In quality control, what does a "control chart" monitor?

A control chart is a statistical tool used in quality control processes to monitor the variability of test results over time. It helps identify trends, shifts, or any unexpected changes in the data, which can indicate potential issues in the production or testing process. By plotting data points on the control chart, engineers and quality control personnel can determine whether the process is stable or if corrective actions are needed. This continuous monitoring plays a crucial role in ensuring that the quality of concrete remains consistent and meets specified standards, thereby contributing to overall project reliability and safety.

The other options pertain to different aspects of concrete production and quality assurance but do not reflect the primary function of a control chart. Monitoring total production costs, assessing the appearance of freshly poured concrete, or recording weather conditions during curing are important but are not the primary focus of a control chart. Instead, these factors can be considered as separate components within a broader quality control strategy.