Aggregate Choice

In the section 3.1.1 it is stated that porous asphalt is a gap-graded mixture (O'Flaherty & Hughes, 2016). This means that that much more large/coarse aggregate is used. Many other asphalt mixtures use a mixture of coarse aggregate, fine aggregate, and even finer materials in combination with bitumen ta achieve the desired qualities. Generally, the fine aggregate and other fine materials fill the voids in between the coarse aggregate to reduce the air voids. Since porous asphalt has very low amounts of the smaller aggregate types, the high void percentages required for porous asphalt are achieved. Figure 23 shows a comparison

of the aggregate compositions of different asphalt types. It can clearly be seen that porous asphalt (PA in the figure) has by far the highest void percentage and the lowest fine aggregate and fines percentage.

SMA, the other type of asphalt of interest in this paper, is very different. SMA has a much lower void percentage (4-6%) (Antonissen, 2019). It is clear to see in Figure 23 that SMA has more fine aggregate and fines than porous asphalt. This, along with sufficient compaction (see section 3.2.2 on compaction) achieves a much lower void percentage. Another noticeable thing is that SMA uses a similar concentration of coarse aggregate. The high coarse aggregate count creates a rigid skeletal structure similar to that of porous asphalt (O'Flaherty & Hughes, 2016). However, the voids in this skeletal structure are filled with the additional fine aggregate, fines, and bitumen content (O'Flaherty & Hughes, 2016).

The aggregate skeleton distributes the traffic loads evenly.

Figure 23: Aggregate Composition of Types of Asphalt (O'Flaherty & Hughes, 2016)

8.1.3.

ASPHALT DISTRESS

Table 1 shows the visible asphalt distress types classified by the US Department of Transportation Federal Highway Administration (Miller & Bellinger, 2014). Information in this table not from that source will be cited to be from another source.

Table 1: Asphalt Distress (Miller & Bellinger, 2014)

Distress Type

Cracking

Fatigue cracking - Many-sides

- Sharp angled pieces - Usually less than 0.3 m - Alligator/chicken wire pattern - Repeated traffic loading - Inadequate pavement drainage or thickness (Roberts, Kandhal, Brown, Lee, & Kennedy, 1996)

- Record the affected area size

Block cracking Pattern of cracking which creates approximately rectangular pieces ranging in size from 0.1 to 10m2

- Low traffic volumes - Low temperature (Roberts, Kandhal, Brown, Lee, & Kennedy, 1996)

- Record affected area size

- Rate in severity

Edge cracking - Only for roads with

unpaved shoulders - Continuous cracking within 0.6 m of pavement edge - Record length - Rate severity

Longitudinal cracking - Cracks predominantly

parallel to the road centerline

- Often between adjacent lanes (Roberts, Kandhal, Brown, Lee, & Kennedy, 1996)

- Low temperature - Heavy loads (Roberts, Kandhal, Brown, Lee, & Kennedy, 1996)

- Record each crack separately

- Rate in severity

Reflection cracking at joints

Cracks in AC overlay surfaces that occur over joints in concrete pavements

Variety of cracks in the underlying surface (Roberts, Kandhal, Brown, Lee, & Kennedy, 1996)

Recorded as longitudinal or transverse cracking

Transverse cracking Cracks predominantly

perpendicular to pavement centerline

Low temperature (Roberts, Kandhal, Brown, Lee, & Kennedy, 1996)

- Record the number and length of the transverse cracks - Rate them on severity

Patching and

Potholes

Part of pavement surface (larger than 0.1 m2_{) that} has been replaced or has had additional pavement added later

- Record the number of patches and surface area

- Rate severity of the patches

- Record the distress to the patches

Potholes Bowl-shaped holes with

minimum diameter of 150mm

- Fatigue Cracking (Roberts, Kandhal, Brown, Lee, & Kennedy, 1996) - Cold spots from lack of temperature homogeneity (Thom, 2014)

- Record the number of potholes and square meters affected

- Rate severity level

Surface Deformation

Rutting Longitudinal surface

depression in wheel path

Traffic loads (Roberts, Kandhal, Brown, Lee, & Kennedy, 1996) - Transverse displacement of vehicles

Record the maximum rut depth to the nearest millimeter at 15.25 m intervals for each wheel path

Shoving Longitudinal displacement of an area of the

pavement surface

Braking or

accelerating vehicles

Record the number of occurrences and the surface area affected

Surface Defects

Bleeding Excess bituminous binder

on pavement surface

Record surface area affected

Polished aggregate Surface binder worn away

to show the coarse aggregate

Record surface area affected

Raveling Wearing away of

pavement caused by loosening of aggregate particles - Deficient asphalt content - excessively oxidized (brittle) binder (Roberts, Kandhal, Brown, Lee, & Kennedy, 1996)

Record surface area affected

Miscellaneous

Distresses

Lane-to-shoulder drop- off

Road surface is higher than outside shoulder Water bleeding and

pumping

Water seeping from cracks Record number of

occurrences with minimum length of 1 m Fat spots (Pavement Tools Consortium, n.d.)

Isolated areas in the mat where excess asphalt binder is visible on the surface

May lead to shoving and rutting and reduce skid resistance (Pavement Tools Consortium, n.d.) - Excessive moisture in the HMA - Construction (Pavement Tools Consortium, n.d.)

Severity Distresses

In this section the method used to assign a severity to the distress areas recorded with the visual inspections will be explained.

The severity of the raveling and cracking found was judged as is explained in the visual inspection guidelines from CROW (Kennisplatform CROW, 2011). Raveling judged by the percentage of the surface area that shows raveling. It is also different for porous asphalt than for other, denser type of asphalts. The severity is judged in the following way:

Table 2: Raveling Severity

Asphalt type Light Medium Heavy

Dense asphalt 5 to 20% of area shows raveling

20 to 50% of area shows raveling

>50% of area shows raveling

Porous asphalt 5 to 10% of area shows raveling

10 to 20% of area shows raveling

>20% of area shows raveling

Cracking is more complicated. Cracking severity is judged by a variety of factors including width, height difference between sides of the crack, length, and other factors. The severy was judged in the following way:

Light Medium Heavy

- Longitudinal with height difference between side <10mm - Width <5mm

- Filled crack

- Multiple cracks parallel - Branching longitudinal cracks - Longitudinal with height difference >10mm but <15mm - Width >5mm but <10mm - Longitudinal and transverse cracks connected to each other

- Longitudinal cracks with height difference >15mm

- Width >10mm