International Concrete Abstracts Portal

This paper presents the characteristics of the surface-layer strength of concrete produced using a special curing sheet to improve the surface layer. The curing sheet is attached to the interior of the forms and has the ability to absorb water from the concrete surface and to retain the absorbed water for curing. The surface layer strength was measured by a pullout test that was carried out by modifying the Michaelis test machine and using truncated conical steel cones embedded in the concrete. The 14-day surface layer strength obtained by using the special curing sheet increased by 1.2 to 1.5 times of that obtained without using the sheet. The improved strength is attributed to the lowering of the water cement ratio in the surface layers produced by using the sheet. After freeze-thaw tests, the surface layer strength of air-entrained concrete showed less decrease independent of the use of special curing sheet. The surface layer strength might be a useful index for the evaluation of the quality of the surface layer of concrete.

The influence of mix constituents on the quality of concrete is discussed. Results demonstrate that the assessment of concrete performance based on the traditional parameters such as water-cement ratio and 28-day strength may be misleading. One way to insure a durable structure is to specify concrete by performance criteria. This would overcome not only the influence of constituents, but would also place emphasis on proper curing. For the protection of reinforcement against corrosion, performance criteria based on carbonation and water sorptivity are suggested.

An experimental study of precast concrete beam-column connection assemblies is presented. The study was initiated to provide data for the development of a rational design procedure for such connections in high-seismic regions. The objective of the study is to develop a moment-resistant precast concrete connection that is economical and easily constructed. All test specimens were 1/3-scale models of the prototype. The monolithic concrete specimens were designed to 1985 Uniform Building Code (UBC) seismic zone 2 and 4 criteria. The designs of the precast specimens were similar to those for the monolithic specimens designed to UBC seismic zones 2 and 4. Results from the monolithic specimens provide a benchmark for comparison with results from the precast tests. Comparisons of the performances of the monolithic beam-column connections with those of the precast, post-tensioned connections were based on the connection strength, energy absorbed, ductility, and failure mode for the two types of beam-column connections.

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.