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Urban Mining: What Is It?

Recycling and Better Asphalt Replacement

This is an example of deteriorating asphalt. In the 70's as the crude oil shortages spread across America, think tanks began to consider recycling oil-containing asphalt.

This is an example of deteriorating asphalt. In the 70's as the crude oil shortages spread across America, think tanks began to consider recycling oil-containing asphalt.

Swedes Are Conservative

Have you ever been driving down a highway and approached an open area on the side of the road where there is a large mound of broken concrete? Snaggled rebar reaches to the sky and you may even see wire mesh emanating at other angles. When I see that I say to myself, "What a waste!" Wouldn't it be wonderful to be able to reuse all of that material?

Well, in the case of concrete and rebar, it is all being reused in Switzerland. The Swiss are committed to using the metals and the concrete from all torn down buildings. In fact, they have been able to crush the concrete and reuse it to make roadway bedding. Some crushed concrete can be used as aggregate for new concrete. The rebar is all collected by cranes with large electromagnets. They have been so successful that they can replenish their new construction needs for bedding and rebar by doing this.

An Old Road May Become New Roadbed

Lime from limestone is mixed with aggregate and silica (sand) and water forming concrete. You may have noticed that the concrete you've poured yourself requires spraying with water for a period of time to slow down the curing time, which results in better strength and fewer cracks. This process is known as hydration. Once concrete is used, the cement cannot be extracted because of the chemical change that it undergoes. Cement is a material that must be mined for all new uses.

But concrete that has been broken and discarded from roads and bridges can be crushed to specific sizes (depending on the structural requirements) and used for underlayment of roadways. In addition, crushers have been developed that effectively crunch up the concrete and allow the steel rebar to fall away. That rebar can then be sent to a steel mill to be melted down and turned into new rebar.

Why Do It? For the Economy and Environment

Anytime you can avoid using new materials, you are conserving energy, and many times saving money. Mining requires a great deal of fuel resulting in CO2 pollution. The faster mining proceeds, the faster the land can be scarred. Some people believe that slowing that process down gives us time to discover alternatives to carbon fuels. And as we know, fuel is expensive. Many times crushed concrete can be used on site or closer to a project rather than having to transport the virgin material to a building area.

Mixing the New With the Old

If crushed concrete is being used as an aggregate in new concrete, some newly-mined aggregate is added to the mix.

In my own community, old concrete has been cobbled together to stabilize the banks of arroyos. I have even seen the concrete fashioned like a mosaic and mortared in place.

Large sections of broken concrete can make very good reinforcement of embankments.

Large sections of broken concrete can make very good reinforcement of embankments.

Asphalt Is Not A Dirty Word

With nearly 4.1 million miles of paved roads and roadways, and 94% surfaced with asphalt, this material, if recycled, has a significant impact on our economy. RAP, or reclaimed asphalt pavement, can be a cost-saving influence for contractors. Lab studies discovered that typical RAP contains about 4-6% of asphalt content. That eliminates the percentage of virgin asphalt required. A second cost-savings is from using less asphalt cement. RAP can also be crushed and ground and added to asphalt. RAP then becomes a binder. This reduces the amount of asphalt cement mix binder required. State guidelines generally dictate percentage of RAP used in pavement. Here are some estimates based on state surveys.

  1. 10-50 percent mixes are accepted in most states depending on the type of pavement being laid.
  2. Technologies have been and are being developed to use 90% to 100% RAP.
  3. A 20 percent or less mix of RAP in hot mix does not change the grade of the asphalt.
  4. WMA (Warm Mix Asphalt) can contain up to 30% RAP.
  5. 20% - 25% RAP is added to Cold Asphalt Recycling,
  6. It has been discovered that 25% to 35% RAP in hot mix asphalt can result in comparable stiffness to virgin asphalt. The difference in stiffness his correlated to how fine the material additives are in the mix.

Interesting Facts

  • The binder in asphalt for paving is 70% of the cost of the mix. RAP can be used as a binder.
  • Of 1800 tons of landfill recycled asphalt, 1700 tons is crushed rock.
  • Of the 1800 tons of landfill recycled asphalt, 29,120 gallons of oil wasn't refined or delivered.
  • Of the 1800 tons of landfill recycled asphalt, 70 tons of greenhouse gas was not expelled into the atmosphere.
  • New asphalt contains 10 -30% RAP.
  • In 2016, more than 76.9 million tons of RAP and nearly 1.4 million tons of recycled asphalt shingles (RAS) were put to use in new pavements in the United States, saving taxpayers more than $2.1 billion.
  • At the job site, hot mix asphalt is between 300 and 350 degrees F. Rolling temps are between 220 and 290 degrees F.
  • Warm mix asphalt is between 200 and 250 degrees F.
  • Cold mix is appropriate if it doesn't drop below 150 degrees F. It is the most economical asphalt to use, mainly for cracks and potholes.
  • In California, where used tires are shredded and added to asphalt, a mix called rubberized asphalt is used for resurfacing roads. It is claimed to be more durable.
  • In 2017, there was a 36% increase in the use of RAP over 2009.
  • Blast furnace slag, steel slag, and cellulose fibers are also mixed with asphalt for binders.

Constructive Advice

You may have seen vehicles that mix RAP with asphalt and then lay it down behind them on the roadway. The mix is dropped on the roadway and compacted in one drive-through pass. The asphalt is in the form of asphalt emulsion (or foamed asphalt) and it's referred to as cold mix asphalt.

Hot mix asphalt uses RAP as an aggregate, not unlike rock content in cement. Here the chunks are mixed with hot asphalt and used as a binder lowering the cost of material.

What Does Virgin Asphalt Require?

To replace the Recycled Asphalt Pavement above would require a lot of pristine material including:

  1. 1870 pounds of quarried rock per ton
  2. 16 gallons of oil per ton
  3. 75 pounds of greenhouse gas per ton for plant power

What About the Future? estimates the savings range from $30 to $80 a ton by taking advantage of reclaimed asphalt.

Technologies will be developed to increase warm mix asphalt (WMA). By being able to reduce the temperatures of the mix, energy can be saved when applying the asphalt to the road. Most importantly, it lengthens the paving season and allows for greater compaction.

Smart robots are in the conceptual stage and are anticipated to separate materials in old buildings without pulverization. They will eliminate the dust created from current demolition models and will separate materials into piles and perform inventory functions. These robots will work from the top of a building down to the bottom.

Recycling in the future stands on three prongs. We will more and more reduce, recycle, and reuse. With growing world population, the concept of limited resources has come quickly to the forefront. Turning something whose value has waned into something new might involve no new consumption, or very little. It is a powerful tool in helping the planet. Recycling materials saves amazing amounts of time, energy, and money. It has been estimated that recycling one aluminum can saves enough energy to power a TV for three hours.

This savings will increase as we continue to try to change the way we think.


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This content is accurate and true to the best of the author’s knowledge and is not meant to substitute for formal and individualized advice from a qualified professional.

© 2019 John R Wilsdon


John R Wilsdon (author) from Superior, Arizona USA on October 20, 2019:

Thanks so much for commenting. We have so much raw material in our great country that could be recycled. Too many times it isn't because it is a bit more expensive than mining new. With old asphalt, that is not the case.

Pamela Oglesby from Sunny Florida on October 20, 2019:

This is such an interesting article. I think recycling asphalt is a great idea. I appreciate this information.