International Concrete Abstracts Portal

Production of high performance concrete (HPC) depends on several factors including the use of low water-to-cementitious material ratio, proper dosage of high-range water-reducer (superplasticizer), and a careful selection and dosage of a mineral pozzolanic admixture such as silica fume. Most of the improvement in the strength and durability characteristics of the hardened concrete is attributed to the filler effect and pozzolanic action of the fine size silica fume. Results of tests conducted at the American University of Beirut (AUB) on tension lap splices in full-scale beam specimens and on reinforcing bars anchored in eccentric pullout specimens, indicate that the replacement of part of the cement by an equal weight of silica fume resulted in reduction in bond strength. The reduction was independent of specimen type, bar size, super-plasticizer dosage, and casting position. The mode of failure of the high strength concrete beam specimens was a very brittle bond splitting failure. Tests were conducted to check the effect of transverse reinforcement in the splice region on the bond strength and mode of failure. This paper will provide an overview of the research performed at AUB.

Inhomogeneous stress situations are prevailing in some engineering problems, such as around corroding steel bars or around aggregate particles expanding due to alkali-silica reaction. Micro-mechanics and macromechanics will significantly differ in those cases. The present article focuses on experiments simulating the effect of steel bar corrosion on four cementitious composites. For that purpose, prismatic specimens containing an excentrically located slightly tapered cylindrical hole, where subjected to controlled push-through of a metal cone of similar shape. Behaviour in the pre-peak range was reflected by strain gauges, and in the post-peak region by clip gauges. Simulateneously, acoustic emission measurements were performed. Various aspects of the tests have been highlighted before in publication to which is referred when relevant. This paper merely presents illustrative data, evidencing typical micro- and macro-mechanical processes taking place under the given conditions. Successively, on macro-level, the elastic range terminates at Discontinuity Point (DP), at ultimate (BOP) a stage of quasi-plastic yielding is obtained, and a mechanism is formed at Crack Opening Point (COP), after which energy due to bar expansion is stored in opening up of only the leading crack. On micro-level, dispersed crack initiation and coalescence in radial direction starts at the interface. this process gradually concentrates in one of the two ‘weakest’ sections of the covercrete. This process slows down, whereupon cracks are initiated at the exterior of the second weakest section. Here they propagate to coalesce with the interor microcracks to form the major crack.

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.