Nature of Concrete 



Concrete is a complex composite consisting of Portland cement, aggre- 

 gate of varying sizes and types, and sufficient water to permit a series of 

 chemical reactions to occur. Concrete structures are usually strengthened 

 by embedding reinforcement in the composite. Reinforcements such as steel 

 rod (rebar) , wire and wire mesh, and various fiber materials increase its 

 tensile strength. Concrete, as it is usually made, is more or less porous 

 and permeable to moisture. Its resistance to penetration by moisture can 

 be much increased by choosing appropriate types of cement and aggregate 

 for its preparation and by the njethod of emplacement. Relatively short- 

 term protection against moisture and water absorption can be provided by 

 coatings applied to the outer surface of the concrete. 



The durability of concrete is defined as its long-term resistance 

 to deterioration and to corrosion of embedded steel reinforcement. Factors 

 affecting concrete durability have been identified by Browne, ^ Haynes and 

 Rail, ^ and Lorman.-*- Sulfate ions in seawater react with certain cements 

 to form calcium sulfo-aluminate with resultant crystal growth and concrete^ 

 disruption. Cements containing more than 8-percent tricalcium aluminate 

 (C3A) have poor resistance to sulfates in seawater. H ASTM Type V Portland 

 cement contains an allowable maximum of 8 percent by weight of C3A. 

 Although ASTM Type II Portland cement performs satisfactorily in seawater, 

 either ASTM Type V or a Pozzolanic cement is preferable for marine instal- 

 lations in tropical or semitropical regions. 



Another aspect, the water-cement ratio (W/C) is an important factor 

 in the durability of concrete. To ensure a low permeability the W/C should 

 not exceed 0.5 by weight." For example, with concretes having a W/C of 

 0.4 by weight and subject to a hydrostatic head of 1 ft, saturation to a 

 depth of 2 in. occurs in about 5 yr. 



Closely related to W/C is the cement content or weight of cement per 

 cubic yard of concrete. Concrete structures exposed to seawater should 

 be dense, impervious, relatively nonabsorbent , and have a minimum cement 

 content of 6 1/2 and a maximum of 7 1/2 bags per cubic yard (one bag 

 contains 94 lb of cement) . Another important factor affecting the dura- 

 bility of concrete is corrosion of embedded steel (rebar) reinforcement. 

 An excellent discussion on the corrosion of reinforcing bars in concrete 



