- Email: [email protected]
cement ratio and hydration
Tunnelling and Underground Space Technology 45 (2015) 34–42
Contents lists available at ScienceDirect
Tunnelling and Underground Space Technology journal homepage: www.elsevier.com/locate/tust
In-line rheological measurements of cement grouts: Effects of water/cement ratio and hydration Mashuqur Rahman a,⇑, Ulf Håkansson a,b, Johan Wiklund c a
KTH – Royal Institute of Technology, Brinellvägen 23, 100 44 Stockholm, Sweden Skanska AB, 169 83 Solna, Sweden c SIK – The Swedish Institute for Food and Biotechnology, P.O. Box 5401, 402 29 Gothenburg, Sweden b
a r t i c l e
i n f o
Article history: Received 11 April 2013 Received in revised form 3 June 2014 Accepted 11 September 2014
Keywords: Cement grout In-line rheology UVP + PD Grouting Ultrasound velocity profiling Cement rheology Hydration
a b s t r a c t The rheological properties of cement based grouts change with water/cement ratio and time, during the course of hydration. For this reason, it is desirable to be able to measure this change continuously, in-line, with a robust instrument during the entire grouting operation in the field. The rheological properties of commonly used cement grouts were determined using the Ultrasound Velocity Profiling combined with the Pressure Difference (UVP + PD) method. A non-model approach was used that directly provides the properties, and the results were compared with the properties obtained using the Bingham and Herschel–Bulkley rheological models. The results show that it is possible to determine the rheological properties, as well as variations with concentration and time, with this method. The UVP + PD method has been found to be an effective measuring device for velocity profile visualization, volumetric flow determination and the characteristics of the grout pump used. Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction Grouting with cement based suspensions is commonly used in tunneling and dam construction. The reason for grouting is to change or improve the physical properties of a rock or soil formation, i.e. making it less permeable, stronger or stiffer. A prerequisite for predicting the result is knowledge of the rheological properties of the grouts used (Hässler, 1991). As cement based suspensions change their properties with time due to the hydration of the cement, it is necessary to know not only the instantaneous properties but also the effect of time. This time dependency is needed for design purposes and as a quality control to determine when the grout becomes too thick to pump or inject. Today’s grouting design is based primarily on empirical knowledge and measurement of volumetric flow and pressure, sometimes in combination with determination of the yield stress (Lombardi, 1985). A new grouting design methodology has been proposed based on estimation of the penetration length for cement grouts in rock fractures and the so called characteristic grouting time (Gustafson and Stille, 2005). This methodology, called Real Time Grouting Control – RTGC, relies upon knowledge of the yield stress and the viscosity throughout the grouting operation (Kobayashi ⇑ Corresponding author. Tel.: +46 (0)765832595. E-mail address: [email protected] (M. Rahman). http://dx.doi.org/10.1016/j.tust.2014.09.003 0886-7798/Ó 2014 Elsevier Ltd. All rights reserved.
et al., 2008). Nevertheless, although it is clear that the rheological properties play a fundamental role in grouting, no method is currently available to measure these properties continuously in-line. To address the current deficiencies, a new methodology for flow visualization and rheological characterization based on ultrasound has been successfully introduced for cement based grouts (Håkansson and Rahman, 2009; Håkansson et al., 2012; Wiklund et al., 2012a). The methodology is based on the combination of an Ultrasound Velocity Profiling (UVP) technique and Pressure Difference (PD) measurements. The UVP + PD method has also been found to be a tool that accurately determines grout pump characteristics (Rahman et al., 2012). In this work, the UVP + PD method was used for in-line measurements of the rheological properties of cement grouts and their change with concentration and time. Earlier measurements in cement grouts were made using delay line ultrasound transducers, which reduce the near field distance and enable measuring immediately in contact with the pipe wall (Wiklund et al., 2012a; Rahman et al., 2012). A limitation of the UVP + PD method has until now been associated with the transducer technology, leading to weak performance in industrial applications. Typical examples are acoustic fluctuations in the near field region, leading to blind zones, and the limited penetration depth in concentrated suspensions. In this work, however, a novel non-invasive ultrasound sensor unit recently developed by SIK (Wiklund et al., 2012b) and
35
M. Rahman et al. / Tunnelling and Underground Space Technology 45 (2015) 34–42
capable of emitting sufficient acoustic energy was successfully used to measure instantaneous velocity profiles in cement grout through a stainless steel pipe. The rheology of cement based suspensions is complex, and the properties are difficult to measure with conventional rheometers (Håkansson, 1993; Banfill, 2006). Testing challenges include lubricating layers or slip at fluid/solid interfaces, sedimentation, separation, flocculation and time dependency owing to the hydration of the cement. This implies that it is often very difficult to compare results from various research teams and authors that use different methods and geometries. Despite these weaknesses, current measurements of grouts are still made with simple devices in the field or with rotational rheometers in a laboratory. The linear Bingham model is widely used in the grouting industry due to its simplicity and its inclusion of the basic rheological properties, viscosity and yield stress. Understanding the rheological behavior of cement based grouts is important for the design and control of the grouting process. For example, assuming the Bingham model, the yield stress and the viscosity will determine the relationship between pressure and flow and consequently the time necessary to reach a certain distance into the grouted formation; the yield stress will also set a limit to the possible penetration length. Håkansson et al. (1992) showed that typical values for the Bingham yield stress and viscosity for commonly used micro cement grouts are in the range of 1–10 Pa and 50–200 mPa s, depending on the water/cement ratio and admixtures used. In contrast to many other industrial fluid flow applications, the detailed geometry of jointed rock or porous soil is never fully known. This implies that it does not make sense to seek overly complex rheological models when modeling flow situations in the grouting process. Although it is known that the true rheological behavior of dense cement suspensions is shear thinning with a yield stress, i.e. in principle fitting the Herschel–Bulkey model, the Bingham model is still used since the geometrical considerations are more influential than the choice of rheological model. In this work, the yield stress and viscosity of the grouts were determined using the Bingham and Herschel–Bulkley models as well as the gradient method (Birkhofer et al., 2008). In addition, the yield stress and viscosity were determined for different flow rates using the tube viscometry concept and were subsequently compared with the results obtained using the rheological models. 2. Materials and method 2.1. Materials Cement is used in grouting because of its wide availability, ease of preparation and relatively low cost. Cementa IC30 micro cement is commonly used in practice and was used in this work. IC30 has a particle distribution where more than 95% of the cement particles are less than 30 lm, which has been shown to be the optimum particle size to avoid clogging inside rock fractures (Eklund and Stille, 2008). Cementa IC30 is a sulphate resistant, chromate reduced and low alkaline injection cement, with a compact density of approximately 3100–3200 kg/m3 and a bulk density of 800–1500 kg/m3. The chemical composition includes MgO (max 5% by weight), SO3 (max 3% by weight) and Cl (max 0.1% by weight). A commercial additive, Cementa SetControl II, was used with a dosage of 2% (w/w) in order to maintain grout properties similar to those in the field. A total of eight batches were used in the experiments. The numbers of performed tests are shown in Table 1. Cement grouts with water to cement ratios of 0.6, 0.7, 0.8 and 1.0 were tested with a mixing time of four minutes. Each batch consisted of 15 L of cement grout. Weight of cement and corresponding weight of water was measured prior to the test. Water was poured
Table 1 Number of tests performed for the experiments. W/C ratio
SetControl II (%)
No of tests
0.6 0.7 0.8 1.0
2 2 2 2
1 3 2 2
into the mixer and cement was added afterward, while the mixer was rotating at a pre-defined speed. A Dispermat CV-3 dissolver from VMA-GETZMANN Gmbh was used to mix the cement. A speed of 2000 rpm was used to mix the cement with water. SetControl II was added after two minutes. 2.2. The UVP + PD methodology for in-line rheometry 2.2.1. Ultrasound Velocity Profiling (UVP) Ultrasound Velocity Profiling (UVP) is a technique for measuring an instantaneous velocity distribution along the pulsed beam axis. It was first developed in medicine to measure the flow of blood and subsequently extended to measure the velocity profiles of opaque fluids (Takeda, 1995). The method has now been successfully extended and is widely accepted as an important tool for measuring the velocity profile in other fields of research and engineering. The working principle of the ultrasound velocity profiling technique is based on emission of successive ultrasound pulses and reception of echoes along the beam axis. A transducer mounted at a specific inclination angle, ensuring the measurement of the velocity component in the direction of the measuring line, emits ultrasound pulses through the acoustic coupling and the wall material. Part of the ultrasound energy is scattered and reflected back from the surface of the moving suspended particles inside the pipe. The reflected Doppler shifted echo is recorded using the same transducer. The time intervals between pulse emissions are used to sample consecutive echo receptions at specific times in a narrow time window, called a gate or channel. The distance of the suspended particle reflection inside the pipe is determined by a time of flight measurement, and the local velocity can be obtained by determining the Doppler shifted frequency for each gate or channel. The Doppler frequency is calculated from successive pulses that are received, and an instantaneous velocity profile can be constructed (Wiklund et al., 2007). Visualization of the velocity profiles directly in-line in the ongoing process is a unique feature available in UVP. When conventional offline rheometers are used, a data fitting procedure is needed to determine the rheological behavior and the velocity profiles are then constructed from the measured data. In the curve fitting procedures, there can be errors in the data themselves and errors while fitting the data to a mathematical model. In contrast, as the UVP + PD method measures the velocity profiles directly, the necessary simplifications and the errors described above are significantly reduced. 2.2.2. Tube viscometry and the UVP + PD method UVP + PD is a methodology for determining rheological properties in-line in pipe flow and is based on the combination of Ultrasound Velocity Profiling (UVP) and the Pressure Difference (PD), based on the principle of tube viscometry. The tube viscometry is well described in the literature and can be found in Barnes (1999), Chhabra and Richardson (1999). However, the problem associated with the traditional tube viscometry method is that it provides only single point measurements; as a result, its applicability for non-Newtonian fluids is questionable, provided that only one flow rate is used. The pressure difference, DP, is measured over a certain distance and the wall shear stress, sw, is given by
