CHAPTER 2: HOT MIX DESIGN

Transcription

1 CHAPTER 2: HOT MIX DESIGN

2 Premix - Materials

3 Aggregate Gradation Distribution of particle sizes expressed as a percentage of total weight (total % passing various sieve sizes) Determined by sieve analysis Gradation affect stiffness, stability, durability, permeability, workability, fatigue, skid, and moisture damage resistance >> limits on the agg gradation to be used in HMA HMA need to have sufficient air voids in grading mix for durability (permits enough AC to be incorporated) and avoid bleeding and rutting (yet still have enough air space in mixture) Graphically presented on semi-log graph 4 gradations : well (dense), uniform (single), gap graded, open graded (air space in mixture) 3

4 1. Well graded (dense) Good interlock Low permeability Near maximum density AC14/AC10/AC28 Types of Gradations 2. Uniformly graded (single size) Few points of contact Poor interlock (shape dependent) High permeability Surface dressing 3. Gap graded Small % of agg in the mid size range Only limited sizes Good interlock Low permeability Stone Mastic Asphalt (SMA) 4

5 0.45 Power Grading Chart Percent Passing max size maximum density line Sieve Size (mm) Raised to 0.45 Power NCAT 5

6 Aggregate Gradation 6

7 Aggregate Size Definitions Two designation for max size 1. Max size 2. Nominal max size Mix designations in spec typically use nominal max size Nominal Maximum Aggregate Size Largest sieve size that retains some of the agg, but not more than 10% Maximum Aggregate Size Smallest sieve size which 100% of the agg pass

8 Steps in Gradation Analysis Mechanical sieve analysis Place dry aggregate in standard stack of sieves Place sieve stack in mechanical shaker Determine mass of aggregate retained on each sieve 8

9 Mechanical Sieve Individual Sieve Stack of Sieves 9

10 Mechanical Sieve Stack in Mechanical Shaker 10

11 Exercise 2.9.1

12 Analisis Ayakan 100 Sieve Analysis Percent Passing, % Sieve Size, mm 12

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14 Aggregate Stockpile

15 Aggregate Blending Two or more stockpile need to be blended to get max density and desired void for HMA (or meet spec envelope) Reasons for blending: 1. Obtain desired gradation 2. Single natural or quarried material not enough 3. Economical to combine natural and processed materials Normally three or more stockpiles plus mineral filler Most common method for determining proportion trial & error Blended aggregate specific gravity 15

16 Blending Stockpiles Basic formula for combining stockpiles to achieve a target gradation is: p = Aa + Bb + Cc +. where: p = percent of material passing given sieve size for the combined agg A, B, C,.. = percent passing given sieve for each agg. a, b, c, = proportion (decimal fraction) of A, B, C, to be used in blend, a + b + c + =

17 Trial and Error Aided by experience and plots of indiv. gradation curves and spec limits Calculated grading compared with spec adjust until pass Guided by reasoning, maths, experience Use of spreadsheet now common 17

18 Blending of Aggregates Material % Used U.S. Sieve Agg. A % Passing % Batch Agg. B % Passing % Batch Blend Target 3/8 No. 4 No. 8 No. 16 No. 30 No. 50 No No

19 Blending of Aggregates Material % Used U.S. Sieve 3/8 No. 4 No. 8 No. 16 No. 30 No. 50 No. 100 No. 200 Agg. A % Passing % % Batch Agg. B % Passing % % Batch 100 * 0.5 = * 0.5 = * 0.5 = 15 7 * 0.5 = * 0.5 = * 0.5 = * 0.5 = 0 0 * 0.5 = 0 First Try (remember trial & error) Blend Target

20 Blending of Aggregates Material % Used U.S. Sieve Agg. A % Passing 50 % % Batch Agg. B % Passing 50 % % Batch Blend Target 3/8 No. 4 No. 8 No. 16 No. 30 No. 50 No Let s Try and get a little closer to 47 the middle 23.5 of the 32target values No

21 Blending of Aggregates Material % Used U.S. Sieve Agg. A % Passing 30 % % Batch Agg. B % Passing 70 % % Batch Blend Target 3/8 No. 4 No. 8 No. 16 No. 30 No. 50 No No

22 Blended Aggregate Specific Gravities Once the percentages of the stockpiles have been established, the combined aggregate specific gravities can also be calculated Combined G = 100 P 1 + P 2 +. P n G 1 G 2 G n 22

23 Exercise and 2.9.3

24 Mix Design Design objectives Develop an economical blend of aggregates and asphalt that meet design requirements Historical mix design methods 1. Marshall use impact hammer 2. Hveem use kneading compactor, Hveem Stabilometer New 1. Superpave gyratory use gyratory compactor to simulate field compaction, able to accommodate large size aggregate

25 Mix design methods

26 Requirements in Common Sufficient asphalt to ensure a durable pavement Sufficient stability under traffic loads Sufficient air voids Upper limit to prevent excessive environmental damage Lower limit to allow room for initial densification due to traffic Sufficient workability

