Material Selection and Plate Fabrication
A pre-mixed ECC material (JSCE 2007), of which the mix proportion is listed in Table 11.2, was used. It contained PVA fibers in the volume of 2%. This premix ECC was packed in 25 kg paper bags, and sent to Concrete Laboratory, University of Tokyo, for fabrication of the test plates.
The coarse aggregate was a crushed type available in the market. Its gradation is shown in Table 11.3. Considering the thickness of the test plates, the aggregate size larger than 9.5 mm was omitted. Prior to casting, the aggregate was washed properly and made in the saturated surface dry (SSD) condition. The amount of aggregate included in the Group 1 and 2 was about 30% and 15% of ynx, respectively, where
Fig. 11.3 Illustration of the aggregate used in Group 1 and 2 plates: (a), (b) in 9-mm depth, (c) and (d) in a sliced section
уша is the maximum packing density of the aggregate used (about 63%). These two aggregate amounts were decided based on the distribution of the aggregate and the workability when mixed with the ECC paste. Note that the aggregate distribution is important to ensure the presence of aggregate at any potential crack interface, whilst the workability is related to the ease of production. The aggregate used in the Group 2 plates had a minor impact on the workability, while the relatively large amount used for Group 1 had a significant effect. Relatively large addition of aggregate in Group 1 plates was intentionally made for the purpose of investigating the improvement of crack-shear transfer.
Mixing of the ECC was in all cases done as specified in Reference 9 for about 10 min. The air content and slump flow was then measured to ensure the property of the ECC matrix after hardening. The slump flow and air content was 490 mm and 9% for plates in the first batch, 510 mm and 15% for the second batch, and 515 mm and 7% for the third batch. After this measurement, coarse aggregate was added slowly to the ECC slurry in the third bath. They were then remixed for about 2 min.
After mixing, the ECC slurry was poured at once to the middle of each framework, made of plywood with aluminum frames. The slurry was then shredded in a radial pattern, rodded, finished with a steel trowel and finally covered with a plastic sheet. After 2 days, the plates were demolded and air-cured under 60% relative humidity and 20°C until the test date (started from the 28th day after casting). The average compressive strength and modulus elasticity were 33 MPa and 15 GPa, and 39 MPa and 20 GPa respectively, for ECC in the first and third batch, respectively. No measurements were made for ECC in the second batch.
The aggregate included in a 9-mm depth framework is shown in Figs. 11.3a and b. It should be noted that this is a 2D illustration; the distribution in a 3D space was much sparser. To better illustrate the actual distribution, shown in Figs. 11.3c and d are the aggregate distributions at cut sections of two representative plates. Since the aggregate amount is much less than that used in ordinary concrete, the aggregates appeared to be suspended well in the ECC matrix.