Measurement of temperature and early age shrinkage of alkali activated metakaolin using fiber Bragg grating sensors

Shrinkage is one of the main cause of early age cracking in civil engineering structures and cracking represents a threat to materials durability through service life. Dealing with traditional cementitious composites, we have at the state of the art a good knowledge of the phenomenon and several techniques to mitigate crack risk: 1) internal curing (lightweight aggregates, superabsorbent polymers, natural fibers); 2) shrinkage reducing admixtures; etc. In the case of alkali activated binder composites, the available data are relatively few and there is still need of understanding. In this paper shrinkage and temperature are monitored in the early age by means of fiber Bragg grating (FBG) sensors, which allow an accurate measurement of both parameters. This kind of assessment is particularly critical for alkali activated metakaolin which undergoes a strongly exothermic polycondensation reaction and an accurate measurement method such as FBG technology represents a highly interesting solution. The binding system here studied exhibited relevant shrinkage after 7 days (0.22%) and high temperature peak (63 °C) at 2.5 h.