Whether in tunneling, mining, hydraulic engineering or road construction, sprayed concrete is a construction material of major importance. The demands made on the concrete are high: it must provide the necessary strength, stability and impermeability. For WACKER application engineers in Burghausen this was not enough. They wanted to reduce rebound during spraying while also making the sprayed concrete water resistant. They developed polymeric additives for sprayed concrete for tunnel construction, making a major contribution toward building a new tunnel for the Stetten salt mine.
The subterranean world of the Stetten salt mine is white and covers an area greater than ten square kilometers – the size of Italy’s Isle of Capri. The Stetten salt mine is one of the oldest and yet most modern salt mines in Germany. It was built back in 1852 at the suggestion of Prussia’s king Frederick William IV. “Initially, the mine produced table salt, then industrial salt. Today, the Stetten salt mine produces some 500,000 metric tons of industrial and road salt annually,” says mine director Alfred Höllerbauer, summarizing the mine’s 150-year history.
But the mine has more to offer than just road salt: “A large portion of the rock salt is shipped to WACKER’s Burghausen plant for producing silicones, hyperpure silicon, organic intermediates and pyrogenic silica. And to ensure the site stays economically viable, we supplement our mining activities by backfilling the worked out tunnels with mineral refuse,” continues Höllerbauer. For this to be done efficiently and safely, however, an additional tunnel into the mine was needed, the Clara tunnel, which has connected the salt mine with the surface since August 12, 2008.
Let’s look back at summer 2008: WACKER engineers at the Burghausen construction-applications lab were developing and testing polymeric dispersions and dispersible polymer powders in new applications. As tunneling and mining are interesting fields, the researchers tried out a novel WACKER product in the new Stetten tunnel: polymer-modified sprayed concrete.
This concrete stabilizes tunnel walls. Very soon after blasting, tunnel builders spray it onto the exposed rock. “One problem here was that we had to penetrate the impermeable layers above and below the rock – and there, of course, we came across ground-water,” explains Höllerbauer. The water leaves dark patches on the concrete and lots of mud on the tunnel floor. A few hundred meters further along the tunnel, however, the concrete suddenly becomes more uniform and dryer. “This is where we had a massive inrush of water,” explains Höllerbauer. “Fresh concrete is very fragile and wouldn’t stick to wet walls.”
WACKER application engineers had already been in the tunnel to try out the new polymer-enhanced sprayed concrete on a small area of tunnel wall. Though only because it was a good opportunity to try out the new product under realistic conditions in WACKER’s own tunnel. “However, the tests showed the new polymer-enhanced sprayed concrete to be so promising that it could be the ideal solution to impregnate our tunnel,” recalls Höllerbauer, as WACKER experts are drilling small holes in the new tunnel wall in order to take samples back to Burghausen.
“We sprayed 40 meters of tunnel with this concrete," recalls construction manager Christian Schönlechner of Jäger Bau GmbH, the company commissioned to build the tunnel. “ETONIS-modified sprayed concrete is a little thicker and stickier than the concrete we usually use. One advantage is that we need less accelerator to make the concrete set and harden.” The wetter the concrete is, the more accelerator it must contain. However, concrete only tolerates a limited amount of this additive. If the complex formulation contains too much accelerator, the concrete gets too hot and doesn’t bind to form the desired hard, impervious layer. Schönlechner stresses another advantage of the novel material: “ETONIS-modified sprayed concrete is softer, and there is significantly less rebound – far less of the concrete we sprayed fell off the wall.”
The core samples taken from the tunnel are carefully analyzed in Burghausen. “The concrete we used in the Stetten tunnel is based on normal sprayed concrete,” explains Klaus Bonin, head of a WACKER applications lab in Burghausen. “The only difference is that we used better quality ingredients because we wanted the best possible concrete for the conditions in the tunnel,” he continues. It was thanks to the many years of research that this new concrete additive was so easy to use. WACKER specialists have been working on these formulations for four years. Originally, this work was only intended to reduce rebound when concrete is sprayed onto tunnel walls. “What is a construction material one moment is just wasted rebound only seconds later, and that is very annoying and costly,” says Bonin. “Because the polymer is sticky, we hoped to significantly reduce the rebound." The Stetten tests confirmed what construction engineer Schönlechner had observed in practice: only half as much ETONIS-modified concrete ended up as rebound on the tunnel floor. It came as a pleasant side effect that the enhanced sprayed concrete also adhered better to wet walls and provided a superior shield against wetness in the tunnel.
Tests performed while the novel material was under development likewise showed ETONIS-modified concrete to be a powerful problem solver in tunnel construction. “I use core samples to measure the concrete’s increase in strength over time,” says Bonin, as he clamps a flashlight-sized concrete cylinder from Stetten in a large press. “Pressure is exerted on the cores until the material yields and the first cracks appear,” he explains before starting the press. This is what happens in the tunnel, too, when the tunnel tubes exert pressure on the new concrete. “In six months’ time we’ll take new core samples, and then we’ll be able to comment on the improvement of the concrete’s longterm stability.”
The WACKER specialists in Burghausen are well able to confirm one thing already: with its waterproofing properties and reduced rebound, the polymer-modified sprayed concrete is a promising and cost-cutting solution for many tunneling projects.