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Course 815 - Demolition Safety

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Demolition of Pre-stressed Concrete Structures

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

“Pre-stressed Concrete” Defined

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:

  • Pre-tensioning: In pre-tensioning, the tendons are first stressed to a given level and then the concrete is cast around them. The tendons may be composed of wires, bars or strands.
  • Post-Tensioning: In post-tensioning, the concrete member is first cast with one or more post-tensioning ducts or tubes for future insertion of tendons. Once the concrete is sufficiently strong, the tendons are stressed by jacking against the concrete. When the desired prestress level is reached, the tendons are locked under stress by means of end anchorages or clamps. Subsequently, the duct is filled with grout to protect the steel from corrosion and give the added safeguard of bond.

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.

Pre-stressed Concrete Structures

accidents
Pre-stressed concrete beams with 0.6 inch diameter strands spaced at 2 inches.

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.

Categories of Pre-stressed Construction

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

Pre-tensioned members usually do not have any end anchors, the wires being embedded or bonded within the length of the member.

  • Demolish simple pre-tensioned beams and slabs of spans up to about 23 feet (7 meters) in a manner similar to ordinary reinforced concrete.
  • Lift and lower pre-tensioned beams and slabs to the ground as complete units after the removing composite concrete covering to tops and ends of the units.
  • Turn the members on their sides to facilitate breaking up.
  • Lift the structure from points near the ends of the units or from lifting point positions.
  • Whenever possible, reuse lifting eyes if they are in good condition.
  • When units are too large to be removed, consider temporary supporting arrangements.

Precast Units Stressed Separately

precast units
Precast and pre-stressed concrete bridge

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.

  • After lowering, the units can be turned on their side with the ends up on blocks after any composite concrete is removed. This may suffice to break the unit and release the prestress; if not, erect a sand bag screen, timbers, or a blast mat as a screen around the ends.
  • Clear the area of any personnel.
  • Remember the end blocks may be heavily reinforced and difficult to break up.

Monolithic Structures

monolithic

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.

  • It will usually be necessary to provide temporary supports so that the tendons and the anchorage can be cautiously exposed.
  • Do not indiscriminately attempt to expose and destress the tendons and anchorages.

Progressively Pre-stressed Structures

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.

Instructions

Before beginning this quiz, we highly recommend you review the module material. This quiz is designed to allow you to self-check your comprehension of the module content, but only focuses on key concepts and ideas.

Read each question carefully. Select the best answer, even if more than one answer seems possible. When done, click on the "Get Quiz Answers" button. If you do not answer all the questions, you will receive an error message.

Good luck!

1. In which of the pre-stressing methods are the tendons first stressed to a given level and then the concrete is cast around them?

2. In which of the pre-stressing methods is the concrete member first cast with one or more ducts or tubes for future insertion of tendons?

3. The most important aspect of demolishing a pre-stressed concrete structure takes place during the _____.

4. In which of the following pre-stressed construction categories are members pre-stressed before the application of superimposed loads?

5. Workers may demolish simple pre-tensioned beams and slabs of spans up to about _____ in a manner similar to ordinary reinforced concrete.


Have a great day!

Important! You will receive an "error" message unless all questions are answered.