International Concrete Abstracts Portal

In this work a paste viscometer with concentric cylinders is used to evaluate the mineral- and chemical admixtures’ impact on fresh fine mortar rheology. Rheology results have been evaluated according to the Bingham model, which describes the rheology with the parameters; yield stress and plastic viscosity. Seven dolomite fillers have been investigated in combination with one superplasticizer and two cement types. The fillers originate from the same deposit, i.e. having the same mineralogy but different particle size distribution. Relationships have been established between rheology results and particle size analysis of cements and fillers. Increasing fineness leads to higher yield stress and plastic viscosity. Addition of superplasticizer results in a parallel displacement of these relationships towards lower rheology values. The results also show a significantly large difference in rheology depending on type of cement. Rheologically, the finer cement exhibits lower yield stress and plastic viscosity than the coarser cement, when adding the same amount of filler. The results for the reference mixes, when no filler is added, are the opposite. Rheology tests using a paste viscometer with concentric cylinders on the fine mortar part of the concrete is a effective method to describe additives’ influence on fresh concrete rheology.

Recently in Japan, foamed cement paste and foamed mortar have frequently been used as embankment material on soft ground because of their low unit weight, flowability, and ease of strength control. Conventional uses of these foamed materials include grouting for tunnel voids and for securing pipelines in tunnels. Since each material for foamed cement milk and foamed mortar have a different specific gravity, segregation can occur during mixing, pumping and placing. The mixture proportions of these foamed mixtures is very important, and are usually determined by trial and error. The authors carried out experiments on the rheological properties of cement past, mortar, foamed cement milk and foamed mortar. Compressive strength of these materials were also examined. Obtained results are as follows.(l)Plastic viscosity of foamed cement pastes or mortars are as same as that of cement pastes or mortars. (2)Yield value of foamed mixture decreases with addition of foam less than 30% by volume, and decrease with addition of foam more than 40%. (3)Relation between plastic viscosity and yield value is almost the same as mixtures without foam.(4)Compressive strength of foamed mixtures decreases with increase of foam content , and the tendency is apparent in lower w/c.

This paper describes the effects of properties of powder such as specific surface and solid volume percentage on the plastic viscosity of fresh paste with admixtures. Eight types of blended powder and three types of high-range water-reducing admixtures (HRWRA) were used, and the plastic viscosity of paste mixed with these materials was measured, keeping the yield value of paste constant. The fresh paste is considered to be a type of highly concentrated suspension, so the authors propose a method to predict plastic viscosity based on this concept. In this method, the ratio of powder content, the specific surface, and solid volume percentage of powder, the type of admixture, and other factors are taken into account.

The significant increase in large high-rise concrete structures has dictated diverse requirements for concrete. Concrete technology has improved, widening the range of applications of this material. In this regard, the technology of polycarboxylate-based superplasticizers has made remarkable progress; a number of new superplasticizers have been developed and applied to big construction projects. To elucidate the fluidizing mechanism of superplasticizers in cement, we focused on the fact that the chemical structure of these superplasticizers changes in an alkali environment. We determined the absolute molecular weight of the superplasticizers using the light scattering method, measured the amount adsorbed, zeta potential, and nuclear magnetic resonance. Based on our finding pertaining to polycarboxylate-based superplasticizers, we discuss the fluidizing mechanism of cement compositions with relevance to dispersibility, dispersibility retention, and flowability. We refer to DLVO theory, steric repulsion effect theory, depletion effect theory, tribology effect, as well as the results of mortar and concrete tests. We also report on the applications at big projects, such as the anchorage of the Akashi Channel Bridge and the deep ground continuous wall of the Tokyo Trans-Bay Highway.

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.