Concrete-filled Steel Tube Specimens Research Articles

A Concrete Core Void Imaging Approach and Parameter Analysis of Concrete-Filled Steel Tube Members Using Travel Time Tomography: Multi-Physics Simulations and Experimental Studies.

Concrete-filled steel tube (CFST) members have been widely used in civil engineering due to their advanced mechanical properties. However, internal defects such as the concrete core voids and interface debonding in CFST structures are likely to weaken their load-carrying capacity and stiffness, which affects the safety and serviceability. Visualizing the inner defects of the concrete cores in CFST members is a critical requirement and a challenging task due to the obvious difference in the material mechanical parameters of the concrete core and steel tube in CFST members. In this study, a curved ray theory-based travel time tomography (TTT) with a least square iterative linear inversion algorithm is first introduced to quantitatively identify and visualize the sizes and positions of the concrete core voids in CFST members. Secondly, a numerical investigation of the influence of different parameters on the inversion algorithm for the defect imaging of CFST members, including the effects of the model weighting matrix, weighting factor and grid size on the void's imaging quality and accuracy, is carried out. Finally, an experimental study on six CFST specimens with mimicked concrete core void defects is performed in a laboratory and the mimicked defects are visualized. The results demonstrate that TTT can identify the sizes and positions of the concrete core void defects in CFST members efficiently with the use of optimal parameters.

Interface debonding defect detection for CFST columns based on EMI measurements: Experiment, numerical simulation and blind inspection in practice

Concrete-filled steel tube (CFST) members have been widely used in skyscrapers and long-span bridges. Non-uniformly distributed hydration heat during curing, inadequate compaction during construction and unavoidable shrinkage and creep of mass concrete in service most likely lead to interface debonding defects between concrete core and steel tube, which has been a common concern. The development of effective non-destructive inspection methods for interface bonding condition of existing CFST members is critical. In this study, an interface debonding defect detection method based on electromechanical impedance (EMI) measurement using surface-mounted piezoelectric-lead-zirconate-titanate (PZT) sensors is proposed at first. Then, experimental study on the feasibility of the proposed approach is carried out with scaled CFST specimens with artificially mimicked interface debonding defects. EMI of surface-mounted PZT sensors at different frequency bands are measured and compared to verify the detectability of the proposed approach for the mimicked interface debonding defects. Thirdly, a multi-physics CFST-PZT coupling finite element model (FEM) with interface debonding defects is established, and EMI of surface-mounted PZT sensors at different locations is simulated. The influence of interface debonding defects on EMI measurements is illustrated. Finally, a blind inspection on the interface bonding condition of selected CFST columns in an existing skyscraper in service using the proposed approach is carried out. Both bonding and debonding defect in the tested CFST columns are detected correctly and validated with drilling hole observation. Experimental, numerical and engineering application results show the proposed approach is effective for interface debonding detection of CFST members and sensitive to minor debonding defect of 0.3 mm in CFST members.

Research on the interfacial bonding performance of novel composite L‐shaped concrete‐filled steel tubes

SummaryThis study aims to investigate the bonding performance of novel composite specially shaped concrete‐filled steel tubes (CFSTs). Seven novel composite L‐shaped CFST specimens were designed utilizing variations in steel tube wall thickness, steel tube length, and concrete strength as the primary parameters. Their failure modes, load–slip relationships, and longitudinal strain distribution patterns were examined through push‐out tests. Additionally, finite element models of the members were established using nonlinear spring elements based on the experimental data and subjected to numerical analysis. The research findings indicate that the ultimate bond strength of the composite L‐shaped CFSTs is positively correlated with steel tube wall thickness, steel tube length, and concrete strength. The strain distributions on the concave and convex faces of the L‐shaped steel tubes are identical. The results obtained from the finite element analysis closely match the experimental findings.

