Study on durability of concrete under alternating positive and negative temperature curing in plateau area

Abstract

In order to investigate the durability of concrete under alternating curing from −20℃ to 20℃, different antifreeze (JB-1 type/HD-type) and different dosages (1.5 wt%/2 wt%) were chosen to carry out the study. This study used a variety of tests such as rapid freeze-thaw test, freeze-thaw-sulfate coupling test, dry and wet cycle test, electrical flux test, and drying shrinkage test, combined with NMR techniques, to comprehensively evaluate the properties and pore structure of the concrete. The results show that adding antifreeze can optimize concrete durability and pore structure under alternating curing from −20°C to 20°C. This study showed that 1.5 wt% was the optimal dose to maximize the improvement of concrete durability. Comparative analysis revealed that JB-1 type antifreeze is superior to HD-type antifreeze in enhancing the frost resistance, the freeze-thaw-sulfate coupling resistance, and the chloride ion permeation resistance of concrete, which is manifested in the relative dynamic modulus of elasticity increase of 1.5 %, compressive strength increase of 1.5 MPa, mass loss decrease of 0.12 % and electric flux decrease of 86 C. In-depth investigation reveals that the JB-1 type antifreeze agent promotes the generation of C-S-H gel by accelerating the hydration process of concrete, fills and shrinks the micropores of concrete, reduces the porosity, and improves the microstructure. However, over-addition has a reverse effect, which enhances the content of tiny bubbles and leads to a 2.4 % decrease in the relative dynamic modulus of elasticity of concrete. Meanwhile, the increase in porosity leads to an increase in the electrical flux of concrete by 151 C and shrinkage by 17.2 μm.