PORTLAND CEMENT CONCRETE PAVEMENTS. 11 
of machine mixers which have proved to be satisfactory for such 
work. The self-propelled batch type with a distributing device is 
probably the most economical to use where the amount of work to 
be done is sufficient to warrant the purchase of such a machine. 
The distributing device may consist of a bucket and boom attach- 
ment or of a chute or a revolving tube which conveys the concrete 
from the drum of the mixer to its place in the road. The chute is 
objectionable, because if the concrete is mixed to such a consistency 
that it will readily flow down the chute it is too wet for best results ; 
and, furthermore, there is a tendency for the mortar to separate 
from the coarse aggregate. This is especially true when the mixer 
is working down a steep grade. No matter what kind of distributing 
device is used, however, steep grades are liable to interfere with the 
proper working of the mixer, and if such grades occur on any par- 
ticular piece of work that is to be undertaken this point should be 
investigated before the concrete mixer is purchased. 
Even when the very best type of concrete mixer is employed it is 
necessary to exercise considerable care to see that the concrete is 
mixed thoroughly and to a uniform consistency. Tests have shown 
that increasing the time during which a batch of concrete remains 
in the revolving drum of a mixer, within reasonable limits, has very 
much the same effect as increasing the proportion of cement. It is 
also almost certain that varying amounts of water in successive 
batches will tend to cause cracks to develop in the pavement. It is 
impracticable to state definite rules for determining the number of 
turns of the mixer drum or the exact quantity of water which each 
batch should be given, because these features are considerably af- 
fected by the condition of the mixer and the materials. In general 
it may be said that each batch should be mixed until there are no 
uncoated particles of sand or coarse aggregate remaining, and the 
amount of water should be such that the resulting concrete will be 
quaky or jellylike, but not sufficiently wet to flow readily while it 
is being handled. On steep grades somewhat less water should be 
used in mixing the concrete than when the grade is level. A com- 
paratively wet concrete is easier to handle on level grades, but is 
liable to flow on steep grades after the pavement has been struck off 
and tamped, causing irregularities to develop in the surface. 
Immediately after the concrete is mixed it should be deposited in 
the pavement. Otherwise the materials of which it is composed will 
begin to separate, and if it is permitted to stand an appreciable length 
of time before being placed the heavy materials will settle to the bot- 
tom of the containing vessel, so that when it is emptied a core will be 
formed in the center of the space occupied by the batch. Concrete 
mixed in a stationary mixer and hauled to its place in the road is 
especially subject to this objection. 
