Changes in rheology of self-consolidating concrete induced by pumping

2.1.1 Circuit at Ghent University

The concrete pump used for the experiments at Ghent University was a truck-mounted piston pump (Schwing P2023), capable of delivering a maximum pressure of 95 bar, or a maximum flow rate of 41.5 l/s. The working action of the pump is as follows: two cylinders with a volume of 83.1 l each alternately pull concrete from the hopper and push concrete inside the pumping circuit.

Behind the pump, a loop circuit was installed with 100 mm diameter pipes, allowing the concrete to flow back inside the hopper of the pump. The circuit was 103 m long for SCC A and B, and 81 m long for SCC C and D (Fig. 1). It consisted of five horizontal straight sections, connected by 180° bends, while a 6^th final straight section was inclined to complete the loop. Pressure sensors were installed flush in the last straight horizontal section, with a separation distance of 13 and 10 m in the 103 and 81 m circuit, respectively (Fig. 1). A set of three strain gauges was attached to the outer pipe wall, acting as a back-up for in case the pressure sensors would fail [20, 25]. All sensors were connected to a data acquisition system registering data at a frequency of 10 Hz [25].

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The flow rate was determined by measuring the time needed to complete a certain number of pumping strokes. A calibration procedure has revealed, in this case, that the correction needed for the incomplete filling of the cylinders was compensated for by the correction imposed by the dead time of the pumping stroke. As a result, the flow rate estimated by determining the time was equal to the real flow rate during which pressure was registered [20, 25].

2.1.2 Pumping circuit at the Université de Sherbrooke

The pump used at the Université de Sherbrooke was a Schwing BPL 900 truck-mounted piston pump. The maximum pressure that the pump can deliver is 60 bar, while the maximum flow rate is 25 l/s. The volume of one pumping cylinder is 68.1 l. Behind the pump, a 30 m long loop circuit was installed. The first horizontal straight section was constructed with 100 mm diameter pipes. After making a 180° turn and enlarging the diameter, a second straight horizontal section in 125 mm pipes was installed, followed by a vertical part enabling the concrete to flow back inside the reservoir of the pump (Fig. 2) [8, 14, 26]. Both straight sections, with 100 and 125 mm diameter pipes, were equipped with pressure sensors spaced 10 m apart, and strain gauges acting as back-up [8]. All data were registered at a frequency of 10 Hz, which was similar to the experiments at Ghent University.

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The flow rate was assessed using the same strategy done at Ghent University, but the calibration procedure has revealed that a correction factor was necessary for each concrete mixture pumped [8].

2.2.1 Testing procedure at Ghent University

The procedure employed was specifically designed to investigate the effect of pumping on fresh concrete properties (Fig. 3). Flow rates could be varied in discrete steps by the pumping operator. The procedure consisted of:

• Bringing the concrete to its “reference state” at each flow rate [27], meaning that equilibrium in pressure was awaited for.

• Taking a sample of the pumped concrete

• Stepwise decreasing the flow rate from the current step to the lowest step.

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The lowest flow rate was first examined (around 4 l/s), and logically, there was no decreasing curve. The flow rate was then increased to around 7 l/s, equilibrium in pressure was awaited for and the down-curve (7 and 4 l/s) was determined. The down-curve consisted of minimum five strokes or 60 s at each flow rate. Consecutively, this procedure was repeated for flow rates around 10, 14 and 16–18 l/s (if time and pressure allowed for the latter one). As a result, for each imposed flow rate (except the 4 l/s), an equilibrium value for the pressure loss was obtained. The pressure loss at each previous flow rate was also determined. Connecting the equilibrium Δp − Q points could be considered as the upper part of a loop curve, while the quick stepwise decrease in Q delivers different lower parts of a loop curve.

After reaching equilibrium at each flow rate, a sample of the concrete was taken to assess the fresh properties: slump flow, V-Funnel flow time, density and air content as well as the rheological properties using a Tattersall Mk-II rheometer [28].

