Spalling Of Cover Research Articles

Phase-field simulations of cover cracking in corroded RC beams

Corrosion of steel reinforcement bars is a crucial issue for the reinforced concrete structures. Being one of the most common deterioration mechanisms, corrosion severely affects the serviceability and durability of the structure as the steel reinforcement suffers from a reduction of the cross section and deterioration of the mechanical properties. Additionally, due to the steel oxidation process, residual material is formed, and the volume occupied by the reinforcement increases, leading to cracking and spalling of the concrete cover. A predictive model coupling concrete carbonation process and phase-field approach for brittle fractures is proposed to simulate the carbonation induced corrosion of rebar and the subsequent cracking phenomenon. First, a Fick diffusion law is used to describe the carbon dioxide (CO2) diffusion within the concrete which leads to a generalized corrosion process. As the corrosion evolves, the steel mass loss is converted into expansion strain due to the formation of residual material, and the damage evolution within the concrete cover is determined. After validation against examples from the literature, numerical simulations are performed, presenting a phase-field strategy capable to describe the cover cracking of corroded RC beams.

Seismic performance of seawater and sea sand concrete-filled ultra-high performance concrete tubes under low-cycle reversed lateral loading

Seawater and sea sand concrete (SWSSC) filled ultra-high performance concrete (UHPC) tube (SFUHPC tube) column is a cement-based tubular composite column, which combines the excellent compressive strength and toughness of UHPC and lateral confining action from fiber reinforced polymer (FRP) hoops. The novel composite system has the potential to be used in marine engineering. The aims of this paper focus on evaluating the seismic performance of SFUHPC tube columns for being designed in costal and marine engineering. A series of low-cycle reversed lateral loading tests were conducted on five relatively large-scale specimens. FRP hoop volumetric ratio, compressive strength of filling SWSSC, and the types of FRP bar were selected as test parameters in this investigation. The failure modes, hysteretic responses and effects of main parameters were studied and discussed. SFUHPC tube columns exhibited flexural failure mode without visible spalling of the UHPC cover. It is noteworthy that the limit plastic drift ratios of all SFUHPC tube columns exceed the specified limits (0.02) in accordance to the rare earthquake requirement in seismic design code. The current study reveals that the proposed composite columns have acceptable ductility and relatively reliable lateral resistant performance for being used in the marine engineering. From the point of view of seismic performance, filling high strength SWSSC in UHPC tube is acceptable for the proposed composite system.

Dynamic behavior of SRC columns with built-in cross-shaped steels subjected to lateral impact

This paper presents an investigation on the dynamic behavior of SRC columns with built-in cross-shaped steels under impact load. Seven 1/2 scaled SRC specimens were subjected to low-speed impact by a gravity drop hammer test system. Three main parameters, including the lateral impact height, the axial compression ratios and the stirrup spacing, were considered in the response analysis of the specimens. The failure mode, deformation, the absorbed energy of columns, as well as impact loads are discussed. The results are mainly characterized by bending-shear failure, meanwhile specimens can maintain an acceptable integrity. More than 33% of the input impact energy is dissipated, which demonstrates its excellent impact resistance. As the impact height increases, the flexural cracks and shear cracks observed on the surface of specimens were denser and wider. The recorded time-history of impact force and mid-span displacement confirmed the three stages of relative movement between the hammer and the column. Additionally, the displacements had a notable delay compared to the rapid changes observed in the measured impact load. The deflection of the mid-span did not exceed 5.90mm while the impact load reached peak value. The impact resistance of the specimen can be improved by proper design for stirrup ratios and increasing the axial load. However, the cracking and spalling of the concrete cover at the impact point was obvious with the increasing in stiffness.

