CONCRETE LINING FOR IRRIGATION CANALS. 57 
In summing up the results of the experiments at Logan, Utah, and 
those at the State Universit} r 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 lining 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 lining 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. 
