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Carbon footprint of asphalt vs cement parking lot

Mike Haasl

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posted 6 years ago

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Our church is considering upgrading the gravel parking lot to asphalt. Another proposal is to turn one small section to cement. I'm on the committee that is assessing these options and I'd like to add carbon footprint to the evaluation.

Does anyone out there know the effective carbon footprint of paving by square yard? Or cement? Both would be residential type thicknesses, nothing like a highway.

In looking around online, some asphalt sites are saying that since they recycle the asphalt, they are actually sequestering the carbon in the pavement and since it will always be reused, it's not contributing much ickyness. I'm guessing that could be the case for highways and big cities but I'm not so sure about here in the country. I'll ask the contractor where they get their raw material and what % is recycled input.

My church is pretty progressive so if I could say that paving the parking lot is the equivalent of 275 F150s driving around for a year, that would certainly kill the proposal.

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Eric Hanson

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posted 6 years ago

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Mike,

This is an especially tricky energy-budget problem. I don't know about the asphalt actually being recycled, but it is essentially oil mixed with a gravel base, There is certainly an energy budget in heating up the mass prior to application, but i don't think there is much of an energy budget to make the asphalt itself. Cement on the other hand is very energy intensive to make. The process involves heating up limestone so that it undergoes a chemical reaction that is then partially reversed when the water is added back in. Right now, I can't provide exact chemical specifics, nor the desired energy budget, but I can think of a couple of anecdotes. The ancient Romans built a LOT with cement, because the parent limestone had already been cooked by volcanic activity. This might give you an idea of the types of energies we are talking about here. Modern methods for making Portland cement involve baking the parent material at high temperatures in a rotating drum. I cannot say for certain, but I am leaning towards cement as being the more energy intensive of the two.

Complicating the matter is how the two substrates will affect albedo (or surface reflectivity). Obviously, a parking lot of black asphalt is going to absorb a LOT more solar energy than cement. If you are (and I think you are) trying to put this in the perspective of global warming, even though the asphalt might use less energy to create, it will doubtlessly absorb more heat over its lifetime than cement. I don't know of this will offset any energy savings from going with the possibly lower energy intensive material in the first place.

As if that were not enough, cement will last longer and won't raise your A/C bills as much in the summer. I am pretty certain that the A/C will be a significant carbon emitter once factored in.

I did my masters research in the history of energy and now I have an unhealthy tendency to see energy in absolutely everything. I would like to say that I have helped you, but I am pretty certain I have confused you. If you have any further questions, please feel free to ask. I won't try to be intentionally confusing, but that will probably happen by accident anyway.

Eric

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Mike Haasl

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Thanks Eric, that does help. To make matters less complicated, the parking lot is surrounded by tall trees so most of the sunlight doesn't hit the lot. It's to the north of the building and we don't have A/C so it shouldn't affect the building in that respect.

One of the asphalt sites (secretly promoting the product I believe) had some damning numbers for cement. For residential streets asphalt is 65 tonnes of CO2 vs 295 tonnes for cement for a mile of road.

If they aren't recycling the asphalt to apply to our site, what is is made from? Is it really just thick oil and gravel mixed together with some heat?

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Chris Kott

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posted 6 years ago

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Well here's a crazy thought.

I know that crushed limestone is often compacted into paths that solidify over time. That would work in your situation, but might have interesting effects on the pH downstream of the parking lot. As it is a large area that doesn't hold onto water, and as it is lime, it will alter the pH of all the water that falls on and passes through it.

I was wondering how gypsum might work in a similar application.

-CK

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Eric Hanson

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Mike,

They may be recycling the base materials. Personally I would ask them from where they are getting their products. I have seen oil and chip roads be put in place new, and all they were was pea gravel and oil. Honestly, I just don’t know where they get their parent material.

I don’t want to sound like an add for the asphalt guy, but his numbers sound believable. The only point I might consider is how many times will the asphalt be redone? Cement does last a lot longer, but with the numbers you got, I would be inclined to go asphalt. But that’s just me.

Good luck,

Eric

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Chris Kott

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Another crazy sideways, or permaculture, approach to this, if the in-situ material is suitable is rammed earth. Theoretically, if the material can be amended to the right ratios, you could use a tenth of the portland as cement and, as long as it is shaped to shed water properly, it will perform as well as concrete structurally.

I would personally line the excavated slab area with landscape fabric, just to cut down on eventual erosion through the soil, and consider treating the surface with a dilute waterproof natural plaster sealant to cut down on water infiltration.

All you'd need is a tamper.

-CK

Mike Haasl

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Thanks Eric, I'll ask them and take it from there. So it sounds like there isn't an excessively huge greenhouse gas component of putting down asphalt. It is consuming oil and I wouldn't want to plant edibles near the lot but in a way I guess it's sequestering that carbon. Maybe...

Thanks for the ideas Chris! Since this is for a church, I have to stick with standard-ish approaches or I'll lose any credibility I have when it's time to do other energy improvements.

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F Agricola

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It is also about how much life that will be achieved by each material.

