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.