Strain and Temperature Monitoring in Early-Age Concrete by Distributed Optical Fiber Sensing

Abstract

In the way towards a carbon neutral construction industry, the partial substitution of cement clinker by alternative binders is becoming increasingly popular in the design of low-carbon concrete mixes. However, as new concrete mixes are developed, the parameters governing the risk of early-age cracking arising from restraint forces due to thermal and shrinkage deformations need to be investigated for each mix. This paper reports the results of a field test in which distributed optical fiber sensors (DOFS) were used to monitor strain and temperature in two large-scale prisms cast against the ground. One of the specimens was cast with a reference concrete mix with CEM I whereas in the other mix cement was partly replaced by fly ash. After casting, mineral wool was used to insulate the specimens in order to reproduce realistic conditions in large mass concrete elements. Temperature measurements enabled a direct comparison of the heat generated by each mix as well as the estimation of the strength development. Strain measurements, on the other hand, gave an indication of the variation of the degree of restraint along the height of the specimens. Using available models for concrete creep, the tensile stresses along the specimens were calculated at different heights and compared to the expected tensile strength in order to assess the crack risk. After removing the insulation, measured strains exhibited a strong dependence on external temperature variations. The test results proved useful to analyse the early-age behaviour of concrete.