CHAPTER ONE

INTRODUCTION

1. BACKGROUND OF THE STUDY

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.

Flexural strength is a measure of the tensile strength of concrete; it is a measure of an unreinforced concrete beam or slab to resist failure resulting from bending stresses. Reinforcements are provided to enhance the tensile strength of concrete. The ability of concrete to induce tensile stresses to reinforcement depends mostly on the bonding force between the two materials and also on the size of aggregates. Inability of concrete to adequately induce tensile stresses to the reinforcement results in cracking at the bottom fibre; cracks will open up bond that exist between the two materials (concrete and steel). This eventually reduces the stiffness of the whole composite, and reinforcement will be exposed to corrosion agents (water, chloride, air etc.). This study therefore focused on the effect of coarse aggregate size on the flexural strength of concrete. To evaluate the flexural strength (the theoretical maximum tensile stress reached in the bottom fibre of a test beam during a flexural strength test) of concrete implies subjecting concrete to loading on flexural testing machine in order to measure it’s resistance to tensile stresses.

The constituents of concrete are cement, water, aggregates (fine and coarse aggregates), aggregates take about three-quarter of the volume of concrete with the coarse aggregates taking between 50 and 60% of the concrete mix depending on the mix proportion used (Waziri et. al., 2011). The larger percentage of coarse aggregate in concrete mix makes it to contribute a lot to the strength of concrete. Its properties like toughness, hardness, shape, size, soundness, density, and specific gravity also affect the strength of concrete.

1. STATEMENT OF THE PROBLEM

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).

Many researchers (Wu et. al., (1997), Zhang et. al. (2010), Waziri et. al. (2011), Abdullahi (2012), and Joseph et. al. (2012) have carried out studies on the strength characteristics of concrete produced using different aggregate materials and using different brands of cement but little or no attention have been focused on the flexural bond stress (which is the stress in structural concrete members between the concrete and the reinforcing element that results from the application of external loads) between reinforcement and concrete. Use of poorly graded coarse aggregate in concrete matrix also has it share in the causes of structural failure due to the development of horny comb in the concrete. This also results in cohesion less composite of concrete and steel with poor flexural bond.

It is in the light of the above that the study intends to look at the relationship effects of aggregate sizes on concrete strength, the factors affecting aggregate sizes, the determinants of aggregate sizes on concrete strength and the way forward to adequate aggregate sizes on concrete strength.

1. AIMS AND OBJECTIVES

The aim of this study is to evaluate the relationship existence between aggregate sizes on concrete strength.

The specific objectives will be:

1. To examine the effects of aggregate sizes on concrete strength
2. To ascertain the factors affecting aggregate sizes
3. To examine the determinants of aggregate sizes on concrete strength
4. To find out the way forward to adequate aggregate sizes on concrete strength.

1.4 RESEARCH QUESTIONS

Arising from the objectives, the following research questions will be address in the study:

1. What are the effects of aggregate sizes on concrete strength?
2. What are the factors affecting aggregate sizes?
3. What are the determinants of aggregate sizes on concrete strength?
4. What measures can be taken to obtain adequate aggregate sizes on concrete strength?

1.5 SIGNIFICANCE OF THE STUDY

This is significant to the fact that modern brick layers, molders and house builders to know the required aggregate sizes to have a concrete strength in their production and building structure. It will also be of important because the study will investigate the effects of aggregate sizes on the properties of structural concrete so as to establish the aggregate size that will improve the properties of structural concrete.

1.6 RESEARCH HYPOTHESIS

H0 there is no significant relationship between effects of aggregate sizes on concrete strength.

H1 there is significant relationship between effects of aggregate sizes on concrete strength.

1.7 SCOPE OF THE STUDY

The study is limited to how effect of aggregate sizes determines concrete strength.

1.8 LIMITATION OF THE STUDY

TIME CONSTRAINTS: One the challenges experienced by the researcher is the issue of time; the research will simultaneously engage in departmental activities like seminars and attendance to lectures. But the researcher was able to meet up with the deadline for the submission of the project.

FINANCIAL CONSTRAINTS: Every research work needs funding; however lack of adequate funds might affect the speed of the researcher in getting materials for completion of the project.

1.9 DEFINITION OF TERMS

Concrete

 Is a composite material composed of fine and coarse aggregate bonded together with a fluid cement (cement paste) that hardens (cures) over time. ... The cement reacts with the water and other ingredients to form a hard matrix that binds the materials together into a durable stone-like material that has many uses.

Compressive strength or compression strength

 Is the capacity of a material or structure to withstand loads tending to reduce size, as opposed to Tensile strength which withstands loads tending to elongate. In other words, compressive strength resists being pushed together, whereas tensile strength resists tension (being pulled apart). In the study of strength of materials, tensile strength, compressive strength, and shear strength can be analyzed independently.

Strength of materials

 Also called mechanics of materials, deals with the behavior of solid objects subject to stresses and strains. The theory began with the consideration of the behavior of one and two dimensional members of structures, whose states of stress can be approximated as two dimensional, and was then generalized to three dimensions to develop a more complete theory of the elastic and plastic behavior of materials.

The water–cement ratio 

Is the ratio of the weight of water to the weight of cement used in a concrete mix. A lower ratio leads to higher strength and durability, but may make the mix difficult to work with and form. Workability can be resolved with the use of plasticizers or super-plasticizers.