Impact force curve feature and failure mode identification method of concrete-filled steel tube members under lateral impact

A collision accident caused by derailed trains is a potential safety hazard for the structure adjacent to high-speed rail lines. Concrete-filled steel tube (CFST) columns are frequently utilized in modern high-speed railway stations, however CFST station columns may suffer severe damage from train collisions because of their high impact energy. Therefore, the method of identifying CFST members' failure modes under large impact energy was investigated in this paper. Adopting numerical simulation by LS-DYNA, the corresponding relationship between the local indentation and impact process was researched, the characteristic impact force time-history curve was proposed, and the effects of impactor and CFST specimen parameters on impact force feature were analyzed. The plateau proportion was established and quantified, then the mapping relationship between plateau proportion and failure degree was acquired. Research results indicated that local indentation developed primarily at impact force plateau stage. CFST specimens showed bending deflection when plateau proportion was below 67.5%, experienced fracture failure when it was above 77%, and exhibited crack failure when it was in between respectively. Finally, the empirical formulas for plateau proportion and maximum deflection were established, and a simplified approach for predicting the failure modes was put forward. The research provides suggestions for identifying damage patterns and strengthening CFST columns subjected to lateral impact loads.

Investigation on behavior of concrete-filled double steel tubular columns infilled with nanomaterial-based concrete subjected to axial compression

Nanotechnology has been applied in construction to enhance the mechanical properties of concrete by using nanomaterials. A new stiffening scheme called catty-cornered propped concrete-filled steel tube (CFDST) column was proposed and analyzed under axial compression. The steel tube of CFDST specimens was filled with normal and nano material-based concrete, including nano-silica (NS), carbon nanotubes (CNT), and nano-titanium dioxide (NT). The study collected data on ultimate capacity, load vs strain behavior, and load vs deformation response, as well as ductility index (DI), secant stiffness, composite interaction, and confining effect variation. The results showed that the proposed stiffening scheme increased the ultimate capacity of unstiffened CFDST by approximately 14%, and the use of nanomaterials in CFDST infill concrete resulted in an approximately 7% increase in load capacity. Increasing the number of stiffening bars improved the ductility and stiffness of the column section, while the inclusion of nanomaterials decreased the ductility index and improved the stiffness of the section. The proposed stiffening scheme resulted in better composite interaction and increased confinement. The study concluded that the utilization of nanomaterial-based concrete as an infill in the stiffened CFDST column could enhance its performance under axial compression loading.

Damage analysis of axially compressed concrete-filled steel tube stub columns based on acoustic emission

Concrete-filled steel tube (CFST) widely applied in engineering structures due to its superior behavior requires monitoring and assessment for damage level. Here, the acoustic emission (AE) technique was used to monitor the fracture process of concrete core in CFST stub columns subjected to axial compression. Test and analyzed results show that the damage process of CFST specimens can be divided into four stages: compaction, elastic-plastic stage, strengthening and secondary strengthening. In the elastic-plastic stage, the evolutionary features of the AE event rate, cumulative energy, Ib-value and crack classification are capable of providing an early warning for cracked concrete core. Full crack propagation can be identified by the rapid increase in the proportion of shear cracks near the inflection point of load, which is impermissible in engineering structures. According to the analyses of the AE event rate and signal intensity in the elastic-plastic stage, the confinement of steel tube with thicker wall thickness or higher strength is delayed, which implies that this confinement is suggested to be triggered early. It is indicated that the AE technique has the potential to monitor and evaluate the damage process of CFST stub columns under axial compression, which can provide additional insight into the failure mechanism and assist in the scheme of repairs.

