Long term creep and shrinkage of nano silica modified high volume fly ash concrete

Abstract

The long-term creep and shrinkage behaviour of two High-Volume Fly Ash (HVFA) concretes incorporating nano silica with 65% and 80% replacement of cement has been investigated. This comprised a detailed analysis of the microstructure, pore structure and chemistry of the two HVFA systems up to a period of 450 days. The compressive strength and modulus of elasticity of HVFA-65 concrete increased from 32 to 73 MPa and 30.3 to 40.5 GPa, respectively between 7 and 450 days. The HVFA-80 concrete achieved compressive strength values of 22 and 71 MPa and elastic modulus values of 28.9 and 37 GPa. After a total loading period of 450 days, HVFA-65 and HVFA-80 concretes displayed creep parameters, which were significantly below the values predicted by AS 3600, ACI 209 and CEB-FIP standard model equations. After a total drying period of 450 days 28-day cured specimens showed significantly reduced shrinkage compared to 7-day cured specimens. On the other hand, HVFA-80 concrete displayed higher shrinkage compared to the HVFA-65 specimens throughout the period. All specimens except for 7-day cured HVFA-80 concrete were within the maximum permissible shrinkage of 800 microns recommended for Australian construction practices. HVFA-65 concrete showed a denser microstructure and a stronger, better packed interfacial transition zone (ITZ) compared to HVFA-80 at all ages. The XRD and FTIR analysis data identified the formation of hydration products including C-S-H and C-A-S-H which contributed towards both the strength gain as well as the creep and shrinkage properties displayed by the HVFA concrete by minimizing the total porosity and pore size.