INDIAN GEOTECHNICAL SOCIETY CHENNAI CHAPTER

ABSTRACT: The physical properties of coarse aggregates are more significant in new generation bituminous mixtures. The strength and serviceability requirements of bituminous mixture such as Stability, Flow, Voids in Mineral Aggregate (VMA), Voids Filled with Bitumen (VFB), Air Voids (Va) and Tensile Strength Ratio (TSR) highly depend on the physical properties of aggregates. Flakiness is an important physical property of mineral aggregates which affects the quality of bituminous mixes. In this study, the influence of flaky aggregates on the properties of most commonly used bituminous mixes in India such as Dense Bituminous Macadam (DBM) and Semi Dense Bituminous Concrete (SDBC) mixes are analysed with different proportions of flaky aggregate and conclusions are drawn.

KEYWORDS: Flakiness, Tensile strength ratio, Mineral aggregate, Bituminous concrete, Bituminous Mecadam

Influence of Aggregate Flakiness on Dense Bituminous Macadam & Semi Dense Bituminous Concrete Mixes

D. Sakthibalan1

1M.E (Urban Engineering) student, Division of Transportation Engineering, Anna University Chennai, Chennai - 25

Introduction

The shape of aggregate particle has a significant influence on the performance of the bituminous pavement. Particle shape can be described as cubical, flat, elongated and round. The presence of flaky aggregates is considered as undesirable in bituminous mixtures because of their tendency to break down during construction and subsequent traffic operations. The voids present in a compacted mix depend on the shape of aggregates. Highly flaky aggregates have more voids and reduce the workability. Hence it was felt that the study on the effect of the flaky aggregates on bituminous mixtures is relevant and essential.

Objectives

The objectives of this study is investigate the effect of flaky aggregates on the following properties of DBM and SDBC mixes

> Stability

> Flow

> Percent air voids

> Voids in mineral aggregate percent

> Percent voids filled with bitumen

> Tensile strength ratio

Methodology

In this study, the behaviour of DBM and SDBC mixes was studied with aggregates having different flakiness indices (0%, 20%, 40%, 60%, 80%, and 100%). Since the aim of this study is to quantify the effects of flaky aggregates alone, elongated aggregates were completely removed from the aggregate samples to avoid the influence of elongated aggregates. The following properties were investigated in this study by conducting Marshall tests.

> Stability

> Flow

> Percent air voids (Va)

> Voids in Mineral Aggregate (VMA percent)

> Percent Voids Filled with Bitumen (VFB)

> Tensile Strength Ratio (TSR)

From the experimental results, analysis was made and conclusions were drawn.

Experimental Investigations

The aggregates used in this study were 20 mm, 12 mm, 6 mm and stone dust. Bitumen of penetration grade 60/70 was used. The properties of the materials used are given in Table 1 and Table 2.

Test results Sl.No Test Test method Unit

20mm 12mm 6mm Dust

1. Aggregate Impact value IS:2386 % 15.01 19.88 - -

2. Water absorption IS:2386 % 0.92 0.96 - -

3. Stripping IS:2386 % Nil Nil - -

4. Specific gravity IS:2386 - 2.71 2.76 2.68 2.59

Table 1 Physical Properties of Aggregates

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The aggregate gradation for DBM and SDBC mixes was selected from Ministry of Shipping Road Transport & Highways (MoSRT&H) specifications. The proportions of aggregate sizes were determined by trial and error method to achieve the combined gradation as per the requirements. The proportions arrived for different aggregate sizes for DBM mix are as given below

> 20 mm aggregate - 20% > 12 mm aggregate - 25% > 6 mm aggregate - 30%

> Stone dust - 25%

The proportions arrived for different aggregate

Stability

The variation of stability with increase in flakiness index is shown in Figure 1. It is observed that stability decreases with increase in proportion of flaky aggregates. The stability of mix with non - flaky aggregates is found to be 18.922 kN against the minimum requirement of 9kN which indicates that non -flaky aggregates yield much higher strength than the minimum requirement. Eventhough, the stability value gradually decreases with increase in the proportion of flaky aggregates, it is found to be satisfying the minimum requirement of 9 kN upto flakiness index of 89%.

sizes for SDBC mix are as given below

> 20 mm aggregate - 5% > 12 mm aggregate - 5% > 6 mm aggregate - 55%

> Stone dust - 35%

The mix design was carried out for DBM and SDBC mixes of 50mm thickness (Grading II in MoSRT&H specifications) with non - flaky aggregates and non elongated aggregates and optimum bitumen content was observed as 4.6 % for DBM and 5.0 % for SDBC by weight of mix.

Analysis of Data

DBM Mix with Different Flakiness Indices for Aggregates

Marshall specimens were prepared at optimum bitumen content with varying flakiness indices for aggregates (0%, 20%, 40%, 60%, 80%, and 100%) and tested in the laboratory. The above proportions of flaky aggregates were maintained after carefully removing all the elongated aggregates to avoid the influence of elongated aggregates and to assess the influence of flaky aggregates alone accurately. The properties ofDBM mix with various flakiness indices are given inTable 3.

