PSI - Issue 13

Myroslava Hredil / Procedia Structural Integrity 13 (2018) 1657–1662 Author name / Structural Integrity Procedia 00 (2018) 000–000

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1. Introduction Reinforced concrete (RC) structures are long-term operated objects with service life of 50–100 years. During their operation they subject to continuous ambient effects (cyclic temperature changes, acid rains, de-icing salts) and service loads (e.g. traffic) which influence on the structural integrity of the composite and lead to worsening of structures serviceability. One of the reasons for strength loss of RC members is bond degradation between rebar and concrete. It could be caused by two different factors: reinforcement corrosion and overprotection of RC. Song et al. (2007) reviewed that corrosion of reinforcing steel is one of the main reasons of RC structures deterioration. Initially, the bond strength between reinforcement and concrete increase a little at low corrosion levels, as reported by Ma et al. (2017), and this increasing tendency will be changed when corrosion-induced cracks occur. Their experimental results show that the corrosion influence on bond strength can be ignored when the corrosion loss is less than 2.4%. However, further accumulation of corrosion products at the steel–concrete interface, which volume is larger than original iron volume, leads to internal stresses promoting cracking and spelling of concrete cover. Cathodic protection of RC structures by impressed current is commonly accepted practice which is efficient for corrosion suppression of reinforcement especially when an installation is operated in chloride-contaminated environment. Nevertheless, application of cathodic current can lead to another problem. Hydrogen generated on the steel surface as a result of electrochemical processes, in atomic state can be absorbed by metal. The role of hydrogen in structural integrity of RC structures usually is reduced to consideration of hydrogen embrittlement of steel reinforcement especially actual and for high strength steels, studied comprehensively by Toribio et al. (2007, 2016) since it could lead to their fracture due to hydrogen assisted cracking under service stresses. Taking into account long term service of RC structures, time-dependent processes such as material degradation should not be neglected. For instance, in-service degradation of hot-rolled reinforcing steel rods was reported by Mikryukov et al. (2006). It revealed in dislocations substructure transformations and their density increase, cementite plates failure in pearlite grains, the second phase inclusions formation in near-surface layer on the basis of interstitial elements, and microcracks formation growing preferentially along the grain boundaries. These processes leaded to significant decrease in strength and plasticity of reinforcement, and hydrogen could facilitate them. However it is well known that only a small part of electrochemically generated hydrogen penetrates a metal while most of it recombines and evolves as a gas at a rebar surface surrounded by concrete. Page et al. (1978) reported about cohesion weakening due to steel hydrogenation in a course of RC specimens curing under potentials lower than 1.1 V, which corresponds exactly to the potentials of hydrogen evolution in the alkali environment as shown by Hredil et al. (2014). It seems to be important to analyze a possible destructive effect of just this hydrogen to the rebar–concrete interface as well as to concrete itself. Cylindrical specimens 100 mm in diameter and 200 mm in height (depicted as R1) were prepared using a mortar based on Portland cement of Type I according to ASTM C150 with water-to-cement ratio w/c = 0.5, and a cement/sand ratio was 1/3. Gravel was added as a coarse aggregate with the maximum particles size equal to 20 mm. Smooth steel roods with a diameter of 12 mm conforming the requirements to grade 40 reinforcement (ASTM A615) were used. A steel rod was fixed along the specimen axis in such a way that its upper end protruded out from the top flat surface of the specimen while lower end was of the same level as the bottom plane surface of the specimen (Fig. 1 , left ). Steel molds were filled in 3 steps with a mortar, after each step the mixture was compacted in a mold by vibration for 10 min. To investigate the influence of cathodic polarization and hydrogen evolved in the process on bond strength at the steel–concrete interface, a special specimen (R2) was developed (Fig. 1 , right ) in the form of cylinder (100 mm in diameter and 100 mm in height), where two smooth steel rods of different length (40 and 60 mm or 20 and 80 mm) were placed axially with an isolating gasket (2 mm) between them. The rods went out of concrete from the both sides of the specimen to fix it in the grips of a testing machine. The end of the shorter one was isolated. The mortar composition and casting procedure were the same. Both types of specimens were de-molded when 24 h passed after casting, and then put into limewater. They were tested after a curing period of 28 days. 2. Materials and methods 2.1. Specimens preparation