CHAPTER 1

INTRODUCTION

BACKGROUND OF STUDY

            Concrete is widely used in the construction of buildings, bridges and other infrastructure around the world. Concrete is affected by the components involved in production, and since coarse aggregate occupies a large part of the volume. The concrete is affected by the maximum size of a well graded coarse aggregate, a concrete mixture containing large coarse aggregate particles has a small surface area compared to small coarse aggregate particles on the other hand, and the smaller coarse size aggregates give larger surface area for bonding with the mortal matrix. 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 saggregate 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, loadinduced cracks developed primarily at the matrix-aggregate interface, while in concretes containing limestone, nearly all of the coarse aggregate particles were fractured.  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 2 parameters. 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 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 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).

AIM AND OBJECTIVES

      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.

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.

SCOPE OF STUDY

         This research is carried out on crushed burnt bricks produced from Araromi Village, Osun State and river washed gravel Idanre water fall Ondo state, as Coarse Aggregate. The investigation is limited to the workability and compressive strengths of concrete cubes made from different mixes of sand, gravel and crushed burnt bricks, Araromi Village. 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.

REFERENCES

 ACI Committee 211. (1991) "Standard Practice for Selecting Proportions for Normal, Heavyweight, and Mass Concrete (ACI 211.1-91)," ACI Manual of Concrete Practice, 1997 Edition, Part I, Farmington Hills, MI. ACI Committee 363. (1992) "State-of-the-Art Report on High-Strength Concrete (ACI 363R-92)," ACI Manual of Concrete Practice, 1997 Edition, Part I, Farmington Hills, MI. Bayasi, Z. and Zhou, J. (1993) "Properties of Silica Fwne Concrete and Mortar," ACI Materials Journal, V. 90, No.4, July-August, pp. 349-356. Bentur, A. and Mindess, S. (1986) "The Effect of Concrete Strength on Crack Patterns," Cement and Concrete Research, V. 16, No. I, January, pp. 59-66. Bloem, D. L. and Gaynor, R. D. (1963) "Effects of Aggregate Properties on Strength of Concrete," ACI Journal, Proceedings V. 60, No. 10, October, pp. 1429-1456. Carrasquillo, R. L., Nilson, A. H., and Slate, F. 0. (1981) "Properties of High-Strength Concrete Subject to Short-Term Loads," ACI Journal, Proceedings V. 78, No.3, MayJune, pp. 171-178. Carrasquillo, R. L., Slate, F. 0., and Nilson, A. H. (1981) "Microcracking and Behavior of High-Strength Concrete Subject to Short-Term Loading," Concrete Fracture," Fracture Mechanics of Concrete: Material Characterization and Testing, Eds. A. Carpinteri and A. R. Ingraffea, Martinus NijhoffPublishers, Boston, pp. 31-65.