Citation: Farooq, K.; Mujtaba, H.;

Munir, S.; Ashiq, S.Z.; Kazmi, S.M.S.;

Munir, M.J. Evaluation of the Impact

of Fines on the Performance of

Sub-Base Materials. Appl. Sci. 2022,

12, 4513. https://doi.org/10.3390/

app12094513

Academic Editor: Adrián

Mota Babiloni

Received: 22 March 2022

Accepted: 26 April 2022

Published: 29 April 2022

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applied sciences

Article

Evaluation of the Impact of Fines on the Performance ofSub-Base MaterialsKhalid Farooq 1,*, Hassan Mujtaba 1 , Shojat Munir 1, Syed Zishan Ashiq 1 , Syed Minhaj Saleem Kazmi 2

and Muhammad Junaid Munir 2,*

1 Civil Engineering Department, University of Engineering & Technology, Lahore 54890, Pakistan;hassanmujtaba@uet.edu.pk (H.M.); shojatmunir@hotmail.com (S.M.); pd@must.edu.pk (S.Z.A.)

2 School of Engineering, RMIT University, Melbourne, VIC 3001, Australia; syed.kazmi@rmit.edu.au* Correspondence: kfch@uet.edu.pk (K.F.); muhammad.munir@rmit.edu.au (M.J.M.)

Abstract: The study aims to evaluate the change in the behavior of sub-base materials being used inroad pavements through blending fines of different types in different amounts. Fines are added inaggregate samples as part of gradations proposed by the American Association for State and HighwayTransportation Official (AASHTO). Composite samples conforming to AASHTO gradations B and Cwere prepared by mixing coarse aggregates in varying proportions, ranging from 0 to 15%. Laboratorytests—including aggregate quality tests (abrasion test, flakiness index and elongation Index), physicaltests (particle size analysis and specific gravity), and strength test (modified Proctor, Californiabearing ratio, and permeability test)—were performed on the control as well as the modified samples.It was observed that the material with 0% fines yielded the highest CBR values (greater than 98%) andcoefficient of permeability of 4.4 × 10−4 cm/s. However, with the increasing of the fines up to 15%, asubstantial reduction in CBR value up to 10% and coefficient of permeability to 1.62 × 10−7 cm/swas noticed. Based on these results, the modulus of rigidity (MR) and the corresponding structuralnumbers were determined for each layer. Conclusively, the required thickness of the base course wasincreased from 11 cm for the samples with 0% fines to 24 cm (118%) for the samples with the additionof 15% fines according to the AASHTO Design method.

Keywords: gradation; sub-base; fines; pavement; structural number

1. Introduction

Sub-base is an important part of the flexible pavement structure and road serviceabilitythat largely depends upon the quality of sub-base and base materials. The functions ofthe sub-base layer in flexible pavement are to: reduce subgrade stresses; encase subgradeagainst the punching of the base material; provide a working platform for constructionmachinery; preventing the intrusion of fines from the subgrade to the wearing course; andserve as a free draining material in the absence of a drainage layer below it. This studyis focused on evaluating the impact of increasing the percentage of fines on the strengthbehavior of the sub-base layer. Previously, various researchers [1–16] studied the effect ofextra fine material on the behavior of aggregates and, some of them, concluded that thebehavior of aggregate fine mixtures mainly depends on the type, shape, and gradation ofthe aggregates in addition to the quantity of fines, moisture content, and testing procedure.Additionally, some researchers [3,17–19] also noticed that the presence of extra fines didnot have an impact upon the subgrade performance if proper drainage was providedthrough the aggregate mixture. Studies carried out by different researchers in this contextare summarized below.

The prime function of the granular sub-base is to provide uniform support to thebase course and compensating the inadequate subgrade strength rather than increasing thestability of the pavement [1]. Uthus et al. [2] studied the impact of fines upon the defor-mation characteristics and frost heave of unbound aggregates used in road construction,

Appl. Sci. 2022, 12, 4513. https://doi.org/10.3390/app12094513 https://www.mdpi.com/journal/applsci

Appl. Sci. 2022, 12, 4513 2 of 12

and concluded that the frost susceptibility of aggregates depends on their mineralogicalcomposition, quantity of fine particles, and hydraulic conductivity. Lekarp et al. [3] re-ported that the deformation behavior of unbound aggregates under repeated traffic loadingcan be interpreted through their resilient response and availability of fines up to specificlimit. However, these fines will have little effect on performance if the drainage conditionsare excellent. Tian et al. [4] determined the resilient modulus (MR) for base materials byselecting finer, medium, and coarser gradation limits as specified by the Oklahoma Depart-ment of Transportation. In their study, three varying moisture contents, namely, optimummoisture content (OMC, 2% below and 2% above the OMC), were considered. The reportedvariability in MR values ranged between 10 and 50% for three different gradations, andbetween 10 and 20% for three different moisture contents. Additionally, they proposed apredictive model for MR values based upon multiple linear regression. Kamal et al. [5] con-ducted research on an unbound granular base using eight different grain size distributionsto identify a free draining material to dissipate excess pore water pressure quickly andreduce pumping, and that had a high in situ stiffness and adequate resistance to permanentdeformation. Raad et al. [6] compared the effect of fines on the dynamic response for openand dense graded base courses and observed that, under saturated conditions, the damageprobability for the pavement with open graded aggregates is lesser. This is owing to factthat they offer more resistance to pore pressure build-up than dense-graded aggregates.

