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Fig. 6: All mixtures tested in this study Fig. 5: Rebound chamber before spraying (a) (b) Fig. 7: (a) Expansion of a mixture close to the expansion line; and (b) expansive mixture compare to a stable mixture Table 4: Average compressive strength Compressive strength, MPa Specimens 1 h 2 h 3 h 6 h 1 d 7 d 28 d Prisms 12.9 24.9 29.6 — — — — Cores — — 35.6 47.6 54.6 51.9 52.1 Note: 1 MPa = 145 psi. Phase 2—Compressive Strength response. Structural damages caused by fire in The spraying was successful using standard tunnels or by natural disasters (e.g. flooding or dry-mix shotcrete equipment. Particularly, the earthquakes) on essential structures could very use of a hydromix nozzle where water is intro- rapidly be alleviated using such a mix design duced 10 ft (3 m) prior to its delivery through in combination with the dry-mix shotcrete the nozzle allowed the production of homoge- application method. In fact, although not mea- neous good quality shotcrete with no clogging sured because of its rapid onset, the setting time in the equipment. Table 3 presents the average of the mixture used is very short (<10 minutes) compressive strength obtained using the end which translates most probably to a few mega- beam test (Fig. 4) and the average compressive pascals of compressive strength with minutes strength using three cores taken from panels. of its placement. The results presented in Table 4 are actually Obviously, the use of dry-mix shotcrete repre- quite impressive and are deemed a success for sents an extremely well-adapted method for the mining and tunneling industry; indeed, placing such potent mixtures. Indeed, the fact that typical early-age and compressive strength all of the dry materials are conveyed with air obtained with ordinary Portland cements based through the hose to the nozzle where water is mixture design and set accelerators are usually added gives a very short contact time between the around 435 to 580 psi (3 to 4 MPa). Compres- water and the cementing materials before it is sive strengths over 4350 psi (30 MPa) at 3 hours sprayed onto the surface. By using pre-blended with 28-day compressive strengths of some bulk bags that can be stored for long periods of 7250 psi (50 MPa) are extremely promising. time, it becomes easy to have concrete material On the other hand, the 1-hour results also show ready in case of emergency. Combined with the very interesting potential for emergency very simple mobilization of dry-mix shotcrete and 18 Shotcrete • Fall 2014


2014FallShotcreteEMag
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