What critical parameter must be measured during slump testing to accurately characterize concrete workability?

The critical parameter that must be measured during slump testing to accurately characterize concrete workability is the vertical distance that the concrete settles or 'slumps' after the slump cone is removed. The slump test, according to ASTM C143 or AASHTO T 119, is a widely used method for assessing the consistency and workability of fresh concrete. The test involves filling a slump cone with concrete in a specified manner, removing the cone, and then measuring the vertical distance between the top of the slumped concrete and the top of the mold. This vertical distance is the slump value, typically measured in inches or millimeters. A higher slump value indicates a wetter, more flowable mix with higher workability, while a lower slump value indicates a stiffer, less flowable mix with lower workability. The slump value is directly related to the water content and the properties of the concrete mix, and it provides a quick and easy way to assess whether the concrete meets the desired consistency for the intended application. It's also important to observe the slump shape. A 'true slump' is a uniform settling of the concrete mass. A 'shear slump' indicates that part of the concrete mass slides or shears off, suggesting a lack of cohesion in the mix. A 'collapse slump' indicates a very wet and unstable mix. These observations, in conjunction with the measured slump value, provide a comprehensive assessment of the concrete's workability. For example, a slump of 4 inches might be suitable for a slab pour, while a slump of 1 inch might be more appropriate for a concrete pavement.