4_Design of Bitumen Mixes

8/13/2019 4_Design of Bitumen Mixes

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KAAF UNIVERSITY COLLEGE

Civil Engineering DepartmentCollege of Engineering

__________________________________

Highway Engineering IICIV 467

Lecture 4_ Design of Bitumen Mixes

Kwasi Agyeman Boakye ( [email protected])

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Bituminous Mix Design

The bituminous mix design aims to determine the proportion of bitumen, filler, fine aggregates, andcoarse aggregates to produce a mix which is workable, strong, durable and economical. Theobjective of the mix design is to produce a bituminous mix by proportioning various components soas to have:1. Stability2. Durability3. Impermeability

4. Workability5. Flexibility6. Fatigue resistance7. Skid resistance

Different kinds of bituminous mix design may occur which include;Premix carpet Thin surfacing

Surface dressing Thick surfacingMixed seal surfacing Thick binder courseBituminous Macadam Cold Mixes and Hot MixesDense Bituminous MacadamSemi dense Bituminous Concrete (SDBC)Bituminous Concrete (BC)Stone Matrix Asphalt

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Bituminous Mix DesignRequirement of Bituminous Mixes

Sufficient Binder: To ensure a durable pavement by coating thoroughly the aggregateparticles and water proofing and binding them together under suitable compaction.

Sufficient Stability: This is for providing resistance to deformation under sustained orrepeated loads. This resistance in the mixture is obtained from aggregate interlocking andcohesion which usually develop due to binder in the mix.

Sufficient Flexibility: This is to withstand deflection and bending without cracking. To obtaindesired flexibility it is important to have proper amount and grade of bitumen.

Sufficient Voids: This is needed in the total compacted mix to provide space for slight amount

of additional compaction under traffic loading.

Sufficient Workability: This is for an efficient construction operation in laying the pavementmixture.

Sufficient Skid Resistance: This offers resistance to vehicles from skidding. A bleeding

surface (rich in bitumen) results in reduction in skid resistance. 3


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Bituminous Mix DesignVolumetrics of Bituminous Mixes

A bituminous mix has;-Bituminous binder-Aggregates of different sizes (coarse, fine and filler)It is identified by the maximum size of the aggregate used. (Maximum aggregate Size orNominal Maximum Aggregate Size)

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Bituminous Mix DesignVolumetrics of Bituminous Mixes

Vma = Volume of Voids in mineral aggregatesVmb = Bulk Volume of Compacted MixVmm = Void less Volume of Paving MixVfa = Volume of Voids Filled with AsphaltVa = Volume of Air Voids

Vb = Volume of AshpaltVba = Volume of absorbed AsphaltVsb = Volume of mineral aggregates (by bulk Sp.Gr)Vse = Volume of mineral aggregates ( by effective

Sp. Gr.)

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Bituminous Mix DesignCase 1: Non absorptive Aggregates

Given InformationVmb = Bulk volume of compacted Mix ( say 100cm 3 )Vsb = Volume of mineral aggregates ( Gsb = 86cm 3 )Vb = Volume of asphalt ( say 10cm 3 )

Vba = Volume of absorbed asphalt ( 0cm 3 )

CalculationVse = Volume of mineral aggregate (by effective sp. Gr.) = 86 0 = 86cm 3 Va = Volume of air voids = 100 86 10 = 4cm 3 (4%)Vma = Volume of voids in mineral aggregates = 100 86 = 14cm 3 Vmm = Void less volume of paving mix = 86+10 = 96cm 3 Vfa = Volume of voids filled with asphalt = 10cm 3 (10*100/14)= 71.43%

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Bituminous Mix DesignCase 2: Absorptive Aggregates

Given InformationVmb = Bulk volume of compacted Mix ( say 100cm 3 )Vsb = Volume of mineral aggregates ( Gsb = 86cm 3 )Vb = Volume of asphalt ( say 10cm 3 )

Vba = Volume of absorbed asphalt ( 2cm 3 )

CalculationVse = Volume of mineral aggregate (by effective sp. Gr.) = 86 2 = 84cm 3 Va = Volume of air voids = 100 84 10 = 6cm 3 (6%)Vma = Volume of voids in mineral aggregates = 100 86 = 14cm 3 Vmm = Void less volume of paving mix = 86+8 = 96cm 3 Vfa = Volume of voids filled with asphalt = 10-2= 8cm 3 (8*100/14)= 57.14%

