Non-air entrained HPC/HSC in wet freeze/thaw

Abstract In non-air entrained High Performance Concrete (HPC) with low w/b-ratio and high degree of hydration a saturated pore system can have no freezable water even at -20°C. However, the self desiccation porosity is high in HPC. Water may enter such pores during wet curing, long time absorption or pumping during wet freeze/thaw. Absorption measured during water curing of HPC with w/b = 0.35 – 0.40 and 0 – 8 % condensed silica fume corresponds roughly to theoretical self desiccation porosity. There is however, a risk that natural or entrained air voids are filled if present. Freezable water content in “saturated” HPC is hypothesized to be necessary for frost damage to occur. Wet frost exposure, which leads to an accelerated absorption - a pumping effect, is a very aggressive exposure situation. Freeze/thaw experiments on HPC with w/b = 0.30 – 0.40 including the above mentioned ones show that the pumping effect works in both undamaged and damaged concrete. Theoretically w/c = 0.36 – 0.41 should result in a pore system without capillary space at sufficient curing. However, for HPC with ordinary Portland cement and condensed silica fume even w/b = 0.30 gives only limited protection against internal cracking in wet frost exposure. Clearly, hydration must be sufficient since freezable water can be forced into some kind of pore space during wet freeze/thaw so that frost damage occurs. Some simple calculations of this transport for materials with and without ice formation are discussed. After internal damage starts the pumping effect is mainly filling cracks. The w/b-limit for frost durability depends on material and exposure. For HPC with silica fume it seems that w/b less than 0.25 gives high probability for both frost and salt/frost resistance without air entrainment. Most other pozzolans give less conclusive results and more research is needed for the design of a capillary pore free and frost immune HPC for severe exposure. Limited field experience on HPC without air entrainment with w/b = 0.23 indicates very good frost durability in the Norwegian tidal zone after 10 years of exposure. Key words: HPC, frost, durability, transport, freezable water, pores, nano structure