36
sw ¼
M. Rahman et al. / Tunnelling and Underground Space Technology 45 (2015) 34–42
R w DP 2L
ð1Þ
where Rw is the radius of the pipe and L is the distance between the pressure sensors. In addition, the yield stress can be calculated if the radius of the plug flow is known. The shear rate and shear stress distribution along the pipe radius can be determined by
c_ ¼
dv dr
ð2Þ
r DP 2L
ð3Þ
sðrÞ ¼
The rheological properties of the fluid under investigation can be determined using different approaches. A simple method is the rheological model fitting approach, in which the velocity profile is fitted with mathematical models, e.g. Bingham, Herschel Bulkley etc., and the rheological parameters are obtained. The volumetric flow rate is determined by integrating the velocity profile. The gradient method is a unique non-model approach offered by the UVP + PD method developed by SIK for direct measurement of the rheological properties. The advantage associated with the gradient method is that it does not require a priori knowledge of the rheological model of the material being tested. In the gradient method, the shear rate is obtained from the gradient of the measured velocity profile and the shear stress is obtained from the pressure drop over a certain length of the pipe section, Furthermore, the yield stress can be determined from the measured plug radius of the velocity profile. Therefore, in contrast to other methods, it is the true shear viscosity and yield stress data that are presented, see Birkhofer et al. (2008), Wiklund et al. (2007). An accurate measurement of the velocity profile is a prerequisite for using the gradient method, as the results depend on the quality of the measured profiles. The pipe wall position and the Doppler angle must thus be precisely determined. In addition, the acoustic energy emitted by the ultrasound transducer must be sufficient to measure the velocity profile at least up to the center of the pipe. Since the wall shear stress and the wall shear rate can be measured directly in-line with the UVP + PD method, the rheological properties can also be determined using the traditional tube viscometry concept, provided that data from different flow rates are used. The yield stress and viscosity can be determined using a Bingham fitting to the measured data. The UVP + PD method used in this work was developed at SIK, The Swedish Institute for Food and Biotechnology; greater detail on the methodology can be found in Wiklund et al. (2007). The UVP + PD technique has been successfully used on a variety of industrial suspensions, such as food, paper pulp and mine tailings e.g. Wiklund et al. (2006), Birkhofer et al. (2008), Kotzé et al. (2008), Wiklund and Stading (2008), Windhab and Ouriev (2002). The UVP + PD method has also been found successful for cement based grout (Wiklund et al., 2012a). 2.2.3. Experimental flow loop and equipment The cement grout was circulated in a flow loop consisting of a storage tank, agitator, progressive cavity single screw pump, LOGAC™ flow meter, electromagnetic flow meter, temperature sensors and an integrated UVP + PD sensor unit. This type of pump was used to achieve a stable flow condition without fluctuation. The integrated sensor unit consisted of a differential pressure sensor with remote seals and one pair of noninvasive ultrasound transducers capable of measuring through high-grade industrial stainless steel pipes. The sensor unit also produces a more well defined beam with higher acoustic energy compared to previous set-ups (Wiklund et al., 2012a). The flow loop was connected by stainless steel pipes with a 22.6 mm inner diameter. This was a
novel application and experimental set-up for measuring the time dependent behavior of cement based grouts. A schematic illustration of the experimental set-up is shown in Fig. 1. A progressive cavity pump type MAE 50-2/BB.BBNT32 PompeRaccorderia was used in the laboratory flow loop. Progressive cavity pumps are also known as single screw pumps, and the major functional components are the rotor and the stator. The stator is made of vulcanized rubber, placed inside a steel pipe. The stator has a circular cross section single start screw with a pitch. The pump was designed for suspension materials, and it is possible to obtain very stable flow conditions. The pump can be operated at flow rate ranges up to 10 L/min with a pressure limit of maximum 1.2 MPa. The LOGAC™ 4000 is a computer based data recording system manufactured by Atlas Copco and consists of an electromagnetic flow meter and a pressure sensor. It is used for storing and sampling data during field grouting operations. The parameters that can be logged and stored are flow, pressure, volume, time and real time. The data can be recorded on the card every 1st, 5th or 10th second. The flow meter operates at a range of 0–200 L/min with a maximum allowed pressure of 4 MPa. LOGAC™ was used as a reference to compare the volumetric flow rate obtained by the UVP and the electromagnetic flow meter. A Proline Promag 55S electromagnetic flow measuring instrument manufactured by Endress + Hauser was installed in the flow loop. The operating flow range was 0.01–10 L/s, and the end frequency output was from 2–10,000 Hz. Electromagnetic flow meters utilize Faraday’s law of induction, where a voltage is induced in a conductor moving in a magnetic field and the flowing medium corresponds to the moving conductor. The induced voltage is assumed to be proportional to the flow velocity. As a consequence, when measurements are made in a non-Newtonian fluid, this assumption can result in inaccuracies due to the unknown shape factor of the velocity profiles. The Proline Promag 55S electromagnetic flow measurement system was used to compare the flow rate determined with UVP and LOGAC™, since an earlier work (Wiklund et al., 2012a) showed that the LOGAC™ flow meter determines the average value over the sampling period and lacks accuracy in comparison with the UVP. 2.2.4. Conventional off-line rheometer The off-line measurements were made with an ARES G2 (Advanced Rheometric Expansion System) rheometer (TA Instruments, USA). Measurements were made over a shear rate range of 0.1–1000 s1 with 10 points per decade for comparison with the in-line results. A 27 mm DIN concentric cylinder geometry, rotating outer cylinder diameter of 30 mm and inner cylinder diameter of 27 mm was used. An equilibration time of 5 s were used for the measurements. Off-line measurements were performed only for the w/c ratio of 0.7. 2.2.5. UVP + PD instrumentation and sensor unit This work tested, for the first time, a pair of custom made, noninvasive, robust sensor units, clamped on a pipe section and capable of emitting sufficient acoustic energy to perform non-invasive measurements of the velocity profiles through the stainless steel pipe. The sensor unit consists of transducers, wedges, absorbers, acoustic couplants etc. To minimize the effect of sedimentation, the two transducers were clamped in a horizontal plane opposite to each other with an angle of inclination (between the flow direction and the transducer axis) of 20 and 110, respectively. A 3D model of the non-invasive sensor unit is shown in Fig. 2. The UVP + PD sensor unit is comprised of two customized ultrasound transducers. The unit further consists of a differential
M. Rahman et al. / Tunnelling and Underground Space Technology 45 (2015) 34–42
37
Fig. 1. Schematic illustration of the flow loop used for the experimental work.
pressure sensor with remote seals to measure the differential pressure over a distance of 1.98 m. The in-line velocity profile measurements were made using a pulser/receiver instrument, UVP-DUO-MX Monitor (Met-Flow SA, Lausanne, Switzerland). The instrument firmware and driver software were modified to allow access to the Demodulated Echo Amplitude Data (DMEA or I-/Q-Data) via an Active X library. The UVP-Duo instrument and the other hardware devices were connected to a master PC via Ethernet and a DAQ card (National Instruments Sweden AB, Solna, Sweden). A high-speed digitalizer was used for data acquisition in the simultaneous measurements of velocity profiles and acoustic properties. Versatile MATLAB based (The MathWorks Inc., Natick,MA, USA) software with a graphical user interface (RheoFlow™), developed by SIK, was used to control the hardware devices for data acquisition, signal
Fig. 2. 3D-model of the non-invasive sensor unit with mounting device (Wiklund et al., 2012b).
processing and visualization of the data. The volumetric flow rate was obtained from integration of the velocity profiles obtained. 3. Results and discussion 3.1. Velocity profiles Since the determination of the rheological properties depends on an accurate measurement of the velocity profiles, it was important to be able to measure velocity profiles at least up to the center of the pipe. Velocity profiles measured by the previously used flush mounted transducers and by the newly developed non-invasive sensor unit for cement grout of a water to cement ratio of 0.7 are shown in Figs. 3 and 4, respectively. Velocity profiles measured by the flush mounted transducers are distorted at the center of the pipe, which indicates that the emitted energy was not sufficient and the penetration depth was reduced due to the strong attenuation of the ultrasound energy in the cement grout. On the
Fig. 3. Velocity profiles of w/c ratio 0.7, measured by delay line 4 MHz transducers (Rahman et al., 2012).
38
M. Rahman et al. / Tunnelling and Underground Space Technology 45 (2015) 34–42
Fig. 4. Velocity profiles of w/c ratio 0.7, measured by non-invasive sensor unit.
other hand, it was possible to measure the velocity profiles nearly for the full diameter of the pipe by the non-invasive sensor unit, shown in Fig. 4. A mirror image is combined and shown in the figures for a complete illustration of the velocity profiles. In Fig. 4, continuous profiles were obtained by the new noninvasive sensor unit because of its capability of emitting high acoustic energy. This is a significant development in transducer technology, and it is the first time such a device has been used to measure velocity profiles of cement grouts in-line. An interesting feature that can be observed in Fig. 4 is the non-uniform shape, shown with a circle in the velocity profile, which might occur due to the change in the viscosity of the cement grout as a consequence of a change in the shearing procedure, since the shearing is different at the different positions in the pipe section. In addition, a wall slip phenomenon is common, provided that a smooth surface pipe is used (Barnes, 1995; Dogan et al., 2002; Isa et al., 2007). As a result, a non-zero velocity near the pipe wall is expected, and this can be seen in Fig. 4. The average velocity profiles for w/c ratios of 0.6, 0.7, 0.8 and 1.0 are shown in Fig. 5. While measuring the velocity profiles, the flow rate was controlled and the pump pressure was synchronized subsequently. As a result, due to different applied flow rates, a higher velocity was observed for a thicker suspension, i.e. a w/c ratio of 0.6. It can be seen that the velocity profiles were successfully measured up to the center of the pipe. The velocity profile for w/c 0.8 is slightly distorted at the near wall region, which can be explained by the presence of air bubbles, indicating the need of an improved mixing and agitation of the cement grout for a thicker suspension. Moreover, the fluid near the wall is influenced and subjected to the strongest shear; this might therefore result in a distorted shape of the profile in the near wall region. Accurate determination of the true wall shear rate and
Fig. 5. Average velocity profiles measured by non-invasive sensor unit for w/c ratio 0.6, 0.7, 0.8 and 1.0.