27 Design bitumen content (JKR, 2008) Mix type Bitumen content (%) AC AC AC

28 Volumetric Properties of Asphalt Mix

29 Aggregate Specific Gravity Ratio weight of mat. to water of equal volume at 23 C, useful in making weight-vol conversion In metric units, G simply: G = weight / vol Four Gs apparent, bulk, effective, bulk impregnated 1. Apparent weight / vol solid Dry 2. Bulk weight / overall vol SSD 3. Effective weight / (overall vol asp asorb. pores) 4. Bulk impregnated eff. but immerse in asphalt G sb < G se < G sa 29

30 Apparent Specific Gravity G sa = Mass of oven dry agg Vol of agg 30

31 Bulk Specific Gravity Surface Voids G sb = Mass of oven dry agg Vol of agg, + perm. pores Vol. of water-perm. pores 31

32 Effective Specific Gravity G s, eff = Mass oven dry agg Vol of agg, + perm. pores not absorb. asphalt Surface Voids Solid Agg. Particle Vol. of water-perm. voids not filled with asphalt Absorbed asphalt

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34 Property When Asphalt cement is varied in mix design VTM VMA VFA VTM 3-5% <3 rutting, bleeding, loss of friction > 5- permebility, oxidation (aging, asphalt get stiffened) Near optimum asphalt cement VMA Low VMA- loss of durability, not enough asphalt.change in asphalt cement content could be more critical High VMA- loss of strength, increased cost VFA- flip of VTM, VTM<3 VFA goes up, VTM>5 VFA goes down

35 MARSHALL MIX DESIGN

36 Marshall Design Method Bruce Marshall, 1939, Mississipi Highway Department, refined-us army. WES began to study it in 1943 for WWII Evaluated compaction effort No. of blows, foot design, etc. Decided on 10 lb.. Hammer, 50 blows/side 4% voids after traffic Initial criteria were established and upgraded for increased tire pressures and loads

37 Aggregate and bitumen test Aggregate blend Lab Mix - Material

38 Course Aggregate

39 Fine Aggregate

40 Sample preparation Mix 160 C 4 sample at each bitumen content 75 blows/face compaction Compact 145 C Lab Mix - compaction

41 Bulk SG ASTM D 2726 Lab Mix - density

42 Calculations G mb = A / ( B - C ) Where: A = mass of dry sample B = mass of SSD sample C = mass of sample under water

43 Lab Mix Marshall Test Stability- max load (kn), loading rate 50.8mm/min Flow- diff sample height (mm)

44 SG and voids analysis Lab Mix Marshall Form

45 Exercise 2.9.4

46 OBC determination 4 graphs (JKR, 2008): Peak stability Peak bulk SG VFB= 75% WC, 70% BC VTM= 4% WC, 5% BC

47 2.370 Lab Mix OBC Determination 1400 Density (g/cu.cm) Stability (kg) a Bit. Content (%) 800 b Bit. Content (%) VTM (%) c Bit. Content (%) VFB (%) d Bit. Content (%)

48 Lab Mix OBC Determination OBC =(a + b + c + d)/4 = e Check OBC - Stability - Flow - Stiffness - VTM - VFB

49 Lab Mix OBC Stability (kg) e Bit. Content (%) VTM (%) VFB (%) e Bit. Content (%) 55.0 e Bit. Content (%)

50 Lab Mix OBC Flow (mm) e Bit. Content (%) Stiffness (kg/mm) e Bit. Content (%) Compare parameters with specification Pass? >> OBC = e Fail? >> redesign

51 Mix design test and analysis parameters (JKR, 2008) Parameter WC BC Stability, S >8000N >8000N Flow, F 2-4mm 2-4mm Stiffness, S/F >2000N/mm >2000N/mm VTM 3-5% 3-7% VFB 70-80% 65-75%

52 Exercise 2.9.5

53 WHAT IS PREMIX PRODUCTION? Premix production is a process of mixing the aggregates and asphalt in the hot mix facilities, to be used as road material regardless whether it s an ACW, ACB or DBM. Stock Pile HOT MIX FACILITES a) Drum mix b) Batch mix Asphalt (Bitumen)

54 Agregate Stockpile

55 Premix - Materials

56 HOT MIX ASPHALT FACILITIES Purpose of an HMA facility is to properly proportion, blend, and heat aggregate and asphalt to produce an HMA that meets the requirements of the job mix formula.

57 Types Of Plants BATCH DRUM MIXER

58 DIFFERENCE drum mix plants dry the aggregate and blend it with asphalt in a continuous process and in the same piece of equipment. batch plants dry and heats the aggregate and then in a separate mixer blend the aggregate and asphalt one batch at a time

59 Drum Mixer Plant typical layout Aggregate Bins Conveyor Belts Asphalt Storage Dryer Burner Storage Silo

60 Batch plant typical layout Asphalt Storage Aggregate Bins Conveyor Belt Dryer Batch Tower Burner Hot elevator Storage Silo

61 61 Drim Mix Plant

62 62 DRUM MIX

63 Drum Dryer Mixer

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65 65 DRUM MIX

66 66

67 67 BATCH PLANT

68 THE END