Basics of Concrete Cracks
So you noticed a crack in some concrete - Now what?
The easy answer is call an Engineer and have them look at it. This may sound easy, but unless you have one living in your house or it happens to be between the hours of 8am and 5pm on a weekday, it might not be so easy. The purpose of this article is to outline the basics of cracks in concrete, and in some cases, provide some motivation to get a hold of one of us Engineers, although it is admittedly difficult sometimes.
It is hard to imagine anyone who has never seen a concrete crack at some point in their life. Structural Engineers can sometimes be guilty of thinking about concrete cracks for hours or even days at a time. As a result, this topic is far bigger than can be written about in this forum.
Not all cracks are bad. It might be a stretch to call any concrete cracks good, but maybe the word "expected" is better suited here. Of these two types:
"Bad" cracks are those that indicate a present or future problem, and
"Expected" cracks are accounted for in the design.
Being able to tell the difference between bad and expected is part of what keeps Engineers busy. The moral of this part of the story is: concrete does crack, and sometimes it isn't a bad thing.
If you are still reading this, you might be wondering: What causes concrete to crack? While there is some highly technical answers to that question, the simplest reasons I have found are:
There are more complete lists than this, but these 3 factors cover the most common causes of concrete cracking.
One type of cracking I would like to explore in a bit more detail here is shrinkage cracks. These are tied to water in the concrete mix. Kosmatka et al, 2002 suggest that the volume or length change in concrete from the time it was poured is between 0.01% to 0.08%. Just to picture what this looks like, let's look at a sidewalk 100 ft long. That would be about 30m for the metric world. Let's use an number in the suggested range of 0.05%. This will make the math easy:
100 ft x 0.05% = 0.05 ft or 0.6 inches (close enough to 5/8")
Or the metric:
30m x 0.05% = 0.015m or 15mm
So this 5/8" (15mm) is the amount that the sidewalk is expected to shrink. If all of this shrinkage forms a single crack in the middle of the sidewalk, that is a crack you could lose a cell phone in. No more math, I promise.
Now you might wonder, how can concrete not crack? Or what can be done about these cracks? Some super smart engineers long before my time have worked out solutions for these problems. Some of those solutions included using steel bars to reinforce the concrete. The Canadian Standards Association design standard for concrete and the National Building Code of Canada 2015 both have methods built into them to control the cracking of concrete. The basic idea is that the shrinkage should be split up amongst many smaller cracks rather than amalgamated in one large crack.
So what are you going to do about the crack you noticed? If it is new, big, or concerning in any other way, talk to an Engineer about it. Keep in mind that not all cracks are the same, and not all cracks are bad. I also can't give you an Engineering Degree for reading all the way to the end of this article but you definitely deserve a gold star!
Design and Control of Concrete Mixtures 7e, Kosmatka, Kerkhoff, Panarese, MacLeod and McGrath, Cement Association of Canada, Portland Cement Association 2002
CSA A23.3 Design of Concrete Structures, 2010 Canadian Standards Association