Probabilistic chloride diffusion modelling in cracked concrete structures by transient BEM formulation

Reinforcement corrosion is a concern in the structural engineering domain, since it triggers several pathological manifestations, reducing the structural service life. Chloride diffusion has been considered one of main causes of reinforcements' corrosion in reinforced concrete. Corrosion starts when the chloride concentration at the reinforcements interface reaches the threshold content, leading to depassivation, whose assessment of its time of starts is a major challenge. This study applied the transient Boundary Element Method (BEM) approach for modelling chloride diffusion in concrete pores. The subregion BEM technique effectively represented the cracks inherent to the material domain, and environmental effects were also considered. Because of the inherent randomness of the problem, the service life was evaluated within the probabilistic context; therefore, Monte Carlo Simulation (MCS) assessed the probabilistic corrosion time initiation. Three applications demonstrated the accuracy and robustness of the model, in which the numerical results achieved by BEM were compared against numerical, analytical, and experimental responses from the literature. The probabilistic modelling substantially reduced the structural service life when the cracks length was longer than half of concrete cover thickness in highly aggressive environments.

Influence of the void ratio of cellular concrete on the corrosion of steel reinforcement

The objective of this study was to understand whether voids intensify the triggering of reinforcement corrosion in cellular concrete, for slabs with light specific masses. The methodology was based on four tests: visual inspection, corrosion potential, electrical resistivity, and mass loss. In relation to the L1 family, the L2 and L3 families (higher air content) were shown to be more susceptible to reinforcement corrosion and mass loss from the steel bars in 90% of cases. However, the behavior of some slabs indicates the possibility of the process being asymptomatic with regard to staining, considering that the influence of the cover on the corrosion of the steel bars was verified

Durable Steel-Reinforced Concrete Structures for Marine Environments

Even in harsh marine environments, concrete structures reinforced with steel can show excellent long-term durability, with little or no reinforcement corrosion. Very few actual reinforced concrete (RC) structures have been closely scrutinized over many years and subject to interpretation using recent state-of-the-art understanding gained from detailed laboratory observations. Such a case is described for an 80-year-old RC structure observed annually over about 30 years in what is essentially an extraordinary long experiment. Despite very high chloride concentrations, field excavation evidence showed that reinforcement corrosion overall remains minimal, except where insufficient concrete compaction permitted air-voids to initiate quite severe, very localized corrosion even with still high concrete pH. It is possible that the use of blast furnace slag as aggregate may have assisted the observed durability. The case study supports other studies that show that it is possible to achieve long-term durable and therefore sustainable RC structures without additives and using only conventional reinforcement steels and conventional cements and aggregates. However, the potential dangers of deep narrow cracking extending to the reinforcement and the potentially deleterious effects of alkali–aggregate reactivity of some aggregates needs to be considered.

Manual Application versus Autonomous Release of Water Repellent Agent to Prevent Reinforcement Corrosion in Cracked Concrete

Cracks in reinforced concrete are preferential ingress paths for aggressive substances such as chlorides. As soon as a critical amount of chlorides has reached the steel reinforcement, corrosion will occur. Therefore, crack healing is of utmost importance. However, manual crack repair is very labour intensive. Therefore, the potential of self-healing through the release of a water repellent agent from embedded capsules was compared with the effectiveness of applying this agent on the concrete surface before or after cracking and the effectiveness of injection of the agent into a crack. From the electrochemical corrosion measurements, it was shown that only uncracked samples were able to withstand 135 weekly cycles of exposure to a 33 g/L chloride solution without corrosion initiation of the reinforcement. While samples with manually injected and autonomously healed cracks resisted the exposure conditions for about 50 cycles or more, samples for which the water repellent agent was applied onto the surface after cracking resisted the exposure conditions for 5–42 cycles, while samples for which the agent was applied onto the surface before cracking showed an immediate corrosion initiation similar as was noted for the untreated cracks. From a visualization of the chloride ingress front and determination of the chloride content in the vicinity of the crack, it was noticed that none of the crack treatment techniques performed as well as the uncracked series. Visual inspection of the corroded rebars and determination of the corroded volume of the rebars through computed tomography and macro-cell corrosion current measurements proved again that the uncracked series outperformed the other series. While the corroded volume of the rebars from the uncracked series was almost zero, this value ranged from 15–95 mm3 for the rebars of the other series. However, the latter investigations also showed that release of the agent into the crack, whether this was done in a manual way or autonomously through release from embedded capsules, resulted in a delayed corrosion initiation and lower corrosion propagation rate compared to the application of a water repellent agent onto the surface. This is a beneficial outcome for the further implementation of self-healing approaches, more specifically though the release of encapsulated water repellent agent, in the market.

