Fortunately, care in the selection of aggregate sources, and use of low 

 alkali cement and pozzolans, where appropriate, will minimize this problem 

 significantly. 



All aggregates should be sound, non-reactive and abrasion resistant, 

 and free from salt or alkalis. Particular care should be taken when 

 working with aggregates from new sources, especially those with siliceous 

 rocks and in desert areas. Sands from deposits several miles from the 

 shores of the Persian Gulf are heavily contaminated with salt from salt 

 fog; their use, unwashed, has led to serious corrosion in mild steel 

 reinforcing . 



Aggregates should meet the requirements of ASTM Standard C33 and, in 

 addition, should be judged for their durability by an engineer, based on 

 prior experience with the particular aggregates involved and tests. Tests 

 are especially necessary when working with new aggregates. These tests, 

 listed in ASTM C33, include tests for soundness (sodium-sulfate soundness 

 test), alkali-aggregate reactivity, cement-aggregate reactivity, and 

 freeze-thaw durability. Washing aggregates with freshwater will remove 

 salt and dust from sand and aggregates. 



(2) Reinforcing Steel Protection . The spalling of concrete in 

 bridge decks and marine structures, such as reinforced wharf decks, piles, 

 groins, and concrete anchors, has been a serious problem for many years. 

 The principal cause is corrosion of the reinforcing steel, which is largely 

 due to the use of deicing salts, exposure to seawater, or inadequately 

 embedded reinforcing steel (Fig. 16). The corrosion products produce an 

 expansive force which causes the concrete to spall out about the steel. A 

 minimum cover over the steel of 76 millimeters (3 inches) and use of a low- 

 permeability, air-entrained concrete will ensure good durability in the 

 great majority of cases, but more positive protection is needed for very 

 severe exposures. 



(a) Sufficient Cover. The concrete cover protects the steel 

 by creating a passive condition of high pH at the surface of the steel. Too 

 thin a cover allows carbonation, usually around the surface of the coarse 

 aggregate particles. Carbonation lowers the pH. Oxygen is necessary to 

 the corrosion mechanism; a thicker cover minimizes the movement of oxygen 



to the steel surface. In seawater, chloride ion movement is also inhibited 

 by thicker covers. The cover should properly be related to the density and 

 cement content. The exact relationships have not been thoroughly established, 

 so arbitrary values are usually used as guides or standards. Thicker 

 covers make it possible to achieve better compaction, fewer voids, and less 

 permeability. 



(b) Reinforcing Steel Coating . Concrete may not provide 

 permanent protection of reinforcing bars under many conditions. Cracks in 

 the concrete surface contribute to corrosion in providing access to moisture, 

 air and contaminants. Hydrated Portland cement is subject to chemical 

 reaction with carbon dioxide of the atmosphere. Carbonation reduces the 

 alkalinity of concrete thus reducing its effectiveness as a protecting 

 medium. Concretes will also deteriorate from other causes such as freeze- 

 thaw cycles, sulfate attack, reactive aggregates, or other causes; it will 

 crack or weaken and thus become less able to protect embedded reinforcing. 



105 



