Quality Waterjet Newsletter – 06/23/2009 Hydro-Demolition of Concrete Surfaces Using waterjets for hydro-demolition is not news any more. A review of Schmid’s article* of 20 years ago refreshes our memory and provides some basic facts about this technology for those who are interested. But keep it in mind that some numbers may need updating. There were about 250,000 bridges that needed to be repaired in the USA and the money for the repair work amounted to approximately 4.4 billion dollars. Freeze and thaw cycling causes micro-cracking in the concrete structure. These micro-cracks allow water and chloride based de-icers to reach and corrode the steel reinforcing bar lattice. The expanding force of the corrosion further cracks the surrounding concrete. To repair the damaged concrete structure, the top layer of concrete needs to be removed to expose the steel rebar from top and below. The traditional tools for this type of repair work are jackhammers. The size of jackhammer is limited to 30 bounds to prevent additional damage caused by the impact. Hydro-demolition equipment includes a high pressure water pump, a cutting head mounted on a cutting robot (tractor), and a carrying truck. The high pressure pump is diesel-driven and runs at a pressure from 13,000 to 35,000 psi and a water flow rate between 13 to 70 GPM (300 – 500 horsepowers). The high pressure cutting head can move on a linear traverse mechanism back and forth perpendicular to the direction of travel of the tractor. The multiple waterjets are positioned such that they can remove the concrete above and below the steel rebar. The productivity of a single hrdro-demolition unit is about 20-30 cubit feet per hour, equivalent to approximately 20 jackhammers. It costs about 12 US dollar per cubit foot. It is more cost effective than the labor-intensive jackhammering where the labor costs more than 12 US dollar per hour. It is more suitable for large projects that need to remove 5,000 cubic feet or more of concrete and when the project schedule is tight. Tests were done to compare the bond strength of an overlaid surface that was prepared with hydro-demolition versus with jackhammering. The test results indicated 2.3 times higher in shear strength and 3.1 timers higher in pull-off strength in favor of hydro-demolition. Peterographic examination also indicated the presence of micro-cracks below the jackhammer prepared surface while none was found below the hydro-demolition prepared surface. A real test of bridge work was done on a freeway bridge in Seattle with a hydro-demolition unit operating at 25,000 psi and between 15 and 44 GPM. A volume removal rate from 12 and 24 cubic feet per hour was achieved by changing the nozzle diameter and therefore the flow rate and horsepower (from 260 to 580 HP) as well as by fixing the horsepower and changing the pressure. (from 13,000 to 55,000 psi). * Schmid, R.F. (1989) High pressure hydromilling of concrete surfaces, Proceedings of the 5^th American Water Jet Conference, Toronto, Canada, August 29-31, Paper 15.

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