Creep of concrete masonry walls strengthened with FRP composites

The objective of the research was to examine the creep behavior of masonry walls strengthened with FRP composites compared to that of conventional reinforcement. Eight full-scale (40 in wide by 96 in tall [1.02 m × 2.44 m]) unreinforced concrete masonry walls were constructed for testing long-term deflections out-of-plane. The walls were strengthened with externally bonded CFRP or GFRP composites. Two additional walls were constructed with mild steel reinforcement grouted in the center cell of the specimens. Long-term deflections due to creep in FRP reinforced walls were shown to be ≈22–56% higher than those of steel reinforced walls.     

Creep effect on the behavior of concrete beams reinforced with GFRP bars subjected to different environments

The effect of different environmental conditions on the creep behavior of concrete beams reinforced with glass fiber reinforced polymer (GFRP) bars under sustained loads is investigated. This is achieved through testing concrete beams reinforced with GFRP bars and subjected to a stress level of about 20–25% of the ultimate stress of the GFRP bars. Reference beams were loaded in the temperature-controlled laboratory (24 ± 3 °C). Other test beams were either completely or partially immersed in different environments (tap-water and sea-water) at elevated temperature (40 ± 2 °C) to accelerate the reaction. During the exposure period, which lasted for ten months, strains in concrete and GFRP bars as well as the midspan deflections were recorded for all considered environmental conditions. The results show that the creep effect due to sustained loads was significant for all environments considered in the study and the highest effect was on beams subjected to wet/dry cycles of sea-water at 40 ± 2 °C.     

Damaged RC beams repaired by bonding of CFRP laminates

This paper summarizes the results of experimental studies on damaged reinforced concrete beams repaired by external bonding of carbon fiber reinforced polymer (CFRP) composite laminates to the tensile face of the beam. Two sets of beams were tested in this study: control beams (without CFRP laminates) and damaged and then repaired beams with different amounts of CFRP laminates by varying different parameters (damage degree, CFRP laminate width, concrete strength class). All beams were tested in four-point bending over a span of 1800 mm. The tests were carried out under displacement control. The most investigated parameter in this experimental study is damage degree (ratio between pre-cracked load and load capacity of control beam). Repairing damaged RC beams with externally bonded CFRP laminates were successful for different degrees of damage. The observed failure modes were peeling off and interfacial debonding. These failure modes depend only on the laminate width.The results indicate that the load capacity and the rigidity of repaired beams were significantly higher then those of control beam for all tested damage degrees. The authors remarked that for a load capacity improvement, reinforcement with a CFRP having about a half width of the beam is satisfactory. Finally, the contribution of CFRP laminates on the load capacity and rigidity of repaired RC beams is significant for any concrete strength class.     

Experimental study on CFST members strengthened by CFRP composites under compression

The concrete filled steel tubular (CFST) members become very popular in the construction industry and, at the same time, aging of structures and member deterioration are often reported. The actions like implementation of new materials and strengthening techniques become essential to combat this problem. This research work aimed to investigate the structural improvements of CFST sections with normal strength concrete externally bonded with fibre reinforced polymer (FRP) composites. For this study, compact mild steel tubes were used with the main variable being FRP characteristics. Carbon fibre reinforced polymer (CFRP) fabrics were used as horizontal strips (lateral ties) with several other parameters such as the number of layers, width and spacing of strips. Among thirty specimens, twenty seven were externally bonded with 50 mm width of CFRP strips with a spacing of 20 mm, 30 mm and 40 mm and the remaining three specimens were unbonded. Experiments were undertaken until column failure to fully understand the influence of FRP characteristics on the compressive behaviour of square CFST sections including their failure modes, axial stress–strain behaviour, and load carrying capapcity. From the test results, it was found that the external bonding of CFRP strips provides external confinement pressure effectively and delays the local buckling of steel tube and also improves the load carrying capacity further. Finally, an analytical model was proposed herein for predicting the axial load carrying capacity of strengthened CFST sections under compression.     

