Durability of Bituminous MixDurability of Bituminous Mix

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Durability of bituminous mix refers to the ability of the mixture to retain the original properties

These include the resistance to load and abrasion

Resistance to load can be impaired when:

(i)The bitumen becomes hard and brittle and thus cannot withstand strains without fracturing

(ii) The bitumen debonds from the aggregate (truly strips) causing the surface to lose strength and subsequently crack and disintegrate

Durability of bituminous mix is also refers to the ability of Durability of bituminous mix is also refers to the ability of of the mixture to resist abrasion of the surface due to of the mixture to resist abrasion of the surface due to scraping action of tires combined with waterscraping action of tires combined with water

The surface is more susceptible to abrasion if:The surface is more susceptible to abrasion if:

(i) The void content is high allowing air and water to (i) The void content is high allowing air and water to

prematurely harden the bitumenprematurely harden the bitumen

(ii) Incompatibility between aggregate and bitumen, (ii) Incompatibility between aggregate and bitumen,

making it easier to strip bitumen from the aggregatemaking it easier to strip bitumen from the aggregate

(iii) The asphalt film thickness is not sufficient to protect (iii) The asphalt film thickness is not sufficient to protect

the mix from the abrasive action of tires and water the mix from the abrasive action of tires and water

For a given bitumen and aggregate mixture, the For a given bitumen and aggregate mixture, the durability can be improved if adequate bitumen film durability can be improved if adequate bitumen film thickness is providedthickness is provided

This can most effectively be accomplished by This can most effectively be accomplished by decreasing or minimizing the percentage of finesdecreasing or minimizing the percentage of fines

Establishing an adequate VMA during mix design and in Establishing an adequate VMA during mix design and in the field will help establish adequate film thickness the field will help establish adequate film thickness without excessive bitumen bleeding or flushingwithout excessive bitumen bleeding or flushing

Voids in Mineral Aggregate

(VMA)

It is the volume of intergranular void space b/w the It is the volume of intergranular void space b/w the aggregate particles of a compacted paving mixtureaggregate particles of a compacted paving mixture

It includes the air voids and the volume of the bitumen It includes the air voids and the volume of the bitumen not absorbed into the aggregatenot absorbed into the aggregate

VMA can also be described that the portion of space in VMA can also be described that the portion of space in a compacted bitumen pavement or specimen which is a compacted bitumen pavement or specimen which is not occupied by the aggregatenot occupied by the aggregate

It is expressed as a percentage of the total volume of It is expressed as a percentage of the total volume of the mixthe mix

When aggregate particles are coated with bitumen, a When aggregate particles are coated with bitumen, a portion of the bitumen is absorbed into the aggregateportion of the bitumen is absorbed into the aggregate

The remainder of the bitumen binder forms a film on the The remainder of the bitumen binder forms a film on the outside of the individual aggregate particlesoutside of the individual aggregate particles

Since the aggregate particles do not consolidate to form Since the aggregate particles do not consolidate to form a solid mass, air pockets also appear within the a solid mass, air pockets also appear within the bitumen-aggregate mixturebitumen-aggregate mixture

Therefore, the four general components of bituminous Therefore, the four general components of bituminous mix are: aggregate, absorbed bitumen, bitumen not mix are: aggregate, absorbed bitumen, bitumen not absorbed into the aggregate (effective bitumen content), absorbed into the aggregate (effective bitumen content), and airand air

Air and effective bitumen, when combined, are defined Air and effective bitumen, when combined, are defined as VMAas VMA

Fig.?Fig.?

VMA is calculated according to the following VMA is calculated according to the following relationship:relationship:

VMA = 100 – [(Ps x Gmb)/Gsb]VMA = 100 – [(Ps x Gmb)/Gsb]

Where, Ps = Aggregate content, % by total mass of mixtureWhere, Ps = Aggregate content, % by total mass of mixture

Gsb = Bulk specific gravity of total aggregateGsb = Bulk specific gravity of total aggregate

Gmb = Bulk specific gravity of compacted mixtureGmb = Bulk specific gravity of compacted mixture

The importance of designing VMA into bituminous mix The importance of designing VMA into bituminous mix has been recognized for many yearshas been recognized for many years

It was first discussed and used by McLeod in 1956It was first discussed and used by McLeod in 1956

