomial regression analysis0 1 (mm,2 is the constant determined from second-order polynomial regression analysis (mm, mm/MPa, and mm/MPa, and mm/(MPa)2, respectively). The determined curves and constants are illustrated with mm/(MPa)2, respectively). The determined curves and constants are illustrated with the test results the test results shown in Figure 7. In Figure 7, “a” and “b” represent the duplicate of the tests (Figure shown in Figure7. In Figure7, “a” and “b” represent the duplicate of the tests (Figure6). The values 6). The values of Ev1 and Ev2 were determined from the first and second regression curves, respectively, of E and E were determined from the first and second regression curves, respectively, as follows: as followsv1 : v2 11 E = 1.5·r· (2) v= 1.5 ∙ ∙ + · (2) a1 +a2σ∙0max wherewhere Evis is the the strain strain modulus modulus (MPa), (MPa),r is the is radiusthe radius of the of plate the plate (mm), (mm),σ0max is the maximum is the maximum normal stressnormal below stress the below plate the (MPa). plate (MPa). For the the calculation calculation of of the the modulus modulus of subgrade of subgrade reaction reaction at 2.5-mm at 2.5 displacement-mm displacement (k2.5), the (k average2.5), the slopeaverage between slope between the displacement the displacement of 0 and of 2.5 0 and mm 2.5 on mm the on regression the regression curve ofcurve the of first the loading first loading cycle (Figurecycle (Figure7) was 7) used. was Theused. following The following is the equationis the equation for calculating for calculatingk2.5: k2.5:

∆ . =∆σ0 (3) k2.5 = (3) s where ∆ is the difference between the preloading stress and the stress corresponding to 2.5-mm where ∆σ is the difference between the preloading stress and the stress corresponding to 2.5-mm displacement0 and is the displacement (therefore, 2.5 mm or 0.0025 m). displacement and s is the displacement (therefore, 2.5 mm or 0.0025 m). Sustainability 2018, 10, 3817 8 of 13 Sustainability 2018, 10, x FOR PEER REVIEW 8 of 13

0 0 a - 1st loading cycle a - 1st loading cycle k2.5 at a a – 2nd loading cycle a – 2nd loading cycle b - 1st loading cycle k2.5 at a b - 1st loading cycle 2 b – 2nd loading cycle 2 b – 2nd loading cycle

y = −7.99 x2 + 20.48 x − 0.37 y = −9.55 x2 + 17.29 x − 0.80

2 (mm) (mm) 4 y = −7.65 x + 21.62 x − 0.86 4 s s y = −8.77 x2 + 17.92 x − 0.85

6 6 y = 1.22 x2 + 1.30 x + 8.21

2 Settlement, Settlement, Settlement, Settlement, y = 1.28 x + 1.08 x + 5.91 8 8 y = 1.34 x2 + 0.97 x + 5.39

y = 1.11 x2 + 1.05 x + 8.47 10 10 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7

Normal stress, σ0 (MPa) Normal stress, σ0 (MPa) (a) (b)

Figure 7. Examples of stress stress-settlement-settlement curves curves in the first ﬁrst lift lift:: ( (aa)) D25 D25 at at locations locations “ “a”a” and “ “b”b” after 4 rollerroller passes; ( b) D25 at locations “ “a”a” and “b”“b” after 12 roller passes.

