There are many other cements used for special purposes, such as expan- 

 sive cements, which cause the concrete to expand after setting (not used 

 for marine concrete), waterproof Portland cement, plastic cement (used for 

 making stucco and plaster], oil well cement, and special-blended custom and 

 polymer cements. 



3. Properties . 



The basic properties of concrete are placeability, consistency, strength, 

 durability and density. These properties will vary depending on the 

 specific components and ratios of components added during mixing. Well- 

 established relationships governing these properties are discussed below. 



a. Placeability . Placeability (including satisfactory finishing 

 properties) encompasses traits loosely accumulated in the terms "worka- 

 bility" and "consistency." Workability is considered to be that property 

 of concrete which determines its capacity to be placed and consolidated 

 properly and to be finished without harmful segregation. It is affected by 

 the grading, particle shape, and proportions of aggregate, the amount of 

 cement, the presence of entrained air, admixtures, and the consistency of 

 the mixture. These factors are to be taken into account to achieve satis- 

 factory placeability economically. 



b. Consistency . Consistency, loosely defined, is the wetness of the 

 concrete mixture. It is measured in terms of slump--e.g., the higher the 

 slump the wetter the mixture--and it affects the ease with which the 

 concrete will flow during placement. In properly proportioned concrete, 

 the unit water content required to produce a given slump will depend on 

 several factors. Water requirement increases as aggregates become more 

 angular and rough textured (but this disadvantage may be offset by improve- 

 ments in other characteristics such as bond to cement paste) . Required 

 mixing water decreases as the maximum size of the well-graded aggregate is 

 increased. It also decreases with the entrainment of air. Mixing water 

 requirement may often be significantly reduced by certain admixtures. 



c. Strength . Strength is an important characteristic of concrete, but 

 other characteristics such as durability, permeability, and wear resistance 

 are often equally or more important. For a given set of materials and 

 conditions, concrete strength is determined by the net quantity of water 

 used per unit quantity of cement. The net water content excludes water 

 absorbed by the aggregates. Differences in strength for a given water- 

 cement ratio may result from changes in maximum size of aggregate; grading, 

 surface texture, shape, strength, and stiffness of aggregate particles; 

 differences in cement types and sources; air content; and the use of 

 admixtures which affect the cement hydration process or develop cementitious 

 properties themselves. However, in view of their number and complexity, 

 accurate predictions of strength must be based on trial batches or experience 

 with the materials to be used. 



d. Durability . The ability of concrete to withstand environmental 

 exposure is called durability. Concrete must be able to endure those 

 exposures which may deprive it of its serviceability--e. g., freezing and 

 thawing, wetting and drying, heating and cooling, chemicals, and deicing 

 agents. Use of a low water-cement ratio will prolong the life of concrete 



72 