stress thus depends on the accuracy in determining the actual wall interface (when measuring through material layers) or liquid-wall interface (when measuring with direct contact to the test fluid), which is of great importance for determining the velocity profile and the rheological properties. The determination of the interface is complicated, especially when measuring velocity profiles with limited spatial resolution or when attenuation distorts the quality of near wall velocity data. It has been shown that the rheological parameters determined using the UVP + PD method vary significantly by changing the wall position by less than 0.37 mm (or one channel) (Wiklund et al., 2007). However, the non-invasive sensor unit, in combination with the gradient method, has the advantage that knowledge of the wall position is not a prerequisite. Since the velocity profiles were accurately measured up to the center of the pipe, it was possible to visualize the shape of the velocity profile and the development of the plug radius with time for a certain concentration of cement grouts. The data in Fig. 5 allow a visualization of the flow pattern, e.g. the development of a plug flow. For a w/c ratio of 1.0, a behavior similar to Newtonian fluid behavior is observed due to the lower concentration; in contrast, for w/c ratios of 0.6 and 0.7, a plug flow is visible at the center of the pipe. 3.2. Comparison of the volumetric flow rate obtained by the UVP method and conventional flow meter As the UVP method offers a visualization of the velocity profile itself, the volumetric flow rate can be determined by integration of the velocity profile. The volumetric flow rate, measured by the field equipment LOGAC™, electromagnetic flowmeter and the UVP, is shown in Fig. 6 for the piston pump and the progressive cavity pump. The pulsation of the flow due to the back stroke of the piston is clearly visible, which is due to the much faster data acquisition and velocity estimation with the UVP technique. As shown in the figure, the true flow rate measured by the LOGAC™ remains unknown; however, the average value of the flow rate was obtained. The electromagnetic flow meter yielded a fixed volumetric flow rate slightly higher than the UVP. The flow measuring principle of the electromagnetic flow meter is based on the assumption of a velocity profile for a Newtonian fluid, which makes it fairly unreliable for non-Newtonian shear thinning fluids such as cement grouts. The progressive cavity pump was used to achieve a stable flowing condition for the cement grout. As can be seen in Fig. 6, UVP provided faster data acquisition and slight fluctuations, whereas the LOGACTM yielded results of the same order of magnitude. The pressure is high and the flow is low during the later stage of grouting, and the commercial coriolis flow meters are not capable of measuring such a low flow rate. Since the stop criteria for grouting are based on the flow of grouts in rock fractures, a more
Fig. 6. Comparison of volumetric flow rate obtained by the LOGAC™ and the UVP. Conversion factor 1 L/min = 1.67 104 m3/s.
M. Rahman et al. / Tunnelling and Underground Space Technology 45 (2015) 34–42
39
accurate measurement of the flow rate will lead to an improved application of these stop criteria. When using a piston pump, a negative velocity can be seen, i.e. a backward flow due to the suction force when the piston is moving backward (Wiklund et al., 2012a). It is not possible to measure such negative velocity by using a commercial coriolis flow meter since the principle of the coriolis flow meter is based on the determination of the phase difference of signals originating from the oscillation of the sensors (Vetter and Notzon, 1994). 3.3. Rheological observations The rheological properties were determined by curve fitting the Bingham and Herschel–Bulkley rheological models to the measured velocity profiles. In addition, a non-model approach by the so called gradient method was used for direct determination of the rheological properties from the velocity profiles. The tube viscometry concept was used with different flow rates to determine the wall shear rate and wall shear stress, from which the yield stress and viscosity could be obtained assuming the Bingham model. It must be noted that the earliest measurement was made 25 min after the mixing of the cement. 15 L of grout was used in the flow loop for each test. Since five liters of grout could be prepared every time using the high speed mixer; due to practical reasons, measurements could not be started before 25 min. For the off-line measurements, the samples were collected from the flow loop and tested in the laboratory, which implies a time lag between the sampling and the experiments. In contrast, in-line measurements are performed while the cement grout is in a continuously flowing condition. As a result, discrepancies between the in-line and off-line results can occur and have also been observed in earlier work (Wiklund et al., 2007). 3.3.1. Rheological properties determined by the Bingham model The yield stress and the viscosity, determined by the Bingham model, are shown in Figs. 7 and 8, respectively. As can be seen, the increases in the yield stress and viscosity due to hydration with time and for different concentrations are visible in the figures. The highest value of yield stress was observed for the w/c ratio of 0.6, which was expected owing to the highest concentration of cement particles. For easier illustration, the increase in the yield stress and viscosity is shown by linear trend lines. The trend of increased yield stress with time was observed for w/c 0.6. However, some discrepancies, e.g. decreased yield stress, can be seen for the time period from 60–80 min, which can be explained by the shape of the velocity profile. Since the measured velocity profile was fitted using a linear mathematical model, a slight distortion in the measured data will yield a different gradient at the velocity profile and will therefore result in an inaccurate rheological property value. In addition, the effect of SetControl II is to decrease the initial viscosity of the cement material to increase the flowability inside the
Fig. 7. Yield stress determined by Bingham model for different w/c ratios and time.
Fig. 8. Viscosity determined by the Bingham model for different w/c ratios and time.
rock fractures. An expected trend of the increased yield stress was observed for w/c ratios of 0.7, 0.8 and 1.0. The yield stress determined for the w/c ratio of 0.7 is in good agreement for both the in-line and off-line measurements. The progress of yield stress with time was mildest for w/c ratio 1.0, which was due to the thinner suspension of cement. In Fig. 8, the increase of viscosity with time is seen for w/c ratios of 0.6, 0.7, 0.8 and 1.0. A rapid change in viscosity was observed with time for w/c ratios of 0.6 and 0.7, which was due to the higher concentration. A slight increase in the viscosity was observed for the w/c ratios of 0.8 and 1.0. The viscosity measured off-line for cement grout with a w/c ratio of 0.7, 125 min after mixing, showed a lower viscosity in comparison with the in-line measurement, which can be explained by the range of the shear rate. During the off-line measurement, the grout was sheared from 0.1 s1 to 1000 s1; in contrast, during in-line measurements, the cement grout was sheared in a lower shearing rate, i.e. from 0.1 to 400 s1. Since the grout was in a dynamic state, continuously in motion inside a flow loop and subjected to structural breakdown different from that in the off-line measurements, development of the yield stress and viscosity would be different from that in the off-line measurement systems. As shown in Figs. 7 and 8, a continuous shearing process slows the development of the yield stress and viscosity for thinner grouts, e.g. w/c ratios of 0.8 and 1.0.
3.3.2. Rheological properties determined by the Herschel–Bulkley (H–B) model The H–B model was chosen to simulate the rheological behavior of cement grout since it consists of a yield stress and exhibits a shear thinning behavior consistent with many dense suspensions. As can be seen from the shear stress vs. shear rate curve, fitted by the H–B model in Fig. 9, a higher shear stress was observed for a higher concentration of cement grout. Time dependent behavior was also observed, since a higher shear stress was observed after a longer period of time, owing to hydration of the cement grout. A comparatively higher yield stress and shear stress were observed for the w/c ratio of 0.6, in comparison with w/c ratios 0.7, 0.8 and 1.0, which indicates a shorter setting time resulting from a thicker concentration. A similar shear stress for the w/c ratio 0.8 after 138 min was observed as compared to w/c 0.7 after 63 min, which was due to the longer time after mixing for the w/c ratio of 0.8. Off-line measurements were made for w/c ratio 0.7, and very good agreement was found between the in-line and offline measurements. Since viscosity is obtained from the gradient of the shear stress curve vs. shear rate curve, Fig. 9 illustrates that the difference in viscosity was more pronounced for different w/c ratios at a lower shear rate, which means that the viscosity increases at lower velocity due to the lower shearing of the cement suspension.
40
M. Rahman et al. / Tunnelling and Underground Space Technology 45 (2015) 34–42
Fig. 9. Shear stress vs. shear rate curves fitted to the H–B model for different w/c ratios and time.
The rheological parameters obtained by the H–B model for different w/c ratios are shown in Table 2. A higher yield stress was observed for the w/c ratio of 0.6. The flow index, n, did not show any significant change for w/c 0.6, which means that the shear thinning behavior did not change significantly. However, the consistency index increased with time for the w/c ratio of 0.6, which indicates a progress in shear stress. A slight increase of the yield stress was observed with time for w/c ratio 0.7. The flow index, n, was similar to that of w/c 0.6, although a lower consistency index was observed, which indicates a lower shear stress in comparison with the w/c ratio of 0.6. The yield stress determined for w/c 0.8 was lower as compared with w/c ratios of 0.6 and 0.7, which was expected. A lower consistency index was observed for the higher w/c ratio, which indicates a lower increase of the shear stress. In addition, a higher value of the flow index, n, was observed, which indicates the decrease in the viscosity. As can be seen for w/c 1.0, the consistency index, k, was the lowest and the flow index, n, was the highest, which can be explained by a lower viscosity due to continuous shearing of the thinner grout. The offline results yield a slightly higher yield stress and consistency index, which can be explained by a different measuring condition and the time lag between the experiments. Although the H–B model successfully explains the shear thinning behavior of cement grout, as shown in Table 2, the difficulty is associated with explaining time dependent behavior, and this difficulty has to do with the number of rheological parameters involved in the fitting procedure. Table 2 Rheological parameters obtained by H–B model. W/C ratio
Time after mixing
Yield stress (Pa)
n
k
R2
0.6
22 65 84 123
5.9 4.8 6 6
0.3 0.3 0.4 0.3
2.7 5 3.3 5.1
0.95 0.9 0.96 0.93
0.7
63 86 96 132 125
1.1 1.2 1.3 1.6 2.2
0.3 0.4 0.3 0.4 0.4
2.1 1.4 2.3 2.2 1.9
0.96 0.99 0.95 0.92 0.95
0.8
49 104 138 150
0.8 2.1 5.6 3.4
0.4 0.4 0.4 0.2
1.0 0.9 0.7 2.2
0.97 0.98 0.95 0.97
1.0
25 64 105 129
1.2 0.9 0.8 0.8
0.3 0.4 0.4 0.4
1.5 1.0 1.0 1.0
0.86 0.97 0.97 0.95
0.7 off-line
3.3.3. Rheological properties determined by the Gradient method The gradient method is a non-model approach available in the UVP + PD method that is capable of performing direct measurements of rheological properties instead of using any rheological fitting procedure. Fig. 10 shows time dependent behavior, i.e. increasing shear stress with time, for w/c ratios 0.7 and 1.0. As expected, a higher shear stress was exhibited by w/c ratio 0.7 in comparison with w/c ratio 1.0 because of a thicker suspension. The distorted shapes at the higher shear rate region of the shear stress vs. the shear rate curve are due to the distortion in the shear rate distribution. Detection of the correct wall position and an accurate velocity profile measurement in the near wall region can eliminate distortion at the higher shear rate region. However, a shape preserving smoothing filter was used to eliminate noise in the near field region. For the w/c ratio 1.0 after 66 min, a lower shear stress was observed as compared with w/c ratio 1.0 after 24 and 26 min. It must be noted that the flow rate was not the same for both samples. As can be seen, a lower flow rate, i.e. a lower shearing rate, yielded a lower shear stress, which indicates a different structural breakdown and recovery of the material at different velocities, i.e. flow rates. With the gradient method, the yield stress is measured directly and determined from the plug radius. The yield stress for w/c ratios of 0.7 and 1.0 is shown in Table 3. As can be seen, the yield stress increased with time. A lower yield stress was observed for w/c ratio 1.0, 66 min after mixing, in comparison with 24 and 26 min. However, the plug radius that was measured was similar. Since the flow rate range for w/c ratio 1.0, 66 min after mixing, was lower, a lower pressure drop was observed, which yielded a lower yield stress. An accurate measurement of the velocity profile is a prerequisite in the gradient method since the determination of the shear rate is highly sensitive to noise. As a result, the gradient method was applied for two w/c ratios, 0.7 and 1.0, which provided smooth velocity profiles. 3.3.4. Rheological properties determined by tube viscometry The tube viscometry approach was also used in this work to determine the yield stress and viscosity of the grout used. Tube viscometry has been used earlier for mineral suspensions, and good agreement with the UVP + PD method has been reported (Kotzé et al., 2008). However, instead of using different diameter pipes, different flow rates were employed in this work to obtain the wall shear rate and wall shear stress. The yield stress and viscosity determined using the tube viscometry concept for different flow rates are shown in Fig. 11. The wall shear rate and wall shear stress were measured at three different flow rates, 5 L/min, 7 L/min and 9 L/min. In Fig. 11, the yield stress is the intercept with the y-axis and the slope is the viscosity. In contrast, the viscosity according to the H–B model and the gradient method was determined by taking
Fig. 10. Shear stress vs. shear rate curve measured by the Gradient method.
