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2014FallShotcreteEMag

To correctly assess the strength development of ture design. Later coring of the test panels con- the ternary mixture, compressive strength tests firms the homogeneity of the in-place material. were conducted. Early-age compressive strength Overall, the material shot very well and allowed tests (up to 3 hours) were conducted using beam placement on vertical surfaces without the need specimens. These beam specimens are sprayed in of accelerator; the in-place mixture showed suf- steel mold to realize end beam test (Heere and ficient cohesion and adhesion to stay in place with Morgan 2002), which is a common test in the more than 5 in. (125 mm) thickness while pre- mining industry. Figure 4 shows the setup used senting rebound losses of approximately 20%. to perform the test and an actual specimen tested. To evaluate compressive strength after 3 hours, (a) (b) 3 in. (75 mm) diameter cores were extracted from test panels (ASTM International 2003, 2005). Figure 5 shows the different test panels ready for shotcreting in the rebound chamber in the shotcrete laboratory. Results Phase 1—Results To obtain an accurate representation of the expan- sion pattern of these types of ternary binder, over 49 different mixtures were tested. The fast-setting nature of the OPC-CAC-CS binder ended up requiring the assistance of three persons to cast the Fig. 2: (a) Spraying gun; and (b) hydromix nozzle cubes. Doing otherwise resulted in the setting of the mixture prior to proper placement. Initial setting of less than 5 minutes has been observed on some mixtures. Figure 6 presents all the mixtures tested in Phase 1. Figure 7 presents the wide range of expansion observed for the unstable mixtures. The line leading to expansion is not a straight line because of the variability in the OPC, CAC, and CS oxide in the binders. Because of this difference and the very sensitive nature of the expansion, a straight and clear expansion limit cannot be identi- fied. This means that with any composition change in any of the three components of the binder, this limit must be investigated again. Doing otherwise may lead to catastrophic expansion of the mixture. It should be noted that the limit observed is very similar to what was observed by Lamberet (Lam- beret 2005) with a minor difference in the CS content Fig. 3: Schematic of the electronic acquisition system leading to expansion on the CAC side of the line. This is most likely due to the difference between the Table 3: Dry proportion of sprayed mixture material used in Europe and Canada. For instance, Aggregate, % a minor change in sulphate contents (S03) can have Ternary binder, % Sand Rock an important change in the mixture stability. 19.0 52.7 28.3 Based on the results from Phase 1, a non-expan- sive mixture was chosen for the shotcreting opera- tions. The results and discussion on this selection can be found in previous work (Lemay et al. 2014). Phase 2—Placement and Adhesion The first goal of this shotcreting session was to establish whether the new type of binder allows for proper shooting and placement of dry-mix shotcrete. Using the standard equipment described above, the shotcreting was a success. Indeed, the use of the hydromix nozzle was efficient in reducing dust emission and no plugging was observed; dust and rebound were similar to those obtained with traditional dry-mix shotcrete mix- Fig. 4: End beam test apparatus Shotcrete • Fall 2014 17


2014FallShotcreteEMag
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