Parking Entropy 6/20/20

Science with Richard Bleil

The pile of rubble was quite the surprise when I stepped outside. The apartment complex is doing a lot of repair work on the parking lots, and I personally have no idea what their schedule is. I’m hoping that my roommate does, but one car has been basically blocked in by the gaping hole left behind on the concrete that was left behind it, the source of the rubble blocking the walkway out of the apartment door. I truly hope I don’t find such a dilemma holding my car in.

If you think about it, concrete is basically nothing but a huge artificially formed flat stone. The process typically involves laying a stone base designed to allow water runoff, and the concrete is sealed to protect it from water getting into it. Unfortunately, eventually water does seep into the concrete. In the cold, the water will freeze and, as water does, expand. This expansion pushes the imperfection or cracks that initially allowed it to enter the concrete in the first place even further apart creating larger cracks and potholes. This winter was especially brutal on the parking lot, and eventually the concrete, like in the parking lot, needs to be repaired.

There are a couple of ways to repair concrete depending on the extent of the damage. Sometimes a tar filler can be used as a patch, but this filler is weaker than the original concrete, and while it’s surely a less expensive alternative to more extensive repairs, it doesn’t last as long. It’s obvious that the concrete in the parking lot had been patched several times before, but these patches have been breaking up and leaving large rocks behind.

Now, they are using jackhammers to break large sections of concrete out of the ground in preparation of pouring new concrete in its place. Often, when I see repairs like this in progress, it makes me think of the second law of thermodynamics which states that the entropy of the universe, that is the tendency towards disorder, can never decrease. This is often a misunderstood law as people believe that the law says that as a law, entropy cannot decrease, but clearly this isn’t true. The entropy of a system, such as the parking lot, can decrease by doing work on the system as it is being done here. In this process, the entropy of the surroundings must increase more than the entropy of the system is decreased to make up for the decrease of entropy of the system, but so long as the net entropy of the system and surroundings increase the second law is maintained.

Looking at the mess in the parking lot, it’s pretty clear that the entropy of the surroundings is increasing. The pile of rocks at the end of the walkway are probably about two or three feet in width, and the pile is probably about four feet tall. Eventually it will be hauled off and either discarded or recycled, but this, in and of itself, is not enough to offset the entropy once the new concrete is poured.

We could have defined the system to be the parking lot in its original state of disarray, and if we were to do so, this rubble would indeed be counted towards the entropy of the surroundings. But we can also define our system to be the hole left behind. Currently, this hole is filled with nothing but air, and as such, is high entropy. Gases have no order in their molecular arrangement, velocities of these molecules or the direction in which they are moving. As such, gases are the highest entropic forms of matter there are, and as air is a mixture, that just adds to the entropy as there is no way of knowing the arrangement of the nitrogen, oxygen or argon (the third most common gas in air) in the hole. Once the concrete is poured and cured, the entropy will drop precipitously as the molecules will arrange themselves into fixed positions. Because these positions are fixed, the entropy is low. In fact, solids are the lowest entropic forms of matter.

So where is the increase in entropy in the surroundings from the poured concrete? Much of this comes from the processing of and work during the pouring. The fuel burned to process and create the entropy, the gasoline of the concrete truck, the respiration of the workers. All of these create entropy in the surroundings. What’s more, most of the time that matter goes to a more condensed state, like a fluid-like slurry of concrete to the solid form, heat is released. This heat increases the velocity of gas molecules, causes evaporation, and involves other processes all of which means increased entropy in the surroundings.

So here’s to increased entropy in the surroundings, and I truly hope the entropy of the system lasts a long time.

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