RC Members Research Articles

Effect of concrete cover on the pure torsional behavior of reinforced concrete beams: Analytical model

Experimental data on RC members subjected to pure torsion is compiled from published studies so far. Interestingly, the past studies have concentrated largely on members with small to moderate concrete covers, with handful data on members with thick concrete cover. Widely accepted torsion models including the diagonal compression field theory (CFT), softened truss model (STM) and softened membrane model for torsion (SMMT) are verified on the domains of experimental data missing-in specimens with thick concrete cover. With increasing demand for durability design of RC structures use of thick concrete cover, say up to 75 mm, is being introduced in practice. Recent experimental investigation by the authors demonstrated that spalling of concrete cover, particularly in cases of thick covers, significantly impacts the torsional behavior of RC members. The present study uses these set of experiments to look at the response prediction capability of the existing advanced models and assess the reliability of existing concrete cover spalling theories. For cases with thick cover, the existing models either did not adequately capture the ultimate capacity or erroneously predicted the torsional behavior of the members due to the different mechanics between RC beams with thin and thick covers. Similarly, the spalling theories failed to fully explain the physically observed spalling behavior. Guided by the recent experimental observations and the apparent gaps, the study provides a rational theory for the spalling of concrete cover. The initiation and gradual evolution of concrete cover spalling is traced by observing the acting spalling moment and spalling resistance formulated in the proposed spalling model. The proposed spalling model inherently assumes members susceptibility to spalling of cover concrete and is governed by, the thickness of the concrete cover, tensile strength of concrete, presence of rebar cage and their location, and size of the member. The theory is unified and can explain the spalling of cover due to torsion as well as shear. Its capability is examined by integrating the approach into existing truss model. When compared with state-of-the-art models, the proposed method provides consistent prediction with a relatively smaller scatter, for cases with thick concrete cover as well.

Shear resistance of RC members with closed FRP jacket for Eurocode 8

Shear resistance of RC members with closed FRP jacket for Eurocode 8

Structural Performance of RC Members Affected by Alkali–silica Reaction According to Crack Patterns

Structural Performance of RC Members Affected by Alkali–silica Reaction According to Crack Patterns

Bond stress-slip models and behavior of externally bonded aluminum alloy (AA) plates to concrete surface

This paper aims to assess the feasibility of employing high strength aluminum alloys (AA) with a rough (R) surface as externally bonded reinforcement (EBR) material. A comprehensive experimental investigation was carried out to evaluate the bond strength and behavior through single shear tests. The variables considered were concrete strength and bonded length, while the remaining parameters were kept constant. The study measured the ultimate load, extension, and strain values, and subsequently calculated the bond stress and slip. It was observed that the ultimate load, bond stress, and failure modes exhibited variations depending on the concrete strength and bond length. For a concrete strength of 60 MPa and a bonded length of 80 % of the concrete prism length, the ultimate load and maximum bond stress increased by 54.0 % and 66.0 % respectively, compared to those with a concrete strength of 20 MPa and the same bonded length. Additionally, when comparing a bonded length of 80 % (200 mm) of the concrete prism length to a bonded length of 20 % (50 mm), the ultimate load and maximum bond stress increased by 20.0 % and 99.0 % respectively for a concrete strength of 60 MPa. The study also developed bond stress-slip models for the AA-concrete interface, as well as models for predicting the ultimate load based on concrete strength and bonded length. These models displayed high accuracy in their predictions. In conclusion, the results indicate that AA plates are effective and suitable as EBR materials for the reinforcement of RC members.

An alternative modeling approach to incorporate bond-slip effects in fiber-based elements

Fiber-based elements can provide accurate estimates of the global and local inelastic response of RC members subjected to reversed cyclic loads. However, fiber-based elements per se do not account for the localized deformations that arise from strain penetration or bond-slip effects at the member ends. To account for bond-slip effects, some researchers have suggested the use of modified properties for the steel reinforcement fibers, or that a zero-length element be attached at the element ends. In this study, a modified modeling procedure is proposed to include anchorage-slip effects in reinforced concrete frame members modeled with fiber-based beam-column elements. The proposed modeling approach is based on local bond-slip laws that can be implemented in general-purpose fiber-element software, and it differs from existing approaches in that it does not require the use of a zero-length element (though it could be used), and that it can be used with any local bond-slip relationship representative of a variety of bar embedment conditions. As such, the model can be used to simulate the effects of bond-slip of short or long embedment lengths in confined or unconfined concrete. The proposed modeling procedure is validated by comparing the calculated response with experimental data from four independent investigations. The test data included three columns and two beam specimens with various confinement and reinforcement configurations and included members reinforced with conventional and high-strength steel reinforcement. The results from the analyses show that the proposed modeling procedure provides a very good estimate of both the global and the local responses (anchorage-slip rotation) of the specimens.

