Hello All
I wonder if Light Weight Aggregate could be considered as a sustainable material and if there is any LCA or any scientific evidence to support this. I am talking about manufactured Light Weight Aggregates that is half the weight of typical aggregate that is expandable by a heating process similar to cement manufacturing but at much lower temperature. It is used for brick blocks manufacturing, and other hard surfaces that does not require loads on it and some prefabricated concrete panels as input to their manufacturing process.
Thanks
Sid Baz
Capital Projects Procurement Program
Category Manager
Holcim (US)
Hi Sid!
When comparing concrete mixtures utilizing normal weight aggregates to ones that use lightweight aggregates, the NRMCA industry-average baseline demonstrates a 30-50% decrease in GWP per cubic yard. Generally speaking, lightweight aggregate is not technically considered a sustainable material when compared directly to normal weight aggregate. However, I would consider utilizing lightweight concrete a sustainability-focused design strategy when the weight reduction significantly downsizes the supporting structural elements. This strategy would only make sense if the embodied carbon reduction associated with the downsizing of the supporting structural elements outweighs the increase associated with the use of higher impact concrete.
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I love the answer “It depends!”.
Same like Wood or Steel. or Concrete or Wood. It depends!
Thanks
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Sid -
Check out this LinkedIn post from @chrisdrew: Christopher Drew, PhD on LinkedIn: CIC recommends BBC 39 Ways to Save the Planet podcast on concrete
He discusses his research with carbon negative aggregate with Ozinga.
@JMartinez - I don’t think I saw similar impacts from just aggregate. Someone in the link above said that crushed coarse aggregate is roughly 8.5 kgCO2e/cy of concrete, which is a very small percentage of the overall concrete mix (about 3% for a good performing concrete at 300 kgCO2e/cy).
@ScottFarbman, I took the GWP values for comparison from the NRMCA EPD (see attached resources and analysis). I’m not quite sure where this drastic difference is coming from, but perhaps more cement is needed to reach the required strength when lightweight aggregates are used?
Would love to hear if anyone has additional insight on the topic!
Lightweight Concrete Study_NRMCA EPDV3-2.pdf (65.7 KB)
NRMCA_EPDV3-2_20220301.pdf (1.9 MB)
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I’d love to learn more about it too. I am not a structural engineer. It would be great to know more directly about the impact aggregate has.
Manufactured lightweight aggregates have high GWP because of the process Sid mentioned: “expandable by a heating process similar to cement manufacturing”. The impact of aggregate becomes significant in a concrete mix using this type of aggregate.
Regular aggregate and natural lightweight aggregate don’t use this process, hence the very low impact when compared to cement in a concrete mix.
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Interesting! I’d be curious to understand how common natural lightweight aggregates are in comparison to manufactured lightweight aggregates.
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Update: I reached out to my contact at the NRMCA and got lots of additional information on the topic!
"You are largely correct in your analysis that higher GWP is realized when manufactured lightweight aggregate is used, a process in which clay-like materials are extruded/pelletized and then fired in a kiln to cause expansive bubbles to form within the material as it hardens. Further, you are correct that there are natural lightweight aggregate sources that would not have to be processed as such.
Manufactured lightweight aggregate:
- Higher inherent strength potential and since lightweight concrete is considered a more premium product, these materials are shipped longer distances which also impact GWP.
- More material consistency as it is manufactured and frequently partially pre-saturated.
Natural lightweight aggregate:
- Expanded naturally when formed from cooling volcanic materials. Much more variability and in many cases, much large voids and weaker material, limiting the strength potential. Some pumice materials can be partially crushed in your hand for instance.
- Not easily pre-saturated, resulting in more plastic variability and lower strength potential.
- Potential to be more locally sourced, but not many viable natural products are available, especially when you get away from mountainous regions.
