Residual Load-bearing Capacity Research Articles

Comparative experimental analysis of the effect caused by artificial and real induced damage in composite laminates

This paper presents the results of an extensive experimental campaign aimed to examine the effect upon the vibration response and on the residual load-bearing capacity caused by both: isolated artificially induced interlaminar damage and low-velocity impact induced damage in composite laminates. The experimental programme included modal testing, drop-weight impact testing, ultrasonic inspection, transverse quasi-static loading testing and compression testing conducted on a set of 72 carbon fibre-reinforced composite laminated coupons. Both types of damage caused measurable changes in laminate performance, however marked divergent trends were observed. The results allowed for conclusions to be drawn regarding the adequacy of the artificial damage approach and highlighted the importance and role of other forms of degradation upon damage tolerance of laminated composites containing damage.

Failure mechanism of compressed reinforced and non-reinforced stone columns

The article deals with the issues of the strengthening and stabilisation of stone masonry columns reinforced by fabrics based on inorganic fibres under concentric compression. These structures represent a frequent design solution of load-bearing structures of historic buildings. High-strength carbon fibres fabric placed along the perimeter of a masonry column—in thirds of the column height—prevents the undesirable redistribution of compressive normal stresses along the compressed element’s cross section in the phase of crack propagation and crack development. In this phase, despite growing horizontal and vertical deformations (mainly in the central part), a masonry element is able to transfer the growing compressive load. In this case, a gradual exhaustion of the ultimate strength of the individual masonry components occurs as a consequence of wrapping in a composite based on high-strength fibres. The experimental research of the failure mechanism of stone columns made of coursed masonry of regular sandstone blocks and coursed masonry of irregular (freestone) blocks under concentric compression and the research of the performance of non-reinforced as well as CFRP-reinforced stone columns completed to-date reveals the necessity of a different approach to the assessment of the load-bearing capacity, or residual load-bearing capacity, of masonry composed of stone blocks.

Assessment of Structural Reliability of Bridge Beams Based on Measured Symptoms

The evaluation of the safety and the estimation of the remaining service life of existing structures need a thorough knowledge of the effective operational conditions, achievable implementing a health monitoring system. The measured quantities, if opportunely chosen, allow to follow the evolution of the structural behavior in the lifetime and to recognize the occurrence of damages. Hence the collected data can be considered as symptoms and used to assess the reliability and the prognosis of the monitored structure. This paper presents the application of symptom-based reliability formulation to a real case study: a set of precast bonded post-tensioned concrete bridge beams, recently dismounted after a service life of 50 years. Both static and dynamic tests were carried out at the aim of evaluating the residual load-bearing capacity and investigating the effects of deterioration on their dynamic properties. The experimental results of test campaigns have been used as symptoms in order to estimate the reliability function of the beams and the evolution of their resistance in time.

Fracture behavior of carbon nanotube/carbon microfiber hybrid polymer composites

Growing carbon nanotubes (CNTs) on the surface of fibers has the potential to modify fiber–matrix interfacial adhesion, enhance composite delamination resistance, and possibly improve toughness. In the present study, aligned CNTs were grown upon carbon fabric via chemical vapor deposition. Continuously monitored single-fiber composite fragmentation tests were performed on pristine and CNT-grafted fibers embedded in epoxy, and single-laminate compact-tension specimens were tested for fracture behavior. A significant increase (up to 20 %) was observed in the interfacial adhesion, at the cost of a decrease in the fiber tensile strength. As a result, the maximum load of the composite was decreased, but its residual load-bearing capacity more than doubled. The likely sources of these effects are discussed, as well as their implications.