碳纤维片材加固钢筋混凝土梁的可靠度分析

建立了 FRP 加固钢筋混凝土梁的极限状态方程;讨论了碳纤维片材的耐久性、纤维布与混凝土间界面的粘结耐久性对加固有构件抗力的影响;通过一次二阶矩法计算了《碳纤维片材加固混凝土结构技术规程》(CECS146:2003)加固的钢筋混凝土梁抗弯可靠度指标,计算结果大于3.7,表明满足《建筑结构可靠度设计统一标准》(GB50068-2001)对抗弯可靠度指标的要求。     

CFRP strengthened openings in two-way concrete slabs – An experimental and numerical study

Rehabilitation and strengthening of concrete structures with externally bonded fibre reinforced polymers (FRPs) has been a viable technique for at least a decade. An interesting and useful application is strengthening of slabs or walls where openings are introduced. In these situations, FRP sheets are very suitable; not only because of their strength, but also due to that they are easy to apply in comparison to traditional steel girders or other lintel systems. Even though many benefits have been shown by strengthening openings with FRPs not much research have been presented in the literature.In this paper, laboratory tests on 11 slabs with openings, loaded with a distributed load are presented together with analytical and numerical evaluations. Six slabs with openings have been strengthened with carbon fibre reinforced polymers (CFRPs) sheets. These slabs are compared with traditionally steel reinforced slabs, both with (four slabs) and without openings (one slab). The slabs are quadratic with a side length of 2.6 m and a thickness of 100 mm. Two different sizes of openings are used, 0.85 × 0.85 m and 1.2 × 1.2 m.The results from the tests show that slabs with openings can be strengthened with externally bonded CFRP sheets. The performance is even better than for traditionally steel reinforced slabs. The numerical and analytical evaluations show good agreement with the experimental results.     

Experimental and finite element analysis on the steel fiber-reinforced concrete (SFRC) beams ultimate behavior

Steel fiber-added reinforced concrete (SFRC) applications have become widespread in areas such as higher upper layers, tunnel shells, concrete sewer pipes, and slabs of large industrial buildings. Usage of SFRC in load-carrying members of buildings having conventional reinforced concrete (RC) frames is also gaining popularity recently because of its positive contribution to both energy absorption capacity and concrete strength.This paper presents experimental and finite element analysis of three SFRC beams. For this purpose, three SFRC beams with 250 × 350 × 2000 mm dimensions are produced using a concrete class of C20 with 30 kg/m³ dosage of steel fibers and steel class S420 with shear stirrups. SFRC beams are subjected to bending by a four-point loading setup in certified beam-loading frame, exactly after having been moist-cured for 28 days. The tests are with control of loads. The beams are loaded until they are broken and the loadings are stopped when the tensile steel bars are broken into two pieces. Applied loads and mid-section deflections are carefully recorded at every 5 kN load increment from the beginning till the ultimate failure.One of the SFRC beams modeled by using nonlinear material properties adopted from experimental study is analyzed till the ultimate failure cracks by ANSYS. Eight-noded solid brick elements are used to model the concrete. Internal reinforcement is modeled by using 3D spar elements. A quarter of the full beam is taken into account in the modeling process.The results obtained from the finite element and experimental analyses are compared to each other. It is seen from the results that the finite element failure behavior indicates a good agreement with the experimental failure behavior.     

