Few exposed sealants have a life as long as that of the structure 

 whose joints they are intended to seal. Fortunately, buried sealants such 

 as waterstops and gaskets have a long life because they are not exposed to 

 weathering and other deteriorating influences. 



Most field-molded sealants will, however, require renewal sooner or 

 later if an effective seal is to be maintained and deterioration of the 

 structure is to be avoided. The time at which this becomes necessary is 

 determined by service conditions, by the type of material used, and whether 

 any defects of the kind already enumerated were built in at the time of the 

 original sealing. 



10. Delivery and Placement . 



a. Batching . 



(1) Objectives . During measurement operations aggregates should 

 be handled in a manner to maintain their desired grading, and all materials 

 should be weighted to the tolerances required for desired reproducibility 

 of the concrete mix selected. In addition to accurate weighing another 

 important objective of successful batching is the proper sequencing and 

 blending of the ingredients during charging of the mixers. The final 

 objective is to obtain uniformity and homogeneity in the concrete produced 

 as indicated by such physical properties as unit weight, slump, air content, 

 strength, and air-free mortar content in successive batches of the same mix 

 proportions. 



(2) Tolerances . Most engineering organizations, both public and 

 private, issue specifications containing detailed requirements for manual, 

 semiautomatic, and automatic batching equipment for concrete. 



(3) Plant Type . Factors affecting the choice of the proper 

 batching systems are (1) size of job, (2) required production rate, and (3) 

 required standards of batching performance. The productive capacity of a 

 plant is determined by a combination of such items as the materials handling 

 system, bin size, batcher size, and plant mixer size and number. The 

 available batching equipment falls into three general categories- -manual, 

 semiautomatic, and automatic. 



(a) Manual Batching . As the name implies, all operations of 

 weighing and batching of the concrete ingredients are done manually. 

 Manual plants are acceptable for small jobs having low batching rate 

 requirements, generally for jobs up to 3 800 cubic meters (5 000 cubic 

 yards) and rates up to 20 cubic meters per hour (25 cubic yards per hour) . 

 As the job size increases, automation of batching operations is rapidly 

 justified. Attempts to increase the capacity of manual plants by rapid 

 batching invariably result in excessive weighing inaccuracies. 



(b) Semiautomatic Batching . In this system aggregate bin 

 gates for charging batchers are opened by manually operated pushbuttons or 

 switches. Gates are closed automatically when the designated weight of 

 material has been delivered. 



(c) Automatic Batching . Automatic batching of all materials 

 is electrically activated by a single starter switch. Interlocks interrupt 



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