Precast Plank Spalling

[WiSEiwish]

I am working on a project in which something interesting came up regarding existing precast. The precast plank was installed per 1950s drawings. On top of the exterior of the precast there is/was a concrete curb. The curb was going to be demolished, and a new one installed. In the process of the demo, the plank started spalling on the underside with chunks of precast falling to the basement floor. The reinforcing was visible from below the plank as were the hollow cores.

I'm trying to figure out the source of the spalling. It is a single span condition and there is no cracking in the midspan of the plan indicating that it is not a moment failure. Since it is happening now rather than decades ago, I want to rule out it being a shear failure. It seems like the demolition process could have put an axial load on the precast which would cause localized compressive stress at the support.

I have uploaded a photo of the drawing.

 

[msquared48]

Corbell reinforcing pattern is to be desired too.

If there was ever any water intrusion, with freeze/thaw, the presence of the corner bars at the curb and the confinement of the precast end could have prevented any spalling being seen until it was removed.

Mike McCann
MMC Engineering

 

[Ingenuity]

I would examine the spalled concrete segments and check for any evidence of water damage, contamination, efflorescence etc - i.e. visually check the fractured surfaces of the spalls.

If the cracks/spalls are fresh then I would check how the demo was undertaken - hand-held demo hammers or large equipment. It is possible that larger equipment could penetrate the hollow-core voids and spall the bottom in a local manner.

Is the spalling localized over part of a panel width, or across the full or several panels widths?

As is typical with problems of this nature - more questions than answers!

 

[oldestguy]

Was the top cover exposed to salt? If so were the planks air entrained concrete? The top parts of the planks are covered, so salt damage there might not show?