If the slump test fails, then we should reject that concrete.
Because slump test gives an idea about workability and water-cement ratio
If the water-cement ratio is high, then the workability of concrete is increased. Still, the durability of concrete is decreased due to excessive water, which forms voids in the concrete and reduce the strength of the concrete.
Generally slump value is wearing from 25mm to 175 mm it depends on how much workability we required