Aggregate is the name given to pieces of inorganic material with various mineral structures, such as sand, gravel and crushed rock that is then combined with water and cement to make concrete, mortar, etc. Approximately 75% of the volume of a cement system consists of aggregate. Aggregates are cheap ingredients in the creation of a cement system. Their characteristics reduce the cost of the mixture. Aggregate makes important contributions to the technical characteristics of cement systems.
What are the properties of aggregate?
Cement paste is a substance that dries out over time and has a contracting effect. Aggregate prevents the change in volume that might occur due to contraction in the cement system. This also prevents cracks.
Aggregates are usually strong and hard materials, so they contribute to the strength of the mixture.
They may also give the mixture other properties depending on gradation, size of the largest grains, water absorption capacity, apparent specific gravity and specific gravity, the shape of the grains, the type and amount of foreign matter in the surface properties, resistance to freezing and thermal properties. Based on these properties, they can be used in different mixtures.
Figure 1. Different types of aggregate
Aggregates can be classified by source:
- Natural and artificial
- By apparent specific gravity and specific gravity: normal, light and heavy
- By grain size: fine versus coarse
- By the shape of the grains: round, angular, flat and long
- By chemical properties: reactive, non-reactive
Normal, Light and Heavy Aggregates
Normal aggregates are any aggregate with an apparent specific gravity between 2000 kg/m3 and 3000 kg/m3 for grains oven dried in accordance with EN 1097-6.
Light aggregates are mineral based aggregates with an apparent specific gravity less than 2000 kg/m3 for grains oven dried in accordance with EN 1097-6, or an apparent specific gravity less than 1200 kg/m3 in bulk when oven dried according to EN 1097-3.
Heavy aggregates are any aggregate with an apparent specific gravity greater than 3000 kg/m3for grains oven dried in accordance with EN 1097-6.
Natural and Artificial Aggregates
Natural aggregates are those taken from riverbeds, old glacier beds, lake shores and stone quarries which have not been processed in any way apart from breaking, washing and grading. An example of this is crushed rock aggregate.
Artificial aggregates are those that are manufactured for decorative purposes or with specific characteristics, or those that are a by-product of another sector. Examples of manufactured aggregates include expanded clay or expanded perlite.
Figure 2. Bulk aggregate
How Aggregates Affects Concrete Strength
Aggregates are imperfect materials used in cement systems and mortar blends. According to standards, the size of the largest aggregate cannot be more than 3 times that of the smallest aggregate. The percentage of the total mixture should not exceed 10-15%. The geometric shape of the aggregate affects the workability parameters of the concrete or mortar because it can change the amount of cement paste used in the mixture.
The surface area ratio with aggregate consisting of round or cubic pieces is less than that of flat or long aggregate, and therefore requires less cement paste to achieve the same degree of workability. The surface area to volume ratio with flat or long aggregate is significantly greater than that of angular pieces. As a result, a lot more cement paste is required to cover the surface of the aggregate pieces. Different sizes of aggregate in the sample will reduce the space between the aggregate in a specific volume. When making calculations for concrete and mortar production, the main goals are for the mixture to exceed the minimum level of strength and to have good workability characteristics.
The distribution of aggregate pieces has a direct effect on the workability of fresh concrete. Without the proper distribution of pieces in a mixture, it becomes necessary to use more water to achieve workability, and the result is a failure to achieve the performance expected from the mixture. As the diameter of the aggregate pieces grows, the amount of water, fine aggregate and trapped air decreases. As a result, using less cement decreases the amount of contraction in the cement system, and the strength of the cement system increases because it needs less water.
Prior to creating the concrete and mortar mixtures, the quality parameters of the aggregate must be checked for suitability. The aggregates that are used for laboratory tests must be a true representation of the aggregates that will be used in the project. The methods that must be used when taking aggregate samples must be conducted as outlined in TS EN 932-1, ASTM C 33 and TS 707.
Key Consideration in Aggregate Experiments
- The amount of aggregate required for the test methods to be applied in the laboratory setting should be reduced in a way that represents the true aggregate bulk.
- The amounts required for tests should be determined using the quartering method or a riffle splitter.
- After the amounts required for the test are obtained, tests may be conducted, including those for the physical characteristics of the aggregate.
- A sieve analysis must be conducted for the aggregate that is prepared for concrete or mortar mixture to observe whether or not granulometry is appropriate.
- Aggregate gradation is determined with sieve analysis. Gradation is determined using the amount obtained by passing the aggregate over the square-hole sieve used in Turkish standards.
- Gradation curve graphs can be helpful when one or more aggregates will be combined in a mixture.
- The ratio of large to small aggregate can be determined using these graphs so that the amount to use is clear. These ratios should be adjusted by taking into consideration the cement system’s workability, segregation and the surface areas in the unit volume.
Figure 3. Representative sieve sizes
The granulometric acceptance criteria for cement systems or mortar mixtures are as follows: It must not cause segregation of large and small pieces in the tumbling, transport and placement of fresh concrete. Furthermore, it should be homogeneous, achieve the desired level of workability and density. The distribution of aggregate size is what achieves these aims.
The best method for determining whether or not an aggregate gradation is suitable is to create a sample of the cement system using an aggregate with the same gradation and to measure the performance criteria of this cement system. The distribution limits for the pieces with the largest size are provided by the Turkish Standard Institute.
Graphic 1. Limits for Dmax 8 mm.
Graphic 2. Limits for Dmax 16 mm.
Graphic 3. Limits for Dmax 31.5 mm.
Proper limits must be established by determining the aggregate usage ratios based on the provided limit values. The cement system mixture should be created and checked to see whether or not it conforms to the characteristics required of the mixture. No matter how big the largest size in an aggregate mixture is, the surface areas of the pieces that comprise that aggregate mixture will be small than the total as long as it conforms to the limit values.
The Importance of Aggregate
Aggregate is just as important as the cement and additives used in the preparation of cement-based products. The right aggregate must be selected for proper concrete and mortar design. In this article, we have discussed different types of aggregate and methods for sampling bulk aggregate. Once the proper aggregate has been selected, the next issue of importance is the distribution of grain size in the aggregate. To achieve maximum performance in concrete and mortar, it is critical that the size of the aggregate pieces be evenly distributed.