Durability of bituminous mix ce 463
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Durability of bituminous mix ce 463

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Durability of bituminous mix ce 463 Durability of bituminous mix ce 463 Presentation Transcript

  • Durability of Bituminous Mix .
    • 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:
    • 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 of the mixture to resist abrasion of the surface due to scraping action of tires combined with water
    • The surface is more susceptible to abrasion if:
    • (i) The void content is high allowing air and water to
    • prematurely harden the bitumen
    • (ii) Incompatibility between aggregate and bitumen,
    • making it easier to strip bitumen from the aggregate
    • (iii) The asphalt film thickness is not sufficient to protect
    • the mix from the abrasive action of tires and water
    • For a given bitumen and aggregate mixture, the durability can be improved if adequate bitumen film thickness is provided
    • This can most effectively be accomplished by decreasing or minimizing the percentage of fines
    • Establishing an adequate VMA during mix design and in the field will help establish adequate film thickness without excessive bitumen bleeding or flushing
  • Voids in Mineral Aggregate (VMA)
    • It is the volume of intergranular void space b/w the aggregate particles of a compacted paving mixture
    • It includes the air voids and the volume of the bitumen not absorbed into the aggregate
    • VMA can also be described that the portion of space in a compacted bitumen pavement or specimen which is not occupied by the aggregate
    • It is expressed as a percentage of the total volume of the mix
    • When aggregate particles are coated with bitumen, a portion of the bitumen is absorbed into the aggregate
    • The remainder of the bitumen binder forms a film on the outside of the individual aggregate particles
    • Since the aggregate particles do not consolidate to form a solid mass, air pockets also appear within the bitumen-aggregate mixture
    • Therefore, the four general components of bituminous mix are: aggregate, absorbed bitumen, bitumen not absorbed into the aggregate (effective bitumen content), and air
    • Air and effective bitumen, when combined, are defined as VMA
    • Fig.?
    • VMA is calculated according to the following relationship:
    • VMA = 100 – [(Ps x Gmb)/Gsb]
    • Where, Ps = Aggregate content, % by total mass of mixture
    • Gsb = Bulk specific gravity of total aggregate
    • Gmb = Bulk specific gravity of compacted mixture
    • The importance of designing VMA into bituminous mix has been recognized for many years
    • It was first discussed and used by McLeod in 1956
    • The Asphalt Institute mix design procedures have used a minimum VMA criteria that is dependent upon the maximum aggregate size
    • Low VMA can be increased by modifying the gradation, bitumen content, or particular angularity
  • Minimum VMA recommended by Asphalt Institute 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
    • One of the key elements in the durability and moisture susceptibility of an bitumen mixture is bitumen film thickness
    • Bitumen film thickness describes the dimension of the bitumen coating of the aggregate particles
    • A thin bitumen coating on aggregate particles is one of the primary causes of premature aging of the bitumen and is one definition of lack of durability
    Asphalt Film Thickness
  • Fig. ?
    • Inadequate film thickness of bitumen can create a lack of cohesion between aggregate particles and would create a “dry” mix
    • Also, if the bitumen film is too thin, air which enters the compacted bituminous mix can more rapidly oxidize the bitumen, causing the pavement to become brittle
    • Additionally, if the aggregates are hydrophilic, thin bitumen films are more easily and rapidly penetrate by water than thick ones, causing stripping or deboning of bitumen from the aggregate
    • Bitumen film thickness is measured in microns
    • It can be calculated by dividing the effective volume of bitumen by the total estimated surface area of the aggregate particles
    • Film thickness has been shown to be a function of size distribution, particle shape, and the amount of bitumen in the mix
    • According to Aljassar and Haas, the bitumen film thickness can be determined using the formula stated below by knowing the approximate surface area of an aggregate gradation
    • A T = a s x N = (6Ws)/(Gsb x ρ w x d)
    • Where: A T = Total surface area of all aggregate particles (m2)
    • a s = Surface area of an aggregate particle (m2)
    • N = Number of aggregate particles
    • Ws = Mass of aggregate (kg)
    • Gsb = Specific gravity of the aggregate
    • ρ w = Density of water (assume 1000 kg/m3)
    • d = Diameter of aggregate particle (m)
    • According to Hveem at the California DOT
    • The film thickness can be estimated knowing surface area of the
    • aggregate. This formulae provides only an estimation of the average film thickness on an aggregate particle
    • T F = Vbe/ (A T x Ws) x 304800
    • Where: T F = Average film thickness, microns
    • Vbe = Volume of effective bitumen (ft3)
    • AT= Surface area of the aggregate (ft2/lb of aggregate)
    • Ws = Mass of aggregate (lb)
    • (304 800 is a conversion factor used to express T F in microns)
    • Effective Bitumen?
    • The quantity of bitumen on the aggregate surface excluding the absorbed into the aggregate particle is defined as the effective bitumen
    • Effective bitumen can be calculated by subtracting the % bitumen absorption from the total bitumen content
    • In order to ensure durable flexible pavements, it is necessary to design mixes with adequate film thickness for which determination of effective bitumen is necessary
      • According to Hveem at the California DOT
    • The average film thickness can be estimated by knowing the
    • surface area of the aggregate particle
    • T F = Vbe/ (A T x Ws) x 304800
    • Where: T F = Average film thickness, microns
    • Vbe = Volume of effective bitumen (ft3)
    • AT= Surface area of the aggregate (ft2/lb of aggregate)
    • Ws = Mass of aggregate (lb)
    • (304 800 is a conversion factor used to express T F in microns)
      • MOISTURE SUSCEPTIBILITY TESTING
    • The susceptibility of bituminous mixtures to moisture is another
    • measure of durability of bituminous mixture
    • Lottman test (ASTM D 4867) is commonly adopted for evaluating moisture susceptibility and stripping of bituminous mixture
      • MOISTURE SUSCEPTIBILITY TESTING
    • Test Procedure:
    • Six specimens are to be compacted to 7± 1 % air voids
    • The six specimens are divided into two groups of three so that the average air void content of the group are approximately equal
    • One group is tested dry and the other is tested after a period of moisture conditioning
    • The conditioning consists of vacuum saturating the specimens to between 55 to 80% saturation
    • They are then placed in a 60 0 C water bath for 24 hours followed by a 25 0 C bath for 1 hour
    • Indirect tensile strength is then determined for the dry and wet samples
    • Moisture susceptibility is reported as a tensile strength ratio (TSR) which is calculated using eqn:
    • TSR = (Stm)/Std x 100
    • where, Stm = Average tensile strength of the moisture-conditioned
    • samples, kPa
    • Std = Average tensile strength of the dry samples, kPa
    • The bituminous mixture properties can be improved by increasing film thickness, reducing air voids, and minimizing the use of the use of marginal aggregate sources