For this purpose, the rheological properties of different cement paste samples are measured with a parallel plate sensor under different conditions, which include material storage condition, mix volume, mixer type and superplasticizer dosage. Simulation of gravity-induced flow requires a flow curve, measuring rheological properties including from 0.1 to 50 s-1. The prediction of concrete flow under certain boundary conditions and the automation of processes in the construction industry would be useful problems of the approach.
A correlation was developed between the rheological properties and field tests of fresh concrete. Saak et al., 2004) Slurry models that consist of a multilayered particle system have been introduced to predict the flow behavior of concrete. An analysis technique for predicting concrete drawdown or formwork pressure was also investigated. Some techniques such as single fluid simulation are progressing to casting prediction applications.
Application of these technologies in the field must obviously be accompanied by an understanding of the rheological properties of materials. The aim of this research provides deep understanding of the rheological properties of cement-based materials, and as a result an optimized protocol to measure its flow curve corresponding to the gravity-induced flow (such as slumping). First, using a reference sample of non-Newtonian non-thixotropic fluid, the effective range of the rate of shear deformation was determined to precisely simulate a field test result such as (gravity-induced flow of cement paste).
Finally, for stable measurement of the non-thixotropic flow curve, the optimized protocol was investigated under different conditions related to the mixture and the condition of the samples.
Rheological Simulation
We can inject the 1L sample into the left box space and raise the vertical plate, the flow progresses to the right horizontally. As well as the mini-drop flow test, it is possible to measure the spread diameter (D) over time to obtain the spread curve. In the case of the channel flow test, the spread length (L) can be easily measured because the flow is advanced in one direction, unlike the mini-drop flow test.
Reflecting on the field experiments, the mini draft cone was configured as a quarter symmetric and the channel box is a 1/2 y-axial symmetric model. To determine the number of elements, a preliminary simulation was carried out according to the mesh size, targeting the flow behavior in the channel of the reference sample. While some researchers heavily patterned the mesh at the bottom of the model to study finite expansion, we designed a 10 mm mesh on the model uniformly.
So the mini slump cone model is divided into 3808 pieces of EC3D8R elements, and the channel has 2700 pieces. The dispersion curve was determined by using an air/liquid volume ratio of more than 0.5 for soil elements as a reference.
Experiment
Sample Preparation
Standard Guide for Flow Curve Measurement
Systematic error with non-Newtonian fluid
Viscosity margin under specific shear rate in carbomer with (a) 25 mm diameter concentric cylinder sensor, (b) 20 mm diameter parallel plate sensor, and (c) 35 mm diameter. Although the carbomer gel sample is non-thixotropic, it takes time to obtain shear stress convergence. In the case of a shear rate of 0.01 s-1, the time to measure the steady shear stress was taken from 21 to 58 s.
At each level of shear rate, the magnitude of shear stress fluctuations was recorded for 20 s, and in Figure 6 it is depicted with a bar at each rate of shear strain. We can confirm that the spurious fluctuation is not negligible for all sensors at such low shear rates (0.01 s-1 or 0.1 s-1). For the shear stress measurement at 1 s-1, the spurious fluctuation almost disappears and the shear stress measurement became stable after 4 s as shown in Figure 5.
To investigate the effect of thixotropy, the single step of specific shear rate is also applied in the cement pastes for 5 min. With the increasing shear rate of 0.1 s-1, the sample exerts a high shear stress in the initial stage, and then the stress is gradually broken down. At the low shear rate of 0.01 s-1, the spurious fluctuation is similar to that observed in the carbomer gel.
Therefore, it could be noted that the shear stress measurement at low speed below 1 s-1 inevitably possesses a systematic error (spurious fluctuation) due to non-Newtonian fluid characteristics. Further investigation of the thixotropic effect on the flow curve was performed with a multistep protocol. The range of the shear rate is from 1 s-1 to 80 s-1, followed by the previous remark: reliable stress measurement could be obtained from 1 s-1.
