Degradation Of Load Carrying Capacity Research Articles

Durability of externally strengthened concrete prisms with CFRP laminates and galvanized steel mesh attached with epoxy adhesives and mortar

The long-term bond degradation and strength retention of flexural bond prisms strengthened with carbon fiber-reinforced polymer (CFRP) composite and galvanized steel mesh (GSM) systems, bonded to concrete using epoxy adhesives and cement-based mortar under aggressive conditioning regimes are compared in this paper. Notched prisms strengthened using low-cord density steel mesh (LSM) with epoxy (SME-strengthened) and cement mortar (SMM-strengthened), and CFRP-epoxy systems were weathered under saline water and direct sunlight for a period of 28 and 540 days. The results of the study were analyzed based on experimentally obtained ultimate load (Pu) values and empirically calculated average bond shear stress (τavg) and prism strength retention (Rp) values. The bond strength degraded by 39 and 34 % in CFRP strengthened, 2.9 and 33 % in SME strengthened, and 2.8 and 10.8 % in SMM-strengthened specimens following the 540-day exposure to saline water and direct sunlight, respectively. The average prism retention ratio was calculated to be 0.61 and 0.66 for CFRP-strengthened, 0.97 and 0.67 for SME-strengthened, and 0.97 and 0.89 for SMM-strengthened specimens after 540 days of saline water and direct sunlight exposure. Flexural prism environment strength reduction factors (CE) were proposed as 0.60, 0.95, and 0.95 for CFRP, SME, and SMM-strengthened specimens under saline water exposures and 0.65, 0.65, and 0.85 for CFRP, SME, and SMM strengthened specimens under direct sunlight exposures. Saline water exposure was observed to be most critical to all strengthening systems. Although CFRP-strengthened specimens showed minimum degradation in load-carrying capacity under both conditioning regimes, they showed maximum bond strength reduction in contrast to SMM-strengthened specimens. It was observed that the choice of bonding agent significantly influenced the extent of bond strength degradation under extreme exposure regimes, like those that prevail in the UAE and the Persian Gulf.

Flexural durability of BFRP bars reinforced geopolymer-based coral aggregate concrete beams conditioned in marine environments

Geopolymer is an environmentally friendly material that utilizes industrial solid waste, boasting reduced greenhouse gas emissions and energy consumption. It exhibits superior performance in terms of corrosion resistance and thermal stability compared to ordinary Portland cement (OPC). These excellent properties are crucial for marine engineering construction, offering significant value and application potential in marine engineering materials. In this study, geopolymer was employed as the cementitious material, with marine resources like seawater and coral aggregates sourced from remote island areas effectively utilized, and basalt fiber-reinforced polymer (BFRP) bars were incorporated as reinforcements to develop BFRP bars reinforced geopolymer-based coral aggregate concrete (GPCAC) beams. Laboratory aging procedures were employed to investigate the deterioration patterns and damage mechanisms affecting the flexural behavior of GPCAC beams under seawater immersion and dry-wet cycling conditions, comparing them with cement-based coral aggregate concrete (CAC) beams. The experimental results indicated that both CAC and GPCAC beams under simulated marine environments experienced a reduction in the number of cracks upon damage, but their crack spacing and width increased. Moreover, the flexural stiffness of CAC and GPCAC beams after seawater attacks tended to increase, but this increased stiffness did not translate to a higher loading capacity. Conversely, as the corrosion age increased, the ultimate load and deflection values of both CAC and GPCAC beams decreased to varying extents, with the load-carrying capacity degradation more pronounced in seawater dry-wet cycling environments than in seawater immersion conditions. Compared to CAC beams, GPCAC beams exhibited superior resistance to seawater erosion. Following 12 months of seawater dry-wet cycling, the ultimate loading capacity of CAC beams decreased by 12.2%, while that of GPCAC beams only degraded by 9.5%. Predictions of the flexural capacity of GPCAC beams using equations from existing FRP-reinforced normal aggregate concrete (NAC) specifications indicated slightly higher values than the experimental values, with an error margin of less than 15%. Overall, the formulas for the flexural capacity of NAC beams in existing FRP-reinforced NAC codes remained applicable to CAC and GPCAC beams.

Seismic Performance of a Full-Scale Precast Concrete L-Shaped Column Frame Structure with a Grouted Connection

Precast concrete frame structure with nonrectangular columns are becoming attractive to researchers and engineers owing to advantages such as avoiding column protrusion from walls and saving space. This paper presents the experimental investigation of a full-scale precast concrete frame structure with L-shaped columns (PCFS) subjected to reverse cyclic loading. The precast connection adopted grouted anchor connection. As a comparison and benchmark, one full-scale monolithic sample (RCFS) was also tested. The results indicates that the greater steel ratio in the grouted anchor connection region leads to a high local stiffness of the elements, which contributed to delaying occurrence of the first crack at the grouting holes. The failure mode, lateral load capacity, stiffness degradation, load-carrying capacity degradation, energy-dissipating capacity, ductility and interlayer residual deformation were compared between two specimens. PCFS specimen exhibited comparatively better hysteretic behavior, energy-dissipating capacity, residual deformation-resisting capacity and ductility. For RCFS and PCFS specimens, the peak load difference was within 7%. About 15% increases in ductility were observed for PCFS specimen compared with RCFS specimen. The test results would provide reference for the application of the precast concrete structure with nonrectangular columns in the new residential buildings.

