Mechanism of asphalt wear under paint

[Pete from Boston]

Somewhere in some pretty extensive driving around the northeast in the past week I found myself noticing areas where pavement is particularly deteriorated at lane divider dashes.

How does this happen? Is it related to scoring of the pavement before paint is applied? In the places I notice this it correlates pretty well to where the dashes are. It's not a coincidental pothole here or there.

[formulanone]

Somewhere in some pretty extensive driving around the northeast in the past week I found myself noticing areas where pavement is particularly deteriorated at lane divider dashes.

How does this happen? Is it related to scoring of the pavement before paint is applied? In the places I notice this it correlates pretty well to where the dashes are. It's not a coincidental pothole here or there.

Curious to know this, too. It's definitely not just the northeast, but perhaps a bit more pronounced due to the freeze/thaw cycles.

I've had a pet theory that tire rubber helps preserve pavement somewhat under normal conditions, but since there's less of that naturally occurring at the lines, it tends to wear down towards the margins of the lane. I've also seen that some rarely-used sections of asphalt tends to fall apart quicker.

Experts: Feel free to knock down this theory as pure bunkum.

[pderocco]

Lanes are usually paved in separate runs, and while the boundaries between them don't always coincide with the stripes, they often do. Any discontinuity in density or thickness will concentrate stress at that point, even though the pavers attempt to make the joints perfect.

[Big John]

^^That also happens in painted areas where pavement  isn't at lane lines.

[triplemultiplex]

I know what you speak of and have seen that at times along the right fog line of asphalt roads where the 1 or 2 foot right shoulder was paved in the same pass as the travel lane.

I think what you are seeing is the result of an actual protective effect the paint lines are providing.  Counterintuitive as that may seem, but because the paint is putting something between vehicle tires and the pavement itself, that causes the pavement there to wear slower than the rest of the lane.  You're creating an abrupt change in elevation at the edge of the paint line.  As that gap increases, it provides a pathway for water to get in and freeze to keep widening and failing the pavement.  In that way, the paint could cause cracks to form and worsen regardless of whether or not there's a seam between lifts of pavement.

If one were to examine closely a road with a situation like this, you'd probably be able to notice an elevation different between the top of the paint line and the unpainted lane nearby.

The other thing that might be going on is because asphalt will absorb some amount of water when wet, the paint is providing a cap that prevents water from soaking in directly under the paint line.  That difference in moisture creates a potential weak point when that moisture freezes.

Might also have to consider thermal expansion and contraction.  The dark surface of the unpainted asphalt will expand slightly more under direct sunlight than the more reflective paint surface. That difference will cause small cracks to form eventually and once they do, that's another opening for water to get in and freeze and expand those cracks.

Some combination of these three mechanisms is going to give you those paint line potholes.

[hbelkins]

Around these parts, the paving companies don't take great care in matching up the seams in lanes of pavement. Therefore, water seeps into the seams, causing potholes along the seams.  It seems (no pun intended) to me that if they'd put some crack sealer along those seams/joints, the problem would lessen.

[Pete from Boston]

I know what you speak of and have seen that at times along the right fog line of asphalt roads where the 1 or 2 foot right shoulder was paved in the same pass as the travel lane.

I think what you are seeing is the result of an actual protective effect the paint lines are providing.  Counterintuitive as that may seem, but because the paint is putting something between vehicle tires and the pavement itself, that causes the pavement there to wear slower than the rest of the lane.  You're creating an abrupt change in elevation at the edge of the paint line.  As that gap increases, it provides a pathway for water to get in and freeze to keep widening and failing the pavement.  In that way, the paint could cause cracks to form and worsen regardless of whether or not there's a seam between lifts of pavement.

If one were to examine closely a road with a situation like this, you'd probably be able to notice an elevation different between the top of the paint line and the unpainted lane nearby.

The other thing that might be going on is because asphalt will absorb some amount of water when wet, the paint is providing a cap that prevents water from soaking in directly under the paint line.  That difference in moisture creates a potential weak point when that moisture freezes.

Might also have to consider thermal expansion and contraction.  The dark surface of the unpainted asphalt will expand slightly more under direct sunlight than the more reflective paint surface. That difference will cause small cracks to form eventually and once they do, that's another opening for water to get in and freeze and expand those cracks.

Some combination of these three mechanisms is going to give you those paint line potholes.

Thanks, this is a very thought out answer.

[HighwayStar]

I have seen this in areas where the lane divider included a rumble strip, which is a pretty sensible mechanism for how it would deteriorate faster. Usually that is a 2 lane road feature, but I seem to recall a few places where they were used on interstates as well.