Vibration tests on dismounted bridge beams and effects of deterioration

This paper aims at describing the tests campaign carried out on five precast bonded post-tensioned concrete bridge beams, recently dismounted after a service life of 50 years. The girders were part of the deck of a recently dismounted viaduct of an Italian motorway. The beams showed different deterioration levels, mainly due to the different exposure to corrosive agents. The test campaign were designed for evaluating the residual load bearing capacity of the members. Dynamic measurements were acquired before and after the static tests by using different excitation sources. This experimental research highlights that the natural frequencies of the beams can be used as a predictor of the ultimate bending moment of an existing structure.

On the orientation of fibres in structural members fabricated with self compacting fibre reinforced concrete

The incorporation of fibres into concrete produces important benefits, mainly on the residual load-bearing capacity. These improvements depend on the type, content and orientation of the fibres, being a strong relationship between the number of fibres in the fracture surfaces and the post peak parameters. Although the fibres could be homogeneously distributed after mixing, the casting and compaction processes can significantly affect the fibre distribution and orientation, and consequently the mechanical performance of the material. In the case of Fibre Reinforced Self Compacting Concrete (FR-SCC) the existence of significant flow and wall effects may influence fibre orientation. This paper analyzes the fibre orientation in thin structural elements cast with FR-SCC and its effects on the residual mechanical properties. A slab of 0.90 × 1.80 × 0.09 m, a wall of 0.50 × 2.00 × 0.08 m, and a beam of 0.15 × 0.15 × 2.50 m were selected as representative elements where different concrete flow conditions take place. A strong heterogeneity in the orientation of the fibres was found. The fibre orientation varied with the flow rate and with the wall effect; the thickness of the elements or the proximity to the bottom of the moulds appeared as important variables. It was demonstrated that in thin elements the residual mechanical properties can be quite different when diverse zones and/or directions of the structural elements are considered.

The Study of Post-Fire Load Bearing Capacity of Steel Column under Axial Compression Exposure to one-Side Fire

The section of steel column under axial compression presents heterogeneous material and its load condition changes after experiencing fire damage of one side. Taking H-section steel column under axial compression commonly used in engineering for study object in this thesis in order to study post-fire residual load bearing capacity, the post-fire eccentric effect of steel column under axial compression exposure to one-side fire is analyzed using the method of equivalent section which considering the change of the material strength after high temperature, the influence of non-uniform temperature field on material properties and the stiffness degradation of member section. Calculation methods of the eccentricity and additional moment are proposed and further given a formula of residual load bearing capacity.

Experimental Study on the Mechanical Properties of Reinforced Concrete Columns after Exposure to Fire

This paper provided three test data pertaining to the mechanical properties of reinforced concrete columns after exposure to ISO834 standard fire and three comparative test data pertaining to the mechanical properties of reinforced concrete columns at room temperature, mainly concerning the influence of fire on failure mode, distortion performance and ultimate load bearing capacity of reinforced concrete columns under axial and eccentric compression. Test results show that the failure mode of reinforced concrete columns after exposure to fire is basically same with that at room temperature. With the same concrete strength and heating condition, the bearing capacity of specimens reduces as the eccentricity increases. Strain along the section height of eccentric columns after fire basically agree with the plane section supposition while the flexural rigidity and ultimate load bearing capacity decreases obviously. The residual load bearing capacity of reinforced concrete columns after exposure to fire is only about 25% to 37% of that at room temperature.

Time-Dependent Load Bearing Capacity of Corroded R.C. Columns in Atmosphere Environment

As a supporting member of concrete structures, the reliability of the columns is of critical importance. To have models targeted to estimating the residual load-bearing capacity of corroded reinforced concrete (R.C.) columns so that inspection procedures and maintenance interventions can be well conducted, a time-dependent capacity model for corroded R.C. column is presented. The model was based on equilibrium equations on which load-bearing capacity of R.C. column at given time (year) can be predicted considering the overall deteriorations of rebar, concrete section, mechanical properties and bond behavior between corroded reinforcement and concrete. The model was verified by published literature test results of corroded R.C. columns served in atmosphere environment for years. The comparison of the model predictions with the test results shows the validity of the model. In the end, considerations for use of the model were suggested.