Strut-and-Tie Method for GFRP-RC Deep Members

The current code provisions in ACI 440.11 are based on the flexural theory that applies to slender members and may not represent the actual structural behavior when the shear span-to-reinforcement depth ratio is less than 2.5 (i.e., deep members). The Strut-and-tie method (STM) can be a better approach to design deep members; however, this chapter is not included in the code. Research has shown that STM models used for steel-reinforced concrete (RC) give satisfactory results when applied to glass fiber-reinforced polymer-reinforced (GFRP)-RC members with a/d less than 2.5. Therefore, this study is carried out to provide insights into the use of STM for GFRP-RC deep members based on the available literature and to highlight the necessity for the inclusion of a new chapter addressing the use of STM in the ACI 440.11 Code. It includes a design example to show the implications of ACI 440.11 code provisions when applied to GFRP-RC deep members (i.e., isolated footings) and compares it when designed as per STM provided in ACI 318-19. It was observed that current code provisions in ACI 440.11 required more concrete thickness (i.e., h = 1.12 m) leading to implementation challenges. However, the required dimensions decreased (i.e., h = 0.91 m) when the design was carried out as per STM. Due to the novelty of GFRP reinforcement, current code provisions may limit its extensive use in RC buildings, particularly in footings given the water table issues and excavation costs. Therefore, it is necessary to adopt innovative methods such as STM to design GFRP-RC deep members if allowed by the code.

Structural behaviors of different corroded RC members strengthened by different types of concrete jackets

Structural behaviors of different corroded RC members strengthened by different types of concrete jackets

Behavior of Circular Reinforced Concrete Columns Strengthened by Different Techniques Subjected to Axial Loading

Abstract In recent years, CFRP is the most popular material in strengthening RC members (columns, beams, slabs…etc.) as well as the textile-reinforced mortar (TRM) was also used as an alternative method for strengthening RC columns. The present study includes fifteen circular RC columns: One control column, ten columns strengthened by one or two layers of CFRP using EBR or EBROG methods, two columns was strengthened by textile-reinforced epoxy on grooves and last two columns were strengthened by textile reinforced mortar without grooves. The parameters of this investigation are type of strengthening technique (EBR, EBROG and TRM), number of CFRP layers, and the grooves configuration. The results showed that the columns strengthened longitudinally with one or two layers of CFRP strips using EBR technique enhanced the load-carrying capacity by 2 and 7.13%, respectively, when compared with control column. However, the load carrying capacity of RC columns strengthened by EBROG increased by the range of 11.65% to 71.4%. in comparison with control column. Finally, TRM strengthening method enhanced the load carrying capacity by about 21% to 58.5%. Also, the results showed that the EBROG and TRM approaches were improved the stiffness and ductility of the columns in comparison with EBR method.

Estimation of load-carrying capacity of cracked RC beams using 3D digital twin model integrated with point clouds and images

Digital twin (DT) is capable of managing, simulating, and future performance prediction of existing infrastructures by integrating three-dimensional (3D) geometric model and finite element (FE) model. This study aims to automatically generate geometric DT of existing RC members using point clouds and incorporate damages detected from images into FE models. An edge-based modeling method was proposed to extract sharp features for geometry reconstruction with incomplete point clouds, and an automated procedure was established for registering images to point clouds, mapping cracks detected in images to FE models, and assigning reduced material properties. Four RC beams were tested in a laboratory to validate the proposed method. Existing cracks were found to have a significant effect on structural capacity and failure mode. The results also showed that the developed FE model produced a good agreement between experimental and predicted failure loads while providing a more efficient and time-saving assessment protocol.

FIRE RESPONSE OF CIRCULAR RC COLUMNS STRENGTHENED WITH PBO FRCM

The use of externally bonded fiber-reinforced system, specifically fiber-reinforced polymer (FRP) system, has increased over the past decades due to their demonstrated efficacy in enhancing the structural integrity of various RC members. However, the vulnerability of FRP systems to elevated temperatures has raised a major concern and led researchers to explore different strengthening systems. This paper investigates the fire performance of circular RC columns strengthened with poly-paraphenylene-ben-zobisoxazole (PBO) fabric-reinforced cementitious matrix (FRCM) system. Four identical columns were cast, three of which were strengthened with varying number of layers of PBO-FRCM. The fire exposure followed ASTM E119 standards. The temperature response curves were analyzed, and post-fire conditions were assessed. Initially, columns with a greater number of PBO-FRCM layers exhibited significantly lower temperatures. As the fire exposure time increased, FRCM strengthened columns experienced more significant spalling than the control column, where the reinforcement was visible from all sides. This excessive concrete spalling resulted in matching temperature readings in both the strengthened and non-strengthened columns.

