Test Specimen Pre-cracking
Test specimens pre-cracking is done in a 4 PBT set up at the loading rate proposed in EN 14651:2007 , but when the nominal crack opening value CMODpn is reached the test is stopped and the specimen unloaded. The load-crack opening evolution during the loading, unloading and recovery process is registered. Figure 4 presents an idealization of the pre-cracking process which include the following main parameters: first crack stress (fL), maximum actual crack opening value on pre-cracking process (CMODp), stress at CMODp (fRp) and residual crack opening value after pre-cracking process (CMODpr).
After the pre-cracking stage, the test specimens are placed inside a chamber with controlled environmental conditions where the creep frames are located. The temperature of the chamber is 20 °C and the relative humidity is 50 %.
Each frame is able to test simultaneously a column of three test specimens (see Fig. 2). Once the specimens were ready, the creep test stage starts when the load is applied. The sustained nominal stress level or nominal index of stress in creep (/„) is
Fig. 4 Pre-cracking stage parameters definition
Fig. 5 Creep tests phase parameters definition
defined as a percentage of the residual strength at CMODpn fRp). Due to the multiple specimen set-up, the load applied is not the same for all the specimens. Therefore, as their residual strength is also variable, the real stress level or real index of stress in creep (Ic) for each specimen is calculated and defined as the ratio between the sustained stress during creep phase (fR c) and the residual strength at CMODpn tf^) by the Eq. (1).
The test continues without interruption until it is decided to unload the specimens. Once specimens are unloaded they remain in the creep frame and the recovery of deformations is recorded for two weeks. Figure 5 represents an idealisation of the creep stage, where it can be identified some parameters: the sustained stress during creep phase (fR c), the instantaneous crack opening deformation (CMODci), the deferred crack opening deformations at different ages (CMODjcd) and the total deformations during creep test (CMODjct) as the addition of instantaneous deformations and deferred deformations.
Post-creep Flexural Bending Test to Failure
When the creep test is over, the test specimens are subjected to post-creep flexural tests to failure, following the same methodology exposed in 3.1 reaching crack opening values higher than 4 mm.
By assembling the experimental results of the previously mentioned stages and plotting Stress-CMOD, the evolution of the measured test parameters is represented considering the complete test to establish a common origin for crack opening values for all test stages.