The different forms of construction used in a number of more or less conventional structures built during the last few decades will give rise to a variety of problems when the time comes for them to be demolished.
Pre-stressed concrete is defined as concrete in which there has been introduced internal stresses of such magnitude and distribution that the stresses resulting from given external loadings are counteracted to a desired degree. In reinforced concrete members, the pre-stress is commonly introduced by tensioning the steel reinforcement.
This internal stress is induced into the member by either of the following two pre-stressing methods:
Read the WisDot Bridge Manual, Chapter 19 – Prestressed Concrete for a detailed discussion.
Also, check out this slide show by Parag Pal on Pre-tensioning and Post-tensioning Concrete.
The most important aspect of demolishing a pre-stressed concrete structure takes place during the engineering survey. During the survey, a qualified person should determine if the structure to be demolished contains any pre-stressed members.
It is the responsibility of the demolition contractor to inform all workers on the demolition job site of the presence of pre-stressed concrete members within the structure. They should also instruct them in the safe work practice which must be followed to perform the demolition safely. Workers should be informed of the hazards of deviating from the prescribed procedures and the importance of following their supervisor's instruction.
There are four main categories of pre-stressed members. The category or categories should be determined before attempting demolition, bearing in mind that any pre-stressed structure may contain elements of more than one category.
Category 1: Members are pre-stressed before the application of the superimposed loads, and all cables or tendons are fully bonded in the concrete or grouted within ducts.
Category 2: Like Category 1, but the tendons are left ungrouted. This type of construction can sometimes be recognized from the access points that may have been provided for inspection of the cables and anchors. More recently, unbonded tendons have been used in the construction of beams, slabs, and other members; these tendons are protected by grease and surrounded by plastic sheathing, instead of the usual metal duct.
Category 3: Members are pre-stressed progressively as building construction proceeds and the dead load increases, using bonded tendons as in Category 1.
Category 4: Like Category 3, but using unbonded tendons as in Category 2.
Examples of construction using members of Categories 3 or 4 are relatively rare. However, they may be found, for example, in the podium of a tall building or some types of bridges. They require particular care in demolition.
Pre-tensioned members usually do not have any end anchors, the wires being embedded or bonded within the length of the member.
Before breaking up precast units stressed separately from the main frames of the structure, they should be lowered to the ground, if possible. It is advisable to seek the counsel of a professional engineer before carrying out this work, especially where there are ungrouted tendons. In general, this is true because grouting is not always 100% efficient.
A monolithic structure is something carved or cast from a single piece of a material. Usually (and literally, from the translation of monolith being "one stone") the material is stone, but it could equally be applied to a structure cut from a single block of metal, or cast in metal in a single piece. Most domed structures, like sports stadiums, are considered monolithic.
The advice of the professional engineer experienced in pre-stressed work should be sought before any attempt is made to expose the tendons or anchorages of structures in which two or more members have been stressed together.
In the case of progressively pre-stressed structures, it is essential to obtain the advice of a professional engineer, and to demolish the structure in strict accordance with the engineer's method of demolition. The stored energy in this type of structure is large. In some cases, the inherent properties of the stressed section may delay failure for some time, but the presence of these large pre-stressing forces may cause sudden and complete collapse with little warning.
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