The background is that I have chosen to repair a badly-deteriorated stretch of asphalt pavement myself, rather than have it properly replaced by a paving professional.
Next step is killing the roots of the plants that were growing in my driveway.
Recap
Above is yesterday’s test using QPR cold asphalt patch.
Unlike its owner, the patch is flat and firm.
Also unlike its owner, a few days of weathering should cure its problem with tackiness.
What’s next?
Next, I’m going to salt my pavement.
This is a completely logical thing to do. As I now explain.
First, to get a good look at the driveway, I shoveled off the surface vegetation, hosed it down, then weed-whacked the residual, and swept. Without that, I would not have realized what poor shape the pavement is in. (N.B., a cement shovel or square-point shovel, filed to a sharp edge, turned out to be the right tool for the task of removing surface vegetation growing through the alligatored driveway pavement.)
My driveway had weeds growing in it because all my prior attempts at killing those weeds failed. My wife objects to the use of Round-Up, and really, to chemical weed killers in general. And I agree, mostly. But in this case, several less-globally-toxic treatments — solarization, vinegar, and one of the more benign weed killers — failed. They knocked the vegetation back temporarily, but it came back.
Second, the roots of those plants are still in the cracks of the pavement, and still alive. They will try to regrow.
But so what? Surely I’ll take care of that when I cover them with asphalt cold patch. I mean, it’s not as if those plants can grow right through fresh pavement, right?
Eh …
Third, I now find out that buried plants, particularly grasses, can grow right through fresh asphalt. As in, grow up through inches of hot-laid asphalt. So my naive notion of patching over them to kill them was …naive.
(FWIW, one good explanation I read is that the binders in fresh asphalt will slowly flow if subjected to steady pressure at a single point. E.g., if I were to place a chest-of-drawers on fresh asphalt, the legs would slowly sink into the surface. Blades of grass will do the same thing, in the opposite direction.)
Fourth, once you’ve chopped off all the greenery, consumer-grade broad-spectrum weed killers are useless for killing plants down to the roots. That is, glyphosphate (Roundup), diquat (Spectracide), glucosinate (Bioadvanced) and similar only work if the plants are green and growing. (That’s my reading of the “mechanism of action” of each of those.) Spraying those chemicals on the root stubs does no good.
Fifth, I have a bag of halite (rock salt, sodium chloride) that’s been kicking around my garage for years. I have no use for it. Yet it was too useful a substance just to throw away. I’d be glad to be rid of it.
Sixth, gardening websites say to use salt as a weed killer with extreme caution. That’s not because it’s particular toxic to humans, but because it effectively poisons the soil long term . Enough salt makes it so that nothing will ever grow there again.
To which I say, that’s not a bug, that’s a feature.
So, I’m going to salt the formerly-overgrown sections of my driveway, before proceeding. The idea is to poison the soil and kill the remaining plant roots. Then I can proceed to patch over the (formerly) weedy sections.
I have combined the two ways that websites suggest using salt as a weed killer.
Websites devoted to pavement suggest sweeping rock salt into the cracks between pavers or bricks, rather than using sand for that purpose. That should keep those cracks weed-free for years. Or so they say.
But gardening websites want you to use salt water. The maximum solubility of salt in water at room temperature is about 360 grams of salt per liter of water (reference). Bagged rock salt (as opposed to a solid chunk of salt) is listed as weighing about 1 kilogram per liter (calculated from this reference). So the saltiest water you can make would require just over 5 cups of salt per gallon of water. I figure a few cups of rock salt, per gallon, should be sufficiently lethal to plants.
So I first broadcast rock salt over the area and swept it into all the cracks. Then I wet down the area with very salty water. I’ll soak it with salt water again later today. And then — under the assumption that the salt will kill anything living in that soil — I can pave over it, once the salt water has dried. I think.
Edit: Next day, in hindsight, just go with salt water, skip spreading the rock salt. The rock salt just makes more work, as I now have to sweep that up before I lay down asphalt patch.
A few other considerations
Let me assume the salt will permanently kill the residual vegetation in this section of my driveway. Here’s a few more things I need to work out.
My alligatored pavement may be too far gone for a thin layer of spread-on goop.
