3- Highway Materials Aggregates

Transcription

1 3- Highway Materials Aggregates

2 Highway Materials/ Aggregates Aggregates are granular mineral particles that are widely used for highway bases, subbases, and backfill. Aggregate are also used in combination with cementing materials (Portland cement and asphalt) to form concretes for bases, subbases, wearing surfaces, and drainage structures.

3 Aggregate Sources Natural deposits of sand and gravel. Pulverized concrete and asphalt pavements. Crushed stone Blast furnace slag

4 Aggregate processing Excavations Transportation Crushing Sizing Stockpiling

5 Properties of Aggregates 1. Particle size and gradation. 2. Hardness or resistance to wear. 3. Durability or resistance to weathering. 4. Specific gravity & absorption. 5. Chemical stability 6. Particle shape and surface texture. 7. Freedom from deleterious particles or substances.

6 Particle Size & Gradation Gradation: Blend of particle sizes in the mix. Gradation affects: Density; Strength; Economy of pavement structure. Particles are separated by sieve analysis. Sieve analysis: Determination of particle size distribution of fine and coarse aggregates by sieving, expressed as %. Grain size analysis data are plotted on aggregate gradation chart. Using the gradation chart engineer can determine a preferred aggregate gradation that meet spec.. Coarse : Retain # 4. Fine : pass # 4 Retain # 200 & fines : pass # 200

7 Aggregate Size Definitions Nominal Maximum Aggregate Size one size larger than the first sieve to retain more than 10% Maximum Aggregate Size one size larger than nominal maximum size

8 %P by wt 0.45 Power Chart 100% 0% max size k 0.45

9 TOTAL PERCENT PASSING (arithmetic) SIEVE NO. (log scale)

10 Typical Gradations Dense or well-graded. Refers to a gradation that is near maximum density. The most common HMA mix designs in the U.S. tend to use dense graded aggregate. Gap graded. Refers to a gradation that contains only a small percentage of aggregate particles in the mid-size range. The curve is flat in the mid-size range. These mixes can be prone to segregation during placement. Open graded. Refers to a gradation that contains only a small percentage of aggregate particles in the small range. This results in more air voids because there are not enough small particles to fill in the voids between the larger particles. The curve is flat and near-zero in the small-size range. Uniformly graded. Refers to a gradation that contains most of the particles in a very narrow size range. In essence, all the particles are the same size. The curve is steep and only occupies the narrow size range specified.

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12 Types of Gradations * Uniformly graded - Few points of contact - Poor interlock (shape dependent) - High permeability * Well graded - Good interlock - Low permeability * Gap graded - Only limited sizes - Good interlock - Low permeability

13 Sieve Analysis Example Sieve No. Sieve Size Wt. Retained (g) % Retained Cumulative Retained % Com. % Passing inch mm (wt. ret./ Total) 100% Sum % Retained Com. Ret. 1" /4" /2" /8" # # # # # # # Pan Pan Total

14 % Passin Sieve Analysis Example Plot 120 Sieve Analysis Example in mm Com. % Passing Sieve Size ^ Sieve Size (0.45 Power) 1" /4" /2" /8" # # # # # # #

15 Superpave Aggregate Gradation Percent Passing 100 Design Aggregate Structure Sieve Size (mm) Raised to 0.45 Power

16 Gradation Considerations Max. size < 1/2 AC lift thickness Larger max size Increases strength Improves skid resistance Increases volume and surface area of agg which decreases required AC content Improves rut resistance Increases problem with segregation of particles Smaller max size Reduces segregation Reduces road noise Decreases tire wear

17 Blending of Aggregates Reasons for Blending Obtain desirable gradation Single natural or quarried material not enough Economical to combine natural and process materials

18 Blending Stockpiles Plot individual gradations Plot specification limits Can be used for initial assessment Can blend be made from available materials? Identification of critical sieves Est. trial proportions

19 Blending Stockpiles Percent Passing, % Gradation B Control points for nominal max. size Gradation A Sieve Size, mm

20 Blending Stockpiles Percent Passing, % Gradation B Gradation A Control points for nominal max. size Sieve Size, mm

21 Combined Specific Gravities G = 1 P 1 + P 2 +. P n 100 G G G n

22 Blending of Aggregates P = Aa + Bb + Cc +. Where: P = % of material passing a given sieve for the blended aggregates A, B, C, A, B, C, = % material passing a given sieve for each aggregate A, B, C,.. a, b, c,. = Proportions (decimal fractions) of aggregates A, B, C, to be used in Blend

23 Blending of Aggregates Material % Used U.S. Sieve Agg. #1 % Passing 50 % % Batch Agg. #2 % Passing 50 % % Batch Blend Target 3/8 No. 4 No. 8 No. 16 No. 30 No. 50 No No

24 Aggregate Properties

25 Resistance to Wear Material should be hard & resist wear due to: 1. The loading from compaction equipments. 2. The polishing effect of traffic. 3. Internal abrasive effects of repeated loading. Measure used for hardness of aggregate is Los Angelos (LA) abrasion test.

