PORTLAND €EME1S T T 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 miser 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 

 neeessary 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. 



