As described below, large-scale tests prove that polymer-modified shotcrete slashes costs for tunnel lining construction, and offers additional properties such as water-proofing properties and increased adhesion on difficult surfaces.

Sprayed concrete is a construction material of major importance, and its technical requirements are high. It must provide the desired strength, stability and impermeability. However, application specialists from Wacker Chemie had an additional idea; they wanted to reduce rebound during spraying while also making the sprayed concrete water resistant. They developed polymeric binders for sprayed concrete to be used in tunnel construction. First tests in practice in a new tunnel for a salt mine proved the efficiency of the new product.

Less rebound
An important quality factor in the performance of sprayed concrete is rebound. Rebound excess material cannot be reused and must be disposed of as waste. Reducing rebound is therefore important for both technological and economic reasons.

The newly developed product line Etonis ensures a major enhancement in rebound properties. Moreover, Etonis-modified sprayed concrete has some additional interesting concrete properties. Besides outstanding processability without the risk of separation, it offers good compaction characteristics and high compressive strength. The polymer-modified sprayed concrete adheres excellently, even to difficult substrates such as wet tunnel walls and has an improved carbon dioxide resistance. The new polymer binders can be shipped in either liquid or powder form.

Development tests
First experiments started more than four years ago in Wacker’s application labs for construction materials in Burghausen, Germany. Based upon many years of experience using polymers in mortar applications, a variety of polymer binders in sprayed concrete formulations have been analysed and tested intensively. The first development tests were designed to determine the amount of polymer that reduces sprayed concrete rebound and still provides the desired concrete properties.

The tests were carried out in the Hagerbach Test Gallery in Switzerland, to work out the effects of polymer on the properties of the concrete compared to a commercially similar, arbitrarily selected reference mixture. Various wet-spray concrete formulations were applied in standardised tests to study their effects on the sprayed concrete’s properties. Design of experiments was carried out to determine the parameters needed for the planning of the next level of tests. Two to three cubic metres of concrete were sprayed in each trial. To ensure a homogeneous mixture, only midstream concrete was used.

In the course of the investigations it became apparent that the addition of the new polymeric binders significantly reduced rebound. As indicated in the figure below, tests with polymer-modified concrete formulations show that with increasing amounts of polymeric binder, the rebound first declines, reaches a minimum and then increases again (Fig. 1). The figure shows a comparison between the solid powder polymer FSZ and the liquid polymer type FLZ, relative to the reference mixture (0-mixture). Additionally, two different plasticisers were tested; a standard plasticiser FM and a combination of plasticiser and retarder FMVZ.

In general sprayed concrete loses aggregates through rebound, which causes the cement concentration in the applied sprayed concrete to rise. As the cement concentration rises, the strength of the concrete increases as well. Therefore, rising rebound equates to increased ultimate strength. Due to process reasons, most convenient were the concrete formulations that had been modified with liquid polymeric binders.

Tunnelling application
To test the new product under realistic conditions, large-scale testing had to be done. An adequate opportunity was given when a new access gallery was built in a salt mine in Stetten (southwestern Germany), which is owned by Wacker Chemie. In 2007 the construction of a new, 905m-long access route allowed standard road trucks to pass through the tunnel. During the construction work the underground construction workers had to penetrate the impermeable layers above and below the rock. There they came across groundwater. Usually, very soon after blasting, concrete is sprayed onto the exposed rock. Unfortunately fresh concrete does not stick effectively to wet tunnel walls. That’s where the novel polymer-modified sprayed concrete proved its additional value-added characteristics.

For this large-scale application aqueous polymer was used for the concrete mix. Inspection of the sidewall surfaces revealed that the standard sprayed concrete exhibited a large number of wet patches. In contrast the polymer-modified sprayed concrete demonstrated much better adhesion to the wet substrate, did not show any wet patches and proved to be less prone to crack formation.

In addition to the decrease in rebound these trials showed that the polymer-modified sprayed concrete leads to improved adhesion on wet ground and to an increased impermeability against water. Thus the new polymer-modified sprayed concrete did not only stabilise tunnel walls, but is also a promising solution to impregnate the tunnel walls and to provide a barrier against water leakage.

The tests in the tunnel confirmed significantly less concrete ended up as rebound on the tunnel floor. It came as an important additional effect that the polymer-modified sprayed concrete also adhered better to wet walls and provided a superior shield against wetness in the tunnel.

Polymer-modified concrete thus possesses valuable potential, both technical and commercial. The additional costs of modifying the sprayed concrete are offset by the time and material saved, and converted into significant profits.

The polymer-modified sprayed concrete is easy to handle and versatile; it can be applied vertically to tunnel crowns, is suitable for both tunnelling and mining, as well as for slopes and canals or culverts. In all these applications the novel Etonis–brand polymers for sprayed concrete speed up construction and reduce system costs immensely. Furthermore they reduce the amount of further additives required in the formulation.

With its waterproofing properties and reduced rebound, the polymer-modified sprayed concrete is a promising and cost-cutting solution for many infrastructure projects like tunnelling and underground construction.

Analysing tunnel shotcrete core samples in the Wacker Chemie Burghausen applications lab to determine the sprayed concrete’s waterproofing properties and its strength increase over time Spraying polymer-modified concrete onto a tunnel wall in the Stetten salt mine to stabilise the tunnel and prevent water leakage