The Asphalt Institute mix design procedures have used The Asphalt Institute mix design procedures have used a minimum VMA criteria that is dependent upon the a minimum VMA criteria that is dependent upon the maximum aggregate sizemaximum aggregate size

Low VMA can be increased by modifying the gradation, Low VMA can be increased by modifying the gradation, bitumen content, or particular angularitybitumen content, or particular angularity

Nominal Max Particle Size (mm)

Minimum VMA, percent

Design Air Voids, %

3.0 4.0 5.0

1.18 21.5 22.5 23.5

2.36 19.0 20.0 21.0

4.75 16.0 17.0 18.0

9.5 14.0 15.0 16.0

12.5 13.0 14.0 15.0

19.0 12.0 13.0 14.0

25.0 11.0 12.0 13.0

37.5 10.0 11.0 12.0

50 9.5 10.5 11.5

63 9.0 10.0 11.0

Minimum VMA recommended by Asphalt Institute

One of the key elements in the durability and moisture One of the key elements in the durability and moisture susceptibility of an bitumen mixture is bitumen film susceptibility of an bitumen mixture is bitumen film thicknessthickness

Bitumen film thickness describes the dimension of the Bitumen film thickness describes the dimension of the bitumen coating of the aggregate particlesbitumen coating of the aggregate particles

A thin bitumen coating on aggregate particles is one of A thin bitumen coating on aggregate particles is one of the primary causes of premature aging of the bitumen the primary causes of premature aging of the bitumen and is one definition of lack of durabilityand is one definition of lack of durability

Asphalt Film Asphalt Film ThicknessThickness

Fig. ?

Inadequate film thickness of bitumen can create a lack Inadequate film thickness of bitumen can create a lack of cohesion between aggregate particles and would of cohesion between aggregate particles and would create a “dry” mixcreate a “dry” mix

Also, if the bitumen film is too thin, air which enters the Also, if the bitumen film is too thin, air which enters the compacted bituminous mix can more rapidly oxidize the compacted bituminous mix can more rapidly oxidize the bitumen, causing the pavement to become brittlebitumen, causing the pavement to become brittle

Additionally, if the aggregates are hydrophilic, thin Additionally, if the aggregates are hydrophilic, thin bitumen films are more easily and rapidly penetrate by bitumen films are more easily and rapidly penetrate by water than thick ones, causing stripping or deboning of water than thick ones, causing stripping or deboning of bitumen from the aggregatebitumen from the aggregate

Bitumen film thickness is measured in micronsBitumen film thickness is measured in microns

It can be calculated by dividing the effective volume of bitumen by It can be calculated by dividing the effective volume of bitumen by the total estimated surface area of the aggregate particlesthe total estimated surface area of the aggregate particles

Film thickness has been shown to be a function of size distribution, Film thickness has been shown to be a function of size distribution, particle shape, and the amount of bitumen in the mixparticle shape, and the amount of bitumen in the mix

According to Aljassar and Haas, the bitumen film thickness can be According to Aljassar and Haas, the bitumen film thickness can be determined using the formula stated below by knowing the determined using the formula stated below by knowing the approximate surface area of an aggregate gradation approximate surface area of an aggregate gradation

AATT= a= ass x N = (6Ws)/(Gsb x x N = (6Ws)/(Gsb x ρρw x d)w x d)

Where: AWhere: ATT= Total surface area of all aggregate particles (m2)= Total surface area of all aggregate particles (m2)

aass = Surface area of an aggregate particle (m2) = Surface area of an aggregate particle (m2)

N = Number of aggregate particlesN = Number of aggregate particles

Ws = Mass of aggregate (kg)Ws = Mass of aggregate (kg)

Gsb = Specific gravity of the aggregateGsb = Specific gravity of the aggregate

ρρw = Density of water (assume 1000 kg/m3)w = Density of water (assume 1000 kg/m3)

d = Diameter of aggregate particle (m)d = Diameter of aggregate particle (m)

According to Hveem at the California DOTAccording to Hveem at the California DOT

The film thickness can be estimated knowing surface area of the The film thickness can be estimated knowing surface area of the

aggregate. This formulae provides only an estimation of the aggregate. This formulae provides only an estimation of the average film thickness on an aggregate particleaverage film thickness on an aggregate particle

TTFF = Vbe/ (A = Vbe/ (ATT x Ws) x 304800 x Ws) x 304800

Where: TWhere: TFF= Average film thickness, microns= Average film thickness, microns

Vbe = Volume of effective bitumen (ft3)Vbe = Volume of effective bitumen (ft3)

AT= Surface area of the aggregate (ft2/lb of aggregate)AT= Surface area of the aggregate (ft2/lb of aggregate)

Ws = Mass of aggregate (lb)Ws = Mass of aggregate (lb)

(304 800 is a conversion factor used to express T(304 800 is a conversion factor used to express TFF in microns) in microns)

Effective Bitumen?Effective Bitumen?

