Bond strength of deformed bars in large reinforced concrete members cast with industrial self-consolidating concrete mixture

The bond strength of reinforcing bars embedded in full-scale heavily reinforced concrete sections made with industrial self-consolidating concrete (SCC) was investigated and compared with that of normal concrete (NC). The flowability of SCC mix through the dense reinforcement was visually monitored from a transparent formwork. The bond stress was tested for bars located at three different heights (150 mm, 510 mm, and 870 mm from the bottom of the pullout specimens) and at different tested ages (1, 3, 7, 14, and 28 days). The bond stress-free end slip relationship, the top bar effect and the effect of age on bond stress was investigated in both SCC and NC pullout specimens. Bond stresses predicted based on some major codes were compared with those obtained from experiments. The results indicated that casting SCC was much faster and easier and could be done with less labor effort and no concrete blockage among the heavy reinforcements compared to NC. The results also indicated that the bond stress was slightly higher in the SCC pullout specimen compared to the NC pullout specimen. The difference was more pronounced in the top bars and at 28 days of testing.     

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.     

Bond between carbon fibre-reinforced polymer (CFRP) bars and ultra high performance fibre reinforced concrete (UHPFRC): Experimental study

Carbon fibre-reinforced polymer (CFRP) bars are currently used to reinforce concrete in an attempt to overcome the corrosion issue encountered with ordinary steel. In order to exploit more efficiently their tensile capacity, it is interesting to use CFRP bars as prestressing tendons. This application requires a high quality concrete matrix. The advantageous characteristics of UHPFRC, such as high strength, good ductility and durability, mean that a UHPFRC structure prestressed with CFRP bars may be lighter and require less maintenance. Since the flexural behaviour of prestressed concrete members reinforced with CFRP bars is highly dependent on the bond between the two materials, an experimental program was carried out in order to investigate the bond of CFRP bars embedded in UHPFRC. Two types of surface, smooth and sand-coated, were investigated. Pullout tests were performed to examine the effect of varying parameters such as embedment length, bar diameter and concrete age. The results clearly show that the bond strength of macroscopically smooth bars embedded in UHPFRC is close to that of sand-coated bars. It was also found that ultimate bond strength decreases with bar diameter and with embedment length. Moreover, the bond strength can be expected during early age (3 days). A post-test examination revealed that damage occurred only in the outer layers of the CFRP bars.     

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.     

Effect of W/C ratio on covering depth of fly ash concrete in marine environment

This investigation studied the effect of W/C ratio on covering depth required against the corrosion of embedded steel of fly ash concrete in marine environment up to 4-year exposure. Fly ash was used to partially replace Portland cement type I at 0%, 15%, 25%, 35%, and 50% by weight of cementitious material. Water to cementitious material ratios (W/C) of fly ash concretes were varied at 0.45, 0.55, and 0.65. The 200-mm concrete cube specimens were cast and steel bars with 12-mm diameter and 50 mm in length were inserted in the concrete with the covering depth of 10, 20, 50, and 75 mm. The specimens were cured in water for 28 days, and then placed to the tidal zone of marine environment in the Gulf of Thailand. Subsequently, the concrete specimens were tested for the compressive strength, chloride penetration profile and corrosion of embedded steel bar after being exposed to tidal zone for 2, 3, and 4 years. The results showed that the concrete mixed with Portland cement type I exhibited higher rate of the chloride penetration than the fly ash concrete. The chloride penetration of fly ash concrete was comparatively low and decreased with the increasing of fly ash content. The increase of fly ash replacement and the decrease of W/C ratio could reduce the covering depth required for the initial corrosion of the steel bar. Interestingly, fly ash concretes with 35% and 50% cement replacement and having W/C ratio of 0.65 provided better corrosion resistance at 4-year exposure than the control concrete with W/C ratio of 0.45. In addition, the covering depth of concrete with compressive strength of 30 MPa (W/C ratio of 0.65) could be reduced from 50 to 30 mm by the addition of fly ash up to 50%.     