Sl.No Test Test method Unit Results

1. Penetration IS: 1203 mm 65

2. Ductility IS: 1208 cm 75

3. Specific Gravity IS:2386 - 1.01

4. Softening Point IS 1203 °C 50

5. Flash Point IS 1209 °c 240

Table 2 Properties of 60/70 Grade Bitumen

Flakiness index % 0 20 40 60 80 100 MoSRT&H

requirement

Stability (kN) 18.922 17.017 15.299 12.990 9.959 8.097 9.000

Flow (mm) 3.800 3.703 3.510 2.997 2.760 1.757 2-4

Va (%) 3.200 3.387 3.431 3.971 4.018 5.302 3-5

VMA (%) 13.000 13.072 13.103 13.969 14.186 15.691 Min 13

VFB (%) 83.180 81.531 81.446 78.731 78.725 72.586 65-85

TSR (%) 92.03 89.64 63.41 58.60 52.65 49.43 Min 80

Table 3 Test results of DBM mix with different flakiness indices for aggregates

Fig 1. Variation of Stability with Flakiness Indices

INFLUENCE OF AGGREGATE FLAKINESS 27

Flow

The variation of flow value with increase in flakiness index is shown in Figure 2. It is observed that flow value decreases with increase in proportion of flaky aggregates. The flow of mix with non - flaky aggregates is found to be 3.8 mm against the requirement of 2 - 4 mm which indicates that non - flaky aggregate satisfy the requirement recommended by MoSRT&H.Eventhough, the flow value gradually decreases with increase in the proportion of flaky aggregates, it is found to be satisfying the specification requirement of 2 - 4 mm upto flakiness index of 92 %.

Air Voids (Va)

The variation of Va with increase in flakiness index is shown in Figure 3. It is observed that Vaincreases with increase in proportion of flaky aggregates. The Va of mix with non - flaky aggregates is found to be 3,2 % against the requirement of 3 - 5 % which indicates that non - flaky aggregate yield better. Eventhough, the Va value gradually increases with increase in the proportion of flaky aggregates, it is found to be satisfying the specification requirement of 3 - 5% upto flakiness index of 94 %.

Voids in Mineral Aggregate (VMA)

The variation of VMA with increase in flakiness index is shown in Figure 4. It is observed that VMA increases with increase in proportion of flaky aggregates. The VMA of mix with non - flaky aggregates is found to be 13.0% against the minimum

requirement of 13.0 % which indicates that non - flaky aggregate yield better mix. Eventhough, the VMA value gradually increases with increase in the proportion of flaky aggregates, it is found to be satisfying the specification requirement of 13.0 % for all proportions.

Voids Filled with Bitumen (VFB)

The variation of VFB with increase in flakiness index is shown in Figure 5. It is observed that VFB decreases with increase in proportion of flaky aggregates. The VFB of mix with non - flaky aggregates is found to be 83.18 % against the specification requirement of 75 - 85 % which indicates that non -flaky aggregates yield better mix. Eventhough, the VFB value gradually decreases with increase in the proportion of flaky aggregates, it is found to be satisfying the minimum requirement of 75 - 85 % upto flakiness index of 92 %.

Tensile Strength Ratio

The variation of&tensile strength ratio with increase in flakiness index is shown in Figure 6. It is observed that tensile strength ratio decreases with increase in proportion of flaky aggregates. The tensile strength ratio of mix with non -flaky aggregates is found to be 92.03 % against the requirement of minimum 80 % which indicates that non - flaky aggregates yield much higher tensile strength than the minimum requirement. Eventhough, the tensile strength ratio value gradually decreases with increase in the proportion of flaky aggregates, it is found to be satisfying the minimum requirement of 80 % upto flakiness index of 32 %.

Fig 2. Variation of Flow with Flakiness Indices

Fig 3. Variation of Flow with Flakiness Indices

Fig 4. Variation of VMA with Flakiness Indices

Fig 5. Variation of VFB with Flakiness Indices

Fig 6. Variation of Tensile Strength Ratio with Flakiness Indices

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SDBC Mix with Different Flakiness Indices for Aggregates

Marshall specimens were prepared at optimum bitumen content with varying flakiness indices for aggregates (0%, 20%, 40%, 60%, 80%, and 100%) and

Stability

The variation of stability with increase in flakiness index is shown in Figure 7. It is observed that stability decreases with increase in proportion of flaky aggregates. The stability of mix with non - flaky aggregates is found to be 18.60 kN against the minimum requirement of 8kN which indicates that non - flaky aggregates yield much higher strength than the minimum requirement. Eventhough, the stability value gradually decreases with increase in the proportion of flaky aggregates, it is found to be satisfying the minimum requirement of 8 kN upto flakiness index of 80%.

tested in the laboratory. The above proportions of flaky aggregates were maintained after carefully removing all the elongated aggregates to avoid the influence of elongated aggregates and to assess the influence of flaky aggregates alone accurately. The properties of SDBC mix with various flakiness indices are given in Table 4.

