Civil Engineering Materials Lecture 5 (Aggregates Properties)

Specific sets of sieves are used for coarse and fine aggregates. Using larger aggregate particles can be more economical due to less binder (cement) consumption, as they have a smaller surface area. However, excessively large aggregates can lead to voids and reduced concrete strength, particularly in reinforced concrete where they might get stuck between rebar.

After sieve analysis, a semi-log graph is plotted with sieve size on the logarithmic x-axis and percentage passing on the linear y-axis. This graph helps evaluate the aggregate's distribution. An example details the calculation steps for percentage passing, starting from the amount retained on each sieve to cumulative amount retained and finally, percentage passing. The resulting curve is then used to classify different types of aggregate gradation, which will be discussed in the next lecture.

The video starts by reviewing the four moisture conditions of aggregates: bone dry, air dry, saturated surface dry (SSD), and moist. It then introduces specific gravity as a critical aggregate property for concrete mix design, noting that there are four types of specific gravity corresponding to the moisture conditions. Specific gravity is defined as the mass of the aggregate divided by the mass of an equal volume of water.

The lecture focuses on three main types of specific gravity: bulk dry, bulk saturated surface dry (SSD), and apparent specific gravity. These types differ based on how voids (permeable and impermeable) within the aggregate particles are considered. The determination of aggregate volume is challenging due to its nature, so water displacement methods are used, comparing the weight of aggregate in air versus in water. An example illustrates how specific gravity is calculated directly if aggregate volume is known, and indirectly using water displacement methods.

The process for determining the specific gravity of coarse aggregate involves using a basket to measure weight in water, achieving SSD condition, and drying in an oven for a constant weight. For fine aggregates, a pycnometer is used due to their small size, following a similar principle of measuring weight differences with and without water.

Bulk unit weight, essential for concrete mix design, is defined as the weight of aggregate filling a unit volume. This can be either loose or rodded. The test involves filling a container of known volume with aggregate and measuring its weight. From the bulk unit weight and specific gravity, the percentage of voids within the aggregate sample can be calculated, which is crucial for assessing the compaction and density of the aggregate.

Aggregate strength is generally a less important property in normal concrete as aggregate strength is typically higher than that of concrete. However, it becomes significant in high-strength concrete applications. Tensile strength of aggregates ranges from 0.7 to 16 MPa, and compressive strength from 35 to 350 MPa. The modulus of elasticity of aggregate is rarely measured.

Aggregate gradation, or particle size distribution, is a highly important property affecting concrete qualities like density. Sieve analysis is performed using a series of sieves arranged in descending order of opening size. A representative aggregate sample is shaken through the sieves, and the amount retained on each sieve is measured. This process helps determine the distribution of particle sizes.