Partially replacing sand with waste plastic. Thoughts? 4

[MorganA]

Currently writing a research paper on the above topic. It is said that around 10%, by weight, of sand can be replaced by granulated waste plastic (to roughly the size of sand)and be almost as strong.
My question to you all is, would yourselves, or do you think companies would make this small change? I understand where strength is the main concerns for the application of concrete then one would rather be safe, however in other applications such as paving slabs and other things you may think of (please say below) then making this change would be greatly beneficial in helping solve the waste plastic epidemic and also estimating to save using roughly 820M.t of sand a year.

 

[ACtrafficengr]

"It is said" - can you give a reference?

The first issue that comes to mind is segregation during mixing because of different densities. For concrete mixing, fill subject to flooding, etc., buoyancy could also be a concern.

Also, how well do cement, pozzolans, etc. adhere to plastic?

My glass has a v/c ratio of 0.5

Maybe the tyranny of Murphy is the penalty for hubris. -

http://xkcd.com/319/

 

[JedClampett]

If there's any potential long term issues, like leaching chemicals, off gassing, aging of the plastic and changing its properties, etc., the concrete batch plants aren't going to take the risk. They're selling a commodity, pretty cheaply, and they can't afford the risk to add mysterious, loosely controlled materials to their product. If the stuff deteriorates, the liability (your building is no good anymore, tear it down) is immense.
Your best bet is to go to ACI, have them research it, and give the suppliers cover.

 

[STrctPono]

I will admit that I have not done a lot of research on this topic but I do always wince a bit when someone discusses dumping waste material into concrete. At best they are a knowledgeable researcher in the field of concrete but haven't the faintest clue of how the concrete industry actually works in their local community and at worst are merely an environmental idealist who has no respect for the intricacies of this finely engineered product. Concrete has come a LONG WAY in the past few decades. You wouldn't believe how high tech this stuff actually can get.... Adding plastic to the mix honestly feels like we would be losing ground. Trust me, this is not a SMALL change that you are talking about.

Strength is not always the main concern. For outdoor structures or environmental structures, strength may be 2nd or 3rd on the list of concerns. Adding plastic to the concrete mix could affect a multitude of other properties that you haven't considered. For instance, if not properly graded with the correct particle size, the addition of the plastic may in fact require the addition of cement to compensate for workability and slump. The addition of cement (a massive CO2 emitter) is now being increased so that we can hide plastic waste. Let's say I have a prestressed element with staged post tensioning and fixed boundary conditions.... Element is under constant sustained compressive loads. I run the analysis for the concrete assuming the cement type and aggregate type and produce a creep coefficient function. But with the addition of the plastic, the concrete creeps more than expected which increases my internal stresses beyond what was designed for. Let's say I have a tall retaining wall or a thin suspended slab that I check deflection limits for but now with the inclusion of the plastic, the elastic modulus values for the concrete are lower and I have a harder time getting the member sizes to work out and meet service limit requirements.

Concerns for durability have now been raised and chemically how the plastic reacts with the concrete mix

The biggest obstacle however is not overcoming the technical obstacles or convincing the Engineers but rather getting the local ready-mix operations to change. What is the incentive? Ready-mix companies are not going to want to take this on unless there is monetary incentive. The only way I see this happening is if this is forced on them through either government intervention or a strong push from the local Architects organization due to the possibility of earning LEED points. Either way is going to take many many years.

I am certainly not trying to be a stick in the mud that doesn't like change, in fact I welcome it but I have a pretty good sense of reality and how that this type of stuff is not a "small change" and has reverberating impacts on the industry.

ACtrafficengr brings up a great point about segregation concerns due to different densities.

 

[r13]

In my memory, plastic waste has log been studied, and used in concrete making. You should do more research and hook up with ACI and DOT to draw more interested audients, and those who has the authority to promote, and/or force the change in industry practices, if proven works. I am with your idea 100%.

 

[hokie66]

Intuitively, recycled plastic is best used to make products which are solely plastic, rather than incorporated in concrete. There are myriad uses for recyled plastic, growing all the time.

https://www.replas.com.au/

 

[JedClampett]

One of our company eggheads came to me with the idea of using waste water softening lime (from large water scale softening) in concrete. One of his egghead buddies wanted to study it. I explained to him that the scale of concrete production is so large, that the product needs to be wasted in such large quantities (fly ash; blast furnace slag; etc.) that most recycling materials isn't worth the hassle. Even if it helped the concrete (not sure on that), passed all the tests, then getting enough of the material is a challenge. You can't count on supply, plus, once you commit to it on a project, it needs to be used everywhere (not a requirement, but most engineers want that), so you need a lot of it.
It's hard to beat materials that come out of large holes in the ground.

 

[r13]

New technology often came much earlier before gaining popularity and support. It needs the right timing, before that, someone needs to start thinking, and to do research.

