Figure 23. Pumped concrete applied with a pressure nozzle. 



To establish the optimum slump for a pump mix and to maintain control 

 of that particular slump through the course of the job are both extremely 

 important factors. Experience indicates that slumps below 50 millimeters 

 (2 inches) are impractical for pumping, and slumps above 152 millimeters (6 

 inches) should be avoided. In mixtures with high slump, the aggregate will 

 separate from the mortar and paste and may cause blocking in the pump line. 

 Overly wet mixes also bleed and increase shrinkage. It is more important 

 to obtain a truly plastic mix through proper proportioning than to try to 

 overcome deficiencies by adding more mortar. 



11. Environmental Considerat ions. 



The following environmental features are generally to be considered in 

 the use of concrete in coastal structures. Portland cement concrete is a 

 durable material and well suited to use in the coastal environment. When 

 properly designed, placed and cured, it will resist most coastal environ- 

 ments for many years. 



a. Corrosive and Pollutant Attacks on Exposed Surfaces . Concrete is 

 rarely attacked by solid, dry chemicals. In order to significantly attack 

 concrete, corrosive chemicals must be in solution form and above some 

 minimum concentration. Chemical attack on concrete is generally the 



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