Asphalt Concrete Aggregates Soil Definition Engineering refers to
- Slides: 35
Asphalt Concrete Aggregates • Soil Definition (Engineering) – “refers to all unconsolidated material in the earth’s crust, all material above the bedrock” • mineral particles (gravel, sand, silt, clay) • organic material (top soil, marshes) – Aggregates • mineral particles of a soil – specifically, granular soil group • gravel, sand, silt ENCI 579 5 1
Asphalt Concrete Aggregates • Granular Soil Group (Aggregates) – Physical weathering • • action of frost, water, wind, glaciers, plant/animals. . particles transported by wind, water, ice soils formed are called granular soil type “grains are similar to the original bedrock” – Larger grain sizes than clays – Particles tend to be more or less spheres/cubes – Bound water is small compared to overall mass ENCI 579 5 2
Asphalt Concrete Aggregates • Granular Soil Group (Aggregates) – ability to achieve greater densities • well graded granular material – increased soil strength – lower permeability – reduced future settlement • These improvements dictate the use of aggregates in pavement layers where wheel loads are greater ENCI 579 5 3
Asphalt Concrete Aggregates • Aggregates for asphalt concrete include: – coarse aggregates • aggregate particles larger than the 4. 75 mm sieve – fine aggregates • aggregate particles smaller than the 4. 75 mm sieve – mineral filler • aggregate particles smaller than the 75 um sieve ENCI 579 5 4
Asphalt Concrete Aggregates • Mineral Filler – provides the fines that are important in producing a dense-graded strong material – however the amount of mineral filler must be limited • covering them would require excess asphalt cement • strength of the concrete mix would be reduced as the mixture would depend on friction between smaller particles, which is less than between larger particles – limestone dust is the most common material ENCI 579 5 5
Asphalt Concrete Aggregates • Aggregate Properties – Well graded-dense including mineral filler – Hard-resistance to wear and traffic polishing – Sound-resistance to breakdown due to freeze/thaw cycles – Rough Surfaced-crushed rough surfaces • higher friction strength • better adhesion to asphalt cement – Avoid cubical-thin elongated particles that break easier ENCI 579 5 6
Asphalt Concrete Aggregates • Aggregate Properties (cont’d) – Free from Deleterious Substances • clay, dust, dirt, lightweight pieces • lower quality of asphalt film on the particle • breaking of some particles – Hydrophobic “water hating” • siliceous aggregates such as quartz are hydrophilic • greater affinity for water than asphalt cement due to surface charges • stripping, asphalt coating comes away from the particle in the presence of water ENCI 579 5 7
Asphalt Concrete Aggregates • Stripping Tests – Visual tests (Saskatchewan Highways) – samples saturated under vacuum / % stripped – Strength Tests • ASTM D 1075, Effect of water on Compressive Strength of Compacted Bituminous Mixtures – samples are submerged for 4 days • ASTM D 4876, Effect of Moisture on Asphalt Concrete Paving Mixtures – samples are submerged under vacuum 80% saturation • AASHTO T 283, Modified Lottman Test – 80% saturation / freeze thaw ENCI 579 5 8
Asphalt Concrete Aggregates ENCI 579 5 9
Asphalt Concrete Aggregates ENCI 579 5 10
Asphalt Concrete Aggregates ENCI 579 5 11
Asphalt Concrete Aggregates ENCI 579 5 12
Asphalt Concrete Aggregates ENCI 579 5 13
Asphalt Concrete Aggregates • Superpave Aggregate Properties – three critical distress mechanisms • rutting • fatigue cracking • low temperature cracking – central role in overcoming permanent deformation ie. Rutting – lesser role in pavement fatigue and low temperature cracking ENCI 579 5 14
Asphalt Concrete Aggregates • Consensus Aggregate Properties – pavement experts agreed that these aggregate properties were critical to well performing mixes and wide agreement in their use and specified values – criteria are based on traffic levels and position within pavement structure ENCI 579 5 15
Asphalt Concrete Aggregates • Source Aggregate Properties – pavement experts agreed that there were other critical aggregate properties that were dependant on local sources and experience and were left to local agencies to specify ENCI 579 5 16
Asphalt Concrete Aggregates • Superpave consensus aggregate properties – Coarse aggregate angularity • ensures a high degree of internal friction and rutting resistance • percent by weight of aggregates larger than 4. 75 mm with one or more fractured faces – Fine aggregate angularity • ensures a high degree of fine aggregate internal friction and rutting resistance • percent air voids in loosely compacted aggregates smaller than 2. 36 mm ENCI 579 5 17
