Suspended Slab Back Propping for Concrete Remediation

Project: Back propping of a suspended concrete slab for remediation works
Location: Ultimo, NSW
Sector: Multi-residential / building remediation
Client: Dapcor
Services Provided: Temporary works design, propping supply and installation
Equipment Used: Titan Props, hardwood timber sleepers

Project Overview

Dapcor engaged Legend Hire to back prop a suspended concrete slab at a multi-residential building in Ultimo, NSW, so that concrete remediation works could be carried out safely on the slab above. Remediation exposes reinforcement and removes sections of the structural concrete, temporarily reducing the slab’s capacity to carry its own load. Legend Hire installed Titan Props on hardwood timber sleepers beneath the affected slab to provide a continuous vertical load path to the level below for the duration of the works, with all propping installed inside a carpark that remained in use by residents.

The Challenge

Concrete remediation temporarily weakens the element being repaired. Removing delaminated concrete and exposing corroded reinforcement reduces the effective section of the slab, so the structure must be independently supported before remediation begins rather than after a defect is found.

The constraint that shaped this job was operational. The propping zone fell inside a live residential carpark, across bays allocated to residents. The carpark could not be closed for the duration of the works, so every prop position had to be set out clear of vehicle movements and reversing paths, delineated against impact, and coordinated with residents’ access to their own parking spaces. Legend Hire routinely installs propping in operational spaces, including back propping inside a live retail loading dock that stayed in service throughout.

Site conditions inside the carpark added further constraints. The soffit carries fire services, sprinkler pipework and lighting at low level, so prop positions had to suit the actual clear height available at each location rather than a nominal figure. Stored resident property adjacent to the works also had to be protected and isolated from the propping zone.

Engineering Considerations

Load path continuity, back propping only works if load travels in an unbroken vertical path from the slab being remediated down to a level capable of receiving it. Establishing and verifying that path is the core of any large-scale structural propping design. Props were positioned so that load was transferred to supporting structure below and not reintroduced into spans not designed to carry it.

Support before removal, propping was installed and pre-loaded before any concrete was removed, so the temporary works were carrying load at the moment the slab section was reduced, rather than picking up movement after the fact.

Bearing pressure distribution, hardwood timber sleeper cribbing was placed beneath each prop to spread the reaction into the slab below over a wider area, reducing localised bearing demand at the prop base and providing a level, stable founding platform on a carpark deck graded to drainage.

Headroom and prop selection, Titan Props are adjustable at the head and base by screw jack, allowing each prop to be extended to the exact clear height at its position and then fine-adjusted to take up load against the soffit.
Handling in a live environment. Titan Props are man-handleable, so the system could be carried in, positioned and adjusted by hand without plant movements through an operating carpark.

The Solution

Legend Hire installed Titan Props on hardwood timber sleepers to support the suspended concrete slab during remediation.

The system combined two elements:

• Titan Props: installed in lines beneath the slab to provide adjustable vertical support at the clear height available at each position, with screw jack adjustment used to take up load against the soffit.
• Hardwood timber sleepers: cribbed beneath each prop to distribute the reaction into the supporting deck and establish a level, stable base for accurate prop installation.

Prop lines were set out clear of trafficked routes and delineated with barriers and cones so the propping remained visible and protected from vehicle movement for the duration of the works.

Legend Hire coordinated set-out and installation sequencing with Dapcor so that propping was in place and loaded before remediation commenced, and so that carpark bays were released back to residents progressively as the works allowed.

Equipment Used

• Titan Props: adjustable telescopic props with screw jack adjustment at head and base, allowing each prop to be set to the exact clear height at its position and fine-adjusted to take up load against the soffit. Selected here for their load capacity relative to weight, which allowed the system to be carried in and installed by hand without plant movements through an operating carpark.
• Hardwood timber sleepers: cribbed beneath each prop to distribute the reaction into the supporting deck over a wider area and establish a level, stable founding platform on a surface graded to drainage.

The Outcome

The back propping system supported the suspended concrete slab for the full duration of the remediation programme, allowing Dapcor to carry out the works safely.

• Suspended slab independently supported throughout concrete remediation
• Continuous vertical load path maintained to the level below
• Carpark remained operational for residents during the propping period
• Zero incidents recorded
• Remediation works completed as programmed

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