4.2. Relationship Relationship Between Between Roller Roller Pass Pass and Modulus Figure8 summarizes the changes in E , E , k and E /E for the successive roller passes at Figure 8 summarizes the changes in Evv11, Evv22, k2.52.5 and Evv1/Ev2v 2for the successive roller passes at which the PLTs were conducted. Each point in Figure 88 isis thethe averageaverage of two test results (duplicate). The values of E , E , and k steeply increase up to four roller passes for most of the test materials The values of Ev1, Evv22, and k2.52.5 steeply increase up to four roller passes for most of the test materials and lifts. It is clear that E keeps increasing as the number of passes raises after four passes but with and lifts. It is clear that Evv11 keeps increasing as the number of passes raises after four passes but with declined trends. E and k seem to somewhat reflect the change in materials with increasing number declined trends. Ev22 and k2.52.5 seem to somewhat reflect the change in materials with increasing number of passes, but they are not as sensitive or consistent to the number of passes as E . It is noted that of passes, but they are not as sensitive or consistent to the number of passes as Ev1. vIt1 is noted that Ev2 isEv the2 is thestrain strain modulus modulus during during reloading reloading process process and and the the material material may may be be compressed compressed and and become denser during the first loading cycle, making E the consequence of the first loading rather than the denser during the first loading cycle, making Ev22 the consequence of the first loading rather than the measure of roller compaction. k is the stiffness measure of the aggregate at relative smaller strain measure of roller compaction. k2.52.5 is the stiffness measure of the aggregate at relative smaller strain level than E , so that the effect of increased compaction level on highly nonlinear material may not level than Ev11, so that the effect of increased compaction level on highly nonlinear material may not be captured well by k . From Figure8a–c, it can be found that the minimum of four roller passes be captured well by k2.5. From Figure 8a–c, it can be found that the minimum of four roller passes recommendedrecommended by by the the ICPI ICPI and and ASCE ASCE [1,28] [1,28] is is an an efficient efficient one one,, but one can expect hardening of the openopen-graded-graded materials for larger number of roller passes than four.four. The underlying subgrade layer could affect the results of PLTs, and the values of E , E and k The underlying subgrade layer could affect the results of PLTs, and the values of vE1v1, Evv22 and k2.52.5 of subgrade were 69 MPa, 196 MPa, and 143 MPa/m, MPa/m, higher higher than than those those of of all all open open-graded-graded aggregates. aggregates. Because the subgrade has a considerably higher stiffness than the permeable base layers, the E values Because the subgrade has a considerably higher stiffness than the permeable base layers,v 1the Ev1 valuesof the firstof the lift, first built lift, directly built directly on the subgrade,on the subgrade, are slightly are slightly higher than higher those than of those the second of the lift second for most lift forcases most (Figure cases8 a),(Figure but certainly 8a), but notcertainly significant. not significant These differences. These differences are not captured are not bycaptured other modulus by other measures such as E and k . modulus measures vsuch2 as 2.5Ev2 and k2.5. The strain modulus at reloading cycle (E ) showed high dependency on the material type. The strain modulus at reloading cycle (Ev2)v s2howed high dependency on the material type. The permeableThe permeable base baselayers layers composed composed of large of large particle particless at the at lowest the lowest compaction compaction level, level, in some in some cases, cases, has has higher E than the smaller particles at the highest compaction level. For example, D13 with 12 higher Ev2 thanv2 the smaller particles at the highest compaction level. For example, D13 with 12 passes passes showed considerably lower E than D25 with 2 passes (Figure8b). These results indicate that showed considerably lower Ev2 thanv 2D25 with 2 passes (Figure 8b). These results indicate that the selectionthe selection of materials of materials is very is very important, important, and the and level the levelof compaction of compaction does doesnot compensate not compensate the mis the- selectionmis-selection of the of materials. the materials. When the values of E of open-graded aggregates are compared to those of conventional When the values of Ev11 of open-graded aggregates are compared to those of conventional (densely-graded) ground materials in literature (Table1), E of open-graded aggregates are smaller (densely-graded) ground materials in literature (Table 1), Evv1 of open-graded aggregates are smaller thanthan those those of of dense dense-graded-graded materials with with similar particle sizes (g (gravel),ravel), but but rather rather close close to those of dense-graded materials with smaller sizes (sand and clay). The values of E are, however, closer to dense-graded materials with smaller sizes (sand and clay). The values of Ev22 are, however, closer to denselydensely-graded-graded gravel gravel materials materials rather rather than than the densely-graded the densely-graded sand or sand clay. orAs such,clay. more As such, permanent more permanentdeformation deformation is expected is in ex open-gradedpected in open aggregates-graded thanaggregates in dense-graded than in dense aggregates,-graded butaggregates, the resilient but the resilient deformation may be similar for these materials. When they are compared to the requirements in German [32] and Korean [33] guides, all the materials tested meet the requirement Sustainability 2018, 10, 3817 9 of 13 deformation may be similar for these materials. When they are compared to the requirements in Sustainability 2018, 10, x FOR PEER REVIEW 9 of 13 German [32] and Korean [33] guides, all the materials tested meet the requirement for Ev2 but not for Ev2/Ev1 and k2.5. This would be partially because the current requirements are setup focusing on for Ev2 but not for Ev2/Ev1 and k2.5. This would be partially because the current requirements are setup focusingdense-graded on dense materials.-graded For materials. open-graded For open aggregates,-graded it aggregates, will be necessary it will tobe develop necessary more to develop suitable morerequirements suitable requirements for QC. for QC.

20 140

120 16 100

12 80 (MPa) (MPa) 1 2 v

v 60

E 8 E

D40 (1st lift) D40 (2nd lift) 40 4 D40+D25 (1st lift) D40+D25 (2nd lift) D25 (1st lift) D25 (2nd lift) 20 D25+D13 (1st lift) D25+D13 (2nd lift) D13 (1st lift) D13 (2nd lift) 0 0 0 2 4 6 8 10 12 14 0 2 4 6 8 10 12 14 Number of Roller Passes Number of Roller Passes (a) (b) 100 14

80 12

60 1 10 v /E 2 v (MPa/m) 40 E 8 2.5 k

20 6

0 4 0 2 4 6 8 10 12 14 0 2 4 6 8 10 12 14 Number of Roller Passes Number of Roller Passes (c) (d)

FigureFigure 8. 8. RelationshipRelationship between between number of roller passespasses andand modulusmodulus measures:measures: ( a(a))E Evv11;(; (bb)) EEvv2;;( (cc)) kk2.52.5; ; ((dd)) EEvv2/2E/vE1.v 1.

