Shape of aggregate, bulk density, voids, and moisture content of Aggregate | Concrete Technology

Lecture-4 Shape of aggregate, bulk density, voids, and moisture content of Aggregate

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

Shape of Aggregate:

(a) Flakiness Index:
• The flakiness index of aggregate is the percentage by weight of particles in it whose least dimension (thickness) is less than three-fifths of their mean dimension.
• The flakiness index is not applicable to sizes smaller than 6.3 mm.
• The flakiness index is taken as the total weight of the material passing the various thickness gauges expressed as a percentage of the total weight of the sample taken.
(b) Elongation Index:
• The elongation index on an aggregate is the percentage by weight of particles whose greatest dimension (length) is greater than 1.8 times their mean dimension.
• The elongation index is not applicable to sizes smaller than 6.3 mm.
• The elongation index is the total weight of the material retained on the various length gauges expressed as a percentage of the total weight of the sample gauged.

Specific Gravity:
• Specific gravity of an aggregate is defined as the ratio of the mass of solid in a given volume of sample to the mass of an equal volume of water at the same temperature.
• Since the aggregate generally contains voids, there are different types of specific gravities.
• The absolute specific gravity refers to the volume of solid material excluding the voids and therefore, is defined as the ratio of the mass of solid to the weight of an equal void – free volume of water at a stated temperature.
• If the volume of aggregate includes the voids, the resulting specific gravity is called apparent / bulk specific gravity.
• It is therefore, the ratio of the mass of the aggregate dried in an oven at 100 to 110oc for 24 hours to the mass of the water occupying a volume equal to that of solids including voids or pores.
• Specific gravity of major aggregates falls within the range of 2.6 to 2.9.
• It is not a measure of aggregate quality. It is used for certain computations in a mix design.
Bulk Density:
• The bulk density of an aggregate is defined as the mass of the material in a given volume and is expressed in kg/ litre.
• Bulk density of aggregates depends upon three factors: (i) Degree of compaction, (ii) Grading of aggregates and (iii) Shape of aggregate particles
Voids:
• The empty spaces between the aggregate particles are termed voids.
• It is the difference between the gross volume of aggregate mass and the volume occupied by the particle alone.
Porosity and Absorption of Aggregates:
• The entrapped air bubbles in the rocks during their formation lead to minute holes or cavities, which are called as pores.
• The porosity of rocks is less than 20%.
• The porosity of aggregate contributes to porosity of concrete and it becomes permeable and affects bond between the aggregate and the cement paste.
• The porous aggregate absorbs more moisture, resulting in loss of workability.

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[Tech_and_nature_amit]

Dear sir,
I want to know about pondash and flyash.
We are normally use in PPC
Flyash 29%and pondash 6%.
So I want to know about what is effect in strength. If I am reduce flyash 25% and increase pondash 10%. so what is empact in strength. 1Day.