Effect of internal moisture content on the TG values of CFRP rods
DMA tests are used for the material charaterisation of CFRP tendons for civil engineering applications and to assess the high temperature behavior of CFRP prestressed structures by measuring the glass transition temperature Tg. The glass transition temperature is sensitive to the moisture content of the CFRP tendons and standard test methods (e.g. ASTM D7028 (ASTM 2007)) have not yet qualitatively addressed the effect of small moisture content variations from environmental conditions in the Tg values. The effect of the internal moisture content on the Tg values of two CFRP rods with different diameters and manufacturing process is evaluated. Lab conditioned specimens with varying drying time (9, 15, 36 and 210 days) and thus moisture content are tested to study the effect of small lab conditions variations in the glass transition temperature and the sensitivity of the DMA testing. Exposed specimens at 60°C for roughly 3 years are also investigated to record effect of greater moisture absorption on the Tg values. Two heating runs were conducted for every test to differentiate postcuring effects and mass weight measurements were recorded before and after each heating run. A linear relationship between the mass loss of the specimens due to drying at vacuum and during the heating runs with the glass transition temperature Tg was observed. Post-curing effects could not be clarified even for the dry specimens. The exposed specimens showed a reduction in Tg of 38°C that was reversible after drying. It is recommended that the use of Tg values to infer the degree of curing should be carried out on dry specimens.
Swiss Federal Laboratories for Materials Testing and Research for Industry, Construction and Commerce (Empa) (unknown)