Importance of Specific Gravity of Aggregates in Concrete Mix Design

Specific Gravity is defined as the ratio of the Weight of Aggregate to the Weight of an equal volume of water. The specific gravity of an aggregate is considered a measure of the material's strength or quality. 

The specific gravity of aggregates is mainly influenced by the size, shape, and pore structure of the aggregate. According to ACI (American Concrete Institute) standards, the specific gravity of coarse and fine aggregates used for construction should be within the following ranges: 

• Coarse Aggregate: The specific gravity should be between 2.5 and 3.0. If the specific gravity is not known, it can be determined by the displacement method, ASTM C127
• Fine Aggregate: The specific gravity should be between 2.6 and 2.7. If the specific gravity is not known, it can be determined by the pyknometer method, ASTM C128.

Aggregates with low specific gravity are generally weaker than those with high specific gravity. This property helps in the general identification of aggregates. 

In this article, we will discuss how the specific gravity of aggregates affects the volume of concrete, density, and compressive strength and why it is important to be determined. 

Specific Gravity Affects the Compressive Strength of Concrete

Aggregates with higher specific gravity imply higher strength. Higher dense concrete implies higher strength concrete. The density of the concrete is also affected by the specific gravity of the aggregates, with higher specific gravity implying higher strength and density. This, in turn, leads to higher-strength concrete, which is essential for supporting the weight of buildings and infrastructure.

Here are some ways in which aggregate specific gravity can affect the compressive strength of concrete:

• Water-Cement Ratio: The specific gravity of aggregates can affect the water-cement ratio of the concrete. Aggregates with higher specific gravity require less water for a given workability, leading to a lower water-cement ratio. A lower water-cement ratio typically results in higher compressive strength.
• Workability: The specific gravity of aggregates can also affect the workability of concrete. Aggregates with higher specific gravity can result in a stiffer mix, which can be harder to place and finish. This can lead to a reduction in the overall strength of the concrete due to issues with compaction and consolidation.
• Porosity: The specific gravity of aggregates can also affect the porosity of concrete. Aggregates with lower specific gravity typically have higher porosity, which can result in more water absorption and a higher water-cement ratio. This can lead to a reduction in compressive strength due to decreased density and increased voids in the concrete.
• Particle Shape: The specific gravity of aggregates can also affect the shape of the aggregate particles, which can in turn affect the compressive strength of the concrete. Angular and rough-textured aggregates with higher specific gravity can provide greater interlocking and friction between particles, resulting in higher compressive strength.

The variation of compressive strength with different values of specific gravities of coarse aggregates is given in the figure below. When the specific gravity of the coarse aggregate was the highest, the compressive strength increased

Variation of the compressive strength with cement content and specific gravity of Coarse Aggregate
Ref: Investigation of the effect of mineral additives on concrete strength using ANN Majid Al‑Gburi1  · Salim A. Yusuf, Asian Journal of Civil Engineering (2022)

The Table below shows the variation of compressive strength of normally vibrated concrete (NVC), and different types of self-compacting concrete (SCC) which includes, low volume fly ash concrete (LVF – SCC) and high-volume fly ash concrete (HVF – SCC) with different aggregate size. 

Table: Variation of the strength of concrete with various aggregate sizes
Image Credits: ResearchGate

Specific Gravity of Aggregates Affects Concrete Volume

The specific gravity of the aggregates planned to be used in the concrete mix is determined to study the absolute volume of aggregates that will occupy the concrete mixture. The absolute volume of aggregates refers to the space occupied by the aggregate particles, including solid matter and internal aggregate pores, but excluding the voids between particles. 

When aggregates with different specific gravities are combined in a concrete mixture, the volume of the mixture can change. This can result in either the purchaser receiving less concrete than ordered or the producer supplying more concrete than purchased, which can lead to issues with the strength and durability of the final product.

Determination of Deleterious Materials in Concrete

Some deleterious materials are lighter than good aggregates. Hence, tracking the specific gravity can sometimes indicate a change of material or possible contamination. 

The difference in specific gravity can be used during the production to separate the deleterious material from the good using a heavy media liquid. 

Determination of Percentage voids and Solid Volume of Aggregates

The percentage of voids and solid volume of aggregates can be determined using the following formula:

na = (γd - G x γw)/(G x γw - w)

where:

na = percentage voids

γd = dry unit weight of aggregates

G = specific gravity of aggregates

γw = unit weight of water

w = water content

The percentage of voids and solid volume of aggregates can affect the strength and durability of the concrete. 

In conclusion, the specific gravity of aggregates plays a critical role in ensuring the strength and durability of concrete, which is essential for supporting the weight of buildings and infrastructure. By carefully selecting aggregates with the right properties, and using specific gravity as a guide, engineers and builders can ensure that concrete structures are safe, durable, and long-lasting.

Also Read: Determination of Specific Gravity/Relative density of Coarse Aggregates- ASTM Standards