EVALUATION OF THIXOTROPY OF SELF-CONSOLIDATING CONCRETE AND INFLUENCE ON CONCRETE PERFORMANCE

Transcription

1 EVALUATION OF THIXOTROPY OF SELF-CONSOLIDATING CONCRETE AND INFLUENCE ON CONCRETE PERFORMANCE K.H. Khayat (1, 2); A. Omran (1); W.A. Magdi (1) (1) Missouri University of Science and Technology, Rolla, MO, USA (2) Université de Sherbrooke, Sherbrooke, QC, Canada Abstract Several properties of self-consolidating concrete (SCC) are affected by the thixotropic behavior of the material, including segregation resistance, lateral pressure exerted by the concrete, and the bond strength between successive lifts of concrete cast without any mechanical consolidation. This paper discusses the rheological behavior of SCC and presents a number of proven, field-oriented test methods that can be employed to evaluate thixotropy, or the structural build-up of the concrete at rest. The paper highlights the positive aspects of thixotropy where SCC with higher thixotropy can reduce the initial lateral pressure exerted by SCC and increases the rate of pressure drop after casting. It also highlights the negative aspects of thixotropy that affects bond strength across a joint in multi-layer casting. Prediction models to predict both the formwork pressure of SCC and the residual bond strength from thixotropic properties of SCC evaluate using the field-oriented test methods are discussed. Keywords: Bond strength, formwork pressure, multi-layer casting rheology, self-consolidating concrete, structural build-up at rest, thixotropy ANAIS DO 54º CONGRESSO BRASILEIRO DO CONCRETO - CBC CBC 1

2 1 Thixotropy One of the first definitions of thixotropy was given by Freundlich and Rawitzer (BARNES, 1997) who stated that By thixotropy is meant the phenomena of concentrated gels which solidify to gels which may again liquefied to sols. The re-solidification occurs repeatedly, at constant temperature with a constant speed. Pryce-Jones (BARNES, 1997) stated after the true meaning of thixotropy was an increase of viscosity in a state of rest, and a decrease of viscosity when submitted to a constant shearing stress. This can be referred to as the structural build-up at rest and structural breakdown, respectively. It has been postulated that thixotropy is caused by the rupture of intermolecular bonds that are probably electrical in nature (BRAUN; ROSEN, 2000). A finite time is required for the rupture of these bonds because they vary in magnitude or distribution throughout the fluid. After shearing at a constant rate for a given time, equilibrium is reached. In this state a balance exists between the applied shear stress and the strength of an appreciable number of bonds. A decrease or an increase of the applied stress results in a new equilibrium, and the effect is reversible. In some cases, even after a long time, the apparent viscosity may only return to a fraction of its making measurements on thixotropic fluids, the shearing history of the sample and span of time required for the measurements is very important. Therefore, thixotropy is a time-dependent process where a material is subjected to the continuous decrease of apparent viscosity with time under shear and the subsequent recovery of viscosity when the flow is discontinued (MEWIS, 1979). The structural behavior, build-up or breakdown, of cement-based materials, is partially reversible over time. In addition to the reversible and truly thixotropic process, there is the hydration aspect, which is an irreversible structural change with elapsed time following cement contact with mixing water. SCC, like any other cement-based material, can undergo structural breakdown after a given shearing action, and that the equilibrium flow curve conforms to the Bingham model:, where is the shear stress at a given shear rate, and and are the yield value and plastic viscosity, respectively. In addition to the rheological characteristics that influence the ability of SCC to deform in a homogeneous manner, the structural build-up of yield stress (static yield stress) as well as in viscosity following a given period of rest is special interest for SCC. The rate of recovery of the concrete can indeed affect workability of the concrete, including segregation resistance, drop in lateral pressure after casting, and bond strength in multi-layer casting with SCC. 2 Evaluation of structural build-up at rest Thixotropy of cement-based materials can be characterized based on the degree of the material structural build-up or breakdown. The structural breakdown is often determined from a material undergoing various shear rates, while structural build-up is determined when a material is at different periods of rest. A number of test approaches have been proposed in the literature. This includes the hysteresis loop of the up-curve and downcurve of a concrete shear in a rheometer (FERRON et al., 2007). Another approach that involves the determination of the structural breakdown of the concrete at a constant shear ANAIS DO 54º CONGRESSO BRASILEIRO DO CONCRETO - CBC CBC 2

