Durability of bituminous mix ce 463


Published on

Published in: Technology, Business
1 Like
  • Be the first to comment

No Downloads
Total views
On SlideShare
From Embeds
Number of Embeds
Embeds 0
No embeds

No notes for slide

Durability of bituminous mix ce 463

  1. 1. Durability of Bituminous Mix . <ul><li>Durability of bituminous mix refers to the ability of the mixture to retain the original properties </li></ul><ul><li>These include the resistance to load and abrasion </li></ul><ul><li>Resistance to load can be impaired when: </li></ul><ul><li>The bitumen becomes hard and brittle and thus </li></ul><ul><li>cannot withstand strains without fracturing </li></ul><ul><li>(ii) The bitumen debonds from the aggregate (truly strips) causing the surface to lose strength and subsequently crack and disintegrate </li></ul>
  2. 2. <ul><li>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 </li></ul><ul><li>The surface is more susceptible to abrasion if: </li></ul><ul><li>(i) The void content is high allowing air and water to </li></ul><ul><li>prematurely harden the bitumen </li></ul><ul><li>(ii) Incompatibility between aggregate and bitumen, </li></ul><ul><li>making it easier to strip bitumen from the aggregate </li></ul><ul><li>(iii) The asphalt film thickness is not sufficient to protect </li></ul><ul><li>the mix from the abrasive action of tires and water </li></ul>
  3. 3. <ul><li>For a given bitumen and aggregate mixture, the durability can be improved if adequate bitumen film thickness is provided </li></ul><ul><li>This can most effectively be accomplished by decreasing or minimizing the percentage of fines </li></ul><ul><li>Establishing an adequate VMA during mix design and in the field will help establish adequate film thickness without excessive bitumen bleeding or flushing </li></ul>
  4. 4. Voids in Mineral Aggregate (VMA) <ul><li>It is the volume of intergranular void space b/w the aggregate particles of a compacted paving mixture </li></ul><ul><li>It includes the air voids and the volume of the bitumen not absorbed into the aggregate </li></ul><ul><li>VMA can also be described that the portion of space in a compacted bitumen pavement or specimen which is not occupied by the aggregate </li></ul><ul><li>It is expressed as a percentage of the total volume of the mix </li></ul>
  5. 5. <ul><li>When aggregate particles are coated with bitumen, a portion of the bitumen is absorbed into the aggregate </li></ul><ul><li>The remainder of the bitumen binder forms a film on the outside of the individual aggregate particles </li></ul><ul><li>Since the aggregate particles do not consolidate to form a solid mass, air pockets also appear within the bitumen-aggregate mixture </li></ul><ul><li>Therefore, the four general components of bituminous mix are: aggregate, absorbed bitumen, bitumen not absorbed into the aggregate (effective bitumen content), and air </li></ul>
  6. 6. <ul><li>Air and effective bitumen, when combined, are defined as VMA </li></ul><ul><li>Fig.? </li></ul>
  7. 7. <ul><li>VMA is calculated according to the following relationship: </li></ul><ul><li>VMA = 100 – [(Ps x Gmb)/Gsb] </li></ul><ul><li>Where, Ps = Aggregate content, % by total mass of mixture </li></ul><ul><li>Gsb = Bulk specific gravity of total aggregate </li></ul><ul><li>Gmb = Bulk specific gravity of compacted mixture </li></ul>
  8. 8. <ul><li>The importance of designing VMA into bituminous mix has been recognized for many years </li></ul><ul><li>It was first discussed and used by McLeod in 1956 </li></ul><ul><li>The Asphalt Institute mix design procedures have used a minimum VMA criteria that is dependent upon the maximum aggregate size </li></ul><ul><li>Low VMA can be increased by modifying the gradation, bitumen content, or particular angularity </li></ul>
  9. 9. 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
  10. 10. <ul><li>One of the key elements in the durability and moisture susceptibility of an bitumen mixture is bitumen film thickness </li></ul><ul><li>Bitumen film thickness describes the dimension of the bitumen coating of the aggregate particles </li></ul><ul><li>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 </li></ul>Asphalt Film Thickness
  11. 11. Fig. ?
