Structural concrete with incorporation of coarse recycled concrete and ceramic aggregates: durability performance

The growing difficulty in obtaining natural coarse aggregates (NCA) for the production of concrete, associated to the environmental issues and social costs that the uncontrolled extraction of natural aggregates creates, led to a search for feasible alternatives. One of the possible paths is to reuse construction and demolition waste (CDW) as aggregates to incorporate into the production of new concrete. Therefore, a vast and detailed experimental campaign was implemented at Instituto Superior Técnico (IST), which aimed at determining the viability of incorporating coarse aggregates from concrete and ceramic brick wall debris, in the production of a new concrete, with properties acceptable for its use in new reinforced and pre-stressed structures. In the experimental campaign different compositions were studied by incorporating pre-determined percentages of recycled coarse concrete aggregates and recycled coarse ceramic plus mortar particles, and the main mechanical, deformability and durability properties were quantified, by comparison with a conventional reference concrete (RC). In this article, these results are presented in terms of the durability performance of concrete, namely water absorption, carbonation and chlorides penetration resistance.     

Probabilistic evaluation of initiation time of chloride-induced corrosion

The paper presents a model for chloride ingress into concrete. The model accounts for two mechanisms which control the chloride ingress—diffusion and convection. Using one-dimensional (1-D) formulation of the model, the influence of chloride binding and ambient humidity on chloride ingress into concrete has been investigated. Based on results of this investigation parameters for probabilistic analysis have been selected. Probabilistic evaluation of the time to corrosion initiation has then been carried out for a reinforced concrete (RC) wall (1-D problem) and a RC column (2-D problem) in a marine environment. Results of the analysis show that for the same thickness of the concrete cover the probability of corrosion initiation in the corner reinforcing bars of the RC column is much higher than in reinforcing bars in the middle part of the RC wall. The results demonstrate the importance of 2-D modelling for correct prediction of corrosion initiation in such RC elements as columns and beams.     

Efficacy of surface hydrophobic agents in reducing water and chloride ion penetration in concrete

Hydrophobic agents are surface protection materials capable of increasing the angle of contact between the water and the concrete surface. For this reason, hydrophobic agents reduce water (in liquid form) penetration in concrete. Therefore, many European construction regulating agencies recommend this treatment in their maintenance policy. Nonetheless, there continues to be a gap in the understanding about which transport mechanisms of the concrete are modified by the hidrophobic agents. The aim of this study was to fill this gap in regards to reinforced concrete structures inserted in a marine environment. To this end, certain tests were used: Two involving permeability mechanism, one determining capillary absorption, and the last, a migration test used to estimate the chloride diffusion coefficient in saturated condition. Results indicated the efficacy of the hydrophobic agents in cases where capillary suction is the mechanism of water penetration (reduced by 2.12 and 7.0 times, depending of the product). However, when the transport mechanism is permeability this product is not advisable. Moreover, it was demonstrated that the chloride diffusion coefficient (in saturated condition) is reduced by the hydrophobic agents, however, the magnitude of this reduction is minor (reduced by 11% and 17%, depending on the product).      

Recycled aggregate concrete as structural material

The use of recycled aggregates in concrete opens a whole new range of possibilities in the reuse of materials in the building industry. The utilisation of recycled aggregates is a good solution to the problem of an excess of waste material, provided that the desired final product quality is reached. The studies on the use of recycled aggregates have been going on for 50 years. In fact, none of the results showed that recycled aggregates are unsuitable for structural use. However, some hypothetical problems related to durability aspects resulted in recycled aggregates being employed practically only as base filler for road construction. This paper focuses on the possibility of the use of recycled aggregate concrete as a structural material. For that purpose an experimental study of the shear behaviour and strength of beams made with recycled aggregate concrete was studied. Twelve beam specimens with the same compression strength, four concrete mixtures using different percentages of recycled coarse aggregates (0%, 25%, 50% and 100%) and three different transverse reinforcement arrangements were cast and tested up to failure. Analytical predictions of the experimental results were carried out using a numerical model based on the modified compression field theory and simplified models such as those proposed by Cladera & Mari, the Canadian standard CSA and the Eurocode-2. The results obtained indicate that a substitution of less than 25% of coarse aggregate, scarcely affects the shear capacity of RC beams, provided that all measures related to dosage and durability aspects have been adopted.     

Interactive Effects of Fly Ash and CNI on Corrosion of Reinforced High-Performance Concrete

The interactive effects of fly ash and CNI in corrosion of reinforced concrete were investigated. A 3⁴ full factorial design was developed considering water to cement ratio, fly ash percent, CNI and cracked condition as factors. The response factors were the weight loss calculated from Linear Polarization Resistance measurements and the pit depth of the steel bars embedded in concrete. Small-scale concrete slabs containing steel reinforcement with a cover depth of 20 mm were cast for this purpose. The slabs were subjected to a simulated marine environment with two cycles of wetting and drying per day during one year; after the exposure, the slabs were broken, the bars were cleaned and the pith depth measured by using SEM. Under the studied conditions, it was found that CNI alone does not provide corrosion protection of the steel reinforcement even for uncracked silica fume concrete in a 0.45 w/c ratio; however, the combination of CNI and fly ash can be useful to overcome this problem. The results indicate that low w/c ratio concrete in its crack state creates conditions suitable for the development of pitting corrosion.     