Self-compacting concrete filled steel tube specimens with RHA and M-Sand replacement subjected to axial compression

ABSTRACT Because the cement content is inexorably growing, CO2 emissions from the cement industry are becoming unavoidable. The use of rice husk ash (RHA) makes the system environmentally benign. This paper investigates the effect of RHA and manufactured sand (M-Sand) on the axial capacity of circular Concrete Filled Steel Tube (CFST) specimens subjected to axial compression. Thirty-two CFST specimens infilled with self-compacting concrete (SCC) with a replacement of RHA and M-Sand to cement and fine aggregate in 0%, 15%, 20% and 25% proportions are subjected to axial compression. Results show that use of RHA beyond 20% decreased the compressive strength of concrete and eventually decreased the axial capacity of CFST and occurred higher local buckling irrespective of their L/D ratios. Failure modes, strength index, confinement along with steel and concrete contribution on axial capacities of CFST specimens are studied. The axial capacities of the specimens were compared with various design code results such as: American concrete institute, Australian Standards, Chinese code, Eurocode, and Japanese code. Among all the codes, Chinese code overestimated the test results and hence, the axial capacity equation is altered and compared the results with test data along with 278 circular CFST data available from literature and found the results to be accurate.

Axial compression experiment and numerical simulation of CFST short columns at an ultra early age

To study the axial compression performance of ultra-early-age concrete-filled steel tube (CFST) short columns, axial compression tests were carried out on short columns of CFSTs with different wall thicknesses of steel tubes and different concrete ages. The load-displacement curves and failure modes of CFST short column specimens under different concrete ages were obtained. The influence of wall thickness and concrete age on the mechanical properties of ultra-early-age CFST specimens was analyzed. The calculation formula for the bearing capacity of mature CFST short columns in GB50936–2014 was extended, and a new calculation formula for calculating the bearing capacity of ultra-early-age CFST short columns was proposed. Considering that there is no known concrete stress-strain expression for ultra-early-age CFST short columns in existing literature, this paper has adjusted the constitutive model of the core concrete in ordinary CFSTs and advanced a stress-strain relationship suitable for the core concrete in ultra-early-age CFST short columns.

Compressive performance of bamboo scrimber and concrete-filled steel tube columns

The light and high strength bamboo scrimber was buried in the core of concrete-filled steel tube (CFST) to form bamboo scrimber and concrete-filled steel tube column (BCFST), which was expected to reduce the amount of concrete and weight in CFST and promote the use of bamboo scrimber. To investigate the influences of key design parameters on the compressive performance, namely bamboo scrimber dimension, design concrete strength, and diameter to thickness of steel tube, ninety BCFSTs and eighteen CFSTs specimens were tested under axial compression. According to the experimental results, the improvement of the BCFST bearing capacity resulted from the contribution of the inner sandwich concrete and the bamboo scrimber under the constraint from the steel tube. The ultimate load rose with the increase of the bamboo scrimber dimension, concrete strength, and steel tube thickness. Note that the increase range of ultimate load of BCFSTs is 3.9% higher than that of CFSTs by increasing the dimension of bamboo scrimber, with a maximum increase at 43.9%. The damage modes of BCFSTs can generally be categorized into local buckling failure mode and shear failure mode. In addition, owing to the different compactness between the bamboo scrimber and the concrete, the stress-strain curve of the C80 concrete characterized the strain hardening tendency rather than the strain softening tendency in the C50 concrete. According to the load-strain curves of the BCFSTs and the contribution of the composite actions between the three parts of the member to the load-carrying capacity, the characteristic points were defined. A calculation model for accurately predicting the ultimate capacity of BCFSTs was proposed by considering the boundary conditions for both non built-in and fully built-in bamboo scrimber in CFSTs.

