Kamal H. Khayat

Viscosity-enhancing admixtures for cement-based materials — An overview

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.

WORKABILITY, TESTING, AND PERFORMANCE OF SELF-CONSOLIDATING CONCRETE

Self-consolidating concrete (SCC) is a new category of high-performance concrete that exhibits a low resistance to flow to ensure high flowability and a moderate viscosity to maintain a homogeneous deformation through restricted sections, such as closely spaced reinforcement. This paper reviews the benefits of using SCC to facilitate the casting of densely reinforced sections and improve productivity and onsite working conditions. Workability requirements necessary to secure self-consolidation and the principles involved in proportioning such highly flowable concrete are discussed. Field-oriented tests useful in evaluating the deformability, filling capacity, and stability of SCC are presented. The performance of concrete mixes proportioned according to two main approaches needed to ensure high deformability, low risk of blockage during flow, and proper stability are compared. Such approaches involved the proportioning of concrete with a moderate water-to-cementitious material ratio (w/cm) of 0.41 and using a viscosity-enhancing admixture to increase stability, as well as mixes without any viscosity-enhancing admixture, but with lower w/cm of 0.35-0.38 to reduce free water content and provide stability. Mixes with both moderate and high contents of ternary cementitious materials were evaluated. The performance of each concrete was compared to that of a flowable concrete with 250-mm slump.

USE OF VISCOSITY-MODIFYING ADMIXTURE TO ENHANCE STABILITY OF FLUID CONCRETE

This paper presents the results of a study carried out to investigate the effects of viscosity-modifying admixture concentration, placement height, and mode of consolidation on enhancing the stability of mixtures made with various water-to-cementitious material ratios and consistency levels. In the first phase, bleeding and settlement are determined using 70-cm high columns cast with concrete containing various viscosity-modifying admixture dosages, water-to-cementitious material ratios (0.50 to 0.70), and slump values (140 to 220 mm). In the second phase, bleeding, settlement, and segregation are evaluated for concretes with 220-mm slump cast in 50-, 70-, and 110-cm high columns. The mixtures are made with a water-to-cementitious material ratio of 0.50 and various contents of viscosity-modifying admixture. The effect of excessive external vibration on stability is also evaluated.

Analytical models for estimating yield stress of high-performance pseudoplastic grout

Abstract The yield stress of cement grout is commonly determined by extrapolating the shear stress–shear rate flow curve to a zero shear rate using an analytical model. High-performance structural cement grouts, made with relatively low water-to-cementitious materials ratio (W/CM) incorporating various supplementary CMs and rheology-modifying admixtures (RMAs), can exhibit rheological behavior different than that of conventional grout. Such mixtures can exhibit high pseudoplastic shear thinning characteristics. Therefore, the degree of error in estimating the yield value of high-performance grouts can be greater than for conventional ones. In this paper, yield stresses of cement grouts made with 0%, 0.03%, 0.05%, and 0.075% of welan gum RMA, by mass of binder, and various high-range water-reducer (HRWR) concentrations were evaluated. Mixtures with different replacement values of silica fume and blast furnace slag were also investigated. All grouts had a constant W/CM of 0.40. Yield stress values obtained using various rheological models are compared. The results showed that, depending on the adopted analytical model, the deducted yield stress can be quite different. A new method to estimate yield stress of high-performance, pseudoplastic grout is proposed and shown to result in lower yield stress estimates than the other models. For mixtures made with 100% cement, the estimated values of yield stress given by the proposed model are found to be close to those estimated using the De Kee model.

OPTIMIZING SELF-CONSOLIDATING CONCRETE WITH LIMESTONE FILLER BY USING STATISTICAL FACTORIAL DESIGN METHODS

Self-consolidating concrete (SCC) is typically proportioned with relatively high contents of cementitious materials and chemical admixtures, leading to relatively high material costs. Such costs can be tolerated in high-value-added applications, especially if cost savings can be realized from using SCC. However, efforts are still needed to reduce material costs for SCC to gain wider acceptance in a variety of applications. In addition to proper material selection and sound mixture proportioning, incorporation of readily available fillers can enable reduction in cement and admixture contents, leading to savings in costs. This paper presents results from an experimental program in which response surface methods were employed to optimize a 4-component concrete containing limestone filler subject to 8 performance criteria.

Effect of Welan Gum-High-Range Water Reducer Combinations on Rheology of Cement Grout

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.