The quantity of bitumen on the aggregate surface The quantity of bitumen on the aggregate surface excluding the absorbed into the aggregate particle is excluding the absorbed into the aggregate particle is defined as the effective bitumendefined as the effective bitumen

Effective bitumen can be calculated by subtracting the Effective bitumen can be calculated by subtracting the % bitumen absorption from the total bitumen content% bitumen absorption from the total bitumen content

In order to ensure durable flexible pavements, it is In order to ensure durable flexible pavements, it is necessary to design mixes with adequate film thickness necessary to design mixes with adequate film thickness for which determination of effective bitumen is for which determination of effective bitumen is necessarynecessary

According to Hveem at the California DOTAccording to Hveem at the California DOT

The average film thickness can be estimated by knowing the The average film thickness can be estimated by knowing the

surface area of the aggregate particlesurface area of the aggregate particle

TTFF = Vbe/ (A = Vbe/ (ATT x Ws) x 304800 x Ws) x 304800

Where: TWhere: TFF= Average film thickness, microns= Average film thickness, microns

Vbe = Volume of effective bitumen (ft3)Vbe = Volume of effective bitumen (ft3)

AT= Surface area of the aggregate (ft2/lb of aggregate)AT= Surface area of the aggregate (ft2/lb of aggregate)

Ws = Mass of aggregate (lb)Ws = Mass of aggregate (lb)

(304 800 is a conversion factor used to express T(304 800 is a conversion factor used to express TFF in microns) in microns)

MOISTURE SUSCEPTIBILITY TESTINGMOISTURE SUSCEPTIBILITY TESTING

The susceptibility of bituminous mixtures to moisture is another The susceptibility of bituminous mixtures to moisture is another

measure of durability of bituminous mixturemeasure of durability of bituminous mixture

Lottman test (ASTM D 4867) is commonly adopted for evaluating Lottman test (ASTM D 4867) is commonly adopted for evaluating moisture susceptibility and stripping of bituminous mixturemoisture susceptibility and stripping of bituminous mixture

MOISTURE SUSCEPTIBILITY TESTINGMOISTURE SUSCEPTIBILITY TESTING

Test Procedure:Test Procedure:

Six specimens are to be compacted to 7± 1 % air voidsSix specimens are to be compacted to 7± 1 % air voids

The six specimens are divided into two groups of three so that the The six specimens are divided into two groups of three so that the average air void content of the group are approximately equalaverage air void content of the group are approximately equal

One group is tested dry and the other is tested after a period of One group is tested dry and the other is tested after a period of moisture conditioningmoisture conditioning

The conditioning consists of vacuum saturating the specimens to The conditioning consists of vacuum saturating the specimens to between 55 to 80% saturationbetween 55 to 80% saturation

They are then placed in a 60They are then placed in a 6000C water bath for 24 hours followed by C water bath for 24 hours followed by a 25a 2500C bath for 1 hour C bath for 1 hour

Indirect tensile strength is then determined for the dry and wet Indirect tensile strength is then determined for the dry and wet samplessamples

Moisture susceptibility is reported as a tensile strength ratio (TSR) Moisture susceptibility is reported as a tensile strength ratio (TSR) which is calculated using eqn:which is calculated using eqn:

TSR = (Stm)/Std x 100TSR = (Stm)/Std x 100

where, Stm = Average tensile strength of the moisture-conditioned where, Stm = Average tensile strength of the moisture-conditioned

samples, kPasamples, kPa

Std = Average tensile strength of the dry samples, kPaStd = Average tensile strength of the dry samples, kPa

• The bituminous mixture properties can be improved by increasing The bituminous mixture properties can be improved by increasing film thickness, reducing air voids, and minimizing the use of the use film thickness, reducing air voids, and minimizing the use of the use of marginal aggregate sourcesof marginal aggregate sources