FIRE RESPONSE OF CIRCULAR RC COLUMNS STRENGTHENED WITH PBO FRCM

The use of externally bonded fiber-reinforced system, specifically fiber-reinforced polymer (FRP) system, has increased over the past decades due to their demonstrated efficacy in enhancing the structural integrity of various RC members. However, the vulnerability of FRP systems to elevated temperatures has raised a major concern and led researchers to explore different strengthening systems. This paper investigates the fire performance of circular RC columns strengthened with poly-paraphenylene-ben-zobisoxazole (PBO) fabric-reinforced cementitious matrix (FRCM) system. Four identical columns were cast, three of which were strengthened with varying number of layers of PBO-FRCM. The fire exposure followed ASTM E119 standards. The temperature response curves were analyzed, and post-fire conditions were assessed. Initially, columns with a greater number of PBO-FRCM layers exhibited significantly lower temperatures. As the fire exposure time increased, FRCM strengthened columns experienced more significant spalling than the control column, where the reinforcement was visible from all sides. This excessive concrete spalling resulted in matching temperature readings in both the strengthened and non-strengthened columns.

Improvement of impact acoustic inspection for high-speed railway track slabs using time–frequency analysis and non-defective machine learning

Slab track is one of the most highly used ballastless track forms in Japan's high-speed railway system (Shinkansen) and is composed from track slabs (precast RC or PRC member), a filling layer (mixture of cement, asphalt emulsion and fine aggregates) and concrete bed. As they age, it has been reported that the supporting conditions of track slabs change due to damage in the filling layer that in turn affects running stability. It is necessary to evaluate supporting conditions of track slabs accurately during maintenance of slab tracks prior to signs of cracking. Some previous works show the usefulness of impact acoustics and non-defective machine learning using frequency response spectrums as feature values for evaluating supporting conditions of track slabs. In this study, we examine a method of non-defective learning to improve the evaluation of supporting conditions: time-frequency response characteristics of impact acoustics using time-frequency analysis and resulting spectrograms as feature values. As a result, we have improved accuracy rate (from 81.90 % to 86.72 %) and F-value (from 70.79 % to 78.75 %) more precisely than conventional method based on frequency response functions as feature values.

Derivation of pressure-impulse diagrams for blast-loaded RC members

Derivation of pressure-impulse diagrams for blast-loaded RC members

A Generalized Nonlinear Beam Element for Slender RC Members Using a Polygonization Section Approach—Part II: Verification of Analytical with Experimental Results

The proposed mathematical model, described in detail in Part I of the present paper, aims to solve the common numerical problems that currently characterize classic fiber models when used with a moderate fiber grid for reasons of computational cost. To confirm these objectives, comparisons were made between analytical predictions of the model with experimental results of RC specimens and of a 2D scaled frame for a wide range of parameters that mainly concern slender specimens without significant influence of the bond slip phenomenon, different types of cross section shapes, and various loading histories. The agreement between analytical to experimental results was found to range from very good to excellent in some cases.

The investigation and estimation of flexural strength in shear-critically designed RC beams considering the steel fiber content

Despite numerous studies investigating the effectiveness of steel fibers in RC members subjected to various loading conditions, the majority ignored the impact of the interaction between the tensile reinforcement ratio, compressive strength of fibrous concrete, and steel fiber dosage. This paper aimed to inspect and quantify the effects of hooked-end steel fiber dosage on the bending behavior of RC beams considering the interaction arising from various longitudinal reinforcement ratios. For this purpose, an extensive and hybrid investigation involving both experimental and numerical techniques was conducted. In the experimental part, an investigation was carried out to evaluate the contribution of a specific amount of fiber ratio and various aspect ratios to the behavior of large-scale RC beams having a shear-deficient design. Three beams with a 0.5% hooked-end steel fiber ratio and one without fibers but with a minimum amount of stirrups were produced, and three-point bending tests were conducted. All beams having fibers utilized full flexural strength as in the beam with stirrups. Once the experimental studies were completed, the numerical models were then developed and validated with those of experimental three-point bending tests using the non-linear finite element code VecTor2 to establish a basis for the comprehensive parametric study. The parametric study consisted of 33 analyses. Rebar and volumetric fiber ratios were selected to be variables, and the ratios adopted were within the typical range used in structural engineering applications. Based on the regression analysis a linear and a quadratic model were proposed for estimating the yield and peak strength depending on the steel fiber content. Finally, the obtained models were verified using work from the literature and found to be effective in forecasting the strength of shear-deficient RC beams with 0.8%, 1.0%, and 1.2% tensile reinforcement ratios and hooked end steel fibers (0.5 to 1.5%).