Investigation on axial load-shorting behaviour of high strength concrete encased steel composite section

Experimental work on steel H-section fully encased by high strength concrete showed the occurrence of premature failure due to concrete cover spalling. This failure mode limits the utilization of full material strength and hence impedes the attainment of anticipated load-carrying capacity. This paper proposes an analytical model to predict the full range axial load-displacement behaviour of concrete encased steel composite section made from concrete grade up to C130 and steel section grade up to S690. The analytical model captures the compression behaviour of concrete encased steel composite section at the constitutive level including premature cover spalling, concrete confinement from stirrups and steel section. Concrete fracture energy is used to estimate the cover spalling strain, and a confinement model is proposed to evaluate the confining pressure provided by the steel section. Good agreement is observed between the analytical predictions and test results in terms of initial stiffness, peak load, residual strength, and the full range of load-displacement response. Based on the findings, recommendations are put forward for practical design of high strength concrete encased steel composite columns.

Determining Criteria for Assessment of RC Structures Affected by Carbonation-Induced Corrosion

The construction industry is now facing expanding and extensive activities in the area of assessing and retrofitting buildings and bridges that aligns with the sustainable construction strategy. These activities recognise the importance of extending the life of existing construction works thereby delivering environmental, economic and socio-political benefits. Reinforced concrete structures and their reliability are currently receiving considerable attention as a significant part of these structures reaches the design service life. Degradation processes such as carbonation- and chloride-induced corrosion have a major influence on the reliability and serviceability of concrete structures. The submitted study is primarily focused on reinforced concrete structures whose main degradation factor is carbonation of the concrete cover. Examples of such structures are cooling towers or industrial chimneys. Structures in the power industry are usually designed for service life of 40 years. Carbonation-induced corrosion results in visible cracks and unacceptable spalling of concrete cover. The aim of the study is to improve predictions of carbonation-induced corrosion propagation and to critically compare the criteria for degradation level assessment used in practice. The probabilistic analysis is based on measurements of concrete cover and carbonation depths and continuous observations of signs of corrosion on structural surfaces. The example of an industrial chimney shows that the limit of a severe failure, which requires (possibly repeated) minor repairs, is exceeded after about 17 years. The critical failure limit (30% of structural surface with visible signs of corrosion) is reached after 50 years, which seems to be sufficient as it is after 10 years than the usual design service life.

Axial compression behavior of ultra-high performance concrete columns confined by high-strength stirrups

Based on the axial compression test of 5 ultra-high performance concrete (UHPC) columns confined by high-strength stirrups and 4 UHPC columns confined by normal-strength stirrups, the bearing capacity, failure mode, steel strain and stress-strain curve for the confined UHPC columns were studied, and the influences of volume stirrup ratio, strength, spacing and configuration of stirrup on the axial compression behavior of confined UHPC were analyzed by combining ductility and toughness index. The results show that all the specimens have ductility damages, and the damage degree of UHPC columns confined by high-strength stirrups is lighter. The confined UHPC columns with high volume stirrup ratio, closely spaced, high-strength and complex ties configuration have good confinement efficiency, bearing capacity and deformation performance, which also leads to an ideal axial compression behavior. The effect of volume stirrup ratio on the axial compression behavior of specimens is greater than that of stirrup strength. Among the three factors affecting the volume stirrup ratio, such as spacing, configuration and diameter of stirrup, the stirrup spacing contributes most to the improvement of confinement performance, followed by the configuration and diameter of stirrup. The UHPC confined by high-strength stirrups presents better axial compression performance and residual bearing capacity than the normal-stirrups. The micro-bending of the longitudinal bars accelerates the cover spalling, while the high-strength stirrups with close space effectively delays the buckling of longitudinal bars, and significantly improves the overall performance of the confined UHPC. The micro-bending of longitudinal bars weakens the confine effect of the high-strength stirrups on the core UHPC, and the combination of high-strength longitudinal bars and stirrups is suggested. Based on the test data, the formulas to determine the bearing capacity of UHPC columns confined by stirrups are drawn.

