On October 23rd 2014, we updated our
Privacy Policy
and
User Agreement.
By continuing to use LinkedIn’s SlideShare service, you agree to the revised terms, so please take a few minutes to review them.
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
The surface is more susceptible to abrasion if:
(i) The void content is high allowing air and water to
prematurely harden the bitumen
(ii) Incompatibility between aggregate and bitumen,
making it easier to strip bitumen from the aggregate
(iii) The asphalt film thickness is not sufficient to protect
the mix from the abrasive action of tires and water
For a given bitumen and aggregate mixture, the durability can be improved if adequate bitumen film thickness is provided
This can most effectively be accomplished by decreasing or minimizing the percentage of fines
Establishing an adequate VMA during mix design and in the field will help establish adequate film thickness without excessive bitumen bleeding or flushing
It is the volume of intergranular void space b/w the aggregate particles of a compacted paving mixture
It includes the air voids and the volume of the bitumen not absorbed into the aggregate
VMA can also be described that the portion of space in a compacted bitumen pavement or specimen which is not occupied by the aggregate
It is expressed as a percentage of the total volume of the mix
5.
When aggregate particles are coated with bitumen, a portion of the bitumen is absorbed into the aggregate
The remainder of the bitumen binder forms a film on the outside of the individual aggregate particles
Since the aggregate particles do not consolidate to form a solid mass, air pockets also appear within the bitumen-aggregate mixture
Therefore, the four general components of bituminous mix are: aggregate, absorbed bitumen, bitumen not absorbed into the aggregate (effective bitumen content), and air
6.
Air and effective bitumen, when combined, are defined as VMA
Fig.?
7.
VMA is calculated according to the following relationship:
VMA = 100 – [(Ps x Gmb)/Gsb]
Where, Ps = Aggregate content, % by total mass of mixture
Gsb = Bulk specific gravity of total aggregate
Gmb = Bulk specific gravity of compacted mixture
8.
The importance of designing VMA into bituminous mix has been recognized for many years
It was first discussed and used by McLeod in 1956
The Asphalt Institute mix design procedures have used a minimum VMA criteria that is dependent upon the maximum aggregate size
Low VMA can be increased by modifying the gradation, bitumen content, or particular angularity
One of the key elements in the durability and moisture susceptibility of an bitumen mixture is bitumen film thickness
Bitumen film thickness describes the dimension of the bitumen coating of the aggregate particles
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
Asphalt Film Thickness
11.
Fig. ?
12.
Inadequate film thickness of bitumen can create a lack of cohesion between aggregate particles and would create a “dry” mix
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
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
13.
Bitumen film thickness is measured in microns
It can be calculated by dividing the effective volume of bitumen by the total estimated surface area of the aggregate particles
Film thickness has been shown to be a function of size distribution, particle shape, and the amount of bitumen in the mix
14.
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
A T = a s x N = (6Ws)/(Gsb x ρ w x d)
Where: A T = Total surface area of all aggregate particles (m2)
a s = Surface area of an aggregate particle (m2)
N = Number of aggregate particles
Ws = Mass of aggregate (kg)
Gsb = Specific gravity of the aggregate
ρ w = Density of water (assume 1000 kg/m3)
d = Diameter of aggregate particle (m)
15.
According to Hveem at the California DOT
The film thickness can be estimated knowing surface area of the
aggregate. This formulae provides only an estimation of the average film thickness on an aggregate particle
T F = Vbe/ (A T x Ws) x 304800
Where: T F = Average film thickness, microns
Vbe = Volume of effective bitumen (ft3)
AT= Surface area of the aggregate (ft2/lb of aggregate)
Ws = Mass of aggregate (lb)
(304 800 is a conversion factor used to express T F in microns)
16.
Effective Bitumen?
The quantity of bitumen on the aggregate surface excluding the absorbed into the aggregate particle is defined as the effective bitumen
Effective bitumen can be calculated by subtracting the % bitumen absorption from the total bitumen content
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
17.
According to Hveem at the California DOT
The average film thickness can be estimated by knowing the
surface area of the aggregate particle
T F = Vbe/ (A T x Ws) x 304800
Where: T F = Average film thickness, microns
Vbe = Volume of effective bitumen (ft3)
AT= Surface area of the aggregate (ft2/lb of aggregate)
Ws = Mass of aggregate (lb)
(304 800 is a conversion factor used to express T F in microns)
18.
MOISTURE SUSCEPTIBILITY TESTING
The susceptibility of bituminous mixtures to moisture is another
measure of durability of bituminous mixture
Lottman test (ASTM D 4867) is commonly adopted for evaluating moisture susceptibility and stripping of bituminous mixture
19.
MOISTURE SUSCEPTIBILITY TESTING
Test Procedure:
Six specimens are to be compacted to 7± 1 % air voids
The six specimens are divided into two groups of three so that the average air void content of the group are approximately equal
One group is tested dry and the other is tested after a period of moisture conditioning
The conditioning consists of vacuum saturating the specimens to between 55 to 80% saturation
They are then placed in a 60 0 C water bath for 24 hours followed by a 25 0 C bath for 1 hour
Indirect tensile strength is then determined for the dry and wet samples
20.
Moisture susceptibility is reported as a tensile strength ratio (TSR) which is calculated using eqn:
TSR = (Stm)/Std x 100
where, Stm = Average tensile strength of the moisture-conditioned
samples, kPa
Std = Average tensile strength of the dry samples, kPa
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