Selective Removal v. Hydrodemolition Operating Pressure and Flow Rate

Hydrodemolition is often presented as a concrete removal method that will selectively remove deteriorated concrete without removing the surrounding sound concrete.  Selective removal is defined as the removal of low strength, damaged, or deteriorated concrete while not removing any of the surrounding sound concrete.  This concept is based on preexisting conditions including:

  • *  Spalled, delaminated, or deteriorated concrete;
  • *  Low-strength concrete resulting from poor consolidation, improper mix-design, honeycombing, ASR, and other defects that result in areas or zones of lower strength concrete.

All hydrodemolition equipment currently in use in the United States can remove sound and unsound concrete.  According to ICRI 310.3 2,the rate of concrete removal is inversely related to the compressive strength of the concrete”.  Therefore, if the compressive strength of the sound concrete is 4000 psi and the adjacent deteriorated/unsound concrete is 1000 psi, all hydrodemolition equipment will remove the 1000-psi concrete at a rate four times greater than the 4000-psi concrete.

The hydrodemolition “selective removal” concept was further developed when compared to the use of mechanical methods such as jackhammers, chipping hammers and milling machines.  Mechanical removal does not distinguish between sound and unsound concrete and simply removes the concrete that is directly impacted by the tool.  Milling of the surface leaves a flat plane without removing any low strength or deteriorated concrete below the plane of removal.

It has been suggested that certain operating pressures or flow ranges are more effective at performing “selective removal”.  ACI, ICRI, and the FHWA indicate a broad range of pressure and flow rates for the hydrodemolition process 1, 2, & 3.  These documents do not indicate any difference in the capabilities of the various types of equipment relative to the removal of sound or unsound concrete.  The flow rate, operating pressure, equipment HP, nozzle type, etc. of the equipment will determine the speed at which the hydrodemolition equipment is capable of removing concrete.  How much concrete is removed (sound or unsound) is dependent on how long the waterjet is applied to the surface.

ICRI 310.3 2 further states “scarification may not remove all delaminated concrete due to the rapid speed which the waterjet moves over the surface”.  Delaminated concrete would be defined as “unsound” concrete.  If a sample of the “delaminated concrete was taken and tested, it would be found to have the same strength as the surrounding concrete.  As a result, if the hydrodemolition equipment is calibrated to remove ½” (scarification) of sound concrete, it will remove ½” off the top of the delamination without removing the delamination.  If the delamination is close to the surface (i.e. 1” deep) then it is probable that the hydrodemolition will “pop” the delamination out thus performing ‘selective removal”.

There is a contradiction between the concept of “hydrodemolition” and “selective removal”.  Hydrodemolition equipment currently in use in the U.S. operates at a wide range of pressures and flow rates ranging from 16 – 90 gpm and 14,000 psi to 40,000 psi.  All of the equipment is capable of removing sound concrete.  The ability to remove unsound concrete “selectively” therefore is not a feature of the equipment; rather it relies on a defect in the concrete and not the capability of any specific type of hydrodemolition equipment.

Since the operating pressure and flow rate do not affect the hydrodemolition equipment’s ability to remove unsound concrete, specifications, which include hydrodemolition, should not specify an operating pressure or flow.  Furthermore, specifying a pressure or flow rate is “means and methods” and places the responsibility for the success or failure of the technique on the specifier.  Simply specifying the use of hydrodemolition and describing the expected results eliminates this issue for the specifier and places the burden on the hydrodemolition providers to achieve the desired results.

References:

1. Concrete International – ACI Fellow James Warner states “Typical water pressures used in hydro-demolition will generally be within a range of 55 to 350 MPa (8000 to 50,000 psi) or more, with water flow rates on the order of 19 to 300 L (5 to 80 gal.) per minute. The amount of work accomplished is dependent on the hydrodemolition energy that is basical­ly the product of the pressure and the flow rate. Also of some influence are the nozzle design, trajectory, and dis­tance from the impingement surface. Low flow rates generally require rela­tively high pressures, whereas a similar amount of removal can be accom­plished at lower pressures combined with higher flow rates. There are mini­mum pressures required, however, that are dependent on the concrete strength and condition.” (http://www.concrete.org/Publications/InternationalConcreteAbstractsPortal.aspx.aspx?m=details&i=163)

2. International Concrete Repair Institute – Technical Guideline 310.3 states “The high-pressure pumps used for hydrodemolition are capable of generating pressures from 10,000 psi to 40,000 psi (70 to 275MPa) with flow rates from 6 to 100 gpm (25 to 380 lpm).”  (ICRI Technical Guideline No. 310.3R)

3. Standard Specifications for Construction of Roads and Bridges on Federal Highway Projects, FP-14, UNITED STATES DEPARTMENT OF TRANSPORTATION, Federal Highway Administration; Section 560. REMOVAL OF CONCRETE BY HYDRODEMOLITION makes no reference to the pressure or flow rate required to perform hydrodemolition.