A custom-built waterblasting unit used for concrete removal shaved months off a tunnel repair schedule. Combining the fast removal with wet shotcrete repair methods helped workers to repair up to 25 feet of tunnel every 24 hours. The tunnel repairs were made in a section of a Los Angeles aqueduct where the 76-year-old concrete lining had cracked and was starting to spall.
A hydrodemolition equipment manufacturer developed a small remote hydrodemolition unit that fit into the 8x8-foot tunnel space. Operated from a remote location, it had an arm that could rotate 360 degrees and was equipped with two high-pressure water nozzles. Using 17,000-psi water pressure, it could remove deteriorated concrete up to 20 inches thick in 30-inch--wide sections.
To remove the deteriorated concrete, the machine made two passes through the tunnel. The first pass cut through a 30-inch-wide section, moved forward 30 inches, skipping a section of concrete, and removed another 30-inch-width. The remaining concrete ribs supported the tunnel. After the first pass, each cut-out area between the ribs of old concrete was shotcreted with a concrete containing silica fume, a superplasticizer, and 1 «-inch deformed steel fibers. The second pass with the hydrodemolition unit removed the deteriorated concrete ribs between the new concrete. Then those spaces were shotcreted with new concrete to complete the job.