Desalination of a brick by application of an electric DC field

Salts in masonry can cause various problems as decay of the masonry itself, lost adhesion of plaster and hygroscopic moisture. Chlorides are among the most common building salts and the present paper is focused on removal of chlorides from a brick in an applied electric field as a step towards developing an electrochemical desalination method for brick masonry. Experiments were conducted in laboratory scale with one type of bricks that were contaminated with either NaCl or KCl through submersion in salt solutions prior to application of current. It was seen that NaCl was slower supplied to the brick during submersion and slower removed in the applied electric field than KCl. This indicates that the removal rate of chloride depends on the associated cation and this must be taken into account when desiding the duration of full scale actions. The electrochemical desalination was very efficient and 99% removal of chloride was obtained. The final concentration in the brick after treatment was less than 10 mg Cl/kg and this concentration is unproblematic. When low salt concentrations were reached during the electrochemical treatment, electroosmotic dewatering of the brick started, showing that electroosmotic dewatering occurs at low ionic concentrations.     

Multi-species ionic diffusion in concrete with account to interaction between ions in the pore solution and the cement hydrates

The penetration and leaching of ionic species in concrete are studied by using a model based on the Nernst–Planck equations. A finite element procedure is used to solve the coupled non-linear governing equations. A numerical example is performed in which the results are compared to measured electron probe micro analysis (EPMA) data. A close agreement of the simulated results to measured data is found for the specific studied example. The model includes the ionic species Cl⁻, Na⁺, OH⁻, Ca²⁺, K⁺ and SO₄²⁻ and solid phases with variable composition. From the EPMA measurements the total concentration profiles of Cl⁻, CaO, SiO₂, Na₂O and SO₃ are evaluated on samples exposed to a 3 wt% sodium chloride solution for 1 year. The main task in this investigation is to quantitatively understand the underlying mechanisms and find an accurate model that gives good correlation with the experimental results concerning the multi-species action during chloride penetration. In the model the chemical interaction between ions in solids and in pore solution is assumed governed by simple ion exchange processes only. The drawback using this approach is that the chemical part is lacking important physical relevance in terms of standard solubility thermodynamics. On the other hand the presented model is capable of accurately simulate the well documented peak behavior of the chloride profiles and the measured high content of calcium ions in pore solution under conditions when also chlorides is present. In this sense the established multi-species models for concrete based on standard solubility calculations alone is still incomplete.     

Performance updating of concrete bridges using proactive health monitoring methods

Uncertainties associated with modelling of deteriorating bridges strongly affect management decisions, such as inspection, maintenance and repair actions. These uncertainties can be reduced by the effective use of health monitoring systems, through which information regarding in situ performance can be incorporated in the management of bridges.The objectives of this paper are twofold; first, an improved chloride induced deterioration model for concrete bridges is proposed that can quantify degradation in performance soon after chlorides are deposited on the bridge, rather than when initiation of corrosion at the reinforcement level takes place. As a result, the implications of introducing proactive health monitoring can be assessed using probabilistic durability criteria. Thus, the second objective of the paper is to present a methodology for performance updating of deteriorating concrete bridges fitted with a proactive health monitoring system.This methodology is illustrated via a simple example of a typical bridge element, such as a beam or a part of a slab. The results highlight the benefits from introducing ‘smart’ technology in managing bridges subject to deterioration, and quantify the reduction in uncertainties and their subsequent effect on predictions of future bridge performance.     

Relationship between concrete resistivity and corrosion rate – A literature review

The process of reinforcement corrosion in concrete is partially controlled by the transport of ions through the concrete microstructure. Ions are charged and the ability of a material to withstand transfer of charge is dependent upon the electrical resistivity. Thus, a connection could be expected between the corrosion process of steel embedded in concrete and the electrical resistivity of concrete. This paper reviews research concerning the relationship between corrosion rate and concrete resistivity. Overall, there exists an inverse proportional correlation between the parameters. However, the dependency varies between studies and one single relationship cannot be established between corrosion rate and resistivity. To address the variation, the article reviews and evaluates the influence of factors including the experimental setup, the concrete mix design and the cause of corrosion.     

The influence of corrosion on bond properties between concrete and reinforcement in concrete structures

The rebar corrosion in reinforced concrete is one of the most extensive pathologies affecting the performance of concrete structures. Chloride-induced rebar corrosion damage results mainly from the use of de-icing salts in cold climates and/or exposure to marine environments. Carbonation damage is a further important degradation mechanism. The internal consequences of corrosion are the modification of the steel behavior and degradation of the steel–concrete bond. This work is devoted to the influence of the controlled corrosion on the adherence between steel and concrete. A new geometry of specimen has been designed to: (i) avoid the lateral confining stresses that appear during the classical pullout tests and (ii) permit to impose a known confinement for the study of its influence on the behavior of the interface. Five specimens with different levels of corrosion have been tested in this contribution. Reinforcing bars embedded in concrete were submitted to accelerated corrosion using an external current source. The magnitude of corrosion was measured using both Faraday’s law and the weight loss method. The level of corrosion varied from 0% to 0.76%. The geometry of the specimens allowed us to take series of digital pictures during the tests, which were analyzed using a digital image correlation, the procedure named CORRELI^LMT. The results of pullout tests proposed in this contribution indicate that: (i) levels of corrosion that are less than 0.4% of weight loss improves the bond stress and (ii) levels of corrosion resulting in more than 0.4% of weight loss lead to a reduction of the bond stress value.     

模拟混凝土孔隙液中钢筋腐蚀行为的恒电量法监测

利用恒电量法监测了钢筋在不同ｐＨ 值的模拟混凝土孔隙液中的极化电阻、钝化膜电阻Ｒｐ、Ｒｆ 值,并与电位法监测值进行了比较,结果表明,恒电量法更加准确和更具定量性,适用于钢筋的腐蚀监测。