M. Rahman et al. / Tunnelling and Underground Space Technology 45 (2015) 34–42
41
which might influence the yield stress determined for the w/c ratio of 1.0.
Table 3 Yield stress measured by the Gradient method. W/C ratio
Time after mixing (min)
Yield stress (Pa)
Pressure difference (Pa/m)
Plug radius (mm)
0.7 0.7 1.0 1.0 1.0 1.0
69 75 3 24 26 66
3.9 4.1 1.0 2.0 1.8 0.5
1137 1172 299 614 577 140
6.8 6.9 6.8 6.4 6.3 6.8
Fig. 11. Yield stress and viscosity determined by the Tube viscometry principle for different flow rates.
Table 4 Comparison of yield stress determined by different methods. W/C ratio
Time after mixing
Bingham
H–B
Gradient
Tube viscometry
0.6 0.6 0.7 0.7 0.8 0.8 1.0
33 65 65 75 49 74 25
14.7 16 4.9 4.9 3.4 4.3 3.2
5.1 4.8 1.1 1.2 0.9 0.8 0.9
– – 3.9 4.1 – – 2
– – 5.5 – – – 0.1
the gradient at individual points of the shear stress vs. shear rate curve and is therefore dependent on shear rate. The trend of increased yield stress and viscosity for thicker grouts was observed, however. The lack of data in the lower shear rate region was due to the pressure drop measurements, since the lowest shear rate was obtained for the flow rate of 5 L/min. 3.4. Comparison of the yield stress determined by different methods Table 4 shows a comparison of the yield stress determined using different models and methods for different w/c ratios. As expected, the yield stresses were overestimated with the Bingham model, due to the linear assumption of the shear stress and shear rate. The H–B model always yielded a lower value than the Bingam model and gradient method due to the non-linear relationship between the shear stress and shear rate. Yield stress measured by the gradient method for the w/c ratio of 0.7 was in good agreement with the Bingham model, although a lower value was observed for w/c 1.0. A comparatively higher yield stress was observed for the w/c ratio of 0.6, which would be due to the thicker grout. For the tube viscometry concept at different flow rates, the yield stress determined for w/c 0.7 was in good agreement with the Binghan yield stress; however, for the w/c ratio 1.0, it was lower. The effect of hydration with time was not taken into consideration when data were recorded for different flow rates,
4. Conclusion The newly developed non-invasive sensor unit was capable of generating sufficient energy to provide velocity profiles exceeding half of the pipe diameter, which is necessary to be able to determine the change in the rheological properties in-line for watercement ratios down to 0.6. This is a significant improvement as compared with transducers that have been used earlier. The visualization of the velocity profile and the change in its shape caused by an increase in both yield stress and viscosity were seen as a function of concentration and time. The velocity profile resembled Newtonian behavior for low concentrations, while it changed to Bingham and H–B models at higher concentrations. Visualization of the velocity profile also has a potential in determining the existence and extent of slip, which is commonly seen for suspensions at a fluid/solid interface. The volumetric flow rate was determined by integrating the velocity profiles and was subsequently compared with the flow rates obtained using a commercial electromagnetic flow meter. Since the UVP is capable of faster data acquisition, and providing accurate measurements at lower velocities (i.e. 1 L/min), it can not only be used for control during grouting but also for optimizing the characteristics of the grout pump. It can be concluded from this work that the UVP + PD method can successfully be used continuously, in-line, to determine the time dependent behavior of cement based grouts for different concentrations. Future investigations should focus on different phenomena, such as the wall slip, thixotropy of cement grout and the robustness of the UVP + PD method for field use. Acknowledgements This work was financed by the Swedish Construction Industry Research Foundation (SBUF), the Swedish Rock Engineering Research Foundation (BeFo) and the Swedish Research Council (Formas). Their support is highly appreciated. AtlasCopco is acknowledged for providing the LOGAC™ flow meter. References Banfill, P., 2006. Rheology of fresh cement and concrete. Rheology Reviews, 31–130. Barnes, H.A., 1995. A review of the slip (wall depletion) of polymer solutions, emulsions and particle suspensions in viscometers: its cause, character and cure. J. Non-Newtonian Fluid Mech. 56, 221–231. Barnes, H.A., 1999. On-line or process viscometry – a review. Appl. Rheol. 9, 102– 107. Birkhofer, B., Jeelani, S., Windhab, E., Ouriev, B., Lisner, K., Braun, P., Zeng, Y., 2008. Monitoring of fat crystallization process using UVP-PD technique. Flow Meas. Instrum. 19, 169. Chhabra, R.P., Richardson, J.F., 1999. Non-Newtonian Flow in the Process Industries: Fundamentals and Engineering Applications. Butterworth-Heinemann. Dogan, N., McCarthy, M.J., Powell, R.L., 2002. In-line measurement of rheological parameters and modeling of apparent wall slip in diced tomato suspensions using ultrasonics. J. Food Sci., 67. Eklund, D., Stille, H., 2008. Penetrability due to filtration tendency of cement-based grouts. Tunn. Undergr. Space Technol. 23, 389–398. Gustafson, G., Stille, H., 2005. Stop criteria for cement grouting. Felsbau 25, 62–68. Håkansson, U., Hässler, L., Stille, H., 1992. Rheological properties of microfine cement grouts. Tunnel. Undergr. Space Technol. 7, 453–458. Håkansson, U., 1993. Rheology of Fresh Cement based Grouts- PhD Thesis, Stockholm: KTH Royal Institute of Technology. Håkansson, U., Rahman, M., 2009. Rheological properties of cement based grouts using the UVP-PD method. In: Proceeding of Nordic Symposium of Rock Grouting, Helsinki. Håkansson, U., Rahman, M., Wiklund, J., 2012. In-line measurements of rheological properties of cement based grouts – introducing the UVP-PD method. In: Proceeding of 4th International Conference on Grouting and Deep Mixing, New Orleans. Hässler, L., 1991. Grouting of Rock – Simulation and Classification- PhD Thesis, Stockholm: KTH Royal Institute of Technology.
42
M. Rahman et al. / Tunnelling and Underground Space Technology 45 (2015) 34–42
Isa, L., Besseling, R., Poon, W.C.K., 2007. Shear zones and wall slip in the capillary flow of concentrated colloidal suspensions. Phys. Rev. Lett., 98. Kobayashi, S., Stille, H., Gustafson, G., Stille, B., 2008. Real Time Grouting Control Method – Development and Application Using ÄspÖ HRL Data. Swedish Nuclear Fuel and Waste Management Co (SKB), Stockholm, Sweden, R-08-133. Kotzé, R., Haldenwang, R., Slatter, P., 2008. Rheological characterization of highly concentrated mineral suspensions using ultrasonic velocity profiling combined pressure difference method. Appl. Rheol. 18, 62114. Lombardi, G., 1985. Some Theoretical Considerations on Cement Rock Grouting. Lombardi Engineering Ltd. Rahman, M., Håkansson, U., Wiklund, J., 2012. Grout pump characteristics evaluated with the Ultrasound Velocity profiling. In: Proceeding of ISRM International Symposium, EUROCK, Stockholm. Takeda, Y., 1995. Velocity profile measurement by ultrasonic Doppler method. Exp. Thermal Fluid Sci. 10, 444–453. Vetter, G., Notzon, S., 1994. Effect of pulsating flow on Coriolis mass flowmeters. Flow Meas. Instrum. 5, 263–273. Wiklund, J., Rahman, M., Håkansson, U., 2012a. In-line rheometry of micro cement based grouts – a promising new industrial application of the ultrasound based UVP+PD method. Appl. Rheol. 22, 42783.
Wiklund, J., Kotzé, R., Haldenwang, R., Stading, M., 2012b. Development of an industrial UVP+PD based rheometer-optimisation of UVP system and transducer technology. In: Proceeding of 8th International Symposium on Ultrasonic Doppler Methods for Fluid Mechanics and Fluid Engineering. Dresden. Wiklund, J., Shahram, I., Stading, M., 2007. Methodology for in-line rheology by ultrasound Doppler velocity profiling and pressure difference techniques. Chem. Eng. Sci. 62, 4277–4293. Wiklund, J., Stading, M., 2008. Application of in-line ultrasound Doppler based UVPPD rheometry method to concentrated model and industrial suspensions. Flow Meas. Instrum. 19, 171–179. Wiklund, J., Stading, M., Pettersson, A.J., Rasmuson, A., 2006. A comparative study of UVP and LDA techniques for pulp suspensions in pipe flow. AIChE J. 52, 484– 495. Windhab, E.J., Ouriev, B., 2002. Rheological study of concentrated suspensions pressure-driven shear flow using a novel in-line ultrasound Doppler method. Experiments in Fluids 32, 204–211.
