Early-age Autogenous Shrinkage Measurements of High Performance Concrete

摘要

This paper discusses results and analyses from a series of early-age autogenous shrinkage measurements for several high performance concrete mixtures developed for bridge applications. These tests on ternary blends of HPC were performed under temperature-controlled conditions to investigate the influence of mixture proportions and binder composition on early-age shrinkage measurements.Corrections for measured strains resulting from temperature effects are also discussed. The problem of autogenous shrinkage measurement at early ages is complicated by the nebulous definition of strains during the transition from a semi-liquid to a solid. It is hence necessary to complement autogenous shrinkage measurements with measurements of concrete setting times. The presence of free water within the concrete matrix (as indicated by setting times) greatly influences the thermal expansion of concrete at very early age. In order to accurately represent very early-age measurement of autogenous shrinkage, the thermal expansion of the concrete due to heat of hydration must be corrected for. A tri-linear model was used where the coefficient of thermal expansion (CTE) varies linearly from initial to final set. Restraint effects can be different for bridge girders compared to deck slabs, and hence the influence of restrained autogenous shrinkage needs to be better understood. A good understanding of coupled effect of temperature and autogenous shrinkage strains is necessary to mitigate potential cracking in continuous bridge deck slabs made using HPC mixtures.Results show good correlations with previously published data. Ternary blends are shown to take advantage of the best features of both silica fume and fly ash, offering the durability improvement typical of silica fume, along with the early-age shrinkage-reducing effects of fly ash.