Hydrodemolition “robots” are machines that basically hold the high-pressure water jet nozzle and during the hydrodemolition process, and uniformly move the water jet over the surface.  The width of the cutting path ranges from 3 inches to 12 feet.  Once the robot starts the hydrodemolition the nozzle moves back and forth over the surface at a set speed.  This nozzle speed is continuously reproduced as the robot moves over the surface.  If the concrete strength is uniform and there is no deterioration then the removal should be uniform with the depth ranging ± ½ the diameter of the coarse aggregate.  However this situation rarely exist.  Normally the concrete is being removed because the structure is deteriorating.  There may already be delaminations as well as previous repairs.  In areas where there is deterioration, delaminations or spalls, the compressive strength maybe very low or zero.  In these areas the concrete will be removed more rapidly then the surrounding area resulting in removals deeper then expected or specified.  If an area was previously patched with a high strength fast setting repair material the compressive strength could reach 7,000-8,000-psi whereas the surrounding material is 4,000-psi.  This results in a wide variation of the compressive strength in the repair area. 

Since the hydrodemolition nozzle is moving at a uniform rate over the surface it cannot adjust to meet these changing conditions in strength and deterioration.  To resolve this issue, the hydrodemolition is generally set to remove the specified depth in good concrete.  This insures that the original surface as well as deteriorated surfaces will be removed to the minimum depth specified (deteriorated surfaces will be removed deeper).

However if the surface was previously repaired the patch material may not be removed to the specified depth.

If these areas are large it may be possible to move the hydrodemolition robot back and recut these areas.  However there are two possible problems with this procedure.  First, since the patch material is much harder then the surrounding material the speed of the hydrodemolition equipment must be slowed significantly to remove the harder material.  If the water jet runs past the edge of the repair it is possible that the water jet will blow-through the deck.  Second, once the water jet cuts through the hard material and is still traveling at a slow speed, it may blow-through the deck below the patch.  Consideration should be given to removing the patches using conventional methods prior to the start of hydrodemolition.  This eliminates the hard material and allows the water jet to properly prepare the surface under the patch.