Quasi-instantaneous and Long-term Deformations of High-Performance Concrete with Some Related Properties

University dissertation from Division of Building Materials, LTH, Lund University

Abstract: This report outlines an experimental and numerical study on quasi-instantaneous and long-term deformations of High-Performance Concrete, HPC, with some related properties. For this purpose about two hundred small cylinders and about one thousand cubes of eight types of HPC were cast. The age at loading varied between 18h and 28 days. Other principal properties of HPC were studied up to 4 years' age. Creep deformations of the HPC were studied from 0.01 s of loading time until 5 years' age. The work also includes observations of recovery at unloading and transversal deformations (Poisson's ratio). Elastic modulus and dynamic modulus of elasticity were studied on both young and mature concrete. Parallel studies were performed on shrinkage, strength, hydration, carbonation and on internal relative humidity of HPC. Supplementary dimensional studies were performed on twenty larger cylinders. Field studies were carried out on almost thirty prestressed beams. The results show good correlation between maturity and the quantity of creep of HPC. The results also show good correlation between, on one hand, the stress level and the mix design of the HPC and, on the other hand, the creep properties of HPC given a constant loading time. The phenomenon named autogenous shrinkage was observed during the study. The autogenous shrinkage was related to the self-desiccation and chemical shrinkage in HPC. Also the elastic properties of HPC were dependent on the moisture state at testing. The field studies on beams confirmed the findings in the laboratory. The short-term studies indicated that the creep rate (related to unit stress) of mature HPC was fairly independent of the compressive strength. The short-term basic creep rate (related to unit stress) of HPC after heat curing was observed to be twice as large at -1°C as at other temperatures. Besides loading time, the long-term compliance of HPC was mainly dependent on the maturity and the compressive strength both when loading the HPC and at 28 days' age. The creep was slightly reduced by use of 10% silica fume instead of 5%. The calculated long-term total compliance of the present study coincided reasonably well with previous research, taking into account that the exact HPC mix was unknown. However, the observed creep compliance was slightly larger than previously seen. The initial compliance calculated according to the previous studies was more affected by the strength than were the results of the present study. The following principal results were obtained: · Elastic modulus was related to compressive strength and porosity as demonstrated in normal strength concrete, NSC. · The creep rate (related to unit stress) was dependent on both the duration of loading and the maturity of the HPC. · Creep properties of the HPCs obtained by the quasi-instantaneous loading were applicable to the long-term studies performed with a much lower loading rate. · Specific creep was shown to be reduced with increase in silica fume content. Autogenous shrinkage was reduced by the use of granulated silica fume instead of silica fume slurry due to the greater fineness.