460 rhoades. CONCRETE AGGREGATE [Ch. 24 



Durability of concrete is controlled by the quality and gradation of the 

 aggregate; the quality, composition, and fineness of the cement; the mix 

 proportions; the methods of mixing, handling, placing, and curing; and the 

 physical and chemical conditions that affect the concrete throughout its life. 

 Aggregates which are physically unsound or chemically unstable, so that the 

 particles disintegrate or change volume inordinately, can cause failure of 

 concrete in service. Unsoundness or chemical instability of aggregate particles 

 may be inherent to the constituent minerals or rock substances, or they may 

 have been engendered by secondary processes of alteration. 



Elasticity 



Young's modulus of elasticity of rock commonly used for aggregate ranges 

 from 2 X 10 6 to 15 X 10 6 . Young's modulus of elasticity of hydrated neat 

 cement paste ranges from about 1 X 10 6 to 4 X 10 6 as the load is sustained or 

 rapidly applied and relieved. Although high elasticity is beneficial in particles 

 used for ballast or road metal, aggregates of lower elasticity tend to reduce 

 stress in concrete resulting from volume change or strain. 



Particle Shape 



Angular particles make "harsh" concrete that is difficultly workable; 

 rounded particles contribute to smooth, workable mixes. High proportions of 

 flat or elongated particles also decrease workability and hence necessitate the 

 use of more sand, cement, and water to produce satisfactory concrete. Also, 

 they pack poorly, thus reducing bulk weight and decreasing compressive 

 strength. Moreover, flat particles tend to orient themselves horizontally in 

 concrete, permitting the accumulation of water beneath them, a condition that 

 prevents development of good bond on their lower surfaces. 



Maximum and Minimum Gradation 



The gradation of an aggregate is its particle-size distribution and is con- 

 veniently expressed as the percentage of material retained on standard screens. 

 The grading of concrete aggregate has very pronounced influence on the 

 workability of concrete and on the proportion of cement and water needed 

 to produce concrete of desired quality. Because of economic considerations, 

 variation in particle shape and texture, variation in different brands of 

 cement, and variation in size and design of structures, no one gradation is 

 satisfactory for all purposes. However, to be suitable for concrete manu- 

 facture, aggregate gradation must fall within certain limits in order to achieve 

 satisfactory packing of the particles. In practice, the gradation of aggregate 

 is controlled by specifying the maximum and minimum quantities that can 

 be retained on screens of various sizes of mesh. The grading of aggregate is 

 so important that, when natural grading of an aggregate does not fall within 

 specification limits, screening into size fractions and recombining to produce 

 a satisfactory grading are justified. 



