3.3 Research tools and methods
3.3.1 Build-up sample collection
A varied range of road build-up sample collection methods have been used in past studies. For example, Chang, et al. (2009) used plastic brooms and brushes and Vaze and Chiew (2002) used brushes and a vacuum system to collect build-up samples. It is reported that brushing and sweeping can create extra materials due to abrasion of the road surface. A wet and dry vacuum system has been used by researchers such as Mahbub (2011) and Gunawardana (2011) with minimal generation of additional
material. They have noted that wet and dry vacuuming has the capability to collect fine solid particles efficiently, compared to the other methods. Furthermore, Mahbub (2011) noted that a dry and wet vacuuming system demonstrated 90% efficiency in collecting and retaining particles. For this study, the collection system used by Mahbub (2011) was used. The system was verified for its collection and retention efficiency under controlled field conditions prior to use in the field sampling. Brief details on the dry and wet vacuum system and the testing procedure adopted are discussed below.
a. Dry and wet vacuum system
The dry and wet vacuum system consisted of a vacuum cleaner (Delonghi Aqualand model) equipped with a water filtration system and a High Efficiency Particulate Air (HEPA) filter, and a pressure controllable water sprayer (12 volts Swift 60 L Compact Sprayer). The presence of the water filtration enables the retention of particles in a water column with minimal changes to the physical and chemical composition. The HEPA filter ensures the retention of fine particles in the collection compartment. The maximum suction power of the vacuum cleaner was 1500W and it was powered by a generator in the field.
The water sprayer was used to enhance the collection efficiency as fine particles that have adhered to the road surface can dislodge due to wetness of the road surface Water was applied at a predetermined pressure to avoid dislodgement of parent particles from the surface and to avoid blowing particles away while spraying. In this study, water was sprayed at 2 bar pressure for 3 min as recommended by Mahbub (2011).
b. Sample collection and retention efficiency of the dry and wet vacuum system
The collection and retention efficiency of the system was verified using the following procedure:
Four adjacent 1 m x 1 m plots were selected on an asphalt surface so that they are equivalent in conditions compared to test plots in the field.
Each plot was first cleaned by vacuuming the surface. Vacuuming was repeated after spraying water to remove the available fine particles on the plot surface and allowing them to dry for two hours.
A known weight of sandy loam (100 g) was uniformly applied on the first
plot (Plot 1). The sample was collected using the vacuum cleaner filled with 3 L of de-ionised water. The plot surface was vacuumed three times perpendicular to plot boundaries. Then, the collected sample in the vacuum cleaner was transferred into a container and the remaining particles in the vacuum cleaner and its hoses were also carefully transferred to the same container using an additional known quantity of de-ionised water.
Once the dry vacuuming was completed, de-ionised water was sprayed on
plot 1 at 2 bar pressure for 3 min. Then, wet vacuuming was done and the collected sample was transferred to a different container.
The same procedure was repeated in plot 2 and 3 as replicates.
Plot 4 was kept as a blank plot during the sample collection period to account for the natural build-up. A natural build-up sample was also collected following the dry and wet vacuuming procedure described above.
Weights of solids in each sample (plot 1, 2, 3, and 4) were measured. Weight of solids in plot 4 was subtracted from weight of solids in plot 1, 2 and 3 in order to determine the efficiency of the vacuum system. The test results are shown in Table 3-2.
Table 3-2 Sample collection efficiency of the dry and wet vacuum system
Parameter Plot 1 Plot 2 Plot 3 Plot 4
Wt. of sand (g) 100.00 100.00 100.00 0.00
Wt. of dry vacuumed solids (g) 93.43 94.66 94.87 0.52
Wt. of wet vacuumed solids (g) 4.57 4.55 4.54 0.13
Collection efficiency in each plot (%)
97.35 98.56 98.76
Efficiency of the system (%) 98.22 ± (0.76%)
As shown in Table 3-2 the total collection efficiency of the dry and wet vacuum system was 98%. Based on this, the sampling equipment was deemed appropriate for sample collection.
c. Build-up sample collection at the field
It was critical to establish a clear protocol for build-up sample collection in order to maintain consistency throughout the investigation. Based on the experience gained from the calibration exercise and reviewing the procedures adopted by past researchers (Gunawardana, 2011; Mahbub et al., 2011), the following protocol was developed.
A plot area of 3 m2 (2 m x 1.5 m) was selected in the middle of the traffic lane (where possible) of the selected road site and demarcated using a wooden frame. In some road sites, this was not possible due to difficulties in having a partial road closure for sample collection.
Vacuum cleaner and accessories were cleaned thoroughly by using de- ionised water. The vacuum cleaner was then filled with 3L of de-ionised water and dry vacuuming was undertaken three times in perpendicular directions in the demarcated area (Figure 3-2(a)).
Then, water was sprayed over the plot using the water sprayer under 2 bar pressure for 3 min (Figure 3-2(b)).
Wet vacuuming was performed in two perpendicular directions to ensure the collection fine particles dislodged by the water spray (Figure 3-2(c)).
The collected sample in the vacuum cleaner was carefully transferred to a 25 L plastic container. The vacuum cleaner and its accessories were cleaned by washing using an additional known quantity of de-ionised water (normally 3-4 L) and transferred to the same container.
The same procedure was followed for collecting samples from the selected road sites and the collected samples were properly labelled including the, date of collection, name of the road site and volume of de-ionised water used.
In addition to the road build-up samples collected, 1 L of de-ionised water was taken into a clean polyethylene bottle as a field blank for each road and labelled accordingly as a quality control/quality assurance measure. All the samples were transported to the laboratory and preserved as stipulated by Australia /New Zealand Standards (AS/NZS, 1998).
Figure 3-2 Build-up sample collection procedure (a) Dry vacuuming (b) Water spraying (c) Wet vacuuming