CONCRETE LINING FOR IRRIGATION CANALS. 57 



111 summing up the results of the experiments at Logan, Utah, and 

 those at the State University of Iowa, it may be stated that for con- 

 crete slab construction such as canal lining, where only one side is 

 exposed and with the other side in contact with earth, a coefficient of 

 expansion of 0.0000045 should be used, but in the case of concrete 

 construction where all sides are subject to equal temperatures and 

 are not under the conditions of moisture and earth contact found in 

 canal fining and similar construction, a coefficient of 0.0000063 can 

 be used with safety. 



JOINTS IN CONCRETE LINING. 



Owing to the fact that concrete lining expands in warm weather 

 and contracts in cold weather, joints would seem to be an essential 

 feature of such construction. Where no provision is made for expan- 

 sion and contraction by means of joints, the concrete lining is certain 

 to be subjected to high internal stresses, which increase in intensity 

 until the lining is ruptured. These ruptures occur at the weakest 

 points, and following the directions of least resistance result in 

 irregular fractures which are difficult to repair. They frequently are 

 so small and so irregular that it is practically impossible to introduce 

 any filler into the seams. Even when this is done the alternate open- 

 ing and closing of the fracture, due to changes in temperature, lowers 

 and in time destroys the effectiveness of such repairs. 



On the other hand, joints in concrete lining constitute a weak fea- 

 ture. In strength, durability, and water-tightness the best formed 

 joint is inferior to the continuous lining. For these reasons, to which 

 may be added that of extra cost, the distance between joints should 

 be as great as possible consistent with changes in volume due to tem- 

 perature and the adoption of suitable forms and proper methods of 

 construction. 



When forms are used their length is usually limited to the size and 

 weight which can be readily shifted by hand without the aid of special 

 equipment. Even when the forms are of the simplest kind the 

 methods of construction commonly employed place other limitations 

 on the distance between joints. Again, in fining curves the frequency 

 of the joints depends upon the degree of curvature, the sharper the 

 curve the shorter the distance between joints. Notwithstanding 

 these limitations, the tendency in the past has been to insert too 

 many joints, particularly on straight portions of canals. 



The subject of joints in concrete lining is also closely related to the 

 manner in which the lining is laid, whether continuously or in alter- 

 nate sections. Joints which are adapted to one of these methods 

 may be a misfit when applied to the other. In using either method 

 it is advisable to break joints between the floor and the sides, as indi- 

 cated in figure 3, a. 



