Thermography-Based Deterioration Detection in Concrete Bridge Girders Strengthened with Carbon Fiber-Reinforced Polymer.

In bridge structures worldwide, carbon fiber-reinforced polymer (CFRP) sheets are applied to strengthen weak components, especially concrete girders that are at a high risk of rapid degradation during the bridge's operation owing to impacts from the superstructure's weight and traffic loads. Regarding the thermography-based method (TM), although deteriorations in the concrete core are some of the main defects in concrete structures strengthened with CFRP, these do not receive as much attention as damage in the CFRP. Therefore, the interpretation of the structural health in terms of these defects using TM is still unclear. The problem presented in this work addresses the quantification of delamination inside the concrete part of a specimen with a CFRP sheet installed on the surface (assumed to be the girder surface strengthened with CFRP) via step heating thermography. Additionally, the empirical thermal diffusivity of concrete girders strengthened with a CFRP sheet (CSC girder), has not been provided previously, is proposed in the present study to predict delamination depths used for field investigations. Moreover, the effect of the CFRP sheet installed on the structure's surface on the absolute contrast of delamination is clarified. Finally, advanced post-processing algorithms, i.e., thermal signal reconstruction and pulsed phase thermography, are applied to images obtained with step heating thermography to enhance the visibility of delamination in CSC girders.

Enhanced Strain Measurement Range of an FBG Sensor Embedded in Seven-Wire Steel Strands.

FBG sensors offer many advantages, such as a lack of sensitivity to electromagnetic waves, small size, high durability, and high sensitivity. However, their maximum strain measurement range is lower than the yield strain range (about 1.0%) of steel strands when embedded in steel strands. This study proposes a new FBG sensing technique in which an FBG sensor is recoated with polyimide and protected by a polyimide tube in an effort to enhance the maximum strain measurement range of FBG sensors embedded in strands. The validation test results showed that the proposed FBG sensing technique has a maximum strain measurement range of 1.73% on average, which is 1.73 times higher than the yield strain of the strands. It was confirmed that recoating the FBG sensor with polyimide and protecting the FBG sensor using a polyimide tube could effectively enhance the maximum strain measurement range of FBG sensors embedded in strands.

Prediction of the Rheological Properties of Fresh Cementitious Suspensions Considering Microstructural Parameters.

Supplementary cementitious materials (SCMs) are commonly used to partially replace cements. Although it is necessary to investigate the rheological properties of the individual supplementary cementitious materials (SCMs) for understanding complex rheological behaviors of the blended mixes, the study on the investigation of rheological properties of various SCMs such as fly ash, blast-furnace slag, and silica fume, according to various solid volume fractions and prediction models is fairly limited. This study investigated the rheological properties of non-blended cementitious suspensions with Portland cement (PC), fly ash (FA), blast-furnace slag (BS), and silica fume (SF) materials in the experiments and predicted using YODEL (Yield stress mODEL) and Krieger-Dougherty's (K-D's) equation. Experiments were designed with various solid volume fractions (ϕ) from 0.28 to 0.44, and the rheological properties of all cementitious suspensions were noted to increase with increasing ϕ, showing an improved flowability at low ϕ. YODEL, derived from the first principles considering particle-size distributions, interparticle forces and microstructural parameters predicted the yield stress. The YODEL predictions were consistent with the experiments with a positive correlation coefficient of above 0.96. K-D's equation with the maximum particle fractions and intrinsic viscosity as key parameters predicted the plastic viscosity. The K-D's equation predictions match up with the experiments with a positive correlation coefficient of above 0.94. Both models showed more quantitative predictions without any fitting parameters and could be applied to any multimodal powder suspensions.

Comparison of Mechanical and Crack-Healing Properties of PE-PVA Hybrid Fiber-Reinforced SHCCs in Natural and Underwater Conditions.

