Water reducer

Abstract: Viscosity-enhancing admixtures, also known as anti-washout admixtures, are water-soluble polymers that increase the viscosity and cohesion of cement-based materials. Such enhancement of the liquid-phase viscosity is essential in flowable systems in order to reduce the rate of separation of material constituents and improve the homogeneity and performance of the hardened product. Viscosity-enhancing admixtures are mostly used along with a high-range water reducer to obtain a highly fluid, yet cohesive cement-based material that can flow readily into place with minimal separation of the various constituents of different densities and minimal intermixing with the surrounding water whenever cast under water. This paper reviews the types and modes of action of commonly used viscosity-enhancing admixtures and highlights their influence on the rheological properties of water and cement paste. An overview of the influence of various types of viscosity-enhancing admixture on high-range water reducer demand, resistance to water dilution, static and forced bleeding, segregation, settlement, setting time, and air entrainment is presented. The influence of such admixtures on bond to anchored reinforcing bars, frost durability, mechanical properties, and rapid-chloride permeability is also highlighted. Special applications where such relatively new admixtures can significantly enhance performance are highlighted, including their incorporation in concrete intended for underwater placement and repair, self-consolidating and segregation-free concrete for abovewater construction, and structural grout for filling post-tensioning ducts.

Abstract: The effects of combined additions of welan gum, a commonly used rheology modifier, and naphthalene-based high-range water reducer on the rheological properties of cement grouts are investigated for mixtures made with 0.40 water-to-cement ratios. Grouts with dosages of rheology-modifying admixture varying from 0 to 0.075 percent by mass of cement were prepared. For each group of grout, the concentration of high-range water reducer was varied to obtain four mixtures of various fluidity levels. Measured properties included apparent viscosities at different shear rates, and estimates of plastic viscosity and yield value. Other measurements of consistency included the ease of spread and flow of grout evaluated using the mini-slump and Marsh cone tests, respectively. The grout stability was evaluated by measuring its resistance to water dilution when cast in water as well as its ability to retain water when subjected to sustained pressure (forced bleeding). Initial setting times were determined for selected mixtures. In all, a total of 27 grout mixtures were evaluated. Test results show that the increase in the dosage of rheology-modifying admixture increases significantly the yield value and plastic and apparent viscosities of cement grouts. Combined with an adequate dosage of high-range water reducer, losses in fluidity are regained without significant reduction in stability. With the increase in high-range water reducer dosage, the apparent viscosity at low rates of shear decreases more dramatically than that at high rates of shear due to the pseudo-plastic behavior of such grouts. The combined use of proper dosages of rheology-modifying admixture and high-range water reducer is shown to clearly contribute to securing high-performance cement grout that is highly fluid, yet cohesive enough to reduce water dilution and enhance water retention. For equal fluidity level, greater stability is obtained with mixtures containing high contents of viscosity modifying admixture. The initial setting time is shown to be delayed by the incorporation of high-range water reducer and rheology-modifying admixture with the latter additive exhibiting greater influence on retardation of setting.