Experimental study of bond behavior between concrete-filled steel tube and UHPC-encased

This paper examines the bond behavior between steel and ultra-high-performance concrete (UHPC) in members made of UHPC-encased concrete-filled steel tubes (CFST). For the push-out test, a total of ten UHPC-encased CFST specimens, eight normal concrete-encased CFST specimens, and two CFST specimens were designed. The specimen parameters considered for variation include steel fiber content, thickness of the steel tube, anchorage length, thickness of outer concrete, and concrete type. Through experimental observation and measured data, the force failure process and crack development pattern of the specimens were observed, and load-slip curves were obtained for the loading end and the free end. A comprehensive analysis was conducted to examine the influence and mechanism of different parameters on the bonding performance of the specimens. Furthermore, an equation was derived to calculate the bonding strength at the interface between the steel tube and the outer concrete. The findings reveal that both UHPC-encased CFST and normal concrete-encased CFST specimens exhibited bond-anchorage failure, with the load-slip curve exhibiting a sudden change at the peak value, indicating a high degree of brittleness. The bond strength between steel and outer UHPC was found to be twice that between steel and core UHPC, and 1.4 times that between steel and outer normal concrete. Increasing the steel fiber content and the thickness of outer concrete within a certain range effectively improved the bonding performance of the specimens. The thickness of the steel tube did not have a significant effect on the bonding performance of UHPC specimens. Additionally, when the anchorage length reached a certain value, further increases did not significantly enhance the bond strength. Moreover, the influence range of the steel tube's anchorage length and outer UHPC on the bonding performance was smaller compared to outer normal concrete.

Experimental and analytical study on the dynamic response of rectangular concrete-filled steel tube under transverse impact loading

Rectangular concrete-filled steel tubes (CFSTs) have been widely employed as prominent structural members in modern constructions. During their life cycles, rectangular CFST members may suffer from impact loads, leading to a need for investigations into their dynamic performance. In this context, 23 rectangular CFSTs subjected to transverse impact loading were experimentally investigated using a drop weight testing system. The instantaneous impact force responses and motion states of the specimens were recorded at high frequencies. The aspect ratio, the steel ratio and the clear span of the specimen, as well as the impact energy conditions, were the key parameters in the test scheme, and their influence on the dynamic response of the rectangular CFST specimens was analyzed. A finite element (FE) model was developed to predict the impact response of rectangular CFSTs, in which the strain rate effects of steel and concrete were well considered, and good accuracy was achieved. Then, an equivalent relationship between rectangular and square CFSTs under transverse impact was proposed and verified, which achieves access for rectangular CFSTs to existing design methods.

Performance of circular concrete filled steel tubes after fire exposure: Experiments

This study investigated the performance of postfire concrete-filled steel tubes (CFSTs) to obtain some information on fire safety of CFST structures. To achieve this aim, 18 CFST specimens that were divided into six groups were used in the tests. One group represented the control group, and five groups were exposed to fire for 15, 30, 45, 60, and 75 min. After the fire exposure, these specimens were axially loaded until failure. The results indicated that 15 to 45 min of fire caused damage in the form of swelling of the steel tubes, whereas longer fire caused explosive failure. Explosive failure resulted in zero load-carrying capacity, leading to the collapse of CFST structures. Fire shifted the global buckling of control specimens to the local buckling of postfire specimens under axial loading. Fire detrimentally affected the loadxdisplacement behavior of the CFST specimens. After exposure to fire for 15–45 min, the ultimate load of the postfire CFST specimens was reduced by 6.0 %−55.3 %, the yield load was reduced by 13.8 %−66.0 %, and the stiffness was reduced by 15.1 %−36.3 % compared with those of the control specimens. An analytical model to predict the load-carrying capacity of postfire CFST columns was proposed. This model could be a useful tool for evaluating postfire CFST columns with short fire durations.

