Bitumen and Asphalt Mixes Flashcards
Asphalt Types
- asphalt cements: generally refinery produced material, air blown asphalt cements
- cutbacks: asphalt cements “cut” with petroleum solvents
- emulsions: mixture of asphalt cement, water, and emulsifying agent
Problems with Asphalt Pavements
- fatigue cracks
- thermal cracks
- transverse cracks
- shoving
- pothole and ravelling
- bleeding
- rutting
bitumen consistency
used to describe the degree of fluidity or plasticity of bitumen
soft asphalt
low viscosity - used in cold climates to avoid thermal cracking
hard asphalt
high viscosity - used in hot climates to avoid rutting
viscosity
ratio between applied shear stress and rate of shear
Penetration Testing: advantages
- grades asphalt near average in-service temp.
- fast
- precision well established
- temperature susceptibility can be determined
Penetration Testing: disadvantages
- empirical test
- does not reflect differences in asphalts
Viscosity Testing: advantages
- fundamental property
- based on max. pavement surface temp,
- test method precision established
- temperature susceptibility is controlled
Viscosity Testing: disadvantages
- not applicable for non-Newtonian materials (polymer modified binders)
- wide range of properties for same grade
Ductility Test
measures the distance in centimeters that a standard briquette of asphalt cement will stretch before breaking
Softening Point Test
measures the temperature at which the bitumen will flow under its own weight
Solubility Test
used to measure the bitumen purity and quantify the amount of impurities
temperature susceptibility
the rate at which the consistency of bitumen changes with a change in temperature (denoted by A)
P.I
- lower P.I. = higher temperature susceptibility
- higher P.I. = less sensitive to temperature variation
- most paving bitumens have P.I. between -1 and +1
viscosity-temperature susceptibility (VTS)
- the slope of the log log (viscosity a 135oC) and log (temperature in K)
- the higher the VTS, the higher the temperature susceptibility
Types of Mix Design
- Marshall Mix Design
- Hveem Mix Design
- Superpave Mix Design
Marshall Mix Design
- uses impact hammer to prepare specimens
- determine stability with Marshall stabilometer
- uses volumetrics to select optimum asphalt content
Hveem Mix Design
- uses kneading compactor to prepare specimens
- determine stability with Hveem stabilometer
- visual observation, volumetrics and stability used to select optimum bitumen content
Superpave Mix Design
- uses gyratory compactor to prepare specimens
- uses volumetric analysis to select optimum bitumen content
Vse
effective volume of aggregate
Vsb
bulk volume of aggregate
Vb
total volume of bitumen
Vba
volume of absorbed bitumen
Vbe
volume of effective bitumen
Vmb
total or bulk volume of mix
Vmm
volume that yields maximum specific gravity - voidless volume of mix
Pb
percent of all bitumen by total weight of mix
Pbe
percent of effective bitumen
Pba
percent of absorbed bitumen
Ps
percent of aggregate in total mix
Ps = 1 - Pb
Gsb
bulk specific gravity of aggregate
Gse
effective specific gravity of aggregate
Gmb
bulk specific gravity of mix
Gmm
maximum specific gravity of mix
VFB
voids filled with bitumen
VA
volume of air voids in mix
VMA
volume of voids in mineral aggregate
Key components of mix design
- laboratory compaction of trial mix specimen
- stability (strength) and volumetric testing
- analysis of results
- evaluation of moisture susceptibility
Purpose of compaction
To simulate the inplace density of hot mix asphalt after it had endured several years of traffic
Compaction Methods
- impact compaction (Marshall Mix Design)
- kneading compaction (Hveem Mix Design)
- gyratory compaction (several forms available)
- vibratory impact hammer
Marshall Specimen
102mm diameter and 64mm height specimens
Marshal mix aggregate size
max. 25mm
marshall mix - bulk specific gravity determination
calculated by finding: - dry weight of specimen in air, A - submerged weight of specimen in water, C - Saturated Surface Dry (SSD) weight, B Gmb = A/(B-C)
marshall mix - Stability and Flow Test
- Immerse specimen in water bath (60oC) for 30 to 40 mins
2, Make sure breaking head clean and well lubricated - Remove specimen from water bath, towel dry and centre on breaking head
- apply constant rate of deformation 51mm/min and record max load to be Marshall stability and max deformation to be flow.
(time from removing from water bath to end of testing
Bitumen Testing
- Consistency Testing
- Durability
- Safety
- Purity
Consistency Testing
- Penetration Testing
- Viscosity
- Ductility
Durability
- Short Term Aging (RTFOT)
- Long Term Aging (PAV)
Safety
Flash Point Cleveland Open Cup
Purity
Solubility Test
Short Term Binder Aging
Rolling Thin Film Oven (RTFO)
- simulates aging from hot mixing and construction
- bitumen kept in high temp and viscosity and penetration measured before and after testing
Long Term Aging
Pressure Aging Vessel (PAV)
- simulates aging of a bitumen binder after 7 to 10 years
- specimen kept at high temperature and high pressure
Asphalt Grading
HISTORY 1. Penetration Grading 2. Viscosity Grading 3. Aged Residue Grading CURRENT Performance Grading (PG)
Emulsion
- mix of asphalt cement, water and emulsifier
- emulsifier gives surface charge to asphalt droplets suspended in water medium
- safe
- cheap
- -85oC (safe)
anionic aggregate
limestones
cationic aggregate
silica gravels
Cutback
- asphalt cement “cut” with petroleum solvents
- bad for environment
- flammable
- expensive
- high temperatures of application (150oC)
- dangerous
Asphalt Cements
- generally refinery produced material
- air blown asphalt cements
Newtonian Fluid
constant viscosity (water)
non-Newtonian Fluid
changing viscosity
- polymer modified binders
- cornflour
Compaction Viscosity
280+-30 MPa.s
Mixing Viscosity
170+-20 MPa.s