Thom and Brown [7] conducted a series of repeated load triaxial tests on aggregatesby varying the gradation, degree of compaction, and moisture contents and concluded that,with the increase in moisture content, the elastic stiffness decreases slightly in the case ofbroadly graded materials. They also observed a reduction in stiffness with the increase inthe fine proportion. Hicks and Monismith [8] studied the influence of density, gradation,and degree of saturation on the modulus of resilience and concluded that the value of theresilient modulus was directly proportional to the confining pressure, whereas a variationin resilience modulus was insignificant with an increase in axial stress. Moreover, theeffect of density, fines, and the degree of saturation on the resilient modulus and Poisson’sratio was marginal. Yoder and Witczak [11] mentioned that, for a specific particle sizedistribution of coarser materials, the proportion of fines affect the performance of thebase course. Rigid pavements experiencing pumping have 3–4 percent or more finesthan pavements that do not experience this phenomenon, which revealed the significanceof fines on the performance of base or sub-base materials. Siswosoebrotho et al. [12]observed that the maximum dry density and California bearing ratio (CBR) increases withthe increase in the fine content and the maximum value is attained at unique fine andmoisture contents. The peak values reported were different for the varying type of fines.Cheung [13] stated that the CBR test can be used as a performance indicator for materialsunder traffic load. Additionally, limiting the value of the plasticity index for fines presentin aggregates does not ensure the suitable performance of road materials under traffic load.Haung [14] stated that the sub-base layer contributes almost 45% of the total pavementstructure for stability against failure. Furthermore, the engineering characterization of roadmaterial is necessary prior to use in any rehabilitation or reconstruction of road works.Cedergen [15] proposed that the useful life of a pavement can be increased three timesby using a free draining base/sub-base layer. Gandara et al. [16] conducted a study toevaluate the engineering properties of granular materials with an emphasis on the particlesize and fine content. From their study, it was revealed that fines have a large impact onthe engineering performance of base materials and the fine content be limited to 10% inthe base course. If the fines remain within the range of 5% to 10% in the base course, itpossesses a high compressive strength and modulus of resilience. The Federal HighwayAdministration [20] stated that the granular unbound layers provided in the pavementhave a significant contribution in the overall structural capacity of the pavement. Thethickness of the pavement layer above the subgrade decreases significantly as the CBRvalue of the subgrade increases.

Appl. Sci. 2022, 12, 4513 3 of 12

2. Materials and Methods

The samples used in this study comprised coarse aggregates and fines. Coarse ag-gregates were collected from Sargodha quarry, whereas non-plastic fines were preparedfrom the locally available silty soil (Ghassu). Fines with medium and high plasticity wereobtained from the locally available and Nandipur clay, respectively. These fines wereblended with coarse aggregates in different proportions, i.e., 5%, 10%, and 15%, to preparecomposite samples as per AASHTO gradations ‘B’ and ‘C’, as shown in Table 1.

Table 1. Grading requirement for samples as per the AASHTO standard.

ASTMSieve Sizes Grading B Grading C

50 mm 100 -25 mm 75–95 100

9.525 mm 40–75 50–85No. 4 30–60 35–65No. 10 20–45 25–50No. 40 15–30 15–40

No. 200 5–20 5–15

Composite samples with non-plastic fines were designated with letters from A1 to A4,while those with medium- and high-plasticity fines were denoted by letters from B1 to B4and from C1 to C4, respectively. The details of sample designation and classification aresummarized in Table 2.

Table 2. Laboratory test results for the composite samples.

CompositeSample

Fines(%)

AASHTO Gradation B AASHTO Gradation C

* Gmb ** LL *** PI Classification byUSCS System * Gmb ** LL *** PI

Classificationby USCSSystem

A(n

on-p

last

icfin

es

A1 02.65

- **** NP

(GW) 2.64

- **** NP

(GW)

A2 5 (GW–GM) 2.64 (GW–GM)

A3 10 2.64 (GW–GM) 2.66 (GW–GM)

A4 15 2.65 (GW–GM) 2.63 (GW–GM)

B(M

ediu

m-

plas

tici

tyfin

es)

B1 0

2.65

- **** NP (GW–GM) 2.64 - **** NP (GW–GM)

B2 5 18.4 **** NP (GW–GC) 2.63 20.4 **** NP (GW–GC)

B3 10 19.2 3.3 (GW–GC) 2.62 22.2 3.26 (GW–GC)

B4 15 2.61 22.3 4.9 (GW–GC) 2.65 24.3 6.9 (GW–GC)

C(H

igh-

)pl

asti

city

fines C1 0

2.65- **** NP (GW) 2.64 - NP (GW)

C2 5 23.5 6.3 (GW–GC) 2.65 24.1 5.9 (GW–GC)

C3 10 2.63 25.5 5.7 (GW–GC) 2.64 25.5 6.6 (GW–GC)

C4 15 2.65 28.8 9 (GW–GC) 2.62 31.4 10.6 (GW–GC)

* Bulk specific gravity; ** Liquid limit; *** Plasticity index; **** Non-plastic.