Effect of alkali-silica reaction and freezing and thawing action on concrete–steel bond

An experimental program was conducted to investigate the effect of stresses and cracks, caused by alkali-silica reaction (ASR) and freezing and thawing (F/T), on bond between reinforcing steel and concrete. Pullout test cylinders, reinforced with 18 mm steel bars, were used to evaluate bond behavior. Concrete prisms (50 × 50 × 300 mm) were also cast to evaluate expansion and reduction in ultrasonic velocity due to ASR and F/T cycles, respectively. Specimens were cured for 40 days before being either immersed in sodium hydroxide solution of 0.5 normality in order to accelerate ASR, or subjected to different cycles of F/T. Bond behavior, expansion, and ultrasonic pulse velocity tests were carried out as ASR progressed or under F/T cycles.The progress of ASR resulted in significant losses in critical bond stress and ultimate bond strength capacity reaching as high as 44% and 24%, respectively, accompanied by a significant increase in free-end slip at failure. The loss in bond due to ASR was higher for specimens prepared using concrete with lower concrete strength and higher percentage of reactive aggregate. F/T action caused a significant reduction in critical bond stress and ultimate bond strength that reached as high as 100% and 55%, respectively, and an increase in free-end slip at failure. Neither ASR nor F/T cycles affected trends in the behavior of bond stress versus free end slip curves.     

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.     

The effect of elevated temperatures on corroded and uncorroded reinforcement embedded in mortar

In this study, the alterations at the reinforcement steels embedded in corroded and not corroded reinforcement mortar specimens after high temperatures were investigated. These reinforced mortar specimens with 3 cm and 5 cm covers were first exposed to 20, 100, 200, 300, 500 and 800 °C temperatures. Subsequently, to determine the mechanical properties tensile tests were applied on the steels taken out of these mortar specimens. With the tensile tests performed stress–strain curves of the steel bars exposed to several temperatures were obtained. Besides, yield and ultimate strengths of the steel bars were also determined. It is obtained that, high temperatures affected the mechanical properties of reinforcement steel considerably negatively. Results of the study have shown that the larger the covers are the better the steel bars are protected against high temperatures.     

Beam test on bond behavior between high-grade rebar and high-strength concrete after elevated temperatures

In this paper, post-heating bond behavior between high-grade rebar and C80 high-strength concrete (hereafter, HSC) is studied. The high-grade rebar is HRBF500 fine grained steel with a yield strength of 500 MPa and the concrete grade C80 denotes compressive strength not lower than 80 MPa. First, the residual mechanical behavior of both high-grade rebar and HSC were tested after fire exposure. Second, the beam bond test was carried out to study the bond behavior between high-grade rebar and HSC after exposed heating at 200 °C, 400 °C, 500 °C and 600 °C, respectively. During the bond test, the influence of temperature, bond length, and some construction measurements on the bond–slip behavior were compared and evaluated. The investigation demonstrates that (1) the bond strength between high-grade rebar and HSC decreases while the peak slip increases with the elevated temperature, especially when the temperature exceeds 400 °C and (2) the confinement effect of steel wire mesh can help to improve rebar׳s bond behavior. Third, the bond–slip model between high-grade rebar and HSC for post-heating is proposed.     

Compressive strength loss and reinforcement degradations of reinforced concrete structure due to long-term exposure

Degradations due to long-term weathering actions on a reinforced concrete structure were investigated. Compressive strength and reinforcement corrosion developments of a prototype RC structure were monitored for 6 years using destructive and non-destructive tests which include periodic coring, compressive strength, rebound hammer, ultrasonic pulse velocity, carbonation, half-cell and tensile strength tests. Eventually, results have shown that more than a quarter of peak compressive strength can be lost within 5 years of continuous exposure. Corrosion of the exposed bars within the range of the testing period was also observed to be quite alarming. Thus, defects caused by prolonged actions of environmental factors may pose serious threats on the integrity of partially completed structures especially abandoned projects.     

Influence of test specimen on experimental characterization of timber–concrete composite joints

Load-carrying capacity of timber–concrete composite joints is usually evaluated using shear tests, which still lack specific standards. Regulations EN 26891 [1] and ASTM D 5652 [2] are usually used, both for timber joints, or EUROCODE 4 [3] for steel–concrete composite joints. Questions about test execution and arrangement of specimens are frequent and recurrent [4], [5], [6]. Steel–concrete composite structures already have a standard shear test for joints (push-out), described in Johnson and Anderson [7]. These authors also discussed the many differences in the results of shear tests because of differences in test methods before EUROCODE 4 [3] standardization.This paper presents some questions about the arrangement of test specimens for shear tests in timber–concrete joints. An experimental program was performed for this reason. The aim of the work was to compare shear test results using two different series of specimens most utilized in a review of the literature: the push-out type with concrete center and timber sides and the push-out type with timber center and concrete sides. 8.0, 10.0 and 12.5 mm diameter corrugated bars were used as connectors. Eucalyptus grandis Brazilian hardwood timber glulam was used. Two-component epoxy adhesive was used to glue the connectors into the timber. Average cylinder compressive strength of the concrete was 25 MPa (28 days old). Reinforcement was 6.0 mm diameter 500 MPa-yield-stress corrugated bars.The results showed that test specimen arrangement influenced the strength and deformation characteristics of timber–concrete composite joints. The specimen with the best shear strength was the concrete–wood–concrete type, similar to those used in steel–concrete composite structures. Since the arrangement of test specimen is an important factor in joint tests, it is recommended that further efforts be made towards standardization.     

