EFFECTS OF AGGREGATE SIZES ON THE CONCRETE STRENGTH

EFFECTS OF AGGREGATE SIZES ON THE CONCRETE STRENGTH

It is well recognized that coarse aggregate plays an important role in concrete. Coarse aggregate typically occupies over one-third of the volume of concrete, and research indicates that changes in coarse aggregate can change the strength and fracture properties of concrete. To predict the behavior of concrete under general loading requires an understanding of the effects of aggregate type, aggregate size, and aggregate content. This understanding can only be gained through extensive testing and observation.  There is much controversy concerning the effects of coarse aggregate size on concrete, principally about the effects on fracture energy. Some research (Strange and Bryant 1979, Nallathambi, Karihaloo, and Heaton 1984) has shown that there is an increase in fracture toughness with an increase in aggregate size. However, Gettu and Shah (1994) have stated that, in some high-strength concretes where the coarse aggregates rupture during fracture, size is not expected to influence the fracture parameters. Tests by Zhou, Barr, and Lydon (1995) show that compressive strength increases with an increase in coarse aggregate size. However, most other studies disagree. Walker and Bloem (1960) and Bloem and Gaynor (1963) concluded that an increase in aggregate size results in a decrease in the compressive strength of concrete. Cook (1989) showed that, for compressive strengths in excess of 69 MPa (10,000 psi), smaller sized coarse aggregate produces higher strengths for a given water-to-cement ratio. In fact, it is generally agreed that, although larger coarse aggregates can be used to make high-strength concrete, it is easier to do so with coarse aggregates below 12.5 mm (Y, in.) (ACI 363-95). There has not been much research on the effects of coarse aggregate content on the fracture energy of concrete. One study, conducted by Moavenzadeh and Kuguel (1969), found that fracture energy increases with the increase in coarse aggregate content. Since cracks must travel around the coarse aggregate particles, the area of the crack surface increases, thus increasing the energy demand for crack propagation. There is controversy, however, on the effects of coarse aggregate content on the compressive strength of concrete. Ruiz (1966) found that the compressive strength of concrete increases with an increase in coarse aggregate content until a critical volume is reached, while Bayasi and Zhou (1993) found little correlation between compressive strength and coarse aggregate content. In light of the controversy, this report describes work that is aimed at improving the understanding of the role that coarse aggregate plays in the compressive, tensile, and fracture behaviors of concrete.

BACKGROUND OF THE STUDY

The role of coarse aggregate in concrete is central to this report. While the topic has been under study for many years, an understanding of the effects of coarse aggregate has become increasingly more important with the introduction of highstrength concretes, since coarse aggregate plays a progressively more important role in concrete behavior as strength increases. In normal-strength concrete, failure in compression almost exclusively involves debonding of the cement paste from the aggregate particles at what, for the purpose of this report, will be called the matrix-aggregate interface. In contrast, in high-strength concrete, the aggregate particles as well as the interface undergo failure, clearly contributing to overall strength. As the strength of the cement paste constituent of concrete increases, there is greater compatibility of stiffness and strength between the normally stiffer and stronger coarse aggregate and the surrounding mortar. Thus, microcracks tend to propagate through the aggregate particles since, not only is the matrix -aggregate bond stronger than in concretes of lower strength, but the stresses due to a mismatch in elastic properties are decreased. Thus, aggregate strength becomes an important factor in high-strength concrete. This report describes work that is aimed at improving the understanding of the role of aggregates in concrete. The variables considered are aggregate type, aggregate size, and aggregate content in normal and high-strength concretes. Compression, flexural, and fracture tests are used to better understand the effects aggregates have in concrete.

STATEMENT OF THE PROBLEM

There is strong evidence that aggregate type is a factor in the strength of concrete. Ezeldin and Aitcin (1991) compared concretes with the same mix proportions containing four different coarse aggregate types. They concluded that, in high-strength concretes, higher strength coarse aggregates typically yield higher compressive strengths, while in normal-strength concretes, coarse aggregate strength has little effect on compressive strength. Other research has compared the effects of limestone and basalt on the compressive strength of high-strength concrete (Giaccio, Rocco, Violini, Zappitelli, and Zerbino 1992). In concretes containing basalt, loadinduced cracks developed primarily at the matrix-aggregate interface, while in concretes containing limestone, nearly all of the coarse aggregate particles were fractured. Darwin, Tholen, Idun, and Zuo (1995, 1996) observed that concretes containing basalt coarse aggregate exhibited higher bond strengths with reinforcing steel than concretes containing limestone.

THE PROJECT OBJECTIVES

The project has some aims of verification. One of the most important purposes of this project is to compare the strength of concrete made with granite of different sizes eg 20, 15, 10 and 5mm. These same types of concrete are made under the same mix proportion.

Another purpose also is to know the compressive strength of concrete at different ages.

SIGNIFICANCE OF THE STUDY

This study will be beneficial to civil engineering firms and experts in the field to know the necessary requirement for building and mixture of cements in order to achieve the expected quality result. This study will also serve as a reference material for further research on this topic and related domain.

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