 

[MorganA]

@ACtrafficengr a lot of papers highlight the decrease in compressive strength, however say that 10% is a just amount that is worth replacing whilst not giving up enough strength to not be practicable,'Engineering Properties of Concrete with Waste Recycled Plastic: A Review' is a report wihch highlights a load of papers on this topic, or there about. Mahaveer Prasad and Devesh Jaysawal ‘Use of Plastic Waste in Concrete Mix’ states that 5%-13% show good enough results to be used in construction as a lightweight material, which has benefits such as reduced cost, simple, energy saving and an effective way to recycle waste plastic.
In terms of the adhering, workability is affected negatively, which i will investigate myself, however based on the research it doesn't seem to be affected negatively enough for too much issue.

 

[epoxybot]

The low surface energy of most plastics means that, while you may be able to entomb plastic in OPC concrete, it is unlikely to bond with the cementitious gel & participate in providing any beneficial strength properties. And as already noted, the density of plastic presents homogeneity of mix design problems.

If I recall correctly, Master Builders, back in the 90's, announced with great fanfare a cementitious flooring material with colorful plastic aggregate. There was even a McDonalds, where the material was spec'd. The flooring material shortly experienced pop outs, of the plastic aggregate and that was the last that was heard of this new wondrous "Green Flooring".

There are polymer flooring materials which can be used with recycled plastic because the surface tension of the polymer is closer in range to the surface energy of the plastic. Most useful plastics for these applications have surface energies of around 43 to 49 dyne/cm or lower. Water, with a surface tension of 72 dyne/cm beads up on plastic. Hence the problem with concrete bonding. And then there is the order of magnitude difference in compressive modulus & creep, plus expansion & contraction behavior.

 

[Canuck65]

Thinking ahead to the future, all concrete has a life span. At present when concrete is removed/disposed it is considered clean fill and can be disposed of safely and cheaply (in my area the waste facilities accept clean concrete for free). In some cases reuse of the rubble is possible after processing. If there is plastic in the concrete it will no longer be considered clean fill, disposal fees will apply and the likelyhood of reuse will diminish. While you may be trying to solve a problem now (which I applaud), you may just be delaying the problem by plus or minus 50 years, and creating a bigger problem by contaminating the 90% concrete matrix that is currently considered clean fill.

 

[msquared48]

Seems to me that plastics could be incorporated into CDF and Lightweight concrete for floor infills with little structural impact where strength is not an issue, but it would have to be tested.

I would be very reticent to use it in structural concrete though due to its tendency to deform under high temperatures and pressures.

Mike McCann, PE, SE (WA, HI)

 

[NorthCivil]

What is the service life of most things made of concrete? 20 years before demolition? 100 max? in 1000 years, where will most concrete in service today be sitting?

My guess is in 1000 years the vast majority of todays concrete will be rubble in a landfill, or left out rotting somewhere. in which case, you may as well just skip the step of putting your plastic in concrete, and chuck it right in the landfill.

The real civil engineering challenge is diverting the plastics from ever being produced, or creating a way to effectively categorize, clean, sort, and re-use the waste. I understand in japan, they sort their plastics based on type of makeup for ease of recycling.

 

[r13]

Here is a good article on plastic wastes uses in pavement, though not in concrete mix. Link

 

[STrctPono]

r13, you reborn as an Aeronautics Engineer now? I thought that you got banned? What happened?

 

[r13]

Thanks for catching and reminding the glitch, I haven't notice it yet. And thanks for asking, it just a minor miscommunication.

 

[cmoreride]

MIT students fortify concrete by adding recycled plastic Adding bits of irradiated plastic water bottles could cut cement industry’s carbon emissions.

https://news.mit.edu/2017/fortify-concrete-adding-recycled-plastic-1025

 

[BridgeSmith]

Doesn't seem like a cost-effective use of recycled plastic. Sand is plentiful and cheap; plastic can be recycled into new plastic products. Other waste materials actually increase the strength of concrete, where plastic reduces it.

If it could be made into a lightweight, stable fill material for backfilling behind bridge abutments, that is an application where few good solutions have been found, and they're fairly expensive.

Rod Smith, P.E., The artist formerly known as HotRod10

 

[r13]

Rod,

A few disagreements here.
1. The adding small amount of waste plastic doe not meant to replace the sand, but to help the consumption/utilization of the waste, which by itself is cheap.
2. Sand is plenty in the North America, but sand shortage is not rare in many parts of the world, especially developing countries with fast economic growth and construction boom.
3. Recycle and reuse is expansive. My city has long ago lost the original enthusiasm in pushing the recycling program. The collected wastes just piled up wait a place to go. Also, I wonder how many times a plastic bottle can be recycled and reproduced, and what to do when the cycle ends.
4. There are successive stories on adding plastic chips in pavement, especially for patching works. I guess it does not hurt the compressive strength too much, at the meantime, the abrasion characteristic could have improved. However, this is for asphalt pavement so far in my local.

I think we just need to find more ways to delay the inevitable fate - live with plenty of non-decayable non-consumable wastes around.

 

[BridgeSmith]

I agree on most of what you wrote, r13, but I think there are more useful and more cost-effective places for plastic waste materials than in concrete, where it reduces strength and, at present, increases the cost.

Rod Smith, P.E., The artist formerly known as HotRod10