A Generalized Nonlinear Beam Element for Slender RC Members Using a Polygonization Section Approach—Part I: Theoretical Formulation

A mathematical model for RC beam elements is presented that falls into the category of distributed inelasticity models discretizing the cross-section in polygons (trapezoids, triangles). The models falling into these categories are considered to be able to describe in the best manner the inelastic behavior of the element across its whole clear length, since its response results from the numerical integration of the stiffnesses of its cross-sections, while presenting an ideal combination of accuracy, simplicity, and computational cost. The behavior of the cross-section is described through the constitutive relationships σ–ε of its materials for cyclic loading. The main objectives for the development of the proposed mathematical model are as follows: (a) the increased accuracy of the results compared to existing experimental ones; (b) the limitless generalization of its application, regarding of the cross-section shape; and (c) the elimination of the numerical problems presented by the application of other related models, a fact that leads to their impractical use in real three-dimensional structures. The proposed model falls under the category of distributed inelasticity models. This paper focuses on its initial version, which targets slender beam elements with negligible shear and bond-slip effects (i.e., with ribbed bars, sufficiently anchored). Thus, it is applicable to 2D and 3D framed structures that fulfill these conditions, while its modular structure allows for future adjustments for the inclusion of other effects.

Optimized Hybrid Carbon-Glass Textile-Reinforced Mortar for Flexural and Shear Strengthening of RC Members

Optimized Hybrid Carbon-Glass Textile-Reinforced Mortar for Flexural and Shear Strengthening of RC Members

Nonlinear bond-slip model for fiber-reinforced polymer laminates externally bonded to thermally damaged concrete

This paper presents a novel nonlinear local bond-slip model for fiber-reinforced polymer (FRP) laminates externally bonded to thermally damaged concrete substrates. The proposed model is an extension of an existing two-parameter bond-slip model and incorporates two key parameters including interfacial fracture energy ([Formula: see text]) and interfacial brittleness index ([Formula: see text]). To study the variations of [Formula: see text] and [Formula: see text] with different thermal damage levels of the concrete substrate, an extensive experimental database of shear tests on FRP-to-thermally damaged concrete bonded joints was collected from the existing literature. The [Formula: see text] values were calculated from the peak pull loads with proper consideration of the bond length and width effects, while the [Formula: see text] values were obtained by least-squares regression analysis using experimental load-displacement curves or measured strain distributions in the FRP laminates. The results have indicated that the [Formula: see text] values initially exhibit a slight increase accompanied by mild thermal damage of the concrete substrate after exposure to moderately high temperatures; however, these values significantly decrease when the exposure temperature exceeds 300°C. The [Formula: see text] values initially decrease with high-temperature exposure and stabilize at around 50% of the initial values when the temperatures reach around 400°C. Despite the inherent variability in the test database, the proposed temperature-dependent bond-slip model has demonstrated its accuracy, as demonstrated by the comparisons between the theoretical predictions generated by the model and the corresponding shear test results. This interfacial bond-slip model is expected to serve as a constitutive law to characterize the bond behavior between externally bonded FRP laminates and thermally damaged concrete substrate, thus facilitating the practical application of high-performance FRP composites in the repair and strengthening of thermally or fire-damaged RC members.

FEM analysis of FRP RC members

The main reasons for choosing fiber reinforced polymer (FRP) materials in the process of designing reinforced concrete structures is their high strength and low weight-to-strength ratio. Also, the corrosion resistance of FRP is the reason for using this type of reinforcement in concrete in an aggressive environment. Since the number of long-term tests is insufficient, the long-term properties and behaviour of FRP reinforced concrete structures are not well-known, yet. The design codes offer the reduced utilisation capacity of FRP materials and it is becoming obvious that it would be appropriate to modify them according to the experiments of the last decades. Reduction factors limit mechanical properties ranging from 0.95 for CFRP to 0.5 for GFRP. The article presents a long-term experimental study based on a four-point bending test on simply supported concrete beams reinforced with GFRP reinforcement in comparison with the results of the FEM analysis.

Crack Width Comparison between ACI 318, Eurocode 2 and GB 50010 for Flexural RC Members

Crack Width Comparison between ACI 318, Eurocode 2 and GB 50010 for Flexural RC Members