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2016WinterShotcreteEMag

Goin’ Underground Reason 2—Quality Assurance In plants that are storing silica fume in silos, problems may arise when traditional silica fume cakes in the silo or when the absorbed moisture causes the formation of lumps. Due to its liquid form, pozzolanic-based rheology control agent can eliminate such deficiencies, which may cause variations in shotcrete performance. It is more effectively dispersed than silica fume, reducing the mixing time required and minimizing the risk of lump formation in concrete or shotcrete. In addition, rheology control agent is manufactured under stringent quality control in an industrial process using high-quality raw materials to obtain a material with high fineness and purity. The tolerances for the particle size distribution are very tight to ensure that the specific surface area remains the same from batch to batch, unlike for silica fume being a by-product, which can have batch-to-batch variations and can contain impurities causing variations in the shotcrete performance. From a physical point of view, differences between the two materials are shown in Table 1. Reason 3—Dosage Efficiency Silica fume is often used to replace ordinary portland cement within the range of 5 to 10% to improve the performance of shotcrete mixtures. However, the selected grade of pozzolanic-based rheology control agent only requires one-tenth of the silica fume dosage rate (for example, 3 kg/m3 5 lb/yd3 of TYTRO RC 430 is needed to replace 30 kg/m3 50 lb/yd3 of silica fume in shotcrete mixtures) to provide equivalent performance, which makes it a more economical and sustainable solution. Reason 4—Enhancement of Shotcrete Performance The use of a pozzolanic-based rheology control agent provides several performance benefits compared to silica fume (designated as “SF”) and ordinary portland cement (designated as “OPC”) as presented in the following, based on the results obtained from the following case studies: Case study A—0.67% TYTRO RC 430 was used to replace 5% SF in a binary mixture • Mix with 5% SF (water-cementitious materials ratio w/cm of 0.40; total cementitious materials content: 473 kg/m3 29.5 lb/ft3) • Mix with 0.67% TYTRO RC 430 (w/cm: 0.40; total cementitious content: 450 kg/m3 28.1 lb/ ft3) Case study B—0.67% TYTRO RC 430 was used as an addition (due to minimum cementitious materials content requirement) in a plain mixture where OPC was used • Mix with 100% OPC (w/cm: 0.46; total cement content: 459 kg/m3 29.7 lb/ft3) • Mix with 0.67% TYTRO RC 430 (w/cm: 0.46; total cement content: 459 kg/m3 29.7 lb/ft3) Case study C—0.67% TYTRO RC 430 was used to replace 6% SF in a ternary mixture • Mix with 6% SF (w/cm: 0.35; fly ash designated as “FA”: 23%; total cementitious materials content: 521 kg/m3 32.5 lb/ft3) • Mix with 0.67% TYTRO RC 430 (w/cm: 0.37; FA: 23%; total cementitious materials content: 489 kg/m3 30.5 lb/ft3) Note: Test methods were selected based on the project requirements. Because the specified performance characteristics varied based on the case study, the following section presents results from the selected case studies where testing was conducted. a. Enhanced sprayability and reduced rebound For sprayability, a viscous and sticky mixture with high cohesiveness is desired as shotcrete mixtures for reduced rebound. In addition, segregation of the shotcrete mixture under pressure is reduced. However, for pumpability reasons, the viscosity of the shotcrete mixture needs to be limited due to the maximum allowable pump pressure. When added to the shotcrete mixture, rheology control agents provide greater cohesiveness of the shotcrete mixture, lower rebound, and higher thickness buildup (Fig. 1). A possible explanation for these effects could be related to the smaller particle size associated with the higher specific surface area of the rheology control agent working as a nucleation site for the precipitation of calcium silica hydrate (CSH) gel, and having stronger Van der Waals and electrostatic ionic forces between particles.2 Considering that the main source of cohesion in cement paste is the calcium silicate hydrate (CSH) gel,3 it is expected for pozzolanic-based rheology control agent to increase cohesion due to its impact on accelerating Table 1. Comparison of Properties between Silica Fume and Rheology Control Agents Silica fume Rheology control agents By-product Engineered material Contains impurities High purity Powder Liquid Difficult to handle Easy to use Variable particle size and Uniform particle size and distribution distribution Shotcrete • Winter 2016 49


2016WinterShotcreteEMag
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