Author: chogan@directresearch.com

With the final patches in place:

Edit 10/6/2024:  Below is the final surface, after using some tar-based crack filler and Latex-ite 10-year seal coating.  See Post #2029.  The seal coat did more-or-less nothing to hide the patch.  That said, while it ain’t pretty, it’s a lot better than it was.

Edit early 2025:  So far, so good.  Halfway through the first winter, and there’s no evidence of any freeze-thaw damage at all.  The upshot is that QPR seems to work just fine as a “surface-laid” patch, placed on top of the existing deteriorated asphalt.  In fact, a couple of smaller patches that I didn’t top-coat or otherwise seal are also doing just fine.  It’s plausible that QPR doesn’t even need sealing or top-coating to be able to stand up to a Mid-Atlantic winter.

This post summarizes what I learned using QPR cold patch (from Lowe’s) on a badly deteriorated section of asphalt driveway.   A prior post (Post #1974) explains the situation, and go back to Post #1971 for an assessment of options for patching asphalt.  Edit:  Post #2029 describes the final steps of crack-fill and seal-coating.  One heads up:  A squeegee does not work for spreading seal coating on an uneven surface like this.

Above, that may not look so hot to you, but I guarantee you it looks a lot better than it did.  Once I seal-coat this, in the fall, I think it’ll be … acceptable.  Given how torn up the driveway is.  I have no idea yet whether these surface-laid patches will survive the winter, but will update this next spring.

First, it took between 3 and 7 weeks for this to cure fully, in the heat of early summer in Virginia.  The reason I’m a little vague is that the patches seemed to be cured after one week.  At three weeks, a heat wave (near 100F temperatures) re-activated them, and the surfaces were once again sticky in spots, shedding little tarry bits.  At seven weeks, another heat wave (several days at 100F) did nothing.  By seven weeks, they were as solid and tar-free as the asphalt they were laid on, despite the heat.

Second, this stuff varies from batch-to-batch.  As you can see above, I laid mine down as a series of separate patches.  I bought and laid the bags of QPR a few at a time, because that’s all I could handle.  From one batch to the next, the QPR material differed in how “liquid-y/tarry” it was, in the final color once set, and to some degree, in the surface finish once set.  I’m going to seal-coat this in the fall, so the color variations don’t much matter.  But if you doing a big area, and are particular about how this looks, you might want to buy all you need, all at once, from a single batch or lot number. 

But arguing against buying a whole lot at once, see the note below on how hard it may be to estimate what you need, if your driveway surface is as un-level and messed-up as this one was.

Big batch-to-batch variation could also explain part of the strong differences of opinion among on-line reviewers of QPR.  In my case, if I’d stopped with my first first batch, I’d have said “QPR is a dandy product”, period.  With the later batches, that has a huge qualification, that the “walk on it anywhere, any time” cure time is unknown.  And all the hassle that can bring, during a hot spell.

Third, foot traffic across these patches makes a mess, due to the tiny little tarry stones that get tracked everywhere.  It’s tough to state just how much of a pain those are.  The get everywhere.  The surface sheds those rocks for the first few days (again, Virginia, early summer), and then starts shedding again if it gets hot, for some weeks thereafter.   So if this is going to be laid in place where people walk, either lay it in patches so that people can walk around the newest patches, or maybe lay plastic over it.

Fourth, the manufacturer says you can drive across these patches immediately.  And … yeah, technically that’s true.  If this were out in the middle of the street, and looks didn’t matter, I’d have no problem with that statement.

But I’d say that’s mistake, if you can avoid it, if you are picky about how the final product looks.  In my experience, there’s a risk of marking the pavement surface slightly for the first couple of days, no matter how carefully you drive (i.e., don’t turn the wheels when stopped).  And there’s a near-surety of picking up some of the tarry surface stones on your tires for the first few days.  Better to stay off these patches as much as possible until they’ve had a few days to cure.

That said, laying down plastic, then thin ply, and driving over that, did seem to compact the surface finish better than I could do with just a tamper.  So, drive over the plastic-and-ply protected surface to get the best flat-level surface on the patch.  But don’t drive over the unprotected patch for a few days, if you can help it.  If you have to, the patch will survive, but you’ll likely ding up the very top surface a bit.

