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COARSE AGGREGATES IN HIGH STRENGTH CONCRETE
Given the critical role that the interfacial transition plays in high-strength concrete, the mechanical properties of coarse aggregate will have a more pronounced effect than they would in conventional-strength concrete. Important parameters of coarse aggregate are shape, texture, grading, cleanliness and nominal maximum size. In conventional-strength structural concretes, it is common for the aggregates to be stronger and stiffer than the paste, aggregate strength is usually not considered a critical factor; however, aggregate strength becomes increasingly important as target strength increases, particularly in the case of high strength lightweight aggregate concrete. Aggregate properties such as surface texture and mineralogy significantly affect the interfacial paste-aggregate bond and the level of stress at which interfacial cracking commence.
Durability properties notwithstanding, important coarse aggregate properties to consider includes strength, stiffness, bonding potential, and absorption. Caution should be exercised when using extremely stiff coarse aggregates, such as diabase or granite. Depending on the desired concrete properties, stiff aggregates can be either beneficial or detrimental. Several studies have found that using coarse aggregates with greater stiffness can increase the elastic modulus while at the same time decrease the strength capacity. Designing high strength concrete to act more like a homogeneous material could enhance ultimate strength potential. This can be achieved by increasing the similarity between the elastic modulii or coarse aggregate and paste.
As the target strength increases, the properties of aggregates as they relate to water-demand becomes less relevant and the properties that relate to interfacial bond become more important. Even though the water demand of smaller size coarse aggregates is higher, having greater surface area (and correspondingly greater interfacial bonding potential), smaller aggregates become more desirable as the target strength increases.
Rough textured and angular coarse aggregate provide greater mechanical bond and are generally more suitable for use in high strength concrete than smooth textured aggregates. With respect to mechanical properties, even though crushed aggregates usually outperform smooth textured aggregates, smooth textured aggregates should be summarily dismissed from consideration or restricted based on this characteristic alone. Depending on the required strength and other necessary properties, clean, well shaped locally available rounded aggregates might perform satisfactorily.
The crushing process eliminates potential zones of weakness within the parent rock, thereby making smaller sizes more likely to be stronger than larger ones. Smaller aggregate sizes are also considered to produce higher concrete strengths because of less sever concentrations of stress around the particles, which are caused by differences between the elastic module of the paste and the aggregate.
For high-strength concrete, aggregate particles should be generally cubical in shape and should not contain excessive amounts of flat and elongated pieces. Note that the flatness and the elongation are relative terms, and that the definitions vary by location. Coarse aggregates containing more than approximately 20% of the particles having ratios of length to circumscribed thickness greater than three to one, as determined by ASTM D 4791, should be avoided when making high-strength concrete. Aggregate particles should be clean and free of any materials that would degrade, such as organic matter, clay lumps and soft particles, or adhere to surface during mixing and impede interfacial transition zone bond. When finally divided materials (i.e. smaller than 75micron) such as clay, shale or excessive dust of fracture remain on the surface of aggregates after undergoing batching, mechanical bond at the interfacial transition zone decreases.
In the case of high-strength concrete, the effect of a weakened paste to aggregate bond can be extremely detrimental to strength. For this reason, use of clean, washed aggregate in the production of high-strength concrete is highly suggested. Coatings that impair paste-aggregate bond can be identified through petrographic examination of the suspect aggregate and frequently through petrographic examination of concrete produced with the suspect aggregate.
Aggregate binding is the process of intermixing two or more aggregates to produce an aggregate with different set of properties. It is not common industry practice to blend crushed and coarse aggregates, however, blending crushed cubically shaped and smooth naturally rounded coarse aggregates can be advantageous for optimizing the properties of high-strength concrete.
Fig: Effect of aggregate type and blend on mean 28 days compressive strength