V. Складіть англо-українські пари словосполучень, еквівалентних за значенням.

1. load stress; 2. long-term temperature record; 3. moisture content; 4. moisture-

warping stress; 5. temperature stress; 6. net result; 7. thermal conductivity.

1. напруження короблення від вологості; 2. вміст вологи ; 3. напруження при навантаженні; 4. теплопровідність; 5. довгострокові температурні дані;

6. температурне (теплове) напруження; 7. кінцевий результат.

TEMPERATURE-WARPING STRESSES

If a thin sheet of cellophane is placed on the (warm) palm of the hand, it will be found to curl upwards as a result of the thermal expansion of its lower surface. Placing it on a cool surface will cause curling in the opposite direction. A long narrow sheet will curl either up or down about its major longitudinal axis, but as the length to breadth ratio of the sheet approaches unity, the major curling or warping will shift to a diagonal direction.

The action of the cellophane is a very useful analogy, which helps in understanding the warping stresses produced in a concrete pavement by temperature differentials throughout its thickness. When a road slab is maintained at a constant temperature it will rest flat on the supporting soil, whether it is a subgrade or a sub-base. If, however - as is usual in the evenings - the air temperature is lowered, then the initial reaction of the top surface of the concrete slab is to attempt to contract. Since the thermal conductivity of concrete is relatively low, the bottom of the slab remains at the same initial temperature and so the corners and the edges of the slab tend to curl upwards. If the temperature conditions are reversed then the tendency will be for the slab to warp downwards. If, as with cellophane sheet, warping of the slab were allowed without restraint, then the stresses produced in the slab would be negligible and no stress problem would arise. In practice, however, the tendency to curl is resisted both by the weight of the slab itself and by load-transfer devices, or friction, at the pavement joints, and these induce warping stresses in the pavement.

Westergaard also considered the problem of temperature warping. Using the assumption that the temperature gradient from top to bottom of a concrete road slab is in the form of a straight line, Westergaard developed equations for three different cases. In the simplest one of these, he assumed that the slab was infinitely large.

In general, the temperature stress results, which he gives tend to be on the high side when compared to what actually does occur. To account for this it has been shown that, in particular, his assumption that the temperature gradient through the slab is a straight line is only occasionally correct. Studies carried out, have shown that the gradient is in fact closer to a curved line and that use of this form of gradient results in calculated stress values which are considerably lower than those given by Westergaard's analysis.

In practice, the stresses induced by temperature warping are not as detrimental as might otherwise be expected. There are a number of reasons for this:

1. At slab corners, where the load stresses are actually the greatest, the warping stresses are negligible since the tendency of a slab to curl at these locations is resisted by only a very small amount of concrete.

2. At the interior of a slab and along its edges, significant curling stresses may be developed which, under certain circumstances, are additive to load stresses. However, since concrete slabs are normally designed to have a uniform thickness based on the corner-load needs, the margin of strength in the interior and edges of a slab is usually sufficient to offset the warping stresses which are produced at these locations.

3. Long-term temperature record studies which were carried out in some northern hemisphere countries have indicated that the temperature at the bottom of the slab exceeds that of the top more often than reverse, so that curling stresses in the interior and the edges are more frequently subtractive than additive.

Завдання I. Знайдіть в тексті речення, які містять Participle II, Речення перекладіть.

Завдання II. Англійською дайте оцінку експерименту, який ілюструє напруження від температурного короблення.

MOISTURE-WARPING STRESSES

Differences in moisture content between the top and the bottom of a slab also cause warping stresses. This is due to the ability of concrete to shrink when its moisture content is decreased and to swell when the moisture content is increased. Very little is known about the extent to which this type of warping occurs, and as yet it has not been possible to develop a method of analysis which enables the stress produced by this phenomenon to be calculated.

Warping stresses caused by moisture differences are not considered by most engineers utilising calculations to obtain a thickness design since, in fact, the omission of this calculation generally results in a solution, which errs on the side of safety and reduces the possibility of failure of the pavement. The reason for this is that the slab normally lies on a moist subgrade and therefore the bottom will have a moisture content which, on average, is significantly higher than that at the top. Furthermore, because of its higher water content, the bottom of the slab is in a more expanded condition and so there is tendency for the slab to curl upward. The net result is therefore that the stresses produced by the moisture differential act to resist the stresses caused by the applied wheel loads and, very often, the warping stresses caused by the temperature differential within the slab.

Завдання I. Знайдіть в тексті речення, які містять інфінітив. Речення перекладіть.

Lesson 9

Exercises