This paper presents a direct comparison of the mechanical and crack-healing properties of strain hardening cementitious composites (SHCC) under water submersion in a laboratory and in a natural environment outdoors. Portland cement, slag, crumb rubber powder, and hybrid polyethylene and polyvinyl alcohol fibers were used for the SHCC, and mixture proportions were determined. Specimens were exposed to different environmental conditions. A sequence of experimental tests including those for density, compressive strength, and tensile properties was performed to assess the mechanical properties of the SHCC. To confirm the healing feasibility of the SHCC, crack width reduction, stiffness recovery, and tensile performance at post-healing were adopted. The test results showed that underwater conditions are better than natural conditions in improving both the mechanical and crack-healing properties of SHCC. Specifically, the SHCC cured in natural conditions had a lower compressive strength, tensile strength, and tensile strain capacity than that cured in underwater conditions by 10%, 4%, and 3%, respectively. The SHCC cured in underwater conditions had a healing threshold of crack width of 60 µm, while the SHCC cured in natural conditions had very limited crack-healing capacity. Additionally, stiffness recovery of the SHCC cured in underwater conditions was higher than that cured in natural conditions.

Effects of the Replacement Length of Concrete with ECC on the Cyclic Behavior of Reinforced Concrete Columns.

This paper presents an experimental investigation on the effects of the replacement length of concrete with engineered cementitious composites (ECC) on the cyclic behavior of a reinforced concrete (RC) column. A conventional RC column specimen and two RC composite columns designed with ECC were fabricated. To investigate the cyclic behavior of each specimen, a series of cyclic loading tests was performed under a reversed cyclic loading condition with a constant axial load. Test results showed that ECC columns exhibited higher cyclic behavior in terms of load carrying capacity, ductility, and energy dissipation capacity compared to the RC column. It was also found that when applying ECC to the column specimen with a length of 3.6d or more, the energy dissipation capacity was greatly increased.

Strain-Hardening and High-Ductile Behavior of Alkali-Activated Slag-Based Composites with Added Zirconia Silica Fume.

This paper presents an experimental study on the effects of zirconia silica fume on the composite properties and cracking patterns of fiber-reinforced alkali-activated slag-based composites. Four mixtures were prepared with added zirconia silica fume and varying water-to-binder ratio. Polyethylene fiber was used as a reinforcing fiber for all the mixtures at a volumetric ratio of 2.0% cubic specimens and uniaxial tensile specimens were prepared to evaluate their density, compressive strength, and tensile behavior. The test results demonstrated that the compressive strength, tensile strength, and tensile strain capacity of the composite can be simultaneously improved by incorporating zirconia silica fume. A mixture incorporating zirconia silica fume showed high-ductile behavior of 26.5% higher tensile strength, and 13.7% higher tensile strain capacity than the mixture without zirconia silica fume. The composite with added zirconia silica fume also showed excellent cracking patterns, i.e., narrow crack spacing and crack width.

Erratum: Myoung, S.C.; Su-Tae, K.; Bang, Y.L.; Kyeong-Taek, K.; Gum-Sung, R. Improvement in Predicting the Post-Cracking Tensile Behavior of Ultra-High Performance Cementitious Composites Based on Fiber Orientation Distribution. Materials 2016, 9, 829.

1.The authors wish to remove the symbol "†", which indicates the equal contribution from each author. The correct authorship is shown below:[...].

A Seismic Strengthening Technique for Reinforced Concrete Columns Using Sprayed FRP.

