POETLAND 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 1^ 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 clump 

 bodies. (See Fig. 2, PL 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 Roads 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 



