The slump test is a useful tool for determining workability. Changes in the slump value acquired throughout a work may reflect changes in materials, water content, or mix proportions. As a result, it is important in regulating the quality of the concrete produced. Concrete that has not been properly mixed will slump over time. This is normal and does not mean that the concrete is failing. The mixture should be stirred immediately before it is poured to ensure homogeneity. Slump measurements should be taken immediately after pouring; otherwise, the moisture content of the concrete will affect how much it sinks. As concrete cures, it becomes more resistant to internal stress caused by any voids or defects within the material. These stresses can develop as a result of improper mixing or inadequate water content, for example. If these stresses become great enough, they can cause portions of the concrete to crack or break away. This is known as "spalling" and is not desirable in any concrete structure. Spalling can also occur if the fresh concrete is stressed beyond its limits, such as by being loaded too soon after pouring. Concrete that has spalled should never be used again because it will likely contain some amount of air which will lead to poor results when casting further batches. If spalling does occur during production of a single batch, that batch should be discarded rather than used again.