When a substance consisting of two or more bodies which have different coefficients of expansion undergoes any change of temperature, it is subjected to stresses, since its various parts do not expand in an equal degree. Thus, Portland cement, which has a coefficient of expansion of .000011, cannot make a reliable joint under varying temperatures with leading, the coefficient of which is .000028. On the other hand, the coefficient for steel fortunately approaches very closely to that of concrete, so that these materials may be combined to advantage in construction work. In the case of brittle substances fixed together, this unequal expansion is a frequent source of fracture. The cracking of glaze upon tiles and terra cotta may be attributed to this cause. The plastering on walls and the seams of cheap wall-paper sometimes open on account of unequal expansion.

Allowance for expansion in non-metallic bodies, such as stone, brick, or concrete, is not usually of importance because the coefficients of expansion of such bodies is as a rule smaller and the specific heat higher than those of metals. For this reason they require more heat to produce a given rise in temperature than do the metals.

The expansion of a number of common substances used in building construction is given below.

Linear Expansion of Solids at Ordinary Temperature

Substance

For 1° F.

For 1° C.

Aluminum, cast.................

.00001234

.00002221

Brick, best stock.................

.00000310

.00000550

Zinc...........................

.00001407

.00001755

Cement, Roman, dry.............

.00000797

.00001435

Cement, Portland, mixed, pure. .. .

.00000594

.00001070

Cement, Portland: mortar mixed with sand.....................

.00000656

.00001180

Concrete: cement, mortar, and pebbles.......................

.00000795

.00001430

Copper.........................

.00000887

.00001596

Ebonite........................

.00004278

.00007700

Glass, English flint...............

.00000451

.00000812

" French flint...............

.00000484

.00000872

white, free from lead.......

.00000492

.00000886

blown....................

.00000498

.00000896

thermometer..............

.00000499

.00000897

" hard.....................

.00000397

.00000714

Granite, gray, dry...............

.00000438

.00000789

" red....................

.00000498

.00000897

There are a few substances, of which water is perhaps the most common, that do not follow the rule of expansion and contraction. When a body contracts, its density, and therefore its weight per cubic inch or cubic foot, increases and we say that it becomes heavier. Water freezes at 32° F. and ice floats at 34° F. showing that it is lighter than water. Careful investigation reveals that water is heaviest at 39° F. (4° C).