Joseph J. Assaad

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.

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.

Variation of Formwork Pressure with Thixotropy ofSelf-Consolidating Concrete

An experimental investigation was carried out to determine the influence of thixotropy on the development of formwork lateral pressure. Five self-consolidating concrete (SCC) mixtures prepared with different combinations of cementitious materials and set-modifying admixtures were evaluated. An experimental column measuring 2100 mm in height and 200 mm in diameter was used to determine lateral pressure distribution using five pressure sensors mounted at 50, 250, 450, 850, and 1250 mm from the base. This paper offers a comprehensive review of key factors affecting formwork pressure developed by fresh concrete and presents the results pertaining to the influence of thixotropy on variations in lateral pressure of SCC. The results show that the lateral pressure exerted by the plastic SCC is directly related to thixotropy. With the increase in thixotropy, the initial lateral pressure registered following casting decreases, and the rate of drop in lateral pressure is accelerated. This is attributed to the reversible effect of the thixotropy which enables the material to regain its shear strength, internal friction, and cohesion when left at rest without any shearing action. Immediately after casting, any decrease in lateral pressure was attributed to the increased internal friction resulting from the use of ternary cement and greater concentration of coarse aggregate. The use of a set-retarding agent resulted in a delay in cement hydration that led to a slower rate of drop in lateral pressure. Conversely, the incorporation of a set-accelerating agent resulted in an increase in the rate of pressure drop.

Effect of Viscosity-Enhancing Admixtures on Formwork Pressure and Thixotropy of Self-Consolidating Concrete

An experimental program was undertaken to determine the influence of the type and concentration of viscosity-enhancing admixture (VEA) on formwork pressure that can be exerted by self-consolidating concrete (SCC). A liquid polysaccharide, powder polysaccharide, and a cellulose-based VEA were used in this study. Both naphthalene-and polycarboxylate-based high-range water-reducing admixtures (HRWRAs) were employed. Thixotropy of the concrete was evaluated and correlated to the initial lateral pressure and its variations in time. Test results show that the type, combination, and dosage rate of VEA-HRWRA have a marked effect on thixotropy and formwork pressure. SCC made with cellulose-based VEA and polycarboxylate-based HRWRA are shown to yield a greater degree of thixotropy compared to similar concrete prepared with polysaccharide-based VEA and naphthalene-based HRWRA. SCC made with cellulose-based VEA, however, exhibited a lower rate of increase in thixotropy with time. This is associated with the greater degree of fluidity retention when using polycarboxylate-based HRWRA. Irrespective of the VEA type and combination with HRWRA, results indicate that the incorporation of VEA at relatively low concentrations could result in lower formwork pressure compared to reference mixtures made without any VEA and those containing medium or high concentrations of VEA. SCC made with a low VEA concentration necessitated lower HRWRA demand to secure the targeted slump flow. This results in a sharper loss in fluidity, enabling the concrete to develop enough cohesiveness to resist some of the applied vertical stress. A good relationship was established between formwork pressure and thixotropy determined for SCC mixtures containing low concentrations of VEA with mixtures of greater degree of thixotropy exerting lower lateral pressure on the formwork. The increase of VEA concentration, however, necessitated a greater HRWRA dosage, thus leading to higher lateral pressure measured initially and over time, despite the increase in thixotropy.

Effect of Coarse Aggregate Characteristics on Lateral Pressure Exerted by Self-Consolidating Concrete

This article reports on a study undertaken to determine the influence of concentration and nominal size of coarse aggregate on the development of lateral and pore water pressures of self-consolidating concrete (SCC). The study evaluated 9 mixtures prepared with sand-total aggregate ratios (S/A) varying between 1.0 and 0.30. An experimental column measuring 2800 mm in height and 200 mm in diameter was used to determine lateral pressure and pore water pressure during the plastic stage of cement hydration. Results show that lateral pressure is significantly influenced by the S/A value. The pore water pressure affects the development of lateral pressure of SCC in the plastic stage. The authors conclude that the lateral pressure developed by the plastic concrete is directly related to internal friction resulting from the coarse aggregate concentration. The greater the degree of increase in internal friction, which corresponds to mixtures made with relatively low S/A values, the lesser the magnitude of initial lateral pressure becomes and the faster the drop in pressure.