Stability bearing capacity of concrete filled steel tubular columns subjected to long-term load

Concrete-filled steel tube (CFST) are commonly used in modern building and bridge applications. Despite their popularity, studies on the investigation of the influence of long-term load on the stability bearing capacity of such elements are scarce. This study investigates how the key parameters including slenderness ratio (λ), axial load ratio (m), and eccentricity ratio (e/r) affect the stability bearing capacity of a CFST column under sustained load. Twenty three CFST columns were fabricated to investigate the effect of long-term load on the stability bearing capacity. Fourteen specimens were subjected to constant compressive loading for 462 days and then tested for failure. The remaining 9 were companion load-free specimens. A three-stage finite element method was used to predict the stability bearing capacity after creep. The results indicate that the stability bearing capacity of CFST columns decrease after being subjected to long-term load. Both the experimental and numerical results indicated that the load of steel tube for long-term load specimens reaching up to the elastic–plastic and plastic process was lower than that of the load-free specimens. Moreover, the corresponding strain of the creep specimens was greater than that of the load-free specimens when the member reached the maximum load. Benchmarking analyses have shown that the creep reduction coefficient (kcr) proposed for CFST columns can be used to predict the reduction of stability bearing capacity after creep. Furthermore, a collected database comprising 49 CFST specimens subjected to long-term load was used to investigate the proposed formulae for kcr. The results show that the formulae were consistent with the experiment results.

Dynamic compressive behavior of concrete-filled steel tubes under single and multiple impacts

In this study, the dynamic compression behavior of concrete-filled steel tubes (CFSTs) under single and multiple impacts was investigated using the split Hopkinson pressure bar (SHPB) apparatus. Analyses of the failure patterns, dynamic stress-strain relationship and dynamic compressive strength of CFST members were conducted, and the results for CFSTs were compared with those for plain concrete samples. Moreover, the confinement mechanism of the CFST specimens during the axial impact was investigated, and the influences of strain rate, steel tube thickness, concrete strength and height-to-diameter ratio on the dynamic increase factor of the CFST (DIFCFST) were explored. The experimental results indicate that the CFST specimens had excellent impact resistance under single and multiple impacts, and the dynamic compressive behaviors of CFSTs exhibited significant strain-rate dependence. Larger loading gas pressure can strengthen the confinement from steel tube to concrete, leading to an increase in the dynamic compressive strength of the CFST. Dynamic compressive strength reduction generally occurred during multiple impacts, while the strength enhancement could be observed at lower impact level due to insufficient single impact energy. The DIFCFST decreased with increasing steel tube thickness and concrete strength, while the height-to-diameter ratio had less effect on the DIFCFST. Based on the experimental test data, an empirical formula was proposed for the DIFCFST considering the strain rate, the confinement factor and concrete strength to predict the dynamic compressive strength of CFST members under axial impact.

Behavior of stiffened concrete-filled steel tube columns infilled with nanomaterial-based concrete subjected to axial compression

The use of nanotechnology in the field of construction emerged as the utilization of nanomaterials to improve the mechanical properties of concrete. In this study, a novel form of stiffening scheme was suggested, named a catty-cornered propped concrete-filled steel tube (CFST) column. The performance of the suggested stiffened CFST column was analyzed under axial compression. The steel tube of CFST specimens was filled with normal and nanomaterial-based concrete. The three kinds of nanomaterials were utilized viz., nano-silica (NS), carbon nanotubes (CNT), and nano-titanium dioxide (NT). The results of the investigation were collected in terms of ultimate capacity, load vs strain behavior, and load vs deformation response. The ductility index (DI), secant stiffness, composite interaction, and confining effect variation were also discussed further. It was observed that the suggested stiffening scheme was able to increase the ultimate capacity of unstiffened CFST by approximately 14%. The use of nanomaterials in CFST infill concrete also resulted in an approximately 7% increase in load capacity. Further increasing the number of stiffening bars improved the ductility and stiffness of the column section. On the other hand, the inclusion of nanomaterials resulted in a decrease in the ductility index and improved the stiffness of the section. The proposed stiffening scheme resulted in better composite interaction and increased confinement. It was also concluded that the utilization of nanomaterial-based concrete as an infill in the stiffened CFST column could enhance its performance under axial compression loading.

