Without further ado, here are the results of the ten bricks, in pounds-force
3207
2503
2670
2627
2873
3155
2488
2878
1844
3013
Average breaking strength = 2726 pounds
The required average was 1900 pounds. Our 50 percent glass bricks exceed the standard by over 40 percent! And let me repeat, these are all ten bricks that were made, with NO rejections.
This means that the bricks could probably be made substantially thinner and lighter, with further energy savings! Note that one of the bricks broke at 1844 pounds, substantially less than the others. On close examination, that brick had pores as large as 1/16 inch through the middle. So this was a poorly formed brick, but still it met the overall spec! This would indicate that the process of making bricks containing glass is quite robust.
We noticed another thing during the testing. In the center of the broken bricks there appears to be an unfired core, which has a different consistency than the outside of the brick:
Remember, we fired these bricks by ramping up to 1850F, then holding for only 30 minutes. It’s apparent that 30 minutes was not enough time to fully fire the glass and clay brick. And we still exceeded the strength standard by over 40 percent!
So it’s probable that even if we reduced the weight of the brick, we could not reduce the firing time. A better way to make the bricks anyway might be to form them with “frogs.” Frogs are the hollowed out middles that many bricks have. The purpose of frog is to keep the same thickness but make a brick that is lighter and easier to fire. With the glass bricks we could keep the brick at 1.5 inches thick, but hollow out about one half-inch on one side, so the thickness stayed the same but the bricks were lighter. The hollowed out middle is also structurally more sound than a thinner brick.
If we can figure out how to cast frogs into the bricks using our manual molding set-up, we will try it.
To test the sensitivity of the firing profile against the strength, we also broke bricks fired cooler and in less time.
For bricks fired to only 1825F and held for 30 minutes, the breaking strengths were
2419
2401
The average absorption for these bricks was 6.3 percent.
For bricks fired to 1850F but only held for 20 minutes the breaking strengths were:
2508
2562
The average absorption for these bricks was 5.95 percent.
So the 50 percent glass bricks fired faster and hotter both exceeded strength standards by A LOT, and slip by on the absorption test. We recognize that this sample size is not statistically valid, but we are getting to the end of our time available for this portion of the project.
Bricks made using the same clay but regular ceramic grog were also tested for strength. The grogged bricks fired to 2100F and held for 30 minutes had strengths of:
1740
1877
So these bricks FAILED to achieve the average strength requirement of 1900 pounds!
These bricks had average absorptions of 5.8 percent.
We also tested grogged bricks fired to 2075F and held for 30 minutes. Their strength was:
1440
1354
These bricks had an average absorption of 6.4 percent, so they failed on BOTH absorption and strength.
For kicks, we also tested grogged bricks fired to the same profile as the glass bricks. The strengths were:
789
756
And the average absorption was 10.4 percent.
Whimpy, whimpy, whimpy.
Finally, we bought some bricks at Home Depot and tested them. These are made by a regional supplier to Home Depot and do not claim to meet any standard. These results may not reflect the results you would get locally. The strengths were
1650
1683
And the average absorption was 6.4 percent. So these bricks failed on both strength and absorption. I include these results to compare with generic commercial bricks you might find anywhere.
To reiterate, 50 percent glass bricks fired in 90 minutes to 1850 and held at 1850 for 30 minutes, met strength and absorption standards for ASTM C1272-05a, “Standard Specification for Heavy Vehicular Paving Brick.”
50 percent ceramic grog bricks, made from the same clay, and fired to 2100F, 250 degrees hotter than the glass-containing bricks, FAILED to meet the same standard.
Now we move onto an estimation of the energy savings.