The results in this paper are from two different testing campaigns: one carried out at Ghent University in Belgium [20, 25] and the other one at the Université de Sherbrooke in Quebec, Canada [7, 8, 14, 26]. The two test setups are described in the sections below.
Pumping circuits
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 6th 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].
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].
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.
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].
Testing procedures
Two different testing procedures were employed, as described below.
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:
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Bringing the concrete to its “reference state” at each flow rate [27], meaning that equilibrium in pressure was awaited for.
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Taking a sample of the pumped concrete
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Stepwise decreasing the flow rate from the current step to the lowest step.
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].
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.
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.
Concrete mixtures
Tests at Ghent University
Four different SCC mixtures were tested at Ghent University. The mixtures were produced in 3.25 m3 batches at a ready-mix plant located about 30 min away from the laboratory. Table 1 shows the mixture proportioning of SCC A and C. SCC B and D are commercial products of the ready-mix company. The main difference between SCC A and C resides in the amount of water in the mixture, while mixtures B and D were identical, except for a slightly higher superplasticizer (SP) dosage in the latter concrete. All mixtures were made with ordinary Portland cement (CEM I 52.5 N), limestone filler, river sand and a mixture of two gravels with nominal maximum aggregate sizes of 8 and 16 mm. The SP employed was a commercially available polycarboxyl-ether, able to maintain workability for up to 2 h, according to the manufacturer. No air-entraining agents or viscosity-modifying agents were used. Results on fresh concrete kept aside during the test indicate that even after 3.5 h, no large changes in slump flow and V-Funnel flow time can be observed.
Table 1 Mixture proportionings of SCC pumped 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.
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.
The reference mixture was produced with 600 kg of blended cement per m3, w/cm equal to 0.295, a paste volume of 37.5 % (excluding air) and a sand-to-total aggregate ratio (s/a), by mass, of 0.53. The SP-S dosage was adjusted to obtain a target slump flow of approximately 700 mm. SCC 1, 4, 10 and 15 were considered as reference mixtures. For SCC 2 and 3, the SP-S dosage was varied to change the initial slump flow. SCC 5 contained fly ash, SCC 8, 9 and 19 have different w/cm, varying between 0.22 and 0.34. SCC 11 and 12 have different paste volumes, while for SCC 16 and 17, s/a was reduced. SCC 13 was the only mixture that was air-entrained. SCC 14 contained a very small dosage of VMA. All mixtures were produced during winter in Canada, requiring heated water. This has led to some anomalies, as SCC 7 may have lost a part of its paste sticking to the frozen drum of the concrete truck, and SCC 14 is likely to have higher water content than requested as chunks of ice were present in the coarse aggregates. SCC 6 is not reported in this paper as it showed severe segregation and the results could not be used. The mix designs are displayed in Table 2.
Table 2 Mixture proportionings of SCC pumped at the Universite de Sherbrooke
Table 3 Evolution of fresh and rheological properties for mixtures SCC A-D, evaluated after achieving equilibrium at each flow rate