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Fineness Modulus of Aggregate and Deleterious substances in Aggregate Concrete technology
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Lecture-5: Fineness Modulus of Aggregate and Deleterious substances in Aggregate | Concrete technology 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. • Fineness Modulus: • • The Fineness Modulus (FM) is an index of the fineness of the aggregate. • • It gives some idea of the mean size of the particles present in the entire body of the aggregate. • • (Fineness modulus of an aggregate is roughly proportional to the average size of particles of aggregate). • • Fineness modulus is an empirical factor obtained by adding the cumulative percentages of aggregate retained on each of the standard sieves ranging from 80 mm to 150 microns and dividing this sum by 100. • • (The sum of the cumulative percentages retained on each of the standard sieves ranging from 80 mm to 150 microns divided by 100 give the fineness modulus). • • Cumulative % of aggregate retained on each sieve specified up to 150 microns • • FM = _________________________________________________________________ 100 • • The value of fineness modulus is higher for coarser aggregate. • • (The higher the FM, the coarser the aggregate). • • The FM of fine aggregate varies between 2.0 and 3.5, for coarse aggregate it varies between 5.5 and 8.0. • • The FM of different type of sand is given below; • Fine Sand 2.2 – 2.6 • Medium Sand 2.6 – 2.9 • Coarse Sand 2.9 – 3.2 • • Generally, sand with FM 3.2 is not suitable for making good concrete. • • Fineness modulus of fine aggregate is required for concrete mix design since sand gradation has the largest effect on workability. FM of coarse aggregate is not required for concrete mix design purposes. • Deleterious Substances in Aggregates • • Deleterious materials are harmful materials whose presence may adversely affect the strength, workability and long-term performance of concrete. • • Deleterious materials can be divided into three categories: • Impurities which interfere with the hydration of cement such as organic matters. Common examples are decayed vegetables in the form of humus or organic loam. They are more likely found in fine aggregate. • Coatings which prevent development of good bond between aggregate and cement paste such as clay and other fine materials such as silt and crusher dust. • Unsound particles which are weak or bring about chemical reaction between the aggregate and cement paste. Shale, clay lumps, wood, coal, lignite etc. are regarded as unsound particles. These particles lead to pitting and scaling. The presence of mica in fine aggregate has found to considerably reduce the compressive strength of concrete. Iron pyrites and marcasite produce surface staining and pop – outs. • • The total amount of deleterious material should not exceed 5 % as per IS: 383 – 1970. • #FinenessModulus #Deleterioussubstances #Aggregate #Concretetechnology
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