The closer I look at my driveway, the worse it looks. I had hoped to patch the low spots, and apply some sort of squeegeed topcoat to the rest (e.g., Gator-Patch). But the more I look, the more I think I’m going to have to put a thick patch over most of it. There’s too much asphalt already missing.
Patches should ideally be no more than 3′ wide.
Turns out, my limit on the size of an asphalt patch is determined by how far I can comfortably reach with the tamper. The tamper weighs quite a bit, so, for pounding that up and down for an extended period of time, my comfortable reach is the length of my forearm, or about a foot and a half. If I want to stand on old pavement while I tamp the patch, the patch can’t be more than 3′ wide at any point.
I guess I could tamp it “on the fly” — tamp down each bag as I dump it. I guess I could stand on the patch, as I tamp it. But for my test patch, I really liked getting the patch all leveled up in the loose material, from one side to the other, and then pounding it flat. I suspect that with the roller-coaster surface of my driveway, I don’t think I’d end up with something that was level if I tried tamping it on the fly.
Asphalt laid on clay is different from asphalt laid on crushed rock.
As I read through internet advice on how to deal with badly damaged asphalt pavement, I’ve been kind of amazed at how cavalierly many suggest starting by pulling up all of the old, alligatored pavement. I look at my pavement and think, you’ve got to be kidding me. Sure, the asphalt is alligatored, but each piece is firmly embedded in the clay beneath. The surface as a whole still has considerable structural integrity.
Then it hit me: I bet that most internet advice assumes you’re working on properly-constructed asphalt pavement, laid on on a bed of crushed rock. In which case, alligatored pavement would essentially consist of loose chunks of broken-up asphalt with no structural integrity. (And that are easier to remove, to boot.)
So I’m going to trust my instinct on this. The alligatored pavement seems more than sound enough to walk on. There’s nothing loose or moving there. I’m going to limit myself to removing loose stones and earth, and otherwise leaving it alone. I think that the command that all alligatored asphalt should be removed is a good working rule for asphalt what was laid on crushed rock. I question the usefulness of doing that for alligatored asphalt that’s embedded in clay soil.
Pavement base as hole filler, the jury remains out.
If I had all the time in the world, I’d let my test patch sit for a couple of years to see how it holds up. But I don’t. So I need to forge ahead without testing how durable this repair is.
The main structural problem for my asphalt driveway is broad, shallow depressions in the pavement. Not potholes through the pavement. Not fully-formed tire ruts. But places where the asphalt surface has sagged a few inches, where tires of a vehicle might run, or a car might be habitually have been parked.
If I don’t level those out, those will form puddles when it rains. I think the term-of-art is ponding: I have ponding on my driveway. And my understanding is that this is bad, full stop. For example, I don’t think seal coatings will adhere to areas subject to frequent and prolonged ponding. Fixing the alligatored surface, but not fixing the ponding, will just make the ponding more prolonged after each rain.
The upshot is that where the pavement has sunk several inches, I need a broad patch that is several inches deep in the middle.
I think that applying several inches of QPR cold patch is a risk for failure to cure in a timely fashion. As I understand it, the stuff cures through exposure to the air. A thick layer, therefore, should take longer to cure than a thin layer. (But, to be clear, that’s just a guess on my part. I haven’t actually tested that.)
That was the genesis of the idea of applying cold patch over tamped paver base for my test patch. For my test hole, I first filled it to within an inch of the final level with tamped paver base. Then I did my best to cap that with a uniform inch-thick layer of PQR cold patch.
Applying cold patch over compacted paver base, in this application (surface patching of badly damaged asphalt), has three benefits.
First, by capping the deep (paver-base-filled) hole with just an inch of asphalt cold patch, I’m hoping to avoid the dreaded “failure to cure”. But, honestly, unless I test it, I don’t know if the depth of the patch (within reason) matters at all in this regard, as long as you tamp it firmly as you build it up in “lifts”.
Second, fiddling around with paver base is a lot easier, to me, than fiddling around with (messy) asphalt cold patch that sets up over time. Filling the bulk of the hole with paver base lets me do all the “sculpting” of the shape and depth of the patch in sand/gravel mix, first, before I break out the cold patch.
This is fairly important here, because I’m trying to re-construct a level surface over which water can flow. I’m starting from a badly distorted surface with multiple puddles. So the ability to take my time, and construct that new level surface, before anything is set, is a plus.
