Wind Turbine Blades: The Secret Ingredient for Tougher Recycled Concrete?

Hey there! Let’s Talk Concrete and Recycling!

Alright, so you know concrete, right? It’s pretty much everywhere – buildings, bridges, roads. It’s super strong when you push on it (that’s called compressive strength), which is why we use it for things like foundations and columns. But here’s the thing: concrete isn’t so hot when you try to bend it. Think of a beam or a road slab – they experience bending, which creates tensile stress (pulling apart). Plain concrete doesn’t handle that pulling well at all. That’s why we usually stick steel bars in there – to give it that extra backbone, limit bending, and make it less likely to just snap.

Now, being the savvy folks we are, we’re always looking for ways to make concrete more sustainable. And guess what? Construction and demolition create a *ton* of waste. One big source? Old concrete itself! Crushing it up gives us something called Coarse Recycled Aggregate, or CRA. Using this instead of digging up new rock sounds like a win-win, right? Less waste, less mining.

But here’s the catch, and it’s a bit of a bummer: CRA comes with bits of old mortar stuck to it. This bonded mortar isn’t as strong or stiff as natural rock, and it doesn’t bond as well with the new cement paste. This makes the concrete a bit weaker and less stiff, especially when you’re trying to bend it. And these issues are *way* more noticeable when the concrete is young, like just a day or a week old, because the cement hasn’t fully hardened yet.

The Wind Turbine Problem (and Potential Solution)

Okay, so we’ve got this challenge with recycled concrete bending. What else is piling up as waste? Get this: old wind turbine blades! They’re amazing pieces of engineering, super strong and light, often made with Glass Fiber-Reinforced Polymer (GFRP) embedded in resin. But when they reach the end of their life, they’re a nightmare to recycle because separating the glass fibers from the plastic is tough and energy-hungry. So, a lot of them end up… well, in landfills.

But what if we just crushed the whole blade up? You’d get this material we call Raw-Crushed Wind-Turbine Blade (RCWTB). It’s got those strong GFRP fibers, but also bits of the resin, maybe some wood or other stuff from the blade’s layers. Previous research hinted that adding fibers to concrete can really help its bending performance. They act like tiny internal reinforcement, providing a “stitching effect” that holds the concrete together even after it starts to crack. Could the fibers in RCWTB do this for our CRA concrete?

That’s exactly what we wanted to find out! Could adding RCWTB counteract the weaknesses that CRA introduces, especially in young concrete? Because let’s be real, sometimes you need concrete elements to start doing their job pretty quickly after they’re poured.

Setting Up the Experiment

So, we got our hands on some standard concrete ingredients: cement, water, plasticizers (to make it flow nicely), and natural sand and gravel. We also got our two special ingredients: CRA (crushed from old, strong concrete) and RCWTB (crushed wind turbine blade panels, full of those GFRP fibers).

We mixed up a bunch of different concrete batches. Some had no CRA and no RCWTB (our baseline, the ‘control’ group). Some had CRA replacing 50% or 100% of the natural gravel, but still no RCWTB. And the really interesting ones had that 6% dose of RCWTB (based on previous work showing this amount helps bending) combined with 0%, 50%, or 100% CRA.

We made little concrete beams and tested them at super early ages: just 1 day, 3 days, and 7 days old. We also tried two different curing conditions – keeping some nice and moist (like concrete loves) and others in just regular lab air (ambient curing). This was important because concrete strength and stiffness develop differently depending on how much water is available for the cement to react, especially when it’s young.

Our goal was to look at how these different mixes behaved when bent. We measured things like:

• Compliance: How much it bends *before* it breaks under a certain load (lower compliance means stiffer).
• Failure Stress: The maximum load it can handle before it starts to really fail.
• Failure Deflection: How much it’s bent right at that maximum load point.
• Fracture Stress e Deflection: What happens *after* the main failure? Can it still hold any load, and how much can it bend before completely giving up? This tells us how brittle or ductile it is.
• Absorbed Energy: The total area under the load-deflection curve – basically, how much punishment it can take before failing completely. This is a great measure of toughness.