All the samples were subjected to a series of laboratory tests, including grain sizedistribution, Los Angeles abrasion test, shape test, specific gravity, and consistency limittests, to determine the physical properties of the samples. Modified Proctor compactiontests were performed in accordance with the ASTM standard D 1557 to ascertain themaximum dry density (MDD) and optimum moisture content (OMC) for each sample.These samples were compacted in CBR molds using modified Proctor hammer at OMC in

Appl. Sci. 2022, 12, 4513 4 of 12

five layers by applying 10, 30, and 65 blows per layer. The molds were soaked in water for96 h. The CBR test was conducted as per ASTM-D 1883 and CBR values at 0.1′′ and 0.2′′

penetration were determined.

3. Test Results and Discussion

Grain size analysis, specific gravity, and shape tests, including flakiness and elongationindices, Los Angeles abrasion test, modified Proctor compaction test and California bearingratios, were performed on all the samples. The results of the various laboratory tests arediscussed in the following sub-sections.

3.1. Engineering Characteristics of Soil Aggregate Mixtures

The gradation curves of the coarse aggregate samples are shown in Figure 1. Theeffective grain size (D10) of these samples is 0.17 mm, while the median grain size (D50) is inthe range of 5.0~7.0 mm. The gravel content (fraction coarser than US #4) ranges between50 and 57%, whereas the sand content varies from 41 to 48%. The percentage of fines (sizelesser than Sieve No. 200) present in the samples is only 2%. Grain sizes with a reference to60%, 30%, and 10% fines (D60, D30 and D10, respectively) were calculated from the GSDcurves. The coefficients of uniformity and curvature vary from 33 to 55 and from 1.0 to1.12, respectively. Both samples were classified as well-graded gravels (GW), according tothe Unified Soil Classification System (USCS), ASTM D-2487 whereas the inherent finespresent in the samples were non-plastic. These samples were mixed with non-mediumand high-plasticity fines to prepare the composites. The gradation curves of the aggregatesamples used in the study are shown in Figure 1.

Appl. Sci. 2022, 12, x FOR PEER REVIEW 4 of 12

layers by applying 10, 30, and 65 blows per layer. The molds were soaked in water for 96 h. The CBR test was conducted as per ASTM-D 1883 and CBR values at 0.1″ and 0.2″ pen-etration were determined.

3. Test Results and Discussion Grain size analysis, specific gravity, and shape tests, including flakiness and

elongation indices, Los Angeles abrasion test, modified Proctor compaction test and California bearing ratios, were performed on all the samples. The results of the various laboratory tests are discussed in the following sub-sections.

3.1. Engineering Characteristics of Soil Aggregate Mixtures The gradation curves of the coarse aggregate samples are shown in Figure 1. The

effective grain size (D10) of these samples is 0.17 mm, while the median grain size (D50) is in the range of 5.0 ~ 7.0 mm. The gravel content (fraction coarser than US #4) ranges be-tween 50 and 57%, whereas the sand content varies from 41 to 48%. The percentage of fines (size lesser than Sieve No. 200) present in the samples is only 2%. Grain sizes with a reference to 60%, 30%, and 10% fines (D60, D30 and D10, respectively) were calculated from the GSD curves. The coefficients of uniformity and curvature vary from 33 to 55 and from 1.0 to 1.12, respectively. Both samples were classified as well-graded gravels (GW), ac-cording to the Unified Soil Classification System (USCS), ASTM D-2487 whereas the in-herent fines present in the samples were non-plastic. These samples were mixed with non-medium and high-plasticity fines to prepare the composites. The gradation curves of the aggregate samples used in the study are shown in Figure 1.

Figure 1. Gradations for the sub-base samples.

Laboratory tests were performed on these composite samples and the results are summarized in Table 2. All the samples belong to gradings B or C as per the AASHTO gradation. The blended samples were classified as per the Unified Soil Classification Sys-tem (USCS). With the addition of medium-plasticity fines up to 15% and high-plasticity fines up to 5%, the sample’s classification changes from GW–GM to GW–GC (well-graded gravel with clay). Details of the classification tests for all composite samples prepared as per AASHTO gradings B or C are summarized in Table 2.

Composite samples prepared as per the AASHTO gradation ‘B’ have an average spe-cific gravity of 2.65, a flakiness index (FI) of 34, an elongation index (EI) value of 42, and

0

10

20

30

40

50

60

70

80

90

100

0.010.101.0010.00100.00

Perc

ent P

assin

g, (%

)

Grain Size (mm)

Sample A

Sample B

Sample Gravels (%)

Sand (%)

Fines (%)

USCS Classification

A 57 41 2 GW

B 50 48 2 GW

Figure 1. Gradations for the sub-base samples.

