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2013WinShotcreteEMag

• The rough, irregular surface profile provides an (a) excellent mechanical bond for all types of repair materials; • Surface microfracturing caused by mechanical removal methods is eliminated; • Exposed aggregates are not fractured, split, or damaged; • Lower strength and deteriorated concrete (delamination) is selectively removed; • Vibration to the surrounding structure is eliminated; • Reinforcing bar is cleaned, eliminating the need for sandblasting; • Reinforcing and other embedded metal ele- ments are undamaged; and • There is increased speed of concrete removal, which can reduce construction time. The hydrodemolition work was very challenging because the spillway had multi-curved surfaces that were 25.6 ft (7.8 m) high. The robotic cutting head (b) was guided by steel pipe rails, which were curved and installed over the curved spillway surface; then, the head was pulled along the rails by a winch system, as shown in Fig. 2(a). The spillway surface prepared by the hydrodemolition was perfect for bonding of shotcrete, as shown in Fig. 2(b). the First rehabilitation Project with Shotcrete in Korea Wet-mix shotcrete has become a common process in tunneling in Korea since 1997; how- ever, it is very rare to use it outside of tunnel work. This rehabilitation work was the first shotcrete project to take place in Korea using high-perfor- mance shotcrete outside a tunnel environment. The coarse aggregate was a crushed limestone Fig. 2: Challenge of hydrodemolition at a multi-curved spillway: with a maximum size of 0.4 in. (10 mm) and the (a) robotic head at a curved surface; and (b) after hydrodemolition fine aggregate was a natural sand having a specific gravity of 2.60. The fine aggregate to the total weight of aggregates ratio was 65%. Shotcrete mixture designs generally require the use of a well-graded aggregate size distribution. The proposed shotcrete mixture also contained silica fume to reduce rebound, increase resistance to water washout, and decrease the permeability of the mixture. The use of silica fume also allowed the nozzleman to shoot the mixture at its wettest stable consistency, which contributes to proper reinforcing bar encapsulation in addition to facilitating surface finishing. The shotcrete mixture was made with a mobile concrete mixer (refer to Fig. 3). One major advan- tage a mobile mixer has is that it can decrease or increase the slump of the mixture on demand, according to the dry/wet conditions of aggregates. Another advantage is that many shotcrete jobs are performed far away from a ready mix concrete batch plant. It can be extremely difficult to reach these remote sites with workable concrete using conventional means of producing concrete. Fig. 3: A mobile mixer supplying concrete to the shotcrete pump Shotcrete • Winter 2013 49


2013WinShotcreteEMag
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