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QPR asphalt cold patch.

1:  Why QPR.

QPR was a relatively cheap patching material that could be applied overtop the existing asphalt surface.  I cannot over-emphasize how much labor that saves, relative to digging up all the alligatored asphalt that was deeply embedded in the clay soil of my driveway.  And then applying a much thicker patch of some alternative material.  If those patches will just stay stuck down, and don’t get popped up by freeze-thaw this winter, that labor savings alone will make it worthwhile to use QPR over other locally-available materials.

Of the cheap, asphalt-based patching compounds I could buy locally, one (Sackrete, at Home Depot) was for filling deep holes only.  It should not be laid atop existing asphalt, per manufacturer’s directions.  Using that would have meant digging up all that alligatored asphalt.  All of which is firmly embedded in the underlying clay soil, because this broken-up section of driveway had originally been laid directly onto the dirt.

But QPR (Lowes), by contrast, can be laid directly over an existing asphalt surface.  At least, that’s my takeaway from the manufacturer’s minimal instructions, and comments on the Lowes website and elsewhere.  Obviously, that won’t work if the underlying asphalt itself is subject to movement.   But as long as it’s firmly stuck in place, it should fine.

A completely different product, Aquaphalt, is a competitor to QPR that can also be laid directly over an existing asphalt surface.  That’s a water-cured patching material that looks like asphalt, but isn’t.  And while Aquaphalt appears to be a superior product in almost every way — particularly with a 15-minute cure time — it’s also between three and four times as expensive as QPR, per cubic foot.  It also comes in plastic buckets, which then must be disposed of.   (I used one bucket of Aquaphalt, on one particularly ugly stretch of pavement.  I explain that below.)

2:  I used a half-ton of material for this ~105 square foot patch.

Each bag of QPR weighs 50 pounds and costs about $20.  Therefore, my 20 bags of QPR weighed half a ton, and cost a little under $400. 

On net, for the area I patched, I got about five square feet of surface covered, per bag.  But that’s clearly a function of how deep my patch is, on average.

I brought the 50-pound bags of QPR home six to eight at a time, in my hatchback, after lining the back with a plastic sheet.

And it’s a good thing I bought just a few at a time, because I waaaaay over-estimated the amount needed, when I first looked over this section of driveway.  Raising the entire sunken driveway surface back to its original level would have taken about 60 bags of the stuff.  So instead of raising it to be fully level, I just filled in the low spots (the puddles), and raised it as little as I could, beyond that.

I’d have had a mess on my hands if I’d stockpiled the full 60 bags that I thought I’d need, before I started.

3:  Applied in manageable pieces

I put this down over several sessions, over the course of a week and a half.

Each session being maybe three or four bags’ worth of material, applied to one defined section of the driveway.

From start to finish, you:

• sweep the area to be patched,
• haul in a bag of QPR patch,
• Slit the bag bottom, dump the QPR.
• Rake it out/shape it at the edges.
• Haul/slit additional bags as needed.
• Tamp it.
• Tamp it some more.
• Run over it with your car, after covering in plastic and thin plywood.

Some days I went through that two or three times.  Most days on which I worked on the driveway, I only did that once.

One full cycle, from sweeping to running it over, seemed to take me about two hours.  But that includes some time pondering the situation, wondering what I should do next.  Mostly, pondering whether I was maintaining enough slope for water to flow, with the help of a 4-foot level.

In my “puddles first” strategy, the goal was to cover the entire area and not end up with standing water anywhere, after a rain.  With that as the goal, it was helpful to have some rain halfway through the patching, so that I could see what puddles remained after I’d filled in the biggest ones.

4: It makes a mess if there’s foot traffic.

At least it did, in my climate (Virginia, typical day in the mid-70s, sunny).

The freshly-laid patch has a tarry surface.  It will be stickier or less sticky depending on temperature and age.  Fresher and hotter mean tarrier.  As long as the patch is still tarry — either because it’s fresh, or it’s a few days old and in the hot sunshine — if you walk on it, you will pick up and track around tiny little tar-covered rock chips.  Which then stick to everything.

And that’s a pain in the ass.

