Specific Gravity of Aggregate

Specific gravity of aggregate is the ratio of the density of aggregates to the density of distilled water at a given temperature. It depends on whether the aggregate's pores are permeable or impermeable to water. This property is essential for understanding aggregate behavior in concrete mixtures and is differentiated into three distinct types based on how voids are considered in the calculation.

Bulk or gross specific gravity uses the saturated surface-dry mass divided by total volume including solids and voids. Apparent specific gravity uses dry mass divided by volume including only impermeable voids. Absolute specific gravity uses dry mass divided by volume excluding all voids. Each type accounts for porosity differently, making them suitable for different applications in concrete design.

Bulk specific gravity considers the saturated surface-dry state and includes all voids in the total volume calculation. Apparent specific gravity uses only the dry mass and accounts for impermeable voids only. The key difference is that bulk accounts for water-filled permeable pores, while apparent excludes them, making apparent specific gravity more practical for concrete mixture design.

Apparent specific gravity is crucial because natural aggregates are inherently porous, and it accounts for the total volume occupied by aggregate particles including their pores. When preparing concrete mixtures, this measurement reflects how aggregates actually behave in the mix. The apparent specific gravity for most commonly used rocks ranges between 2.60 and 2.70, with granite at approximately 2.69.

Fine aggregates are measured using a pycnometer, a precision instrument that determines volume by water displacement. Coarse aggregates are measured using the wire-basket method, which submerges the aggregate in water to calculate volume. These different methods account for the varying sizes and handling requirements of fine and coarse aggregate particles.

Natural aggregates typically have apparent specific gravity values ranging from 2.60 to 2.70. Specific examples include granite at approximately 2.69, sandstone at 2.65, and dense limestone at 2.60. These values reflect the mineral composition and porosity characteristics of each rock type used in concrete construction.

Aggregate porosity determines whether pores are permeable or impermeable to water, which directly influences which specific gravity type applies. Permeable pores affect bulk specific gravity calculations, while impermeable pores are excluded from apparent specific gravity. Understanding porosity and absorption of aggregate is essential for selecting the correct specific gravity measurement for design purposes.