Lateral deformation of RC beams under simultaneous load and steel corrosion

Cracking of cover concrete due to steel corrosion is one of the clear physical indicators of loss of service life of corroding RC structures. Its prediction is therefore very important for service life modelling of these structures. Models developed to predict the time to cover cracking assume that stresses due to steel corrosion follow the principles of a thick-walled cylinder under internal pressure. Considering the errors in the models, this paper contests the applicability of the thick-walled cylinder approach to model the time to cover cracking as well as the rate of lateral expansion of concrete after cover cracking using experimental results from 12 RC beams (153 × 254 × 3000 mm) corroded under a sustained load. It is shown in the paper that, contrary to the assumptions of uniform expansion made in the thick-walled cylinder approach, before cracking of the cover concrete, tensile strains are applied on the face of beams where corrosion agents are drawn whilst other faces are in compression. Corroded steel coupons are used to verify that this variation of strains is caused by the corrosion process not being uniformly distributed around the steel bar. It is also shown in the paper how cracking and location of cracks affects the rate of lateral deformation of concrete due to steel corrosion.     

Influence of corrosion on the friction characteristics of the steel/concrete interface

This paper reports results of an experimental programme to measure changes in friction characteristics of the interface between reinforcement and concrete as a result of the formation of corrosion products on the surface of the steel. The aim of the study was to determine whether a weak corrosion layer contributes to reported reductions in residual bond capacity. Corrosion was carried out under accelerated conditions using three different conditioning regimes. Principal parameters included in the investigation are conditioning regime, the amount of corrosion, and the stiffness of the restraint to expansion of the corrosion products.Experimental results indicate that corrosion products do not impair friction characteristics of a bar/concrete interface when surface crack width does not exceed 1.0 mm.     

Variation of steel loss and its effect on the ultimate flexural capacity of RC beams corroded and repaired under load

This paper presents results of an investigation on the variation of mass loss of deformed tensile steel bars in RC beams (153 × 254 × 3000 mm) that were corroded whilst under a sustained load using an impressed current, constant wetting cycles with 5% NaCl solution and two different drying cycles. Following the corrosion test, selected beams were patch repaired whilst under a sustained load, but eventually all beams were tested to failure. The results indicated that the highest level of corrosion occurred where there were longer drying cycles, and that the level of sustained load had little effect on the rate of corrosion. Maximum mass loss of steel was found to occur at the centre of the corrosion region. The ultimate flexural capacity of beams was found to be best related to the maximum gravimetric mass loss compared to the average mass loss of steel. A maximum mass loss of steel of 1% was found to reduce the flexural capacity of beams by 0.7%.     

Experimental evaluation of the flexural behavior of corroded P/C beams

Corrosion phenomena and related effects, such as size reduction in both rebars and strands, bond decay at steel–concrete interface, and cracking in the surrounding concrete, are particularly critical in prestressed-concrete members, not only for safety reasons, but also for their huge potential socio-economic effects. As a matter of fact, this technique has been used for the last 50 years in the majority of viaducts and bridges built in many countries like Italy.In order to evaluate the influence of the corrosion on prestressed pretensioned beams, a number of tests has been carried out in the Laboratory of the University of Rome “Tor Vergata”.Nine prestressed beams (section size 200 × 300; total length 3000 mm; clear span 2700 mm) were first subjected to artificial corrosion, to obtain different damage levels, and then were tested in four-point bending.The results clearly show the sizable effects that corrosion has on the ultimate capacity (that is significantly reduced), on the failure mode and on the structural response, that turns from ductile to brittle.     