Contents lists available at ScienceDirect
Tunnelling and Underground Space Technology journal homepage: www.elsevier.com/locate/tust
In-line rheological measurements of cement grouts: Effects of water/cement ratio and hydration Mashuqur Rahman a,⇑, Ulf Håkansson a,b, Johan Wiklund c a
KTH – Royal Institute of Technology, Brinellvägen 23, 100 44 Stockholm, Sweden Skanska AB, 169 83 Solna, Sweden c SIK – The Swedish Institute for Food and Biotechnology, P.O. Box 5401, 402 29 Gothenburg, Sweden b
a r t i c l e
i n f o
Article history: Received 11 April 2013 Received in revised form 3 June 2014 Accepted 11 September 2014
Keywords: Cement grout In-line rheology UVP + PD Grouting Ultrasound velocity profiling Cement rheology Hydration
a b s t r a c t The rheological properties of cement based grouts change with water/cement ratio and time, during the course of hydration. For this reason, it is desirable to be able to measure this change continuously, in-line, with a robust instrument during the entire grouting operation in the field. The rheological properties of commonly used cement grouts were determined using the Ultrasound Velocity Profiling combined with the Pressure Difference (UVP + PD) method. A non-model approach was used that directly provides the properties, and the results were compared with the properties obtained using the Bingham and Herschel–Bulkley rheological models. The results show that it is possible to determine the rheological properties, as well as variations with concentration and time, with this method. The UVP + PD method has been found to be an effective measuring device for velocity profile visualization, volumetric flow determination and the characteristics of the grout pump used. Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction Grouting with cement based suspensions is commonly used in tunneling and dam construction. The reason for grouting is to change or improve the physical properties of a rock or soil formation, i.e. making it less permeable, stronger or stiffer. A prerequisite for predicting the result is knowledge of the rheological properties of the grouts used (Hässler, 1991). As cement based suspensions change their properties with time due to the hydration of the cement, it is necessary to know not only the instantaneous properties but also the effect of time. This time dependency is needed for design purposes and as a quality control to determine when the grout becomes too thick to pump or inject. Today’s grouting design is based primarily on empirical knowledge and measurement of volumetric flow and pressure, sometimes in combination with determination of the yield stress (Lombardi, 1985). A new grouting design methodology has been proposed based on estimation of the penetration length for cement grouts in rock fractures and the so called characteristic grouting time (Gustafson and Stille, 2005). This methodology, called Real Time Grouting Control – RTGC, relies upon knowledge of the yield stress and the viscosity throughout the grouting operation (Kobayashi ⇑ Corresponding author. Tel.: +46 (0)765832595. E-mail address: [email protected] (M. Rahman). http://dx.doi.org/10.1016/j.tust.2014.09.003 0886-7798/Ó 2014 Elsevier Ltd. All rights reserved.
et al., 2008). Nevertheless, although it is clear that the rheological properties play a fundamental role in grouting, no method is currently available to measure these properties continuously in-line. To address the current deficiencies, a new methodology for flow visualization and rheological characterization based on ultrasound has been successfully introduced for cement based grouts (Håkansson and Rahman, 2009; Håkansson et al., 2012; Wiklund et al., 2012a). The methodology is based on the combination of an Ultrasound Velocity Profiling (UVP) technique and Pressure Difference (PD) measurements. The UVP + PD method has also been found to be a tool that accurately determines grout pump characteristics (Rahman et al., 2012). In this work, the UVP + PD method was used for in-line measurements of the rheological properties of cement grouts and their change with concentration and time. Earlier measurements in cement grouts were made using delay line ultrasound transducers, which reduce the near field distance and enable measuring immediately in contact with the pipe wall (Wiklund et al., 2012a; Rahman et al., 2012). A limitation of the UVP + PD method has until now been associated with the transducer technology, leading to weak performance in industrial applications. Typical examples are acoustic fluctuations in the near field region, leading to blind zones, and the limited penetration depth in concentrated suspensions. In this work, however, a novel non-invasive ultrasound sensor unit recently developed by SIK (Wiklund et al., 2012b) and
35
M. Rahman et al. / Tunnelling and Underground Space Technology 45 (2015) 34–42
capable of emitting sufficient acoustic energy was successfully used to measure instantaneous velocity profiles in cement grout through a stainless steel pipe. The rheology of cement based suspensions is complex, and the properties are difficult to measure with conventional rheometers (Håkansson, 1993; Banfill, 2006). Testing challenges include lubricating layers or slip at fluid/solid interfaces, sedimentation, separation, flocculation and time dependency owing to the hydration of the cement. This implies that it is often very difficult to compare results from various research teams and authors that use different methods and geometries. Despite these weaknesses, current measurements of grouts are still made with simple devices in the field or with rotational rheometers in a laboratory. The linear Bingham model is widely used in the grouting industry due to its simplicity and its inclusion of the basic rheological properties, viscosity and yield stress. Understanding the rheological behavior of cement based grouts is important for the design and control of the grouting process. For example, assuming the Bingham model, the yield stress and the viscosity will determine the relationship between pressure and flow and consequently the time necessary to reach a certain distance into the grouted formation; the yield stress will also set a limit to the possible penetration length. Håkansson et al. (1992) showed that typical values for the Bingham yield stress and viscosity for commonly used micro cement grouts are in the range of 1–10 Pa and 50–200 mPa s, depending on the water/cement ratio and admixtures used. In contrast to many other industrial fluid flow applications, the detailed geometry of jointed rock or porous soil is never fully known. This implies that it does not make sense to seek overly complex rheological models when modeling flow situations in the grouting process. Although it is known that the true rheological behavior of dense cement suspensions is shear thinning with a yield stress, i.e. in principle fitting the Herschel–Bulkey model, the Bingham model is still used since the geometrical considerations are more influential than the choice of rheological model. In this work, the yield stress and viscosity of the grouts were determined using the Bingham and Herschel–Bulkley models as well as the gradient method (Birkhofer et al., 2008). In addition, the yield stress and viscosity were determined for different flow rates using the tube viscometry concept and were subsequently compared with the results obtained using the rheological models. 2. Materials and method 2.1. Materials Cement is used in grouting because of its wide availability, ease of preparation and relatively low cost. Cementa IC30 micro cement is commonly used in practice and was used in this work. IC30 has a particle distribution where more than 95% of the cement particles are less than 30 lm, which has been shown to be the optimum particle size to avoid clogging inside rock fractures (Eklund and Stille, 2008). Cementa IC30 is a sulphate resistant, chromate reduced and low alkaline injection cement, with a compact density of approximately 3100–3200 kg/m3 and a bulk density of 800–1500 kg/m3. The chemical composition includes MgO (max 5% by weight), SO3 (max 3% by weight) and Cl (max 0.1% by weight). A commercial additive, Cementa SetControl II, was used with a dosage of 2% (w/w) in order to maintain grout properties similar to those in the field. A total of eight batches were used in the experiments. The numbers of performed tests are shown in Table 1. Cement grouts with water to cement ratios of 0.6, 0.7, 0.8 and 1.0 were tested with a mixing time of four minutes. Each batch consisted of 15 L of cement grout. Weight of cement and corresponding weight of water was measured prior to the test. Water was poured
Table 1 Number of tests performed for the experiments. W/C ratio
SetControl II (%)
No of tests
0.6 0.7 0.8 1.0
2 2 2 2
1 3 2 2
into the mixer and cement was added afterward, while the mixer was rotating at a pre-defined speed. A Dispermat CV-3 dissolver from VMA-GETZMANN Gmbh was used to mix the cement. A speed of 2000 rpm was used to mix the cement with water. SetControl II was added after two minutes. 2.2. The UVP + PD methodology for in-line rheometry 2.2.1. Ultrasound Velocity Profiling (UVP) Ultrasound Velocity Profiling (UVP) is a technique for measuring an instantaneous velocity distribution along the pulsed beam axis. It was first developed in medicine to measure the flow of blood and subsequently extended to measure the velocity profiles of opaque fluids (Takeda, 1995). The method has now been successfully extended and is widely accepted as an important tool for measuring the velocity profile in other fields of research and engineering. The working principle of the ultrasound velocity profiling technique is based on emission of successive ultrasound pulses and reception of echoes along the beam axis. A transducer mounted at a specific inclination angle, ensuring the measurement of the velocity component in the direction of the measuring line, emits ultrasound pulses through the acoustic coupling and the wall material. Part of the ultrasound energy is scattered and reflected back from the surface of the moving suspended particles inside the pipe. The reflected Doppler shifted echo is recorded using the same transducer. The time intervals between pulse emissions are used to sample consecutive echo receptions at specific times in a narrow time window, called a gate or channel. The distance of the suspended particle reflection inside the pipe is determined by a time of flight measurement, and the local velocity can be obtained by determining the Doppler shifted frequency for each gate or channel. The Doppler frequency is calculated from successive pulses that are received, and an instantaneous velocity profile can be constructed (Wiklund et al., 2007). Visualization of the velocity profiles directly in-line in the ongoing process is a unique feature available in UVP. When conventional offline rheometers are used, a data fitting procedure is needed to determine the rheological behavior and the velocity profiles are then constructed from the measured data. In the curve fitting procedures, there can be errors in the data themselves and errors while fitting the data to a mathematical model. In contrast, as the UVP + PD method measures the velocity profiles directly, the necessary simplifications and the errors described above are significantly reduced. 2.2.2. Tube viscometry and the UVP + PD method UVP + PD is a methodology for determining rheological properties in-line in pipe flow and is based on the combination of Ultrasound Velocity Profiling (UVP) and the Pressure Difference (PD), based on the principle of tube viscometry. The tube viscometry is well described in the literature and can be found in Barnes (1999), Chhabra and Richardson (1999). However, the problem associated with the traditional tube viscometry method is that it provides only single point measurements; as a result, its applicability for non-Newtonian fluids is questionable, provided that only one flow rate is used. The pressure difference, DP, is measured over a certain distance and the wall shear stress, sw, is given by
