How does rain affect the freshly placed concrete?

Rain may affect the water cement ratio at top portion of freshly placed concrete provided that the concrete is not properly protected from rain. To substantially change the water-cement ratio of the concrete at the surface of the slab, external energy must be supplied to the system such as troweling passes with excess water on the concrete surface. The energy supplied by the finishing operations pushes the excess water into the slab surface creating a high water cement ratio in the near surface of the concrete so that its strength and durability is reduced. Sometimes, the damage to the concrete surface is apparent since the texture of the surface is easily damaged after the initial curing period. When the surface strength is affected, the long-term durability of the concrete may be reduced. However, the concrete strength and durability below the surface would not be affected.

This question is taken from book named – A Closer Look at Prevailing Civil Engineering Practice – What, Why and How by Vincent T. H. CHU.