Shrinkage properties of normal and high strength fibre reinforced concrete

This paper reports on a study of shrinkage–weight loss relationships for normal and high strength plain and fibre reinforced concrete. Three fibre concentrations (1, 2 and 3% by weight) and five concrete grades (in the range 40–120 MPa) were used in the study. Four test specimen sizes were used to investigate the influence of effective thickness, three being cylinders with diameters of 100, 150 and 200 mm and a height/diameter ratio of two and the fourth being 76 × 76 × 254 mm prisms. All specimens were subjected to standard air drying in a temperature and humidity controlled laboratory (RH 65 ± 5% and temperature 20 ± 2°C) for either 100 days or 1 year. The test specimens were unsealed and hence the contribution of autogenous shrinkage was not evaluated separately. In general, the results show that high strength concrete has a similar long-term variation of shrinkage and weight loss with time to that observed for normal strength concrete and the relationship between shrinkage and weight loss during the first 100 days of drying is approximately linear for all grades of concrete. The main influence of fibre reinforcement is to reduce the development of early shrinkage strains and 1% fibre concentration is sufficient for this purpose. The test results have been compared with results obtained from two prediction models and it is concluded that further work is necessary to improve prediction models for application to high strength concrete and in particular to improve the prediction of early shrinkage strains.