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1.1 Background of study
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 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, load induced 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.
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 RvfPa (10,000 psi), smaller sized coarse aggegate 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 (h 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. 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 high strength 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 deboning 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 normal stiffer and stronger coarse aggregate and the surrounding mortar. Thus, micro cracks 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 PROBLEM
Concrete is one of the most widely used construction materials. The raw material from which it is prepared: cement, aggregates and water affect both the quality and cost of construction. Aggregates are usually cheaper than cement and constitute over 70% of the volume of concrete. The availability and proximity of aggregate to the construction site also affect the cost of construction. At present, the most commonly used coarse aggregates for concrete production in Benue State of Nigeria is river washed gravel due mainly to the presence of River Benue and its deposits. But these are not readily available in some local government areas which are not serviced by the river. Thus the cost of transporting gravel to the areas outside the catchment of the river tends to increase the cost of construction even at relatively low labour. This necessitates the use of alternative coarse aggregates which are locally obtained. One such coarse aggregate is crushed burnt bricks obtained from the production of burnt bricks (Maher, 1987).
1.2 OBJECTIVES OF STUDY
The purpose of this research is to compare the compressive strength, flexural strength, and fracture energy of normal and high-strength concretes with different aggregate types, sizes, and contents.
1.3 SIGNIFICANCE OF STUDY
In many countries, the need for locally manufactured building materials can hardly be overemphasized because there is an imbalance between the demands for 3 housing and expensive conventional building materials coupled with the depletion of traditional building materials. To address this situation, attention has been focused on low - cost alternative building materials (Agbede and Manasseh, 2008 and Waziri et al, 2011). This research is therefore important as it tries to compare the compressive strength of concrete made with the aggregate size.
1.4 SCOPE AND LIMITATIONS
This research is carried out on crushed burnt bricks produced from Naka, Benue State and river washed gravel from River Benue as coarse aggregates. The investigation is limited to the workability and compressive strengths of concrete cubes made from different mixes of sand, gravel and/or crushed burnt bricks, Benue Cement and water. The study does not cover the temperature at which burnt bricks will give optimum strength; neither does it cover the effect of admixtures on the compressive strength of crushed burnt bricks-concrete.
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