Behavior of T-shaped CFST column and U-shaped steel-concrete composite beam joints

This paper intends to study the seismic and shear behavior of joints for T-shaped concrete-filled steel tubular (CFST) column and U-shaped steel-concrete composite beam (USCB). In the joint details, the C-shaped slots cut in the T-shaped steel tube allowed the double-C channels used to connect the thin-walled U-shaped steel (USS) passing through the panel zone directly. A total of six full-scale specimens constructed according to the concept of strong column-weak beam were tested: two specimens were tested under the monotonic loading to obtain the load-carrying capacity and the other four specimens under cyclic loading to evaluate the seismic performance . The variables included the details of USCB, the axial load level, and the beam-column linear stiffness ratio . Test results demonstrated that the proposed joint details exhibited good seismic and shear performance. Three typical failure modes, i.e. welding fracture between the double-C channels and the steel tube, local buckling of the steel tube, and fracture of the double-C channels were observed. Some indexes of seismic performances, such as load-carrying capacity, deformability , degradation of load-carrying capacity, stiffness degradation , and energy dissipation capacity were evaluated. Finally, the joint shear resistance of the specimens was discussed according to the ASCE design guidelines. • An innovative joint details for T-shaped concrete-filled steel tubular column and U-shaped steel-concrete composite beam was proposed. • Some indexes of seismic performances for the joints, such as load-carrying capacity, deformability, degradation of load-carrying capacity, stiffness degradation, and energy dissipation capacity were evaluated. • The joint shear capacity was evaluated according the methods given by the ASCE Task Committee and by the previous findings of the authors.

Mechanical behavior of locally corroded circular steel tube under compression

Localized corrosion often occurs on steel members due to paint peeling led from insufficient durability of the coating material or human collision, which may significantly reduce the load carrying capacity and safety performance of the structures. The effects of the parameters such as localized corrosion penetration, circumferential corrosion ratio, longitudinal corrosion ratio, slenderness ratio and eccentricity of mechanical behaviors of locally corroded circular steel tubes under compression were studied experimentally and numerically. Based on the test results and the finite element analysis, a calculation method was proposed for predicting load carrying capacity of the corroded specimen under compression. It is found that localized corrosion did not change the failure modes of test specimens, and all of them failed in the limit point buckling mode. The factors affecting the degradation of load carrying capacity and stiffness of localized corrosion specimens from high to low were corrosion time, circumferential corrosion ratio and longitudinal corrosion ratio. The most unfavorable localized corrosion modes of load carrying capacity and stiffness were H3L3-270 and H3L1-270, respectively. The load carrying capacity degradation of members increases with the increase of the localized corrosion ratio, circumferential corrosion ratio and longitudinal corrosion ratio, while decreases with the increase of the eccentricity. The formula proposed can be used to predict the load carrying capacity of locally corroded circular steel tube members under compression.

Fatigue Behavior of Concrete Beams Prestressed with Partially Bonded CFRP Bars Subjected to Cyclic Loads.

The lack of ductility is the greatest concern in the applications of carbon fiber reinforced polymer (CFRP) materials, when used as pre-stressing reinforcements. To improve the ductility, a partially bonded FRP system which is intentionally unbonded in the middle part of the beam and bonded in both end parts of the beam has been developed and applied to prestressed concrete beams. While, many researchers investigated the instantaneous performance of partially bonded CFRP prestressed concrete beams, this study intended to evaluate the fatigue performance, the static load-carrying capacity after fatigue loading and ductility. Based on the fatigue loading tests followed by static loading tests, over-reinforced and web-confined partially bonded CFRP prestressed concrete beams exhibited satisfactory fatigue performance without cracks and stiffness degradation during fatigue loading. In addition, no degradation of load-carrying capacity was observed in static loading tests after the fatigue tests. The ductility index of concrete beams, prestressed with partially bonded CFRP bars, is combined with over-reinforcements and web-confinements, similar to that of beams prestressed with steel bars.

Cyclic testing and parametric study of bolted beam-height adjustable steel beam-to-column connections

A kind of bolted beam-height adjustable beam-to-column connections (BHA connections) is proposed for moment-resisting steel frames. It is applicable when the height of beams change in small ranges. A BHA connection is composed of a T-shaped connector, an L-shaped connector with ellipse-type blot holes and bolts. A cyclic test on four full-scaled subassemblies and a series of numerical parametric analyses of BHA connections were conducted to study their mechanical performance. The test results show that the BHA connections with properly stiffened L-shaped connectors possessed good hysteretic properties, stabile secant stiffness and load-carrying capacity degradation and excellent energy dissipation capacity. And the effect of connector slippage and beam web eccentricity was negligible. It can be found in the parametric study that the flange thickness of T-shaped connectors played a pivotal role both in the load-carrying capacity and elastic stiffness of BHA connections, and the elastic stiffness was also affected by the thickness of T-shaped connector webs, L-shaped connector flanges and webs, and column stiffeners. Moreover, the numerical results also demonstrated that the column stiffeners in joint regions could improve the efficiency of load transfer between beams and columns. This kind of connections can be used to assemble steel frames expediently.

