High Strength Concrete: Coarse Aggregates
High Strength Concrete: Coarse Aggregates: The characteristics of coarse aggregates hold more effect in case of high strength concrete (HSC) compared to normal strength concrete, since HSC is significantly influenced by the interfacial transition zone.
Some basic properties of the aggregates are shape, maximum particle size, maximum nominal particle size, and how well the particles are graded.
In case of normal concretes, the paste is what governs the strength of the concrete mixtures. In other words, coarse aggregates have usually higher strength compared to the cement paste. Thus, the value of the strength associated with the aggregate is not very important to the overall mixture; it is the value of the cement paste strength.
In case of high strength concretes, this concept is not usually valid because in such concretes, we target a very high value of strength, thus, care should be given to the values of the strength of both the cement paste and the aggregate.
The shape and texture properties, in addition to the mineral composition of the aggregate, influence the interfacial zone between the paste and aggregate, and the stress level at which cracking in this zone starts to happen.
With respect to durability of HSC, the properties of coarse aggregate including hardness and absorption ratio are important.
Starting with the hardness, stiff aggregates can be of benefit in some cases and harmful in other cases. Numerous researches have shown that by employing coarse aggregates with a higher stiffness, the elastic modulus may be increased but on the other side, the strength capacity is decreased.
Treating HSC as a uniform/homogeneous material during the design process may lead to higher ultimate strength value. One way this can be achieved is by incorporating a paste and aggregates with similar elastic modulus.
Coarse aggregate with angular sides and rough texture gives higher bond with the cement paste, making them more appropriate for using in HSC, relative to gravel and aggregates with smooth surface. In some cases, if the strength requirements are not very strict, rounded aggregates may be sufficient. Also, a combination of crushed aggregate and smooth rounded type can be used to have an optimized strength of HSC.
Aggregates with smaller maximum size are usually chosen in the production of HSC.
The particles are prepared to have a cubic shape without much flat or elongated particles. Coarse aggregates that have > 20% of particles with length / thickness > 3:1, as per ASTM D 4791, should be disregarded from application and HSC.
Another important aspect is the cleanliness of the particles. Aggregates should be clean and do not contain harmful substances such as for example salts or organic material. Clean aggregate also assists in providing better bond between the cement paste and the aggregate particles.