International Concrete Abstracts Portal

This state-of-the-art starts out with a short discussion of the methods for measuring autogenous deformation of cementitious paste and how influence of artifacts (e.g. segregation forming bleed water) can be avoided. The influence of all components of cementitious paste on autogenous shrinkage is reviewed; water-cement ratio, cement type, additives like silica fume and fly ash (including the pozzolanic reaction itself), and ad-mixtures like lignosulphonate and super-plasticizers. The mechanism of autogenous shrinkage is discussed, with emphasis on it's relation to setting time and pore pressure. Finally, remedies to reduce the amount of autogenous shrinkage are briefly mentioned in terms of mechanisms and effects.

This paper presents the introduction of a steel fiber made by a pre-cast manufacturer suitable for plant-produced products and transit-supplied concrete. The fiber con-figuration allows fiber manufacturing to be done in-house as are the other concrete products. Toughness test results indicate equivalent or improved performance with lab mixtures compared with other steel fibers available and tested. Tests were conducted with both wet (laboratory and transit mixture) and dry cast techniques for testing samples and full-scale three-edge bearing tests for dry cast pipe. Performance issues were identifiable for the sample casting techniques, compression strength, maturity, and toughness tests with fiber reinforcement. Pipe tests were conducted for the first visible crack, the first 0.25 mm crack, and the ultimate load with fabric reinforcement only, fiber reinforcement only, and then with both fabric and fiber reinforcements. Concrete mixture proportions for the pipe were constant with three dosages of fiber used: 0.25, 0.50, and 0.75 percent by volume.

The working mechanism of a polycarboxylate superplasticizer (PC) which is a new generation of superplasticizer (NSP) is investigated. This NSP shows a shrinkage reducing effect as well as a water reducing effect with adequate slump retention in a wide range of water cement ratio by introducing a shrinkage-reducing component (SRC) into the molecular structure. Superplasticizers have been thought to be adsorbed on cement hydrates and to show their particle dispersing effects by modifying the inter-particle potentials. On the other hand, shrinkage reducing agents of the organic type have been thought to exist at the interface between the aqueous and the pore phases in hardened cement paste and to show their shrinkage reducing effects by reducing the surface tension, which occasionally results in the degradation of freezing and thawing (F/T) resistance. In this study, the mechanisms of NSP are discussed. By building a SRC into the molecular structure, the entrained air system is expected to be controlled successfully compared to traditional shrinkage-reducing agents (SRA) and so the degradation of F/T resistance can be avoided. With the progress of hydration, SRC is released from NSP and the surface tensions decreases, which results in the reduction of drying shrinkage.

Magnetic Resonance Imaging (MRI) is a nondestructive technique that can be used to spatially resolve distributions of certain nuclei. Lithium is a relatively sensitive nucleus for MRI. Therefore, it is possible to directly measure the distribution of lithium in cement based materials. Lithium salts are used in concrete to suppress alkali-silica reaction. The MRI relaxation parameters associated with lithium in cement-based materials are relatively short by traditional MRI standards. Due to the short relaxation parameters, special MRI measurement techniques and hardware considerations had to be developed in order to quantify lithium distributions in cement based materials. MRI has the potential to play an important role in concrete technology. While this method has been developed for laboratory studies, measurements could be made on cores extracted from existing concrete structures.

The use of precast panels made of new ultra-high performance reactive powder composite materials (RPCM) in civil construction as permanent formwork for concrete structures and buried forms for bridge slabs is a new application that has great applicability prospects due to the high strength and durability of these new materials. The bond at the interface between RPCM and fiber-reinforced ordinary cement concrete is studied in this paper using slant shear tests. The program aimed at testing the bond between a substrate of RPCM with different surface treatments and an overlay of ordinary concrete with different fiber contents. This study concluded that casting the bonding surface of the substrate on air-cell plastic sheets produces a rough surface with concavities and thus provides an easy and effective way to increase the bonding strength. The results of this method of surface preparation are comparable to conventional methods like sand-blasting or grinder treatment. Increasing the fiber content of the concrete overlay in-creases the bond strength with the RPCM substrate if the surface of RPCM is not treated. When the surface of RPCM is roughened to obtain mechanical interlock, the high fiber content of the concrete overlay negatively affects the bond strength. A volumetric fiber content of 0.1% in the concrete overlay is found to be adequate in all surface types.