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Sustainability Table 2: Cost Comparison Mixture component Reference mixture Proposed mixture Dosage, lb/yd3 (kg/m3) Cost, %$/yd3 (%$/m3) Dosage, lb/yd3 (kg/m3) Cost, %$/yd3 (%$/m3) Portland cement 703 (417) 45 (45) 703 (417) 45 (45) Silica fume 56 (33) 22 (22) 0 — Aggregates 2865 (1700) 33 (33) 2983 (1770) 34 (34) TYTRO® RC 430 0 — 6 (3.6) 10 (10) Total — 100 (100) — 89 (89) Outcome 4—Cost Savings Considering that an equivalent performance is obtained with the reduced binder content, the proposed approach would not only be a sustainable but also a cost-efficient solution. A simple cost analysis was done to compare a mixture containing 56 lb/yd3 (33 kg/m3) of silica fume that was replaced with the addition of 6 lb/yd3 (3.6 kg/m3) of TYTRO RC 430. Table 2 shows the cost comparison between the reference mixture containing silica fume and the mixture with the reduced binder content. Because the cost of the materials varies based on the geographical regions, a relative comparison was done based on the percentage of US$ cost per a given material to produce 1 yd3 (0.8 m3) of shotcrete. Based on the comparison, at least 10% less expensive shotcrete production is expected in the mixture with reduced cementitious materials content accompanied with TYTRO RC 430. Proposal 2—Reduction of Rebound Shotcrete has material loss due to rebound of aggregates because the concrete is pneumatically applied.14 Although a certain percentage of rebound is inevitable and even necessary because a higher paste content is needed to create a sticky surface for subsequent shotcrete material to become compacted into the surface, it is desirable to keep the rebound to a minimum.15 The rebound loss is affected by many factors, such as the position of the application, angle of the nozzle, skill and expertise of the nozzleman, air flow, impact velocity, thickness of layer, amount of reinforcement, and mixture design (for example, cementitious materials content, water content, size and gradation of aggregates, and type and dosage of admixtures). In many field applications, it is common to have a rebound rate of 20 to 30%. While high rebound rates are considered to be a waste of material, time, labor, material and removal cost, and a burden on environment, it should be noted that they also adversely affect shotcrete performance by increasing the tendency toward shrinkage cracking because the paste content of the in-place shotcrete is higher due to rebound, resulting in loss of aggregates.16 Outcome 1—Rebound Reduction To reduce rebound, a “sticky” and cohesive shotcrete is desirable, as it will exhibit a lower tendency to bounce off the wall on impact, thus providing a mixture with a better coating of the reinforcing bar surface and fewer voids than shotcrete displaying poorer cohesive characteristics. Considering a high volume of rebound material consists of mainly aggregate particles, paste stickiness and aggregate gradation play a more important role on rebound reduction than the amount of cementitious materials content as long as a sufficient amount of paste is used that would fill the voids between the aggregate. In other words, the “quality of paste” is more important than the “quantity of paste” for evaluating rebound characteristics. According to the previous experiments conducted in the field, plain portland cement mixtures with a high cement content of 760 lb/yd3 (450 kg/m3) exhibited a rebound rate of 20% on average. However, when the cement content was reduced by the addition of TYTRO RC 430, the rebound rate was decreased from 20 to 5% due to the increased cohesiveness promoted by the pozzolanic based rheology control agent. Outcome 2—Reduction of Carbon Dioxide Emissions Carbon dioxide emission factors of various mixture components and construction activities are listed in Table 3. Shotcrete • Spring 2016 35


2016SpringShotcreteEMag
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