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Bituminous Mix DesignVolumetrics Parameters

To calculate the volumetrics of a bituminous mix design the parameters that are needed are;-Specific Gravity of Binder (Gb)-Bulk Specific Gravity of mineral aggregates (Gsb)-Bulk Specific Gravity of Compacted Mix (Gmb)-Specific Gravity of Void-less Volume of Paving Mix (Gmm)

With the above the following parameters can be computed;-Effective Specific Gravity of mineral aggregate-Volume of voids in mineral aggregates(%)-Volume of voids filled with asphalt (%)-Volume of air voids (%)-Volume of asphalt (%)-Volume of absorbed asphalt (%)

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Bituminous Mix DesignVolumetric Analysis of Bituminous Mixes

- Bulk (Dry) Specific Gravity of Aggregates,(Gsb) = mass .

Volume of water replaced by the saturated surface dry aggregate

- Bulk (Dry) Specific Gravity of Compacted Mix,

(Gmb) = Dry mass of mix . Volume of water replaced by the saturated surface dry specimen

- Bulk (Dry) Specific Gravity of Void less Volume of Paving Mix ,(Gmm) = Dry mass of loose mix .

Volume of water replaced by the saturated surface dry loose mix

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Significance of Volumetric ParametersSignificance of Air Void Content

Studies in several countries , especially in hot tropical countries , indicates that mixes who airvoid content reduced to less than 2 to 3% are more likely to fail by rutting and bleeding.

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A i r V o i d M i x

( % )

Days After Construction


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Significance of Volumetric ParametersSignificance of Air Void Content

Low Air Void ContentIn this scenario load is transmitted by the bitumen and not by aggregates. The mix loosesstrength when bitumen is in a continuous phase. It also leads to bleeding mixes due tosecondary compaction and expansion of bitumen.

High Air Void ContentThis scenario allows free circulation of air ( causing oxidation of bitumen)- becomes stiff andbreaks easily. Also it allows free circulation of water ( causing raveling and stripping).

Most agencies design mixes to have 4% air void content after years of traffic.

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Primary Objective of Design Mix Exercise

The primary objective of design mix exercises carried out by most agencies is to selectaggregate gradation and binder content, which when compacted by standard compactioneffort should yield an air void content of 4%.

The compaction effort should correspond to that attained in the field after years of traffic. Themix also has to satisfy other volumetric and strength criteria.

The binder content should have the following characteristics;

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Primary Objective of Design Mix ExerciseEffect of aggregate size and gradation

The effect of aggregate size and gradation may affect the following;-Workability-Layer thicknesses- Stability-Stiffness

-Resistance to deformation-Fatigue strength-Durability-Permeability-Surface texture and frictional resistance

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Primary Objective of Design Mix ExerciseAggregate Size

Aggregate of different sizes are normally used in a combination. Minimum thickness of a layer is 2 to 3 timesthe maximum aggregate size.

Maximum Size Represents the smallest size of sieve through which 100% of the aggregate sampleparticles pass. Nominal Maximum Size Represents the largest sieve that retains some of the aggregateparticles but not more than 10% by weight. Similar to Maximum Size. For example

Sieve Size(mm) % of aggregate passing the sieve19 10013.2 929.5 774.75 622.36 501.18 410.60 320.30 230.15 160.075 7

Maximum aggregate size is 19mmNominal maximum aggregate size is 13.2mm 14


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Aggregate Gradation

Aggregate gradation is obtained by sieving material through successive sieves

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Aggregate GradationDensest

Densest aggregate gradation is mostly desired and could be achieved by several means;-Fuller and Thompson 0.5 Power.

P(%) = 100 ( d ) 0.5

Dwhere P = Percentage of aggregate (by weight) passing the sieve of size d

D = Maximum sieve size-FHWA 0.45 Power Gradation (for crushed aggregates).

where P(%) = 100 ( d ) 0.45

D

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Aggregate Gradation 0.45 Chart

The 0.45 chart enables densest gradation to be obtained for any given sieve size. This is doneby obtaining the percentage passing for any given sieve size by comparing it with the largestsieve size which is 13.2.

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Aggregate Gradation 0.45 Chart

Typically from the dense gradation chart we have;

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Aggregate Gradation

Dense or Well Graded Closely packed, lesser voids, more particle contacts

Gap Gradation Contains only a small percentage of aggregate particles in the mid-size

range, more voids, less workability, seggregation

Open Gradation Contains only small portion of aggregate particles in the small ranges (near vertical in the mid-range and flat near small range)

Uniform Gradation Most of the particles are in a narrow range.

