PTI DC20.2 PDF

INTRODUCTION

Most concrete structures develop cracks for one reason or another. In most cases, cracks do not impair the expected performance of the structure. Some cracks, on the other hand, may be detrimental by leading to excessive deflections, exposure to corrosive environment, and possibly strength reduction. Cracks may also be aesthetically unacceptable. Undesirable cracks should be avoided by design, and if they occur, should be identified and neutralized.

It is important to recognize which cracks are unwanted and should be treated, and what method of repair is best suited. Preventive measures against crack formation are an essential component of the process of design, construction and maintenance.

Richardson [1] has presented a comprehensive overview of different crack types in reinforced concrete structures, their causes and effects.and has concluded with a lucid crack categorization flow chart.

ACI Committee 362 State-of-the Art Report on Parking Structures [2] reviews the cracking problems of parking structures and their rehabilitation. Horn and Kost [3], and Ojha [4] report on crack formation, evaluation and repair of a specific parking structure. For box girder bridges Podolny [5] has presented a detailed account on the causes of cracks and their retrofit procedures.

For the purposes of the present work the causes of crack formation are categorized as follows:

(i)   Inadequate design, such as insufficient reinforcement; deficient detailing; and, poor workmanship.

(ii)   Restrained volume changes which include:

shrinkage;

creep;

elastic shortening; and,

temperature.

This paper is limited in scope to the treatment of cracks of the second category (ii), namely shrinkage, creep, elastic shortening and temperature, as they occur in post-tensioned members and the supporting structural elements. These cracks are commonly referred to as "restraining cracks."

Further, the work is directed primarily to the application of unbonded post-tensioning in commercial buildings, with specific references to parking structures and subterranean structures. A subterranean structure is one in which one or several post-tensioned concrete decks partially or totally below grade are topped with a superstructure built of wood, concrete or steel construction. The architectural outline of the post-tensioned lower levels is generally different from the upper levels as they commonly serve different functions. The lower concrete levels may be for parking or commercial retail, the upper levels for residential or office occupancy. This type of structure is widely used in California metroplexes where high land cost prohibits the provision of off-site parking.

The study reported herein is the outcome of design, review and observation on over three hundred post-tensioned structures between 1982 and 1987.

The work is subdivided into (1) crack causes and types, (2) crack mitigation measures, (3) structural evaluation of cracks, and (4) repair of cracks; the work concludes with (5) remarks and recommendations.