Micro-indentation based study on steel sheet degradation through forming and flattening: Toward a predictive model to assess cold recyclability

Sheet metal forming has always been an important sector of the metal industry thanks to work hardening, however, a complex set of deformation entails evolution of damage in the material through the forming stages. With an outlook to cold recycling of sheet metals, this paper focuses on experimental quantification of damage and degradation in load carrying capacity due to the forming process. Assuming that cold recycling of sheet metals involves an intermediate flattening prior to the secondary forming process, the adverse effects of flattening on the material was also investigated. An industrial cold roll forming process was taken as case study. An experimental investigation using microhardness mapping on the cross-section of fold zones, in conjunction with 3D global-local FE modelling, were the basis of through-thickness damage analysis. Taking advantage of strain-hardness correlation a new method was established to extrapolate hardness for ductile damage characterization. Investigation on sensitivity of the measured microhardness to crystallographic texture and sample surface preparation backed the experimental results. The results particularly outlined the progressive decrease in load carrying capacity of material after forming and after flattening. The possibility of a secondary manufacturing process is discussed.

Axial-Bending Interaction Diagrams of Reinforced Concrete Columns Exposed to Chloride Attack

This paper focuses on reinforced concrete columns load carrying capacity degradation over time due to chloride induced steel pitting corrosion. The structural element is exposed to marine environment and the effects of corrosion are described by the time degradation of the axial-bending interaction diagram. Because chlorides ingress and consequent pitting corrosion propagation are both time-dependent mechanisms, the study adopts a time-variant predictive approach to evaluate residual strength of corroded reinforced concrete columns at different lifetimes. Corrosion initiation and propagation process is modelled by taking into account all the parameters, such as external environmental conditions, concrete mix proportion, concrete cover and so on, which influence the time evolution of the corrosion phenomenon and its effects on the residual strength of RC columns.

STRUCTURAL PERFORMANCE OF A PLANE FRAME FOR COMPOSITE CES STRUCTURAL SYSTEMS

A static loading test was carried out on a two-bay two-story concrete encased steel (CES) frame in which the beams and columns consisted of only steel and fiber reinforced concrete (FRC). In this test, the structural behavior of the frame such as the failure modes, hysteresis characteristics and deformation capacity was examined. Although the frame designed to be a weak beam-strong column system with the flexural yielding at the both ends of beams and the bottom of the first story columns, the shear yielding in the joint panel of the interior beam-column joints also occurred. The results showed that the use of FRC in the CES frame controlled the damage of the concrete. In addition, the frame had stable hysteresis loops with almost no degradation in load carrying capacity until the average story drift angle of 0.05 rad. Static inelastic analysis with reversal loading was also conducted to simulate the story shear versus story drift response of the frame. It is shown that the analytical results agree well with the test results in this paper.

Influence of strain rate on the deformation and fracture response of a 6061-T6 Al–50 vol.% Al 2O 3 continuous-reinforced composite

The compressive mechanical properties of an aluminum–matrix composite unidirectionally reinforced with Al 2O 3 fibers have been measured and characterized as a function of loading orientation. The influence of strain rate and fiber orientation on the deformation and fracture response of a 6061 Al–50 vol.% Al 2O 3 continuous fiber-reinforced metal–matrix composite (MMC) aged to a T6 condition is reported. The stress–strain response of this composite was found to vary substantially as a function of loading orientation; the quasi-static yield changing from nominally 250 MPa transverse to the fibers to ∼1.7 GPa parallel to the fibers under ideal conditions. Increasing the strain rate to 2000 s −1 was observed to only slightly increase the yield strength of the composite for both orientations. The main failure mechanism has been identified to be kinking, although an upper bound seems to be attained when the fibers reach their compressive strength. The experimental results are consistent with a plastic kinking model for strain hardening composites. The failure response of the composite transverse to the fibers, under both uniaxial stress (quasi-static and dynamic) and uniaxial strain loading, displays a protracted but substantial load drop after yield followed by continued degradation in load carrying capacity. Lack of ideal parallel fiber construction was found to lead to systematic buckling failure of the alumina fibers through the sample under uniaxial loading.

The Cracking of Ti-6Al-4V Alloys Under Sustained Load in Ambient Air

This study reveals that degradation in load-carrying capacity owing to sustained-load cracking (SLC) can occur in alloys of the Ti-6Al-4V family. Each of eight alloys tested with fatigue-precracked specimens in ambient room air exhibited the phenomenon, with degradations ranging from 11 to 35 percent. The degree of susceptibility to SLC was seen to be dependent upon crack orientation in rolled plate materials. The threshold stress-intensity factor below which SLC failures will not occur in these alloys, KIt, appears to represent a more conservative fracture-safe design parameter than the plane-strain fracture toughness KIc, which cannot account for time-delayed failures owing to SLC.