International Concrete Abstracts Portal

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.

The scope of this work is to quantify the synergic action of fly ash when mixed with other pozzolans, through the performance of microstructural and durability parameters of HPC. Eleven mixture proportions were tested with fly ash, rice husk ash and silica fume in binary and ternary mixtures, with cement substitution, in mass, from 10 to 50%. The following tests related to HPC durability were made: axial compression strength, elasticity modulus, shrinkage, total chloride content, Cl/OH ionic relationship, water penetration and accelerated carbonation. Some micro structural parameters were determined such as bound water, amount of C-S-H, remaining (C3S+C2S) and C-H. The results were calculated related tot the unitary mass of cement, compared to each other in compressive strength equality of 70 Mpa. Practically all the variables linked to the durability presented better performance in the ternary mixtures, than the arithmetic sums of the respective binary mixtures and this behavior is validated with micro structural evidence. It is suggested that the fly ash have a synergic action in ternary mixtures probably due to the higher dispersion of the cement grains, similar of plasticizing mixture action, resoling in additional nucleation sites and larger amounts of hydration products. A synergic action model of fly ash in ternary mixtures is provided.

The invention of superplasticizers is one of the most important breakthrough that has led to the development of high performance concrete. Superplasticizers can be used for three different purpose, namely (a) to increase workability without changing the mixture composition, (b) to reduce the amount of mixing water in order to reduce the water-cement ratio and then to increase strength and/or improve durability, and © to reduce both water and cement in order to reduce cost in addition to reducing creep, shrinkage and thermal strains caused by heat of cement hydration. Practical examples of these different ways of using superplasticizers are given by referring to the traditional superplasticizers (naphthalene- and melamine-based) and to the recent advances in this area (acrylic polymer-based admixtures). In particular the following topics are examined: composition of superplasticizers, mechanism of action (electrostatic repulsion and steric hindrance), influence of the cement composition (C3A, alkali, SO3) mode of addition of superplasticizer, slump loss, blending of superplasticizers.