Abstract
When assessing the quality of concrete on site, it is necessary to base the interpretation on reliable and representative test results. Generally, core tests are the most reliable and effective method for assessing the quality of concrete. Unfortunately, this type of testing is expensive and time consuming and only a limited number of cores can be carried out in practice. Non-destructive tests (NDT) can be used to overcome these drawbacks. The most popular and widely used NDT methods for assessing concrete strength are rebound hammer (RH) and the ultrasonic pulse velocity (UPV). The use of these methods provide unreliable predictions unless their results are correlated to destructive tests. A sufficient number of cores is needed to accurately predict the compressive strength of concrete. Recent researches have been carried out for identifying and optimizing the number of cores able to stabilize the calibration model on data from recent structures and from synthetic data. However, more case studies are needed to draw conclusions. In addition, the effect of the degradation of reinforced concrete elements on the number of cores needed for obtaining a reliable prediction needs to be investigated. In this paper, RH and UPV in conjunction with core tests are used to evaluate the concrete compressive strength in existing structures built in the 1970s and degraded mainly by steel corrosion. More than 234 elements were tested by RH and 86 elements by UPV. Also, 36 cores were drilled and tested under compression. A regression analysis is adopted to establish the correlations between NDT and strength measurements. The accuracy of the predictive assessment was evaluated using two indicators: the root mean square error (RMSE) and the coefficient of determination (r 2 ). The results of this case study showed that seven to nine cores is the minimal number of cores that guarantees the improvement of concrete strength assessment by combined or single NDT methods.
Highlights
Concrete is the most widely used materials worldwide in the construction industry due to its strength, possibility to realize all desired shapes and low cost
The methodology adopted in this investigation is based on the comparison between two static parameters identifying the reliability of concrete strength prediction: the root mean square error (RMSE) Eq ( 1 ) and the coefficient of determination (r2) Eq ( 2 ) in order to analyse the effect of cores number on the prediction quality and to identify the minimum number of cores sufficient to stabilise assessment models
The accuracy of the estimation at the stages of calibration and prediction is presented as a function of number of cores on Figs. 3 to 5, respectively for rebound hammer (RH) and ultrasonic pulse velocity (UPV)
Summary
Concrete is the most widely used materials worldwide in the construction industry due to its strength, possibility to realize all desired shapes and low cost. For this reason, many researchers proposed various empirical models to relate RH and UPV measurements (separately or in combination) with the compressive strength [7,8,9,10]. The challenge on one hand is reducing the amount of core number without structurally affecting the concrete element or the structure and keeping the measurement at moderate costs and on the other hand to acquire a sufficiently complete and reliable understanding of the concrete strength in structures For this aim, recent research has been directed toward studying the effect of core number on the reliability of prediction. The main purpose of this study is to investigate the needed number of cores for a reliable strength assessment for old degraded structures
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