International Concrete Abstracts Portal

The Second CANMET/ACI International Conference was held in Brazil in 1999 and showcased information on emerging high-performance concrete in Brazil and other South American countries. Over 100 papers were submitted from all over the world and were reviewed in accordance with ACI policy. Forty-five were accepted for publication in this volume. 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. SP186

The present paper provides an example of the application of the holistic model to the study of one of the most complex phenomenon in the science of concrete durability, namely the deterioration caused by delayed ettringite formation (DEF) in a sulfate-free environment. By adopting the holistic approach, a new model to explain this damage is proposed. The model is based on three essential elements: late-sulfate release, microcracking, and exposure to water. Late-sulfate release from a cement with high-sulfate content (especially that with high content of clinker sulfate in less available form) can cause the delayed deposition of ettringite in pre-existing microcracks after sulfate ions diffuse through the pore solution in concrete, either intermittently, or continuously exposed to environmental water. Microcracking may be promoted by alkali-silica reaction, steam curing at high temperatures, localized high stress in prestressed concrete structures or other causes. Theoretically, the DEF-included damage occurrence can be reduced or prevented by controlling at least one of the above three parameters. In practice, the best way of reducing the DEF-induced damage risk is either to avoid cements with high clinker sulfate that are responsible for the late-sulfate release, or to adopt lower and more homogenous stress distribution derived from the prestressing process in precast elements, such as concrete ties.

This paper discusses a new method for the determination of the optimum W/C plus FA for maximum compaction of no slump concrete made with high volumes of fly ash. It explores the effect of fly ash fineness and particularly, carbon content on the explores the effect of fly ash fineness an particularly, carbon content on the compressive strength of the mixtures made with 50% and 70% replacement of normal portland cement with fly ash. By using an appropriate surfactant the no slump concrete mixtures are rendered workable and suitable for structural applications. The strength attained at 28 days is 60 Mpa or more, and therefore these mixtures are considered to yield high-strength concrete. The performance of the high-volume fly ash concrete is assessed in terms of abrasion and fatigue resistance that are the most appropriate performance indicators for concrete that will be used for the construction of pavements.

One of the limitations of high-performance concrete is its susceptibility to autogenously shrinkage, which can considerably compromise its durability and mechanical strength despite diligent cure during the hardening stage. Although calcium-sulfoaluminate admixture (CSA) and shrinkage-reducing admixture (SRA) can play a role in reducing autogenously shrinkage, rapid set and low effectiveness have been associated with their use in concrete with low water content/ Recently, an innovative type of CSA admixture based on pre-hydrated high-alumina cement (H-HAC) has been developed, but its potential in reducing autogenous shrinkage has not yet been evaluated. In this paper, the effects of H-HAC expansive admixture in decreasing the shrinkage during the early hardening and hardening stages are described. It was hoped that the expansion generated from hydration of the expansive components would offset the volume reduction due to autogenous shrinkage. The influences of particle size distribution, dosage and type of H-HAC hydrates in determining expansion are evaluated and discussed. Microstructural and chemical aspects are studied through XRD and SEM techniques. Tests on the evaluation of the H-HAC admixture to offset autogenous shrinkage are described and preliminary results are presented. The results show that in specific dosage and particle size distribution, H-HAC is a suitable admixture for compensating shrinkage, although autogenous shrinkage in high-performance concrete has not been prevented by the use of this admixture.

A total of 15 repaired and unrepaired reinforced concrete beams was tested under static loading in this study. The beams were of rectangular cross-section, 100 by 120 mm, and tested in flexure in simply supported mode on a span of 600 mm under a line load at mid-span. These include control beams, which were unrepaired, and beams repaired by shotcreting prepacked materials, shotcreting mortar, casting prepacked materials and casting prepacked material with steel fibres. The performance of the repaired beams statically-loaded to failure was monitored. The influence of differential shrinkage between repair material and substrate, bonding strength, casting method and the presence of steel fibres on the mechanical behaviour of repaired beams are discussed in this paper. The test results indicated that the mechanical properties of the repaired properties and the mix proportions of the repair materials used, and the repairing methods employed. The flexural stiffness of the repaired beams could be increased by using steel fibres, improving the bond quality between substrate and repair materials, and selecting appropriate repair method.