  12. 12. <ul><li>Inadequate film thickness of bitumen can create a lack of cohesion between aggregate particles and would create a “dry” mix </li></ul><ul><li>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 </li></ul><ul><li>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 </li></ul>
  13. 13. <ul><li>Bitumen film thickness is measured in microns </li></ul><ul><li>It can be calculated by dividing the effective volume of bitumen by the total estimated surface area of the aggregate particles </li></ul><ul><li>Film thickness has been shown to be a function of size distribution, particle shape, and the amount of bitumen in the mix </li></ul>
  14. 14. <ul><li>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 </li></ul><ul><li>A T = a s x N = (6Ws)/(Gsb x ρ w x d) </li></ul><ul><li>Where: A T = Total surface area of all aggregate particles (m2) </li></ul><ul><li>a s = Surface area of an aggregate particle (m2) </li></ul><ul><li>N = Number of aggregate particles </li></ul><ul><li>Ws = Mass of aggregate (kg) </li></ul><ul><li>Gsb = Specific gravity of the aggregate </li></ul><ul><li>ρ w = Density of water (assume 1000 kg/m3) </li></ul><ul><li>d = Diameter of aggregate particle (m) </li></ul>
  15. 15. <ul><li>According to Hveem at the California DOT </li></ul><ul><li>The film thickness can be estimated knowing surface area of the </li></ul><ul><li>aggregate. This formulae provides only an estimation of the average film thickness on an aggregate particle </li></ul><ul><li>T F = Vbe/ (A T x Ws) x 304800 </li></ul><ul><li>Where: T F = Average film thickness, microns </li></ul><ul><li>Vbe = Volume of effective bitumen (ft3) </li></ul><ul><li>AT= Surface area of the aggregate (ft2/lb of aggregate) </li></ul><ul><li>Ws = Mass of aggregate (lb) </li></ul><ul><li>(304 800 is a conversion factor used to express T F in microns) </li></ul>
  16. 16. <ul><li>Effective Bitumen? </li></ul><ul><li>The quantity of bitumen on the aggregate surface excluding the absorbed into the aggregate particle is defined as the effective bitumen </li></ul><ul><li>Effective bitumen can be calculated by subtracting the % bitumen absorption from the total bitumen content </li></ul><ul><li>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 </li></ul>
  17. 17. <ul><ul><li>According to Hveem at the California DOT </li></ul></ul><ul><li>The average film thickness can be estimated by knowing the </li></ul><ul><li>surface area of the aggregate particle </li></ul><ul><li>T F = Vbe/ (A T x Ws) x 304800 </li></ul><ul><li>Where: T F = Average film thickness, microns </li></ul><ul><li>Vbe = Volume of effective bitumen (ft3) </li></ul><ul><li>AT= Surface area of the aggregate (ft2/lb of aggregate) </li></ul><ul><li>Ws = Mass of aggregate (lb) </li></ul><ul><li>(304 800 is a conversion factor used to express T F in microns) </li></ul>
  18. 18. <ul><ul><li>MOISTURE SUSCEPTIBILITY TESTING </li></ul></ul><ul><li>The susceptibility of bituminous mixtures to moisture is another </li></ul><ul><li>measure of durability of bituminous mixture </li></ul><ul><li>Lottman test (ASTM D 4867) is commonly adopted for evaluating moisture susceptibility and stripping of bituminous mixture </li></ul>
  19. 19. <ul><ul><li>MOISTURE SUSCEPTIBILITY TESTING </li></ul></ul><ul><li>Test Procedure: </li></ul><ul><li>Six specimens are to be compacted to 7± 1 % air voids </li></ul><ul><li>The six specimens are divided into two groups of three so that the average air void content of the group are approximately equal </li></ul><ul><li>One group is tested dry and the other is tested after a period of moisture conditioning </li></ul><ul><li>The conditioning consists of vacuum saturating the specimens to between 55 to 80% saturation </li></ul><ul><li>They are then placed in a 60 0 C water bath for 24 hours followed by a 25 0 C bath for 1 hour </li></ul><ul><li>Indirect tensile strength is then determined for the dry and wet samples </li></ul>
  20. 20. <ul><li>Moisture susceptibility is reported as a tensile strength ratio (TSR) which is calculated using eqn: </li></ul><ul><li>TSR = (Stm)/Std x 100 </li></ul><ul><li>where, Stm = Average tensile strength of the moisture-conditioned </li></ul><ul><li>samples, kPa </li></ul><ul><li>Std = Average tensile strength of the dry samples, kPa </li></ul><ul><li>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 </li></ul>