Laboratory tests were performed on these composite samples and the results aresummarized in Table 2. All the samples belong to gradings B or C as per the AASHTOgradation. The blended samples were classified as per the Unified Soil Classification System(USCS). With the addition of medium-plasticity fines up to 15% and high-plasticity fines upto 5%, the sample’s classification changes from GW–GM to GW–GC (well-graded gravelwith clay). Details of the classification tests for all composite samples prepared as perAASHTO gradings B or C are summarized in Table 2.

Composite samples prepared as per the AASHTO gradation ‘B’ have an averagespecific gravity of 2.65, a flakiness index (FI) of 34, an elongation index (EI) value of 42, anda Los Angeles abrasion (LAAV) value of 19, whereas for composite samples prepared asper the AASHTO gradation ‘C’, these values were 2.66, 16, 39, and 42, respectively.

Appl. Sci. 2022, 12, 4513 5 of 12

3.2. Effect of Fines on the Compaction Characteristics of Soil Aggregate Mixtures

The compaction curves of the samples belonging to gradation ‘B’ are shown in Figure 2.The figure depicts that, with the increase in fines from 0 to 15%, the OMC increases from5.5 to 8.8%, whereas MDD decreases from 21.75 to 20.68 kN/m3. Likewise, for the sampleswith the AASHTO gradation ‘C’ that showed similar trends, i.e., with an increase in finesup to 15%, OMC increases in the range of 5.8~9% with a reduction in MDD value from20.90 to 20.20 kN/m3, as presented in Figure 3.

Appl. Sci. 2022, 12, x FOR PEER REVIEW 5 of 12

a Los Angeles abrasion (LAAV) value of 19, whereas for composite samples prepared as per the AASHTO gradation ‘C’, these values were 2.66, 16, 39, and 42, respectively.

3.2. Effect of Fines on the Compaction Characteristics of Soil Aggregate Mixtures The compaction curves of the samples belonging to gradation ‘B’ are shown in Figure

2. The figure depicts that, with the increase in fines from 0 to 15%, the OMC increases from 5.5 to 8.8%, whereas MDD decreases from 21.75 to 20.68 kN/m3. Likewise, for the samples with the AASHTO gradation ‘C’ that showed similar trends, i.e., with an increase in fines up to 15%, OMC increases in the range of 5.8~9% with a reduction in MDD value from 20.90 to 20.20 kN/m3, as presented in Figure 3.

Figure 2. Compaction curves of the composite samples (gradation B).

Figure 3. Compaction curves of the composite samples (gradation C).

124

126

128

130

132

134

136

138

140

142

144

19.50

20.00

20.50

21.00

21.50

22.00

22.50

3.00 5.00 7.00 9.00 11.00 13.00

Dry

Den

sity

(pcf

)

Dry

Den

sity

(KN

/m3 )

Water Content %

0%(Non-Plastic Fines) 5%(Non-Plastic f ines)10% (Non-plastic Fines) 15%(Non-Plastic Fines)5% (Low to Medium Plastic Fines) 10% (Low to Medium Plastic Fines)15% (Low to Medium Plastic Fines) 5% (High Plastic Fines)10% (High Plastic Fines) 15% (High Plastic Fines)

γ(dmax) = 20.68 kN/m3

OMC = 5.5 % OMC = 8.5 %

15% High Plastic Fines

Max. Dry Density = 21.93 KN/m3

γ(dmax) = 21.75 kN/m3

0% Non-Plastic Fines

124

126

128

130

132

134

136

138

140

19.50

20.00

20.50

21.00

21.50

22.00

22.50

3.00 5.00 7.00 9.00 11.00 13.00

Dry

Den

sity

(pcf

)

Dry

Den

sity

(KN

/m3 )

Water Content %

0% (Non-Plastic Fines) 5% (Non-Plastic Fines)10% (Non-Plastic Fines) 15% (Non-Plastic Fines)5% (Low to Medium Plastic Fines) 10% (Low to Medium Plastic Fines)15% (Low to Medium Plastic Fines) 5% (High Plastic Fines)10% (High Plastic Fines) 15% (High Plastic Fines)

Max. Dry Density = 21.30 KN/m3

0% Non-Plastic Fines

γ (dmax) = 20.90 kN/m3

γ (dmax) = 20.20 kN/m3

15% High Plastic Fines

OMC = 5.5 % OMC = 8.9 %

Figure 2. Compaction curves of the composite samples (gradation B).

Appl. Sci. 2022, 12, x FOR PEER REVIEW 5 of 12

a Los Angeles abrasion (LAAV) value of 19, whereas for composite samples prepared as per the AASHTO gradation ‘C’, these values were 2.66, 16, 39, and 42, respectively.

3.2. Effect of Fines on the Compaction Characteristics of Soil Aggregate Mixtures The compaction curves of the samples belonging to gradation ‘B’ are shown in Figure

2. The figure depicts that, with the increase in fines from 0 to 15%, the OMC increases from 5.5 to 8.8%, whereas MDD decreases from 21.75 to 20.68 kN/m3. Likewise, for the samples with the AASHTO gradation ‘C’ that showed similar trends, i.e., with an increase in fines up to 15%, OMC increases in the range of 5.8~9% with a reduction in MDD value from 20.90 to 20.20 kN/m3, as presented in Figure 3.

