International Concrete Abstracts Portal

High-performance concretes (HPC) typically have low w/c to achieve the desired levels of strength and durability. As a result, HPC have a tendency to be stiff and lose their workability rather quickly. Often, high-range water-reducing admixtures (HRWRA) are used to improve the workability of HPC. Care must be exercised when using any admixture, or combination of admixtures, to insure that there are no detrimental side effects that might shorten the life of the concrete. Research has shown that, although retempering concrete with an HRWRA will generally improve workability and maintain the strength of low-w/c concretes, it may also reduce freeze-thaw resistance. Therefore, an experimental study was

The influence of a phosphonate-based admixture called aminotri (methylene phosphonic acid) (ATMP) on the hydration of C 3A, C 3A and 25 percent gypsum, and C 3S and Type I cements (normal, high-alkali, and low-alkali) was evaluated. Isothermal conduction calorimetric investigations of various mixes containing 0.03 to 0.05 percent of phosphonic acid, at water-cement ratios of 0.35 and 1.0, were carried out up to 72 hr. The ATMP acted as a super-retarder for all mixes studied. At a dosage of only 0.05, the exothermal effect for one of the high-alkali Type I cements was delayed by about 16 hr. The hydration of C 3A and C 3S was also retarded by about 40 to 45 min and more than 50 hr, respectively, at a dosage of 0.05 percent. In the C 3A and gypsum system, the third peak corresponding to the reaction of C 3A with ettringite was extended by about 8 to 9 hr.

The corporate use of mineral admixtures, such as slag, silica fume, fly ash, and superplasticizer, in concrete is steadily rising for reasons of economy, enhanced strength, low heat generation, increased durability, and better rheological control. This study reports results of the influence of a cross-linked and NSF type of superplasticizer on the flow properties of blended cements. The cross-linked superplasticizer was comprised of polycarboxylic ether and cross-linked polymer, whereas the NSF type was a modified lignin, alkysulfonate, and polymer. In view of the difference in their molecular structures, their effect was studied on two types of cement: a normal portland cement and a moderate heat portland cement (belite-rich, low in C 3A), to which different proportions of slag, silica fume, and Class C and F fly ash were added to simulate binary and ternary blended cement compositions. Following a detailed chemical and mineralogical characterization of these blending components, the slump flow of 25 mortar blends were tested at a sand-binder ratio of 1.5, with the superplasticizer dosage varying from 2.5 to 3 percent by weight of cement. The water-binder ratio of these mixes ranged from 0.31 to 0.35. Marked differences in flow characteristics (determined by different methods) were recorded as a function of the cement type, blending component, and superplasticizer composition. Viscometric measurements made on the corresponding cement paste mixtures using a rheometer also exhibit pronounced differences in terms of their apparent viscosity. The possible superplasticizer interactions that occur in these blended cementitious systems are discussed. This study reiterates the cement-superplasticizer compatibility factor currently under intense discussion among researchers.

Aim of this study is to estimate the workability of highly superplasticized concrete, which can satisfactorily fill up all the corners of the forms in reinforced concrete without using a mechanical compactor. To place concrete into reinforced concrete members, it must have segregation resistance and high fluidity. These properties are obtained by using very fine powders with an AE-type high-range water-reducing admixture. The highly superplasticized concrete was prepared by mixing blast furnace slag and limestone powder or silica fume with ordinary portland cement. The fluidity was measured by the box flow test (with and without the reinforcement) and by rheological tests of wet-screening mortar. The effects of mix proportioning and spacing of reinforcements on the fluidity of highly superplasticized concrete have been determined.

To place ultra-high-strength concrete with a compressive strength exceeding 100 MPa on site, technology is required to impart high fluidity to the concrete, with a water-to-cementitious material ratio in an extremely low range, 0.25 or less. For this purpose, the authors synthesized a new superplasticizer comprising water-soluble acrylic graft copolymer, which has excellent cement-dispersing capability. Paper reports investigation of the surface chemical properties of the new superplasticizer and those properties of the cement paste and mortar containing it. It was confirmed that the new superplasticizer imparts a higher fluidity to cement paste and mortar, with a lower range of water-to-cementitious material ratio than conventional superplasticizers. It was also found that the surface tension of the solution of the new superplasticizer is similar to that of conventional polycarboxylate superplasticizers, whereas the adsorption by cement and zeta potential of the new superplasticizer is between those of the ¯-naphthalene superplasticizers and the polycarboxylate superplasticizers. The high fluidity of the cement paste and mortar containing the new superplasticizer with a low-range water-to-cementitious material ratio may be particularly attributable to the preceding properties with respect to surface tension as well as molecular weight and chemical structure of the graft copolymer.