Measuring strain and stress distributions along rebar embedded in concrete using time-of-flight neutron diffraction

Abstract

In modern society, architectural and civil engineering structures such as reinforced concrete buildings require high seismic performance to minimize the ‘megarisk’ exposed from urban earthquake hazards. In the reinforced concrete structures, the bond resistance between rebar and concrete is one important parameter for discussing its performance and it has been typically evaluated by measuring the strain distribution along the embedded rebar. Here, we present in-situ strain and stress measurements for the rebar in reinforced concrete using time-of-flight neutron diffraction as a novel alternative technique to typical strain gauges. It was demonstrated in this study that the three-dimensional deformation behavior of the embedded rebar in normal-strength concrete, cured in air, can be accurately measured under pull-out loading using time-of-flight neutron diffraction. Wider applications of neutron diffraction in the structural engineering field are expected for advanced understanding of actual phenomena on reinforced concrete structures.

Structural Engineering Studies on Reinforced Concrete Structure using Neutron Diffraction

“…Lengths of the anchorage region can be predicted to be about 85 mm for σ ap =125 MPa and about 145 mm for σ ap =248.5 MPa, and the average bond stresses are calculated to be about 6.4 MPa and 5.9 MPa, respectively. These values are different from rough estimation in our previous study [7], but more reliable since the bond stress distribution obtained here assists to determine these values accurately. On the other hand, several peaks can be found in the bond stress distribution with about 20 mm in intervals.…”

“…3), and we demonstrated that the neutron diffraction technique can be a novel strain measurement method for rebar embedded in concrete [5,6]. More recently, threedimensional deformation behavior of the embedded rebar including the axial and transverse strains was successfully measured under pull-out loading using Time-of-Flight (TOF) neutron diffraction with the engineering diffractometer, TAKUMI in MLF (Materials and life Science Experimental Facility) of J-PARC (Japan Proton Accelerator Research Complex) [7]. Furthermore, it was demonstrated by some application studies using TAKUMI that the neutron diffraction technique is available to assess the bond deterioration due to rebar corrosion and crack generation in concrete [8].…”

“…approximately 125 MPa and 248.5 MPa. The detailed descriptions of the experimental condition including the sample preparation can be found in the reference [7].…”

“…The average lattice constants, a in the axial and transverse directions were determined by multiple-fitting over 15 peaks from the 110 to 431 reflections by the Rietveld refinement software, Z-Rietveld [12]. The axial stress in a rebar axis, σ A was calculated using the strains in the axial and transverse directions of rebar in assumption of equiaxial-stress condition [7]. On the other hand, the bond stress, τ is equivalent to the shear stress acting at the boundary between rebar and concrete, expressed by following equation,…”

“…In this study, the binomial smoothing was, at first, applied to the measured axial stress distribution to reduce the influence of data scattering, and then the bond stress distribution was obtained by plotting slopes fitted within a range of neighboring data in the smoothed axial stress distribution, as a function of the axial position of rebar. The average bond stress was, in our previous studies [5][6][7][8], roughly calculated by Eq (1) with the slope of the axial stress distribution within a range of the anchorage region, approximately estimated by eye. In contrast, it was obtained accurately in this study normalizing the area of the bond stress diagram in the anchorage region by its length.…”