Reddit mentions: The best layout tools

Got your message, but wanted to put my reply here, in case it could benefit anyone else.

The fact that the cracks were already visible on the initial walk through is probably a good sign. Makes it likely they may just be shrinkage cracks from when the concrete first set.

If you don't see any obvious distress in any other part of the building around it, but still want a more solid answer, read on, and hopefully I can help. If someone called me for cracks like this, the second question I'd be trying to answer (after that first "distress in any other part of the building" question) would be, "Is the crack moving." The best tool for that is something you can buy and use just fine yourself, a crack monitoring gauge. They are stupid expensive for what they are (plastic with some markings and epoxy), but cheaper than getting another structural engineer out to even look at what might just be shrinkage cracks.

I'd apply monitors across whichever cracks looked largest. If, for example, this is one long crack that cuts across a corner of your slab, I'd think about putting one near each end, at least. Since you're looking at slab cracks, I'd mostly just be looking for spots on the floor near the walls where I might be able to glue them down and not trip over them and damage them. Leave on, get good initial readings, and watch them for at least a year (you want a full set of seasons minimum).

A few possible outcomes;

• No movement at all. Shrinkage cracks or some initial settlement that has finished. Either way, yay!
  
• Cracks getting continuously wider. Could be a sign of an ongoing settlement problem.
  
• Cracks that open and close depending on season. This is common on expansive soil (often clays), where the soil moisture can vary seasonally but does so more at the edges of the slab than at the relatively protected middle. Causes the slab to basically bend back and forth over time.
  
  Either of the second or third might be a problem or might not, depending on how much it's moving. If you never see any sort of distress elsewhere in the building around it, either might be no big deal - you could just be getting extended minor settlement or a bit of seasonal swell that won't do any harm. Houses move some. On the other hand, if you have a year worth of info from these crack gauges and do end up calling a structural engineer because you're seeing issues beyond just the slab, they'll be able to do a lot more to help you sooner because you're going to end up bringing them good background info that we don't usually get to start with.
  
  Either way, that's my suggestion for now, at least. Nothing in the photos jumped out at me as immediately scary, so at most I'd consider throwing gauges on it and watching.

To address your first question, install some monitoring gauges:

https://www.amazon.com/CRACKMON-4020A-Concrete-Adhesive-Included/dp/B0049MAVYU

They are very accurate, and will let you know what is moving. Just be aware that there WILL be some movement, because cementious foundations cannot flex, only crack, when the earth around them moves.

The best thing you can do for your foundation is to start at the roof. Do you have gutters and downspouts? If so, are they free and clear? Where do they discharge? How far away from the foundation is the water sent? How is the soil around your home sloped? Can water pool against the foundation at any location? Is water directed away from your home?

It is funny, but a solid foundation starts at the top of the house.

CRACKMON Not going to lie that is a great name. Not sure if its the best deal but a quick search led me to those. You maybe able to find something cheaper and more information on how they work.

There is often no need for electronic measuring if you are prepared to check visually from time to time. You can use tell-tales which allow you to detect movement of oine piece against the other in two dimensions whether it is a crack or a corner. See here for an example from Amazon. If you buy many, you should find them significantly cheaper. The movement is very obvious when checked and I recommend photographing with a mobile phone (as it timestamps).

Strain guages are more for sudden problems like subsidence, heavy road traffic or earthquakes.

Or better yet, throw a crack gauge on the joint so you can quantitatively measure any changes over time.

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