Post-fire load-reversed push-out performance of normal and lightweight concrete-filled steel tube columns: Experiments and predictions

In this study, the push-out experiment with load reversal and up to four load half-cycles was conducted on concrete-filled steel tube (CFST) members to investigate parameters affecting the natural bond capacity (chemical adhesion, micro-locking, and macro-locking) and bond-slip curves before and after heat-induced damage. Variables under study included the exposure temperature, length-to-diameter ratio of steel tube (L/D), diameter to thickness ratio of steel tube (D/t), grade of cementitious materials, and type of concrete (pumice lightweight concrete and normal concrete). The results showed that exposure to heat often had a negative effect on chemical cohesion and friction between the steel tube and concrete core. In this regard, after exposure to 200, 400, and 600 °C, the bond capacity of the lightweight CFST specimens respectively remained relatively unchanged (negligible increase or decrease) and declined by 84 and 94% compared with that at the ambient temperature. An increase in parameter L/D due to an increase in length and a decrease in diameter led to a drop and a rise in the bond capacity, respectively. As the D/t ratio increased (due to a rise in D or a drop in t) in the lightweight CFST specimens, the bond capacity declined after exposure to all the target temperatures. In addition, lightweight concrete specimens with a smaller cement grade showed greater bond capacities compared with the corresponding columns with a higher cement grade for most exposure temperatures. Moreover, the bond capacity of the specimen with normal concrete was significantly higher than the corresponding specimen with lightweight concrete (around 8 times) for the exposure temperature of 600 °C. By comparing half-cycles 1 and 3 of CFST specimens, the share of macro-locking was obtained as 12 and 34% for 600 °C. The initial stiffness and energy absorption of the specimens with the lightweight concrete experience a significant drop after exposure to 600 °C compared with the ambient temperature; however, an inverse trend was seen for the specimens with normal concrete. Finally, two simple models with proper accuracy were proposed to predict the normalized bond capacity of the CFTS specimens with lightweight and normal concrete in terms of temperature.

Flexural behavior of circular concrete filled steel tubes with partially incorporated demolished concrete lumps

For the past decade, researchers have been experimenting with the use of Demolished Concrete Lumps (DCLs) in structural members, as it has been proven to be a promising method to recycle concrete in different field applications. Although there have been several studies on incorporating DCLs in Concrete Filled Steel Tube (CFST) members, there are no studies that evaluate its effect on the flexural performance of CFST beams. Therefore, this paper focuses on studying the flexural behavior of CFST beams with DCLs, where the DCLs are inserted at the center of the cross-section. In total, fourteen circular CFST and two circular Steel Tube (ST) specimens were tested under bending using a four-point loading setup. The specimens are categorized into two different steel tube sections, where two circular sections of D/t = 55 (C1) and D/t = 45 (C2) were considered. The CFST specimens within each steel tube category differ in DCLs particle sizes and DCLs inner occupation area. CFST beams filled completely with normal concrete were also tested as control specimens. In addition, two CFST specimens from the steel section C2 were left partially hollow at the center to study the contribution of DCLs to the overall flexural performance. The results were very promising as the flexural behavior of DCL CFST specimens was very similar to the control CFST specimens. The DCLs’ particle sizes and the inner occupation area had minimal effect on the ductility, stiffness, yielding capacity, and ultimate capacity of the CFST specimens. The maximum percentage reduction in the flexural capacity between DCL CFST and control specimens were less than 2% for both sections whereas the percentage gain was up to 7%. Furthermore, the obtained flexural capacities were compared with nominal predictions from different design codes and models. All codes underestimated the capacities of all CFST specimens, with Han's model being the most conservative, followed by EC4, BS-5400–5, and AISC-LRFD, respectively.

