We conducted a constant load test to evaluate how stress, hydrogen content, and temperature affect time to fracture due to hydrogen embrittlement in rebar for prestressed concrete. We experimented on tempered martensitic steel rebar taken from prestressed concrete structures prepared for the test. Hydrogen charging was performed by cathodic charging at a current density of 10 A/m2 in a 1 M NaHCO3 solution with ammonium thiocyanate (NH4SCN) added. The sub-surface hydrogen content was controlled by changing the NH4SCN concentration in the solution. The experimental results showed that the time to fracture decreased with increasing applied tensile stress in a power law relationship. The time to fracture and sub-surface hydrogen content also demonstrated a power law relationship, while stress and hydrogen content had no effect on the respective power indexes. Temperature affected the coefficient of power law and the power index of hydrogen content, but the power index of tensile stress was mostly independent of temperature.
Rebars used in prestressed concrete structures are constantly subjected to tensile stress, and some rebars have been reported to fracture due to hydrogen embrittlement.1 It is important to know the hydrogen embrittlement behavior in rebars to prevent fractures. The effects of environmental conditions such as tensile stress, hydrogen content, and temperature on time to fracture have been evaluated individually;2,3 however, their combined effects have not been clarified. The purpose of this study is to experimentally clarify the relationship between time to fracture due to hydrogen embrittlement and environmental conditions to which the rebars are subjected.
The specimens were tempered martensitic steel rebar taken from prestressed concrete structures prepared for the test. The rebar was in accordance with Japanese Industrial Standard (JIS) G 3137 SBPDN 1275/1420, with a nominal cross-sectional area of 40 mm2. The specified tensile strength was at least 1420 MPa, and the specified chemical composition was at most 0.030% for P, at most 0.035% for S, and at most 0.30% for Cu. The rebar had been welded to other rebar when it was used in prestressed concrete structures, so there were welding marks on the surface of the specimens at roughly 90-mm intervals (Figure 1). The length of each specimen was 500 mm, and they were masked with epoxy so that the area for hydrogen charging was 180 mm long and contained two welding marks.