5:  The tarry top surface of my patches temporarily went away over the course of a week. 

(I have now rewritten the intro to reflect what actually happened over the course of seven weeks.)

After a week, in my climate, I could walk cleanly across the patch and not pick up anything.

Before that point, though, in addition to shedding rock chips, the surface of the patch tends to pick up any stray organic matter (e.g., leaves, pine needles, wood chips) that will stick to the tar.  I believe this stuff will mostly move along once the surface is no longer tarry.  At any rate, the week-old patches were mostly clear of debris.

In principal, these were “ready for car traffic” almost immediately after they’d been fully tamped.  But only in the sense that the car tire would not squish the patch, much.  But you’d still be well-advised to wait until the next day before driving over these.  I think my car treads lifted some surface stones off the patch, when I drove over the patch on the first day.

The upshot is that, as the manufacturer advertises, you can drive right over the patches on Day 1.  Don’t stop and turn your wheels.   But my take on it is that you shouldn’t drive on the fresh patches if you can avoid it.  Your tire treads are going to pull some tarry stones off the top of the patch when you do that.  Better to minimize that until the top surface of the patch has had a few days to cure.

The other interesting aspect of aging of the patch is the surface gets smoother over time.  I guess it continues to flow a bit.  But, for sure, the fresh patch (dark) has a much rougher surface texture than the week-old patch, despite being laid and tamped the same.

6:  Pros and cons of doing this piecemeal.

Doing this piecemeal, as I did, has several advantages.  First, I don’t think I could have done 20 bags of QPR in one day.  Second, I would walk on the older (cured) patches, as I put in the newer (fresh) patches.  And I could walk on them as a way to walk around that freshly-laid patching material. Third, the only way I could figure to end up with a reasonably level final product was to fill in the low spots — the puddles — first.

Arguing against this approach are the looks and the time.  I believe that the entire patches surface will cure to roughly the same dry and densely-packed finish.  But the joins between the individual “batches” of patching will probably remain visible no matter what.  But in addition, each fresh patch extends the time during which you’re at risk for tracking tarred stone chips around.  For example, I started this more than a week ago, and it’ll be a week from now before the most recent patching material will have a cured, non-tarry surface.

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Conclusion

I’m not sure I’d do this again.  And I’m not sure I wouldn’t, either.

For me, it boiled down to QPR being the easy and cheap solution.  You can drive down to your local hardware store, pick it up by the bag, and (after some significant surface prep) spread over a badly damaged asphalt surface.

This, as opposed to (say) trying to get three bids from pros, to come out, tear that up, and re-lay that section of the driveway correctly.  If I could get a pro around here interested in something that small.

The physical labor wasn’t that big a deal as long as you can lift the 50-pound bags.  I worked up a sweat tamping it, but I’m not even sore from doing that.  (OTOH, I lift weights regularly.)

Sure, it sticks to your tools.  And to your shoes.  And anything else it comes in contact with.  And it stinks faintly of asphalt, for some days afterward.  All depending on the temperature.  But, given that it basically is asphalt, none of that should be a huge surprise.

I have no idea how well it will last.  For now, it all appears to be physically solid and well-attached.  This, despite doing my best to apply it as thinly as I could, in some areas.  And without the best surface prep in the world.

The individual pieces of the patch give it a little bit of a redneck look.  But that should mostly go away as all the patches cure to the same shade and surface finish.

In any case, I have to leave it alone for a couple of months as it cures fully.  So I get to look at that patch until August or so.  At which point I’ll apply some modern miracle crack filler to any remaining cracks, then top coat the entire pavement.

That’s the plan, anyway.

Addendum:  Plus one bucket of Aquaphalt.

I actually started by purchasing a bucket of Aquaphalt 4.0 (smaller stones).  That, before I realized how much of this stuff I needed.  And how much Aquaphalt would cost to do the entire job.

I ended up using the Aquaphalt on one section of pavement that had been heavily colonized by grasses.  Unlike QPR and similar products, Aquaphalt cures by addition of water, and it cures fast (15 minutes) and hard.  No tarry mess.  I figured that if the grass should try to grow back (despite my heavily salting the area per Post #1973), Aquaphalt would stand a much better chance of keeping the buried grass roots from growing through the pavement than would the slow-to-cure QPR.

