Strength Tests

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For our standard definition of a good brick, we have decided to use
ASTM C1272-05a, “Standard Specification for Heavy Vehicular Paving Brick.”
This is a convenient standard because it applies to bricks that are out-of-doors, completely exposed to the elements, and are continuously exposed to varying loads (vehicular traffic).  The most stringent standard within C 1272 is for Type F bricks, which are bricks set in sand rather than concrete, so they have to carry the full vehicular load.

The two parts of the standard that are interesting to us are Cold Water Absorption and Minimum Breaking Load.  The standards for the heaviest duty paving bricks specify that they need to exceed:

1)  Cold Water Absorption less than 6% for an average of 5 bricks

2)  Breaking Strength greater than 475 pounds-force per inch of width for an average of 5 bricks, with the weakest brick exceeding 333 pounds per inch of width.

Our bricks are nominal 4 inches wide, so for convenience we will assume that we need a minimum average breaking strength of 4x475 = 1900 pounds, with the weakest brick exceeding 4x333=1332 pounds.

On a previous page we established that the Cold Water Absorption for the ten bricks we made from 50 percent 12 mesh glass was 4.1%, way better than the standard.

The procedure for testing breaking strength comes from ASTM C67-05, “C67-06 Standard Test Methods for Sampling and Testing Brick and Structural Clay Tile.”  The test requires a fixture.  The fixture we had fabricated looks like this:

The sample brick is supported at the ends.  The test machine applies a load in the middle on top of the steel load spreading plate.  When the brick breaks, the maximum load is recorded.

We take the same ten bricks we made for absorption testing, and have them tested on a Instron test machine in the University of Washington Mechanical Engineering Department. 

Reiterating what we said on the page about absorption, these ten bricks were made consecutively using the process shown on an earlier page.  There were no rejections.  Using our standard firing profile, no bricks had visible cracks or notable defects.  However, this does not eliminate the possibility that the bricks have internal faults or hairline defects invisible to the naked eye.  This is our greatest concern.  Invisible defects may make the bricks fail prematurely, and then it’s back to the drawing board on the firing profile.

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.


Attempt at Thinner Brick

In an attempt to push the envelope, we decided to try making reduced weight bricks with frogs.  We cut a pattern and screwed it to the bottom of our mold:

It appeared to work well with our press mold.  The bricks coming out of the mold were now about 1200 grams, but were the same overall thickness as previously, and looked like this:

We formed and fired a series, starting with the standard profile of 90 minutes to 1850 and holding for 30 minutes.  The cold water absorptions of those bricks were as follows:
4.2%
4.8%
4.6%
4.5%
for an average of 4.5 percent.  This was a little higher than the 4.1% average for the 1600 gram bricks, which was unexpected, but probably not statistically significant.

To push the envelope further, we began reducing the time at maximum temperature, so see how quickly pairs of these thinner bricks would fire.
At 20 minutes soak:
4.7%
4.7%

At 15 minutes soak:
5.6%
5.4%

At 10 minutes soak:
6.6%
6.4%
This average exceeds the minimum required to meet the Heavy Paving Brick spec., so we stopped here.

Taking the bricks to the University of Washington test lab for braking, two things were pointed out to me by Hanson Fong, the post-doc running the test machine:
1) The sharp corners of my relief were conducive to stress concentration, and would probably cause premature failure, and 
2) The ASTM spec required us to break the bricks with the frog down, which was the weakest orientation.

We forged ahead, and got the following breaking strengths:
1388 pounds
1606 pounds
1555 pounds
On the last one we turned it over just for fun and broke it with the frog up, and got
1733 pounds.  All of these were lower than required by the ASTM spec.

So that’s that, we failed with our attempt to further reduce the weight.

We are certain that weight reduction is possible, as is profile optimization for even better energy savings, but we need to move on to the energy calculations.  Besides, optimizing the profile for a kiln that will never actually be used in production would be somewhat of an exercise in naval gazing.

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