When a massive fire erupted in a major tunnel in the Newhall Pass on Interstate 5 near Santa Clarita, Calif., the heat from the flames reached temperatures up to 1400° C (2550° F), causing the concrete in the tunnel to split. Shortly after the fire burned out, the tunnel was inspected by California DOT (Caltrans). The inspection revealed a repair estimate that was approximately $19 million, and included the removal of the entire concrete skin of the fire-damaged tunnel, followed by reskinning with a new sprayed concrete lining.
HydroPressure Cleaning (HPC) Inc., Camarillo, Calif., contacted Caltrans to explain the benefits of using the hydrodemolition technique of high-pressure water jetting to selectively remove only damaged or unsound concrete.
Paul Phelps, general manager of HPC, explains that there is a rule of thumb in the hydrodemolition industry: it takes water pressure that is 3½ times the compressive strength of the concrete to remove concrete. After the parameters have been set for a specific job—based on good concrete—the 15,000- to 20,000-psi water jet will seek out and remove the weaker or inferior concrete and leave the sound concrete intact.
“We were given the go ahead to start with the concrete removal tests with our Conjet 322 hydrodemolition robot,” says Phelps. “We initially used different water pressures of 20,000, 15,000, and 12,000 psi with the robot and our 500 hp high-pressure pump and removed concrete from the walls in 2-feet-wide, 5-feet-high patches every 50 feet. This enabled Caltrans engineers to see the difference in concrete condition, and together with core samples, allowed them to assess and verify the damage and develop the scope of work for the overall repair. We also did a test at 19,000 psi using a nozzle with a narrow fan jet pattern instead of the normal straight jetting nozzle. This was great, especially in those areas where they just wanted to roughen up existing concrete surface to get a bond for the new overlay.”
According to Phelps, repairs done with hydrodemolition can last more than double those of pneumatic methods because there is no microfracturing, which weakens the bond with repair mortars and existing rebar. Also, the remaining surface allows for good bonding similar to surfaces prepared with pneumatic tools.
Henry Kirzhner, Caltran's senior bridge engineer, says, “As we could initially use different pressures we were able to determine how deep we should go to remove just the damaged concrete and quickly evaluate the extent of the damage, which was not consistent. The biggest advantage for us was that it took away just the bad concrete and didn't damage any of the rebar or good concrete left behind. Any conventional removal methods, like jackhammers, could damage reinforcement and it was vitally important for us to do removal without any damage to the portion of the structure remaining in place.”
The tunnel reopened for traffic more than two weeks ahead of schedule. “I would recommend using hydrodemolition robots again on other projects like bridge deck rehabilitation,” says Kirzhner.