International Concrete Abstracts Portal

Shrinkage-reducing admixtures currently available in the market serve for controlling cracks occurring in concrete due to reduction in the shrinkage stresses from drying by relieving the surface tension of free water in concrete. However, it is generally believed that freezing and thawing resistance of concrete would be decreased when shrinkage-reducing admixture is used, and presumably this can cause problems in cold and snowy regions, In this study, analytical examinations were carried out by focusing attention on the molecular structure of a shrinkage-reducing admixture and methods of its application for the purpose of improving freezing and thawing resistance of concrete. As the result, it was found that freezing and thawing resistance could be improved if concrete is mixed with delayed adding method of shrinkage-reducing admixture.

The oxygen diffusion coefficient through hydrophobic cement-based materials fully immersed in water was determined by potentiostatic measurements on concrete and by the use of a diffusion cell on cement pastes and mortars. The results obtained show that very high oxygen diffusion occurs through cement paste, mortar and concrete made with hydrophobic admixture as opposed to negligible diffusion through the reference cement matrix without admixture. Moreover, the oxygen diffusion coefficients measured through hydrophobic cement matrices immersed in water were comparable with those reported in literature for unsaturated cement materials in air. These experimental results appear to confirm that oxygen dissolved in water directly diffuses as a gaseous phase through the empty pores of a hydrophobic cement matrix. This could explain the severe corrosion of steel reinforcement embedded in cracked hydrophobic concrete immersed in an aqueous chloride solution observed in a previous work.

Reactive Powder Concretes (RPC) - in form of superplasticized cement mixtures with silica fume, steel fibers, quartz fine sand (100-400 um) and/or limestone coarse aggregate (0.1-8 um ) - were studied in comparison with modified RPC where artificial aggregates substituted for natural aggregates. Artificial aggregates were obtained by grinding portland clinker coarsely so that fine and coarse aggregate were obtained with approximately the same particle size distribution of natural fine quartz (100-400 um and limestone gravel (O.l-8 mm) respectively. The source of clinker-aggregate was the same as that used for portland cement as binder of RPC. The idea was to improve the bond strength between cement paste and aggregate due to some hydration of the clinker-aggregate surface. RPC specimens were cured at room temperature (2OO C) or steam-cured at low or high pressure at 90°C or 160°C respectively. Compressive strengths were measured as a function of time at 1-28 days. The 28-day compressive strength level was as high as 200 MPa. Regardless of the curing temperature, compressive strength of RPC was increased by about 20 MPa when clinker-aggregate was used instead of natural aggregates. These results indicate that the bond strength of the interface between cement paste and aggregate is improved when clinker particles are used instead of natural stones. Scanning electron microscope observations of the microstructure confirmed this hypothesis and indicated that the interface between cement paste and natural aggregate is the weak point in RPC.

There is presently a strong drive to minimize the amount of Portland cement used in cementitious systems replacing it to the extent possible with supplementary cementitious materials, such as silica fume, blast furnace slag or fly ash. Such an approach would enable the use of more and more environmentally friendly concrete. With respect to applications, construction practices continue to evolve towards minimum-labor technologies which are more reliable, and usually more cost-effective. Most of these applications require the use of chemical admixtures, such as water reducers, high-range water reducers, and in some cases viscosity-enhancing additives. With the increase in the replacement value of portland cement with supplementary cementitious materials and the growing trend towards using flowing concrete with high workability, it is critical to assess the risk of such mixtures to bleeding and segregation. This paper presents the results focusing the evaluation of the influence of dosage rates of high range water reducer, set retarder, and viscosity-enhancing admixture on the changes in stability of highly flowable cement-based materials. This characterization is undertaken using a newly developed method to quantitatively measure continuously and in-situ changes in the homogeneity of cementitious materials with time during their consolidation period. The method is based on monitoring the variation of electrical conductivity with respect to depth of the test sample and time using electrokinetic probe with multiple electrode pairs embedded at various heights and operating using a low voltage pulsating current at 1 kHz.

Chemical admixtures play a central role in modern concrete materials and technologies. In conjunction with mineral additives such as silica fume, chemical admixtures have enabled major improvements in many of the properties of concrete, particularly, compressive strength and durability. Chemical admixtures have also assisted in developing new concrete technologies, for example, concrete pumping and self-leveling, underwater concreting and shotcreting. Chemical admixtures have further promoted the use of secondary industrial materials (blast furnace slag and fly ash) in cementitious systems, contributing to resource conservation and environmental sustainability. In the continuing quest for more cost-efficient and environmentally acceptable materials and technologies, it may thus be expected that chemical admixtures will continue to play an important role in future generations of concrete. Probing into the future, how will concrete chemical admixtures evolve in the coming decades? What trends can be anticipated in future developments and use of these admixtures ? What will be the driving influences for these developments? This paper addresses some of the issues that are considered relevant driving forces to promote changes in the use of currently available chemical admixtures, or in the development of new admixtures. The trends already apparent in cementitious materials and concrete applications provide a reasonable basis for proposing probable trends in the evolution of concrete admixtures into the 2 lSt Century.