International Concrete Abstracts Portal

In many countries there are regions with very cold winter climate. To provide for year-round building and construction works antifreezing admixtures must be used. When suitable technology and high performance antifreezing admixtures have been used, ready mix concrete may be delivered and placed at temperature as low as -25 °C. The transfer from “summer” concrete mix to “winter” one implies the addition of optimal amount of antifreezing admixture. In principal, both separate superplasticizer and antifreezing admixture dosing and usage of blended (combined) admixture is possible. In Russia the usage of blended plasticizing-antifreezing admixtures is predominant historically. The prime blended admixtures on the base of sodium formate didn’t freeze down to -15 oC and allowed secure concrete hardening with dosages about 10% of commercial product. At present day such admixtures are unsuitable neither by the dosage level (danger of ASR) nor by the freezing-point. The method for elaboration of blended antifreezing admixture (BAFA) with extreme freezing-point was developed. These admixtures allow building and construction works at a temperature as low as -25 oC with dosages about 1-1.3% for PCE-based admixtures and 1.8-2.2% for PNS-based admixtures. It is important that these dosages ensure superplasticizing effect. Non-chloride non-plasticizing admixture with freezing-point below -40 oC was developed also. The kinetic of concrete strengthening at various temperature regimes has been thoroughly studied.

In this research several fiber-reinforced mortars (FRMs) were studied. The effectiveness of three different kinds of macro-fibers was tested: brass-coated steel fibers (SF), polyvinyl-alcohol fibers (PM and anti-crack HP glass fibers (GF) were separately added to superplasticized mortar mixtures, at the same rate of about 1,2% by volume of the mortar. Moreover, special FRMs were also manufactured combining a CaO-based expansive agent with a shrinkage reducing admixture (SRA) in order to reduce the risk of cracking induced by drying shrinkage and make more reliable mortars from the durability point of view. All the mortar mixtures were characterized by the same w/c ratio of 0.45, and the same sand/cement ratio of 3.0, as well as the same amount of a polycarboxylate-based superplasticizer (0.6% by weight of cement). A control superplasticized mixture (CM) with the same w/c, the same sand/cement ratio, but without fibers, expansive agent and SRA was also prepared and studied for comparison purpose. Moreover, an expansive mortar (EM) without fibers but with expansive agent and SRA was manufactured and studied. All the mortar mixtures were characterized for the workability in the fresh state (where they showed approximately the same plastic consistency), and in the hardened state by measuring compressive and flexural strength, as well as free or restrained length changes. The results obtained show the effectiveness in the restraint expansion of the combined use of macro-fibers, expansive agent and SRA when glass fibers and steel fibers were used.

The effect of pore solution composition on zeta potential and superplasticizer adsorption has been investigated experimentally. The investigations were conducted on highly concentrated suspensions, containing quartz flour, limestone flour, cement and combinations of these materials. Furthermore cement-limestone suspensions with different types of cements and a varying ratio of cement to limestone were investigated. The results show that the zeta potential is significantly determined by pore solution. In a pore solution of highly concentrated cement suspensions the zeta potential can be characterized by the ratio of calcium to sulfate concentration. Furthermore it was shown that the superplasticizer adsorption is affected the zeta potential. At higher, more positive zeta potentials the superplasticizer molecules are more likely adsorbed onto the solid surfaces. Moreover, superplasticizer adsorption in limestone-cement suspensions is predominantly controlled by the composition of pore solution rather than the ratio of cement to limestone flour. If the ion concentration of the pore solution is artificially kept constant, the polymer adsorption is almost constant independent of the cement to limestone ratio in the suspension.

A series of polycarboxylic ether (PCE) superplasticizers were copolymerized through acrylic acid (AA) and isobutylene polyethylene glycol (IPEG) with different molecular weight (500, 1000, 1500, 2000, 2400 and 2700). The molecular weight, molecular weight distribution and reaction ratio were measured by gel permeation chromatography (GPC). The initial fluidity and flow retaining ability were evaluated through mini slump test at the same dosage (by mole and by weight). The results indicated that the dispersing capability of single PCE molecule firstly improved with the increase of backbone length, and then decreased after reaching a critical value. Both the appropriate backbone length and side chain density of PCE were affected significantly by the side chain length. In general, PCE with longer side chain, shorter backbone length and lower side chain density are appropriate to achieve better comprehensive properties. Finally, regression equations were proposed to predict the suitable backbone length and proper side chain density of PCE based on the length of grafted side chain.

Portland cement is a multi-phase material, which can be simplified as a two-phase system with alite and C3A as the main constituents determining early properties. Alite is the most abundant phase and in a first approximation it is responsible for the development of mechanical strength during hydration, while C3A mainly affects the plastic behavior before set. In portland cement, superplasticizers are preferentially adsorbed onto C3A and its hydrates rather than alite, due to the different interaction with the mineral surfaces. Three different polycarboxylate superplasticizers (PCEs) were studied, based on copolymers of methacrylic acid and MPEG-methacrylate and characterized by different side chain length and different charge density. Their affinity to C3A and alite surfaces was determined through adsorption measurements on alite/gypsum and alite/gypsum/C3A mixtures. The results of the adsorption tests indicated that the charge density of PCEs, expressed as the ratio between carboxylic groups to ester groups, is the main parameter affecting the adsorption of the PCEs: the lower the charge density, the lower the adsorption on both the phases. The same parameter affects the induction period of alite phase, as demonstrated by in situ XRPD dissolution kinetics experiments, both in the presence and in the absence of C3A. These results can be put in relation with both the hindrance of adsorbed PCE molecules in the dissolution kinetic of alite and the concentration of PCE molecules in solution in conditions of saturation.