## 2 Extracted Beams
### Mechanical Characteristics of the Beam Materials  
#### 2.2.1 Compressive Strength of Concrete  
NDTs were carried out on the two beams DB1 and DB2 to evaluate the variation of the compressive strength of concrete along the beams. Measurements of ultrasonic pulse velocities and rebound index values were conducted on the beam surface at 18 and 25 test stations located along the beams DB1 and DB2 (U points in Figure 3a). A rebound hammer was used to measure the rebound index as indicated in UNI EN 12504-2:2001 [62] after surface preparation to remove the more external deteriorated concrete parts. A digital portable detector (50 kHz probes; Low frequency) was used to carry out transparent velocity measurement as indicated in UNI EN 12504-4:2005 [63]. The NDT results were combined and correlated with the cubic compressive strength of concrete (R) by the combined method SonReb to compensate for the limits and the uncertainties of the two NDTs. The cubic compressive strength (R) was obtained by means of Equation (1) in RILEM guideline [64]:  
\[\mathrm{R}=7.695\cdot 10^{-11}\cdot\mathrm{N}^{1.4}\cdot\mathrm{V}^{2.6} \tag{1}\]  
in which N is the rebound index and V the ultrasonic velocity measured by the NDTs. This strength is usually different with respect to the one obtained by destructive tests (DT) on concrete cores extracted by a beam especially in the case of old concretes. The uncertainties in the evaluation of concrete strength by NDT depend on different factors: concrete composition and ages, effect of admixture, water/cement ratio, non-homogeneous concrete structure, and surface deterioration. The compressive strength values obtained by DT on concrete cores extracted from beams are less uncertain than the ones by NDTs. For that reason, DTs were conducted on 8 and 7 concrete cores (diameter 94 mm, height/diameter ratio equal to 2) extracted from the beams DB1 and DB2 at the ST points in Figure 3b after the failure tests (SS4.3) on the retrofitted beams (SS3). The cores were extracted predominantly at some NDT stations to compare the DT and the NDT results. The choice of the stations, where to perform the core extractions, was determined by the local conditions of the beams  
Figure 2: Beams DB1 (**a**) and DB2 (**b**); concrete geometries and steel reinforcement configuration for the sections at the beam ends (S1, S2) and at the beam midspan (SM); rebar diameter [mm]; concrete geometries [cm]. after the beam collapse (regions in tension or in compression during the tests without evident damage), selecting the points that represented better the distribution of the compressive strength of concrete measured by means of the NDTs. A diamond core drill (water cooled) was used for the core drilling made with constant and modest velocity to avoid excessive stresses on the concrete core. The cylindrical compressive strength (\(\mathrm{f_{c}}\)) values obtained by the DTs on the concrete cores were converted in the cubic compressive strength values (R) in Figure 3b and Table 1 by means of Equation (2). This conversion permits comparison of the DT compressive strength values directly with the ones obtained by NDTs using Equation (1) which gives the cubic compressive strength values. \[\mathrm{R}=\mathrm{f_{c}}/0.83 \tag{2}\]  
The mean cubic compressive strength values (\(\mathrm{R_{m}}\)) are 14.7 MPa and 10.9 MPa for the beam DB1 and DB2 whereas the coefficient of variation (CoV) values are 0.28 and 0.12 for the beam DB1 and DB2. The concretes show a wide scatter of the compressive strength values especially along the beam DB1. The comparison between the values of the cubic compressive strength obtained by NDT and DT tests at the same points permitted estimation of the strength correction factor to correct the compressive strength evaluated by NDTs only at each U-test station. The corrected cubic compressive strength values are given in Figure 3a.