What are “shadows” under the rebar?

What causes “shadows” under the rebar?

What can be done to eliminate “shadows”?

Shadows (pictured right) are defined as the concrete left under the reinforcing steel following hydrodemolition.  During hydrodemolition, the high-pressure water jet is directed at the concrete surface.  The water jet can demolish the concrete but it will not damage the reinforcing steel or other embedded metal items.  The reinforcing, however acts to block the water jet from removing the concrete directly under it.  In effect the reinforcing steel shields the concrete from the water jet.

 

 

The effect of shadowing can be minimized or eliminated by directing the water jet under the reinforcing steel.  To accomplish this, Rampart uses an angled rotating water jet. (Pictured left)

 

 

 

There are several factors that will impact the amount of shadowing.

• The removal depth must be adequate in order to insure proper removal of the reinforcing steel.  The size will impact shadows.  Larger rebar, which will shield a wider area under the rebar, will require a greater depth of removal to insure removal of the shadow.

• The spacing of the reinforcing will impact shadows.  Closely spaced reinforcing will work together to shield areas from the water jet.

• The larger the aggregate the deeper the required remove will be if a ¾” uniform removal under the reinforcing is required. 

Eliminating or minimizing shadowing can be accomplished by specifying a deeper removal.  This will insure that only minimal detail chipping is required.  Needless to say the primary purpose of performing hydrodemolition is to prepare the surface for the best possible bond to the repair or overlay material.  Extensive chipping to provide clearance under the rebar damages or bruises the substrate at any location the chipping is performed.  An additional ½” removal is often all that stands between a great job (little to no detail chipping) and mediocre job (significant amount of detail chipping).

However, hydrodemolition cost is directly related to the depth of removal.  The cost for an additional ½” of removal is significant along with the added cost of clean up and disposal and the replacement material.  This cost is offset by the added cost of detail chipping if the original hydrodemolition is inadequate.  The most important factor though, is the reduction in the quality of the restoration caused by the use of jackhammers and chipping hammers.