Freezing, frost

Jiir-entraining cements produce concretes that protect the concrete from frost damage. They are commonly used for concrete pavements subjected to wet and freezing conditions. Cement of low alkah content may be used with certain concrete aggregates containing reactive siUca to prevent deleterious expansions. 

In the tubular process a thin tube is extruded (usually in a vertically upward direction) and by blowing air through the die head the tube is inflated into a thin bubble. This is cooled, flattened out and wound up. The ratio of bubble diameter to die diameter is known as the blow-up ratio, the ratio of the haul-off rate to the natural extrusion rate is referred to as the draw-down ratio and the distance between the die and the frost line (when the extrudate becomes solidified and which can often be seen by the appearance of haziness), the freeze-line distance. 

The molecular structure and dynamics of the ice/water interface are of interest, for example, in understanding phenomena like frost heaving, freezing (and the inhibition of freezing) in biological systems, and the growth mechanisms of ice crystals. In a series of simulations, Haymet and coworkers (see Refs. 193-196) studied the density variation, the orientational order and the layer-dependence of the mobilitity of water molecules. The ice/water basal interface is found to be a relatively broad interface of about... 

Abnormally low suction pressures will lead to high discharge temperatures, owing to the high compression ratio, and possible malfunction of other components. Air cooling coils may frost excessively, or water chillers freeze. 

Prevent frost forming on an air cooling evaporator, where this is close to freezing point, or where a temporary malfunction cannot be permitted to interrupt operation. 

Afurther consideration maybe the possibility of reducing ice buildup on the evaporator, whether this is in the form of frost on fins or ice on the coils of a liquid chilling coil. Where temperatures close to freezing point are required, it may be an advantage to design with an evaporator temperature high enough to avoid frost or ice -either for safety or to simplify the defrost method. 

Frost-heave is prevented hy supplying low-intensity heat to the underside of the insulation, to keep it above freezing point. This may take several forms ... 

For a room condition to BS.2852.C., and at full air flow, the ADP will be just above freezing point. If the unit is fitted with a low fan speed control, the ADP can fall below freezing and the coil frost over. Such units need to be fitted with a defrosting control and an allowance made for the time that the compressor will not be running. 

Filter layers, frost penetration, and cap-liner connections are other factors to consider in designing the closure system for a hazardous waste landfill. Before using geotextiles for filter layers in closures, one should conduct pressure tests and clogging tests on the material. Freeze-thaw cycles probably have little effect on membranes, but their impact on clay is still not known. Because of this lack of knowledge, membrane and clay layers should be placed below the frost penetration layer. Finally, a cap membrane should not be welded to the primary FML. Differential settlement in the cap can put tension on the cap membrane. In such a situation, the seam could separate and increase the potential for integration of the surface water collection system into the LDS. 

The warm-weather climates necessary for citrus orchards generally minimize the risk of frost conditions. Nevertheless, occasionally measures are needed to protect the trees and their fruit from freezing temperatures. Citrus crops become threatened when temperatures fall below 28°F for four hours or moreJ l Heating... 

Preparation of cotton bract extracts. Figure 1 is a flow chart showing our procedures for preparing the various bract extracts. Dried bracts (frost killed) were hand picked just prior to harvest from cotton fields in the Lubbock, Texas area. These were stored at room temperature. Extracts were freeze-dried and stored at -4°C. For inhalation challenge by our subjects each extract was reconstituted with water or saline, as indicated, at a concentration equivalent to the standard crude extract. This Insured that for challenge purposes components were not concentrated as purification progressed. 

Black currants, rich in flavor and full of vitamin C, are hardy and reliable, although late frost can damage blossoms and reduce the crop. The berries can be eaten raw, cooked, or pressed for their juice they freeze well, and make excellent preserves. Black currants are always grown as bushes. You will harvest a moderate crop after two years, and a full crop after three or four years. 

To store a block for later cutting, place in an airtight container, preferably one that is not too mnch bigger than the block. Small Zip-lock bags are good for this. Place into an nltralow-temperature freezer (-70°C) as soon as possible. Avoid freezers with antomatic freeze-thaw cycles snch as frost-free types. 

The recorded data on lignosulfonate water-reducing agents indicate that, as far as freeze-thaw durability is concerned, because of the low water-cement ratios possible, an enhancement to the durability will invariably be obtained. When the admixtures are used to effect a reduction in the cement content, there are strong indications that a considerable enhancement of durability is obtained, presumably due to a reduction in the cement matrix which is the part of the concrete susceptible to frost damage. The higher aggregate content would therefore allow easier dissipation of stresses. 

The reduction in porosity, decreased water content, and air entrainment that results when latexes are used in mortar and concrete mixes make them much more resistant to freezing and thawing conditions than conventional mortar and concrete. Figure 6.17 presents the freeze-thaw durability in water (-18 to 4°C) of combined water-and dry-cured SBR-, PAE- and EVA-modified mortars [98], The frost resistance of mortars made with these latexes is markedly improved even at polymer-cement ratios of 5%. However, an increase in the polymer-cement ratio does not necessarily produce further improvement in freeze-thaw resistance. EMM and EMC, when exposed to outdoor conditions involving freeze-thaw, UV radiation and carbonation show better weatherability when compared with conventional mortar and concrete. 

It has sometimes been argued that concrete is seldom damaged by frost action if low water-cement ratio mixes are used. However, the paramount effect of air entrainment in improving freeze-thaw resistance was clearly demonstrated in a study of the freeze-thaw resistance of both air-entrained and non-air-entrained superplasticized concrete with low water-cement... 

The use of superplasticizers in air-entrained concrete has caused much debate. Two main problems are associated with superplasticized air-entrained concrete (1) a decrease in air content by 1-3% when slump is increased from 75 mm to 220 mm after the addition of the superpiasticizer to create flowing concrete, and (2) a change in the air void system to less desirable values. However, most investigators [10-11, 12] have shown that, although the air-void spacing factor required for adequate frost resistance is altered, the change did not necessarily affect the freeze-thaw durability of... 

In spite of the decades of use of air-entraining agents (AEA) for the improvement of frost resistance of concrete, there has been a significant increase in reported cases of poor performance of concrete exposed to freezing and thawing, particularly where de-icing salts are used. Examination of specimens from a large number of concrete structures showed that only about half of the structures had an air-void system (AVS) that met the... 

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