International Concrete Abstracts Portal

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.

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 Hong Kong Mass Transit Railway Corporation, carrying a daily patronage of over 2 million, has a prime objective of providing a safe and reliable railway transportation system to the public in Hong Kong. The existing Urban Line System is situated mostly underground through soft, marine deposits and/or rock. The aggressive underground conditions in the coastal regions had resulted in various degrees of deterioration to the tunnel linings in a period less than 10 years from completion. The problem was most serious where precast concrete (PCC) lining was adopted because of the existence of segmental joints. A special method of repair was jointly developed by MTRC and Skanska International Civil Engineering AB. A robotic high pressure water jet system (hydrodemolition) is used to remove the surface delaminated concrete, and the surface repaired with a 75 mm layer of low permeability, fibre (polypropylene) reinforced shotcrete. Repair has to be implemented in the short time window during non traffic-hours each night with absolutely no disruption to the daily traffic.

Chloride ion is the main deterioration factor of concrete structures, mainly for external surfaces. These surfaces can be protected by paints, which may reduce the penetration of aggressive agents from external sources. A wide range of organic polymers are used as coatings for concrete and for each generic group a large number of formulations with different permeability against aggressive agents is possible. In order to understand the influence of paint composition on chloride ion penetration, acrylic latex paints with different pigment/binder ratios were studied. The objective of the study was also to evaluate the chloride penetration according to two methodologies. One, based on ASTM Cl202 “Electrical indication of concrete’s ability to resist chloride ion penetration” and the other, according to the previous method with modifications. The ASTM method allows us to obtain results in few hours; however, there are some controversies about this test and on the acceleration rate phenomenon. The tests were carried on applying paints on two different concrete substrates. The results not only show there is a correlation between paint composition and chloride penetration but also that the acrylic latex paints provide an effective barrier to this ion.