Variations of Lateral and Pore Water Pressure of Self-Consolidating Concrete at Early Age

This study investigates the kinetics of variations in lateral pressure and pore water pressure that can be exerted by self-consolidating concrete (SCC) during the plastic stage of cement hydration and until the early stage of hardening. Nine mixtures of constant slump flow values of 650 +/- 15 mm were evaluated by varying the sand-to-total aggregate ratio R from 1.0 to 0.30. The mixture proportioned with an R value of 0.50 was tested with 3 different coarse aggregate gradations of 10-5, 14-5, and 20-5 mm. Each concrete was tested to determine setting time, adiabatic heat rise and temperature development, and pressure variations with time. Test results show that kinetics of lateral pressure drop during the plastic stage are significantly affected by the degree of internal friction that can be related to coarse aggregate concentration. For mixtures with R values greater than 0.50, the concrete behaved as an exclusively viscous material with a limited degree of internal friction, which resulted in greater development of lateral pressure. For mixtures with R values lower than 0.50, the concrete appeared to exhibit greater resistance to shear stress given the higher degree of aggregate friction, which led to considerably lower lateral pressure development. The drop in lateral pressure toward zero occurs after the end of the dormant period as the rate of cement hydration is accelerated. Beyond the dormant period, progressive formation of hydration products leads to the creation of a structural network, and the pore water pressure begins to drop abruptly towards negative values. The time for cessation lateral pressure is shown to be influenced by the R value. For R values greater than or equal to 0.40, the concrete can continue to exert some lateral pressure until the onset of final setting. For mixtures made with R values of 0.36 and 0.30, the cessation of lateral pressure occurred before initial setting as the rate of pressure drop was more accentuated than that resulting from SCC with higher R values. The increase in maximum aggregate size from 10 to 14 mm resulted in a sharper rate of pressure drop, which is associated with an increase in the degree of internal friction. A lower rate of pressure drop, however, can be obtained with a further increase in aggregate size to 20 mm, given the relatively decreased packing density of the 20-5 mm coarse aggregate as compared to that of the 14-5 mm.

Effect of W/CM and High-Range Water-Reducing Admixture on Formwork Pressure and Thixotropy of Self-Consolidating Concrete

An experimental program was undertaken to evaluate the effect of water-cementitious material ratio (w/cm) and type of high-range water-reducing admixture (HRWRA) on the development of form-work pressure that can be exerted when using self-consolidating concrete (SCC). Pressure variation was monitored using an experimental column measuring 2800 mm in height. The tested mixtures were proportioned with a similar initial slump flow consistency of 650 ± 15 mm. Three w/cm of 0.36, 0.40, and 0.46 and three types of HRWRA (polycarboxylate, polynaphthalene sulphonate, and polymelamine sulphonate) were investigated. Variations in lateral pressure were related to the thixotropy of the concrete. Test results show that the variations in lateral pressure and thixotropy of SCC are significantly affected by the w/cm. Irrespective of the HRWRA type, mixtures proportioned with 0.46 w/cm exhibited greater initial pressure and lower thixotropy compared with mixtures made with a w/cm of 0.40 and 0.36. This is related to the higher water content and lower coarse aggregate volume in concrete proportioned with the higher w/cm, which can lead to a reduction in shear strength properties of the plastic concrete. The rate of pressure drop and increase in thixotropy with time, however, were greater in mixtures made with a higher w/cm. This is attributed to the lower HRWRA demand that can lead to sharper fluidity loss with time. For any given w/cm, the type of HRWRA appears to have a limited effect on initial lateral pressure. Compared with naphthalene-and melamine-based HRWRA, the use of polycarboxylate-based HRWRA in SCC resulted in lower rate of pressure drop with time. This is reflected by the greater fluidity retention of the mixtures containing the polycarboxylate-based HRWRA. The incorporation of a water-reducing agent in mixtures made with polynaphthalene sulphonate-based HRWRA is shown to increase lateral pressure development of the plastic concrete over time.