Investigation of insulated FRP-wrapped reinforced concrete columns in fire

Research has demonstrated that fibre-reinforced polymers (FRPs) can be used efficiently and safely in strengthening and rehabilitation of reinforced concrete structures. However, the use of FRPs in buildings has been limited because relatively little is known about the behaviour in fire of reinforced concrete structural members that have been strengthened with FRP systems. This paper presents the recent results of an ongoing experimental study of the fire performance of FRP-wrapped reinforced concrete circular columns. The results of fire tests on two columns are presented, one of which was tested without supplemental fire protection, and one of which was protected by a supplemental fire protection system applied to the exterior of the FRP-strengthening system. The primary objective of these tests was to compare the fire behaviour of the two FRP-wrapped columns and to investigate the effectiveness of the supplemental insulation system. The thermal and structural behaviour of the two columns are discussed. The results show that, although FRP systems are sensitive to high temperatures, satisfactory fire endurance ratings can be achieved for reinforced concrete columns that are strengthened with FRP systems by providing adequate supplemental fire protection. In particular, the insulated FRP-strengthened column in this study was able to resist elevated temperatures during the fire tests for at least 90 min longer than the equivalent uninsulated FRP-strengthened column.     

Behaviour of RC beams corroded under sustained service loads

This paper presents the results of an experimental study conducted to characterize the structural behaviour of reinforced concrete beams corroded whilst subjected to constant sustained service loads. Corrosion of tensile steel bars was induced by an accelerated corrosion process using a 5% solution of NaCl and a constant impressed current. Four RC beams were tested, each with a width of 153 mm, a depth of 254 mm and a length of 3000 mm. Beams were tested whilst under a load equivalent to 1%, 8% and 12% of the ultimate load. Longitudinal tensile and compressive strains were monitored during the corrosion process and used to determine the variation of the depth of the neutral axis, the curvature and the second moment of area of beams with the time of electrolysis. The results indicate that the longitudinal strains, the depth of the neutral axis and the curvature of beams depend on both the level of corrosion and the applied service load whilst the second moment of area is mostly influenced by the level of corrosion.     

Effects of steel fiber addition on mechanical properties of concrete and RC beams

C20 and C30 classes of concrete are produced each with addition of Dramix RC-80/0.60-BN type of steel fibers (SFs) at dosages of 0, 30, 60 kg/m³, and their compressive strengths, split tensile strength, moduli of elasticity and toughnesses are measured. Nine reinforced concrete (RC) beams of 300 × 300 × 2000 mm outer dimensions, designed as tension failure and all having the same steel reinforcement, having SFs at dosages of 0, 30, 60 kg/m³ with C20 class concrete, and nine other RC beams of the same peculiarities with C30 class concrete again designed as tension failure and all having the same reinforcement are produced and tested under simple bending. The load versus mid-span deflection relationships of all these RC and steel-fiber-added RC (SFARC) beams under simple bending are recorded. First, the mechanical properties of C20 and C30 classes of concrete with no SFs and with SFs at dosages of 30 and 60 kg/m³ are determined in a comparative way. The flexural behaviours and toughnesses of RC and SFARC beams for C20 and C30 classes of concrete are also determined in a comparative way. The experimentally determined (mid-section load)–(SFs dosage) and (toughness)–(SFs dosage) relationships are given to reveal the quantitative effects of concrete class and SFs dosage on these crucial properties.     

Composite tubes as an alternative to steel spirals for concrete members in bending and shear

Glass fibre-reinforced polymer (GFRP) tubes are compared to steel spiral reinforcement in circular concrete members with longitudinal reinforcement and prestressing, using six beam tests. Two 324 mm diameter and 4.2 m long prestressed specimens were tested in bending. Four 219 mm diameter reinforced specimens were also tested, including two 2.43 m long beams tested in bending and two 0.6 m long beams tested in shear. In each set, one specimen was essentially a concrete-filled GFRP tube, while the other control specimen included steel spiral reinforcement of comparable hoop stiffness to that of GFRP tube. The strength of control specimens was governed by crushing and spalling of concrete cover. Unlike spiral reinforcement, GFRP tubes confined larger concrete areas and also contributed as longitudinal reinforcement, leading to increases in flexural and shear strengths, up to 113% and 69%, respectively.     