Figure 2. Compaction curves of the composite samples (gradation B).

Figure 3. Compaction curves of the composite samples (gradation C).

124

126

128

130

132

134

136

138

140

142

144

19.50

20.00

20.50

21.00

21.50

22.00

22.50

3.00 5.00 7.00 9.00 11.00 13.00

Dry

Den

sity

(pcf

)

Dry

Den

sity

(KN

/m3 )

Water Content %

0%(Non-Plastic Fines) 5%(Non-Plastic f ines)10% (Non-plastic Fines) 15%(Non-Plastic Fines)5% (Low to Medium Plastic Fines) 10% (Low to Medium Plastic Fines)15% (Low to Medium Plastic Fines) 5% (High Plastic Fines)10% (High Plastic Fines) 15% (High Plastic Fines)

γ(dmax) = 20.68 kN/m3

OMC = 5.5 % OMC = 8.5 %

15% High Plastic Fines

Max. Dry Density = 21.93 KN/m3

γ(dmax) = 21.75 kN/m3

0% Non-Plastic Fines

124

126

128

130

132

134

136

138

140

19.50

20.00

20.50

21.00

21.50

22.00

22.50

3.00 5.00 7.00 9.00 11.00 13.00

Dry

Den

sity

(pcf

)

Dry

Den

sity

(KN

/m3 )

Water Content %

0% (Non-Plastic Fines) 5% (Non-Plastic Fines)10% (Non-Plastic Fines) 15% (Non-Plastic Fines)5% (Low to Medium Plastic Fines) 10% (Low to Medium Plastic Fines)15% (Low to Medium Plastic Fines) 5% (High Plastic Fines)10% (High Plastic Fines) 15% (High Plastic Fines)

Max. Dry Density = 21.30 KN/m3

0% Non-Plastic Fines

γ (dmax) = 20.90 kN/m3

γ (dmax) = 20.20 kN/m3

15% High Plastic Fines

OMC = 5.5 % OMC = 8.9 %

Figure 3. Compaction curves of the composite samples (gradation C).

Specifically for gradation ‘B’, with the addition of 5% of non-plastic and medium-plasticity fines, MDD first increased from 21.64 KN/m3 to 21.92 kN/m3 and 21.74 kN/m3,respectively; however, with the inclusion of 5% of high-plasticity fines, the value of MDD

Appl. Sci. 2022, 12, 4513 6 of 12

reduced to 21.53 kN/m3. Moreover, the addition of 10 and 15% of non-plastic fines reducedthe MDD value to 21.62 kN/m3 and 21.35 kN/m3, respectively. Similarly, MDD values formedium-plasticity fines were 21.33 kN/m3 and 21.15 kN/m3 and, for high-plasticity fines,they were 21.01 kN/m3 and 20.84 kN/m3. The variation in MDD with the change in finecontent for gradations B and C is shown in Figures 4 and 5, respectively.

Appl. Sci. 2022, 12, x FOR PEER REVIEW 6 of 12

Specifically for gradation ‘B’, with the addition of 5% of non-plastic and medium-plasticity fines, MDD first increased from 21.64 KN/m3 to 21.92 kN/m3 and 21.74 kN/m3, respectively; however, with the inclusion of 5% of high-plasticity fines, the value of MDD reduced to 21.53 kN/m3. Moreover, the addition of 10 and 15% of non-plastic fines reduced the MDD value to 21.62 kN/m3 and 21.35 kN/m3, respectively. Similarly, MDD values for medium-plasticity fines were 21.33 kN/m3 and 21.15 kN/m3 and, for high-plasticity fines, they were 21.01 kN/m3 and 20.84 kN/m3. The variation in MDD with the change in fine content for gradations B and C is shown in Figures 4 and 5, respectively.

It can be inferred from Figures 4 and 5 that the addition of 5% of non-plastic and medium-plasticity fines caused an almost 10% increase in the value of MDD for both gra-dations. However, beyond this, the maximum dry density decreased with the further ad-dition of fines. The increase in the value of MDD may be attributed to the filling of void spaces, thus causing the denser packing of the samples, which resulted into a higher unit weight. However, after 5% fines, the aggregates are being replaced by fines that have a lesser specific gravity, resulting in a lower maximum dry density of the matrix.

Figure 4. Effect of fines on the MDD and CBR values for the composite samples (gradation B).

Figure 5. Effect of fines on the MDD and CBR values for the composite samples (gradation C).

3.3. Effect of Fines on the California Bearing Ratio (CBR) of Soil Aggregate Mixture With increase in the percentage of fines (0–15%) for the composite samples with gra-

dation ‘B’, the CBR values decreased from 122 to 39 for non-plastic fines, to 22 for medium-plasticity fines, and to 14 for high-plasticity fines.

Figure 4. Effect of fines on the MDD and CBR values for the composite samples (gradation B).

