Tuesday, 30 May 2017

Assessment of concrete strength in existing structures: Cores and NDTs

Previously I discussed about the necessity of assessment of concrete compressive strength in an existing structures. I tried to present some of the testing methods to assess the concrete strength. The methods were non-destructive test (NDT) methods as they offer no or little damages to the structures. I presented the limitations of using NDTs in this field.
The direct determination of strength assessment of concrete requires concrete specimens to be taken from the structure that can be tested destructively. The samples are known as cores. Core testing is the most direct and reliable method to estimate concrete strength in a structure. The examination and compression testing of cores cut from hardened concrete is a well-established method for strength estimation. The principal limitation of core testing are those of cost, possible damage to the structural integrity, and the whole procedure is time consuming. The number of core that can be taken from a structure is usually limited. Core testing may not always be possible at all parts of the structure. NDTs are considered to be advantageous to supplement the core test. NDTs allow more economical evaluation of concrete in the structures. As NDTs measure compressive strength of concrete indirectly, a valid relationship between test result and compressive strength must be established following a valid statistical procedure.
European Standard (EN 13791 2007) gives methods and procedures for the assessment of the in-situ compressive strength of concrete in structures by coring and provides guidance on the use of NDTs for the strength assessment. Use of NDTs with limited number of cores introduces two sources of uncertainty. One is due to the calibration that is performed from a limited number of cores and another is due to the lack of precision of NDTs. EN 13791 offers two approaches; Alternative 1-Direct correlation with cores and alternative 2-Calibration with cores for a limited strength range using an established relationship. Alternative 1 requires minimum of 18 cores where at least 9 pairs of data is required for alternative 2. According to ACI 228.1R 2003, use of NDTs in the practical field should be preceded by the preparation of calibration curve from the same concrete that is under investigation (ACI 228.1R. 2003 ).
In conclusion, it can be stated that core testing is a must for the assessment of concrete strength in existing structures. NDTs are useful to minimise the number of cores to be taken and to reduce the uncertainty in compressive strength assessment across the structure. NDT allows the collection of data in a short time at a low cost. NDTs are be used in determining the statistically most reliable results of core tests with limited expenses and time.