Axial-moment interaction of high strength concrete encased steel composite columns: Experimental investigation

This paper investigates the failure mechanism and structural behavior of high strength Concrete Encased Steel (CES) composite columns subject to compression and bending. Twelve composite column specimens with C90 concrete and S500 steel section were tested under concentric compression, eccentric compression and four-point bending. All specimens had the same geometry and were reinforced with the same steel section and same amount of reinforcing bars. In some specimens, micro steel fiber was added into concrete mix with 0.5% or 1.0% dosage to delay or prevent the potential premature cover spalling. The load-carrying capacity, load-deformation response, post-peak ductility, flexural stiffness, as well as damage pattern are comprehensively analyzed. Comparison is made between the experimental results and analytical predictions using modern design codes such as EN 1994-1-1 and AISC 360–16. From the four-point bending test, the flexural stiffness obtained from the test seems to be smaller than the codes' predicted values, indicating that modification is needed to achieve better prediction of structural response of composite beams at the service load level. As for the axial force (N)-bending moment (M) strength interaction diagram, it is found AISC 360–16 gives overall more conservative prediction than EN 1994-1-1 method for high strength CES columns with C90 concrete and S500 steel. However, both methods overestimate the axial compression capacity although they can predict their flexural capacity well.

Behavior of Plastic-steel Fiber Reinforced Lightweight Aggregate Concrete Columns Subjected to Concentric Axial Loading

This paper presents the findings of an experimental study on the behavior of plastic-steel fiber (PSF) reinforced lightweight aggregate concrete (LWC) columns under axial compression loading. The experimental variables were concrete compressive strength, main reinforcement percentage, and PSF volumetric ratio. The behavior of the PSF reinforced LWC columns was evaluated in terms of the failure mode, load-bearing capacity, steel reinforcement strain, and ductility. The results showed that the addition of PSFs not only prevented premature spalling of the concrete cover but also strengthened the bond between the LWC and the steel bar. Compared with that of the control specimen, the ductility of the fiber-reinforced columns improved significantly. In addition to the experimental program, a numerical investigation based on nonlinear finite element (FE) analysis was performed using ANSYS 10.0. The experimental and numerical results were compared and found to be in satisfactory agreement. Furthermore, an analytical model developed in a previous study was used to predict the load-bearing capacities of the PSF reinforced LWC columns. The compressive strength of concrete, the spacing, and yield strength of transverse reinforcement were considered in the analytical models, and the analytical predictions were in agreement with the experimental results.

Service life prediction of corrosion-affected reinforced concrete columns based on time-dependent reliability analysis

Reinforced concrete columns in chloride-laden environments are prone to steel reinforcement corrosion, which reduces their capacity to resist applied loads. The current study modeled the time-dependent effect of corrosion as a non-stationary Gaussian process while accounting for the uncertainties of other parameters using random variables. The down-crossing rate of the residual capacity of these structures under combined axial loads and bending moments from the safe domain has been calculated using a recently derived closed-form equation, and the corresponding first passage probability has been estimated over the service life of the column. It was found that corrosion significantly affected the remaining service life of the columns due to the loss of reinforcement, especially in the tensile region of the column interaction curve. This indicates that the probability of failure due to lateral loads and bending moments in corrosion-affected columns can be significantly high; therefore, the load and resistance factors in design codes and standards should be calibrated accordingly. It is also found that if the effects of confinement loss of core concrete of column and spalling of concrete cover due to corrosion of transverse reinforcement are modeled as well the reliability index profiles in tensile and compressive regions of the interaction curve would match more closely. This is a research domain which needs further investigations.

Cyclic response of rectangular RC columns retrofitted by hybrid FRP sheets

In an effort to enhance the seismic capacity of non-seismically designed reinforced concrete (RC) columns, this study proposes a retrofit method for non-seismically designed RC columns using hybrid FRP (HFRP) sheets consisting of glass and carbon FRPs. HFRP sheets can be installed for retrofitting the RC columns in a structure employing electric equipment such as metro stations or tunnels. With the assumption that the cyclic behavior of RC columns retrofitted with wrapping HFRP sheets is dependent on their numbers of plies and their wrapping lengths, full-scale cyclic tests with a total of five specimens were carried out to verify their seismic performance and to confirm the effects of HFRP sheets. In addition, this study aims to construct an analytical model to predict the cyclic behavior of HFRP retrofitted RC columns for more practical applications. The analytical model can properly capture the cracking and maximum strength with reasonable tolerance and also well simulate the hysteresis loops including pinching phenomenon as well as confinement effect of wrapping HFRP sheets. The BC specimen without employing HFRP sheets suffers the lowest inelastic deformation following the strength degradation with the early spalling of concrete cover. The test and analytical results present that wrapping HFRP sheets results in slight increase in the initial stiffness whereas cracking and maximum strength enhanced up to 15% since the HFRP sheet delay the crack propagation. The specimens wrapped with HFRP sheets develops the significant increase in the deformation capacities due to the increase in the confinement of concrete around plastic hinge regions. For the maximum deformation capacity of retrofitted RC columns, the number of plies of HFRP sheets wrapped at the expected plastic hinge regions is more influential than their length. The deformation capacity of RC columns wrapping two plies of HFRP are enhanced up to at least 2.1 times of that of a non-seismically design bare RC column.