Appl. Sci. 2022, 12, x FOR PEER REVIEW 6 of 12

Specifically for gradation ‘B’, with the addition of 5% of non-plastic and medium-plasticity fines, MDD first increased from 21.64 KN/m3 to 21.92 kN/m3 and 21.74 kN/m3, respectively; however, with the inclusion of 5% of high-plasticity fines, the value of MDD reduced to 21.53 kN/m3. Moreover, the addition of 10 and 15% of non-plastic fines reduced the MDD value to 21.62 kN/m3 and 21.35 kN/m3, respectively. Similarly, MDD values for medium-plasticity fines were 21.33 kN/m3 and 21.15 kN/m3 and, for high-plasticity fines, they were 21.01 kN/m3 and 20.84 kN/m3. The variation in MDD with the change in fine content for gradations B and C is shown in Figures 4 and 5, respectively.

It can be inferred from Figures 4 and 5 that the addition of 5% of non-plastic and medium-plasticity fines caused an almost 10% increase in the value of MDD for both gra-dations. However, beyond this, the maximum dry density decreased with the further ad-dition of fines. The increase in the value of MDD may be attributed to the filling of void spaces, thus causing the denser packing of the samples, which resulted into a higher unit weight. However, after 5% fines, the aggregates are being replaced by fines that have a lesser specific gravity, resulting in a lower maximum dry density of the matrix.

Figure 4. Effect of fines on the MDD and CBR values for the composite samples (gradation B).

Figure 5. Effect of fines on the MDD and CBR values for the composite samples (gradation C).

3.3. Effect of Fines on the California Bearing Ratio (CBR) of Soil Aggregate Mixture With increase in the percentage of fines (0–15%) for the composite samples with gra-

dation ‘B’, the CBR values decreased from 122 to 39 for non-plastic fines, to 22 for medium-plasticity fines, and to 14 for high-plasticity fines.

Figure 5. Effect of fines on the MDD and CBR values for the composite samples (gradation C).

It can be inferred from Figures 4 and 5 that the addition of 5% of non-plastic andmedium-plasticity fines caused an almost 10% increase in the value of MDD for bothgradations. However, beyond this, the maximum dry density decreased with the furtheraddition of fines. The increase in the value of MDD may be attributed to the filling of voidspaces, thus causing the denser packing of the samples, which resulted into a higher unitweight. However, after 5% fines, the aggregates are being replaced by fines that have alesser specific gravity, resulting in a lower maximum dry density of the matrix.

3.3. Effect of Fines on the California Bearing Ratio (CBR) of Soil Aggregate Mixture

With increase in the percentage of fines (0–15%) for the composite samples withgradation ‘B’, the CBR values decreased from 122 to 39 for non-plastic fines, to 22 formedium-plasticity fines, and to 14 for high-plasticity fines.

For the composite samples that have the AASHTO gradation ‘C’, the addition ofnon-plastic and medium- and high-plasticity fines up to 15% caused a reduction in the

Appl. Sci. 2022, 12, 4513 7 of 12

soaked CBR from 99 to 65, 35 and 10, respectively. The variation in CBR values with theaddition of fines is shown in Figures 4 and 5. Thus, the composite samples that have a CBRvalue of more than 30% are regarded as suitable samples that can be used for constructionas per the National Highway Authority of Pakistan specifications [21].

3.4. Effect of Fines on the Permeability of Soil Aggregate Mixtures

Falling head permeability tests were carried out on each composite sample. The resultsof the permeability tests are presented in Figures 6 and 7 for gradation B and gradation C,respectively. The coefficients of permeability (k) for non-plastic and medium-plasticity andhigh-plasticity fines reduced from 4.4 × 10−4 cm/s to 4.4 × 10−6 cm/s, 1.1 × 10−6 cm/s,and 1.6 × 10−7 cm/s, respectively, for gradation ‘B’, while for gradation ‘C’, the k valuereduced from 1.2 × 10−4 cm/s to 1.4 × 10−6 cm/s, 6.2 × 10−7 cm/s, and 1.1 × 10−7 cm/swith the addition of non-plastic and medium-plasticity and high-plasticity fines. The kvalues were used for specific road geometry that has a length of 24 ft and slope of 0.02 ft/ftto determine the drainage time for pavement design.

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For the composite samples that have the AASHTO gradation ‘C’, the addition of non-plastic and medium- and high-plasticity fines up to 15% caused a reduction in the soaked CBR from 99 to 65, 35 and 10, respectively. The variation in CBR values with the addition of fines is shown in Figures 4 and 5. Thus, the composite samples that have a CBR value of more than 30% are regarded as suitable samples that can be used for construction as per the National Highway Authority of Pakistan specifications [21].

3.4. Effect of Fines on the Permeability of Soil Aggregate Mixtures Falling head permeability tests were carried out on each composite sample. The re-

sults of the permeability tests are presented in Figures 6 and 7 for gradation B and grada-tion C, respectively. The coefficients of permeability (k) for non-plastic and medium-plas-ticity and high-plasticity fines reduced from 4.4 × 10−4 cm/s to 4.4 × 10−6 cm/s, 1.1 × 10−6 cm/s, and 1.6 × 10−7 cm/s, respectively, for gradation ‘B’, while for gradation ‘C’, the k value reduced from 1.2 × 10−4 cm/s to 1.4 × 10−6 cm/s, 6.2 × 10−7 cm/s, and 1.1 × 10−7 cm/s with the addition of non-plastic and medium-plasticity and high-plasticity fines. The k values were used for specific road geometry that has a length of 24 ft and slope of 0.02 ft/ft to determine the drainage time for pavement design.

