Processing Green Forms by Compression Moulding

Green form specimens were produced using the moulds introduced in Sub-sections 4.5.2 to 4.5.4. The following steps were applied:

1. As shown in Figure 4.6(a) a sheet of filter paper was placed into the bottom of mould. Once the mixing process of Section 4.3 was finished, a 1 to 2 mm layer of the matrix was placed into the mould and evenly spread over the mould area using a spatula. Figure 4.6(b) shows the mould after one of these thin layers had been added. This layer was then tamped repeatedly by a 150 mm long and 20 mm diameter steel tamping rod before the next thin layer is added. When required, a sheet of glass or carbon fabric (Section 3.2.4) is placed on top of the mortar; it was then gently placed into the mortar by hand, as seen in Figure 4.6(c). The layered sample was built-up by repeating this procedure until the required thickness, higher 15 mm was achieved. The number of fabric layers differs dependent on the type of fabric and this feature of slab manufacturer is presented in Chapter 7.

2. Once the mould is full, a sheet of filter paper was placed on the top surface of the sample as seen in Figure 4.6(d).

3. It was especially important when using the six trapezoidal specimen mould to ensure that equal amounts of mix were placed into each cavity. Casting took place in the concrete laboratory of the Engineering Workshop and the loaded tool was transported to the WMG building on a trolley. The weight of material required to fill just one cavity before pressing was measured for each mix. This was achieved by fully filling a cavity, excavating it completely and measuring the weight of the removed cementitious materials. The remaining cavities were then filled with the same volume of mix.

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4. The mould was then placed centrally on the lower platen of the either the Denison 7231 or the DASSET machine; in Figure 4.6(e) the Denison 7231 is shown. It is essential that the mould was positioned centrally in order to achieve a uniform downward pressure. The upper steel platen to the testing machine has a ball joint and is free to rotate when finding the state of static equilibrium.

5. The mould was then subjected to a compression force equivalent to a 9 MPa pressure, over the top surface area of the sample. The compressive stress was kept constant for one minute. During that time, the excess water was squeezed out of the samples and removed by vacuum through filter sheets and holes in the tool. Once the compression settings had been programmed into the machine, the vacuum pump turned on and the machine was instructed to compress the mix.

6. The applied pressure was then released.

7. To prevent a significant upward force that might damage the green form material when the upper platen of either the Denison 7231 or the DASSET was lifted the vacuum pump had to be turned off and the machine’s ram lifted slowly.

8. The mould was then returned to the concrete lab and disassembled. A specimen was always carefully extracted by pushing it using a special tool designed for demoulding purposes. This step is shown in Figure 4.6(f). It is essential to be as gentle as possible when removing a sample as excessive deformation will induce deformation that could lead to undesirable micro-cracking (Miller, 1993). Although these micro-cracks may not be visible, Miller discovered that the in presence weakens the cured material significantly.

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9. Finally, the tool was cleaned and dried thoroughly. Although laborious, due to the large number of drainage holes, this stage was absolutely essential if the mould was to be repeatedly used to produce green samples. Compressed air was used to clear the small holes and was found to minimise the time taken to complete this necessary stage. Once the steel had fully dry a light coating of the oil Aquaseal 777 was applied to prevent oxidation.

10. To ascertain the amount of water extracted during the pressing process, the weight of the specimens was recorded immediately after demoulding.

11. A specimen was then covered with a damp hessian sack and left overnight in the moist environment. Specimens were cured for 26 days using the procedures introduced in Section 4.6.

To determine the optimum pressing condition the author used material CF1 at 2% of

Vf to investigate the influence of compression stress and the pressing time on flexural strength. All the specimens had the same mix design 1C:2A:0.58W. Following this, the specimens were cured for 28 days in hot water at 50 °C following the procedure given in Section 4.6. The aim was to identify when the flexural strength was a maximum. For this, the single trapezoid mould (Section 4.5.1) was used to produce batches of four specimens for the three compression forces of 50 kN (4.50 MPa), 100 kN (9 MPa), and 150 kN (13.5 MPa). The pressing times were of 1, 3, 6 and 9 minutes. To determine flexural strength, the test procedure outlined in Section 3.8 was used.

119 (a) (b) (c) (d) (e) (f)

Figure 4.6: Steps in the fabrication of the FRC green forms by the compression moulding process: (a) laying a filter paper into the base of the mould; (b) laying a thin layer of mortar; (c) laying a sheet of fabric; (d) covering with an upper layer of filter paper when the required slab thickness has been reached; (e) placing the mould in the centre of the Denison 7231 compression machine; (f) demoulding the FRC specimen after pressing.

Plotted in Figure 4.7 are the mean flexural strengths from the twelve batches. It can be seen that as the pressing time increases, the strength decreases; this is contrary to the desired outcome of using compression moulding. The maximum mean flexural strength of

Placing a filter paper on the bottom _{Thin layer of 1 to 2 mm of mortar}

Vacuum pump

Mould

Placing a filter paper on the top Laying a sheet of fabric

Applying the pressure Demoulding the specimen after pressing

Specimen

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8.4 MPa was achieved when the applied compression stress was 9 MPa with a pressing time of 1 minute. This maximum mean can be compared with the hand lay-up flexural strength of 7.6 MPa from having a pressing of time of zero minute. The hand lay-up strength is lower and is found to be similar to the mean flexural strengths for pressing times of 3 to 9 minutes. Based on this experimental study it was decided that one minute was the optimum pressing condition for the programme of characterizing FRC materials reported in Chapter 5 to 7.

Figure 4.7: Flexural strength of CF1 specimens exposed to different pressing regimes in compression moulding process.