Analytical model for torsional behavior of RC members combined with bending, shear, and axial loads

In this study, a torsional behavior analysis model is developed for reinforced concrete (RC) members subjected to combined loads. In the analytical model, a member section is considered to be idealized as panel elements, on which a flexural analysis is first done considering axial forces to derive normal stresses acting on the cross-section. The panel elements are then analyzed with the normal stresses using the smeared truss model that satisfies the equilibrium and compatibility conditions. The various member forces are combined in terms of stress, in which the initial crack angle, the effective thickness affected by the combined stresses, and the distribution of the longitudinal reinforcement are reflected. From the combined stresses in the idealized panel elements, the member strengths are determined by applying multi-potential capacity criteria in which aggregate interlock failure, concrete crushing, and spalling of concrete cover are considered. The analytical model is verified by comparing it with existing test results, and it is found that the proposed model well evaluates the torsional behavior of RC members with and without shear force, bending moment, and axial force.

Experimental investigation on the seismic behavior and damage states of reinforced high strength concrete columns

Eight one-third scale concrete columns designed based on the latest seismic provisions for Special Moment Frame systems were experimentally investigated. To do so, the specimens were subjected to lateral cyclic loading and axial loads up to 35% of the axial capacity of columns, other test variables included the concrete strength, spacing, and diameters of transverse reinforcement. Accordingly, damage states were developed accounting for the specimens and attributed to the backbone curve by which the reinforced High-Strength Concrete (HSC) columns capacity and the performance levels were represented. Moreover, the range of drift ratios and damage index limits were proposed and utilized versus the performance levels of reinforced HSC columns used in Special Moment Frame systems. Finally, the correlation between the calculated drift ratio and observed damages such as initial crack, concrete cover spalling, longitudinal bar buckling, and concrete core crushing were studied. The results indicate that reinforced HSC columns have a more reliable performance at Immediate Occupancy and Damage Control levels compare to other levels.

Seismic strengthening of low-strength RC concrete columns using low-cost glass fiber reinforced polymers (GFRPs)

The main objective of this study is to investigate the performance of low-cost glass fiber reinforced polymer (GFRP) in strengthening the extremely low strength and poorly-detailed concrete columns typically found in a rural area of Thailand. These structures were usually constructed by local people without following any standard guidelines. A detailed survey of these structures revealed that longitudinal reinforcement in reinforced concrete columns is comprised of round bars along with widely spaced stirrups. Further, the compressive strength of concrete is observed very low i.e., 5.0 MPa. This type of column is referred as “Non-engineered Reinforced Concrete Column” or NRCC. To achieve the research objectives, two reinforced concrete columns representative of existing NRCC columns were constructed and subjected to cyclic loadings. One column was tested as built to serve as a control column, whereas second column was strengthened by using locally available low cost bi-directional GFRP. Both columns were subjected to lateral loading along with constant axial load to simulate the effect of earthquake forces. Test results indicate that the ultimate failure mode of the control column is very brittle. Severe crushing of concrete and spalling of concrete cover along with buckling of the longitudinal steel bars was observed at very low drift levels. The maximum lateral strength (14.73 kN) was observed at the drift level of 0.75 %. Following this, lateral load carrying capacity was rapidly dropped and gravity collapse was occurred at 1.75 % lateral drift. In contrast, the counterpart GFRP column performed very well under cyclic loading. The maximum lateral strength (19.47 kN) was observed at the drift level of 1.22 %. The damage was almost invisible up to 3% drift and the lateral strength degradation occurred gradually up to 8% drift. Finally, the well-known Paulay and Priestley’s model based on plastic hinge theory together with the method essentially based on ACI440 equations were used to calculate the lateral resistance and drift of both control and GFRP strengthened columns. The predicted results in terms of lateral forces are found in well agreement with the experimental results, however, predicted results in terms of lateral displacements are found on conservative side especially for lateral displacements at the onset of ultimate lateral load and 20 % drop in lateral load carrying capacity.

