Shrinkage freezing process

The destructive effects, such as differential shrinkage and tensile stress at the gel surface, are of the same kind in both cases, so that for the freezing process, maximum tensile stress can be computed analogously to Eq. 5.17 as... 

In order to minimize rebound, thorough blending of the ingredients should be done. The aggregate should be well coated with the cement in the dry-mix process. Since shotcrete admixtures promote shrinkage, it is essential that the sprayed surfaces be kept moist for at least 7 days. Ambient temperature conditions should be maintained at a level so that air in contact with the shotcrete surface is above freezing - at least for 7 days [106, 115]. 

The freeze-drying process was applied for first time in 1890 by the Leipzig anatomist Altmann. He froze samples of tissue required for microscopic investigations at very low temperatures. Then, the tissues were placed in a desiccator filled with sulfuric acid and cooled on the outside. The air was evacuated while the tissues remained frozen and the water yielded to the sulfuric acid by sublimation. In this way, Altmann obtained anhydrous tissue preparations, without any shrinkage of the cells. 

To avoid excessive shrinkage a freeze drying process was used to prepare the partially saturated samples (Delage et al. 1982). Figure 2 shows the measured pore size density functions for the as-compacted state, which correspond to a packing having /C(( = 1.35 Mg/m for pellets having /C(i= 1.95 Mg/m ... 

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