3 Shear stress (Pa) rate (LAPASIN et al., 1979). ASSAAD et al. (2003) employed this approach to evaluate thixotropy of SCC using a Modified Tattersall Mk-III rheometer operating at four different rotational velocities for breakdown measurements. Each velocity is monitored for 25 s to reach an equilibrium state. The tested mixture is subjected to 5 min of rest before each applying each velocity. The area comprised between i (N)- and e (N)-curve is denoted the breakdown (A b ), figure Breakdown area (Ab) Rotational speed (sec) Rotation speed (rps) (rps) Figure 1 - Example of a breakdown area for a SCC Thixotropy can also be determined through the evaluation of the static yield stress at rest (BILLBERG, 2005; ROUSSEL, 2007). Static yield stress can be considered as the stress that must be surpassed for a material going from a state of rest to a state of flow. The above-mentioned methods for thixotropy and structural build-up measurement are typically carried out using concrete rheometers. Recently, attempts have been made to develop field-oriented test methods to evaluate structural behavior at rest of concrete by using the vane geometry equipped with a torque meter (BILLBERG, 2005; ROUSSEL; CUSSIGH, 2008; KHAYAT et al., 2010; KHAYAT et al., 2012). Some of the developments leading to the use of the portable vane and inclined plane tests are described below. 2.1 Portable vane test The portable vane test is inspired from a field test for in-situ measurement of shear strength of soil (in particular clay soils) (OMRAN et al., 2011). Four-blade vanes of different sizes (table 1) were manufactured from stainless steel to enable the use of one torque-meter to capture shear strength of the plastic concrete after various times of rest (figures 2 and 3). The largest vane is used for the weakest structure, i.e., shortest resting time, and vice versa. A torque-meter measuring with high precision was employed to capture the torque values. ANAIS DO 54º CONGRESSO BRASILEIRO DO CONCRETO - CBC CBC 3

4 Table 1 Vane dimensions. Time at rest (min) Vane # 1 (largest) Vane # Vane # Vane # 4 (smallest) Vane dimensions (mm) R H h Vary according to the filling height of up to total height (H) Figure 2 - Four buckets and four vanes used in the portable vane test Figure 3 - Schematic for the portable vane test Immediately after mixing, the four vanes are centered vertically in the containers. The containers are filled with SCC to a given height (h) indicated in table 2. The rested materials are covered. After 15, 30, 45, and 60 minutes of rest, the torque-meter is attached to the axis of the vane and turned slowly (10 to 15 s for a quarter turn). The maximum torque needed to break down the structure is then noted. The torque values are converted to static shear stress (τ 0 rest ) using equations 1 and 2 as follows: 0 rest T = (Equation 1) G ANAIS DO 54º CONGRESSO BRASILEIRO DO CONCRETO - CBC CBC 4

