Durability of clinker reduced shotcrete: Ca2+ leaching, sintering, carbonation and chloride penetration

Abstract

The reduction of clinker use is mandatory to lower the negative environmental impact of concrete. In shotcrete mixes, similarly to the case of conventional concrete, the use of supplementary cementitious materials (SCMs) and proper mix design allow for the substitution of clinker without compromising the mechanical properties. However, the impact of the substitution on the durability of shotcrete needs to be further assessed and understood. The results from the present study, obtained from real-scale sprayed concrete applications, show a reduction of the Ca2+ leaching and sintering potential of clinker-reduced shotcrete mixes due to the presence of SCMs. This positive effect, crucial for low maintenance costs of tunnels, is mainly related to a reduced portlandite content, which on the other hand negatively affects the carbonation resistance of shotcrete. Additionally, the hydration of SCMs positively influences the chloride penetration resistance presumably due to a combination of microstructural changes and changes in the chloride binding capacity. Differences found in the pore size distribution of the various mixes have low impact on the determined durability parameters, in particular compared to the effect of inhomogeneities produced during shotcrete application.

Prozesse der Versinterung – Kenntnisstand und aktuelle Herausforderungen

“…Versinterung orientieren sich somit einerseits verstärkt an der Entwicklung und dem Einsatz neuartiger Baustoffe, die ein deutlich reduziertes Kalzium-Leaching-Verhalten aufweisen, andererseits an sogenannten Inhibitoren, auch Härtestabilisatoren genannt, um die Bildung von CaCO 3 -Abscheidungen entlang der Fließstrecke im Tunnel zu verzögern und damit zu verringern bzw. zu verhindern [7][8][9]. Ferner wurden große Fortschritte bei der Planung und dem Design der Drainage bezüglich deren räumlicher Anordnung und optimierter Betriebsbedingungen gemacht, z.B.…”

“…Currently successful approaches to prevent such scale deposits thus increasingly focus on the development and use of novel building materials which exhibit significantly reduced calcium leaching behaviour, as well as the use of inhibitors (also referred to as hardness stabilisers) to suppress and thus reduce or prevent CaCO 3 precipitation along the tunnel drainage system [7][8][9]. Furthermore, significant advances have been made in the planning and design of drainage systems in terms of optimising their spatial arrangement and operating conditions, e.g.…”

“…Specific issues and aspects can be investigated and assessed using methods designed specifically for the project in question, such as test tracks and field trials in the tunnel itself, simulation experiments at laboratory scale or the development and use of computer-based modelling tools. Examples include modifying shotcrete and pipework formulations, efficient inhibitor dosing, drainage design and flushing processes [7,8,11,13,16].…”

“…Spezifische Fragestellungen und Aspekte können maßgeschneidert über Teststrecken und Feldversuche direkt im Tunnel, Simulationsexperimente im Labormaßstab oder die Entwicklung und Verwendung von Computer basierten Modellierungswerkzeugen untersucht und bewertet werden. Beispiele hierfür sind die Anpassung von Spritzbeton/Rohrleitungs-Rezepturen, effiziente Inhibitorzugaben, Drainagedesign und Reinigungsvorgängen[7,8,11,13,16].…”