Mechanical behavior of tubular T-joint after fire

This paper presents experimental results of the post-fire behavior of tubular T-joints. The research aims at the failure modes and the residual strengths of the T-joints after fire. Three tests of full-scale tubular T-joints are conducted. The first one is carried out to study the mechanical behavior of T-joints under ambient temperature. The other two tests are performed to study the influence of pre-load, heating and cooling phase on residual load-bearing capacity of the tubular T-joints. The test results show that the sustained axial load on the brace has remarkable influence on the residual deflection of the T-joints which is cooled down to room temperature. The results of the experiments also indicate that the axial load level and heating and cooling history have more significant effects on the compressive stiffness of the T-joints than the residual strength. In the numerical study, the result of finite element model agrees well with the test result. The work in this paper provides a basis for further parametric analysis and theoretical study on the structural evaluation after fire.

Experimental Study on Residual Load Bearing Capacity of SRC Eccentric Columns after Exposure to Fire

This paper provided three test data pertaining to the mechanical properties of steel reinforced concrete (SRC) eccentric columns after exposure to fire and one comparative test data pertaining to the mechanical properties of steel reinforced concrete columns at room temperature. The influence of eccentricity on failure mode, distortion performance and ultimate load bearing capacity are mainly studied. Test results show that the failure modes of steel reinforced concrete eccentric columns after exposure to fire are similar to that at room temperature. Strain along the section height at mid-span section of eccentric columns before loaded to 90% ultimate load bearing capacity is linearly distributed and well agree with the plane section supposition. After exposure to fire, the flexural rigidity and load bearing capacity of specimens are all declined compared with that at normal temperature. In various loading stages from the initial loading to 80% ultimate strength, the ratio of flexural rigidity of SRC eccentric columns after exposure to fire and at normal temperature is ranged from 0.30 to 0.59. With the same concrete strength and heating condition, the ultimate strength of specimens decreases with the increasing of eccentricity. The ultimate bearing capacity of all specimens at normal room temperature is calculated on the method proposed by Chinese regulation JGJ 138-2001. The compared results of experimental values and calculating values show that the residual load bearing capacity of SRC eccentric columns after exposure to fire is about 69% to 81% of that at room temperature.

Investigation on Toughness of Fibre Cocktail Reinforced Self Consolidating Concrete after High Temperature

The effect of different fibres on the residual load-bearing capacity and the failure pattern of high-performance self consolidating concrete (HPSCC) after exposure to high temperature hass been studied in this work. The polypropylene fibers mitigate the spalling of HPSCC element clearly, but did not show clear effect on the mechanic properties of concrete. The macro steel fiber reinforced HPSCC showed higher flexural toughness and ultimate load before and after high temperatures. The mechanical properties of hybrid fibre reinforced HPSCC (HFHPSCC) after heating were better than that of mono-fibre reinforced HPSCC. The failure mode changed from pull-out of steel fibers at lower temperature to broken down of steel fibers at higher temperature. The use of hybrid fibre can be effective in providing the residual strength and failure pattern, and improving the toughness of HPSCC after high temperature.

Experiments on the residual load-bearing capacity of prestressed confined concrete columns

This paper is an experimental study of the prestressed external confinement (as opposed to the unstressed external confinement) of concrete compression members as a means of influencing load-deformation behaviour and residual capacity. Residual capacity is understood to be the load which can be absorbed in the event of ineffective or removed external confinement by a column which has previously been subjected to high loading in a confined state. The prestress and tensile stiffness of the confinement, as well as the loads on the confined system, were systematically varied in order to evaluate influences on residual capacity. 45 cylindrical confined specimens, as well as reference specimens, were subjected to concentric and eccentric, single and cyclic loading. The experiments confirmed the beneficial influence of confinement prestressing on concrete residual capacity. Prestressed confinement proved to be particularly effective in the case of cyclic loading and low confinement modulus, especially when compared to the unstressed variant. Analysis of the experimental results goes on to show that safety considerations are indispensable in the case of ineffective unstressed external confinement.