5 G 2 r h r (Equation 2) where: T = measured torque (N.m), and the other terms are defined in figure 3. Variations of static yield stress (τ 0 rest ) determined using the portable vane test (PV τ 0rest ) with resting time for typical SCC mixtures designed with different thixotropic characteristics are shown in figure 4. The τ 0 rest obtained from the portable vane at 15 min (PV τ 0 ) corresponding to the initial response (R i ) is used as a structural build-up index. Similarly, the rate of change of static yield stress with time [PV τ 0 rest (t)] or [R(t)] and the coupled effect of R i R(t) can also be for structural build-up indices. Figure 4 - Variations of static yield stress with time obtained with portable vane test 2.2 Inclined plane test The inclined plane method (COUSSOT; BOYER, 1995; KHAYAT et al., 2010) involves casting concrete in a cylindrical mould onto a horizontal plate of a given roughness, then lifting the plate to initiate flow of the material, as shown in figures 5 and 6. The corresponding angle the necessary to initiate flow is used to determine the static yield stress, τ 0 rest (Pa), as follows: τ 0rest = ρ.g.h.sinα (Equation 3) where ρ is the density of the sample (in g/cm³), g is the gravitation constant (= 9.81 m/s²), h is the characteristic height (in mm) of the slumped sample, and α is the critical angle of the plane (in degree) when the sample starts to flow. The characteristic height (h) is determined by calculating the mean value of five heights of the slumped sample near the center of the spread. Four tests are performed after different periods of rest (after 15, 30, 45, and 60 min) to evaluate the rate of increase in τ 0 rest at rest. ANAIS DO 54º CONGRESSO BRASILEIRO DO CONCRETO - CBC CBC 5

6 Figure 5 - Inclined plane tests at different rest times SCC concrete specimen Sand paper covered inclined plans 10 mm Angle fixing rod 5 mm Bottom plate 610 mm Inclined plane PVC Plate 10 mm thick C top & lower plate ISOMETRIC VIEW FRONT VIEW 400 mm mm PLAN Surface covered by sand paper (No. 600) to avoid slipping and sliding Figure 6 - Schematic for inclined plane test Variations of τ 0 rest obtained with the inclined plane test (IP τ 0rest ) with resting time for typical SCC mixtures of different structural build-up, or thixotropic, properties are shown in figure 7. Similar to the portable vane test, three structural build-up indices can be obtained using the inclined plane method: (R i ), [R(t)], and [R i R(t)]. Figure 7 - Variations of static yield stress with time obtained with inclined plane test ANAIS DO 54º CONGRESSO BRASILEIRO DO CONCRETO - CBC CBC 6

7 Table 2 Various thixotropic indices obtained from the empirical test methods. Response 1- Initial value at 15 min resting time 2- Rate of change in the response with time (slope of the regression line) 3- Couple effect of initial value and rate (slope) Portable vane (PV) Inclined plane (IP) (τ 0 rest ) (τ 0 rest ) R i, (Pa) R(t), (Pa/min) R i R(t), (Pa 2 /min) R i, (Pa) R(t), (Pa/min) R i R(t), (Pa 2 /min) The portable vane and inclined plane test methods showed good repeatability and low relative error (KHAYAT et al., 2012). The response obtained with the portable vane and inclined plane were compared to rheological measurements obtained using a modified Tattersall-Type MK-III concrete rheometer (YAHIA; KHAYAT, 2006) using 22 SCC mixtures prepared with various mix designs and material constituents. The comparison clearly indicates that the empirical methods can capture the structural build-up of SCC at rest in terms of static yield stress and its rate of gain in time (KHAYAT et al., 2012). 3 Effect of thixotropy on lateral pressure exerted by SCC The structural build-up at rest of SCC can significantly reduce the lateral pressure exerted by the concrete on formwork system. This is shown in figure 8 where SCC cast in a pressure device where a concrete head of 13 m begins to decrease soon after the end of casting. The concrete was cast in portable pressure device, referred to as UofS2 pressure column, at a rate of casting of 10 m/hr. The pressure devise has a circular in cross-section measuring 0.2 m in internal diameter and 0.7 m in total height with a wall thickness of 10 mm. It is initially filled to a height of 0.5 m with SCC at the required rate from the top then sealed from the top to introduce an overhead air pressure to simulate pressure increase up to 13 m at a given placement rate. The column is then sealed, and air pressure is applied in steps equivalent to one-meter of concrete head at the same casting rate of 10 m/hr until a concrete head corresponding to 13 m is reached. As shown in figure 8, the corresponding lateral pressure exerted on the column wall recorded using the sensor at the base of the pressure column, decreases between each consecutive increment. This decrease in lateral pressure profile reflects the restructuring (or structural build-up at rest) of the concrete. This becomes even more apparent at the end of casting where pressure decay is observed. The maximum lateral pressure (P max ) recorded at each increase of simulated concrete height is 40% lower relative lateral pressure (K 0 ) than the equivalent hydrostatic pressure at concrete height of 13 m. ANAIS DO 54º CONGRESSO BRASILEIRO DO CONCRETO - CBC CBC 7