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Blending of Aggregates

After selecting the aggregates and their gradation, proportioning of aggregates has to be doneand following are the common methods of proportioning of aggregates:Trial and error procedure: Vary the proportion of materials until the required aggregate

gradation is achieved.

Graphical Methods: Two graphical methods in common use for proportioning of aggregatesare, Triangular chart method and Rothfuch's method . The former is used when only threematerials are to be mixed.

Analytical Method: In this method a system of equations are developed based on thegradation of each aggregates, required gradation, and solved by numerical methods. With theadvent of computer, this method is becoming popular and is discussed here. The resultingsolution gives the proportion of each type of material required for the given aggregategradation.

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Blending of Aggregates by AnalyticalMethod

Blending of aggregates by weight is done by finding the proportions in which aggregates fromdifferent sources are to be mixed to attain a gradation that is closer to the target gradation.

The basic equation governing the blending process is given by;

P = Aa + Bb + Cc + .

P = % of the combined aggregates passing a given sieve A, B, C, = % of material passing a given sieve for the individual aggregatesa, b, c, ..= Proportion of individual aggregates used in the combination (total = 100)

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Blending of Aggregates by AnalyticalMethodExample

A hypothetical gradation given in table is found in column 1 and 2. The gradation of availablethree aggregate A, B, and C are given in column 3, 4, and 5. To construct the system ofsimultaneous equations, the midpoint of the lower and upper limits of the required gradation iscomputed in column 6. The decision needed to be taken is the proportion of aggregate A, B, Cneeded to be blended to get the gradation of column 6.

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Sieve Size(mm)

RequiredGradation Range

Filler(A)

Fine Aggr.(B)

CoarseAggr.

(C)Extract

Gradation

1 2 3 4 5 625.4 100 100 100.0 100.0 100.0

12.7 90 - 100 100 100.0 94.0 95.0

4.76 60 - 75 100 100.0 54.0 67.5

1.18 40 - 55 100 66.4 31.3 47.5

0.3 20 - 35 100 26.0 22.8 27.5

0.15 12 - 22 73.6 17.6 9.0 17.0

0.075 5 - 10 40.1 5.0 3.1 7.5


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Let x1, x2, x3 represent the proportion of A, B, and C respectively. Equation of the formax1+bx2+cx3 = p can be written for each sieve size, where a, b, c is the proportion ofaggregates A, B, and C passing for that sieve size and p is the required proportion for thatsieve size. This will lead to following system of equation:

x1 + x2 + x3 = 1x1 + x2 + 0.94x3 = 0.95x1 + x2 + 0.54x3 = 0.675x1 + 0.664x2 + 0.313x3 = 0.475x1 + 0.260x2 + 0.228x3 = 0.275

736x1 + 0.176x2 + 0.09x3 = 0.170

401x1 + 0.050x2 + 0.031x3 = 0.075

Solution to this problem is x1 = 0.05, x2 = 0.3, x3 = 0.65. Table 28:1 shows how when theseproportions of aggregates A, B, and C are combined, produces the required gradation.

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Below shows how when these proportions of aggregates A, B, and C are combined, andproduces the required gradation.

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Sieve Filler Fine Coarse Combined

size Aggr. Aggr. Gradation mix

(mm) (A) (B) (C)

1 2 3 4 5

25.4 100x0.5=5.0 100x0.3=30 100x.65=65 100

12.7 100x0.5=5.0 100x0.3=30 94x0.65=61 96

4.76 100x0.5=5.0 100x0.3=30 54x0.65=35.1 70.1

1.18 100x0.5=5.0 66.4x0.3=19.8 31.3x0.65=20.4 45.2

0.3 100x0.05=5.0 26.3x0.3=7.8 22.8x.65=14.8 27.60.15 73.6x0.05=3.7 17.6x0.3=5.3 9x0.65=5.9 14.9

0.75 40.1x0.05=2.0 5x0.3=1.5 3.1x0.65=2.0 5.5


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25

0

20

40

60

80

100

120

0.01 0.10 1.00 10.00 100.00

P e r c e n

t P a s s

i n g

Sieve Size(mm)

Grading Curve

Upper Boundary

Lower Boundary

Exact Gradation

Fines

Fine Aggreg.

Coarse Aggreg.

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