In most cases, aggregates perform very well in Portland cement concrete. Aggregates, because they occupy about 60 to 80% of the total volume of concrete, affect many key fresh and hardened concrete properties and also contribute to the long-term performance of the concrete structure, including its long-term durability and resistance to cracking. Aggregates also allow for the overall cost of concrete to remain lower, thereby making concrete an economic- ally competitive product. Given the importance of aggregates in concrete, it is essential to understand the general aggregate properties needed to ensure quality concrete and to also be aware of potential durability-related issues. Because the emphasis of this chapter and book is durability, only a brief coverage of general requirements is provided. For a more detailed discussion of general aggregate properties, various references are available (Mehta and Monteiro, 1993; Neville, 1996; Kosmatka et al., 2002). This section will briefly discuss general require- ments for aggregates, with an emphasis on how these aggregate properties may impact long-term durability.
Aggregates affect the fresh, hardened, and durability characteristics of concrete. Their impact is quite direct, even at early ages, in some cases (e.g., thermal conductivity, elastic modulus) and less direct or more long-term for other cases (e.g., AAR).
Physical characteristics of aggregates, such as gradation, absorption, and specific gravity are important properties for mixture proportioning and general quality control and assurance. They can also be important to durability in several ways. For example, for some aggregate types, absorption and specific gravity values can serve as useful indices for frost susceptibility (as discussed in detail in Section 7.3). Aggregate grading, shape, and texture have significant effects on water demand for fresh concrete, and any water added to the concrete to offset workability issues will increase the permeability of the concrete and exacerbate frost-related damage. The presence of microfines (or aggregate particles passing the #200 sieve) will particularly influence the water demand of concrete, especially if the microfines contain clay. Aggregates have a direct impact on the thermal properties of concrete, especially thermal conductivity, specific heat, and coefficient of thermal expansion (CTE). The thermal conductivity and specific heat values of aggregates play a key role in early-age mass concrete elements as these values will affect directly the heat transfer process. The CTE affects both the early-age behavior and also the long-term thermal stability of concrete (in terms of effects on daily thermal strains, curling, warping, etc.). Although aggregates mainly affect concrete shrinkage and creep indirectly by providing internal restraint to these causes of volume change, some fine-grained aggregates can, themselves, undergo shrinkage upon drying (Meininger, 1998).
The mechanical properties of aggregates, such as strength, elastic modulus, abrasion (and wet attrition) resistance, and resistance to polishing can all have some impact on durability. The strength of aggregates can have a somewhat minor effect on concrete of relatively low strength (say 20 MPa or less) because the bond strength is typically weak, with failure around aggregates. For denser, stronger concrete mixtures (e.g., over 50 MPa), the aggregate strength maybecome more important as the interfacial transition zone (ITZ) is so high in strength and low in porosity that failure can occur through the coarse aggregates. The elastic modulus of aggregates plays a significant role in early-age proper- ties, especially with regard to cracking potential, and in long-term properties, such as deflections. Because coarse aggregates play such a key role in concrete stiffness, measuring the elastic modulus of concrete containing a subject aggre- gate is a good surrogate for aggregate stiffness determination. The resistance of aggregates to abrasion becomes important when heavy abrasion to the concrete surface is anticipated (e.g. floors of industrial warehouses or surfaces exposed to studded tires, etc.). Abrasion resistance may also be a factor in the breakdown of aggregates during handling, mixing, and placing concrete. Dry abrasion tests, like the Los Angeles (LA) abrasion test (AASHTO T 96), involve rotating aggregates in a test vessel with steel balls as charges. The trend in recent years has been towards wet attrition tests, such as the Micro-Deval test (CSA A23.2- 23A, AASHTO T 327), which tend to better represent the type of distress observed in field concrete. The resistance of an aggregate to polishing is important, especially for pavements and other types of flatwork. Typically, the top layer of concrete is mortar-rich, where the polish resistance of the fine aggregate tends to govern behavior. Although methods exist to measure polishing of aggregates or concrete containing the subject aggregates, they are not commonly available in many locations. Tests that indirectly assess polish resistance, such as the acid insoluble test (ASTM D 3042), focus on the composition of the aggregate and its inherent link to polish resistance. The acid insoluble test estimates the amount of non-carbonate (usually siliceous) material in the aggregate sample; this information can be useful because, generally, the higher the acid insoluble fraction, the higher the polish resistance, and hence, the better the skid resistance on pavements (Meininger, 1998). Generally, limestone sands tend to result in more polishing in the cover concrete than siliceous sands, and some agencies and owners limit the amount of carbonates in sands.
The chemical or mineralogical properties of aggregates can significantly affect concrete performance and long-term durability. The effects of mineralogy, aggregate composition, and pore structure are especially related to alkali- aggregate reactivity (Sections 7.5±7.8) and frost susceptibility (Section 7.3), and the presence of impurities and harmful constituents and their effect on durability are discussed in Section 7.4.