8 Figure 8 - Variations of lateral pressure with time for typical SCC mixtures A comprehensive testing program was undertaken to evaluate key mixture parameters affecting formwork pressure exerted by SCC (KHAYAT; OMRAN, 2009). The investigated parameters included mix design, concrete constituents, concrete temperature, casting characteristics, and minimum formwork dimension. The UofS2 pressure column and the empirical test methods were employed to evaluate the lateral pressure characteristics and relate them to relevant SCC rheological properties. Two analytical models enabling the predicting of relative lateral pressure (K 0 ) are discussed. The first model was derived to predict K 0 as function of concrete height (H in m), casting rate (R in m/hr), concrete temperature (T in ºC), and structural build-up (equation 4). The structural build-up is expressed in terms of static yield stress at rest after 15 min of rest determined using the portable vane test (PV τ 0 min in Pa). Similar models were developed using the inclined plane test method. The influence of maximum size aggregate (MSA) is incorporated in the model (f MSA ) when using MSA other than 14 mm. The effect of different waiting times between successive lifts (f WT ) is also taken into consideration. PV τ 0 min measurements in equation 4 are measured at 22 ± 2 ºC. Thus, a separate factor expressing the influence of concrete temperature is introduced. K 0 = [ H R - 0.6T D min PV τ 0 min ] f MSA f WT (Equation 4) where f MSA is a correction factor for MSA other than 14 mm. For relatively low thixotropic SCC [PV τ 0 min 700 Pa] and H < 4 m f MSA = 1 H = 4-12 m f MSA = 1. when MSA = 20 mm 1.26 H f MSA = when MSA = 10 mm For high thixotropic SCC [PV τ 0 min > 700 Pa], f MSA = 1 ANAIS DO 54º CONGRESSO BRASILEIRO DO CONCRETO - CBC CBC 8

9 f WT is a correction factor reflecting the effect of waiting time (WT) between successive lifts and ranges between 0.85 and 1.0 for a waiting time of 30 min, depending on the thixotropy of the concrete. A second prediction model for K 0 values is shown in equation 5 in which the structural build-up varies with the temperature of the SCC at casting. The f MSA and f WT factors are the same as those defined for equation 4. K 0 = [ H R D min PV τ 0 min ] f MSA f WT (Equation 5) The models proposed in equations 4 and 5 are valid only for the following ranges of tested casting conditions and formwork geometry: H: Concrete height (1 13 m) R: Casting rate (2 30 m/hr) T: Concrete temperature (12 32 ºC) D min : Minimum formwork dimension ( mm) PV τ 0rest@15min : Static yield stress at rest from portable vane test (up to 2000 Pa) An example of abacuses resulting from equations 4 or 5 is shown in figure 9 where K 0 at a given concrete height can be estimated given the structural build-up index obtained after 15 min of rest using the portable vane device. This relationship was constructed for a constant casting rate of 10 m/hr, concrete temperature of 22 ºC, a minimum formwork dimension of 200 mm, and no waiting time between successive lifts. Figure 9 - Correlation between K 0 and static yield stress after 15 min resting time obtained using portable vane test (PV τ 0 rest@15 min ) for SCC cast at rate of 10 m/hr 4 Effect of thixotropy on interlayer bond strength SCC that exhibits high level of structural build-up at rest can yield lower bond strength to a successive layer of SCC consolidation. The resulting bond associated with multi-layer casting can decrease with the increase in waiting period between successive castings, which will result in an increase in static yield stress (and viscosity) of the concrete cast in the lower lift. Therefore, in the absence of mechanical consolidation, that takes place in conventional concrete, the successive upper layer of the SCC may not intermix properly ANAIS DO 54º CONGRESSO BRASILEIRO DO CONCRETO - CBC CBC 9

