April 1, 2012
Consider the impact of the physical and chemical properties of the parts when evaluating the whole.
By Bill Langer
After I retired from the U.S. Geological Survey last November, my wife, Pam, and I moved to Phoenix, Ariz. We currently live with our daughter and her family while we remodel our house located directly across the street. One of the many jobs was to install an electrical outlet in the floor for our table lamp.
This undertaking required cutting a channel in the concrete slab floor for the conduit that would carry the wire. While Pam watered down the diamond saw blade, I made a saw cut on either side of the channel. I then cleaned out the channel with a mason’s chisel and hammer. Occasionally, a hammer blow would dislodge a pebble and send it zinging across the room. Sometimes the projectiles would be pebble fragments; other times they were entirely intact pebbles.
My mind drifted off the task at hand as I thought about how this phenomenon related to geology. The pebbles that were intact were very hard, had very small crystals, were nearly spherical, and had very smooth surfaces. Those that were broken were either soft, or were hard and brittle, or had a rough surface.
The aggregate in the concrete probably came from the source closest to our house — the Agua Fria River channel. A few years ago, I conducted a study of that aggregate.
Some of the pebbles are hard, very smooth, and nearly spherical. Those are the type of pebbles that popped out of the concrete intact.
Some of the pebbles have larger crystals that provide zones of weakness where they fracture. They also have rough surfaces that provide something to which the cement paste can adhere. Those types of pebbles broke apart and came out in chunks attached to the cement matrix.
Some of the pebbles are very soft or brittle. The chisel tended to break them into very small pieces
This is where geology comes to the party. The majority of the Agua Fria River gravel consists of pebbles originating from eight distinct rock types, each with its own physical and chemical properties, but the properties of individual constituents of a mixed gravel are seldom identified using traditional engineering tests. Take, for example, the Microdeval test for durability. When large batch samples of Agua Fria River gravel were subjected to the Microdeval test, the results ranged from 21 percent to 26 percent.
With an understanding of the geologic properties, the eight rock types were lumped into three groups — granitic, metamorphic, and extrusive volcanic pebbles. When tested, the results for the three groups were noticeably different than the results for the bulk samples. Granitic and metamorphic pebbles both had results ranging from 12 percent to 16 percent; noticeably better than the results for the bulk samples. In contrast, the volcanic pebbles had poorer results, ranging from 24 percent to 35 percent; generally worse than the bulk samples. It is easy to imagine how mixing the more durable granitic and metamorphic pebbles with the weaker volcanic pebbles can result in a bulk sample with an average value.
Getting my head back into the job and armed with the properties of individual rocks in mind, I was able to amaze Pam by predicting whether a pebble would split or pop out of the concrete. After a few successful predictions Pam commented, “Sounds like rock science to me.”
Bill Langer is a research geologist who spent 41 years with the U.S. Geological Survey. He can be reached at Bill_Langer@hotmail.com.