Continued from pg 13
constant for all the samples, the one and two hour samples should correspond to this curve. Looking at the 60-minute values, there is a significant amount of water left in the samples (1.9 grams divided by 5.2 grams = 36.5%). At two hours, the samples are still not dry (approximately 0.6 grams moisture left, or 11.5% of the original amount). This helps to explain why both sets of bars (one and two hours) had such lower strengths than the others, and why two hours produced greater strength over one hour. Based on the data in Figure 4, it might be conjectured that a three-hour dry time might be enough to achieve optimum bonding given the same drying conditions. However, it should be noted that the dry times on successive dips will probably be somewhat extended, just as it took ten minutes longer to dry the fourth layer. (Other more complex configurations would probably extend the dry time out much more than ten minutes.) Looking at the average MOR results for the 24 hour final dry samples (Figure 2), the same trend that occurred with 12 hours final dry time is seen. The one and two hour dry between dip samples have much lower green and fired strengths than the other, longer dry times. Only the two hour fired bars had strength approaching that of the four hour bars. The lower strengths with the one and two hour dry time systems are because of the incomplete setting of the binder, as they were not completely dry (from Figure 4). As before, Figure 3 shows the amount of difference between the systems using four hours as the control.
It is evident from the above results, that drying for an additional 12 hours does not make up for too short a dry time between dips. For example, at one hour dry between dips, only a 12 psi increase (9%) was noted (green) when samples were dried for 24 hours instead of 12 hours, and no increase was seen in fired MOR. This is most likely because even though most of the water has been removed from the sample, insufficient bonding occurred because of too fast a dip cycle. Very little extra strength was
24 ❘ June 2020 ®
Figure 2: MOR versus inter coat Dry Time
Figure 3: Percentage change in strength versus a 4-hour control
Figure 4: Weight loss vs. Time on 3rd and 4th layers
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32
