Shape Memory Alloys In Civil Engineering

INTRODUCTION

Today, more and more researchers are focusing on smart materials for their special properties and performance in civil engineering applications. The shape memory alloys (SMAs) is a kind of smart material which can be used as a prestressing tendon. Shape memory alloys can exist in two phases at different temperatures and stresses. Austenite, which exists in high temperature, and Martensite, which exists in low temperature. When the external temperature or stress condition changes, these two phases will transform to the other phase, depending on what change appears. SMA exhibits many special properties during the transformations between these two phases, such as shape memory effect (Fig. 1.1), super elasticity effect, etc.

Properties

The high-strength prestressing material is most often tensioned by means of hydraulic jacks. The tensioning process may occur before or after the concrete is cast and, accordingly, prestressed members are categorised as either pretensioned or post-tensioned.

Pretensioned concrete:

In the pretensioning system, the tendons are first tensioned between rigid anchor blocks cast on the ground or in a column or unit-mould type pretensioning bed, prior to the casting of the concrete in the moulds. The typical pretensioning arrangement is shown in Fig.1.4 The tendons comprising individual wires or strands are stretched with constant eccentricity as shown in (a) or variable eccentricity as in (b) with tendon anchorage at one end and jacks at the other end of the beam. With the forms in place, the concrete is cast around the stressing tendon. When the concrete attains sufficient strength, the jacking pressure is released. The high-tensile wires tend to shorten but are restrained by the bond between concrete and steel. In this way, the prestress is transferred to the concrete by bond, mostly near the ends of the beam, and no superior anchorages are required in the pretensioned members, except for wires of diameter exceeding 7mm. Wires less than 7mm diameter anchor themselves satisfactorily with the aid of the

Use of smart prestressing reduce these losses. Conventional prestressing technology is very difficult to execute and it requires special equipment’s for this purpose. Execution of smart prestressing is comparatively easier and noise free this makes this technology more environmental friendly.

Self compacting concrete, with its unique ability to consolidate under its own weight, is the future of concrete industry with its added efficiency in terms of cost and time management. The flexural strength of self compacting concrete is much higher than that of ordinary concrete. Imparting prestress in self compacting concrete members makes sections even lighter and reduces the overall dead weight of the member. This technology is ideal for rail and highway bridges. By providing parabolic and eccentric tendons moment can be easily counteracted.

OBJECTIVES

Increase the load carrying capacity of a simply supported beam using a Shape Memory Alloy wire as a prestressed tendon, instead of steel