THE EFFECT OF THE ENVIRONMENT ON BOND RESISTANCE BETWEEN CONCRETE AND STEEL REINFORCEMENT

 

CHAPTER ONE

INTRODUCTION

1.1       BACKGROUND OF RESEARCH

Reinforced concrete design is based on the assumption that concrete and steel reinforcement combines to create a new, composite material. Generally, the concrete is designed to carry the compressive loads while the reinforcing bar is designed to carry the tensile loads. There must be bond between the reinforcing bar and the concrete if it is to carry the tensile load and maintain equilibrium. However, bond between concrete and reinforcing bar is been affected by corrosion, a leading cause of structural failure. Reinforcing bar corrosion is a major concern in the world of concrete structures. It develops high pressure within the concrete causing the cracking and spilling of the concrete cover and the exposure of the reinforcing bar to further corrosion activity.

The Bond clause of the BS 8110 –1:1997 code is simply based on the compressive strength of concrete, a factor that could be expected to increase useable bond capacity. All other factors known to affect bond were not taken into consideration. The ACI 318 (2002) code took various factors like concrete cover, diameter of bar, lap length, transverse reinforcement, spacing, transverse steel strength and the cylinder strength into consideration but the environment was not considered. The Eurocode EN 1992–1–1: 2004 (E) took various factors affecting bond into consideration including the environment and a factor of 0.7 of the normal bond strength was recommended in aggressive environment.

A developing country like Nigeria can adopt any code of practice but local specifications must be written to compliment the adopted code. The lifespan and failure of structures especially reinforced structures has been of great concern to professionals and researchers in and out of the construction industry and over time, there has been a list of other factors that supposedly reduces the lifespan of structures or even possible failures. The environments where the reinforced structures are built affect the bond between concrete and steel reinforcement which in turn accelerates the performance loss of reinforced structures especially when reinforced concrete structures are located in aggressive environments, where they are exposed to corrosive fumes, sea water spray, abrasive actions, adverse weather conditions etc. hence an attempt to study the effect of the environment on bond between concrete and steel is desirable and worthwhile.

Not much work has been done on the effect of the environment on bond between concrete and steel reinforcement. This investigation is an attempt to generate data that can be used in developing local specification on bond between concrete and steel reinforcement in aggressive environments like coastal and industrial, to be able to justify the use or otherwise of the recommended reduction factor of 0.7 in the Eurocode or to be able to recommend appropriate reduction factor in severe environments.

1.2       AIMS AND OBJECTIVES

The aim is to determine the effect of the environment on bond resistance between concrete and steel reinforcement. The objectives are to:

        i.            Generate data on bond between concrete and steel reinforcement exposed to different environments (Normal, Coastal and Industrial); and

      ii.            Compare the effect of each environment on the bond between the concrete and steel reinforcement.

1.3       SCOPE OF RESEARCH

Seventy-five (75) RILEM push-in specimens were cast. Twenty-five (25) specimens each were cast in the Normal, Coastal and Industrial environments, using 16mm high yield diameter bars in 160mm cube moulds. Three (3) control cubes were also cast to monitor the strength of concrete cast in each environment. All the materials used were exposed to the respective environment for fourteen (14) days. The RILEM specimens cast were cured in their respective environment for twenty-eight (28) days, and afterwards push-in tests were conducted to determine their bond resistances.

1.4       SIGNIFICANCE OF RESEARCH

The results of this present study would enhance the writing of local specifications for bond in different environments.

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