Classification of the damage condition of preloaded reinforced concrete slabs using parameter-based acoustic emission analysis

A parameter-based acoustic emission (AE) technique is applied to AE signals acquired in physical experiments carried out on a series of predamaged reinforced concrete slabs. Three reinforced concrete slabs without shear reinforcement with dimensions of 1.50 × 1.50 × 0.23 m are subjected to cycles of a concentrated centric load with increasing peak values up to failure. The slabs had been previously exposed to impact loads in rockfall experiments and exhibit an unknown damage condition yet to be determined. Acoustic emissions are recorded during the loading and unloading cycles and evaluated. An analysis of load ratio and calm ratio associated with the Kaiser effect is performed. Damage classification is carried out successfully. Definitions of load ratio and calm ratio are reconsidered and specified. A static preloading of the slabs is approximated. The relationship between cracking process, failure mechanism and the acoustic emissions that occur is described and discussed.     

CFRP片材加固混凝土梁收缩徐变效应研究

按换算截面法推导了FRP片材加固混凝土梁的短期与长期挠曲变形计算公式,在此基础上编制了相应的分析计算程序,该分析方法能较准确预测开裂混凝土梁的长期挠曲变形。制作了10根钢筋混凝土梁,其中6根为普通钢筋混凝土梁,另外4根为CFRP片材加固梁,并对试件的短期与长期受力性能进行了试验观测,持续荷载作用下对试件进行了近700d的收缩徐变试验。运用本文计算方法预测了试件的长期挠曲变形,理论计算值与本文试验结果吻合较好。研究表明:碳纤维片材对混凝土梁的长期挠曲变形影响很小,但碳纤维片材能有效地控制持续荷载作用下裂缝的进一步开展。研究结果可为CFRP加固混凝土结构工程设计与理论分析提供参考。     

Flexural strengthening of RC beams with prestressed NSM CFRP rods – Experimental and analytical investigation

The effectiveness of strengthening reinforced concrete (RC) beams with prestressed near-surface mounted (NSM) carbon fiber reinforced polymer (CFRP) rods was investigated. Four RC beams (254 mm deep by 152 mm wide by 3500 mm long) were tested under monotonic loading. One beam was kept un-strengthened as a control beam. One beam was strengthened with a non-prestressed NSM CFRP rod. Two beams were strengthened with prestressed NSM CFRP rods stressed to 40% and 60% of the rod’s ultimate strength. The test results showed that strengthening with non-prestressed NSM CFRP rod enhanced the flexural response of the beam compared to that of the control beam. A remarkable improvement in the response was obtained when the RC beams were strengthened with prestressed (40% and 60%) NSM CFRP rods. An increase up to 90% in the yield load and a 79% in the ultimate load compared to those of the control beam were obtained. An analytical model was developed using sectional analysis method to predict the flexural response of RC beams strengthened with prestressed NSM CFRP rods. The proposed model showed excellent agreement with the experimental results.     

Behaviour of eccentric loading of FRP confined fibre steel reinforced concrete columns

This paper presents results of testing 16 specimens, 12 of which as columns under different eccentricities and four as beams under four point loading regime. All 16 specimens were circular in cross section and were made of reinforced concrete. Four specimens served as reference specimens and were just made of reinforced concrete. The next four specimens were wrapped with carbon fibre reinforced polymers (CFRP). The next four specimens had steel fibres added to the concrete. The final four specimens were reinforced with steel fibres and wrapped with CFRP. From each group of specimens, one specimen was tested as a column under a concentric load, the second specimen was tested as a column under 25 mm eccentricity, the third specimen was tested as a column under 50 mm eccentricity, and the final specimen was tested as a beam under four point loading regime. The experimental programme proved that the introduction of fibres as well as wrapping the specimens with FRP improve the properties of concrete, especially its ductility.     

