PSI - Issue 62

Alessandro Bellini et al. / Procedia Structural Integrity 62 (2024) 315–322 A. Bellini et al. / Structural Integrity Procedia 00 (2019) 000 – 000

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5.5. Accuracy of results and effects of differential shrinkage Results reported in Table 2, expressed in terms of percentage residual errors, allow to draw some preliminary considerations about the accuracy of results and the error to be expected during the application of the method. It should be noted that these error values have been obtained in ideal conditions, in a controlled laboratory environment. For this reason, the reported results could offer an estimation of the accuracy of the method itself, after completing a first standardization of the procedure and with a simple stress field; they hardly contain additional sources of error that may be present during the application of the technique to a real in situ case. After this clarification, following the analysis of results reported in the table, it can be concluded that all the residual errors are included in an acceptable range (6.0 – 8.6%). The effects of the differential drying shrinkage, which can significantly affect the reliability of the method in relatively young concrete elements (McGinnis and Pessiki, 2007; Romano and Mazzotti, 2022), but are not relevant in real case scenarios (old beams typical of Italian bridges and viaducts), have been also taken into account, observing a slight improvement in accuracy (from about 7-8% to 6% of residual error). 5.6. Distribution of the residual error The statistical distribution of the percentage residual error detected during all the prestress release tests is presented in Fig. 5, using two histograms: the first one concerning the residual error without considering differential drying shrinkage (Fig. 5a), the second one after correcting data with the removal of the effects of shrinkage (Fig. 5b). Fig. 5a shows that the majority of residual errors is included in the 0-10% range, with a predominance of the 5-10% class and only a limited number of cases can be found with errors larger than 15%. After correcting experimental results by removing shrinkage effects, a further improvement is possible in the 0-10% range, with an increase of sample in the 0-5% class. As already discussed, Table 2 shows that also the average value for each group of specimens is affected by a slight improvement (from about 8 to 6%).

10 12 14 16

10 12 14 16

0 2 4 6 8

0 2 4 6 8

n° of samples

n° of samples

0-5% 5-10% 10-15% 15-20% 20-25% 25-30%

0-5% 5-10% 10-15% 15-20% 20-25% 25-30%

Residual error [%]

Residual error [%]

(a)

(b)

Fig. 5. Distribution of the residual error: (a) without considering differential shrinkage; (b) after correcting experimental results by removing the effects of shrinkage on the external faces of the columns.

5.7. Results without detachment of the sample The first tests carried out with only orthogonal saw-cuts (samples P10_6 in Table 1), so without reaching the full detachment of the sample, were also analyzed in terms of percentage residual error. Even if, during the experimental campaign, the orthogonal saw-cuts have always offered the predominant contribution, the impossibility to extract the sample applying this technique did not allow a full prestress release, resulting in errors included in the 15-25% range (see Fig. 5b). This range is higher than that detected in the majority of the other samples, confirming that a full detachment of the specimen is needed for obtaining reliable results. 6. Conclusions Preliminary results of prestress release tests carried out on RC columns, after the application of a preassigned and constant compressive axial stress, are presented and discussed in the paper, with the aim of obtaining a scientific validation of the method in a controlled laboratory environment. Different parameters have been considered during the tests, such as the effects of saw-cuts order, direction and inclination, of different cutting depths and of the differential drying shrinkage, potentially present in relatively young concrete samples.