during loading and reduced to agitating speed or stopped after loading is 

 complete. Elapsed time for discharge of the concrete can be the same as 

 for truck mixing. 



(4) Final Objective. The method of transportation used should 

 efficiently deliver the concrete to the point of placement without signifi- 

 cantly altering its desired properties with regard to water-cement ratio, 

 slump, air content, and homogeneity. Each method of transportation has 

 advantages under particular conditions of use pertaining to such items as 

 mix materials and design, type and accessibility of placement, required 

 delivery capacity, location of batch plant, and others. These various 

 conditions should be carefully reviewed in selecting the type of transporta- 

 tion best suited for economically obtaining quality concrete in-place. 



d. Placing Concrete . A basic requirement for placing equipment and 

 methods, as for all other handling equipment and methods, is that the 

 quality of the concrete, in terms of water-cement ratio, slump, air content, 

 and homogeneity, must be preserved. Selection of equipment should be based 

 on its capability for efficiently handling concrete of the most advantageous 

 proportions that can be readily consolidated in place with vibration. 



Sufficient placing capacity, as well as mixing and transporting capacity, 

 should be provided so that the concrete can be kept plastic and free of 

 cold joints while it is being placed. It should be placed in horizontal 

 layers not exceeding 0.6 meter (2 feet) in depth, avoiding inclined layers 

 and cold joints. For monolithic construction each concrete layer should be 

 placed while the underlying layer is still responsive to vibration, and 

 layers should be sufficiently shallow to permit knitting the two together 

 by proper vibration. Concrete should be deposited at or near its final 

 position in the placement, eliminating the tendency to segregate when it 

 has to be flowed laterally into place. On sloping surfaces, concrete 

 should be placed at the lower part of the slope first, progressing upward, 

 and thereby increasing natural compaction of the concrete. High- velocity 

 discharge of concrete causing segregation of the concrete should be avoided. 



The equipment and method used for placing concrete should avoid separa- 

 ting the coarse aggregate from the concrete. Clusters and pockets of 

 coarse aggregate should be scattered before placing concrete over them to 

 prevent rock pockets and honeycomb in the completed work. 



Requests for increases in mixing water are frequently made on the job 

 when concrete of relatively stiff consistency will not flow down chutes, 

 drop out of buckets or hoppers, or discharge through gates or trunks. If 

 the concrete is readily workable and satisfactorily consolidated in place 

 with proper vibration, these requests for additional water are not valid. 

 A limitation on the use of reasonable mix proportions and slump should not 

 be imposed because inadequate placing equipment is being used. 



(1) Preplaced Aggregate Concrete . In this method of construction, 

 forms are first filled with clean, well-graded coarse aggregate, and then 

 structural quality grout is injected into the voids of the aggregate mass 

 to produce concrete. It is especially adaptable to underwater construc- 

 tion, to concrete and masonry repairs, and in general to new structures, 

 where placement by conventional means is usually difficult or where concrete 



141 