Figure 6. Effect of fines on permeability for the composite samples (gradation B).

4E-08

4E-07

4E-06

4E-05

0.0004

0 5 10 15 20

Perm

eabi

lity

(cm

/sec)

Fines Content %

Non-Plastic FinesMedium Plastic FinesHigh Plastic Fines

Figure 6. Effect of fines on permeability for the composite samples (gradation B).

For sub-base materials with 0~6% of non-plastic fines, the material depicted fairdrainage characteristics. However, as the fine content increased from 6~11%, drainage ispoor and, with the addition of non-plastic fines in excess of 11%, drainage characteristics arevery poor and the material is characterized as nearly impermeable. Similarly, for medium-plasticity fines up to 2%, the sub-base material showed fair drainage characteristics, whiledrainage characteristics are poor for a fine content in the range of 2~5% and are the worstfor medium-plasticity fines with a content of more than 5%. Sub-base materials withhigh-plasticity fines exhibit fair drainage characteristics with fines in the range of 0~1% anddrainage characteristics are poor for a fine content in the range of 1~3%, while for contentsof more than 3% of fines, very poor drainage characteristics are observed.

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Figure 7. Effect of fines on permeability for the composite samples (gradation C).

For sub-base materials with 0~6% of non-plastic fines, the material depicted fair drainage characteristics. However, as the fine content increased from 6~11%, drainage is poor and, with the addition of non-plastic fines in excess of 11%, drainage characteristics are very poor and the material is characterized as nearly impermeable. Similarly, for me-dium-plasticity fines up to 2%, the sub-base material showed fair drainage characteristics, while drainage characteristics are poor for a fine content in the range of 2~5% and are the worst for medium-plasticity fines with a content of more than 5%. Sub-base materials with high-plasticity fines exhibit fair drainage characteristics with fines in the range of 0~1% and drainage characteristics are poor for a fine content in the range of 1~3%, while for contents of more than 3% of fines, very poor drainage characteristics are observed.

4. Field Implications The laboratory test results were utilized to design the pavement in accordance with

the AASHTO 1993 guidelines. The pavement design parameters are shown in Table 3.

Table 3. Road design parameters for AASHTO gradations B and C.

Parameter Description Design Value

Equivalent Single Axle Load ESALs (W18) 5 × 106 Asphaltic layer co-efficient (a1) 0.42

Base layer coefficient (a2) 0.14 Base layer drainage co-efficient (m2) 1.20

Sub-base layer drainage co-efficient (m3) 1.0 Standard deviation 0.35

Reliability (R0) 95% Intital servicibility index (Pi) 4.3

Terminal servicibility index (Pt) 2.5

The required base course thicknesses using layered equations were obtained for each type of sub-base for fines of varying types in different proportions. The required base course thicknesses for the AASHTO gradings B and C are shown in Figures 8 and 9, respectively. The figures depict that, with a maximum fine proportion of up to 15%, the thickness of the base course layer varies between 11 and 22 cm for grading B and between

4E-08

4E-07

4E-06

4E-05

0.0004

0 5 10 15 20

Perm

eabi

lity

(cm

/sec)

Fines Content %

Non-Plastic FinesMedium Plastic FinesHigh Plastic Fines

Figure 7. Effect of fines on permeability for the composite samples (gradation C).

4. Field Implications

The laboratory test results were utilized to design the pavement in accordance withthe AASHTO 1993 guidelines. The pavement design parameters are shown in Table 3.

Table 3. Road design parameters for AASHTO gradations B and C.

Parameter Description Design Value

Equivalent Single Axle Load ESALs (W18) 5 × 106

Asphaltic layer co-efficient (a1) 0.42Base layer coefficient (a2) 0.14

Base layer drainage co-efficient (m2) 1.20Sub-base layer drainage co-efficient (m3) 1.0

Standard deviation 0.35Reliability (R0) 95%

Intital servicibility index (Pi) 4.3Terminal servicibility index (Pt) 2.5

The required base course thicknesses using layered equations were obtained for eachtype of sub-base for fines of varying types in different proportions. The required base coursethicknesses for the AASHTO gradings B and C are shown in Figures 8 and 9, respectively.The figures depict that, with a maximum fine proportion of up to 15%, the thickness of thebase course layer varies between 11 and 22 cm for grading B and between 11 and 24 cmfor grading C. The effect of adding fines to the sub-base material on the overall flexiblepavement thickness is presented in Figures 10 and 11 for the AASHTO gradings B andC, respectively.

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11 and 24 cm for grading C. The effect of adding fines to the sub-base material on the overall flexible pavement thickness is presented in Figures 10 and 11 for the AASHTO gradings B and C, respectively.

Figure 8. Base thickness with the use of composite samples (gradation B).

Figure 9. Base thickness with the use of composite samples (gradation C).