26 L A Abrasion Test Insert aggregate sample in a drum that rotates rpm for 500 revolutions with steel spheres inside as an abrasive charge. Sample removed & #12 sieve. Retained material are washed and dried. Difference between original mass and final mass expressed as percentage of original mass is reported as %wear. %wear = [(Original Final)/ Original] 100%

27 LA Abrasion Test - Approx. 10% loss for extremely hard igneous rocks - Approx. 60% loss for soft limestones and sandstones

28 Durability & Resistance to Weathering (Soundness Test) Soundness Test AASHTO T104, ASTM C88 Measures the resistance of aggregate to disintegration in a saturated solution of sodium or magnesium sulfate (Na 2 SO 4, MgSO 4 ). It simulates the weathering of aggregates that occur in nature. Simulates freeze/thaw action by successively wetting and drying aggregate in sodium sulfate or magnesium sulfate solution One immersion and drying is considered one cycle

29 Durability & Resistance to Weathering (Soundness Test) It measures resistance to breakdown due to crystal growth. Result is total percent loss over various sieve intervals for a prescribed number of cycles specify max % loss after X cycles typical 10-20% after 5 cycles

30 Soundness Test Before After

31 Specific Gravity & Absorption Required for the design of concrete & bituminous mixes. S.G. : Ratio of the mass to volume of an object to that of water at the same temperature. Mass Solid Volume Mass Water Volume

32 Specific Gravity & Absorption Due to permeable voids in aggregates, three types of S. G. are defined apparent (G sa ) bulk (oven-dry) (G sb ) effective (G se ) Density is the unit weight of a material lb/ft 3 or kg/m 3 Unit weight = g w G g w = g/cm 3 = 62.4 lb/ft 3

33 S.G. Cont. Coarse Aggregate W s W ssd W sub Fine Aggregate OD (W s ) or SSD (W ssd ) Container filled Container + agg + H 2 0 aggregate with H 2 O W pyc+w1 W pyc+agg+w2

34 Apparent Specific Gravity (G sa ) Computed based on net volume of the aggregates impermeable voids (ip) permeable voids (pp) G sa W ( V V s s ip )g w G sa W s Ws W sub W Ws W W pyc w1 s pyc agg w2

35 Bulk Specific Gravity (G sb ) Computed based on total volume of the aggregates impermeable voids (ip) permeable voids (pp) G sb Ws ( V V V s ip pp )g w G sb W ssd Ws W sub W W Ws W pyc w1 ssd pyc agg w2

36 Effective Specific Gravity (G se ) Aggregate absorb some asphalt cement (AC). G sa assumes all PP absorb AC (V ab = V pp ) G sb assumes no PP absorb AC (V ab = 0) Neither is correct - G se defined based on overall volume exclusive of those that absorb AC

37 Effective Specific Gravity (G se ), Cont. impermeable voids (ip) G se permeable voids (pp) W s ( V V V V s ip pp ap Calculated from mixture information G se 100 P b 100 P G G mm b b for P b )g by wt mix w

38 Specific Gravity Relationships Gsb < Gse< Gsa

39 Chemical Stability Aggregate surface chemistry affects bonding to asphalt cement. Aggregates that have affinity to water are not desirable in the asphalt mixes. Stripping Hydrophobic Agg.: Water-hating such as limestone and dolomites have a positive surface charge. Work well in asphalt concrete (show little or no strength reduction) Hydrauphilic Agg.: Water-loving such as gravels and silicates (acidic) have a negative surface charge (show reduce strength). Gravels may tend to create a weaker interfacial zone in concrete than lime-stone aggregates. Surface coating (dust of clay, silt, gypsum).tend to reduce bond strength. Immersion stripping test

40 Chemical Stability ITS = 2p / t d

41 Aggregate Shape & Surface Texture Results from Processing Shape: rounded, sub-rounded, flat, elongated. Angularity: sub-angular, angular surface texture: very rough, rough, smooth, polished

42 Percent Crushed Fragments in Gravels 0% Crushed 100% with 2 or More Crushed Faces

43 Flat and Elongated Particles ASTM D4791 Maximum Minimum

44 Disintegration/ Cleanliness Clay Lumbs & Friable Particle (ASTM C142, AASHTO T112). Specify max (typical %). Dries a given mass of agg., then soaks for 24, hr., and each particle is rubbed. A washed sieve is then performed over several screens, the aggregate dried, and the percent loss is reported as the % clay or friable particles.

45 Cleanliness of Aggregates/ SE sand equivalent (ASTM D2419) Used to estimate the relative proportions of fine agg. and clay-like or plastic fines and dust SE = Sand Reading Clay Reading *100 Flocculating Solution specify min Clay Reading Sand Reading Suspended Clay Sedimented Aggregate

46 Things to Remember Aggregates should be clean, tough, durable, and free from :excess flat and elongated particles, dust, clay lumbs, and any other objectionable materials.