As we all know, if it's properly laid, concrete will last several lifetimes. Asphalt, even the stuff used on major roads ('Asphaltic Concrete') won't last more than a few years in a parking lot where vehicles exert severe transverse forces when doing small radius turns on the pavement causing ruts and cracks leading to potholes - more so in a hot summer.

Unless there's $$ to do ongoing maintenance, the larger upfront cost of concrete would be more economical in the long run.

The chemicals in asphalt are nasty and will leech into the soil.

In regards to car park design, it would also be highly beneficial to get an appropriately qualified person to design it to make the most of the space - bay/aisle widths, swept paths, blind aisles, disabled parking, pedestrian safety, landscaping, drainage, etc.

The design cost is peanuts when compared to the benefits it can achieve by increased functionality and safety.

'Every time I learn something new, it pushes some old stuff out of my brain.'

Eric Hanson

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Any time Mike,

Give me a chance to talk about the convoluted nature of energy and I likely won’t shut up. Incidentally, I looked up some specs and asphalt is looking better and better. Manufacturing of Portland cement takes a 2 step heating process. In the first step, the limestone/base material is heated to 1100 degrees F. In the second step, the modified base material is heated to 2600 degrees F. Given the mass of the material and temperatures involved, you can get an idea of just how much energy is required to manufacture cement.

Asphalt on the other hand is oil that is NOT burned. If it is indeed recycled, then the major energy input is in heating up the material to the point of being a semi-liquid for packing into place.

Thanks for bringing this topic up. It sounds so very strange to think that a petrochemical would be more benign than an artificial rock, but there it is.

Eric

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Rufus Laggren

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Water. Always water needs to be considered.

There are permeable solutions which will put some of the precip back into the ground relatively quickly. But it's not a no-brainer because oil from vehicles will clog pores. There are hollow cobbles or blocks that can be used that are less affected by that. In any case, do consider where the runoff is going.

The other issue is snow plowing. You're north WI, right? Probably do a fair amount of that each year and it will impact the surface of your lot. That lone might argue for concrete. And it leads indirectly back to drainage, both onto and off the lot. You need a place to dump large amounts of snow easily w/low spring time impact. Probably with salt and that leads to the need for some careful concrete specifications and more thought about where the runoff goes - salt is hard on everything.

Final thought is make _damn_ sure the area is compacted well and truly properly before placing material. That is the only way you get multi-generational slabs that continue on as planned.

Might like to look around your local area, there must be a few samples of each with some info that could help the decision.

Cheers,
Rufus

Mike Haasl

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Thanks F for the added input. This is for a small church (30 cars on a Sunday) so I think the longevity will be pretty good. Most asphalt driveways around here (daily use) seem to go for a decade or two before major work is needed. Luckily we're in a pretty cold climate so the hottest it gets is 90 and on a 10am church service it's rarely over 80 in August. Half the year it's covered in snow. But it is plowed so that could do more damage than all the cars combined.

Chemicals leaching into the ground is a concern for this group since they're very pro-nature. They're so pro-nature that cutting trees to make the lot a bit bigger was nixed a few years back. I'd love to get a designer to improve the parking lot layout but I don't think they'd spend the money. Especially since some members don't want to cut any of the established trees which would greatly limit design options. I've made several improvements myself to increase the number of cars that can fit in the lot (vs on the road). So I've been elected to be the parking lot engineer :(

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Mike Haasl

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Sorry, just noticed the replies.

Eric, energy math is fun. Not for me but I can see how it could be fun.

Rufus, runoff is something to always consider. In this case, the snow is plowed off and in about half the places, it goes down a hill away from the building. The rest is far enough away and the soil is sandy enough that it hasn't been a problem. I don't think the plowing contractor uses much salt. At least I haven't seen any evidence of melting snow in the winter. They possibly spread sand.

I'll make sure they are planning to compact properly before doing any work. How would I be able to ascertain that they are planning a good enough compaction job? Is a little jiggling steam roller good enough? I'm assuming a walk behind plate compactor wouldn't be good enough?

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Rufus Laggren

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> compaction

I'm sorry, I'm not really the expert. I researched roads/driveways a couple years ago to help my BinLaw with his place, so somewhere I have a list or problems and look-outs. He had a drive that needed a lot of help. That's what passes for my smarts on roads. And some minor embarrassments in a cobblestone area that should have been flat...

Who does roads (and inspects them) for the county? Place to start. Your job is tiny and light duty, but the same principles apply. Ask what is in the contract, how it's done and how it's inspected, verified. Seems like your job might involve an inspector in any case. Your existing parking lot s/b an example of what will happen over time. More of the same going forward. If it's flat as a pancake now you're 3/4 there. If it needs leveling, than the fill (scrapings) needs compacting. I think the vibratory rollers is what I've seen on street, driveway and lot jobs. There have to be county people (city doesn't do much for roads, right?) who know this backwards. And probably they know a few of the contractors as well, although guvmint jobs would be much larger.

