Why is it that when it starts to rain, some blocks of concrete sidewalk are completely soaked, while others appear to be completely dry?

January 12, 2007 12:10 PM

Why is it that when it starts to rain, some blocks of concrete sidewalk are completely soaked, while others appear to be completely dry?

I have been wondering about this since I was a little kid, and I hope I'm not the only one to have noticed this. As it starts to rain, I have noticed that some sidewalk squares immediatly get soaked and glisten with water, while the square right next to it appears to be completely dry. What's interesting is that there is a strict boundary between wet and dry and it is always between squares. I have never seen half a square look dry and half look wet. Now, I realize that statistically, the same number of rain drops are falling on each square of concrete, so why does it seems like ALL the drops are falling on one and NONE of them are falling on another?

The only thing I can think of is that the porosity between different sidewalk squres must be different. Perhaps a more porous square sort of wicks the water down into the concrete, leaving less water visible on the surface. But what's got me confused is how two different squares can have different porosities because I would imagine that two bordering squares would have come from the same batch and dried under the same conditions. Also, I see weird patterns as I walk down a sidewalk where two will look wet, one will look dry, one will look wet and then 2 look dry. Different batches and drying conditions just don't seem to explain it well enough for me.

I realize I am probably overthinking such a trivial question, but these are the kind of things that keep me up at night! Please, MeFites, help a poor guy out!

posted by Paul KC to science & nature (7 comments total)

I've also wondered this same thing before. I always assumed there was something different about the composition of the adjacent squares.
posted by dead_ at 12:15 PM on January 12, 2007

You'd be surprised - depending on how the sidewalk is built, one square of sidewalk can sometimes take the majority of cement that a single mixer can hold.

I haven't poured a ton of concrete, but enough to know that if they used different brands for different batches of concrete, or varied how they mixed/poured it enough, it could make the concrete of different squares appear differently. My guess is that the water hitting it just accentuates the differences and makes them easier for you to notice.
posted by allkindsoftime at 12:20 PM on January 12, 2007

Just a guess, but it would probably be the less-porous squares that look dry since the water is not soaking in and darkening the concrete. Or something.

Also, they just replaced two squares in front of my house so they look different from the rest. If you're seeing this on older streets, chances are the squares were created at different times.
posted by cabingirl at 12:40 PM on January 12, 2007

Remember in sixth grade science class when you classified rocks by hardness, color, and *scratch* color, which is often different? Well, the wet color of rocks, and for that matter, any solid thing, is not a one-to-one function with the dry color. So, yes, composition.

That said, there was a spot on one panel of my parents' backyard sidewalk growing up, that stayed dry. You could watch the water bead up and stay away from that area. After much experimenting, I realized that it was wax from fireworks when we were kids.
posted by notsnot at 12:40 PM on January 12, 2007

Surface porosity can be affected by float technique. Over float a chunk of concrete and excess water rises the during the cure. Over hydration at the top of the slab results in poor quality, porous concrete when cured that is at increased risk of spalling.

Also sidewalks are repaired (due to cracking, spalling or just because they need to access services underneath) as needed, rarely do you see entire swaths of sidewalk replaced at one time. Over time this can result in a real patch work quilt appeance. Different pours can have slightly different compositions even if nominally identical. A common technique when finishing by hand is to alternate sections on different days. IE: every second section is poured one day and then a week later they come back and pour the gaps. This technique not only reduces the amount of form work required by a third it also breaks the sidewalk up into manageable units.
posted by Mitheral at 1:43 PM on January 12, 2007

Couple things it could be:

1. Finishing technique. Troweling the wet concrete makes the surface smoother and less porous. Typically sidewalks aren't troweled too much, especially if they're just replacing one square.

2. Age. New concrete will be darker and absorb less water than older concrete because it has a higher moisture content than when it's fully cured.
posted by electroboy at 1:49 PM on January 12, 2007

Sometimes there are steam pipes or other heat sources which can keep parts of the sidewalk much warmer. If there's just a bit of rain, it will evaporate off the warmer sidewalk for a while.
posted by jefftang at 7:26 AM on January 13, 2007