International Concrete Abstracts Portal

Polycarboxylate polymers as superplasticizers have revolutionized concrete technology in the past years. Today most of SCC and high performance or ultra high strength concrete is produced with this new kind of polymers. Their comb-like structure consisting of an ionic backbone and non-ionic polyalkylene glycol side chains offers a huge "playground" for polymer design. Polymers with different side chain chemistry, length, grafting density as well as with different backbone ionic content, structure and length are on the market, which offers the customer an almost confusing range of superplasticizers. Different concrete applications often require different and chemically optimized superplasticizers. The universal admixture is still a challenge for the researchers. In order to better understand the structure-performance relationship of polycarboxylates, systematic variations of comb type superplasticizers were produced in the laboratory and the influence of the changes in polymer structure on the performance in cement paste, mortar and concrete was studied. Clear effects of the structural changes on the performance regarding water reduction, flow improvement and slump retention can be shown. They stress the importance of a good adsorption and adequate surface coverage to achieve good dispersion. All can be directly influenced by the polymer design depending on the desired properties of the superplasticizer.

In this paper, the influence of high volumes of ultra-fine fly ash, raw fly ash, silica fume and natural zeolites on the properties of self-compacting concrete is evaluated. Three different samples of ultra-fine fly ash obtained after high energy milling of raw material were employed. Concretes were prepared employing various kinds of mineral additions as partial replacement (40%) of cement or in addition to it. Fresh concretes prepared were submitted to technological characterization through the determination of normal slump and J-ring slump flow values. The values of the normal slump flow were found to range between 604 and 785 mm, while the differences with the J-ring slump flow were less than 30 mm. Specimens were tested to evaluate the mechanical properties of the hardened concrete after 7- and 28-day curing. Specifically, the modulus of elasticity and compressive strength were determined. Significant strength increases were observed for the concrete containing ultra-fine fly ash. Finally, no segregation phenomena were observed in the case of cylindrical column specimens (30 x 150 cm). All the results obtained show that environmentally sustainable, high workability concrete could be successfully prepared using large volumes of mineral additions.

A novel slurry rheology modifier (VT) has been developed for slurry, mortar and concrete using an inorganic powder such as cement. This chemical admixture mainly consists of cationic and anionic surfactants. VT can efficiently exhibit unique viscoelasticity in spite of low molecular weight when the cationic and anionic surfactants coexist in slurry at the same time. This technology gives very good segregation resistance to cement slurry and concrete under several conditions, e.g., water, vibration, and high water-cement ratio and without setting retardation. The viscoelasticity mechanism of VT is caused by the formation of higher-order structure of surfactants in slurry. Controlling the viscoelasticity of VT is an effective method to prevent segregation for cement slurry, various grouts for repair and ground improvement. VT has various potential abilities in civil engineering and construction fields because of unique viscoelastic behavior.

To analyze the performance of two shrinkage reducing admixtures (SRA), tests have been performed, in order to evaluate the influence of increasing dosages of the admixtures. Shrinkage, compressive strength and weight loss have been measured. The efficiency of the admixtures in controlling autogenous and drying shrinkage was evaluated using sealed and unsealed specimens. As the accepted mechanism of action of these products is the reduction on surface tension of the capillary liquid of concrete, surface tension of water solutions was also determined to compare the shrinkage reduction with the decrease of surface tension. Mixtures with four different dosages of each SRA were prepared. The results indicate that shrinkage reduction is not proportional to the decrease of surface tension. Secondary effects such as changes in the evaporation rate and in the compressive strength of specimens have to be considered. Results of sealed specimens are not fully consistent with the accepted main mechanism of action of these products.

Testing of admixtures performance is done routinely in industrial laboratories with spread tests in which a cylindrical mould is filled with cement paste and then lifted. The sample spreads and the measured diameter is used to infer the efficiency of the admixture. Recent results link this measurement quantitatively to yield stress. On the other hand, expressions for the dependence of yield stress on interparticle forces and volume fraction of cement have been proposed. In this paper, it is shown that the combination of both approaches brings new insights into the mechanisms by which admixtures modify the rheology of cementitious materials.