Evaluation of the transfer length of prestressed near surface mounted CFRP rods in concrete

The transfer length of a prestressed near surface mounted (NSM) fiber reinforced polymer (FRP) rod is the distance over which the rod must be bonded to the epoxy to develop the prestressing force in the rod. The transfer length is intended to provide bond integrity for the strengthened concrete member. This paper presents experimental results and an empirical equation to estimate the transfer length of prestressed NSM Carbon FRP (CFRP) rod in concrete beams. Twenty-two reinforced concrete specimens were strengthened with NSM CFRP rods. Two types of CFRP rods were used: spirally wound and sand blasted rods. Four prestressing levels were used: 40%, 45%, 50% and 60% of the tensile strength of the CFRP rod. The strain behavior in the CFRP rod was monitored by gauges mounted on the CFRP rod along the length of the beam. The test results showed that the transfer length of the prestressed NSM CFRP rod was about 35 times the diameter of the CFRP rod. The maximum bond stress of the CFRP rod in epoxy was found to range from 11 to 16 MPa for the sand blasted rods and from 12 to 23 MPa for the spirally wound rods. An empirical expression based on curve fitting of the measured data was proposed to predict the prestressing stress in the CFRP rod along the length of the beam.     

Effects of tensile reinforcement ratio and compressive strength on the behaviour of over-reinforced helically confined HSC beams

The technology of high strength concrete has improved over the last decade. High strength concrete (HSC) is more brittle than normal strength concrete. The brittleness increases with the use of over-reinforced section, which fails suddenly without warning. Use of over reinforced sections is restricted in codes of practice of concrete design. This paper presents an experimental study of the behaviour of five HSC beams confined with helical reinforcement. Concrete compressive strength in the range 72–95 MPa and tensile reinforcement ratio in the range 5.24–7.86% were used. The main results indicate that as the concrete compressive strength increases the displacement ductility index decreases and the load at spalling-off the concrete cover increases. Also, the displacement ductility index increases as the longitudinal reinforcement ratio increases and the load at spalling-off the concrete cover decreases.     

Influence of corrosion crack patterns on the rate of crack widening of RC beams

Corrosion crack widths are often used by structural engineers in the field to predict level of steel corrosion as well as residual load-bearing capacities of corroding RC structures. This paper presents further work on this matter but with focus on corrosion crack patterns and how they affect rate of crack widening. It is based on results from a research where 17 quasi-full-scale (153 × 254 × 3000 mm) RC beams were corroded under various levels of sustained loads. The rate of widening of corrosion crack widths was found to be very much dependent on crack patterns. Deformation of cover concrete under each crack pattern was discussed. It was found that at maximum crack widths below 0.6 mm, the majority of beams exhibited nearly similar crack patterns as well as rate of widening of corrosion cracks. A mass loss of steel of 1% corresponded to a maximum crack width between 0.14 and 0.22 mm. At large crack widths (>0.6 mm), various beams exhibited very different rates of crack widening. It was shown that at crack widths above 0.6 mm, to be conservative an increase in mass loss of steel of 1% corresponded to corrosion crack widening of 0.02 mm.     

Preliminary experimental investigation of the fatigue bond behavior of CFRP confined RC beams

This paper presents the results of the first phase of a study on the effect of the confinement provided by transverse carbon fiber reinforced polymer (CFRP) sheets on the fatigue bond strength of steel reinforcing bars in concrete beams. Reinforced concrete bond-beams 150 × 250 × 2000 mm were tested. The variables examined were the area of the CFRP sheets (none or one U-wrap CFRP sheet), the reinforcing bar diameter (20 or 25 mm) and the load range applied to the specimens. The results showed that increasing the bar diameter increased the fatigue bond strength for the unwrapped beams. The CFRP sheets increased the bond strength of the bond-beams with 20 mm bars. However, for the beams with 25 mm steel bars the failure mode changed from a bond splitting failure for the unwrapped beams to a diagonal shear failure for the CFRP wrapped beams, and there was little increase in fatigue strength. Finally, the bond failure mechanism for repeated loading is described.