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Einfluss von Hüttensand und ultrafeinem Kalksteinmehl auf die Hydratation von jungem Spritzbeton
(2018)
The contact of underground water with shotcrete leads to leaching of calcium hydroxide and can increase the precipitation of calcite in the secondary drainage system of tunnels under certain conditions. Indeed precipitations in the drainage pipes lead to high maintenance costs. Within a research project the leaching behavior of new and innovative binder compositions was tested in lab tests. One possible change in mix-design is the reduction in clinker content by using a lower amount of Ordinary Portland Cement together with cementitious materials. Successful combinations were further investigated in sprayed mortar and shotcrete tests to examine the influences of the spraying process and the addition of accelerator. The results indicate how shotcrete mix-design can be improved regarding its potential for precipitations and subsequently enhanced durability of shotcrete.
Im Rahmen des FFG-Forschungsprojektes „Entwicklung neuer dauerhafter und nachhaltiger Spritzbetone (Advanced and sustainable sprayed concrete; ASSpC)“ werden Methoden zur Prüfung der Calcium-Auslaugung von Spritzbeton eingesetzt bzw. weiterentwickelt. Bestimmt wurde dafür das Reduzierte Versinterungspotential nach Regelwerken der Österreichischen Bautechnik Vereinigung und das Auslaugverhalten von Spritzbeton bei leichtem Säureangriff (Salpetersäure und Kohlensäure), um dauerhaftigkeitsrelevante Parameter im Bereich der Wechselwirkungen von Spritzbeton mit leicht sauren (Kohlensäure- und Nitrathaltigen) Oberflächen-, Grund- und Sickerwässern zu untersuchen und potentielle Schädigungsmechanismen zu identifizieren und zu quantifizieren. Die Vor- und Nachteile der verwendeten Prüfmethoden zum lösenden Angriff werden in Bezug auf die Dauerhaftigkeit von Spritzbeton diskutiert.
Improved understanding of the mechanisms underlying deleterious chemical attacks is necessary to better predict the long-term performance and durability of shotcrete in aggressive environments. In the scope of the research project “ASSpC - Advanced and Sustainable Sprayed Concrete”, new durable and sustainable shotcretes and new test methods are developed. In order to increase shotcrete durability, the spray cement (specified in Austrian Guideline “Sprayed concrete”) used is substituted with supplementary cementitious materials (SCMs). However, in the past SCMs have seen only limited use in the application of dry-mix shotcrete. This contribution presents 6 new mix designs intended to be used for dry-mix shotcrete, each with varying amounts and types of SCMs, and compares their performance and sprayability with a commercially available spray cement. As sulfate attack is one of the major threats to shotcrete (e.g. in tunnelling), the resistance of the new mixes against sulfate attack is tested using a modified version of the Swiss sulfate test according to SIA 262/1:2013. This approach provides new insights into damaging mechanisms of dry mix shotcrete in sulfate-loaded environments
Filler Effect in Shotcrete
(2019)
The effects of fine limestone powder on the early hydration of cementitious systems accelerated by means of alkali-free aluminum sulfate based products, commonly used for shotcrete applications, were investigated in the course of laboratory and real scale tests. In binary (CEM I + limestone) and ternary (CEM I + limestone + slag) systems the addition of fine limestone led to an enhancement of the hydration degree and strength development at early times (<24 h). The formation of ettringite, aluminate hydrates, and C-S-H is affected by the joint action of the setting accelerator and the fine limestone. Accelerator and limestone, in combination with the cement, can be optimized to enhance ettringite and silicate reaction, in some cases coupled with aluminate reaction inhibition, to produce mixes suitable for sprayed concrete applications. Such optimization can help to reduce the cement content in the mixes without compromising the early strength development of the shotcrete.
Before new mixes are sprayed in real scale applications, lab experiments have to be carried out to provide a basis for predicting a good performance in real scale. In many cases, to understand the correlations between the behavior in the lab and in real scale spraying, a further set of experiments after the real spraying is required. In this paper we present such a two way process for two new wet mixes, one with a very low tricalcium aluminate Portland cement and the other one with a CEM I with addition of ultra-fine calcite. Calorimetry and Shear-Modulus measurements were used in the lab to monitor hydration evolution. In the real scale spraying compressive strength was measured at periodic intervals up to 24 hours. The hydration of the sprayed samples was stopped at 3, 6 and 24 hours and samples were scanned with X-ray for the identification and quantification of the phases present. The results presented are used to discuss the extent to which lab experiments can predict the behavior of mixes in real scale and how small changes in the raw materials and the mixes can lead to quite different results.