36
sw ¼
M. Rahman et al. / Tunnelling and Underground Space Technology 45 (2015) 34–42
R w DP 2L
ð1Þ
where Rw is the radius of the pipe and L is the distance between the pressure sensors. In addition, the yield stress can be calculated if the radius of the plug flow is known. The shear rate and shear stress distribution along the pipe radius can be determined by
c_ ¼
dv dr
ð2Þ
r DP 2L
ð3Þ
sðrÞ ¼
The rheological properties of the fluid under investigation can be determined using different approaches. A simple method is the rheological model fitting approach, in which the velocity profile is fitted with mathematical models, e.g. Bingham, Herschel Bulkley etc., and the rheological parameters are obtained. The volumetric flow rate is determined by integrating the velocity profile. The gradient method is a unique non-model approach offered by the UVP + PD method developed by SIK for direct measurement of the rheological properties. The advantage associated with the gradient method is that it does not require a priori knowledge of the rheological model of the material being tested. In the gradient method, the shear rate is obtained from the gradient of the measured velocity profile and the shear stress is obtained from the pressure drop over a certain length of the pipe section, Furthermore, the yield stress can be determined from the measured plug radius of the velocity profile. Therefore, in contrast to other methods, it is the true shear viscosity and yield stress data that are presented, see Birkhofer et al. (2008), Wiklund et al. (2007). An accurate measurement of the velocity profile is a prerequisite for using the gradient method, as the results depend on the quality of the measured profiles. The pipe wall position and the Doppler angle must thus be precisely determined. In addition, the acoustic energy emitted by the ultrasound transducer must be sufficient to measure the velocity profile at least up to the center of the pipe. Since the wall shear stress and the wall shear rate can be measured directly in-line with the UVP + PD method, the rheological properties can also be determined using the traditional tube viscometry concept, provided that data from different flow rates are used. The yield stress and viscosity can be determined using a Bingham fitting to the measured data. The UVP + PD method used in this work was developed at SIK, The Swedish Institute for Food and Biotechnology; greater detail on the methodology can be found in Wiklund et al. (2007). The UVP + PD technique has been successfully used on a variety of industrial suspensions, such as food, paper pulp and mine tailings e.g. Wiklund et al. (2006), Birkhofer et al. (2008), Kotzé et al. (2008), Wiklund and Stading (2008), Windhab and Ouriev (2002). The UVP + PD method has also been found successful for cement based grout (Wiklund et al., 2012a). 2.2.3. Experimental flow loop and equipment The cement grout was circulated in a flow loop consisting of a storage tank, agitator, progressive cavity single screw pump, LOGAC™ flow meter, electromagnetic flow meter, temperature sensors and an integrated UVP + PD sensor unit. This type of pump was used to achieve a stable flow condition without fluctuation. The integrated sensor unit consisted of a differential pressure sensor with remote seals and one pair of noninvasive ultrasound transducers capable of measuring through high-grade industrial stainless steel pipes. The sensor unit also produces a more well defined beam with higher acoustic energy compared to previous set-ups (Wiklund et al., 2012a). The flow loop was connected by stainless steel pipes with a 22.6 mm inner diameter. This was a
novel application and experimental set-up for measuring the time dependent behavior of cement based grouts. A schematic illustration of the experimental set-up is shown in Fig. 1. A progressive cavity pump type MAE 50-2/BB.BBNT32 PompeRaccorderia was used in the laboratory flow loop. Progressive cavity pumps are also known as single screw pumps, and the major functional components are the rotor and the stator. The stator is made of vulcanized rubber, placed inside a steel pipe. The stator has a circular cross section single start screw with a pitch. The pump was designed for suspension materials, and it is possible to obtain very stable flow conditions. The pump can be operated at flow rate ranges up to 10 L/min with a pressure limit of maximum 1.2 MPa. The LOGAC™ 4000 is a computer based data recording system manufactured by Atlas Copco and consists of an electromagnetic flow meter and a pressure sensor. It is used for storing and sampling data during field grouting operations. The parameters that can be logged and stored are flow, pressure, volume, time and real time. The data can be recorded on the card every 1st, 5th or 10th second. The flow meter operates at a range of 0–200 L/min with a maximum allowed pressure of 4 MPa. LOGAC™ was used as a reference to compare the volumetric flow rate obtained by the UVP and the electromagnetic flow meter. A Proline Promag 55S electromagnetic flow measuring instrument manufactured by Endress + Hauser was installed in the flow loop. The operating flow range was 0.01–10 L/s, and the end frequency output was from 2–10,000 Hz. Electromagnetic flow meters utilize Faraday’s law of induction, where a voltage is induced in a conductor moving in a magnetic field and the flowing medium corresponds to the moving conductor. The induced voltage is assumed to be proportional to the flow velocity. As a consequence, when measurements are made in a non-Newtonian fluid, this assumption can result in inaccuracies due to the unknown shape factor of the velocity profiles. The Proline Promag 55S electromagnetic flow measurement system was used to compare the flow rate determined with UVP and LOGAC™, since an earlier work (Wiklund et al., 2012a) showed that the LOGAC™ flow meter determines the average value over the sampling period and lacks accuracy in comparison with the UVP. 2.2.4. Conventional off-line rheometer The off-line measurements were made with an ARES G2 (Advanced Rheometric Expansion System) rheometer (TA Instruments, USA). Measurements were made over a shear rate range of 0.1–1000 s1 with 10 points per decade for comparison with the in-line results. A 27 mm DIN concentric cylinder geometry, rotating outer cylinder diameter of 30 mm and inner cylinder diameter of 27 mm was used. An equilibration time of 5 s were used for the measurements. Off-line measurements were performed only for the w/c ratio of 0.7. 2.2.5. UVP + PD instrumentation and sensor unit This work tested, for the first time, a pair of custom made, noninvasive, robust sensor units, clamped on a pipe section and capable of emitting sufficient acoustic energy to perform non-invasive measurements of the velocity profiles through the stainless steel pipe. The sensor unit consists of transducers, wedges, absorbers, acoustic couplants etc. To minimize the effect of sedimentation, the two transducers were clamped in a horizontal plane opposite to each other with an angle of inclination (between the flow direction and the transducer axis) of 20 and 110, respectively. A 3D model of the non-invasive sensor unit is shown in Fig. 2. The UVP + PD sensor unit is comprised of two customized ultrasound transducers. The unit further consists of a differential
M. Rahman et al. / Tunnelling and Underground Space Technology 45 (2015) 34–42
37
Fig. 1. Schematic illustration of the flow loop used for the experimental work.
pressure sensor with remote seals to measure the differential pressure over a distance of 1.98 m. The in-line velocity profile measurements were made using a pulser/receiver instrument, UVP-DUO-MX Monitor (Met-Flow SA, Lausanne, Switzerland). The instrument firmware and driver software were modified to allow access to the Demodulated Echo Amplitude Data (DMEA or I-/Q-Data) via an Active X library. The UVP-Duo instrument and the other hardware devices were connected to a master PC via Ethernet and a DAQ card (National Instruments Sweden AB, Solna, Sweden). A high-speed digitalizer was used for data acquisition in the simultaneous measurements of velocity profiles and acoustic properties. Versatile MATLAB based (The MathWorks Inc., Natick,MA, USA) software with a graphical user interface (RheoFlow™), developed by SIK, was used to control the hardware devices for data acquisition, signal
Fig. 2. 3D-model of the non-invasive sensor unit with mounting device (Wiklund et al., 2012b).
processing and visualization of the data. The volumetric flow rate was obtained from integration of the velocity profiles obtained. 3. Results and discussion 3.1. Velocity profiles Since the determination of the rheological properties depends on an accurate measurement of the velocity profiles, it was important to be able to measure velocity profiles at least up to the center of the pipe. Velocity profiles measured by the previously used flush mounted transducers and by the newly developed non-invasive sensor unit for cement grout of a water to cement ratio of 0.7 are shown in Figs. 3 and 4, respectively. Velocity profiles measured by the flush mounted transducers are distorted at the center of the pipe, which indicates that the emitted energy was not sufficient and the penetration depth was reduced due to the strong attenuation of the ultrasound energy in the cement grout. On the
Fig. 3. Velocity profiles of w/c ratio 0.7, measured by delay line 4 MHz transducers (Rahman et al., 2012).
38
M. Rahman et al. / Tunnelling and Underground Space Technology 45 (2015) 34–42
Fig. 4. Velocity profiles of w/c ratio 0.7, measured by non-invasive sensor unit.
other hand, it was possible to measure the velocity profiles nearly for the full diameter of the pipe by the non-invasive sensor unit, shown in Fig. 4. A mirror image is combined and shown in the figures for a complete illustration of the velocity profiles. In Fig. 4, continuous profiles were obtained by the new noninvasive sensor unit because of its capability of emitting high acoustic energy. This is a significant development in transducer technology, and it is the first time such a device has been used to measure velocity profiles of cement grouts in-line. An interesting feature that can be observed in Fig. 4 is the non-uniform shape, shown with a circle in the velocity profile, which might occur due to the change in the viscosity of the cement grout as a consequence of a change in the shearing procedure, since the shearing is different at the different positions in the pipe section. In addition, a wall slip phenomenon is common, provided that a smooth surface pipe is used (Barnes, 1995; Dogan et al., 2002; Isa et al., 2007). As a result, a non-zero velocity near the pipe wall is expected, and this can be seen in Fig. 4. The average velocity profiles for w/c ratios of 0.6, 0.7, 0.8 and 1.0 are shown in Fig. 5. While measuring the velocity profiles, the flow rate was controlled and the pump pressure was synchronized subsequently. As a result, due to different applied flow rates, a higher velocity was observed for a thicker suspension, i.e. a w/c ratio of 0.6. It can be seen that the velocity profiles were successfully measured up to the center of the pipe. The velocity profile for w/c 0.8 is slightly distorted at the near wall region, which can be explained by the presence of air bubbles, indicating the need of an improved mixing and agitation of the cement grout for a thicker suspension. Moreover, the fluid near the wall is influenced and subjected to the strongest shear; this might therefore result in a distorted shape of the profile in the near wall region. Accurate determination of the true wall shear rate and
Fig. 5. Average velocity profiles measured by non-invasive sensor unit for w/c ratio 0.6, 0.7, 0.8 and 1.0.