COATING FOR PROTECTION OF CONCRETE FROM SULFATES

Durability of reinforced concrete is one of the main demands in civil engineering.Operating conditions, particularly in aggressive mediums, determine durability of constructions.Sulfate mediums are among the most aggressive ones which cause steel reinforcement corrosion. Themodern requirements for high consistency fresh concretes are governed by practice. This way thedisturbance of reinforcement passive state can be caused by changes in hardened concrete especiallyin aggressive mediums. Thus, the restriction of SO42- ions transport in concretes, which are obtainedfrom high consistency fresh mixes and exploited in sulfate mediums, can be considered as an actualproblem.The paper is devoted to protection of concrete surface by coating based on alkali-activatedaluminosilicate binder from SO42- ions transport to prevent steel reinforcement corrosion. It wasshown that the coating with thickness of 3 mm ensures total concrete protection. It was revealed thatpermeability of concrete depending on cation decreases in the row (NH4)2SO4>Na2SO4>MgSO4. Theprotective function of coating was simulated by application of mentioned salts as admixtures. LesspH values of water extracts of the binder during hydration while using of 2.5 % MgSO4 is evidenceof advanced crystallinity of zeolite-like sulfate-containing hydroaluminosilicates with participationof Mg2+ ions equal to Са2+ ions. While content of (NH4)2SO4 was increased up to 5.0 % less pH wasfixed due to decelerated formation of zeolite-like minerals. Stability of pH values in presence ofNa2SO4 (0.5…2.5 %) was caused by no influence on structure formation. Thus, the restriction of SO 2-ions transport in protective coating is due to their binding by alkaline aluminosilicate binder inzeolite-like minerals with higher crystallinity due to presence of Na+, NH + and Mg2+ cations fromsulfates.

Durability Properties of Ultra-High Performance Lightweight Concrete (UHPLC) with Expanded Glass

It is important to ensure the durability and safety of structures. In the case of newly developed materials that are outside the current rules, it is important to investigate all aspects of structural safety. The material studied in the following is a structural lightweight concrete with an ultra-high-performance matrix and expanded glass as a lightweight aggregate. The material, with a compressive strength of 60–100 MPa and a bulk density of 1.5–1.9 kg/dm3, showed high capillary porosities of 12 vol% (ultra-high-performance concretes (UHPC) < 5 vol%). Since the capillary porosity basically enables transport processes into the concrete, the material had to be examined more closely from the aspect of durability. Freeze-thaw resistance (68 g/m2) and chemical attack with sulfate at pH 3.5 for 12 weeks (16 g/m2) showed no increase in concrete corrosion. Targeted carbonation (0.53 mm/year0.5) and chloride penetration resistance (6.0 × 10−13 to 12.6 × 10−13 m2/s) also showed good results against reinforcement corrosion. The results show that most of the measured capillary pores resulted from the lightweight aggregate and were not all present as a pore system. Thus, the durability was only slightly affected and the concrete can be compared to an UHPC. Only the abrasion resistance showed an increased value (22,000 mm3/5000 mm2), which, however, only matters if the material is used as a screed.