The best way to deal with grout shoulders is to make sure you minimize the chances of cracking before you start. There are 5 things you should consider before you proceed with the placement of your grout.
Make sure you choose a design that is less prone to cracking. Minimize the size of the shoulders while still making it easy from a constructability standpoint. Think about how the grout will have to be placed and design access in your base plate or frame to make application and air evacuation easy.
2. Product Selection
Choose a product that will flow and fill your entire space between your base plate and the foundation. Visualize the flow and how it will move air from around the back-side of obstacles like shims and anchor bolts.
Make sure the preparation of the foundation is complete and uniform. Exposed aggregate on the surface is key. Any laitance or cement-rich slurry must be removed. Remove all dust for epoxy grouts and pre-soak for cement grouts.
4. Mixing Water
For cement grouts, make sure you accurately measure the water. Over watering can weaken the resulting grout. Also, make sure the water is potable to prevent contamination of the grout.
Cement grouts have to retain moisture for the curing process. The time required for this process varies widely. Make sure you plan to retain moisture by wet cure, curing compound, polyethylene, etc. for the required time for your grout. Epoxy grouts need to be protected from contamination. For both cement and epoxies, temperature is key to the curing process. As a general rule, strength gain is faster the higher the temperature within an acceptable range.