Experimental research on post-fire behaviour of reinforced concrete columns

This paper reports an experimental research into the effect of fire exposure time on the post-fire behaviour of reinforced concrete columns. Nine full-size reinforced concrete columns (45×30×300 cm) with two longitudinal reinforcement ratios (1.4% and 2.3%) were unexposed and exposed to the ISO 834 standard fire for 2 and 4 h with a constant preload. One month after cooling, the specimens were tested in axial load combined with uniaxial or biaxial bending. The test results show that the residual load-bearing capacity decreases with increase in fire exposure time. This deterioration in strength following an increase in fire exposure time can be slowed down by the strength recovery of hot rolled reinforcing bars after cooling. In addition, the reduction in residual stiffness is higher than that in ultimate load; consequently, much attention should be given to the deformation and stress redistribution of the reinforced concrete building subject to earthquakes after a fire.

Behavior of prestressing steels after a simulated fire: Fire-induced damages

Fire safety should consider not only the performance of the structure during the fire but also the behavior after cooling. Even if a fire does not give rise to apparent damage in the prestressed structure, mechanical properties of materials as well as load distribution can be affected. A verification of residual load-bearing capacity is necessary to determine if the structure can be maintained in use. In this work, the behavior of prestressing steels during and after fire has been studied. We have attempted to clarify two aspects that should be considered when assessing the security of the structures: the reduction of the prestressing loads due to the increment in the stress relaxation losses during exposure to high temperatures and the permanent damage of the residual mechanical properties of steel wires produced by high temperatures. This paper seeks to give a comprehensive and extensive explanation, in addition to a simplified estimation of the non-visible fire-induced damages produced in steel wires, with the hope that this information can be useful for engineers in the structural evaluation after a fire.

Fatigue Resistance of Resin-Bonded Post–Core–Crown Treated Teeth with Flared Root Canal

Here the objective was to evaluate fatigue resistance of anterior teeth with flared root canal restored using resin-bonded individualized fiber-reinforced composite (FRC) posts, prefabricated FRC posts, or prefabricated stainless-steel posts. Twenty-four maxillary central incisors were endodontically treated and prepared as flared root canals. They were divided into three groups of 8 each. In Group 1, individual posts were made of FRC material. Prefabricated FRC posts were used for Group 2 and prefabricated stainless-steel posts used in Group 3. All posts were cemented with a self-adhesive resin into canals. The composite core and porcelain fused to metal crown were made for each. The specimens underwent thermocycling and dynamic mechanical loading and survived specimens were loaded to fracture. The data were statistically analyzed using ANOVA and independent sample t-test. In the fatigue test, Group 1 had no failure; all Group 2 specimens failed; and 7 specimens of Group 3 survived. There were significant differences in numbers of cyclically mechanical loading times between Groups 1 and 2, and Groups 2 and 3 (p < 0.01, p < 0.05). The average of residual load-bearing capacity was 317.5 (±90.2) N for Group 1 and 403.8 (±138.2) N for Group 3 and there was no significant difference between these two groups. The fracture mode of specimens was cervical root fracture with or without vertical crack in both Groups 1 and 3. In Group 2 the debonding mostly occurred at the cement–post interface. This in vitro study indicates that the flared canal teeth restored with individually formed FRC posts and prefabricated serrated stainless-steel posts provide good resistance against cyclic loading. The teeth restored with conventional prefabricated FRC posts may not be strong enough to resist masticatory forces without a ferrule.

