In which situation is slump a reliable indicator?

Slump is a reliable indicator primarily in situations where the same concrete mix is being assessed across different loads. This is because the slump test measures the consistency and workability of the concrete mix. When using the same mix, any variations in slump can be attributed to factors like variations in moisture content, compaction, or even the mixing process.

In contrast, when comparing different concrete mixes, the variations in materials and proportions can significantly affect the slump readings, making it an unreliable metric for comparison. Similarly, determining the water-cement ratio typically requires more precise methods, such as testing samples in a laboratory, rather than relying on slump alone. Finally, when comparing the designs of different concrete structures, slump does not provide insight into structural integrity or performance, since the design aspects might influence the overall concrete composition differently. Thus, slump is most useful and reliable when evaluating the consistency of the same mix across different loads.