The Quality of Fine Aggregate Used in Concrete Construction

The fine aggregate that is used in concrete construction must be clean, free from lumps and organic material. Moreover, it should be strong and durable with tough floor. It must not absorb more than 5% of water and shouldn’t be soft and porous.

Many research institutes aim to discover and characterize alternate materials to replace river sand. However, they lack fundamental knowledge that is essential for field engineers to make a decision on selecting an alternate fine aggregate.

Particle Size

The particle size of fine aggregate is important because it determines how much the material weighs in relation to its volume. The size of the aggregate also affects its specific gravity and absorption rate. This is a very common test that can be performed by almost any lab technician.

To do this test, the aggregate sample is placed in a wire basket and immersed in water. When the aggregate has reached a surface-dry condition, it will slump slightly. This slump is the desired result and indicates that the gradation of the aggregate has been achieved.

There are several different sources of fine aggregates including river sand, surki, stone screenings, burnt clays, and cinders. Most fine aggregate is produced by drilling and blasting a mountainside, but it can also be mined from existing roads or crushed rock from the natural disintegration of larger stones. The gradation of fine aggregate is critical because it must contain enough void space for the cement and other materials to interlock, but not too many spaces that reduce the strength of the concrete. The angularity of the fine aggregate is also important because it gives the concrete more shear strength.

Specific Gravity

Specific gravity is a measure of the relative density of one substance with respect to that of another. For aggregates, this is often compared with water, which has a density of 1000 kg/m3.

The bulk specific gravity, apparent specific gravity and absorption of fine aggregate can be determined using the AASHTO T 84-13/ASTM C 128-13 test methods. These tests compare the mass of an oven-dry aggregate sample with the mass of a saturated surface dry (SSD) aggregate sample, including the volume of water permeable voids.

The test requires a volumetric flask and a pycnometer. The pycnometer should be clean and free of any entrapped air. A balance of capacity not less than 3kg is used to weigh the vessel containing the aggregate and the water. The resulting value is then divided by the specific gravity of the reference material. This is the specific gravity of the aggregate. It is then recorded. The procedure also involves transferring the sample from the pycnometer into a tray with care being taken to avoid removing any water that may have accumulated on the top of the meniscus.

Absorption Rate

The absorption rate of fine aggregate is related to its particle size. The smaller the particle size, the higher the absorption rate. However, it is important to note that the absorption rate also affects the surface texture of the aggregate.

The pycnometer is a highly accurate method for determining the specific gravity and absorption of fine aggregates. The pycnometer features a tamper and a conical mold that are designed for easy loading and removal of the sample. The tamper is equipped with threaded glass to ensure precise volume measurement. The pycnometer is compliant with AASHTO T84 and ASTM C 128, ensuring it meets industry standards.

The pycnometer tests the oven-dry (OD) and saturated surface dry (SSD) relative density of the fine aggregate by weight. The OD and SSD results can be used to determine the workability of the aggregate, as well as its water absorption. This is a crucial factor when assessing the quality of aggregates.

Surface Texture

Fine aggregate texture is affected by the surface roughness of the particles. Rounded particles have less contact area with the cement and will not bond well with it, while coarse, sharply textured aggregates will develop a good mechanical bond.

The coarse and fine particle morphology is also important in determining the workability of the mixture. A coarse, gap graded sand can lead to low cohesiveness and permeability while a fine aggregate with a uniform grading will provide high workability.

A common test for coarse aggregate morphology is the flakiness and elongation gauge, used by many state DOTs. This test uses a flat sheet metal piece with cut-outs to measure aggregate thicknesses for different size fractions.

AIMS, an image capture and detection system, uses a camera, variable magnification microscope, aggregate tray, and back- and top-lighting to evaluate the angularity, surface texture, and shape of fine aggregates. It also measures the loose uncompacted void content of an aggregate sample and indirectly characterizes its shape, angularity, and surface texture.



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