1. Review the Mix Design:

   • Ensure that the mix design used was appropriate for the desired strength and that the proportions of cement, aggregates, and water were correctly measured and mixed.

2. Check the Quality of Materials:

   • Verify the quality of the cement, aggregates, and water used in the mix. Poor quality materials can significantly affect the strength of the concrete.

3. Evaluate the Mixing Process:

   • Hand mixing can sometimes lead to inconsistencies. Ensure that the mixing process was thorough and that all materials were uniformly mixed.

4. Assess the Curing Process:

   • Proper curing is crucial for concrete to achieve its strength. Check if the concrete was kept moist and protected from extreme temperatures during the curing period.

5. Recalculate Water-Cement Ratio:

   • An incorrect water-cement ratio can lead to weaker concrete. Verify that the ratio was within the recommended range.

6. Test Additional Samples:

   • Take additional core samples and perform compressive strength tests to confirm the results. This can help identify if the issue is localized or widespread.

7. Non-Destructive Testing (NDT):

   • Consider using non-destructive testing methods such as rebound hammer tests or ultrasonic pulse velocity tests to assess the quality and uniformity of the concrete.

8. Consult a Structural Engineer:

   • If the issue persists, consult a structural engineer to evaluate the situation and provide recommendations. They may suggest additional testing or alternative solutions.

9. Remediation Options:

   • Depending on the severity of the strength deficiency, remediation options such as applying a surface coating, using chemical hardeners, or even partial or complete removal and replacement of the affected concrete may be necessary.

10. Documentation and Prevention:

• Document the findings and steps taken to address the issue. Implement measures to prevent recurrence, such as improved quality control during mixing, better supervision, and adherence to best practices in concrete production and curing.

By systematically addressing these factors, you can identify the root cause of the strength deficiency and take appropriate corrective actions.