9 Define the terms bulk dry specific gravity and apparent sp

9. Define the terms bulk dry specific gravity and apparent specific gravity. Discuss the differences between these two specific gravities. 10. Discuss what is meant by absorbed moisture in aggregate and how it is determined 11. Discuss the importance of grading of aggregates and describe the different types of grading.

Solution

9)Gs=bulk dry specific gravity

Gs is defined as the ratio of density of substance to the density of equal volume of standard substance at the same temperature.

Generally,water is used as a standard substance.

Gm=apparent specific gravity.

Apparent specific gravity is the ratio of the unit weight of a volume of the substance to the unit weight of an equal volume of the standard substance at same temperature.

difference between Gs & Gm

Gs is specific gravity for solid and is always higher the Gm and Gm in mass specific gravity,which is for whole sample.

10)absorbed moisture in agregate

Since agregates surfaces are rough and can absorb little moisture,which is very important to be taken care because while making design mix it may lead to imbalance,as there is already moisture is water and we need to add more for the proper design mix concrete so a moisture correction is used.

Moisture in aggregate can be easily determined by using oven dry procedure as follows

1)take the container

2)put the moisturized aggregate and weigh it

3)after weighing put container in over for drying

4)after it again weigh the container with dried aggregate

5)difference of 2)-4)= weight of moisture

11)Importance of grading

gradation of concrete helps to make workabe concrete, hence Proper gradation ensures that a sample of aggregates contains all standard fractions of aggregate in required proportion such that the sample contains minimum voids.well graded aggregate containing minimum voids will require minimum paste to fill up the voids in the aggregates. Minimum paste means less quantity of water and cement,leading to higher strength, lower shrinkage & greater durability.

Different types of grading

1)well graded aggregate-contains all size(fine to coarse) of agregates so that it has minimum voids.In gradation cure it is characterized by s shape curve.

2)poorly graded- it contains particles of almost same size so it is also called uniformly graded, In gradation curve it is represented by steep curve.

3)Gap graded-If some intermidiate sizes of aggregates are missing in well graded aggregate it is called Gap graded aggregate.In gradation curve it is reresent by straight line parallel to x axis in between the curved.