Edit 7/19/2024;  And, so far, so good.  Going on eight weeks later, and nothing is poking up through my asphalt patches.  I’m guessing that spraying the alligatored asphalt with a strong salt-water solution, prior to patching, killed the roots of all the vegetation that was there, as intended (Post #1973).

As far as I can tell, other than the high price and the waste stream of plastic buckets, Aquaphalt is a superior product.  It spreads and shapes almost as easily as QPR, and seems to stick to the pavement just as well.  It cures in 15 minutes, as advertised.  The surface finish of the Aquaphalt 4.0 is much finer than that of the QPR, owing mostly to the smaller average gravel size in the Aquaphalt 4.0.  The sole downside I noted to Aquaphalt is that it didn’t flow/rake to the edges of the patch as easily as QPR, and I don’t think I was able to lay quite as thin a patch with Aquaphalt as I was with QPR.

Edit 6/5/2024:  That’s not quite right.  Aquaphalt’s main downside is that it “flows” less well than QPR, at least once you’re at the water-and-compact stage.  I ended up leaving marks in the Aquaphalt in areas where the tamper did not hit squarely onto the surface of the Aquaphalt.  At the time, I thought I had fixed that by tamping these areas flat.  But, in fact, the Aquaphalt’s surface had so little “flow” at that point that it didn’t fill in the little low spots my mis-tamping had created.

But worse, the finer surface finish of Aquaphalt is much less forgiving than the coarser surface finish of QPR.  Little imperfections that are lost in the background roughness of the QPR surface finish stand out in the Aquaphalt surface finish. 

The moral of the story being that if you are not the best at leaving a smooth surface finish on materials like this, Aquaphalt may not be the better choice, relative to a tarry patch such as QPR.  For the reasons described just above.

That’s a lesson that my driveway and I learned the hard way. 

Looking on the bright side, the little dings in the Aquaphalt section get lost in the overall unevenness of the patch. 

I guess that’s a bright side.

We’ll see how it looks with a seal coat.

Otherwise, if I didn’t care about the expense or the waste stream of big plastic buckets, think I’d do the whole thing in Aquaphalt.  It’s as versatile as QPR (in that you can lay it over existing pavement), but lacking all the factors that make QPR a bit of a mess.  You also avoid QPR’s months-long wait prior to seal-coating over the patch and roadway.

Edit 6/5/2024:  But on a raggedy, roller-coaster asphalt surface such as my driveway, you aren’t going to end up with a beautiful finished surface of Aquaphalt as-seen-on-TV.  If nothing else, there’s no flat reference surface to screed to.  Unsurprisingly, the finished surface of my driveway — after QPR top coating — is not flat.  Plus, making it flat (level) with the remaining sound driveway surface would have required laying down three times as much material as I actually used with a “puddles first” patch-application strategy.  I’m pretty sure I’d have done Aquaphalt the same way — in a series of discrete patches — if only because it’s 50 pounds a bucket, and don’t think I could move 1.5 tons of that material in a day.  Let alone get it laid, watered, and tamped.

 

I’m learning a few more things about patching my badly deteriorated driveway using QPR cold patch.  See just prior posts for background.

See also:

Post #1977: Updated: Twenty bags, and done. What I have learned about QPR asphalt cold patch.

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Lesson 1:  Day 2, Still tarry when hot

On Day 1, yesterday, these patches were “walk-able”.  Nice and firm.  I could stand on the edges, as above.

More importantly, I could walk right across them without picking up little tarry stone flakes. (Which would then get carried into the house on my shoes, and make a mess.)

I thought that was the way QPR worked.  Ready for traffic immediately, so the manufacturer says.

I was wrong.  And the manufacturer meant auto traffic.

Turns out, yesterday was a relatively cool day.  Maybe low 70’s F.

By contrast, today, Day 2, is hot and sunny.  And the surface of those patches is now tarry, and they are no longer cleanly “walk-able”.  I picked up little stray bits of tar as I walk on them.  It took me a while to notice that.  With everything that implies.

Lesson learned?  Don’t walk on the patches yet.

