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2016WinterShotcreteEMag

Goin’ Underground Fig. 1: The effect of pozzolanic-based rheology control agent on thickness buildup and forming additional CSH gels, and its reactant surface particles exhibiting stronger tendency for adsorption of ions and increasing the surface adhesion between adjacent particles, and to other materials. Studies are currently being carried out to determine the exact mechanisms of the pozzolanic based rheology control agent leading to the observed enhanced sprayability and reduced rebound characteristics. As shown in Fig. 2, in Case Study A, mixtures containing 5% SF and 0.67% TYTRO RC 430 were both efficient in reducing rebound to as low as 5 to 6%. The reference mixture containing SF was already optimized. Because the rebound loss of the baseline mixture is already considered to be very low, the impact of the rheology control agents in achieving similar low rebound loss at much lower dosage rates is a significant improvement. However, Case Study B shows that the addition of the pozzolanic-based rheology control agent significantly reduced the rebound from 20 to 6%. Results show that the degree of improvement on rebound loss is related with the mixture design and mixture constituents. When the performance of the rheology control agents on rebound is compared with those of mixtures containing portland cement only, or mixtures containing silica fume with rebound losses higher than the one obtained in this study, the decrease in rebound may be more dramatic. b. Higher early-age strength The compressive strength test results based on the penetrometer needle up to 3 hours followed by Hilti stud4 are shown in Fig. 3. The rate of strength development during the first couple of readings is relatively low in both mixtures. However, starting from 2 hours, rapid strength gain has been observed, especially for the mixture containing TYTRO RC 430 compared to the silica fume mixture. This could be due to the following properties of the pozzolanic-based rheology control agent4: • It has high pozzolanic activity because its ultrafine particles have high specific surface area and they are fully hydroxylated; • It reacts with the calcium hydroxide released by the cement hydration and forming additional calcium silicate hydrate (CSH) gel; • It serves as nucleation sites to CSH gel; and • It accelerates the primary CSH gel formation. Figure 4 shows the compressive strength5 tested 1 day after spraying. As expected, mixtures with pozzolanic-based rheology control agent outperformed their corresponding reference mixtures. c. Equal or higher long-term strength Figure 5 shows the impact of the rheology control agent on the compressive strength5 at 28 days. As desired, while accelerating the hydration process, which increased the early-age strength of the shotcrete, the pozzolanic-based rheology control agent provided equal (compared to mixtures with silica fume) or higher (compared to the mixture containing portland cement only) later-age strength based on three case studies incorporating various mixture designs. This trend shows that the impact of the rheology control agent on the compressive strength is more prominent at early ages, and most importantly, unlike other rapid setting materials, it is not harming the later-age strength. This makes it ideal for underground applications, where minimal time for re-entry to the mine or tunnel is desired.7-9 d. Similar durability Figure 6 shows that the water penetration depth10 of the mixture containing TYTRO RC 430 was slightly lower than that of the silica fume mixture. Figure 7 shows the boiled absorption and the volume of permeable voids test results11 from 50 Shotcrete • Winter 2016


2016WinterShotcreteEMag
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