Parking Lot Paving

A pavement is a system, not a surface

The most expensive misunderstanding in this trade is that a parking lot is a layer of asphalt. It is not. It is a structure with three parts, and the visible part is the least structural of the three.

The subgrade is the native ground. Every load on the lot ends up transferred into it, and whatever it does — settle, swell, soften when wet — the pavement above it does too.

The aggregate base is the compacted stone layer that takes a concentrated wheel load and spreads it out across a wider area of subgrade before it gets there. This is where the structural capacity of a parking lot actually lives, and it is the layer nobody ever sees.

The asphalt is a wearing course. It provides a smooth, sealed, durable driving surface, it contributes some structural capacity, and its most important job is to keep water out of everything underneath it.

Once that structure is clear, most of the decisions on a paving project explain themselves. A lot that is failing from the bottom cannot be fixed from the top. A lot built on inadequate base will fail on schedule no matter how good the asphalt above it is. And a lot where water is getting into the base is being destroyed continuously regardless of what the surface looks like.

Front Range ground is not neutral

A great deal of ground along the north Front Range contains expansive clay, and expansive clay changes volume with moisture content. It swells when it takes on water and shrinks when it dries out, and pavement built on top of it moves with it.

That single fact explains a lot of what owners find confusing about their lots. Two properties of the same age on the same road can be in genuinely different condition. Cracking can appear in patterns that have nothing to do with where the traffic goes. A lot can look stable for years and then move noticeably after an unusually wet spring.

It is also why drainage on this ground is structural rather than cosmetic. Anywhere water is allowed to collect and soak in is somewhere the subgrade is being given the moisture it needs to move, and the pavement above will register that movement.

A properly built lot on this ground answers the question before paving: what is the subgrade, how does it behave, does it need to be treated, over-excavated or replaced, and where is the water going to go. Those answers are not visible in the finished product, which is exactly why they are the first thing cut when a bid is being sharpened.

Freeze-thaw is the other half of the problem

Colorado runs a large number of freeze-thaw cycles, and the mechanism is simple and destructive. Water enters a crack or a joint, temperature drops, the water expands as it freezes, and the crack gets wider. Then it thaws, more water gets in, and it happens again.

On a sound, sealed pavement this is slow. On a pavement with open cracks it is fast, and it is the reason crack sealing along the Front Range returns more per dollar than almost anything else on a maintenance schedule.

On a new lot, freeze-thaw is an argument for two things in particular: getting the drainage right so that water is not sitting where it can work into the structure, and specifying joints and thickness so that the surface stays closed as long as possible.

It is also an argument against the cheapest version of everything. A thin lot on a thin base in this climate does not fail gracefully over a long period. It opens up, takes on water, and then degrades quickly.

Traffic class decides thickness

The single most common structural error on commercial property is building the whole site to one specification when the site does not carry one kind of load.

Employee and customer parking is light duty. Passenger vehicles that are stationary most of the day put very little into a pavement. Drive aisles carry everything that moves through the site. Delivery routes, dock approaches, dumpster pads and anywhere a loaded truck turns or stops carry loads that are different in kind, not just in degree.

Building all of that to the parking-stall specification produces exactly the failure pattern most owners eventually describe: the parking is fine and the truck route is breaking up. Building all of it to the truck specification wastes a great deal of money on the stalls.

The right approach zones the site by what actually drives on it, and specifies each zone for the loading it carries. That conversation should happen before anything is priced, because it is the difference between a lot that ages evenly and a lot that produces a repair bill in year three.

Drainage is designed, not hoped for

Water has to leave the lot, and it has to leave it fast enough that it is not standing after a storm and not sitting in low spots afterwards.

That means grade. A lot needs enough fall across it to move water toward inlets or edges, and the grade has to be consistent enough that there are no flat areas where water pools. Flat spots are not a cosmetic defect; they are the place where the pavement will fail first and where the eventual repair bill will concentrate.

It also means the drainage structures themselves. Inlets, curb cuts and any conveyance across the site need to be at the right elevation, clear, and capable of taking what a hard storm delivers. On reconstruction projects, existing inlets are frequently buried under successive resurfacing or damaged by traffic, and the reconstruction is the moment to fix them.

Snowmelt makes this a year-round consideration rather than a summer one. Piles of plowed snow melt slowly and deliver water to whatever is underneath them for days, so where snow is stacked matters and it is worth deciding deliberately rather than by habit.

What a reconstruction actually involves

1. Assessment of what failed and why, including the subgrade rather than only the surface.
2. Removal of the existing pavement, in full depth where the structure has failed.
3. Evaluation and correction of the subgrade, which may mean over-excavation, moisture conditioning or replacement of unsuitable material.
4. Grading, so that water goes where it is supposed to go rather than where the old lot sent it.
5. Placement and compaction of aggregate base to the thickness the traffic class requires.
6. Paving, in lifts appropriate to the specification, with attention to joints and to compaction while the material is in temperature.
7. Curing time before the surface takes traffic, which is temperature-dependent and not negotiable.
8. Striping, always last, on a finished surface.

The season is shorter than people plan for

Hot-mix asphalt has to be placed and compacted while it is hot enough to compact properly, and that is a function of air temperature, ground temperature, wind and the thickness of the lift being placed.

Along the Front Range the practical paving window closes earlier in the autumn and opens later in the spring than a national schedule assumes, and at higher elevation on the west side of the metro it is shorter still. A project that goes out to bid in September is frequently a project that gets built in April.

That matters more than it sounds, because deferring a structural repair through a winter is not neutral. An open, failing area takes on water all winter and cycles through freeze-thaw repeatedly, and the area that needed repair in September is measurably larger in April.

The practical consequence: plan paving work the winter before, not the month before. Assessment, scope and budget approval in the cold months, work booked into the first part of the season, and contingency for the weather rather than optimism about it.

Questions worth asking any paving bid

• What is the subgrade, and was it assessed or assumed?
• What base thickness is specified, and on what basis?
• Is the site zoned by traffic class, or priced as one pavement?
• What is the drainage plan, and does it change the existing grade?
• How many phases, which areas close when, and how is accessible parking maintained?
• What are the reopening criteria and the cure time for each phase?
• Is striping included, and is the layout being reviewed or reproduced?
• What is the plan if weather closes the window mid-project?

Parking Lot Paving — common questions

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