Early age stresses and creep-shrinkage interaction of restrained concrete

Experimental and numerical analyses were performed to characterize the early age tensile creep and shrinkage behavior of concrete. A uniaxial restrained shrinkage test was developed. The experiment tested two identical specimens: restrained and unrestrained. The test was controlled by computer, and the shrinkage deformation was checked continuously and compared to a threshold value of 5 μm, which when exceeded, triggered an increase in tensile load to recover the shrinkage strain in the restrained specimen. Thus, a restrained condition is achieved and the stress generated by shrinkage mechanisms was measurable. The experiment revealed how shrinkage stresses developed and how creep mechanisms reduced shrinkage strain.; The tests revealed that shrinkage stresses in the first days after casting are significant and caused fracture of the concrete. The rate of stress evolution influenced the time and stress of first cracking. The tensile creep of concrete formed a substantial part of the time dependent deformation and reduced the shrinkage stresses by 50%.; A method separating drying creep mechanisms of concrete into stress-induced shrinkage and microcracking was developed. The method required measurement of creep and shrinkage of concrete under drying, sealed, and moist curing conditions. The moist-curing test produce the basic creep; the sealed test provided data on basic creep and stress-induced shrinkage, and the drying test provided data on basic creep, stress-induced shrinkage and microcracking.; The basic creep results of young concrete indicated a high creep rate in the initial 10–20 hours after loading. Then, the rate decreased and the creep function approached a stable value. The initial rate of creep was sensitive to age at loading in the first two days, and became age-independent after a few days.; The analysis revealed stress-induced shrinkage as a major mechanism of drying creep for plain and fiber reinforced concrete (FRC). Microcracking forms a significant portion of drying creep of plain concrete, but it is less significant in FRC. Therefore, creep of FRC is dominated by real mechanisms, whereas apparent mechanisms induced by microcracking is significant in plain concrete. The real creep mechanisms are beneficial because they provide tensile stress relaxation, but the apparent mechanisms are associated with microstructural damage and are detrimental. Therefore, FRC enhances stress relaxation and delays the time of shrinkage cracking.