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Benefits of SCC
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1 |
Segregation Probe Test
u System Configuration
The segregation probe, inspired by the Penetration Apparatus method, is a fast and effective method to measure the thickness of mortar/paste at the top of fresh SCC. A thicker layer of mortar/paste at the surface corresponds to a lower static stability. The results of the segregation probe method and the measured thickness of the mortar/paste layer in hardened concrete were found to be quite similar.
The segregation probe is a 125mm (5 in.) diameter ring connected with a 150 mm (6 in.) high rod marked with scale (Fig. 9). The whole probe is made of 1.6-mm (1/16 in.) diameter steel wire. The total weight of the probe is about 18g. Before the test, fresh concrete is cast into a 150 x 300 mm (6 × 12 in.) cylinder with one lift. The concrete is allowed to rest for 2 min. before the test, during which excessive disturbance is avoided. The segregation probe is then placed gently on the concrete surface allowed to settle for 1 minute. The penetration depth marked on the rod is used to determine the stability rating according to Table 1.
Table 1: Stability Rating for Segregation Probe Method
u Penetration Mechanism
When the segregation probe is suspended at rest in a suspension it experiences two opposing forces, buoyancy force BF and gravitational attraction GF. Due to the higher density of steel compared to the measured liquid, the unbalanced force, (GF - BF), will cause the probe to accelerate downward if yield stress of the liquid is not high enough. The resistance offered by the liquid is called the skin friction. Skin friction results in the development of a drag force, FD, which opposes the motion and increases with increasing particle velocity relative to the liquid (Fig. 10). FD reduces the acceleration and finally becomes equal to the original driving force (GF - BF). Then
there are no more unopposed forces acting on the particle and it continues to travel at a constant settling velocity, v∞.
According to fluid mechanics, this drag
force can be expressed by equation 4, where d denotes the
diameter of the cylindrical cross section of the probe, v denotes
the velocity of the probe, and η represents the viscosity of the
liquid. It is widely accepted that concrete flows as a Bingham material.
Thus, it is necessary to replace the viscosity in equation 4 by the
apparent viscosity as shown in equation 5. In the equation, τ0
represents the yield stress and
(2.4)
(2.5)
Considering the final state of the probe, the equilibrium condition has to be satisfied, which leads to the expression of the final velocity of the probe that shown as follows:
(2.6)
where
(2.7)
u Experimental Results
Fig. 11 gives an example in which the segregation probe was used to determine robustness of SCC mixtures to moisture content. The VMA mix has a lower slope of the curve and a larger margin between target w/cm and maximum penetration depth and thus is more robust to moisture content than the other two mixes.
u Potential for Field Testing
The segregation probe test is simple and rapid and thus is suitable for quality control and other applications such as robustness measurement. The segregation probe is lightweight, and the test does not rely on the experience of the tester. Due to these reasons, this method is a field-friendly method.
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is
the shear rate, which can be related to the velocity of the probe.
and
