PORTLAND CEMENT CONCRETE ROADS. 37 
begun at the point nearest the unloading plant and as it becomes 
sufficiently strong to permit traffic the point of batch-box transfer 
is moved ahead on the new pavement. 
The principal advantage of this method of hauling is that it permits 
the partial use of the industrial railway on work where it could not- 
otherwise be used, thereby securing so far as possible the advantages 
of industrial railway haul. As the point of transfer is moved ahead 
an excellent road is made available for a part of the truck haul, and 
the wear and tear of the trucks is reduced to a minimum. The in- 
creased speed of the trucks on the new pavement over the industrial 
trains compensates for the time lost in effecting the transfer of the 
boxes from the trucks to the industrial cars. The amount of in- 
dustrial railway equipment is reduced to a minimum. Usually not 
more than two locomotives and 1J miles of track are required for the 
industrial railway feature of this operation. 
If the concrete is mixed at the unloading plant and hauled to 
the road, trucks are about the only hauling equipment than can be 
used satisfactorily. Trucks for this purpose should preferably be 
equipped with turn-over dump bodies rather than hoisting dump 
bodies. (See Fig. 2, PI. V.) In hauling, the concrete has a tendency 
to compact and stick to the truck body, making the discharge rather 
difficult. If hoisting dump bodies are used, a high angle of hoist is 
desirable. A comparatively dry concrete is more readily discharged 
from trucks than a wet, sloppy mix. It is generally accepted 
that concrete" mixed at a central plant should be deposited in the 
pavement within 30 to 35 minutes after being mixed, though tests 
made by the Bureau of Public Eoads show that the final placement 
may be delayed by as much as three hours without materially affect- 
ing the strength of the concrete. This limitation of time necessarily 
determines the limit of haul for mixed concrete. Under extremely 
favorable conditions mixed concrete may be hauled as far as 6 miles. 
The hauling of mixed concrete is particularly advantageous on 
work where the supply of water along the road is limited. Its 
principal disadvantages are that the subgrade must be used for haul- 
ing and that considerable delays are caused even by moderate rains. 
HANDLING AND STORING MATERIALS. 
Cement. — Cement for concrete-pavement construction may be pur- 
chased either in bulk or in sacks. Bulk cement is not used to any 
extent; in fact, its use is practically confined to operations where 
proportioned aggregate or mixed concrete is hauled to the road. 
Even for this use it is not recommended on account of the difficulty 
of measuring the proper quantity of cement for each batch. If it 
is used, the proper quantity for each batch should be weighed or 
measured by means of separate compartments placed in the batch 