2.2.2 Testing procedure at the Université de Sherbrooke

This testing procedure aimed at studying the influence of rheology and mix design on pumping pressure [8, 14]. It consisted of awaiting equilibrium at the highest flow rate (between 10 and 18 l/s, dependent on the pressure generated by the pump), and decreasing the flow rate in six or seven steps, maintaining each step for five strokes or 45 s maximum [8, 26]. Figure 4 shows the results for the pressure measurements in the 100 mm diameter section for one test [8]. Such test generally took 5 min and was repeated four or five times at 30 min intervals. After each pumping test, a sample of the concrete was characterized by means of slump flow, V-Funnel, density and air content, sieve stability, rheology (by means of the ICAR) and tribology [7, 8]. A sample of concrete was kept aside and tested once before all pumping tests and once before the last pumping test. This sample served as reference to determine the loss of workability of the concrete and was manually re-agitated prior to testing. By comparing the successive rheological results of the pumped samples, and subtracting the workability loss, assuming yield stress and viscosity evolve linearly with time, the effect of pumping on the rheology can be isolated.

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2.2.3 Rheometers

As mentioned before, two different rheometers were used, both based on the principle of the coaxial cylinders. The inner cylinder of the Tattersall Mk-II consists of an interrupted helicoidal screw. The maximum distance between the edges of the blades of the screw was 160 mm in horizontal direction and 140 mm in vertical direction [28]. The diameter of the container was 250 mm, measured between the outer sides of the ribs, which were installed to prevent the formation of a lubrication layer. The testing procedure in the Tattersall Mk-II consisted of pre-shearing the concrete at approximately 75 rpm. Once equilibrium was achieved, the rotational velocity was decreased in 11 steps of 5 s each. Torque and rotational velocity were registered at the inner cylinder and averaged for the last 4 s of each step.

A correction procedure to obtain reliable rheological measurements was employed [29], and a comparative study revealed that the results from the Tattersall Mk-II are similar to those of the ConTec Viscometer 5, except for very fluid SCC mixtures [30]. Furthermore, most of the mixtures tested with the Tattersall Mk-II rheometer at Ghent University showed important shear-thickening behavior [25, 31, 32], requiring the application of a non-linear rheological model. The modified Bingham model [33] was chosen. The rheological properties obtained with the Tattersall Mk-II rheometer reported in this paper are the modified Bingham yield stress, and the differential viscosity at a shear rate of 5 s⁻¹. This represents the slope of the rheological curve at this shear rate. If the material was too fluid (e.g. SCC D, see further), no reliable rheological properties could be obtained. As this has occurred in this research work, the equilibrium torque at maximum rotational velocity (=ca. 75 rpm) was also reported as an indication of flow resistance.

The ICAR rheometer is based on the same principle, but the inner cylinder is a 4-blade vane [34]. The inner cylinder has a radius of 63.5 mm and a height of 127 mm. The outer radius, measured between the ribs of the container, is 143 mm. Both in horizontal and vertical direction, the gap between the vane and the bucket is at least 80 mm, allowing a maximum aggregate size up to 20 mm. The testing procedure consisted of pre-shearing the concrete at 0.5 rps for 20 s, followed by a stepwise decrease of the rotational velocity from 0.5 to 0.025 rps in 10 steps of 5 s each. Torque and rotational velocity are measured at the inner cylinder and are averaged for the last 4 s of each step, provided the torque was in equilibrium. The Reiner-Riwlin equation was used to calculate yield stress and plastic viscosity according to the linear Bingham model [35]. A correction for plug flow was employed if necessary [36]. Finally, based on a comparative test between the ConTec Viscometer 5 and the ICAR rheometer [37], the values were transformed to “as if obtained with the ConTec rheometer”. The specific reasons for this transformation are described in [8]. For some tests: SCC 2, test 5; SCC 12, test 4 and SCC 19, test 5, the yield stress value is quite elevated, and the rheological values could be doubtful due to inaccuracies in the measurements. However, qualitatively, these three measurements represent concrete with a high yield stress value.

2.3.1 Tests at Ghent University

The insertion of the concrete in the pipes took, especially for SCC A and SCC C, approximately 1 h due to many blockages during pumping start-up. For SCC B and D, as the previous concrete was kept in the pipes, the start-up had a significantly shorter duration (±5 min). The mixtures underwent the testing procedure as follows: SCC A underwent four steps with 12.6 l/s as maximum flow rate, for SCC B, the test was performed twice with a maximum flow rate of 13.1 l/s (four steps) and 16.4 l/s (five steps) respectively, SCC C and D had a maximum flow of 13.9 l/s (four steps) and 18.5 l/s (five steps) respectively.

