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The Hong Kong Housing Authority maintains an extensive portfolio of property that includes over 700,000 flats. In 1985 it embarked on a major structural investigation of 828 housing blocks, and a US $200 million repair program. The investigation has involved over one million visual inspections, testing of 30,000 concrete cores and an extensive nondestructive testing program. It was necessary to develop methods of appraising low strength concrete structures, and unique strengthening and repair techniques. The paper examines the development and implementation of an investigation and maintenance strategy that takes into consideration the condition, life expectancy, and environment of each building. The paper focuses on the way priorities were determined and research undertaken. It discusses the public relations and management of a program that has required the strengthening of 92 high-rise buildings, and 400,000 mý of spalling repairs over a six year period.

Six single-span beams and two two-span beams with preformed cavities simulating concrete spalling were cast and long-term tests were carried out to investigate the performance of structural repair. The early age properties of three types of polymer-modified concrete, i.e., tensile strength, modulus of elasticity, shrinkage and creep, were investigated under the same environmental conditions as the beams. The test results show that the effect of shrinkage of polymer cement concrete on the stress at serviceability of a repaired structure is quite significant. The restrained shrinkage can lead to cracking in the repair patch and to additional tensile stresses in the substrate. Simulation analyses were also carried out to support this experimental investigation. An analytical method has been calibrated by the test results. Tests and analyses show that the major factors influencing the performance of structural repair include free shrinkage, creep coefficient and tensile strength and ultimate tensile strain in early age properties of a repair material. Stiffness of the member and moment redistribution in the repaired structure are also important.

A major rehabilitation of the reinforced concrete approach viaduct to the Pattullo Bridge has been successfully completed over the last nine years. A condition inspection revealed that extensive cracking had taken place over the years,especially at the bridge girder ends due mainly to frozen bearings. A program of bearing replacement was carried out at all girder ends along with seismic improvements and general concrete repairs. An elaborate falsework system was designed which was easily adaptable to the varying geometry along the viaduct. Load transfer to the falsework to implement the rehabilitation was achieved using a hydraulic jacking system. The work was carried out in a series of six contracts and the project was completed without any bridge traffic closures during the assignment.

An intense fire in the antioxidant plant of a chemical factory near Bombay caused extensive damage to the reinforced concrete frame of the building as well as to the flooring, equipment, fixtures, and fittings. Consultants were appointed to investigate the damaged structure and later to design and supervise the rehabilitation work. After the preliminary investigation, the consultants decided that it was not necessary to demolish the structure, as long as it was possible to restore the structural members at a reasonable cost and within a short time span. In addition to visual inspection and sounding, extensive nondestructive tests were conducted to determine the extent of damage and the residual strength of the important load-carrying members of the structures. The findings of the investigations were analyzed and the restoration system was designed by the consultants. The paper outlines the considerations that led to restoration of the structure in preference to demolition, the details of investigations, the analysis of the findings, and the successful rehabilitation techniques that followed.

The crest of Kakabeka Falls, a major tourist attraction near Thunder Bay, Ontario, had changed rapidly within 15 years. Geotechnical evaluation had concluded that, due to the geometry of the rock formation, there was little continuous cap-rock beyond the crest of the falls. Undercutting by water and subsequent ice jacking and freeze-thaw action was eroding the remaining cap-rock. Without prompt remedial work, the falls would degenerate to rapids over 50 years. Rapid recession of the falls also threatened the foundations of two bridges. In 1988 and 1989, remedial works were undertaken to stabilize the erosion process and improve the appearance of the falls. Concrete mixes were designed using powdered color dyes to duplicate the color and appearance of the reddish brown cap-rock and black hales. The crest of the falls was reconstructed incorporating a weir which spread the water flow evenly across the width of the falls. Special formwork and hand-finishing techniques were used to texture the concrete surface to match the existing rock. Two years in planning and eight weeks in construction, the project successfully restored the falls to its former beauty.