The Nature of the Concrete-Steel Rebar Interface in Plain and Silica Fume Concrete
- PDF / 919,326 Bytes
- 7 Pages / 420.48 x 639 pts Page_size
- 86 Downloads / 224 Views
THE NATURE OF THE CONCRETE-STEEL REBAR INTERFACE IN PLAIN AND SILICA FUME CONCRETE ABLA M. ZAYED Department of Civil Engineering and Mechanics, University of South Florida, Tampa; Florida 33620 ABSTRACT In this investigation the nature and morphology of the concrete-reinforcing steel interface is studied. The effect of the partial replacement of cement by silica fume on the interface microstructure was determined. The distribution profiles of percentage porosity, calcium hydroxide and unreacted cement from the interface towards the concrete bulk were measured using the scanning electron microscope operated in the back scattering mode. The findings indicate the similarities between the morphology of the concrete-steel interface and those reported earlier in the literature for the concrete-aggregate interface. Additions of silica fume resulted in a decrease of the interface porosity and calcium hydroxide content. INTRODUCTION In structural concrete applications, reinforcement is a common practice to improve the mechanical performance of concrete, specifically its tensile strength. In marine conditions, steel reinforced concrete shows severe problems of corrosion. In addition to noticeable decrease in strength, the corrosion products introduce volume expansion and internal tensile stresses in the surrounding concrete thus promoting concrete cracking and spalling. Concrete is a heterogeneous complex composite material that can be classified into three phases, namely: aggregates, cement paste and transition zone. Research for the past 40 years has been involved with the characterization of the cement phases hydration products, their properties and different properties of aggregates. Though the transition zone was well known early enough, it has not received much attention until lately. The transition zone, defined as the steel or aggregate-paste interface, extends from 10 - 100 microns and is characterized by a higher water to cement ratio than the bulk matrix and therefore a higher porosity. The existence of this interface explained ambiguous behavior of concrete in terms of strength and permeability [1, 2]. In reinforced concrete structures, subsidence cracking occur due to residual tensile stresses that exist in fresh mixed concrete above reinforcement. These stresses are created from the opposition offered by reinforcement to the body forces of fresh concrete. Microcracks tend to develop and propagate preferentially along the weak interfacial zone, under external applied loads, thus providing easy access of chloride ions to steel. As the chloride ion concentration reaches a threshold limit, the passive oxide layer of steel is destroyed and corrosion is initiated [2]. Investigators [1-4] agree that subsidence microcracking in the concrete-steel interface and the permeability of concrete are the parameters controlling the corrosion process of steel in concrete. This investigation is part of an ongoing research aimed at characterizing the transition zone (reinforcing steel-concrete interface) in plain and silica fume c
Data Loading...