Conventional methods for seismic retrofitting of concrete columns include reinforcement with steel plates or steel frame braces, as well as cross-sectional increments and in-filled walls. However, these methods have some disadvantages, such as the increase in mass and the need for precise construction. Fiber-reinforced polymer (FRP) sheets for seismic strengthening of concrete columns using new light-weight composite materials, such as carbon fiber or glass fiber, have been developed, have excellent durability and performance, and are being widely applied to overcome the shortcomings of conventional seismic strengthening methods. Nonetheless, the FRP-sheet reinforcement method also has some drawbacks, such as the need for prior surface treatment, problems at joints, and relatively expensive material costs. In the current research, the structural and material properties associated with a new method for seismic strengthening of concrete columns using FRP were investigated. The new technique is a sprayed FRP system, achieved by mixing chopped glass and carbon fibers with epoxy and vinyl ester resin in the open air and randomly spraying the resulting mixture onto the uneven surface of the concrete columns. This paper reports on the seismic resistance of reinforced concrete columns controlled by shear strengthening using the sprayed FRP system. Five shear column specimens were designed, and then strengthened with sprayed FRP by using different combinations of short carbon or glass fibers and epoxy or vinyl ester resins. There was also a non-strengthened control specimen. Cyclic loading tests were carried out, and the ultimate load carrying capacity and deformation were investigated, as well as hysteresis in the lateral load-drift relationship. The results showed that shear strengths and deformation capacities of shear columns strengthened using sprayed FRP improved markedly, compared with those of the control column. The spraying FRP technique developed in this study can be practically and effectively used for the seismic strengthening of existing concrete columns.

Improved Sectional Image Analysis Technique for Evaluating Fiber Orientations in Fiber-Reinforced Cement-Based Materials.

The distribution of fiber orientation is an important factor in determining the mechanical properties of fiber-reinforced concrete. This study proposes a new image analysis technique for improving the evaluation accuracy of fiber orientation distribution in the sectional image of fiber-reinforced concrete. A series of tests on the accuracy of fiber detection and the estimation performance of fiber orientation was performed on artificial fiber images to assess the validity of the proposed technique. The validation test results showed that the proposed technique estimates the distribution of fiber orientation more accurately than the direct measurement of fiber orientation by image analysis.

Improvement in Predicting the Post-Cracking Tensile Behavior of Ultra-High Performance Cementitious Composites Based on Fiber Orientation Distribution.

In this paper, the post-cracking tensile behavior of Ultra-High Performance Cementitious Composites (UHPCC) was studied and an improved analytical model to predict the behavior depending on the fiber orientation distribution was proposed. Two different casting methods were adopted to estimate the influence of the casting method on the tensile behavior. The direct tensile test results showed that the post-cracking tensile behavior was considerably dependent on the casting method. The influence of the casting method was quantified by image analysis of the fiber distribution. The fiber orientation distribution obtained by image analysis may sometimes include considerable error according to the image resolution, which may cause inaccuracy when predicting the post-cracking tensile behavior based on the fiber orientation distribution. To overcome this dependency, the tensile bridging behavior by the fibers in UHPCC was simulated considering the obtained fiber orientation distribution as well as the number of fibers detected. The post-cracking behavior was then simulated by combining the bridging behavior and tension softening behavior of the matrix. The approach adopted in this study to simulate the post-cracking behavior of UHPCC showed good agreement with the experimental results.

Bonding Properties of Basalt Fiber and Strength Reduction According to Fiber Orientation.

The basalt fiber is a promising reinforcing fiber because it has a relatively higher tensile strength and a density similar to that of a concrete matrix as well as no corrosion possibility. This study investigated experimentally the bonding properties of basalt fiber with cementitious material as well as the effect of fiber orientation on the tensile strength of basalt fiber for evaluating basalt fiber's suitability as a reinforcing fiber. Single fiber pullout tests were performed and then the tensile strength of fiber was measured according to fiber orientation. The test results showed that basalt fiber has a strong chemical bond with the cementitious matrix, 1.88 times higher than that of polyvinyl alcohol fibers with it. However, other properties of basalt fiber such as slip-hardening coefficient and strength reduction coefficient were worse than PVA and polyethylene fibers in terms of fiber bridging capacity. Theoretical fiber-bridging curves showed that the basalt fiber reinforcing system has a higher cracking strength than the PVA fiber reinforcing system, but the reinforcing system showed softening behavior after cracking.