Effect of spontaneous-combustion coal gangue aggregate on axial performance of square concrete-filled steel tube stub columns

This work experimentally studied the effect of spontaneous-combustion coal gangue aggregate (SCGA) on the mechanical performances of the square concrete-filled steel tube (CFST) stub columns subjected to axial loading. The influences of the substitution level of SCGA and the steel tube thickness on the failure mode, axial load–displacement response, ultimate load, elastic stiffness, and ductile performance were investigated. Trial results showed that the reduction in the ultimate compressive load of the CFST stub columns at an SCGA substitution level of 50% and 100% is within 5.96% and 12.10% respectively, compared to the CFST specimens made of natural aggregate (NA). Moreover, the elastic stiffness of the columns made of SCGA declined by 4.58%–16.67% and 22.44%–24.20% and their ductility coefficient by 1.10%–14.76% and 8.04%–18.27% at an SCGA substitution level of 50% and 100% respectively. To extend the parametric range of the experiment, the finite element models were analyzed in detail for estimating the influences of the SCGA substitution level, the concrete strength, the yielding strength of the steel tube, and the diameter of the stub columns utilizing Abaqus software. The results demonstrated that influences of SCGA incorporation on ultimate compressive load of CFSTs with square section could be presented via concrete strength and confinement coefficient. The design equations described in standard GB50936 were comparable to predict the ultimate load of the square CFST stub columns according to the finite element modeling data and verified by experimental results, and could be used for the routine design of CFSTs with SCGA.

Experimental study and design of bond behavior in concrete-filled steel tubes (CFST)

Steel plate-concrete (SPC) and concrete filled steel tube (CFST) push out test specimens are conducted to study the nature of the bond behavior between the steel tube and concrete and the according contribution of each component (chemical adhesion, micro-interlocking and friction). A total of 10 SPC specimens considering lateral normal pressure applied on the concrete and 18 circular CFST specimens with reversed loading have been carried out, where the surface roughness of the steel tube and the length of the specimens have been considered as variables. The chemical adhesion contributions are obtained through the SPC specimens without normal pressures. The micro-interlocking and friction can be calculated through both the SPC and CFST specimens. The micro-interlocking coefficient concept has been proposed and determined together with the friction coefficient through SPC test specimens with normal pressures. Both the micro-interlocking and friction component contribution to the bond behavior can be determined through the steel tube hoop strain from the CFST specimens and the obtained coefficient accordingly. The bond mechanism of CFST specimen has been systematically and quantitively explained by analyzing the full-history developments of the chemical adhesion, micro-interlocking and friction components. A bond-slip constitutive model considering both the physical significance and prediction accuracy has been proposed and verified, which may promote refined numerical analysis.

Distributed detection of internal cavities in concrete-filled steel tube arch bridge elements

Internal cavities in structural elements, such as in concrete-filled steel tubes (CFSTs) of arch ribs reduce confinement, bearing capacity, and the durability of the arch bridges. Formation of internal cavities may be either materials related, which affects proper consolidation, or construction related, especially in terms of proper timing and delivery of concrete slurry to horizontal and inclined structural members. Nondestructive test methods such as ultrasonics have been employed for detection of internal cavities. Despite their success, point-by-point inspection of structural members becomes time-consuming and inefficient, especially when entire structures need to be inspected. Infrared thermography (IR) provides a more reasonable global approach for detection of anomalies. The accuracy and resolution of IR depend on the local ambient conditions affecting convection and external thermal flux prior to detection by IR. The research described herein pertains to the development of a hybrid thermographic approach by using Brillouin fiber optic sensors (BFOSs) for direct detection of thermal convection at the surfaces of steel tubes. Accurate quantitative detection of internal cavity locations and their dimensions required development of a specific machine learning-based approach through which the distributed thermographic data was converted to a series of grayscale images. A Faster Region-based Convolutional Neural Network (Faster R-CNN) based on the improved VGG-16 (IVGG-16) network architecture was utilized for this purpose. The experimental work involved design and fabrication of a distributed temperature sensing sheet (DTSS) for simultaneous transmission of thermal energy into CFST specimens and acquisition of distributed thermographic data by the BFOS. By using the proposed approach, it was possible to detect the embedded internal cavities during the experiments.