CHAPTER THREE
METHODOLOGY
3.1
Introduction
This chapter describes the method used to prepare specimens and laboratory testing
procedures that were used to assess the porous asphalt properties.
The right method
of sample preparation is important because it will affect all test results. Therefore, the
specimen preparation must be done consistently and in the correct manner to assure
quality data.
3.2
Preparation of Marshall Specimens
Porous asphalt samples were prepared using materials whose properties were
described in Chapter 3. Aggregates and fillers were batched in metal containers to
produce one specimen weighing approximately 1.1 kg. The samples were compacted
by using the Marshall hammer that applied an impact mode of compaction.
3.3
Batching and Preparation of Aggregates
Aggregates and fillers were batched in metal containers. Each batch was sufficient to
manufacture one sample. The batched aggregates were placed in an oven set at the
desired mixing temperature for a period of at least 4 hours.
3.4
Preparation of Bitumen
The bitumen that arrived in bulk was subjected to two cycles of heating. Sufficient
quantity of bitumen during one laboratory session was placed in the oven at the
required mixing temperature until it was fully liquefied for mixing with the
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aggregates. The SBS and 70P respectively required at least 4 and 2 hours of preheating. The cycle of heating of each bitumen was kept consistent.
The mixing and
compaction temperatures are dependent on binder viscosity. The guidelines from
Darin Tech (2000) provides information on mixing and the range of compaction
temperatures for base and rubberized mixes as shown in Table 3.1. Following the
recommendations given in Table 3.1, the chosen mixing and compaction temperatures
for mixes prepared using the three types of bitumen are outlined in Table 3.2.
Table 3.1 Mixing and Compaction Temperatures According to Darin Tech (2000)
Binder Type
Mixing Temperature
Compaction
Temperature
Base Bitumen
150oC
130oC
SBS Modified
185oC
140oC
Base Bitumen + DAMA
180oC  5oC
165oC
Table 3.2 Mixing and Compaction Temperatures Adopted in This Investigation
3.5
Binder Type
Mixing Temperature
Compaction
Temperature
70P
140oC
130oC
SBS
180oC
165oC
70P + DAMA
180oC
165oC
Preparation of Moulds
A typical 101.6 mm inner diameter steel Marshall moulds were used in conjunction
with the Marshall hammer. In addition, moulds for the Marshall hammer must be
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fitted with a collar. Both steel moulds and their corresponding base plates were placed
in the oven together with the aggregates.
3.6
Mixing
An electrically heated paddle mixer was used to blend the aggregates and bitumen.
The mixer was first calibrated and then set to the required mixing temperature.
Mixing of dry aggregates was accomplished in less than 30 seconds. Then, the correct
amount of binder and additives was poured onto the dry mixed aggregate and the wet
mixing continued for a further 1 minute.
The amount of bitumen required was
calculated as a percentage of the total mix. Temperatures of the mix immediately
prior to compaction shall be within the limits of the specified compaction temperature.
Full compaction was then carried out using the Marshall hammer once the mix
temperature dropped to the desired compaction temperature.
3.7
Compaction of Marshall Specimen
Impact specimens were compacted using an automatic Marshall compaction apparatus.
The compaction hammer had a flat circular tamping face and a 4536 g sliding weight
with a free fall of 457.2 mm. The mould holder was mounted on the compaction
pedestal so as to center the compaction mould over the center of the post. It held the
compaction mould, collar and base plate securely in position during compaction of the
specimen. The entire batch was placed in a previously prepared mold assembly and
the mixture vigorously spaded with a heated spatula or trowel. A heated rod was then
used to tamp the mix 15 times around the perimeter and 10 times over the interior.
Temperatures of the mix immediately prior to compaction shall be within the limits of
the specified compaction temperature.
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Two filter papers were placed on the surface of the mix and replaced the mold collar.
The motor was switched on caused the hammers to apply about 50 blows per face and
the compacter was run automatically. When compaction was completed, the sample
was allowed to cool to room temperature overnight and later extruded from the
compaction mould. Specimen was then carefully transferred to a smooth, flat surface
and allowed it to cool to ambient temperature before testing.
