6. DEMOLITION OF SPECIAL STRUCTURES
Special structures are complex and / or unusual because of the nature of their construction or condition. They include:
• pre or post-tensioned construction
• pre-cast concrete panel and framed structures
• stressed skin structures( i. e. buildings that rely on the sheeting, cladding or decking to stiffen and restrain the structural framework)
• slung structures( for example, floors or roofs) that are in some way suspended from a framework, supported by a structural core.
Special structures will require proper planning and care to be demolished safely. An appropriate demolition method and sequence should be selected and documented prior to the work commencing. A demolition plan and an assessment by a competent person of the proposed demolition method may assist with this process. A SWMS must be prepared where structural elements are to be demolished.
6.1 Pre and post-tensioned concrete
Pre-tensioned concrete contains tendons( wires, strands or bars) that have been tensioned before the concrete is placed.
Post-tensioned concrete contains tendons that have been tensioned after the concrete has hardened.
Tensioned tendons require controlled removal because the high level of potential energy stored in the tendons poses a risk to the health and safety and can cause damage to property. It is also important that structural stability is retained during and after tendon removal, prior to the final demolition of the concrete element.
The tendons can be subject to corrosion that weakens them and decreases the structural integrity of the building. Damage is not usually evident externally, even if strand breakage is extensive and conditions can vary widely even within an individual structure.
Before demolishing pre or post-tensioned concrete elements, review all available documentation on the building or structure including:
• building plans, designs and specifications to understand the type of tensioning used, the load carried, anchorage points and number of tendons
• any construction photographs to obtain information on anchorage details, the construction sequencing, and any other measures may affect moisture access.
The condition of the concrete and tendons should be considered before and during demolition, for example, by:
• conducting a visual inspection to confirm loads, obvious deviations from the original design and waterproofing details
• assessing conditions throughout the building, as well as utilising selective testing in representative areas to assess any weakening or breakage of tendons
• measuring humidity within tendon sheathing and analysing any sheathing contaminants
• removing, inspecting and testing a small number of tendons to assess their condition
• continuing to monitor tendon tension.
32 CODE OF PRACTICE | DEMOLITION WORK