Axial-moment interaction of high strength concrete encased steel composite columns: Design recommendation

This paper investigates the axial force-moment (N-M) interaction behaviour of high strength concrete encased steel (CES) composite columns. A numerical method is developed to simulate the nonlinear inelastic behaviour of CES composite beam-columns, which includes concrete cover spalling, concrete confinement effect, buckling of longitudinal reinforcing bars, and stain-hardening of the encased steel section. The numerical model is implemented in a self-developed computer program to generate the load-deflection response, moment-curvature relation, as well as the N-M strength interaction diagrams. The accuracy of the proposed analysis method is validated by comparison with the experimental results report in the companion paper and the test data available in literatures. To further understand the resistance of CES columns consisting of high strength concrete and high strength steel, parametric study is performed to evaluate the effect of concrete compressive strength, steel yield strength, load eccentricity, as well as steel area ratio. Finally, a simplified method is proposed to construct the N-M strength interaction curve of CES composite columns. The proposed method can be used to design CES beam columns for concrete grade up to C100 and steel grade up to S960.

3D non-linear finite element analysis of concentrically loaded high strength reinforced concrete column with GFRP bar

The use of High Strength Concrete (HSC) material in Reinforced Concrete (RC) column has become widely used. HSC was found to be durable, strong in compression, but it has low ductility. This low ductility of HSC can be improved by providing confinement. However, for HSC with concrete strength higher than 70 MPa, additional clause for confinement in ACI 318-19 generates denser arrangement of transverse bars and eventually creates weak planes between the concrete core and the cover. These weak planes can trigger early cover spalling. To reduce the utilization of confining bars, high-strength Glass Fiber Reinforce Polymer (GFRP) bar can be used. However, the performance of GFRP bar varies significantly from their uniaxial behavior in tension or compression to the real performance when it is used as the main reinforcement. For that reason, this paper tries to investigate the behavior of HSC RC column with bars made of conventional steel rebar and with GFRP bars. Due to limited data on the strain gauge reading on the GFRP bars from the available test result, an inverse analysis is carried out to determine the best stress-strain curve for GFRP bars used as the main reinforcement. For that purpose, an inhouse finite element package called 3D-NLFEA is used. From the comparisons, it was found out that the peak load, softening behavior, and the concrete core enhancement prediction agrees well with the test result. From the inverse analysis, only 25% and 45% of the GFRP bar yield strength can be deployed when loaded under compression and tension, respectively.

Mechanical Degradation of Reinforced Concrete Columns Corroded Under Sustained Loads

This study presented an experimental investigation on the degradation of mechanical performance of reinforced concrete (RC) columns with the reinforcements corroded under sustained loads. A total of ten RC column specimens were tested. The effects of different levels of sustained load (0%, 30%, and 60% of the designed ultimate bearing capacity Nu) and reinforcement corrosion (0%, 5%, 10%, and 20%) on the failure modes, ultimate bearing capacity, and axial load–axial deformation relationship were analyzed. The results showed that the coupling adverse effects due to the reinforcement corrosion and sustained load considerably exacerbate the mechanical deterioration of RC columns and turn the failure mode into a much more brittle manner. Compared with the control specimen L-0-0, the ultimate bearing capacity of the specimen L-4-20 could be reduced as much as about 42%. Based on the test results, an improved model was proposed to estimate the ultimate bearing capacity of corroded RC columns, in which the effects of the corrosion of both longitudinal reinforcements and stirrups and the corrosion-induced spalling of concrete cover were taken into consideration. The close agreements between the analytical predictions and test results prove the applicability of the model.