There are two other contributors to the GWP increase in combination with this though:
- Most notably, the relationship between strength and density. Strength of concrete is a function of its constituents. With aggregate constituents, strength can loosely be correlated to density as voids are the largest contributor to density reduction. As such, lightweight materials inherently have a lower strength potential. To offset this loss, cementitious materials are frequently increased which impacts GWP.
- Next, plastic characteristics required for pumping and placing lightweight concrete. Lightweight is frequently on deck at multiple levels above grade, requiring the use of a pump. The voids in lightweight aggregate are always saturated as an operational standard operating procedure with the producer, but never perfectly saturated as that would require a vacuum chamber or immense amounts of time. As such, lightweight mix designs need to account for additional water and binder/paste that will inevitably be taken up by the aggregate voids when pumped at high pressure. This is why slump at the pump hopper can be a 10” and the same mix can come out on the deck at a 5” slump. Increased paste requirements further impact GWP.
When I worked with Central Concrete in the Bay Area, we had 3 lightweight sources: (1) Arcosa Fraizer Park manufactured product from Southern CA for higher strength applications (4-5 ksi higher cost), (2) Glass Mountain Pumice a naturally mined pumice material for lower unit weight and lower strength applications (3 ksi lower cost), (3) a newer natural product called Teichert Lite that had higher strength potential but was in the early stages of evaluation a couple years ago (could be standard now).
Quick examples of the above (EPDs attached):
| Strength (psi) | LWT Type | SCM (%) | GWP (kg-CO2e/m3) |
|---|---|---|---|
| 3000 | Natural | 50 | 278 |
| 3000 | Manufactured | 50 | 406 |
| 4000 | Manufactured | 50 | 444 |
| 5000 | Manufactured | 50 | 559 |
*note that these mix choices may not be current as it’s been a couple years since I was with them"
Let me know if anyone has follow-up questions they’d like me to explore with my contact!
EPD_Central_Concrete_Stockton (wet)_L50PF2P8 Version 4_2022419.pdf (122.9 KB)
EPD_Central_Concrete_Stockton (wet)_L30PL2P8 Version 5_2022419.pdf (122.8 KB)
EPD_Central_Concrete_Stockton (wet)_L40PF8Q8 Version 2_2022419.pdf (123.2 KB)
EPD_Central_Concrete_Stockton (wet)_L30PF2P8 Version 2_2022419.pdf (123.0 KB)
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#JMartinez
Hello Jessica
Thanks a lot for such a detailed answer. I guess my only question that I still have is there is any off set to the carbon consumption by the virtue of using less concrete or less steel structure due to the lighter weight of the structural elements/
Sid
Hi Sid!
I think this question warrants an in-depth structural study to further explore the impacts of utilizing lightweight concrete in lieu of normal weight concrete. I think the results will vary on a case-by-case basis depending on the bay sizes, occupancy type and imposed seismic/wind lateral forces. I’d love to explore some tangible structural designs to provide answers, but this will take some time on our end.
If anyone else has existing studies they could share that could shed some light on this design choice in the meantime, it’d be greatly appreciated! If no one speaks up in the meantime, I’ll plan to circle back on this once we’ve found a candidate project to explore and manipulate.
Hi Jessica, Sid,
We have carried out some studies on using lightweight aggregate concrete for steel-framed composite deck structures. Please feel free to reach out to me at jscarr@hts.uk.com if you would like some more specific details.
Thanks,
Jack
HI @JMartinez, all. Picking up an old thread. We have a project in California that is setting GWP thresholds for mixes, and someone flagged that the lightweight mixes may have higher GWP than others. However, I can see that this is not always the case, and we don’t see separate baselines for lightweight and regular.
Are others differentiating between requirements for lightweight and typical concrete in specs? If so, are you increasing GWP thresholds for lightweight? Either way, can anyone share takeaways for specifying to reduce GWP of lightweight?
(*e.g., is anyone specifically requiring natural lightweight in addition to a threshold to help secure lower GWP sources?)
