International Concrete Abstracts Portal

In September 2000, the American Concrete Institute sponsored the ACI Fourth International Conference in Seoul, Korea. Sixty-two papers are included in this publication. Note: The individual papers are also available as .pdf downloads.. Please click on the following link to view the papers available, or call 248.848.3800 to order. SP193

This paper reports the results of an exploratory study on the effects of curing duration on the variation of mortar strength with distance from the drying surface. A novel, notched cylindrical test specimen was adopted for measuring tensile strength at different depths. Two mortar mixtures with w/c of 0.30 and 0.45 were used; the former was assumed to be representative of the paste system in a high-performance concrete. Specimens were moist cured for (1, 3, or 7) d and then exposed to air at 25 “C and 50 % or 70 % RH. The cylinders were sealed to simulate one-dimensional drying in a large member. Tensile strengths were measured at 28 d. Relationships between tensile strength and depth were compared with those of specimens continuously moist cured. The data tended to show that 1 d of moist curing might be sufficient to ensure adequate strength development at a depth of 25 mm from the exposed surface. The phenomenon of increasing strength with drying may have confounded the results, and recommendations for additional studies are provided.

This contribution deals with the development of slurries for the restoration of brickwork buildings in Northern Germany. Various slurries on the basis of two binders with a high sulphate resistance with an admixture of solid glass globules respectively were developed, one slurry on the basis of lime the other on the basis of cement. Their aggregate is of fine quartz sands and various additives were used. The slurries were developed to restore historical terracotta. Beside the workability and the water retention capacity of the slurries, technological pa-rameters such as dynamic modulus of elasticity, compressive strength, bending and bond strength and hygrical parameters such as water absorption, water desorptions and water vapour diffusion were investigated. Further important characteristics were the resistance to freezing and thawing, the scuff resistance and the efflorescence behaviour. Some of the slurries were successfully applied to the terracotta surfaces.

The paper investigates the shear strength and the failure modes of high strength concrete beams with and without steel fibers ( fC = 110 MPa). Sixteen reinforced high strength concrete beams (3600x350~200 mm) were tested under different combinations of shear force and bending moment. The beams were singly reinforced and without shear (web) reinforcement. The test results indicated that the addition of steel fibers to high strength concrete increases the ultimate shear strength, improves the brittle characteristic and transforms the failure mode into a more ductile one. The average gain of the ultimate shear strength due to the addition of steel fibers varied from about 14% to 14 1% depending on the shear span to depth ratio and the longitudinal steel ratio. Four modes of failure of reinforced high strength concrete are clearly distinguished as; diagonal tension, shear compression, shear flexure, and flexural failure. In general, cracks in fiber reinforced concrete beams are closer, narrower, and more than those in beams without fibers. This reflects the effect of steel fibers in redistributing the stresses beyond cracking.

In this paper, shear strengthening of RC beams by confining shear critical region is experimentally and theoretically investigated. Confinement is implemented using vertical post-compression force in the shear critical region with steel plate, bolts and angles. The loading tests include (1) loading beams to first major shear crack, then shear strengthening, and reloading to complete failure, (2) strengthening beams from beginning and loading to complete failure, and (3) loading unrepaired beam to complete failure. Influences of concrete strength, shear span length, longitudinal tensile reinforcement, level of post-tensioning, presence of shear reinforcement, use of continuous plates, and presence of shear crack are studied by testing twenty-four RC beams. Test results show that in the presence of post-compression stress, as low as 0.04f'c for strengthening, shear strength increases significantly and the mode of failure of the beams changes from brittle shear to ductile bending. Also, for this level of post-compression, influence of all other parameters is limited, while a ductile failure is dominant. Calculations based on Mohr’s theory also indicate that this method of strengthening is very effective, and provides ductile flexural behavior in beams with inadequate shear strength.