PORTLAND CEMENT CONCRETE PAVEMENTS. 15 
joint has varied in common practice from one thickness of two-ply 
tar paper up to about one-half inch. A thickness of one-quarter inch 
seems to give very satisfactory results when the joints are spaced 
about 30 feet apart. Joints of this kind are sometimes provided with 
metal armor, which is intended to keep the adjacent edges of the 
concrete from being spalled off. It is claimed that armored joints 
require less maintenance than other types, but they are more ex- 
pensive to construct. 
The joints are undoubtedly the weakest feature of the concrete 
pavement; and no matter what type of joint is used, they must be 
given frequent and careful attention to prevent rapid deterioration 
of the pavement adjacent to them. 
In the past, contraction joints of all types have usually been placed 
at right angles to the line of the pavement. This method of con- 
struction has the disadvantage that two wheels of a vehicle strike the 
joint at the same time and thus" produce the maximum amount of 
impact. By skewing the joint at an angle of about 15 degrees the 
wheels strike one at a time, and the total resultant impact is reduced 
by at least one-half. This is advantageous to both the traffic and the 
pavement, and since the difficulties involved in constructing skewed 
joints are not at all serious, there is no apparent objection to their 
use. 
PROTECTING AND CUEING THE CONCRETE. 
The quality of the concrete depends to a great extent upon the con- 
ditions under which it sets or hardens. When early exposed to dry 
air, for example, water is evaporated out, thereby greatly accelerat- 
ing the shrinkage of the concrete and delaying the jDrocess of setting. 
It is evident that these results form a very effective combination for 
producing cracks. The effect of freezing on concrete is still more 
harmful; not only are cracks produced, but the internal structure 
of the concrete is also damaged. 
The precautions that must be taken in order to protect a newly 
constructed concrete pavement during the process of curing depend 
largely on the weather conditions. In drying weather small hair- 
like cracks will frequently begin to form almost as soon as the 
surface of the concrete is finished, and unless the concrete is quickly 
covered and protected from the air these cracks increase in size 
very rapidly. At other times, when the atmosphere is moist, the 
concrete may sometimes be permitted to stand for several hours 
before being covered, without any danger of cracks forming. Heavy 
canvas made into sections of convenient length and proper width 
should be used for covering the concrete surface (PL III, fig. 2). 
The canvas should be spread over the pavement as soon as this 
can be done without marring the surface. Under unfavorable 
