Pouring concrete on wet surfaces places the strength and quality of the concrete at risk. Light dampness will not affect the concrete as much as the ground being saturated, but you will still have the potential for problems. Problems develop when the concrete has too much water gathering on the surface.
Curing is a process that allows the concrete to develop its strength. Too much water or premature drying of the concrete weakens the strength of the concrete when it is setting up. Concrete curing requires more than moisture on the surface; it requires an adequate amount of moisture throughout the concrete so cement crystals can grow around the aggregate. Hydration occurs when water reacts with the cement, which causes this growth of the cement crystals.
If you must pour concrete on wet ground, then you can do some things to avoid damaging the concrete. Remove as much of the water from the wet ground as possible. Sand collects the water and does make an effective foundation for the concrete. You can also lay plastic with a covering of sand over the wet ground so it will not penetrate the bottom of the concrete. You may want to use less water when mixing the concrete so the concrete will not be overloaded with water.
When too much water is available to the concrete, the concrete begins to prematurely crack. The weakened concrete causes the cracking. The wet ground is also a softer surface, which allows the concrete to settle unevenly. If one side of the concrete is higher or lower than the other side, then the middle of the concrete will be under constant pressure. The pressure will eventually cause the concrete to crack and break in that section.
The best time to pour concrete is when the weather conditions remain the same for five to seven days. The ground is dry and compacted, which makes a suitable foundation. Pour concrete in the fall or spring because the temperature does not fluctuate that much within a 24-hour period. No matter what you do, you should not pour concrete on ground saturated with water, on extremely hot days, on extremely cold days or if you expect the temperature to change dramatically within a 24-hour period.