How to make concrete more crack resistant used for sidewalks and driveways?

The main reason why concrete cracks is because it shrinks. It shrinks as it loses water. Most of that happens because of evaporation when we let it dry**.

So, the first thing to do is to minimise the amount of water present in the mix. That is done by minimising the workability of the concrete, improving the mix design, using water reducers or high range water reducers.

Another thing to do is to have jointing at realistic distances, and making those joints early. Our state roads authority requires the saw cutting not long after you can first walk on the concrete. Like 6 or 8 hours after placement. NOT days and days. And certainly not after the cracks have already happened!!! Many concreters argue that such is not possible, and I just tell them to go argue with the experts.

Think hard on how deep to cut sawn joints. 10 mm deep out of 100 mm is not going to cut the mustard. Particularly if the reinforcement is continuous across the joint. Many people cut every second wire in the mesh where the joint is going to be. Though that requires remembering where you have cut the joint. If you are really worried, put in dowels, rather than deformed bars across key-formed joints. Those dowels are greased or taped to ensure the joint can actually open, rather than just crack further along the slab.

Remember that a cross-shaped reinforced or dowelled joint will likely lock up, and not work in two directions. there are proprietary dowels for such a situations. They include rectangular sleeves that go over the bars, and also disk shaped dowels.

Some aggregates are more resistant to shrinkage than others, yielding a lower shrinkage mix.

Always put diagonal bars (plural) across reentrant corners, such as around drainage grates and other places where a diagonal crack is likely to form. The bars go at right angles to the expected crack, of course.

Some other kinds of cement other than Portland cement can have a somewhat lower drying shrinkage than Portland cement. Eg CSA cement. But beware. It sets FAST. Like our client which can remove a large slab of freeway, reinstate it and have full traffic flow all in a turn around of 4 hours! But then, they have to mix on site in special Volumetric mixers, or else the concrete would be set before it got 5 km down the freeway.

** Hear ye, hear ye! Concrete hardens by chemical reactions involving the cement and the water. Once the water is gone, strength gain, like Grandfather’s clock “stopped dead, never to go again”. Do not let your concrete dry out prematurely. Keeping it moist is called “curing”. Curing does not happen by accident, or all of the concrete’s doing.

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