Tests on Aggregates used in Concrete Production

Aggregate is a material in granular or particle form, such as sand or gravel which is added to the class of materials known as binder (eg cement, hydraulic limes, plasters and bitumen) to produce a solid mass on hardening. Since most aggregates are inert and undergo no chemical action with the binder, the strength of the combined mass depends on:

1. The specific adhesion or bond which develops between aggregates and binder.
2. The mechanical key or interlock which develops between the constituent particles in virtue of their shape, size and surface texture.
3. The strengths of the aggregate and binder respectively.

Reasons for Mixing Aggregate with Binder

1. To reduce the material costs, using the aggregate as an extender (where the aggregate is less costly than the binder).
2. To offset either the drying shrinkage or setting expansion of the binder.
3. To obtain increased or reduced density (by using high or low aggregates respectively).
4. To alter appearance, such as colour or texture.
5. To obtain better resistance to wear by abrasion or weathering (using hard, abrasion-resistant or non-absorbent aggregates).
6. To import some other special property such as fire resistance, thermal insulation or acoustic characteristics.

Aggregates may be classified as fine or coarse, natural or artificial, dense or lightweight depending on the requirement of the user. Natural aggregates used for building should be hard, durable, clean and free of any harmful matter to an extent which would adversely affect the hardening of the binder, or the strength and durability of the finished material in which they are put. Aggregates for reinforced concrete work should not contain substances which will attack the reinforcement.

Tests on the aggregates depends on whether the aggregates is fine or coarse aggregate.

Fine Aggregates

For fine aggregates, the following tests apply.

1. Soundness: this is measured by their resistance to deterioration under the action of solutions of sodium or magnesium sulphate or under freezing and thawing.
2. Sand equivalent tests: this test is used to determine the presence of undesirable quantities of clay like materials in the sand. A method developed by F.N. Hveem of the California department of transportation is commonly used for this. Permissible average value from the 1978 California specification for concrete sand is 76%.
3. Organic impurities test: this test is used to check the quantity of organic impurities in sand as organic impurities can affect the hardening of the concrete. A field test, caustic soda test is commonly used for it.
4. Fineness modulus test: this test is usually a follow up of grading test. It is used to express the grading test as a single figure by adding together the percentage by weight of material retained on each of the test sieves and dividing the value by 100. The figure allows a comparison of aggregates for their relative overall fineness, and is useful in routine checks on the grading of successive deliveries of aggregate from a particular source.
5. Bulking test: bulking is the behaviour of sand such that it can occupy greater volume when damp than when dry and this is factored when measuring sand by volume. The extent of bulking varies with the moisture content of the sand, but is governed by the film thickness of the water around its particles, and the particle sizes (grading).

6. Silt content tests: this is a test that gives guide to the percentage of silt, clay and fine dust in natural sands. Laboratory methods used here are the decantation test which determines the amount of sample to pass 0.075mm sieve and a sedimentation test that gives the amount of material less than 0.02mm particle size.

Coarse Aggregates

Tests for coarse aggregate include: gradation tests, water absorption tests, bulk specific gravity tests, flakiness index test, elongation index test, los Angeles Abrasion tests, Aggregate crushing test, aggregate impact value test.

1. Gradation test: this test also known as sieve analysis is the test used to assess the particle size distribution of a granular material by allowing the material to pass through series of sieves where the topmost sieve have the largest aperture while the bottommost sieve have the least aperture.
2. Water absorption test: this test is usually combined with that of bulk specific gravity of aggregate.

3. Flakiness index: this test is usually done to get the percentage by weight of aggregate particles whose least dimension is less than 0.6 of their mean dimensions.

4. Elongation index: this is the percentage of weight of particles in a sample with a length exceeding 1.8 times their mean size.
5. Angularity number: this test relates the shape of single sized aggregate particles to spherical particles of same mean size, by measuring the extent to which they occupy the space inside a cylindrical container into which they are packed.

6. Los Angeles Abrasion test: the test was developed for rigid pavement and is also used for measuring the abrasion resistance of aggregates to wear. Aggregate particle size of given range is set in a small tray, are braided by sand in a special machine. At the end of a specified period (about 30 – 33 revolutions per minute for 500 – 1000 revolutions), the loss in weight is determined and expressed as a percentage of the original sample weight. Suitable values are between 35% and 50%
7. Aggregate crushing value: this test is useful when dealing with aggregates of unknown performance in order to determine the value of rock for crushed aggregate. The samples are crushed in a compression testing machine by the application of compressive load of about 40 tonnes gradually through 10 minutes times. The material passing through 2.36 mm is separated. The weight of this material (now fines) expressed as a percentage of the weight of the total sample gives the crushing value. A value less than 10% signifies strong aggregate while more than 35% signifies weak aggregate.
8. Aggregate impact value test: this test measures the aggregate resistance to impact load, as distinct from a gradually applied load. The test is conducted by dropping a hammer weighing 14 kg through 380 mm height onto a weighed amount of oven-dried material (10 mm – 12.5 mm) packed into a steel cup. After 15 blows, the weight of the amount crushed material finer than 2.36 mm sieve is determined and expressed as a percentage of the sample weight. For low-grade aggregate, a maximum of 50% wet aggregate impact value is allowed when used in sub-base. When used as base course, the limit is 40%. For surfacing courses, the limit is 30%.
9. Polished stone value: this test allows a relative assessment of the extent to which different aggregates will polish and become slippery under traffic.