The first rule of concrete is that it cracks. There are thousands of reasons why the concrete cracked. Sometime cracked concrete is unavoidable. Conversely, sometimes cracked concrete is avoidable.
First I will talk about concrete cracks that are generally unavoidable. Generally a surface crack (hairline crack) is unavoidable and the concrete decided to crack there instead of the control joint. In theory the concrete will crack at the weakest spot, and so we put control joints in to let the concrete crack there.
Sometimes the crack happens not at the control joint and these usually occur pretty quickly after the concrete was poured. It is disheartening to have a beautiful concrete patio and then one week in the concrete has a crack. It is as frustrating to me (or more) as it is to the pool owner.
Next I will talk about structural cracks which are avoidable. Structural cracks are where one section of concrete has shifted up or down from concrete next to it. Structural cracks are usually due to poor installation.
The main reason for a structural crack is usually hidden. Hidden under the concrete should be a layer of gravel (or sand). Sometimes the original builder will back fill with dirt and overtime that dirt will settle. This settling leaves space.
Or the sub grade will be the wrong type of gravel. Normally for a concrete patio the sub grade gets compacted with heavy machines. This helps compact the gravel and prevents settling. However, with a swimming pool if you compacted the gravel the compaction forces would bow and push the pool wall out, ruining the pool.
Most pool installers will use either a gravel called grits (or called torpedo fill) which is a fine gravel that is self leveling. The grits can not be compacted and will naturally level themselves out when dumped somewhere. It acts almost like water and eliminates the need to compact it. Luckily in Cincinnati, grits are readily available and easy to purchase.
In areas where grits are hard to obtain, the pool installer will usually use sand. Sand is a small aggregate that wont leave voids, but needs to be water down every 1″ or 2″ requiring extra time and labor.
Anyways lets say the sub-grade wasn’t installed correctly. In theory the concrete has steel re-enforcement bars (rebar) in the middle and could withstand not sitting on a sub grade material in one section, but normally that rebar wasn’t installed or is at the bottom of the concrete and not the middle.
I always install rebar on a pool patio, and normally I have one guy whose only job on concrete day is to make sure the rebar is lifted up in the middle of the concrete pour.
Almost always when I tear out a cracked concrete patio, the rebar (if there is any) will be at the bottom of the concrete doing almost nothing.
The voids from these issues are bad for a couple of reasons, but for Cincinnati it’s usually bad because of our freeze-thaw cycle. Sometimes the new void will get water stuck there, and in winter this water freezes and now there is a lot of pressure on the concrete above. The pressure builds and builds until it pops the concrete to let that pressure escape.
Here is a post from a great concrete company (albit from VA) about concrete and cracking. http://www.salzanoconcrete.com/when-does-concrete-crack.html
Can the concrete crack / separation be fixed?
This is hard to answer over the internet in general terms. A picture emailed can help, but normally I have to physical see the crack or separation and see why and what is going on. Then I can give some different options and find a solution to fit your budget.