International Concrete Abstracts Portal

Recycled-aggregate concrete is known for its higher tendency to shrink with respect to concrete prepared with ordinary aggregate, at least when both coarse and fine recycled aggregate are used. In this work, an attempt was made to manufacture recycled-aggregate concrete that is less sensitive to shrinkage. Seven different concrete mixtures were prepared with the same water-to-cement ratio of 0.45 by various kinds of coarse aggregate: ordinary natural gravel, recycled-concrete aggregate from a suitable treatment of precast-concrete scraps, or a recycled-rubble aggregate from a crushing plant in which rubble from building demolition is treated. Different kinds of water-reducing admixtures were also tested: the first one was based on polycarboxylate polymer, the other two were also based on polycarboxylate polymer incorporating a shrinkage reducing group. The latter two are characterized by a different formulation to assure either a set-accelerating or a retarding effect. The pure superplasticizing admixture was added at a dosage of 0.8%, by mass of cement, while the multifunction admixtures were added at a dosage of 1.6% and 2.0%, by mass of cement, for the accelerating and the retarding types, respectively. Compressive strength tests were carried out at different curing times, and free-drying shrinkage was measured up to 70 days of age. The results were positive, particularly in terms of very low shrinkage of recycled-aggregate concrete containing the shrinkage reducing admixture.

Dynamic and static stability of self-consolidating concrete (SCC) affect the production, transport, and overall performance of the concrete. Viscosity- modifying admixtures (VMAs) are often incorporated to enhance the stability of SCC, especially in the case of cast-in place concrete. The influence of a new type of polysaccharide, diutan gum, on key characteristics of SCC targeted for the construction and repair of concrete infrastructure is examined. The investigation compares the performance of SCC made with diutan gum and different types of high-range water reducing admixtures (HRWRAs), including polynaphtalene sulphonate (PNS)-based and two polycarboxylate polymer (PCP)-based HRWRAs. The effect of admixture combination on workability, rheology, stability, and setting time was investigated. The robustness of optimized VMAHRWRA system and key durability parameters of the concrete were also determined. Test results indicate that the use of diutan gum increases the plastic viscosity and yield stress of SCC. Regardless of the VMA and HRWRA combination, the use of diutan gum can significantly decrease segregation and bleeding, and lead to greater homogeneity of the concrete during the dormant period of cement hydration. The performance of SCC made with diutan gum depends on the type of PCP in use. Out of two PCP-based HRWRA, one system resulted in lower HRWRA demand, increase in viscosity with the increase in VMA content, and greater stability. The use of PCP and diutan gum is shown to lead to adequate robustness where the SCC can tolerate small changes in sand moisture content without significant effect on SCC properties. SCC made with medium dosage of diutan gum and PCP is shown to develop adequate air-void system and excellent frost durability and resistance to de-icing salt scaling.

The relationship between the number of individual functional groups present in lignosulfonate molecular structures and the performance of lignosulfonates in fresh cement pastes was investigated. Lignosulfonate fractions from three different pulping processes (sulfite, sulfate and organosolv) were included in this study. The dispersing, set-retarding and air-entraining effects of these fractions in ordinary Portland cement pastes were studied. Elemental composition, methoxyl and sulfonate group contents were determined analytically. An algorithm was developed to generate model molecular structures representing individual lignosulfonate fractions. Each model structure was based on the results of the chemical analysis, the model structural segment typical of each particular lignin and additional literature data. The numbers of sulfonate, carboxyl, phenolic hydroxyl, aliphatic hydroxyl and methoxyl groups in each proposed molecular structure were determined. The numbers of C-C and C-O-C inter-unit bonds in each molecule were calculated as well. Correlations between selected functional group counts and the dispersing, set- retarding and air-entraining effects of lignosulfonates were determined. For the dispersing effect, methoxyl was the most positively correlated and carboxyl was the most negatively correlated group. Sulfonate had a very low correlation with the dispersing effect. For the set-retarding effect, the C-C inter-unit bond was the most positively correlated and sulfonate was the most negatively correlated. For the air-entraining effect, carboxyl was the most positively correlated and aliphatic hydroxyl was the most negatively correlated group. A low correlation was found between sulfonate and the air-entraining effect. The results are interpreted from the perspective of cement hydration processes and the implications on the understanding of lignosulfonate interactions with cement-water systems are discussed.

Aggregate shape, texture, and grading have been known to have a significant effect on the rheological performance of fresh concrete. Moreover, while the optimization of aggregate selection can provide both technical and economical benefits, the availability of materials and construction operations can often dictate the use and proportioning of certain aggregate sources, such as manufactured sands, which can adversely impact the rheology of cementitious mixtures. The use of certain chemical admixtures has been found to often minimize the need to increase cement and water contents in order to overcome the loss of workability that can accompany aggregate sources which feature flat, elongated, angular, and rough particles. In this study, a wide range of natural and manufactured sands, characterized for gradation, mineralogy, shape, texture, and cleanliness, are evaluated for their effect on mortar rheology, with and without a viscosifying-type chemical admixture. While associations between aggregate characteristics and their impact of mortar rheology may not be readily evident, the ability of this class of admixture can be shown to mitigate the rheological effect of certain sands, and in some cases allow for optimizing the mixture to lower paste contents.

Recently, the concrete workability has become regarded as much more important due to large demands for high durable concrete with low water content and unstable quality of concrete ingredients. The new superplasticizer was found to provide better workability controlling the rheological properties of the fresh concrete than the previous polycarboxylate type superplasticizers regardless of the quality of concrete ingredients. The framework structure of the new superplasticizer was a hyper-branched polymer in which specific function monomers were co-polymerized. The new polymer of the hyper-branched structure was more condensed than that of polycarboxylate type polymers and was highly adsorptive to cement particles, which made a higher density of the polymer adsorption onto cement surfaces possible. This dense and strong adsorption of the superplasticizer polymer molecules provided the unique rheological properties of the concrete or the mortar such as the lower viscosity of the concrete giving much easier handling. This new superplasticizer also provided the robustness against varieties of the concrete ingredients qualities. This superplasticizer for new generations has already been made practicable successfully for several constructions, and this technology will bring various possibilities and merits in concrete construction fields such as shortening the construction period thanks to better workability.