2.3.2 Tests at the Université de Sherbrooke

In total, 18 different SCC mixtures were delivered in 1.25 or 1.5 m³ batches during this testing campaign. All mixtures were produced in a nearby ready-mix plant. The blended cement employed was a mixture of 92 % GU (Portland) cement and 8 % silica fume (GUbSF). All mixtures, except SCC 5 having an extra replacement of cement with class C fly ash, contained no other binder. The sand was a river sand, while the coarse aggregates were a combination of 80 % 5–10 mm and 20 % 10–20 mm crushed aggregates. For SCC 18 and 19, in an attempt to improve the compressive strength, the source of the coarse aggregates was varied, but the grain-size distribution was maintained. Two different types of polycarboxyl-ether based superplasticizers were used, one with long workability retention (SP-L), added at the plant and resulting in a slump flow of approximately 350–450 mm at delivery, and another one with short workability retention (SP-S) added in the laboratory to fine-tune the consistency of the mixture.

4.1.1 Mechanism

The shear stress in a circular pipe evolves linearly from zero at the center to the maximum value at the wall [40]. Due to the presence of a yield stress in concrete, a zone around the center is not sheared. The magnitude of this zone increases with increasing yield stress. For conventional vibrated concrete, it can occur that the yield stress is significantly high causing only the lubrication layer to be sheared [41]. For SCC, however, the yield stress is sufficiently low to shear a significant portion of the bulk concrete in the pipe. Shear rates in the bulk concrete can reach values of 30–60 s⁻¹ [20]. Furthermore, as a pipe is circular, a larger volume of concrete is subjected to high shear rates compared to the volume subjected to low or zero shear rates. As a consequence, a large portion of concrete is exposed to a high shear rate for a relatively long time. For example, in the Université de Sherbrooke circuit, approximately 300 liter is continuously in the pipes. Pumping concrete at a flow rate of 15 l/s leads to a time of 20 s during which the concrete is subject to shearing. This duration is similar to the time typically employed during the pre-shearing in a rheological measurement to eliminate the effect of thixotropy [36, 42, 43]. The shear rates imposed on concrete are in most cases significantly higher than the shear rate in a concrete truck or a regular concrete mixer [44]. Some high-performance concrete mixers are able to introduce an enhanced shear rate on the concrete for a prolonged time. In this case, the conclusions are likely to be altered [45].

As mentioned in Sect. 3.1 for the procedure at Ghent University (Fig. 3), with each increase in flow rate step, a new reference state, corresponding to the applied shear rate, is imposed. As a normal batching plant was used to mix the concrete, the applied shear rate at each flow rate step is expected to be the highest shear rate the concrete has underwent at that time. As equilibrium is awaited for, the imposition of the higher flow rate eliminates the importance of the shear history before the application of this flow rate. Also, with each increase in flow rate, more cement(-itious) particles are (re-)dispersed. This leads to a decrease in plastic viscosity of the mixture. This is confirmed by the rheological and V-Funnel measurements and pressure loss. It has been shown, especially for SCC, that pressure loss is well related to viscosity of the concrete [14, 20, 25]. As viscosity is decreased with each increase in flow rate step, the pressure loss should also decrease. This is observed in Fig. 5 and in Table 4, as the pressure loss at a certain flow rate decreased with increasing maximum flow rate applied before. For example (see Fig. 5 and Table 4 for SCC D), the pressure loss at approximately 4 l/s was 8.7 kPa/m in equilibrium, while it decreased to 6.6 kPa/m (at 4 l/s flow rate) when the flow rate applied before was 6.9 l/s; 5.1 kPa/m (at 4 l/s) with a maximum flow rate of 11.4 l/s applied before, and around 4.0 kPa/m if even higher flow rates were imposed before.

The second test on SCC B confirms the effect of shearing, as no further decrease in pressure loss was observed after the concrete was sheared at a flow rate of 13.1 l/s for the first time. Indeed, as the highest flow rate applied before the second test on SCC B was 13.1 l/s, the reference state corresponding to this flow rate was imposed and maintained. During test 2, the imposed flow rates were lower than or equal to 13.1 l/s, except for step 5, meaning that no new reference state was introduced. As the internal structure needs a lot more time to rebuild than the time needed to break it down, one can assume that the reference state remains approximately constant for the duration of test 2, although some stiffening occurred near the end of the test. As no higher shear rate is applied since step 4 of test 1, no additional dispersion of cement particles is expected and no decrease in viscosity, V-Funnel flow time and pressure loss is observed. However, for step 5 in test 2, such a decrease is expected, but is probably hidden due to the stiffening of the concrete at that time.