stress thus depends on the accuracy in determining the actual wall interface (when measuring through material layers) or liquid-wall interface (when measuring with direct contact to the test fluid), which is of great importance for determining the velocity profile and the rheological properties. The determination of the interface is complicated, especially when measuring velocity profiles with limited spatial resolution or when attenuation distorts the quality of near wall velocity data. It has been shown that the rheological parameters determined using the UVP + PD method vary significantly by changing the wall position by less than 0.37 mm (or one channel) (Wiklund et al., 2007). However, the non-invasive sensor unit, in combination with the gradient method, has the advantage that knowledge of the wall position is not a prerequisite. Since the velocity profiles were accurately measured up to the center of the pipe, it was possible to visualize the shape of the velocity profile and the development of the plug radius with time for a certain concentration of cement grouts. The data in Fig. 5 allow a visualization of the flow pattern, e.g. the development of a plug flow. For a w/c ratio of 1.0, a behavior similar to Newtonian fluid behavior is observed due to the lower concentration; in contrast, for w/c ratios of 0.6 and 0.7, a plug flow is visible at the center of the pipe. 3.2. Comparison of the volumetric flow rate obtained by the UVP method and conventional flow meter As the UVP method offers a visualization of the velocity profile itself, the volumetric flow rate can be determined by integration of the velocity profile. The volumetric flow rate, measured by the field equipment LOGAC™, electromagnetic flowmeter and the UVP, is shown in Fig. 6 for the piston pump and the progressive cavity pump. The pulsation of the flow due to the back stroke of the piston is clearly visible, which is due to the much faster data acquisition and velocity estimation with the UVP technique. As shown in the figure, the true flow rate measured by the LOGAC™ remains unknown; however, the average value of the flow rate was obtained. The electromagnetic flow meter yielded a fixed volumetric flow rate slightly higher than the UVP. The flow measuring principle of the electromagnetic flow meter is based on the assumption of a velocity profile for a Newtonian fluid, which makes it fairly unreliable for non-Newtonian shear thinning fluids such as cement grouts. The progressive cavity pump was used to achieve a stable flowing condition for the cement grout. As can be seen in Fig. 6, UVP provided faster data acquisition and slight fluctuations, whereas the LOGACTM yielded results of the same order of magnitude. The pressure is high and the flow is low during the later stage of grouting, and the commercial coriolis flow meters are not capable of measuring such a low flow rate. Since the stop criteria for grouting are based on the flow of grouts in rock fractures, a more
Fig. 6. Comparison of volumetric flow rate obtained by the LOGAC™ and the UVP. Conversion factor 1 L/min = 1.67 104 m3/s.
M. Rahman et al. / Tunnelling and Underground Space Technology 45 (2015) 34–42
39
accurate measurement of the flow rate will lead to an improved application of these stop criteria. When using a piston pump, a negative velocity can be seen, i.e. a backward flow due to the suction force when the piston is moving backward (Wiklund et al., 2012a). It is not possible to measure such negative velocity by using a commercial coriolis flow meter since the principle of the coriolis flow meter is based on the determination of the phase difference of signals originating from the oscillation of the sensors (Vetter and Notzon, 1994). 3.3. Rheological observations The rheological properties were determined by curve fitting the Bingham and Herschel–Bulkley rheological models to the measured velocity profiles. In addition, a non-model approach by the so called gradient method was used for direct determination of the rheological properties from the velocity profiles. The tube viscometry concept was used with different flow rates to determine the wall shear rate and wall shear stress, from which the yield stress and viscosity could be obtained assuming the Bingham model. It must be noted that the earliest measurement was made 25 min after the mixing of the cement. 15 L of grout was used in the flow loop for each test. Since five liters of grout could be prepared every time using the high speed mixer; due to practical reasons, measurements could not be started before 25 min. For the off-line measurements, the samples were collected from the flow loop and tested in the laboratory, which implies a time lag between the sampling and the experiments. In contrast, in-line measurements are performed while the cement grout is in a continuously flowing condition. As a result, discrepancies between the in-line and off-line results can occur and have also been observed in earlier work (Wiklund et al., 2007). 3.3.1. Rheological properties determined by the Bingham model The yield stress and the viscosity, determined by the Bingham model, are shown in Figs. 7 and 8, respectively. As can be seen, the increases in the yield stress and viscosity due to hydration with time and for different concentrations are visible in the figures. The highest value of yield stress was observed for the w/c ratio of 0.6, which was expected owing to the highest concentration of cement particles. For easier illustration, the increase in the yield stress and viscosity is shown by linear trend lines. The trend of increased yield stress with time was observed for w/c 0.6. However, some discrepancies, e.g. decreased yield stress, can be seen for the time period from 60–80 min, which can be explained by the shape of the velocity profile. Since the measured velocity profile was fitted using a linear mathematical model, a slight distortion in the measured data will yield a different gradient at the velocity profile and will therefore result in an inaccurate rheological property value. In addition, the effect of SetControl II is to decrease the initial viscosity of the cement material to increase the flowability inside the
Fig. 7. Yield stress determined by Bingham model for different w/c ratios and time.
Fig. 8. Viscosity determined by the Bingham model for different w/c ratios and time.
rock fractures. An expected trend of the increased yield stress was observed for w/c ratios of 0.7, 0.8 and 1.0. The yield stress determined for the w/c ratio of 0.7 is in good agreement for both the in-line and off-line measurements. The progress of yield stress with time was mildest for w/c ratio 1.0, which was due to the thinner suspension of cement. In Fig. 8, the increase of viscosity with time is seen for w/c ratios of 0.6, 0.7, 0.8 and 1.0. A rapid change in viscosity was observed with time for w/c ratios of 0.6 and 0.7, which was due to the higher concentration. A slight increase in the viscosity was observed for the w/c ratios of 0.8 and 1.0. The viscosity measured off-line for cement grout with a w/c ratio of 0.7, 125 min after mixing, showed a lower viscosity in comparison with the in-line measurement, which can be explained by the range of the shear rate. During the off-line measurement, the grout was sheared from 0.1 s1 to 1000 s1; in contrast, during in-line measurements, the cement grout was sheared in a lower shearing rate, i.e. from 0.1 to 400 s1. Since the grout was in a dynamic state, continuously in motion inside a flow loop and subjected to structural breakdown different from that in the off-line measurements, development of the yield stress and viscosity would be different from that in the off-line measurement systems. As shown in Figs. 7 and 8, a continuous shearing process slows the development of the yield stress and viscosity for thinner grouts, e.g. w/c ratios of 0.8 and 1.0.
3.3.2. Rheological properties determined by the Herschel–Bulkley (H–B) model The H–B model was chosen to simulate the rheological behavior of cement grout since it consists of a yield stress and exhibits a shear thinning behavior consistent with many dense suspensions. As can be seen from the shear stress vs. shear rate curve, fitted by the H–B model in Fig. 9, a higher shear stress was observed for a higher concentration of cement grout. Time dependent behavior was also observed, since a higher shear stress was observed after a longer period of time, owing to hydration of the cement grout. A comparatively higher yield stress and shear stress were observed for the w/c ratio of 0.6, in comparison with w/c ratios 0.7, 0.8 and 1.0, which indicates a shorter setting time resulting from a thicker concentration. A similar shear stress for the w/c ratio 0.8 after 138 min was observed as compared to w/c 0.7 after 63 min, which was due to the longer time after mixing for the w/c ratio of 0.8. Off-line measurements were made for w/c ratio 0.7, and very good agreement was found between the in-line and offline measurements. Since viscosity is obtained from the gradient of the shear stress curve vs. shear rate curve, Fig. 9 illustrates that the difference in viscosity was more pronounced for different w/c ratios at a lower shear rate, which means that the viscosity increases at lower velocity due to the lower shearing of the cement suspension.
40
M. Rahman et al. / Tunnelling and Underground Space Technology 45 (2015) 34–42
Fig. 9. Shear stress vs. shear rate curves fitted to the H–B model for different w/c ratios and time.
The rheological parameters obtained by the H–B model for different w/c ratios are shown in Table 2. A higher yield stress was observed for the w/c ratio of 0.6. The flow index, n, did not show any significant change for w/c 0.6, which means that the shear thinning behavior did not change significantly. However, the consistency index increased with time for the w/c ratio of 0.6, which indicates a progress in shear stress. A slight increase of the yield stress was observed with time for w/c ratio 0.7. The flow index, n, was similar to that of w/c 0.6, although a lower consistency index was observed, which indicates a lower shear stress in comparison with the w/c ratio of 0.6. The yield stress determined for w/c 0.8 was lower as compared with w/c ratios of 0.6 and 0.7, which was expected. A lower consistency index was observed for the higher w/c ratio, which indicates a lower increase of the shear stress. In addition, a higher value of the flow index, n, was observed, which indicates the decrease in the viscosity. As can be seen for w/c 1.0, the consistency index, k, was the lowest and the flow index, n, was the highest, which can be explained by a lower viscosity due to continuous shearing of the thinner grout. The offline results yield a slightly higher yield stress and consistency index, which can be explained by a different measuring condition and the time lag between the experiments. Although the H–B model successfully explains the shear thinning behavior of cement grout, as shown in Table 2, the difficulty is associated with explaining time dependent behavior, and this difficulty has to do with the number of rheological parameters involved in the fitting procedure. Table 2 Rheological parameters obtained by H–B model. W/C ratio
Time after mixing
Yield stress (Pa)
n
k
R2
0.6
22 65 84 123
5.9 4.8 6 6
0.3 0.3 0.4 0.3
2.7 5 3.3 5.1
0.95 0.9 0.96 0.93
0.7
63 86 96 132 125
1.1 1.2 1.3 1.6 2.2
0.3 0.4 0.3 0.4 0.4
2.1 1.4 2.3 2.2 1.9
0.96 0.99 0.95 0.92 0.95
0.8
49 104 138 150
0.8 2.1 5.6 3.4
0.4 0.4 0.4 0.2
1.0 0.9 0.7 2.2
0.97 0.98 0.95 0.97
1.0
25 64 105 129
1.2 0.9 0.8 0.8
0.3 0.4 0.4 0.4
1.5 1.0 1.0 1.0
0.86 0.97 0.97 0.95
0.7 off-line
3.3.3. Rheological properties determined by the Gradient method The gradient method is a non-model approach available in the UVP + PD method that is capable of performing direct measurements of rheological properties instead of using any rheological fitting procedure. Fig. 10 shows time dependent behavior, i.e. increasing shear stress with time, for w/c ratios 0.7 and 1.0. As expected, a higher shear stress was exhibited by w/c ratio 0.7 in comparison with w/c ratio 1.0 because of a thicker suspension. The distorted shapes at the higher shear rate region of the shear stress vs. the shear rate curve are due to the distortion in the shear rate distribution. Detection of the correct wall position and an accurate velocity profile measurement in the near wall region can eliminate distortion at the higher shear rate region. However, a shape preserving smoothing filter was used to eliminate noise in the near field region. For the w/c ratio 1.0 after 66 min, a lower shear stress was observed as compared with w/c ratio 1.0 after 24 and 26 min. It must be noted that the flow rate was not the same for both samples. As can be seen, a lower flow rate, i.e. a lower shearing rate, yielded a lower shear stress, which indicates a different structural breakdown and recovery of the material at different velocities, i.e. flow rates. With the gradient method, the yield stress is measured directly and determined from the plug radius. The yield stress for w/c ratios of 0.7 and 1.0 is shown in Table 3. As can be seen, the yield stress increased with time. A lower yield stress was observed for w/c ratio 1.0, 66 min after mixing, in comparison with 24 and 26 min. However, the plug radius that was measured was similar. Since the flow rate range for w/c ratio 1.0, 66 min after mixing, was lower, a lower pressure drop was observed, which yielded a lower yield stress. An accurate measurement of the velocity profile is a prerequisite in the gradient method since the determination of the shear rate is highly sensitive to noise. As a result, the gradient method was applied for two w/c ratios, 0.7 and 1.0, which provided smooth velocity profiles. 3.3.4. Rheological properties determined by tube viscometry The tube viscometry approach was also used in this work to determine the yield stress and viscosity of the grout used. Tube viscometry has been used earlier for mineral suspensions, and good agreement with the UVP + PD method has been reported (Kotzé et al., 2008). However, instead of using different diameter pipes, different flow rates were employed in this work to obtain the wall shear rate and wall shear stress. The yield stress and viscosity determined using the tube viscometry concept for different flow rates are shown in Fig. 11. The wall shear rate and wall shear stress were measured at three different flow rates, 5 L/min, 7 L/min and 9 L/min. In Fig. 11, the yield stress is the intercept with the y-axis and the slope is the viscosity. In contrast, the viscosity according to the H–B model and the gradient method was determined by taking
Fig. 10. Shear stress vs. shear rate curve measured by the Gradient method.