Parameter optimization on compressive strength of steel fiber reinforced high strength concrete

This paper illustrates parameter optimization of compressive strength of steel fiber reinforced high strength concrete (SFRHSC) by statistical design and analysis of experiments. Among several factors affecting the compressive strength, five parameters that maximize all of the responses have been chosen as the most important ones as age of testing, binder type, binder amount, curing type and steel fiber volume fraction. Taguchi analysis techniques have been used to evaluate L₂₇ (3¹³) Taguchi’s orthogonal array experimental design results. Signal to noise ratio transformation and ANOVA have been applied to the results of experiments in Taguchi analysis. The confirmation runs were conducted for the optimal parameter level combination, which is obtained from the results of the above methodologies. The maximum compressive strength has been observed as around 124 MPa. By using the optimal parameter level combination, the direct tensile strength and flexural strength tests have been conducted. The mean values at the age of 28 days are obtained as 7.5 MPa and 13 MPa respectively. In this study, it is clearly demonstrated that all main factors except steel fiber significantly contribute to the compressive strength of steel fiber reinforced high strength concrete, yet age and binder type are the most significant contributors.     

The effect of thiosemicarbazide on corrosion resistance of steel reinforcement in concrete

This study describes a laboratory investigation of the influence of thiosemicarbazide (TSC) on the corrosion of reinforcing steel and the compressive strength of concrete. The effect of TSC on the corrosion resistance of steel reinforced concrete was evaluated by carrying out electrochemical tests in NaCl and NaCl + TSC solutions for 60 days. Polarisation resistance (R_p) values of TSC added reinforced concrete were much higher than those without TSC. Similarly, AC impedance spectra revealed that the resistance of TSC mixed electrodes were also quite higher than those without. The compressive strength of concrete specimens containing TSC was measured and an increase of 20–25% was observed.     

Shrinkage characteristics of high-strength concrete for large underground space structures

This study forms part of a research project that was carried out on the development and application of high-strength concrete for large underground spaces. In order to develop 50 MPa high-strength concrete, eight optimal mixtures with different portions of fly ash and ground granulated blast furnace slag, which make the pozzolanic reaction, were selected. For assessments of shrinkage characteristics, free shrinkage tests with prismatic specimens and shrinkage crack tests were performed. The compressive strength was more than 30 MPa at 7 days, and stable design strength was acquired at 28 days. High-strength concrete containing blast furnace slag shows large autogenous shrinkage, while large shrinkage deformations and cracks will occur when mixtures are replaced with large volumes of cementitious materials. Hence, for these high-strength concrete mixtures, the curing conditions of initial ages that affect the reaction of hydration and drying effects need to be checked.     

混凝土中钢筋加速锈蚀试验适用性研究

钢筋锈蚀导致其屈服强度降低、力学行为改变,影响钢筋与混凝土之间的粘结性能,钢筋锈蚀量影响钢筋混凝土的失效模式。为研究锈蚀钢筋混凝土结构的相关性能,需要在较短时间内得到所需的锈蚀构件。通过对4种不同工况下混凝土中钢筋电化学加速锈蚀方法进行对比试验,得到了锈蚀后钢筋表面形态特征,分析了模拟自然环境条件下钢筋锈蚀的适用性。试验表明：全浸泡外加电流加速锈蚀方法使钢筋纵向、径向表面形成均匀锈蚀,而自然环境锈蚀钢筋表面锈蚀相对不均匀,坑蚀更明显,两者差异显著;利用全浸泡外加电流加速锈蚀方法进行锈蚀钢筋与混凝土粘结-滑移本构关系和锈蚀钢筋混凝土构件承载能力等研究不合适;半浸泡和贴面外加电流加速锈蚀方法能较好模拟自然环境锈蚀;加速锈蚀试验方法的理论锈蚀质量高于试验锈蚀质量。图12表1参7     

A study on improvement of durability of reinforced concrete structures mixed with Natural Inorganic Minerals

As a fundamental study on the corrosion resistance of reinforced concrete structures using Natural Inorganic Minerals exposed to carbonation environment. The test specimens were concrete(W/C = 60%) with Natural Inorganic Minerals content of 0% and 10%. Accelerated arbonation and autoclave corrosion accelerated curing were then conducted with them. The corrosion resistance of steel in concrete with Natural Inorganic Minerals content of 0% and 10% was examined by corrosion form, half-cell potential, polarization resistance, corrosion area and weight loss after 24 h of autoclave corrosion accelerated curing.The results of the study showed that as for steel in concrete with Natural Inorganic Minerals content of 10%, the corrosion resistance was more excellent than steel in concrete with Natural Inorganic Minerals content of 0%.