What We Discovered: The RCWTB Effect

Okay, deep breath, here’s the cool part! Adding CRA by itself, just like the literature said, generally made the concrete bendier (higher compliance) and weaker (lower failure stress). The bonded mortar on the CRA particles made the whole thing less stiff and messed up the bond between the new cement and the old rock bits. This was especially true at those super early ages when the cement matrix wasn’t very strong yet.

But then we added the RCWTB. And wow, things changed!

First off, the RCWTB generally increased the compliance (made it bendier) even more. This might sound bad, but it’s tied to the fibers doing their stitching thing – they allow the concrete to deform a bit more before catastrophic failure. This effect was actually *most* noticeable when we combined RCWTB with 50% CRA. It’s like the fibers and the CRA particles found a sweet spot for interacting, maybe the fibers rubbing against the slightly rougher CRA surface helped their stitching action.

Now, for the strength (failure stress): CRA alone reduced it. But adding 6% RCWTB? It actually *improved* the failure stress compared to the mixes with the same CRA content but no RCWTB! Even better, the mix with 50% CRA and 6% RCWTB often had a *higher* failure stress than the mix with 0% CRA and 6% RCWTB. Again, that 50% CRA level seemed to work really well with the RCWTB fibers, maybe because the fibers could interact nicely with the bonded mortar without the overall mix being too weakened by excessive CRA (like at 100% CRA).

The failure deflection (how much it bent at the breaking point) also got a boost from the RCWTB, especially with up to 50% CRA. This is great because it means the concrete can deform more before reaching its peak load, which is usually a good sign of toughness. At 100% CRA, though, the RCWTB didn’t help as much, likely because there were just too many weak points from the high CRA content and the other non-fiber bits in the RCWTB.

Holding Together After the Snap

Here’s where the RCWTB really shone: the post-failure behavior. Concrete without any fiber reinforcement is typically super brittle. It reaches its peak load, maybe bends a tiny bit more, and then BAM! It loses almost all its load-bearing capacity instantly. Our mixes without RCWTB did pretty much that, especially at 3 and 7 days old when the cement matrix was stiffer. Even at 1 day, any post-failure capacity was minimal and over a tiny range of bending.

But the mixes with 6% RCWTB? Totally different story! They showed a *remarkable* ability to still carry load *after* the main failure point. The fracture stress (load capacity after failure) was much lower than the failure stress, but it was still there! And the fracture deflection (how much it could bend while still holding *some* load) was significantly higher – sometimes more than double the failure deflection. This is the stitching effect in action! The GFRP fibers held the cracked concrete together, preventing it from just crumbling.

What’s cool is that with RCWTB, this post-failure capacity didn’t really depend on the CRA content or even the age of the concrete as much. The fibers were doing the heavy lifting after the break, overriding the weaknesses introduced by CRA or the increased stiffness from aging.

The Bottom Line: Tougher Concrete, Less Waste

So, what does all this mean? It means that adding Raw-Crushed Wind-Turbine Blade material to concrete made with Coarse Recycled Aggregate is a pretty fantastic idea, especially for concrete that needs to perform well in bending at an early age. The RCWTB significantly improved the concrete’s deformability (both before and after failure) and, crucially, gave it that much-needed post-failure load-bearing capacity. This translated into a huge jump in absorbed energy – the RCWTB mixes were way tougher, absorbing more than double the energy compared to similar mixes without it, particularly when combined with 50% CRA.

Think about beams, pavements, or other elements that experience bending. Using CRA is already a step towards sustainability. Adding RCWTB not only counteracts the negative effects of CRA on bending performance but actually *improves* it, making the concrete more ductile and safer. And it helps us find a use for those tricky-to-recycle wind turbine blades.

Our statistical analysis backed all this up – the effects of CRA, RCWTB, and age were all significant, and they interacted in interesting ways, confirming that the combination of RCWTB and CRA (especially at 50% CRA) is where the magic happens. The improved bending performance, particularly the increased deflection and post-failure capacity, is key to why the absorbed energy goes up so much.

Ultimately, this research shows that we can potentially take two waste streams from wind farms – the crushed concrete from the foundations (CRA) and the crushed blades (RCWTB) – and use them together to make concrete that’s not only more sustainable but actually performs *better* in bending at early ages than concrete with just recycled aggregate. That’s a pretty exciting step forward!

Source: Springer