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 CURING 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 process of setting. 

 It is evident that these results form a very effective combination f or 

 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 



