Testing of Aggregates for Concrete
1. Sampling of Aggregates for Testing
1. Sampling of aggregates - General
Quality control tests are routinely carried out on representative samples of aggregates to ensure that any variation in quality is within the specification limits. The need for representative samples cannot be over-stressed; otherwise the test results may misrepresent the characteristics of the aggregates. Sampling operations must therefore be conducted by such methods which ensure that the samples obtained are, as far as, possible, representative of the supply. The basic-unit of sampling is called the sample increment. It is a portion of materials taken directly from the conveyor, bin, truck or stockpile. Sample-increment should be taken in sets of five, each approximately equal quantity. A bulk-sample should comprise a set of five sample-increments, mixed thoroughly to give a uniform bulk sample; the bulk-sample may often be subdivided to produce a sample of appropriate size for testing. Further division may be carried out in the laboratory to produce the test portion for the use in a particular test. A referee-sample is that portion reserved for testing in the event of a dispute about test results.
2. Sample size and sampling frequency
Testing standard for the materials specifies the sample size for test as well as the sampling method and sampling frequency. Therefore, in actual testing at the site or production plant, it is necessary to follow the recommendation from standards in order to assess the quality and property of the materials tested.
3. Sample Reduction
When the amount of material has to be reduced it is essential that the representative nature of the sample be maintained. Suitable methods for sample division are by coning and quartering or by the use of sample divider.
2. TESTING FOR LOOSE BULK DENSITY
The loose bulk density, expressed in kg/m3, gives an indication of the volume occupied by a given mass of aggregate at a particular moisture content in the loose state. This information is required if aggregates are being measured by volume.
The purpose of this test is to determine the loose bulk density of fine, coarse or mixed aggregate, and the void content for each type of the aggregate mention earlier. Then determine the fine aggregate content obtaining from the data to create min void ratio, thus producing max bulk density.
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Bulk density is a property of particulate materials. It is the mass of many particles of the material divided by the volume they occupy. The volume includes the space between particles as well as the space inside the pores of individual particles. Bulk density is not an intrinsic property of a material; it can change depending on how the material is handled (loose or compacted).
Bulk Density (Db ) = Weight of aggregate (W) / Bulk Volume (Vb)
The particle density of a particulate solid or grain, is the density of the particles that make up the soil. The particle density is a relatively well-defined quantity, as it is not dependent on the degree of compaction of the solid, whereas the bulk density has different values depending on whether it is measured in the loose state or compacted state.
Particle density (Dp) = Weight og aggregate (W)/Solid volume of aggregates (Vs)
From the two above equations, we can conclude that:
Void content, in materials science, is defined as the volume of voids in a mixture divided by the volume of solids.
Void content (%) = 100 [Bulk Volume – Solid Volume] / Bulk Volume
As proven above.
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