International Concrete Abstracts Portal

The variation in rheological properties of normal portland cement type-10 and blended silica fume (SF) cement pastes was investigated as a function of temperature (0-40°C) in order to elucidate changes in concrete workability with ambient temperature. The rheological parameters measured included the Kantro mini-slump (spreading areas, S) and the dynamic viscosity (r> at various shear rates as a function of superplasticizer concentration (sodium polynaphthalene sulfonate, PNS). To interpret the changes in fluidity of the cement pastes, the concentration of the superplasticizer in the solution phase was monitored as a function of time measurements also measured of the earlv cement hydration rate (0- 3 hrs) in the pastes were in some cases. (O-2 hrs); calorimetric The variations observed in paste fluidity (S, or l/q) at a given PNS dosage exhibit significant non-linear variations with temperature; the rate of change of S and l/q with time (i.e. slump loss rate) are also found to be non-linear, usually with a maximum value in the interval 5-20°C. The non-linear effects are more pronounced with the SF cement than with the type-10 cement. The observations are interpreted tentatively on the basis of coupled physico-chemical effects involving PNS adsorption on cement and on silica, and the influence of PNS on the early hydration rate.

The fluidity of self compacting concrete is influenced by the fluidity of the composed mortar. This paper discusses the rheological estimation of the fluidity of highly-flowing mortars with superplasticizers. The character of the fine aggregate is considered the most effective factor on mortar fluidity. The applicability of the viscosity equation proposed by Mori and Ototake for condensed suspension liquids to highly-flowing mortar is discussed. Mortars were tested with different volume concentration of fine aggregate suspended in highly-flowing paste. The result from rheological test using a Brookfield type viscometer indicated good agreement with the equation. Provided that the fluidity of the mortar behaves as a Bingham fluid, it can be estimated by plastic viscosity and yield value. These rheological tests were applied to two types of highly-flowing mortars composed of self-compacting concrete mixtures ranging from 60 ~ 70 cm of slumping flow value. The effects of the fine aggregate fluctuated with differences in water content, sand content and sand grading of the basic mortar. Test result showed that the character of the sand remarkably affects the 5uidity of highly-flowing mortar.

A series of viscosity agents, some of which have no tendency to adsorb onto cement particles, has been studied for self-compacting concrete. It was found that the mortar rheology of self-compacting concrete is controlled by the adsorption nature of viscosity agents. The adsorptive viscosity agents such as cellulose derivatives form bridge structures and compete with the super-plasticizers (SP) to occupy at adsorption sites once the agents adsorb on the surface of the cement particles in the mortar. This reduces the fluidity of the mortar. A use of the non-adsorptive viscosity agents, on the other hand, accomplishes the adequate fluidity and viscosity of the mortar. In addition, the adsorption sites on the cement particles are reserved for the SP even such agents are used. In order to understand how the viscosity agents work, the adsorption amounts, the zeta potentials and the particle size distributions were observed, while the interaction between the admixtures were examined.

New construction or repair of existing structures dealing with the placement of concrete underwater poses certain challenges that require special placement techniques, and more specifically, specialized mixture proportions. Anti-washout admixtures (AWA), based on water-soluble polymers of various types, have been developed for use in concrete placed underwater in order to minimize cement washout and provide in-place concrete of satisfactory quality. These admixtures create a cohesive paste phase either by binding free mix water or by enhancing inter-particle attraction. Hence, they are finding additional uses such as segregation or bleed control agents, and viscosity modifiers. Recently, the need for a viscosity modifying agent arose in a dam rehabilitation project in Alabama where a grout curtain was being placed to control seepage under the dam. To verify the feasibility of using an AWA in the grout mixtures for this project, a preliminary study was performed in the laboratory to investigate the performance of fluid grouts treated with a unique cellulose-based liquid AWA developed for underwater concreting. The effect of this unique AWA on the plastic properties of fluid and flowable grouts are presented in this paper. The results indicate that it is possible to use this admixture to modify grout viscosity, minimize bleeding characteristics, and improve washout resistance of fluid and flowable grout mixtures. Applications that might benefit from these improvements include: grouting riprap in tidal zones; and placement of grout curtains to minimize water seepage under dams.

A super workable concrete is defined as the concrete which has excellent deformability, high resistance to segregation, and can be filled into heavily reinforced areas without applying vibration. High deformability and high resistance to segregation are important properties of super workable concrete. Since these properties are essentially opposite in nature, they have a tendency to be sensitive to quality fluctuations of materials incomparison to conventional concrete when manufactured at ready mixed concrete plants. For improvement of manufacturing and handling, the authors developed the super workable concrete using ,8 - 1,3-Gluca.n(Curd1a.n) viscosity agent. which i s produced by afermentation process as a The purpose of this study is to investigate the properties of p -1, 3-Glucan which gives a super workable concrete and the mechanism by which it produces such effects. The results confirm that ,8 -1, 3-Glucan exists in the form of independent swollen particles, calcium complex gel which have negative electric charge on their surface. They also indicate that ,4? -1,3-Glucan has the ability to increase the viscosity of cement slurry. Thus, p -1, 3-Glucan gives an effect to increase plastic viscosity without changing yield value of a mortar, which was obtained by wet-screening a concrete.