RHEOLOGICAL PARAMETERS USED FOR DELIBERATE DEFORMATION OF A FLEXIBLE MOULD AFTER CASTING
Roel Schipper'", Steffen Grünewaid' and Prashanth Raghunath'
' Faculty o f Civil Engineering and GeoSciences, TU Deift, THE NETHERtANDS.
* : c o r r e s p o n d i n g a u t h o r , h . r . s c h i p p e r t a t u d e i f t . n i
ABSTRACT
This paper describes how curved precast concrete elements can be manufactured in an open and reusable flexible mould. The proposed method reduces formwork costs of architectural freeform elements in concrete. First, the method is described briefly, then a series of tests are discussed, demonstrating that by measuring the rheological parameters ofthe mixture during the process, the right moment of deformation con be determined. The measurements show that thixotropic behavior for this manufacturing method is very helpful, since it leads to a quick increase of the yield strength of the fresh concrete, but still leaves the concrete deformable in order to prevent cracking caused by the movement ofthe mould.
Keywords: flexible mould; rheology; deformation; thixotropy
INTRODUCTION
Curved elements are often applied in freeform architecture buildings with complex geometry [1], The manufacturing of curved precast concrete elements is expensive, due to the high costs and limited repetition of element shapes and dimensions. The flexible mould method therefore was designed to allow easy production of curved precast concrete elements in an open, flexible and reusable mould [2]. The method is outlined by Figure 1 on the next page: Fiexible materials are used forthe mould, that is supported by a subsystem controlling the desired final shape (step 1). The mould is filled with self-compacting concrete (SCC) (step 2); fibres or textile can be used as reinforcement. During a short period of structural build-up, the yield strength of concrete increases (step 3). Then the mould is carefully deformed into its final shape
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Figure 1. The flexible mould production process for double curved precast concrete panels explained in six steps
(step 4). Concrete hardens in the deformed mould (step 5) and finally the curved element is demoulded (step 6). Compared to the initial horizontal position during casting, fresh concrete remains in its final position in a certain slope. The flexible mould can be reused to produce more elements, with identical or with altered curvature and geometry. A series of experiments were carried out in order to study rheological properties, the feasibility of the method and the thixotropic behavior of freshly mixed concrete.
De Larrard [3] refers to rheology as a suitable tool for solving engineering problems in the case of fresh concrete, more specifically for the determination ofthe yield strength. He describes the equilibrium of a fresh concrete slab under gravity, using an equation often applied in soil mechanics (Fig. 2, left); the behavior of soil under a slope is a fundamental case in soil mechanics, and depends on slope, specific weight, slab thickness and cohesion.
Figure 2. Concrete under slope according to De Larrard (left) and in applied test setup (right)
h =50mm
Based on De Larrard [3] Equation 1 is proposed to estimate the critical yield strength (p: density, g: concrete weight, L: chord length in horizontal direction, h & R: See Figure 3).
Many elements were successfully cast, deformed and demoulded. In some cases the mould was deformed too early (with concrete still being too fluid). Several tests (slump flow, flow time T50 and/or slump) were carried out to determine the workability at different moments after mixing. In order to determine the right moment of deformation in relation to the workability of the mixture, slump tests were executed on the same batch at different moments. Workability tests with slump flow and slump do give a rough idea of the development of the yield strength over time, but have the disadvantage only being empirically linked to the strength in the unit 'Pascal'. The tests can also be slightly inaccurate due to the human factor and instability of the concrete after lifting the cone (Fig. 4). For these reasons, it was decided to perform tests with the BML-Viscometer and to measure the mini-slump as a secondary test. In cases where the mould could not be deformed in a single smooth movement, and vibrations or shocks were accidentally introduced, the concrete started to become more fluid again resulting in a higher variation of the panel thickness. Figure 4 shows two effects on the yield strength: A (left): the effect of vibration and B) the effect of the height of the concrete cone.
Figure 4. Sudden decrease of yield strength due to slightly vibrating the mini slump cone (left) and the same mixture simultaneously tested with Abrams and mini-slump cones (right)
In the following, results of Mixture 1 are discussed that show the evolution of the yield strength over time (Fig. 5). The container for the test with the BML-Viscometer was filled directly after mixing. Directly after filling the container, several up- and down-curves were completed, increasing and decreasing the rotation speed between 0,03 and 0,39 rps in 7 steps of 5 sec each. A maximum torsional moment (resistance) up to 9,5 Nm in the first 1300 sec was recorded (Fig. 5). Using the linear interpolation method of the G-H diagram [5] for the specific geometry of this BML-cylinder, a yield strength between 0-10 Pa, and a plastic viscosity of about 40 Pa-s are obtained (slump flow: 720 mm; T50: 5.7 s). Figure 5 also shows that repeated up-down curves after t=2000 sec yields a slightly higher resistance (up to 11 Nm), which is equal to a yield strength of 23 Pa, caused mainly by thixotropy.
Figure 5. Example ofa test (Mixture 1) with the BML-viscometer indicating thixotropic behavior: A (top): results ofthe complete test (4000 sec = 67 min); B (bottom): measurement in the period 3690-3820 sec (the rest penod was 26 minutes)
M (NmLFIltored R-(rp%)_Filtered
After a rest of 26 minutes, the repeated test shows a distinct rise of resistance to almost 14 Nm, which however is decreasing again due to the shear and movement of the mixture (Fig 5: left-lower diagram). The increase of shear results in a breakdown of the structure that was build-up before. Only at higher shear rates the resistance of 14 Nm is exceeded again. If the up-curve in the G-H diagram would be interpolated linearly, it would not be clear which values represent the actual yield strength, since the rheological characteristics are apparently changing due to the measurement itself. A third-order polynomial curve was used to approximate the yield strength at a shear rate of 0; the interpolated curve crosses the y-axis at 15.38 Nm, which equals a yield strength of about To=600 Pa. This value is in the range of yield strengths discussed in Table 2. To compare the BML-values with the traditional slump test: the slump (with Abrams cone) at this moment was 3 cm. If a similar interpolation is made for the curve after about 2000 sec, the yield value is To=260 Pa (Abrams cone slump: 19 cm). The empirical formula of Ferraris and De Larrard [8], relating slump value and yield strength results in a factor 2 to 3 higher values for the yield strength than the values measured with the BML e.g. slump = 19 cm gives TO=761 Pa according to [8] and