The variation of Va with increase in flakiness index is shown in Figure 9. It is observed that Va increases with increase in proportion of flaky aggregates. The Va of mix with non - flaky aggregates is found to be 3.2 % against the requirement of 3 - 5 % which indicates that non - flaky aggregate yield better. Eventhough, the Va value gradually increases with increase in the proportion of flaky aggregates, it is found to be satisfying the specification requirement of 3 - 5% upto flakiness index of 92%.

Flakiness Index % 0 20 40 60 80 100 MoSRT&H

requirement

Stability (kN) 18.60 15.13 13.56 10.43 8.73 7.89 8.00

Flow (mm) 2.90 2.83 2.61 2.52 2.12 1.92 2-4

Va (%) 3.30 3.67 3.89 4.21 4.65 5.37 3-5

VMA (%) 13.50 13.72 14.03 14.54 14.78 15.02 Min 13

VFB (%) 77.00 76.54 72.46 70.04 69.28 66.28 65-75

TSR (%) 92.17 86.78 74.35 68.31 56.92 50.12 Min 80

Table 4 Test results of SDBC mix with different flakiness indices for aggregates

Fig 7. Variation of Stability with Flakiness Indices

Flow

The variation of flow value with increase in flakiness index is shown in Figure 8. It is observed that flow value decreases with increase in proportion of flaky aggregates. The flow of mix with non - flaky aggregates is found to be 2.9mm against the requirement of 2 - 4 mm which indicates that non – flaky aggregate satisfy the requirement recommended by MoSRT&H. Eventhough, the flow value gradually decreases with increase in the proportion of flaky aggregates, it is found to be satisfyingthe specification requirement of 2 - 4 mm upto flakiness index of 91 %.

  Fig 8. Variation of air voids with flakiness indices Fig 9. Variation of Flow with Flakiness Indices 5.23 Air

Voids (Va)

INFLUENCE OF AGGREGATE FLAKINESS 29

Voids in Mineral Aggregate (VMA)

The variation of VMA with increase in flakiness index is shown in Figure 10. It is observed that VMA increases with increase in proportion of flaky aggregates. The VMA of mix with non - flaky aggregates is found to be 13.0% against the minimum requirement of 13.0 % which indicates that non - flaky aggregate yield better mix. Eventhough, the VMA value gradually increases with increase in the proportion of flaky aggregates, it is found to be satisfying the specification requirement of 13.0 % for all proportions.

Voids Filled with Bitumen (VFB)

The variation of VFB with increase in flakiness index is shown in Figure 11. It is observed that VFB decreases with increase in proportion of flaky aggregates. The VFB of mix with non - flaky aggregates is found to be 83.18 % against the specification requirement of 75 - 85 % which indicates that non - flaky aggregates yield better mix. Eventhough, the VFB value gradually decreases with increase in the proportion of flaky aggregates, it is found to be satisfying the minimum requirement of 65 - 75 % upto flakiness index of 94 %.

The variation of tensile strength ratio with increase in flakiness index is shown in Figure 12. It is observed that tensile strength ratio decreases with increase in proportion of flaky aggregates. The tensile strength ratio of mix with non - flaky aggregates is found to be 92.03 % against the requirement of minimum 80 % which indicates that non - flaky aggregates yield much higher tensile strength than the minimum requirement.

proportion of flaky aggregates for both DBM and SDBC mixes.

> Flakiness index upto 32% for DBM and 33% for SDBC can be permitted without compromising the specification requirements.

References

Ahlrich, Aho and Rismantojo F (2005), 'Investigation of the influence of aggregate properties on performance of heavy duty HMA pavements', NCHRP Report 539.

Bambang Ismanto siswosoebrotho and Kariantoni ginting J (2005), 'Workability and resilient modulus of asphalt concrete mixtures containing flaky aggregates', Journal of the Eastern Asia Society for Transportation Studies, Vol. 6, pp. 1302 -1312.

Erol tutumluer, Tongyan pan C (2005), ‘Investigation of aggregate shape effects on hot mix performance using an image analysis approach', Department of Civil and Environmental Engineering University of Dlinois at urbane champaign urbana, Dlinois.

Isaac K.P, Bhuduru S and (Ms) Mathew BS (2006), 'Influence of flaky and elongated aggregates on the properties of bituminous concrete mix', Journal for Engineers (i) Journal-Cv, Vol 87, pp. 54-58.

Krishnamurthy and Mohammed i Anjum (2004), 'Influence of coarse aggregate shape factors on mixes',, Indian Highways, pp. 51-56.

Eventhough, the tensile strength ratio value gradually decreases with increase in the proportion of flaky aggregates, it is found to be satisfying the minimum requirement of 80 % upto flakiness index of 33%.

Conclusions

From the experiments conducted in this study, the following conclusions are drawn

> The parameters such as stability, flow, voids filled with bitumen and tensile strength ratio decreases with increase in proportion of flaky aggregates for both DBM and SDBC mixes.

> The parameters such as air voids and voids in mineral aggregate increases with increase in

Fig 10. Variation of air Voids with Flakiness Indices

Fig 11. Variation of VFB with flakiness indices 5.2.6 Tensile Strength Ratio

  Fig 12. Variation of Tensile Strength Ratio with flakiness indices