4.3.4.3. Relationship Relationship Between Between Materials Materials and and Modulus Modulus TheThe results in in Figure Figure8 are 8 are reorganized reorganized in Figure in Figure9 to present 9 to present better the better differences the differences among materials. among The values of E are higher for larger size of particles with a few variations (Figure9a). The D13 materials. The valuesv1 of Ev1 are higher for larger size of particles with a few variations (Figure 9a). sample, the smallest particles among the investigated, has the lowest values of E , E , and k for The D13 sample, the smallest particles among the investigated, has the lowest valuesv1 ofv2 Ev1, Ev22.5, and most cases. The values of E are mostly within a band of 80~130 MPa. The values of k do not seem k2.5 for most cases. The valuesv2 of Ev2 are mostly within a band of 80~130 MPa. The values2.5 of k2.5 do not seemconsistent, consistent possibly, possibly because because they representthey represent the slope the slope of stress of stress and displacement and displacement at very at narrowvery narrow range rangeof displacement of displacement (2.5 mm). (2.5 mm). Sustainability 2018, 10, 3817 10 of 13 Sustainability 2018, 10, x FOR PEER REVIEW 10 of 13

20 140

120 16 100

12 80 (MPa) (MPa) 1 2 v 8 v 60 E E

2 pass (1st lift) 4 pass (1st lift) 40 4 8 pass (1st lift) 12 pass (1st lift) 2 pass (2nd lift) 4 pass (2nd lift) 20 8 pass (2nd lift) 12 pass (2nd lift) 0 0 D40 D40+D25 D25 D25+D13 D13 D40 D40+D25 D25 D25+D13 D13 Material Material

(a) (b) 100 14

12 80 10 )

60 1 v 8 E / 2 v MPa/m E ( 40 6 2.5 k 4 20 2

0 0 D40 D40+D25 D25 D25+D13 D13 D40 D40+D25 D25 D25+D13 D13 Material Material (c) (d)

FigureFigure 9. 9.Relationship Relationship between between materials materials and and modulus modulus measures: measures: (a ()aE) vE1v;(1; b(b) )E Ev2v2;(; (cc))k k2.52.5;(; (d) Ev22//EEv1v. 1.

44.4..4. Variations Variations Between Between Modulus Modulus Measurements Measurements FigureFigure 10 10 shows shows the the examples examples of the of modulus the modulus values obtained values before obtained averaging before the averaging measurements the measurementsat two locations at “a” two and locations “b”. It can“a” be and found “b”. thatIt canEv 1be, which found measures that Ev1, slopewhich of measures stress and slope displacement of stress andup to displacement the maximum up stress, to the presents maximum more stress, consistent presents results more comparing consistent to resultsk2.5 that comparing uses smaller to k portion2.5 that usesof the smaller stress-displacement portion of the curve stress (Figure-displacement7). For dense-graded curve (Figure materials, 7). For dense this- gapgraded may materials, be less signiﬁcant; this gap mayfor open-graded be less significant; materials for that open undergoes-graded considerable materials that amount undergoes of displacement, considerableEv1 and amountEv2 give of displacement,more consistent Ev results.1 and Ev2 Table give 5more summarized consistent the results. variations Table of 5 duplicatesummarized measurements the variations for of all duplicate the tests measurementsconducted, and for for all open-graded the tests conducted, aggregates, andE vfor1 and openEv-2gradedwould aggregates, be a more reliableEv1 and measureEv2 would for be QC a morethan kreliable2.5. measure for QC than k2.5.

Sustainability 2018, 10, 3817 11 of 13 Sustainability 2018, 10, x FOR PEER REVIEW 11 of 13

20 100

16 80

12 60 (MPa) 1 (MPa/m) v 8 40 E 2.5 k 4 D40+D25 (a) 20 D13 (a) D40+D25 (b) D13 (b) 0 0 0 2 4 6 8 10 12 14 0 2 4 6 8 10 12 14 Number of Roller Passes Number of Roller Passes (a) (b)

v1 FigureFigure10. 10.Examples Examples ofof modulusmodulus measurementsmeasurements before before averaging: averaging: ( a(a))E Ev1 measurementsmeasurements atat locationlocation aa 2.5 andand bb inin D40+D25D40+D25 ofof ﬁrstfirst lift;lift; ((bb))k k2.5 measurements at location a and b in D13 of second lift.lift.