How do you drive in/out of the parking? If there's a curb, is the curb cut OK? That involves the authorities, at least it does around here. Also, around here, the city controls about 15-18' from road edge into "private" property. Might check that - surprises aren't always nice. Are you going to edge the lot in any way (including any water control)? Or maybe use parking stops at each space (probably not, I guess)? They've just started using porous concrete and open cobble stones and block for certain areas of the lots around here; permeable surface goes under the parking space and the drive-through access gets regular concrete. It's too early to tell how the different systems will pan out, so something like that would be a bit experimental. Is the lot surface above the road grade and above the landscape grade on site? That helps to keep it civilized when it gets wet or melts a lot. How does the lot surface meet the walkway access from the buildings? Where is water going to flow there? Need any steps there or sloped walks? Which way will the lot be sloped (in contract and inspect, if you care!)?

> not much salt
That sure makes things a whole lot more benign chemically. I forgot - you folks in WI drive on hardpack in the winter, right?

Ah, almost forgot. You maybe want to carefully and thoroughly check what your legal responsibilities are vis the ADA law. Maximum height of any curbs, ramps, etc, signage, number, clearance around, handicapped slots... That stuff. Also, considering more laws: Any required lighting in your city/county?

As long as I'm talking to the Parking Engineer, I'll request generous width specifications for the spaces. Narrow skinny parking slots are just plain hateful! <g>

Sounds like real fun! Cheers,
Rufus

Greg Mamishian

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posted 6 years ago

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Mike, had you considered interlocking concrete pavers?

The pattern is produced by using only one kind of block and there are also many other shapes and colors. They are durable, highly water permeable, and can withstand heavy vehicle traffic.

Mike Haasl

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Thanks Rufus, good point on the ADA. I'm not sure if it applies to a small church but I'm guessing so. Our access is currently confusing and I plan to pave the handicapped spots regardless of doing the rest of the lot. The challenge is that the current folks who are mobility restricted enter through the front door (flatish thresholds and wide doors) while the "official" handicapped access is a ramp on the side of the building that is rarely used.

Greg, I haven't considered pavers. We'd just barely be able to afford asphalt. I'm sure we can't afford cement. And I'm certain we couldn't afford a beautiful paver system.

Members are torn between leaving it gravel and doing asphalt so that's why I'm doing my homework on the environmental impact of the asphalt.

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Chris Kott

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Does anyone in your area use crushed limestone?

If it's not available, well that would be a moot point, wouldn't it, but if it is, a pad of finely crushed limestone grit, just coarse enough that it won't be picked up by the wind before it gets soaked down, will pack densely and eventually settle into solidity, almost monolithically. If it settles improperly, or if low spots form, add more and tamp down. And no leaking of toxic gick into the groundwater or the soil.

-CK

Mike Haasl

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I don't think crushed limestone is common around here. At least I haven't seen it. The normal "fancy" gravel driveway material here is crushed granite (I think). It's reddish. One guy in the church uses that for his driveway so I'll check with him on Sunday.

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Chris Kott

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I don't know how granite would work, although theoretically, I could see any fine grit packing well enough to support the weight and action of cars atop it, especially if it was essentially a four to six-inch compacted slab of it recessed to be flush to ground level. On the fine side of things, as long as it remains well-permeable to water, it should act as a large three-dimensional sieve that happens to be sturdy enough to support cars.

-CK

Casie Becker

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Crushed granite is very popular for long lasting foot paths and patios around here. Eventually we need to repair and expand our driveway and it's on the short list of options for the extension due to water permeability. I don't know how durable it is, but repairs are easy.

Rufus Laggren

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> leaving it gravel

That may be a perfectly sensible solution. Cheap and practical has it's own beauty. Provided:
- It's leveled and compacted properly
- The winter plow season doesn't damage it too much (what's the history there?)
- The regular maintenance needed actually gets done.

Sometimes less is more and somehow I get the impression that there are no "fires" to put out by redoing the parking lot - just a bit of smoother ride for people going slow anyway. Worth considering saving on the "field" and spending money on some targeted details: Repairing the gravel lot properly, then maybe upgrades to the handicapped slots, a nice ramp or other mods to the front entrance if that's where people really feel best about entering church... A little drainage control if there are problem areas. Maybe a little landscaping if people think it has a fighting chance to survive. A covered and wind protected waiting hutch near the drop off spot at the lot/sidewalk join - big enough to keep 6-10 people out of the sun and rain... Well, maybe that's a little grandiose! <g>

Cheers,
Rufus

Mike Haasl

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Yup, I'm definitely leaning towards leaving it gravel. Hopefully the quote for asphalt is higher than expected so it's nixed without me even having to try to influence things

With the gravel we have now it hasn't been an issue. We have it maintained whenever potholes show up (every other year). Plowing does remove some material but it's not too bad. Once things freeze it stays put pretty good. The biggest risk is heavy snow in the spring after the frost is out of the ground. But plow operators around here know about that and tend to not plow to avoid removing all the gravel.

I'll take another look at the front door on Sunday and see what might need to be done there.

Thanks team!

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James Alun

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Mike, have you looked at plastic grids to hold the gravel in place?

Something like this http://gridforce.co.uk/gravel-stone-plastic-grid

I think it would stop your potholes but I don't know what a plow would do to it.

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I vote to leave it gravel esp for just two dozen cars.
As stewards of the earth lets vote with our church dollars.

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