It’s still a matter of faith that these will eventually cure to the point where they are clean to walk on regardless of the temperature.  And I can always toss sand or mortar over them if that fails to happen.  (But that permanently alters the appearance of the top of the patch, so I’m hoping I don’t have to resort to that.)

The bottom line is that in a warm climate, and in warm sunshine particularly, you can’t count on having a cleanly walkable surface on QPR cold patch for at least a few days.

Lesson 2:  Plan your patches accordingly

So, obviously, you need to plan your patches so that you don’t need to walk on them for a while.  At least in my climate — USDA Zone 7 in Virginia.

But I didn’t do that.  I’m able to walk around on my driveway now purely as a matter of luck.

My patching strategy defaulted to filling in the biggest puddles first.  Each patch in the first picture above corresponds to an area of the driveway that formed a puddle when it rained.

Just by chance, the resulting isolated patches give me plenty of old pavement to move around on.  I would like to claim that I though of that ahead of time.  But I didn’t.  It’s purely luck, that filling puddles gave me places to walk on the old asphalt.

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Lesson 3:  Use in whole-bag increments.

The recommended strategy for getting QPR onto the road surface is to slit the bottom of the bag, pull up on the top (using the handles built into the bag), and let the contents slide out of the bag.

But the contents are a) heavy and b) semi-liquid.

The result is that everything in the bag spills out, and fast.  One moment, you have a bag of cold patch sitting on nice clean asphalt.  Three seconds later, you have a pile of cold patch on the asphalt, and an empty bag.

As a result, you have to patch in whole-bag increments.  Once you open the bag as the manufacturer directs, there’s no going back.  Move it around with a shovel, maybe.  But one way or the other, you’re placing a whole bag of it somewhere.

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Lesson 4:  Estimating the quantity needed is harder than you’d think.

It’s not just that the holes to be patched are irregular in shape and depth.

It’s that, with a surface-laid patch, on an irregular (not-flat) driveway surface, you have some leeway on the depth of the resulting patch.

In particular, I’m trying to shape these so that water will drain off my driveway.  I want to avoid puddles.

But the driveway slope itself is so low, and varies so much from place-to-place owning to the uneven surface, that shaping the finished patch to do that involves a lot of guesswork.  Or, at least, it did for me.

In my case, I ended up using vastly less patching material, so far, than I originally estimated.  And that’s because I’m not filling the driveway up to some theoretical original surface level.  I’m just filling the puddles enough to get water to flow across it.

I hope.  I won’t really know if I’ve succeeded so far until the next hard rain.

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Lesson 5:  Patch size may be limited by plywood size (4′ width).

Common advice is to do the final tamping of asphalt cold patch by laying down a sheet of plastic, then a piece of thin plywood, then driving over it.  I can vouch that this works with QPR.  I hand-tamped mine as firmly as I could, then ran it over.  Running over it, covered by thin plywood, definitely appears to make the patch surface more compact, and to make the patch more firmly compressed.

If you rely on this method, then the largest patch you can make well is one that can be covered by a sheet of plywood.  And that you can conveniently drive your car over.

You can, I guess, finagle it, by sliding the plywood around and only driving on a part of the patch at a time.  But that’s asking for the plywood edge to leave an imprint in the patch.

Conclusion

At this point in my driveway rehab, I have filled in the biggest puddles and coincidentally covered up the largest places where pavement was outright missing.

There’s still a lot of badly-alligatored pavement, with chunks of pavement missing, that I don’t quite know what to do with.

Right now, the patches look fine, but are tacky due to the heat and sunshine.  Will they cure?  Will water flow off the driveway without puddling?  Will these patches last?

Making an isolated patch, like the ones above, is easy.  The QPR material flows easily at my ambient temperature (say, 75F).  And it’s not even tiring, as long as you have the strength to lift the 50 pound bags.  Move a bag to a hole, slit it, rake the patch material out flat. pound it flatter.  Repeat.

Whether the final patched driveway is going to function well, or look right, I have no clue.

It’s going to be rain and threat of rain for the next few days, so at this point, I’ll let it be until we get some sunnier weather.

 

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.

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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?

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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.

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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.

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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.