Assessment of Thixotropy of Flowable and Self-Consolidating Concrete

This study gives results of a comprehensive research project conducted to evaluate the influence of thixotropy on formwork pressure. The study is divided into 3 phases, with results of the first one reported in this work. Five self-consolidating concrete (SCC) mixtures made with binary and ternary binders incorporating different classes of chemical admixtures as well as 2 flowable mixtures prepared with or without viscosity-modifying admixture (VMA) were investigated. Testing methods and protocols adopted to quantify thixotropy of concrete are presented. Modifications to the concrete rheometer necessary to quantify thixotropy are discussed. Results will be related in a future publication to variations in lateral pressure determined from experimental column formworks measuring 2100mm in height and 200 mm in diameter, and then a larger scale formwork of 3600 mm in height and 900 mm in diameter.

OPTIMIZATION AND PERFORMANCE OF AIR-ENTRAINED, SELF-CONSOLIDATING CONCRETE

The use of self-consolidating concrete (SCC) can enable the reduction of labor demand for vibration and surface finishing, accelerate placement rate of concrete, and secure superior surface quality. Despite the low yield value required for deformability, SCC is characterized by a moderate viscosity to enhance cohesiveness and stability of the fresh concrete. The air entrainment of SCC for frost durability can reduce viscosity, leading to greater risk of segregation and blockage of concrete flow upon spreading between closely spaced obstacles. This paper investigates the mixture proportioning of air-entrained SCC suitable for filling congested sections, such as in the case of repair of the underside of bridge deck girders and conventional nonrestricted elements. The results of a laboratory study undertaken to optimize and evaluate properties of air-entrained SCC are presented. The mixtures were proportioned with 370 kg/cu m, 450 kg/cu m, and 550 kg/cu m of cementitious materials and water-cementitious material ratios of 0.45 to 0.50. Ternary binders containing 20% Class C fly ash or 40% ground blast-furnace slag with 3% silica fume were used. The mixtures were evaluated for slump flow consistency, restricted deformability and surface settlement, strength development, elastic modulus, temperature rise, shrinkage, permeability, and frost durability. Examples of the use of such concrete for repair of a densely reinforced beam in a parking structure and a moderately reinforced beam-wall element with restricted access in a powerhouse are also discussed. Test results clearly indicate the feasibility of proportioning air-entrained SCC of high stability and resistance to blockage. Optimized mixtures exhibited adequate engineering properties and durability. Field studies demonstrated the effectiveness of such high-performance concrete to repair damaged sections presenting difficulties for placement and consolidation.

Performance-Based Specifications of Self-Consolidating Concrete Used in Structural Applications

Proper selection of test methods and workability specifications are key concerns in the optimization and control testing of self-consolidating concrete (SCC). An experimental program was carried out to evaluate the suitability of various test methods for workability assessment and to propose performance specifications of such concrete used in structural applications. Various workability characteristics were determined for approximately 70 SCC mixtures made with water-cementitious material ratios (w/cm) of 0.35 and 0.42. Workability responses included the slump flow, J-Ring, V-funnel flow time, L-box, filling capacity, and surface settlement tests. Comparisons of various test methods indicate that the L-box blocking ratio (h 2 /h 1 ) and the J-Ring flow diameter can be related to filling capacity values determined using the caisson test. It is recommended that SCC used in structural applications should have slump flow values of 620 to 720 mm. To ensure proper filling capacity greater than 80%, such concrete should have high passing ability that corresponds to L-box blocking ratio (h 2 /h 1 ) ≥ 2 0.7, J-Ring flow of 600 to 700 mm, slump flow minus J-Ring flow diameter ≤ 50 mm, or V-funnel flow time ≤ 8 seconds. Such SCC should have a settlement rate of 0.16%/h at 30 minutes, corresponding to 0.5% maximum settlement.

Air-Void Stability in Self-Consolidating Concrete

Ensuring an adequate air-void system in concrete is essential to obtain proper resistance to freezing/thawing and deicer scaling. With increasing usage of highly flowable and self-consolidating concretes, it is important to ensure that these concretes can secure a proper air-void system that remains stable during agitation, placement, and setting. This paper gives information regarding the effect of mixture composition on the stability of an air-void system during agitation. Several mixtures made with various contents of cementitious materials and proportioned to ensure stability by different means were studied. The results will be of interest to engineers and concrete technologists involved in producing highly flowable, air-entrained concrete.