Figure 8. Base thickness with the use of composite samples (gradation B).

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11 and 24 cm for grading C. The effect of adding fines to the sub-base material on the overall flexible pavement thickness is presented in Figures 10 and 11 for the AASHTO gradings B and C, respectively.

Figure 8. Base thickness with the use of composite samples (gradation B).

Figure 9. Base thickness with the use of composite samples (gradation C). Figure 9. Base thickness with the use of composite samples (gradation C).

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Figure 10. Thickness of the various pavement layers for gradation B.

Figure 11. Thickness of the various pavement layers for gradation C.

5. Conclusions The current study was carried out to study the effect of fines on the performance of

sub-base materials. Classification and quality assessment tests were performed on com-posite sub-base samples. The laboratory test results were utilized to design a pavement following AASHTO 1993 guidelines. On the basis of the study, the following conclusions were drawn:

i. The impact of blending fines with sub-base materials on the compaction characteris-tics is subtle as the maximum dry unit weight decreased only by about 4%, whereas the optimum moisture content increased by about 3% with the addition of fines up to 15%.

ii. The permeability characteristics of sub-base materials showed a drastic change with the increase in fines and the drainage characteristics of the blended samples changed

Figure 10. Thickness of the various pavement layers for gradation B.

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Figure 10. Thickness of the various pavement layers for gradation B.

Figure 11. Thickness of the various pavement layers for gradation C.

5. Conclusions The current study was carried out to study the effect of fines on the performance of

sub-base materials. Classification and quality assessment tests were performed on com-posite sub-base samples. The laboratory test results were utilized to design a pavement following AASHTO 1993 guidelines. On the basis of the study, the following conclusions were drawn:

i. The impact of blending fines with sub-base materials on the compaction characteris-tics is subtle as the maximum dry unit weight decreased only by about 4%, whereas the optimum moisture content increased by about 3% with the addition of fines up to 15%.

ii. The permeability characteristics of sub-base materials showed a drastic change with the increase in fines and the drainage characteristics of the blended samples changed

Figure 11. Thickness of the various pavement layers for gradation C.

5. Conclusions

The current study was carried out to study the effect of fines on the performanceof sub-base materials. Classification and quality assessment tests were performed oncomposite sub-base samples. The laboratory test results were utilized to design a pavementfollowing AASHTO 1993 guidelines. On the basis of the study, the following conclusionswere drawn:

i. The impact of blending fines with sub-base materials on the compaction characteris-tics is subtle as the maximum dry unit weight decreased only by about 4%, whereasthe optimum moisture content increased by about 3% with the addition of fines upto 15%.

ii. The permeability characteristics of sub-base materials showed a drastic changewith the increase in fines and the drainage characteristics of the blended sampleschanged from fair to very poor. The plasticity of the fines present in the sub-basematerials has a significant impact on the prediction of the drainage characteristicsof the sub-base materials.

iii. The California bearing ratio (CBR) of the sub-base materials decreased with theincrease in fine content. The sub-base materials exhibited a maximum strengthand fair drainage characteristics at a 0% fine content. With the addition of non-plastic fines up to 10%, the sub-base material exhibited a good strength (CBR > 30);however, the drainage characteristics were poor. With the addition of 15% ofnon-plastic fines and more than 5% of medium-plasticity fines, the strength wassubstantially reduced (CBR < 30) with very poor drainage characteristics, thusmaking the material unsuitable for road construction. The addition of only 5% ofhigh-plasticity fines or more than 10% of medium-plasticity fines resulted in verypoor strength/stiffness and drainage characteristics of the sub-base material.

iv. The results of the pavement design using AASHTO 1993 guidelines using thecomposite samples of the sub-base materials depict that the required thickness ofthe base course layer increases by 72~118% with the respective addition of 5~15% ofplastic fines as compared to the case of not adding fines. However, the same amountof non-plastic fines results in a 27~55% increase in the base course thickness.

v. Based on the overall evaluation, we conclude that the mixing of non-plastic fineswith a sub-base material up to 5~6% results in improved compaction, good strength,fair drainage, and a minimum requirement of increased base course thickness.

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Furthermore, the composite samples prepared as per the AASHTO gradation “B”exhibited a better strength and drainage characteristics compared to the compositesamples prepared as per the AASHTO gradation “C”.

Author Contributions: K.F.: conceptualization, formulation of the methodology and top supervisionof the entire study. H.M.: formal analysis and writing—original draft preparation. S.M. providedhis input for the investigation and data curation. S.Z.A. contributed to the writing—review andediting. S.M.S.K. carried out the final review and visualization. M.J.M. performed the visualizationand writing—review and editing. All authors have read and agreed to the published version ofthe manuscript.

Funding: This research received no external funding.

Institutional Review Board Statement: Not applicable.

Informed Consent Statement: Not applicable.

Data Availability Statement: Not applicable.

Acknowledgments: The authors highly acknowledge the support of the Department of Civil En-gineering, University of Engineering and Technology, Lahore, Pakistan, for providing laboratoryfacilities, library access, and a research environment to accomplish the study.

Conflicts of Interest: The authors declare no conflict of interest.

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