You often hear people saying things like “I used gravel for my garden path, because I’ve tried concrete before but it always cracks”. Is this actually true or is it just a DIY Myth? Concrete can crack of course; we’ve all seen it, but how? And why?
Like many other cement based building materials, Concrete expands and shrinks due to changes in temperature and moisture content. If it gets wet it expands and as it dries out it will shrink again. In the long term most concrete dries out and shrinks as it loses some of the original mixing water. This in itself is not a problem, but if something stops the concrete from shrinking (such as friction between the concrete path and the hardcore base), stresses build up in the concrete. If this stress gets high enough, the concrete will eventually crack. If you cast concrete on to Teflon (hypothetically of course!), it will still shrink, but it won’t crack. It is the restraint to shrinkage that causes cracking.
So, how do you stop a concrete path from cracking? Firstly, put in some joints dividing it into bays (typically divide the path into bays that are no more than 1 – 2 times as long as the width). If there is a bend in the path, also put a joint here. Wait until you have poured and tamped the concrete and then push thin wood strips vertically into the concrete at appropriate spacings. Remove them when the concrete has hardened a vertical groove in the concrete surface. If the concrete wants to crack, it will crack at the bottom of the groove and you won’t be able to see it.
Secondly, make sure that when you are mixing up the concrete you only use just enough water to make the concrete workable to place and tamp down. The less water that is in the concrete on day one, the less water there is to be lost through shrinkage in the longer term.
Thirdly, that old chestnut – ‘Curing’. If you protect the concrete from evaporation early in its life, by covering it with polythene for a few days, the concrete gains strength more rapidly, meaning it can withstand a lot more shrinkage without cracking.
By following these tips, you can make sure that concrete rarely cracks, and if it does, you won’t see it.