Factors affecting the reliability of assessing the concrete strength by rebound hammer and cores. (1st June 2017)

• Record Type: Journal Article
• Authors: Alwash, Maitham; Breysse, Denys; Sbartaï, Zoubir Mehdi; Szilágyi, Katalin; Borosnyói, Adorján
• Is Part Of: Construction & building materials. Volume 140(2017)
• Page Start: 354
• Page End: 363
• Publication Date: 2017-06-01
• Subjects: Superscript ¯ mean value of the variable under consideration -- s( ) standard deviation of the variable under consideration -- CV( ) coefficient of variation of the variable under consideration -- Test location limited area selected for measurements used to provide one test result -- fc core (or cube) compressive strength corresponding to one test location -- fcest estimated strength of concrete corresponding to one test location -- R Rebound number, test result, it is the mean of rebound hammer readings corresponding to one test location -- CVR within-test variability of rebound measurements (in terms of the coefficient of variation) -- NC represents the number of cores (or cube) used to identify the model, in present study, one core is considered to be provided by each test location, consequently NC also refers to number of test locations for cores -- NI number of repetitions -- NR number of test locations for rebound hammer measurements -- NT total number of test locations of the reduced dataset -- U accepted level of uncertainty, relative value % -- T true reference value -- U×T accepted level of uncertainty, absolute value; Concrete -- Assessment reliability -- Mean strength -- Strength variability -- Rebound hammer -- NDT techniques -- Cores; Building materials -- Periodicals; 624.18
• DOI: 10.1016/j.conbuildmat.2017.02.129
• Languages: English
• ISSNs: 0950-0618

Abstract

Highlights: Selecting core locations based on rebound measurements improves the assessment reliability. Reducing repeatability of rebound measurements improves the assessment reliability. Increasing the number of cores improves the assessment reliability. Concrete intrinsic variability plays an important role on the assessment reliability. Abstract: To assess concrete strength in a structure, nondestructive technique (NDT) like rebound hammer is combined with destructive technique (coring tests) in order to implement a relationship ''conversion model" between the compressive strength and, NDT measurements. The conversion model is used to estimate the local strength value at each test location using the corresponding NDT value. Then the estimated mean strength and/or estimated strength standard deviation (concrete strength variability) values are calculated. However, the reliability of these estimated values is always a questionable issue because of the uncertainties associated with the strength predictions based upon NDT measurements. To improve the reliability, the uncertainties must be reduced by specifying and controlling their influencing factors. The objective of this paper is to study the reliability of assessment by analyzing the effects of several influencing factors: number of test locations used to identify a conversion model between strength and rebound measurement NC (number of cores), true value of concrete strength variability, within-test variability of rebound measurements, accepted uncertainty level, quantity to be assessed (mean strength, strength variability), model identification approach (like regression) and the way of selection core locations (random or conditional i.e. selection based on NDT measurements from preliminary investigation). To this end, a large campaign of laboratory studies datasets (1700 test results) was considered for the analysis in the present study. Results show that NC, within-test variability of rebound measurements and true concrete strength variability have significant effects on the assessment reliability. Conditional selection of cores has also an important effect on improving the reliability so it is strongly recommended.