Thies van der Wal is convinced that concrete has an eco-friendly future ahead of it. In his view, it remains a good alternative to wood. “Because wood has to be imported—we don’t have any of that here at all.” We’re visiting VBI’s concrete plant in Huissen.
For 35 years, Thies has been championing the reduction of CO2 emissions in concrete production at VBI. “It’s in our DNA.” Despite all the criticism of concrete, he continues to believe in a bright future for the material.
Hollow Construction
When it comes to making concrete more environmentally friendly, VBI is pursuing two approaches. The first—and the company is already fully engaged in this at its factory in Huissen and the new one in Koudekerk aan de Rijn—is hollow concrete. This is anything but new. “It’s been around for a hundred years,” says Thies. “And the Netherlands is a leader in this field. But our biggest competitor has just stopped producing it.” He considers this anything but a good thing, because in his view, we actually need more hollow-core concrete, not less. “Fortunately, a number of other companies have started producing it again; they keep us on our toes.”
“Hollow-core construction saves a huge amount of material,” Thies continues. “A hollow-core slab is almost 50 percent lighter than conventionally poured concrete,” adds André Derksen. He’s joined us because he’s in charge of safety as we’re about to take a look inside the factory. He’s just as passionate and proud, and knows just as much about the process as veteran Thies.
Wood and Concrete
VBI is the market leader and a pioneer, Thies adds. “Over the past few decades, we’ve fully optimized and standardized our hollow-core slabs. We’ve thought through and refined every detail. The shape of the hollow, the tension of the reinforcement, and its configuration. As a result, our slabs can now span more than 20 meters without supports. This allows you to build large structures that are still very lightweight.”
That means large spaces without pillars in the middle. But it also means, for example, apartment buildings that can be freely divided into larger and smaller units, since no load-bearing walls are needed, as Thies explains to us.
Strands Under Tension
A short while later, we’re allowed to take a look inside the factory (where, incidentally, we’re not allowed to film). There, sections of hollow-core slabs measuring over 150 meters are produced, after which they’re immediately cut to size according to customer specifications. First, steel strands and wires are stretched along the entire length. The strands, which consist of multiple wires, are attached to the underside of the hollow-core slabs and are pulled extremely tight. André points out the amount of concrete these strands hold in place. “There’s as much concrete in there as in the foundation of the Eiffel Tower. We need that to absorb the forces during tensioning.” Because those strands are so taut, the hollow-core slabs themselves eventually even bulge slightly, which in turn makes them stronger.
At the top of the hollow-core slabs, the concrete is reinforced with several wires that are under much less tension. As a result, they serve no structural purpose. Their main purpose is to make the slabs movable. “If they weren’t there, the slabs would break when lifted by a crane.”
The steel in the panels is exactly what the panels need—no more, no less. “That’s very different from cast-in-place reinforced concrete,” says André. “It contains much more steel, which is why it’s also much heavier.”
As little water and energy as possible
A machine rolls along the tracks where the cables are tensioned at a speed of a few meters per hour. Concrete for the bottom of the slab is poured into the front opening, while the rest goes into the rear opening. An ingenious system of tubes is used to carve out the hollow section in the concrete. The concrete contains as little water as possible—just enough for it to take the correct shape, but also to hold that shape immediately once the machine has passed.
Everything is precisely calibrated and measured. That also applies to the heating of the curing concrete. “We used to heat it much more than necessary,” André explains. “But over the past thirty years, we’ve fully optimized the process, and now we use as little energy as possible for this as well. And that, in turn, reduces CO2 emissions.”
Going Against the Grain
But hollow concrete is only part of the story, as they realized at VBI. Because no matter how hollow and optimized it may be, the production of the Portland cement currently in use still releases a significant amount of CO2. So VBI has opened a second path toward more eco-friendly concrete: replacing that cement with a byproduct, so that virtually no CO2 is emitted during the production of their concrete.
The technology is here—it’s even already being used in a number of homes in Roosendaal—and according to Thies, it actually makes the concrete stronger. But regulations are still standing in the way for now. “According to the rules, concrete must contain a certain amount of cement. That’s nonsense, of course, because what matters is its strength and usability. But that’s just how it is right now.” Still, Thies and André are convinced that this hollow, cement-free concrete is the future. Going against the grain of convention.
In this episode of “Innovation in 3 Minutes,” watch Thies van der Wal discuss VBI’s cement-free concrete.