M. Rahman et al. / Tunnelling and Underground Space Technology 45 (2015) 34–42
41
which might influence the yield stress determined for the w/c ratio of 1.0.
Table 3 Yield stress measured by the Gradient method. W/C ratio
Time after mixing (min)
Yield stress (Pa)
Pressure difference (Pa/m)
Plug radius (mm)
0.7 0.7 1.0 1.0 1.0 1.0
69 75 3 24 26 66
3.9 4.1 1.0 2.0 1.8 0.5
1137 1172 299 614 577 140
6.8 6.9 6.8 6.4 6.3 6.8
Fig. 11. Yield stress and viscosity determined by the Tube viscometry principle for different flow rates.
Table 4 Comparison of yield stress determined by different methods. W/C ratio
Time after mixing
Bingham
H–B
Gradient
Tube viscometry
0.6 0.6 0.7 0.7 0.8 0.8 1.0
33 65 65 75 49 74 25
14.7 16 4.9 4.9 3.4 4.3 3.2
5.1 4.8 1.1 1.2 0.9 0.8 0.9
– – 3.9 4.1 – – 2
– – 5.5 – – – 0.1
the gradient at individual points of the shear stress vs. shear rate curve and is therefore dependent on shear rate. The trend of increased yield stress and viscosity for thicker grouts was observed, however. The lack of data in the lower shear rate region was due to the pressure drop measurements, since the lowest shear rate was obtained for the flow rate of 5 L/min. 3.4. Comparison of the yield stress determined by different methods Table 4 shows a comparison of the yield stress determined using different models and methods for different w/c ratios. As expected, the yield stresses were overestimated with the Bingham model, due to the linear assumption of the shear stress and shear rate. The H–B model always yielded a lower value than the Bingam model and gradient method due to the non-linear relationship between the shear stress and shear rate. Yield stress measured by the gradient method for the w/c ratio of 0.7 was in good agreement with the Bingham model, although a lower value was observed for w/c 1.0. A comparatively higher yield stress was observed for the w/c ratio of 0.6, which would be due to the thicker grout. For the tube viscometry concept at different flow rates, the yield stress determined for w/c 0.7 was in good agreement with the Binghan yield stress; however, for the w/c ratio 1.0, it was lower. The effect of hydration with time was not taken into consideration when data were recorded for different flow rates,
4. Conclusion The newly developed non-invasive sensor unit was capable of generating sufficient energy to provide velocity profiles exceeding half of the pipe diameter, which is necessary to be able to determine the change in the rheological properties in-line for watercement ratios down to 0.6. This is a significant improvement as compared with transducers that have been used earlier. The visualization of the velocity profile and the change in its shape caused by an increase in both yield stress and viscosity were seen as a function of concentration and time. The velocity profile resembled Newtonian behavior for low concentrations, while it changed to Bingham and H–B models at higher concentrations. Visualization of the velocity profile also has a potential in determining the existence and extent of slip, which is commonly seen for suspensions at a fluid/solid interface. The volumetric flow rate was determined by integrating the velocity profiles and was subsequently compared with the flow rates obtained using a commercial electromagnetic flow meter. Since the UVP is capable of faster data acquisition, and providing accurate measurements at lower velocities (i.e. 1 L/min), it can not only be used for control during grouting but also for optimizing the characteristics of the grout pump. It can be concluded from this work that the UVP + PD method can successfully be used continuously, in-line, to determine the time dependent behavior of cement based grouts for different concentrations. Future investigations should focus on different phenomena, such as the wall slip, thixotropy of cement grout and the robustness of the UVP + PD method for field use. Acknowledgements This work was financed by the Swedish Construction Industry Research Foundation (SBUF), the Swedish Rock Engineering Research Foundation (BeFo) and the Swedish Research Council (Formas). Their support is highly appreciated. AtlasCopco is acknowledged for providing the LOGAC™ flow meter. References Banfill, P., 2006. Rheology of fresh cement and concrete. Rheology Reviews, 31–130. Barnes, H.A., 1995. A review of the slip (wall depletion) of polymer solutions, emulsions and particle suspensions in viscometers: its cause, character and cure. J. Non-Newtonian Fluid Mech. 56, 221–231. Barnes, H.A., 1999. On-line or process viscometry – a review. Appl. Rheol. 9, 102– 107. Birkhofer, B., Jeelani, S., Windhab, E., Ouriev, B., Lisner, K., Braun, P., Zeng, Y., 2008. Monitoring of fat crystallization process using UVP-PD technique. Flow Meas. Instrum. 19, 169. Chhabra, R.P., Richardson, J.F., 1999. Non-Newtonian Flow in the Process Industries: Fundamentals and Engineering Applications. Butterworth-Heinemann. Dogan, N., McCarthy, M.J., Powell, R.L., 2002. In-line measurement of rheological parameters and modeling of apparent wall slip in diced tomato suspensions using ultrasonics. J. Food Sci., 67. Eklund, D., Stille, H., 2008. Penetrability due to filtration tendency of cement-based grouts. Tunn. Undergr. Space Technol. 23, 389–398. Gustafson, G., Stille, H., 2005. Stop criteria for cement grouting. Felsbau 25, 62–68. Håkansson, U., Hässler, L., Stille, H., 1992. Rheological properties of microfine cement grouts. Tunnel. Undergr. Space Technol. 7, 453–458. Håkansson, U., 1993. Rheology of Fresh Cement based Grouts- PhD Thesis, Stockholm: KTH Royal Institute of Technology. Håkansson, U., Rahman, M., 2009. Rheological properties of cement based grouts using the UVP-PD method. In: Proceeding of Nordic Symposium of Rock Grouting, Helsinki. Håkansson, U., Rahman, M., Wiklund, J., 2012. In-line measurements of rheological properties of cement based grouts – introducing the UVP-PD method. In: Proceeding of 4th International Conference on Grouting and Deep Mixing, New Orleans. Hässler, L., 1991. Grouting of Rock – Simulation and Classification- PhD Thesis, Stockholm: KTH Royal Institute of Technology.
42
M. Rahman et al. / Tunnelling and Underground Space Technology 45 (2015) 34–42
Isa, L., Besseling, R., Poon, W.C.K., 2007. Shear zones and wall slip in the capillary flow of concentrated colloidal suspensions. Phys. Rev. Lett., 98. Kobayashi, S., Stille, H., Gustafson, G., Stille, B., 2008. Real Time Grouting Control Method – Development and Application Using ÄspÖ HRL Data. Swedish Nuclear Fuel and Waste Management Co (SKB), Stockholm, Sweden, R-08-133. Kotzé, R., Haldenwang, R., Slatter, P., 2008. Rheological characterization of highly concentrated mineral suspensions using ultrasonic velocity profiling combined pressure difference method. Appl. Rheol. 18, 62114. Lombardi, G., 1985. Some Theoretical Considerations on Cement Rock Grouting. Lombardi Engineering Ltd. Rahman, M., Håkansson, U., Wiklund, J., 2012. Grout pump characteristics evaluated with the Ultrasound Velocity profiling. In: Proceeding of ISRM International Symposium, EUROCK, Stockholm. Takeda, Y., 1995. Velocity profile measurement by ultrasonic Doppler method. Exp. Thermal Fluid Sci. 10, 444–453. Vetter, G., Notzon, S., 1994. Effect of pulsating flow on Coriolis mass flowmeters. Flow Meas. Instrum. 5, 263–273. Wiklund, J., Rahman, M., Håkansson, U., 2012a. In-line rheometry of micro cement based grouts – a promising new industrial application of the ultrasound based UVP+PD method. Appl. Rheol. 22, 42783.
Wiklund, J., Kotzé, R., Haldenwang, R., Stading, M., 2012b. Development of an industrial UVP+PD based rheometer-optimisation of UVP system and transducer technology. In: Proceeding of 8th International Symposium on Ultrasonic Doppler Methods for Fluid Mechanics and Fluid Engineering. Dresden. Wiklund, J., Shahram, I., Stading, M., 2007. Methodology for in-line rheology by ultrasound Doppler velocity profiling and pressure difference techniques. Chem. Eng. Sci. 62, 4277–4293. Wiklund, J., Stading, M., 2008. Application of in-line ultrasound Doppler based UVPPD rheometry method to concentrated model and industrial suspensions. Flow Meas. Instrum. 19, 171–179. Wiklund, J., Stading, M., Pettersson, A.J., Rasmuson, A., 2006. A comparative study of UVP and LDA techniques for pulp suspensions in pipe flow. AIChE J. 52, 484– 495. Windhab, E.J., Ouriev, B., 2002. Rheological study of concentrated suspensions pressure-driven shear flow using a novel in-line ultrasound Doppler method. Experiments in Fluids 32, 204–211.