For a situation such as slump flow (gravity-induced flow), the upper limit of shear rate has been reported to be less than 50 s-1 (Nathan Tregger et al., 2008). At low rates, in the first two steps (less than 10 s-1), the thixotropic response of shear stress reduction is easily observed. However, at high rates (conservatively higher than 50 s-1), shear stress convergence is quickly reached.
Reproducibility
The average of the measurement parameters shows a lot of difference in the K, while there is a small gap in the τ0. In the upward curve, a decrease in yield point is apparently observed, as well as a consistency index, while the flow behavior increases in the case of the downward curve. In the fresh cement-based materials, the material property expressed in the flow curve is affected by the shear rate range.
In the field experiment, the final spreading diameter (Df) was calculated by counting the average value of the points through wide and long lines, and the final spreading length (Lf) was measured based on the center. In the simulation results, a mesh size of 10 mm, a final spreading diameter of 21 cm and a final spreading length of 47 cm were designed for which model. This range is the shear rate range in the gravity-induced flow condition.
In the fresh cement paste, the sample was prepared with 15.5 cm final diameter in the mini slip flow test and 35.5 cm final length in the channel flow test at 40% w/b ratio and 21 cm final diameter and 49.5 cm final length at 40% w/b ratio adding 0.2% sp dose. In the simulation, the final diameter in the mini-slip flow test is 17 cm and the final length of the channel flow test is 37 cm. The results of the mixture simulation by adding superplasticizer did not match the real behavior due to the yield stress which was measured less than the intrinsic property of the material.
In Figure 20-(a), the reason why the final diameter was reduced to zero is because of the lost volume of the liquid. The rheological properties of a cement paste fit well with the Herschel-Bulkey model in the up stage, while the measured values of the down stage follow the power-law model. Incorporation of HRWAR also provides good reproducibility due to the homogenous cement dispersion in the suspension.
The flow curve in the range of 0.1 to 50 s-1 is critical for simulating mini-slump flow or channel flow. With the parallel plate sandpaper attached, the flow curve measured with 10 steps for every 5 seconds in the range from 0.1 to 50 s-1 accurately simulates the gravity-induced flow. The measured yield stresses ultimately provide an accurate simulation within the error of 1.5 cm spread in the pure paste.
Shear-induced migration results in a lower than realistic yield strength. Regardless of whether the sandpaper is attached to the plates, there is a large difference between yield stress values (Banfill et al., 2003).
Optimized protocol for flow curve
Conclusions
Gravity flow was measured using a cement paste having a water-cement ratio of 0.4 and containing a superplasticizer. Channel flow test has higher sensitivity than mixture proportions or admixtures instead of mini drop. When conducting a rheological experiment, the best sensor appears to be concentric cylinders due to their high repeatability and no shear-induced migration.
In particular, slippage between the sensors and the samples can be prevented by attaching sandpaper to the plates. When the cement powder has been stored in a closed state (decreasing the moisture content of the powder), the rheological test shows high reproducibility. This is because the paste becomes stiff and very thixotropic under the influence of high shear.
For example, the sample with a water-cement ratio of 40% and that with 0.2% HRWRA have a yield stress (the result of curve fitting to the Herschel-Bulkey model) of 55.36 Pa and 8.14 Pa, respectively. But more conditions need to be imposed on measuring the rheological properties in non-Newtonian fluids with thixotropy under different mixture designs. Paper presented at the Rheology of Fresh Cement and Concrete: Proceedings of an International Conference, Liverpool, 1990.
On the identification of rheological properties of cement suspensions: rheometry, computational fluid dynamics modeling and field measurements. ASTM C1749-12, Standard Guide for Measuring the Rheological Properties of Hydraulic Cement Pastes Using a Rotary Rheometer. In this study, the water-cement ratio of 35% of the sample was probably measured with parallel plates of 35 mm diameter.
