Chapter 5: Sample size for determining the in situ strength of concrete

One of the most important tasks in the appraisal of an existing concrete structure is to establish the characteristic in situ strength to be adopted for checking the structural adequacy of the existing structure for its intended usage. The existing codes of practice on the assessment of in situ concrete strength have addressed most of the important issues in carrying out this task. One of the gaps is the appropriate size for the random sample for determining the in situ concrete strength of the existing structure. The approach taken by the codes at present is to leave this important decision to the judgement of the engineers. This is not satisfactory. Kog (2012) presented a statistical study using a non-parametric method based on order statistics to establish the appropriate sample size of cores and NDTs for determining the in situ concrete strength. The statistical study has established a relationship for the size of a random sample so that a desired fraction of the sample yields in situ strengths between a required range with a desired probability. An example is included to show the application of this relationship to determine the sample size for the assessment of in situ strength for a multistorey concrete building.