A sensor measures slump and rheological characteristics of the concrete and is connected to a system that adjusts the slump by monitoring the sensor within the interior surface of a concrete mixer and controlling liquid additions. Data is analyzed by a computer processing unit to determine the slump and rheological characteristics of the concrete, liquid required to meet the slump requirements. The measurement done by the sensor is more accurate then the existing methods because it brings into consideration the effect of the helix inside the mixer on the movement of the concrete mixture inside the mixer. Furthermore, this method also allows the operation of the sensor in “real” life situations where the rotation speed of the mixer can't be maintained at a fixed value. The fact that the sensor rotates with the drum and that the concrete mixture is pushed to the bottom of the mixing drum guaranties that all the concrete is “sampled” by comparing results collected from each revolution of the mixing drum.