Experimental Study and Analytical Model of Concrete Cover Spalling Induced by Steel Corrosion

AbstractConcrete cover spalling induced by steel corrosion is generally assumed to be the limit state of concrete structures when considering their structural safety. A new method for testing the r...

Core confinement and cover spalling in square and rectangular SCC and SFRC columns

To ensure adequate strength and deformability, modern concrete codes require the use of closely-spaced confinement reinforcement in columns. In ductile columns, these requirements result in congested columns which are difficult to construct; the use of self-consolidating concrete (SCC) and steel fiber reinforced concrete (SFRC) can facilitate the construction of such columns. However, limited research exists on the axial behaviour of SCC columns. Moreover, further research is needed to understand the influence of fibers on core confinement in SCC columns. This research aims at better understanding the influence of transverse reinforcement and fibers on core confinement in SCC columns. To achieve this objective, this paper presents the results from thirteen columns built with SCC, steel fibers and varied amounts of transverse reinforcement. The columns, which had square (250 mm × 250 mm) and rectangular (350 mm × 175 mm) cross sections, were built without cover in order to examine the competing influence of transverse reinforcement and fibers on core confinement. The results confirm that an increase in transverse steel-ratio improves the axial strength and ductility of SCC columns. The findings also show that fibers improve core behaviour in SCC columns, with a more significant effect in columns with low transverse steel-ratios compared to well-confined columns. The results also point to a relative loss in confinement and fiber efficiency when the sectional shape changes from square to rectangular. The influence of fibers on the cover contribution was also studied using the results from companion columns containing identical reinforcing cages but constructed with cover. The results show that fibers transform the cover spalling mechanism from sudden to gradual, allowing the cover to contribute significantly to column capacity and ductility. However, bar buckling plays an important role in the SFRC cover spalling mechanism, particularly in columns with large tie spacing. The paper also presents an analytical study which predicts the axial response of the columns using different confinement models. The results show that the Cusson-Paultre and Aoude models can predict the axial response of plain and fiber-reinforced SCC columns, however recalibration of the models is recommended for columns having rectangular cross sections.

Model-Based Assessment of Long-Term Serviceability and Fire Resistance for Underground Reinforced Concrete Ducts

The collapse simulation of underground reinforced concrete (RC) ducts surrounded by soil foundations in a fire is conducted using multi-scale finite element analysis. The underground RC ducts are found to be deteriorated by both long-term soil subsidence and creep and shrinkage of concrete. The long-term shear crack risk is clarified by site inspection of some underground RC ducts. To conduct the structural risk assessment, the multi-scale simulation is applied and its performance upgraded to 1000°C. The release of chemically bound water from hardened cement paste is developed in the scheme of micropore structural modelling. It is linked with the mesoscale thermodynamics of vapour and the mechanics of the solid skeleton, and its fracture is integrated with the macroscopic cracked concrete constitutive models. This framework is upgraded to include the spalling of concrete cover in the event of a fire, and the exposure of reinforcing bars to high temperatures during a fire is reproduced. As the moisture dynamics inside the micropores of concrete can be used to determine the creep and shrinkage of concrete, long-term RC deformation can also be consistently taken into account for its normal use.

Structural performance of novel SCARC column under axial and eccentric loads

A novel spiral confined angle-steel reinforced concrete (SCARC) column was developed in this study. A total of 16 specimens were prepared and tested (eight of them were tested under axial loading, the other eight were tested under eccentric loading). The failure processes and load-displacement relationships of specimens under axial and eccentric loads were examined, respectively. The load-carrying capacity and ductility were evaluated by parametric analysis. A calculation approach was developed to predict the axial and eccentric load-carrying capacity of these novel columns. Results showed that the spiral reinforcement provided enough confinement in SCARC columns under axial and low eccentric loads, but was not effective in that under high eccentric loads. The axial load-carrying capacity and ductility of SCARC columns were improved significantly due to the satisfactory confinement from spirals. The outer reinforcement and other construction measures were necessary for SCARC columns to prevent premature spalling of the concrete cover. The proposed calculation approach provided a reliable prediction of the load-carrying capacity of SCARC columns.