TableTable5. 5.Coefﬁcients Coefficients ofof variationvariation ofof modulusmodulus measures. measures.

CoefficientCoefﬁcient of of Variation Variation (CV) ModulusModulus MaximumMaximum (%) (%) M Meanean (%) (%)

Ev1 Ev1 16.116.1 5.3 5.3 Ev2 Ev2 22.422.4 4.5 4.5 k 40.6 10.5 2.5 k2.5 40.6 10.5

5.5. ConclusionsConclusions TheThe mechanicalmechanical behaviorbehavior ofof open-gradedopen-graded aggregatesaggregates hashas notnot beenbeen aa majormajor interestinterest ofof pavementpavement industryindustry and and society,society, and and therefore therefore there there is is no no much much information information available available for for behavior behavior ofof compactedcompacted open-gradedopen-graded aggregates. aggregates. Sets of PLTs have have conducted conducted to to investigate investigate the the mechanical mechanical behavior behavior of of compactedcompacted permeablepermeable oror open-gradedopen-graded aggregates.aggregates. BasedBased onon thethe testtest results,results, thethe followingfollowing ﬁndingsfindings werewere made:made:

• TheThe values values of ofE Ev1v1,, EEvv22, and k2.5 steeplysteeply increased increased up to four roller passes passes for for most most of of the the test test materialsmaterials andand liftslifts consideredconsidered here.here.E Evv11 clearlyclearly keepskeeps increasingincreasing asas thethe numbernumber ofof passespasses raisesraises afterafter fourfour passespasses butbut withwith declineddeclined trends.trends. TheThe minimumminimum ofof fourfour rollerroller passespasses recommendedrecommended byby thethe ICPIICPI andand ASCEASCE [[1,28]1,28] isis efﬁcient,efficient, butbut oneone cancan expectexpect hardeninghardening ofof thethe open-gradedopen-graded materialsmaterials forfor largerlarger number number of of roller roller passes passes than than four. four.  v2 • TheThe strainstrain modulusmodulus atat thethe second second loading loading cycle cycle ( E(Ev2)) showedshowed highhigh dependencydependency onon thethe materialmaterial type.type. LargeLarge particles particles at at the the lowest lowest compaction compaction level, level, in in some some cases, cases, hadhad higherhigherE Ev2v2 thanthan the the smallersmaller particles particles at at the the highest highest compaction compaction level. level. The T selectionhe selection of materials of materials is very is very important, important, and theand level the level of compaction of compaction does does not compensate not compensate the mis-selection the mis-selection of the of materials. the materials. • ForFor open-gradedopen-graded aggregatesaggregates thatthat undergoesundergoes largelarge deformationdeformation duringduring PLT,PLT, thethe strainstrain modulusmodulus Ev1 and Ev2 measures much wider range of stress and displacement than k2.5. As such, Ev1 and Ev2 Ev1 and Ev2 measures much wider range of stress and displacement than k2.5. As such, Ev1 and give more consistent results. For open-graded aggregates, Ev1 and Ev2 would make a more Ev2 give more consistent results. For open-graded aggregates, Ev1 and Ev2 would make a more reliable measure for QC than k2.5 reliable measure for QC than k2.5. • WhenWhen the the results results are are compared compared to the to requirements the requirements in German in German and Korean and guides, Korean all guides, the materials all the materials tested meet the requirement for Ev2 but not for Ev2/Ev1 and k2.5. This would be partially tested meet the requirement for Ev2 but not for Ev2/Ev1 and k2.5. This would be partially because thebecause current the requirements current requirements are setup focusing are setup on focusing dense-graded on dense materials.-graded It materials.will be necessary It will tobe developnecessary more to develop suitable more QC requirements suitable QC requirements for the open-graded for the aggregateopen-graded base. aggregate base.

Author Contributions: Conceptualization and methodology Y.-J.C., D.A., and J.A.; Validation, Y.-J.C., T.H.N., D.A., and J.A.; Formal Analysis, Y.-J.C.; Investigation, Y.-J.C. and J.A.; Resources, J.A.; Data Curation, Y.-J.C.; Writing—Original Draft, Y.-J.C.; Writing—Review & Editing, J.A.; Visualization, Y.-J.C.; Supervision, J.A.; Project Administration, J.A.; Funding Acquisition, J.A. Funding: This research was supported by a grant from the Technology Advancement Research Program (grant No. 18CTAP-C132363-02) funded by the Ministry of Land, Infrastructure, and Transport of the Korean government. Acknowledgments: The authors would like to thank the Ministry of Land, Infrastructure, and Transport of Korean government for the grant from Technology Advancement Research Program (grant No. 18CTAP-C132363-02). Conﬂicts of Interest: The authors declare no conﬂict of interest.

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