A simplified methodology for the evaluation of the residual life of corroded elements

Steel corrosion is one of the most common causes of deterioration of reinforced concrete structures. In fact, the chemical attack causes cracking in the concrete and a number of related effects that weaken the capacity of the composite, as well as reducing the cross-section of the rebars. It is therefore extremely valuable to have methods targeted to the evaluation of the structural damage induced by corrosion for estimating the residual load bearing capacity of a structure, and then for planning the maintenance interventions. The aim of this paper is the development of a simplified methodology capable of providing conservative estimates of the residual life of corroded structures. Because of the great amount of results obtained by the authors, general relationships between the corrosion level and the residual strength and ductility of beams are derived, and their evolution is predicted using conventional deterioration models. The methodology proposed is validated using experimental results from literature.

Quenched reinforcement exposed to fire

Idealised data are derived for the tensile strength of quenched and tempered prestressing steel and of quenched and self-tempered reinforcing bars for fire safety design. 0·2% stresses are derived as a function of the maximum temperature and, in addition, 2·0% stresses are provided. A strain of 2·0% is seldom found in ‘slack’ (not prestressed) reinforcement, but 2·0% stresses might be relevant for reinforcement in T-shaped cross sections and for prestressed structures, where large strains can be applied. All data are provided in a ‘HOT’ condition during a fire and in a ‘COLD’ condition after a fire. The COLD condition is relevant for analyses of residual load-bearing capacity of a structure after a fire exposure. It is also relevant for analyses of concrete structures exposed to fully developed fire courses. The reason is that compression zones of concrete are always the weakest in the cooling phase after a fire, and therefore data for the reinforcement are needed in this phase as well as in a HOT condition in order to assess the minimum of the load-bearing capacity of a structure. Quenched and self-tempered steel is widely used, but its properties at fire exposure are rarely investigated. Therefore, two test series have been made, which are presented in the present paper. In one test series, transient properties are found in a HOT condition during a fire and in the other, residual properties are found in a COLD condition after a fully developed fire course.

Anchorage and shear strength properties for composite tendons used in earthwork support systems

When employing advanced excavation techniques (e.g. continuous mining methods, mechanized tunnelling, etc.) it may be often necessary to excavate through previously supported areas. This means that the cutting heads of the excavation machine have to come into contact with supporting elements. In such situations, use of conventional steel support systems may damage the cutting bits of the machine, and thus, the support system must have the ability to be cut by standard cutters. As an alternative, composite tendons have the potential to be used as a support system in such conditions. The advantages of this type of bolts are; their cuttability, resistance against corrosion, and increase in rate of installation due to their lighter weight. The aim of this study is to explore the potential for use of composite tendons in situations that automated excavating techniques are to be used in grounds previously supported. As part of a research project with the Amirkabir University of Technology, a literature survey was conducted to explore the possibilities of the use of composite material as a support system in underground infrastructures. Secondly, the results of experimental study conducted by the author on two types of composite tendons are presented. Moreover, experiments were carried out on resin, as grouting material, and a summary of test results are presented. The experimental findings show that Weldgrip tendons have a very good capacity under shear loading. Also Weldgrip outer surface profile provides good pull-out strength as well as high residual load-bearing capacity.

Damage Propagation in a Composite Sandwich Panel Subjected to Increasing Uniaxial Compression after Low-velocity Impact

This paper is focused on an analytical model to simulate the damage propagation behavior and to predict the onset of damage propagation of a low-velocity impacted composite sandwich panel subjected to increasing uniaxial compressive load. Major damage modes due to impact, such as the residual indentation and the locally crushed core, are included into this model. Composite sandwich panels with unsymmetrical angle-plied facesheets are studied. A consequential core-crushing mechanism is incorporated into the analysis. Good correlations between the numerical results and the experimental data have been obtained. The far-field stress corresponding to the onset of damage propagation at a characteristic location near the damage zone has been captured accurately. This quantity can be used to represent the residual load-bearing capacity of an impact-damaged sandwich panel under compression. It serves as a characteristic value in a newly proposed event-driven failure criterion named damage propagation criterion. This criterion can be used in both the damage tolerant design and the safety evaluation of composite sandwich structures.