When w/c ratio is very high, concrete segregation and water bleeding might occur immediately after casting. In this case, coarse aggregates settle to the bottom (segregation) and water crops out on the top surface (bleeding). The resulting concrete will be not homogeneous and thus weaker.
For example, when ready-mixed concrete is used, any arbitrary addition of water in the truck mixer during transport and in the construction site should be avoided. Also pouring concrete into framework where water from rain accumulated may cause a severe damage.