Basically, it makes the cold patch part of the process easier. After tamping the paver base, I end up with a smooth flat place, in the right shape, at the right height, on which I want to spread an inch of cold patch. All I’m trying to do, with the cold patch, is apply it in a uniform flat layer.
Third, there’s a small cost saving, as it substitutes relatively inexpensive paver base for more-expensive cold patch. Paver base costs about a third of what QPR cold patch costs, per volume. For my test hole, using paver base as the filler cut my use of QPR not-quite-in-half. (Obviously, it’ll vary with, among other things, the depth of the hole being filled.) In the case of my test hole, that works out to … call it a one-quarter reduction in cost per area patched.
But there are some possible downsides to using tamped paver base filler for low spots, under asphalt cold patch.
First, nobody on the internet even mentions the possibility of doing this for surface-patched asphalt. Either it’s too fussy to be used by pros, or pros don’t do this kind of half-assed surface patching, or it’s a bad idea.
Or all three. Or yet something different.
But generally, if nobody does it this way, there’s probably a reason for it.
In my defense, I note that many asphalt crack fillers explicitly tell you to fill a deep crack with sand, and only apply filler to the top half-inch or so. My pounded paver-base plateau is the same concept, just for a larger area.
Second, having a paver-base core means there’s the potential for freeze-thaw damage. If I saturate the paver base with water, and the asphalt below the paver base keeps that from draining, a hard freeze could turn that to ice and heave the patch up. I’m hoping that the patch itself will keep out most of the water, and and that any residual water entry will drain out through the cracks in the asphalt pavement. So I’m not sure I’d try this, if the asphalt at the bottom of the hole is still sound enough to retain water.
The potential for trapped water is a strong argument for making the patch all-asphalt, no matter how deep the dip in the pavement. Asphalt can’t absorb and trap water as paver base can.
Third, the patch has less area to adhere to the road. Done this way, the patch is only stuck to the roadway around its perimeter. Is that enough? The middle of the patch is well-supported against compressive (pushing-down) loads, because compacted paver base will not compress. But it’s more-or-less stuck to sand. It’s not firmly adhered to anything solid, and so is not protected from lifting loads, such as (e.g.) high winds.
Surface patching and water flow: An inch of thickness is a mile, in driveway slope terms.
Surface patches stand above the existing pavement, at least somewhat. So, by definition, they will impede the flow of water off that pavement. Somewhat.
With QPR, I can’t manage to make the edge of the patch any thinner than about half-an-inch. That doesn’t sound like much, but a) water doesn’t flow uphill and b) this driveway section doesn’t have much slope to begin with. I’m guessing 1″ per 8′, or thereabouts.
If nothing else, these raised patch edges will direct the flow of water. So patch edges have to be oriented so as to try to get the driveway to shed water.
Surface patch edge thickness also raises the issue of connecting or overlapping adjacent patches. Based on what I see on YouTube, it should be easy enough to “graft” a fresh patch onto the edge of an existing patch just by placing the cold patch and pounding it out with a tamper. The upshot is that I should be able to do a string of separate patches while still only worrying about a minimum of a half-inch increase in pavement height for the patch.
Patching in layers still seems off the table.
The natural way for me to try to restore the asphalt surface back to its original level is to start by filling in the lowest spots, then working up and outward from there. But I just don’t think that’s a viable approach, because that would require me doing layers of patches, one on top the other, as I built up the surface back to level.
I don’t know if that will work. I don’t know that it won’t. The fact is, nobody talks about doing that — stacking patches vertically. So either it’s a dumb idea, or the typical users of these products does not face the issue I’m facing, with sagged pavement areas.
At any rate, near as I can tell, the approved method is to bring each patch up to the finished pavement level, in one pass. (Maybe several “lifts” or compacted layers to bring the patch to full thickness, but doing those layers all those layers one-after-another until the full required height is reached in a single session.) Coming back later and adding another layer of patching, weeks later, just doesn’t seem to be done.
I’m not sure I can do that, and respect my maximum-three-foot-width rule.
Conclusion
In hindsight, the decision to patch this pavement myself may not have been the best decision I’ve ever made. With pavement this screwed up, there’s a lot to be said for having a pro tear it out and replace it with hot asphalt.
But having started down this path, I’m going to finish it. The results don’t have to look spectacular. I just need a reasonably sound repair.
