Table 7. Typical analyses of city water supplies, and seawater. 



Dissolved Solids 



In 



tapwatei 



• (analysis no 



.1 





In 

 seawater 1 



1 



2 



3 



4 



5 



6 











(ppm) 









Silica (SiQ 2 ) 



2.4 



0.0 



6.5 



9.4 



22.0 



3.0 



_ 



Iron (Fe) 



Q.l 



0.Q 



Q.Q 



Q.2 



0.Q 



0.0 



- 



Calcium (Ca) 



5.8 



15.3 



29.5 



96.0 



3.0. 



1.3 



50-480 



Magnesium (Mgl 



1.4 



5.5 



7.6 



27.0 



2.4 



0.3 



26Q-1 410 



Sodium (Na) 



1.7 



16.1 



2.3 



183.0. 



215.0. 



1.4 



2 190-12 200 



Potassium (K) 



Q.7 



0.0 



1.6 



18.0 



9.8 



0.2 



70-550 



Bicarbonate (HC0 3 ) 



14.07 



35.8 



122.0 



334.0 



549.0 



4.1 



- 



Sulfate (SO^) 



9.7 



59.9 



5.3 



121.0 



11.0 



2.6 



580-2 810 



Chloride (CI) 



2.0 



3.0 



1.4 



280.0 



22.0 



1.0 



3 960-20 000 



Nitrate (N0 3 ) 



0.5 



0.0 



1.6 



0.2 



0.5 



0.0 



- 



Total dissolved 

















solids 



31.0 



250.0 



125.0 



983.0 



564.0 



19.0 



35 000 



different seas contain different amounts of dissolved salts. 



When suitable freshwater is not available, seawater can also be used 

 for making reinforced concrete. Its use may increase the risk of corro- 

 sion, but the risk is reduced if the reinforcement has sufficient cover and 

 if the concrete is watertight and contains an adequate amount of entrained 

 air. Reinforced concrete structures made with seawater and exposed to 

 marine environment should have a water-cement ratio of less than 0.45 and 

 the reinforcement cover should be at least 75 millimeters (3 inches). Seawater 

 should not be used to make prestressed concrete in which the prestressing 

 steel is in contact with the concrete. Sodium or potassium salts present 

 in seawater used for mix water can produce substances that combine with 

 alkali-reactive aggregates in the same manner as when combined with cement 

 alkalies. Therefore, seawater should not be used as mixing water for 

 concrete with known potentially alkali-reactive aggregates, even when the 

 alkali content of the cement is low. 



(3) Impurities . The following resume discusses the effects of 

 certain impurities in mixing water on the quality of plain concrete. 



(a) Alkali Carbonate and Bicarbonate . Sodium carbonate can 

 cause very rapid setting, bicarbonate can either accelerate or retard set. 

 In large concentrations the salts can materially reduce concrete strengths. 

 When the sum of these disolved salts exceeds 1 000 parts per million, tests 

 for their setting time and 28-day strength should be made. 



(b) Chloride and Sulfate . Concern over a high cloride 

 content in the water is chiefly due to the possible adverse effect of 

 chloride ions on the corrosion of reinforcing steel or prestressing strands. 

 The chloride level at which corrosion begins is about 7.6 newtons per cubic 

 meter (1.3 pounds per cubic yard). Placing an acceptable limit on chloride 

 content for any one ingredient, such as mixing water, is difficult consider- 

 ing the several sources of chloride ions in concrete. An acceptable limit 



76 



