Aggregate properties | Sieve Analysis | Gap graded aggregate by Dr K Mohan

Lecture-3: Aggregate properties, Sieve Analysis, Gap graded aggregate by Dr. K Mohan. Dr K Mohan is an internationally renowned and an acclaimed cement scientist. He is a former Director General of National Council for Cement and Building Materials (NCCBM). He holds a Ph.D. degree in Cement Chemistry from University of Aberdeen, Scotland, U.K. • This is the name given to the operation of dividing a sample of aggregate into various fractions each consisting of particles of the same size. • The sieve analysis is conducted to determine the particle size distribution in a sample of aggregate, which we call gradation. • The sieve analysis conducted to determine the particle size distribution in a sample of aggregate is called gradation. • A convenient system of expressing the gradation of aggregate is one in which the consecutive sieve openings are constantly doubled, such as 10 mm, 20 mm, 40 mm etc. • Under such a system, employing a logarithmic scale, lines can be spaced at equal intervals to represent the successive sizes. • The aggregate used for making concrete are normally of the maximum size 80 mm, 40 mm, 20 mm, 10 mm, 4.75 mm, 2.36 mm, 600 microns, 300 microns and 150 microns. • The aggregate fraction from 80 mm to 4.75 mm are termed as coarse aggregate and those fraction from 4.75 mm to 150 microns are termed as fine aggregate. The size 4.75 mm is a common fraction appearing both in coarse aggregate (C.A.) and fine aggregate (F.A.) • Grading pattern of a sample of C.A. or F.A. is assessed by sieving a sample successively through all the sieves mounted one over the other in order of size, with larger sieve on the top. • The material retained on each sieve after shaking, represents the fraction of aggregate coarser than the sieve in question and finer than the sieve above. Grading of Aggregate • The particle size distribution of an aggregate, as determined by sieve analysis, is termed as grading of aggregate. • If all the particles of an aggregate are of uniform size the compacted mass will contain more voids whereas an aggregate comprising particles of various sizes will give a mass containing lesser voids. • The particle size distribution of a mass of aggregate should be such that smaller particles fill the voids between larger particles. • Typical aggregate gradations are shown in Fig. • The proper grading of an aggregate produces dense concrete and needs less quantity of fine aggregate and cement paste. It is, therefore, essential that the coarse and fine aggregates be well graded to produce quality concrete. • Grading of an aggregate is expressed in terms of percentages by weight retained or passing through a series of sieves taken in order of; • 80 mm, 40 mm, 20 mm, 10 mm, and 4.75 mm for coarse aggregate. • 4.75 mm, 2.36 mm, 1.18 mm, 600 microns, 300 microns and 150 microns for fine aggregate. • The sieves are arranged in such an order that the square openings are half for each succeeding smaller size sieves. • The curve showing the cumulative percentages of the material passing the sieves represented on the ordinate with the sieve openings to the logarithmic scale represented on the abscissa is termed as grading curve. • The grading curve indicates whether the grading of a given sample conforms to that specified, or is too coarse or too fine or deficient in a particular size. • The grading of the aggregate affects the workability which, in turn, controls the water and cement requirements, segregation, and influences the placing and finishing of concrete. • The main factors governing the desired aggregate grading are: the surface area of aggregate, the relative volume occupied by the aggregate, the workability of the mix and the tendency to segregate. • The grading of fine aggregate has a much greater effect on workability of concrete than does the grading of the coarse aggregate. • Usually very coarse sand or very fine sand is unsatisfactory for concrete. • Fine grading conforming to the specifications laid by IS: 383 – 1970 shall be satisfactory for most concrete. #Aggregateproperties #SieveAnalysis #Gapgradedaggregate #cementpdm