This can be explained by means of the PFI theory developed by Wallevik [46]. Cement particles can be connected in two ways: by means of physical attraction forces (causing thixotropy), or by means of chemical bonds caused by the initial hydration (causing structural breakdown, as defined by Tattersall [47]). The thixotropic forces only act on the “smaller” particles and are fully reversible: the connections can be broken when increasing the shear rate while at lower shear rates, they build back up. The chemical links though are easily broken down due to shearing, but they take a longer time to build up. Monitoring the workability loss gives a good indication on the evolution of the chemical links over time. Important to know is that some connections, whether physical or chemical, can be considered permanent, or impossible to break. However, this depends on the amount of work put in the system, which depends on the shear rate applied. Permanent connections at a specific shear rate can be broken at higher shear rates, leading to more dispersion.

In both testing campaigns, after mixing the concrete at the plant, it took 15 to 45 min to transport it to the job site. Once on site, the concrete is remixed in the truck, but typically, shear rates in a truck are not elevated, and the time the concrete is exposed to this shear rate is not long either, as the highest shear rates occur in very distinct areas. Some of the thixotropic connections which have formed during transport are broken down, but not all of them. Exposing the concrete to the high shear rates in the pipes could further break down the thixotropic connections, thus re-dispersing the cement particles.

However, as the mixer at both batching plants is not expected to deliver very high shear rates for prolonged times, the shear rate in the pipes can cause additional dispersion of cement particles, whether they were connected via thixotropic bonds or chemical links, and each increase in shear rate could cause more of the “permanent” connections to break down, thus further fluidifying the concrete.

For the mixtures pumped at the Université de Sherbrooke, a decrease in relative plastic viscosity, with increased product of flow rate and time was observed. The fact that the viscosity still decreased after the first tests is probably due to the fact that perfect equilibrium was not achieved during the first test. Equilibrium was judged visually based on the pressure values displayed on the screen of the data acquisition system.

4.1.2 Most influential parameters

The two main mix design parameters which influence the change in viscosity due to pumping are the w/cm and the paste volume. Decreasing the w/cm and decreasing the paste volume can lead to a greater level of decrease in viscosity with increasing product of flow rate and pumping time. Figure 7 shows the relative plastic viscosity as a function of Qt, for SCC 9 (w/cm = 0.34), Reference SCC 1, 4, 10 and 15 (w/cm = 0.30), SCC 8 (w/cm = 0.25) and SCC 19 (w/cm = 0.22). In general, decreasing the w/cm results in a more important decrease in relative plastic viscosity, despite the results of SCC 8. A reduction in w/cm makes it more difficult to disperse cement particles in the concrete mixer, as the relative particle fraction (volumetric concentration of solids divided their maximum packing density) is increased in the cement paste [49, 50]. This leads to an enhancement of the inter-particle forces, an increase in the yield stress (when keeping the admixture dosage constant), requiring more energy to disperse the particles. As the energy of the pumping action is expected to be larger than the energy delivered by the mixer, more dispersion would occur during pumping. Furthermore, a lower w/cm is also the cause of a faster thixotropic re-build [51, 52], which could result in more re-dispersion of cement particles during pumping.

Figure 9 shows the effect of paste volume for SCC 12 (paste volume = 35 %), SCC 4 (paste volume = 37.5 %) and SCC 11 (paste volume = 40 %). Decreasing the paste volume from 37.5 to 35 % led to a more substantial decrease in relative plastic viscosity. This can be attributed to the higher shear rate in the cement paste at constant flow rate in the concrete. However, increasing the paste volume has not significantly altered the decrease in relative plastic viscosity, up to a point at which the relative plastic viscosity is increased with increased pumping action. SCC 11 is the only concrete where this important increase has been observed (the increase in plastic viscosity due to pumping is larger than the increase due to workability loss for test 5: see hollow points in Fig. 9). The reason for this behavior is currently unknown.

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4.1.3 Influence of shear-thickening?

Shear-thickening is not expected to influence the changes in viscosity and yield stress, as it is caused by a different mechanism. As stated in [32], shear-thickening of SCC is thought to be caused by the formation of hydroclusters of small cement(-itious) particles when subjected to a high shear rate. This means that the forces induced by the flow push small cement particles (or small agglomerates) sufficiently close together to create a temporary cluster. Slowing down the flow releases the particles from this cluster, and the “viscosity” decreases with decreasing shear rate. Furthermore, large agglomerates of particles are less likely to form clusters than individual particles or small agglomerates. In fact, this means, roughly stated, that the breakdown of the connections must happen before shear-thickening can occur. Therefore, it is expected that shear-thickening does not affect the dispersion process caused by the pumping.