Use of Thixotropy-Enhancing Agent to Reduce Formwork Pressure Exerted by Self-Consolidating Concrete

This paper seeks to evaluate the impact of a thixotropy-enhancing agent (TEA) on the variations in thixotropy and formwork lateral pressure of self-consolidating concrete (SCC) with 650 ± 15 mm (24.3 ± 0.6 in.) slump flow consistency. Six mixtures containing various TEA concentrations along with either a melamine- or polycarboxylate-based high-range water-reducing admixture (HRWRA) were tested. The results are compared with similar SCC mixtures made with conventional viscosity-enhancing admixtures (VEAs), including a liquid polysaccharide, powder polysaccharide, and cellulose-based. Test results show that the use of TEA can significantly increase the degree of thixotropy and reduce the formwork pressure compared with similar mixtures containing conventional VEAs. This is attributed to the thixotropic nature of this agent that enables the material to rapidly recover its cohesiveness following some time at rest. The combination of TEA with either powder polysaccharide or cellulose-based VEA at low concentration was found to reduce the maximum initial pressure and increase the rate of pressure drop with time compared with SCC containing only conventional VEA at similar concentration. The TEA/VEA combinations resulted in better fluidity retention with time. A good relationship exists between the lateral pressure and thixotropy determined from SCC mixtures containing various concentrations of TEA and/or low concentrations of conventional VEA. The higher the degree of thixotropy, the less the mixture develops lateral pressure. This is attributed to the reversible effect of thixotropy that enables the material to increase its shear strength properties after some resting time.

Formwork pressure of self-consolidating concrete made with various binder types and contents

Self-consolidating concrete (SCC) is a high-performance concrete that flows readily under its own weight and achieves good consolidation with a minimum degree of segregation. This article reports on a study undertaken to determine the effect of binder type and content on variations in the lateral pressure of SCC. The mixtures were prepared with five binder types incorporated at various contents varying from 400 to 550 kg/m3. The influence of thixotropy, determined from concrete and concrete-equivalent-mortar (CEM) mixtures, on the variations of lateral pressure development was also investigated. Results showed that, for a given binder content, the initial lateral pressure and rate of pressure drop with time are significantly affected by the binder type in use. Self-consolidating concrete made with 450 kg/m3 of Type 10 CSA cement (GU) and no supplementary cementitious materials exhibited the highest initial pressure, corresponding to 98% of hydrostatic pressure. Test results also indicate that the rate of pressure drop following casting is dependent on the degree of increase in cohesion. The increase in the degree of thixotropy of SCC and CEM can lead to lower initial pressure. The increase in thixotropy determined from concrete mixtures is highly affected by internal friction resulting from the presence of coarse aggregate. This can overshadow the development of cohesion resulting from the binder phase that controls the rate of pressure drop with time. Thixotropy evaluated using CEM mixtures is more adequate to assess the decrease in lateral pressure development with time than thixotropy determined from SCC mixtures.

Relationships Between Key ASTM Test Methods Determined on Concrete and Concrete-Equivalent-Mortar Mixtures

Concrete batching requires considerable amounts of materials, energy, and time for testing. This highlights the importance of using alternative easier approaches based on mortars to simplify and speed up the experimental testing programs. This paper seeks to establish relationships between responses of ASTM Test Methods conducted on concrete and concrete-equivalent-mortar (CEM). Different series of mixtures having various cement contents of 300, 350, 400, and 450 kg/m3 and water-to-cement ratios of 0.4, 0.45, 0.5, 0.55, and 0.6 were tested. Test results showed that the CEM approach can adequately predict the slump, slump variations, water reduction, air content, setting time, and compressive strength of concrete with coefficients of correlation (R2) greater than 0.86. Conversely, moderate relationships were obtained when correlating the flexural strength and length change responses of concrete to those determined on CEM. This was mainly attributed to variations in the specimen dimensions and effect of the interfacial transition zone resulting from the presence of coarse aggregates.