3.8
Determination of Permeability
Measurement of coefficient of permeability constituted a major part of the study. A
new permeameter similar to the one developed at Leeds University by Hamzah (1995)
was designed and fabricated. The coefficient of permeability was calculated using
Equation (3.1).
k  2.3
aL  h1 
log  
At
 h2 
Equation (3.1)
Where:
k
= The coefficient of permeability (cm/s)
a
= The tube cross sectional area (cm2)
A = Sample cross sectional area (cm2)
L = Height of sample (cm)
t
= Time (s)
h1 and h2 = Initial level (cm) and final level (cm)
In general, a hydraulic gradient was created across the specimen and then the water
flow over a period of time was measured. During the test, water in the standpipe fell
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from heights h1 to h2 over a time period t. The time taken, to the nearest 0.1 s, for
water to pass through the two marks on the standpipe was noted using a stopwatch.
The average of three readings was calculated for the determination of permeability.
Having determined the permeability, each specimen was manually extruded and the
physical dimensions measured.
3.9
Determination of Density and Porosity
3.9.1
Marshall Specimen
The density of porous asphalt specimens were measured based on the specimen
geometry. The heights of specimen were measured using a digital vernier caliper at
three different points. Specimen diameter was assumed to be equal to the standard
Marshall mould inner diameter.
The average of 3 readings was taken for
measurements of heights and diameters of specimens. Mix density was calculated
from Equation (3.2).
D
4M a
d 2 h
Equation (3.2)
Where:
D = Compacted specimen density (g/cm3)
Ma = Mass of specimen in air (g)
d
= Diameter of specimen (cm)
h
= Average height of specimen (cm)
Knowing the density, specimen porosity was calculated from Equation (3.3).
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D 

P  1001 

 SG 
Equation (3.3)
Where:
P = Porosity of specimen (%)
D = Density of specimen (g/cm3)
SG = Specific gravity of mix, computed from the specific gravities of
individual aggregate size range
3.10
The Marshall Test
In the stability test, the specimens were prepared with the specified temperature by
immersing in a water bath at a temperature of 60ºC ± 1ºC for a period of 45 minutes.
It was then placed in the Marshall stability testing machine and loaded at a constant
rate of deformation of 50.8 mm/minute until the maximum load was reached. The
stability result was recorded on the Marshall testing machine in kN. The total time
elapsed between removing the specimen from the bath and completion of the test was
not more than 30 seconds.
3.11
The Resilient Modulus Test
The resilient modulus test was conducted using the Universal Asphalt Testing
Machine, MATTA according to the ASTM test method D4123 (ASTM, 1999). Each
specimen was tested at 25oC after 4 hour-conditioning. A 1200 N peak load was
applied along the vertical diameter of the sample. With a fixed level of applied peak
force, the test sequence consists of a 5 count of conditioning pulses followed by 5
loading pulses. The pulse period and pulse width were respectively 3000 ms and 100
ms while the rise time was 50 ms. The rise time was measured at the points of 10 %
and 90 % of peak force and force pulse diagram is shown in Figure 3.1.
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Peak Load
Load
Rise Time
Time
Pulse Width
Pulse Period
Figure 3.1 The Force Pulse Level and Timing Diagram (BSI, 1993)
3.12
The Dynamic Creep Test
The dynamic creep testing was carried out using the Universal Testing Machine,
MATTA and shown in Plate 3.1. The test conditions were pre loading for 2 minutes at
0.01 MPa as a conditioning stress to obtain a proper bedding of the specimen due to its
shape that might not really flat. Then, 2 minutes of recovery time was allocated before
actual testing. Actual dynamic creep test was conducted at 40ºC for 1 hour loading
time and 0.1 MPa applied stress. The results were given as permanent deformation
after 1800 pulses or 1 hour expressed in strain.
Periodically repeated loading
consisting of 1 second loading and another 1 second unloading before application of
the next pulse. Hence, the pulse period and pulse width were respectively 2 and 1 sec.
The dynamic creep test parameters adopted above was in accordance to Nils Ulmgren
(1996). The loading cycle for dynamic creep test is shown in Plate 3.3.
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Stress (kPa)
Plate 3.1 The Dynamic Creep Test in Progress
110
10
10 min
1s
1s
Strain ()
T
irr
A
T
A = Strain after conditioning
irr = Irreversibel strain (permanent def.)
Figure 3.2 Load and deformation (Nils,1996)
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3.13
Summary
This chapter describes the laboratory testing methods and the instrumentation that
were used to prepare and test the specimens. Slab and Marshall specimens were
prepared for testing the engineering properties and performance of porous asphalt.
Marshall specimens were compacted by the Marshall hammer while slab specimens
were compacted by a Kango hammer.
Apart from permeability, two main
performance tests conducted on porous asphalt specimens included stiffness modulus
and permanent deformation.
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