International Concrete Abstracts Portal

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.

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.

Because of the shortage of aggregate, the reinforced concrete -containing sea sand and/or sea water can probably be used for marine structures. These marine structures may experience corrosion problems due to salt attack that can result in early deterioration. Therefore, such structures should be protected against corrosion damage induced by chloride ion to extend the service life. This study was focused on a basic counter-plan against concrete corrosion related to marine infrastructures, especially the evaluation of effects of various corrosion inhibitors with different cover thicknesses. Several electrochemical/corrosion test techniques, i.e. half -cell potential, linear & cyclic polarization behaviors & autopsy inspection, were included. Based on this long-term(7 years) study, it was concluded that a chloride ion accelerated reinforcement corrosion seriously, however, a increase of cover thickness and adoption of some inhibitors could provide positive effects against the chloride-induced corrosion. In addition, it was recognized that the electrochemical corrosion tests generally agreed with those of visual autopsy examinations in these long-term experiments.

One of ways to improve the durability of the reinforced concretete structures is sealing with various coating materials on the surface of the concrete that represses the penetration of moisture, chloride ions and carbon dioxide gases which may cause the reduction of durability. Up to now epoxy and urethane-based coating materials are widely used for coating. However, these amterials have the characteristics of poor crack bridging, large thermal expansion and vulnerable color by ultraviolet rays. Also their coating effect depends on application methods that it is dubious of maintaing long-term durability so the development of new alternative coating materials is in demand.Test were performed on an aluminum oxide-isocyanate-based material for coating in order to improve the weather resistance, adhexiveness to concrete surface, resistance of chloride penetration and carbonation by forming the aluminum oxide coating on the surface. Aluminum oxide-isocyanate-baced coating material is compared with other coating materials, and shows higher color retention against ultraviolet rays and resistance of chloride penetration and carbonation and protection of reinforcing steel against corrosion.

This study focused on the investigation of durability characteristics of very early strength concretes incorporating polypropylene fibers and application feasibility of these into the highway pavements. A series of laboratory tests were performed to assess the strength development and scaling resistance. The test of surface scaling resistance was performed according to ASTM C 672 exposed to frost in the presence of de-icing chemicals. The experimental variables included cement types, fiber contents and concrete mixtures. The compressive strength of very early strength concrete at 3 hours was measured as 22 MPa, which corresponded to the strength at 7 days or 28 days of ordinary portland cement concrete. This may enable the repaired concrete pavement to be opened to traffic 3 hours after concrete placement. There was little effect of polypropylene fiber reinforcement on compressive strength. The fiber reinforced concretes exhibited noticeably higher flexural strength than the reference concrete at all ages. The plain concrete slabs were given a visual rating of 3 or 4, and according to ASTM Standard C 672. This corresponds to moderate to severe scaling. The fibrous concrete performed better than the plain concrete slab which was given a visual rating of 0 or 1. This corresponds to no scaling or very light scaling. As the amount of fiber reinforcement increased the surface scaling resistance increased. The scaling resistance of very early strength concrete was comparable to that of ordinary concrete from the visual rating. Thus, the developed very early strength concrete may be satisfactory for use in repair works for pavements.