10 with lower lift that can support the mass of the upper lift. This can result in fold-lines that lead to surface defects and weakened bond strength between the successive lifts that fail to be intermixed properly (ROUSSEL; CUSSIGH, 2008; ROUSSEL, 2007). ABD EL MEGDI (2012) evaluated the effect of structural build-up at rest on bond strength and water permeability that can be developed across a lift-line following a certain period of rest. Thixotropy was measured using the portable vane test after different resting times up to 60 minutes. The residual bond strength was measured under flexure stress with various delay times between layers from 0 to 60 minutes. To evaluate the residual bond strength between two SCC shifts, bond strength under flexure stress was evaluated. In this method, small beams with dimensions of 100 mm in width and height and 400 mm in length were cast, figure 10. Small notch were formed during casting at mid-length point to be sure that the failure takes place at mid-span. For each tested concrete, three reference beams were normally cast in one layer, and 12 beams were cast in two layers considering the interface between layers vertical at midspan. The delay time between casting the first and the second layer is varied from 15 to 60 minutes with 15 minutes interval. Three beams for each delay time are considered. The beams were tested flexure, as illustrated in figure 10c-d. Casting point 100 mm 3.94 in (a) mm 3.94 in 2 Small notch (b) (c) P/2 200 mm 7.87 in P/2 200 mm 7.87 in 1 (c) (d) Figure 10 - Casting and testing flexural beams with distinct casting fold line The variation of residual bond under flexure stress with thixotropy and delay time (DT) is plotted in figure 11 (ABD EL MEGID, 2012). The residual flexure bond strength between two layers at certain delay time Rb f(t) is calculated by dividing flexural strength of specimen of the same delay time f f(t) by flexural strength of reference specimen f f(o). The results clearly show that for all tested SCC mixtures that were proportioned with various thixotropic levels, the increase in thixotropy and time at rest before the casting of a successive layer can negatively affect residual bond strength. For rest time of 15 minutes, the residual bond strength of SCC with low thixotropy can be as high as 95% under flexure ANAIS DO 54º CONGRESSO BRASILEIRO DO CONCRETO - CBC CBC 10

11 RBf, % stress. At rest time of 60 minutes, the residual bond strength of SCC of high thixotropy index can be nearly 50% SCC1 SCC3 SCC5 SCC7 SCC2 SCC4 SCC6 SCC DT, min Figure 11 - Variation of residual flexure bond with casting delay time for SCC of different thixotropic levels (SCC1: lowest thixotropy and SCC8: highest thixotropy) Figure 12 shows typical variations of residual bond strength in flexure with thixotropy and delay time between castings of successive layers. Equation 6 presents the statistical model that can be used to determine the residual bond strength in flexure as a function of delay time between successive layers and thixotropy index. RB % DT Athix DT +100 (Equation 6) f where, RB f % = Residual bond strength under flexure stress; DT = Actual delay time between layers in minute; Athix1 = Thixotropy index, in Pa determined using the portable vane method after 15 minutes of rest ANAIS DO 54º CONGRESSO BRASILEIRO DO CONCRETO - CBC CBC 11

12 Figure 12 - Model of residual flexure resistance as a function of thixotropy of concrete and waiting time to cast successive layers The critical delay time (t c ) that can lead to 10% reduction in residual bond strength can be estimated using equation 7. Again, this relationship clearly shows that t c decreases with the increase in structural build-up at rest. The structural build-up at rest is measured as the coupled effect of the initial static yield stress after 15 minutes of rest multiplied by the rate of increase in static yield stress (Athix2). t = (0.38 RB %-38.46) ln Athix2-5.6 RB % (Equation 7) c f f where, t c = critical delay time between layers in minute; RB f % = Residual bond strength under flexure stress; Athix2 = Thixotropy index 2 in Pa.Pa/min ANAIS DO 54º CONGRESSO BRASILEIRO DO CONCRETO - CBC CBC 12

