Blast loading and impact action are occasionally considered for some structures in the field of civil engineering. Typical scenarios include a nuclear containment subjected to an aircraft crash, a control room in a chemical plant under blast loading, protective structures, and bridge piers impacted by ships. The involved facilities are usually constructed using reinforced concrete, due to its excellent blast resistant performance [
1]. Consequently, high strain rates can occur in materials made of reinforcing steel bars (also called rebars) and concrete after being subjected to blasting and other severe dynamic loads. In case of such events, similar to concrete and other metallic materials, reinforcing steel bars exhibit a strain rate effect, which means their mechanical properties are different from those under static load conditions. Generally, the strain rate effect of rebars should be considered in the dynamic analyses of reinforced concrete structures.
Efforts have been made to investigate the strain rate behavior of various types of steel alloys and a few types of reinforcing steels in terms of strengths and deformation capacities [
2,
3,
4,
5,
6,
7,
8,
9,
10]. Mainstone [
2] summarized a quantity of test results for several types of steels before 1975 with strain rates
ranging from 10
−2 to 10
3 s
−1. He used dynamic increase factors (DIF) based on the ratio of dynamic strength to static strength with connections to strain rates to describe the strain rate effect. After that, investigations on the strain rate effect continued. Hu et al. recently demonstrated strain rate dependent behavior of AerMet 100 steel, which had excellent mechanical properties [
4]. They used a Split-Hopkinson pressure bar which could generate strain rates in a range from 560 to 4200 s
−1. In addition, the effects of strain rates on the tensile properties of a TRIP (transformation induced plasticity) -aided duplex stainless steel were studied by Choi et al. [
5] using the Instron 4484 hydraulic testing machine with strain rates of 10
−3, 5 × 10
−2, and 10
−2 s
−1. On the other hand, for reinforcing steel bars that are widely used in concrete structures, relatively few studies were found in the literature. Early experimental studies were conducted by Brandes et al. [
6] in 1986. They tested the reinforcing steel bars BSt 420/500 RU (hot rolled) and BSt 420/500 RK (cold draw) which were broadly used in Germany at that time. These test data were then adopted to build DIF expressions by Eibl [
11]. Later, Malvar [
12] presented a review of rebar properties with consideration of strain rates that varied from 10
−4 to 10 s
−1, and the yield strengths of the rebars were in the range of 290 to 710 MPa. He also proposed DIF formulations to describe the strain rate effect in terms of rebar strengths. More recently, the strain rate behavior of several particular types of reinforcing steel bars were investigated, including cold formed steel B500A (
= 250, 500, and 1000 s
−1) [
13], quenched and self-tempered reinforcing steel B450C (
= 250, 500, and 1000 s
−1) [
7], stainless steel AISI304 (
= 10
−3, 5, 30, 250, 500, 1000 s
−1) [
8], and low-alloy structural steel S355 (
= 10
−3, 5, 35, 300, 500, and 850 s
−1) [
9]. Based on these studies, the following observations could be made for steel alloys and rebars under high strain rates:
An increase in yield strengths and ultimate strengths have been confirmed;
Yield strength is more strain rate-sensitive than ultimate strength;
Steels with relatively low strengths are susceptible to strain rates compared to those with relatively high strengths;
No changes have been found in the elastic modulus;
The magnitude of the strain rate effect is different for various types of steel materials;
The variation tendency of plateau lengths (only for hot rolled rebars) and ultimate strains under high strain rates are inconsistent based on different test sources.
The reinforcing steel bars HPB235, HRB335, HRB400, and HRB500 are different rebar grades and have been widely used in reinforced concrete structures in China. Their strain rate behavior has not been systematically and experimentally investigated so far. These four types of rebars belong to hot rolled and low-carbon structural steels and possess good properties of strength, ductility, and processability. The rebars HPB235 and HRB335 are usually used as stirrups to resist shear forces, while rebars HRB400 and HRB500 commonly serve as longitudinal reinforcements to avoid bending failure. For each steel grade, the number (e.g., 235 in HPB235) denotes the yield strength used for design. As a matter of fact, the analyses of reinforced concrete structures under blasting and impact loads were short of full confidence due to the lack of the appropriate constitutive models of rebars. Evidently, the test results of other rebars in the literature were only of reference value for understanding the strain rate behavior of the rebars HPB235, HRB335, HRB400, and HRB500, because the dynamic behavior of steels is type-dependent. In this sense, to fill the current knowledge gap and enrich the database, the strain rate behaviors of rebars HPB235, HRB335, HRB400, and HRB500 were experimentally investigated in this study. The strain rates in testing ranged from 2 to 80 s
−1, which are the strain rates that typically occur in blast [
14,
15] and impact events [
10]. The parameters in DIF formulations and in the Johnson-Cook model were derived, and the results were compared with existing formulations. The primary objective of this study was to propose realistic material models of rebars for their application in dynamic analyses of reinforced concrete structures. Therefore, engineering stress and engineering strain were used throughout this paper.