Smart Stressing of Concrete Beams with Nitinol Shape Memory Alloy Cables

Abstract

In this study, a novel method is presented to smartly prestress concrete beams using Nitinol Shape Memory Alloy (SMA) cables. Concrete beams having different compressive strengths and steel fiber contents prestressed with SMA cables are studied in comparison with concrete beams prestressed with steel and carbon fiber reinforced polymer (CFRP) cables. SMA cables with Nickel: Titanium ratio of 0.558:0.442 are embedded in concrete beams and prestrained at temperatures below their martensite start temperature, M s =20°C±1°C. Smart prestressing is accomplished by heating the Nitinol SMA cables to temperatures above their austenite finish temperatures, A f =28°C±1°C, and thus subjecting the concrete to compressive forces as the SMA cables attempt to regain their parent form. The study shows that SMA prestressed concrete beams have improved cracking behavior with fewer cracks, larger crack spacing, partial crack closure upon load removal, and an increase in cracking loads. The 35MPa beams prestressed with SMA had 24.2% and 6% higher cracking loads than beams prestressed with steel and CFRP cables, respectively. Meanwhile, SMA prestressed concrete beams had reduced failure loads varying from 4% to 34% in comparison with beams prestressed with steel and CFRP cables. The addition of steel fiber increased the magnitudes of the failure loads in SMA loaded beams. Further, SMA loaded beams possessed large load deflections accompanied with a unique shape retaining capability. This study adverts that SMA cables are attractive alternatives to steel and CFRP cables in post-tensioned beams and slabs, where the use of a complicated jacking system could be substituted by a self-prestressing cable.