13 5 References ABD EL MEGID, W. (2012). Effect of Rheology on Surface Quality and Performance of SCC. PhD Dissertation, University of Sherbrooke, Department of Civil Engineering, Sherbrooke, Qc, Canada. ASSAAD, J., KHAYAT, K.H., MESBAH, H. (2003). Assessment of Thixotropy of Flowable and Self-Consolidating Concrete. ACI Materials Jr., V. 100, No. 2, pp BARNES, H.A. (1997). Thixotropy - A Review. Journal of Non-Newtonian Fluids Mechanics, V. 70. pp BRAUN, D.B., ROSEN, M.R. (2000). Rheology Modifier Handbook Practical Use and Application. William Andrew Publishing N.Y. BILLBERG, P., (2005). Development Of SCC Static Yield Stress At Rest And Its Effect on the Lateral Form Pressure. Proceedings of the Second North American Conference on the Design and Use of Self-Consolidating Concrete and the Fourth International RILEM Symposium on Self-Compacting Concrete, RILEM Publications, Chicago, USA, Ed. S. Shah. COUSSOT, P., BOYER, S. (1995). Determination of Yield Stress Fluid Behaviour from Inclined Plane Test. Rheol Acta, 34, pp FERRON, R., GREGORI, A., SUN, Z., SHAH, S.P. (2007). Rheological Method to Evaluate Structural Build-up in Self-Consolidating Concrete Cement Paste. ACI Materials Journal, Vol. 104, No. 3, pp KHAYAT, K.H., OMRAN, A.F. (2009). Evaluation of SCC Formwork Pressure. Proceedings of the 2nd International Symposium on the Design, Performance and Use of Self-Consolidating Concrete (SCC 2009), Beijing, China, Ed. C. Shi, Z. Yu, K.H. Khayat and P. Yan, pp KHAYAT, K., OMRAN, A., PAVATE, T. (2010). Inclined Plane Test Method to Determine Structural Build-Up at Rest of Self-Consolidating Concrete. ACI Materials Journal, V. 107, No. 5, pp KHAYAT, K.H., OMRAN, A.F., NAJI, S., BILLBERG, P., YAHIA, A., (2012). Field- Oriented Test Methods to Evaluate Structural Build-Up at Rest of Flowable Mortar and Concrete. RILEM Materials and Structures, V. 45, No. 10, pp LAPASIN, R, LONGO, V., RAJGELJ, S., (1979). Thixotropic Behaviour of Cement Pastes. Cement and Concrete Research, Vol. 9, pp MEWIS, J. (1979) Thixotropy A General Review, Journal of Non-Newtonian Fluid Mechanics, 6, pp ANAIS DO 54º CONGRESSO BRASILEIRO DO CONCRETO - CBC CBC 13

14 OMRAN, A.F., NAJI, S., AND KHAYAT, K.H., (2011). Portable Vane Test to Assess Structural Build-up at Rest of Self-Consolidating Concrete. ACI Materials Journal, V. 108, No. 6, 2011, pp ROUSSEL, N. (2007). A Thixotropic Model for Fresh Fluid Concretes: Theory And Applications. 5th International RILEM Symposium on Self-Compacting Concrete, Ghent, Belgium, pp ROUSSEL, N. AND CUSSIGH, F. (2008) Distinct-layer Casting of SCC: The Mechanical Consequences of Thixotropy, Cement and Concrete Research, V. 38, pp YAHIA, A., KHAYAT, K.H. (2006). Modification of the Concrete Rheometer to Determine Rheological Parameters of SCC Vane Device, Proc. 2nd Int. RILEM Symposium on Advances in Concrete through Science and Engineering, pp ANAIS DO 54º CONGRESSO BRASILEIRO DO CONCRETO - CBC CBC 14