Asphalt Concrete Aggregates ENCI 579 5 18
Asphalt Concrete Aggregates Superpave consensus aggregate properties ENCI 579 5 19
Asphalt Concrete Aggregates ENCI 579 5 20
Asphalt Concrete Aggregates • Superpave consensus aggregate properties – Flat and Elongated Particles • percentage by mass of coarse aggregates that have a maximum to minimum dimension ratio greater than five • particles are undesirable because of their tendency to break during construction and under traffic • procedure uses a proportional caliper device to measure the dimensional ratio of a representative sample of aggregate particles • percentage of flat + percentage of elongated particles ENCI 579 5 21
Asphalt Concrete Aggregates Flat and Elongated Particles ENCI 579 5 22
Asphalt Concrete Aggregates Flat and Elongated Particles ENCI 579 5 23
Asphalt Concrete Aggregates • Superpave consensus aggregate properties – Clay content • percentage of clay material contained in the aggregate fraction that is finer than the 4. 75 mm sieve • sand equivalency test is used in which a sample is mixes with a flocculating solution forcing clay particles into suspension • after a settling period the height of suspended clay and sedimented sand is measured • sand equivalent is the ratio of sand to clay readings ENCI 579 5 24
Asphalt Concrete Aggregates Clay Content ENCI 579 5 25
Asphalt Concrete Aggregates Clay Content ENCI 579 5 26
Asphalt Concrete Aggregates • Superpave source aggregate properties – Toughness • percent loss of material from an aggregate blend larger than 2. 36 mm using a Los Angeles Abrasion Test • estimates the resistance of coarse aggregate to abrasion and mechanical degradation during handling, construction and in service • maximum loss values typically range from 35 to 45 percent ENCI 579 5 27
Asphalt Concrete Aggregates • Superpave source aggregate properties – Soundness • percent loss of material from an aggregate blend during the sodium or magnesium sulfate soundness test • can be used on both coarse and fine aggregates • resembles freeze/thaw in the field • test result is total percent loss over various sieve intervals for a required number of cycles • maximum loss values typically range from 10 to 20 percent for five cycles ENCI 579 5 28
Asphalt Concrete Aggregates • Superpave source aggregate properties – Deleterious materials • defined as mass percentage of contaminants such as clay lumps, shale, wood, mica, and coal in a blended aggregate • performed on both coarse and fine aggregates • mass percentage of material lost as a result of wet sieving is reported as the percent of clay lumps and friable particles • values range as low as 0. 2 percent to 10 percent depending on the exact composition of the contaminant ENCI 579 5 29
Asphalt Concrete Aggregates • Superpave aggregate properties – Gradation Chart • the 0. 45 power gradation chart is used in which sieve sizes are raised to the power 0. 45 on the horizontal axis (normal sieve gradation graph already in use) • maximum density gradation is a straight line from the maximum aggregate size to the origin • maximum size is defined as one sieve size larger that the nominal maximum size • nominal maximum size is defined as one sieve size larger than the first sieve to retain more than 10 % ENCI 579 5 30
Asphalt Concrete Aggregates • Superpave source aggregate properties – Gradation Chart (cont’d) • specifies two new features to the gradation chart • Control Points placed on the nominal maximum sieve, an intermediate sieve (2. 36 mm), and the smallest sieve (75 um) through which the gradation must pass • Restricted Zone placed on the maximum density line between an intermediate sieve and the 0. 3 mm sieve through which the gradation cannot pass. Gradations that pass through the restricted zone are called “humped gradations” ENCI 579 5 31
Asphalt Concrete Aggregates • Superpave source aggregate properties – Gradation Chart (cont’d) • Humped gradations indicate an: • oversanded mixture and/or a mixture that contains too much fine sand in relation to total sand • restriction zone discourages the use of fine natural sand encourages the use of a clean manufactured sand ENCI 579 5 32
Asphalt Concrete Aggregates • Superpave source aggregate properties – Gradation Chart (cont’d) – Results : • mixtures that poses compaction problems during construction “tender mixes” • offers reduced resistance to rutting • gradations that follow to close to the maximum density line for fine aggregates often have inadequate VMA to allow enough asphalt for adequate durability ENCI 579 5 33
Asphalt Concrete Aggregates Gradation Chart ENCI 579 5 34
Asphalt Concrete Aggregates Gradation Chart ENCI 579 5 35
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