Holdfast Training Services Trial Blog

A quick reminder… I know all C’s have this covered but for the E&Ms, unlike conventionally reinforced and mass concrete, the modification of pre-stressed concrete is complicated due to the potential energy stored in the member. If suspended, all members contain gravitational potential energy but prestressed concrete stores elastic potential energy in its stressed tendons. This makes the modification, demolition and decommission of prestressed concrete members a more complex operation.  Moreover, an alteration to any part of a pre-stressed member is highly likely to effect the entire structure.

With this in mind, you can imagine my horror as I was recently told I was responsible for the modification of a post tensioned slab which formed part of an exisiting structure adjacent to my project.

Sitch… In brief, I have got to co-ordinate the removal of a strip of PT slab (2m x 17m x 250mm thick), which cantilevers from ground level, over our basement, obstructing progress.  Our structure will tie into the existing slab once we reach ground level.  The image below shows the offender, north is up.

The western most 12m of the slab is 5m above our current rock level, the remainder is 10m above a lower rock level.

Thoughts so far… Having consulted almost everyone in the office, the structural engineer consultant and the PT sub-contractor I have come up with the following methodology…

Confirm effect of these works on remaining structure, temporarily back-prop remaining structure if required, back-prop the slab with falsework, provide edge protection, confirm tendon locations, expose tendons and confirm grouting, anchor the tendons with new ‘onions’ if required, cut strands, cut slab into manageable sections, sling and crane out, treat exposed edge of remaining slab with epoxy paint to prevent corrosion.  Once the new structure ties in to the existing, any temporary back-propping will be removed.

I’ll take some photos and let you know how we get on.