Investigation on the effect of supplementary cementitious materials on the critical chloride threshold of steel in concrete

open3noThe critical chloride threshold is a key parameter in the service life design of reinforced concrete structures exposed to chloride-bearing environments. This paper investigates the role of concrete composition, and particularly the effect of supplementary cementing materials, on the chloride threshold. To simulate real exposure conditions, ponding tests were carried out on reinforced concrete specimens with bars in free corrosion conditions and corrosion initiation was detected through corrosion potential and corrosion rate measurements. After two and a half years, the ponding was followed by an ageing period and the initiation of corrosion was further detected with anodic potentiostatic polarisation tests. Results of the tests showed several limitations of the approach based on chloride penetration and monitoring of free corrosion parameters to investigate the chloride threshold. In spite of this, a possible role of natural pozzolan and coal fly ash additions in leading to higher values of the chloride threshold and ground limestone in promoting lower values of the chloride threshold could be observed.Lollini, Federica; Redaelli, Elena; Bertolini, LucaLollini, Federica; Redaelli, Elena; Bertolini, Luc

Cathodic protection with localised galvanic anodes in slender carbonated concrete elements

A combined experimental and numerical investigation was carried out with the aim of determining whether few localised galvanic anodes per unit length could protect the reinforcement of slender carbonated concrete elements, exposed to atmospheric conditions, which could not be repaired with traditional methods. Initially, the cathodic behaviour of steel under galvanostatic polarisation was determined on small-size specimens obtained from a real element. A correlation of potential versus applied current was obtained. The current distribution in slender elements was then determined through finite elements simulations, considering various scenarios of carbonation and humidity. Results showed that, in spite of the high electrical resistivity of carbonated concrete, anodes with spacing of 0.45 m are enough to protect corroding reinforcement in most exposure conditions, even in thin parts of element. Estimated anode durations were of the order of several years or even decades; however, it was shown that also reinforcement in dry (carbonated or alkaline) concrete, which does not need to be protected, contributes to anode consumption. Although other aspects play a role on the performance of a cathodic protection system (such as the effectiveness of anode-encasing material and of electrical connection to reinforcement), the results obtained are supportive of a repair strategy based on the use of localised galvanic anodes and can be generalised to slender elements exposed to atmospheric conditions suffering carbonation induced corrosion

The challenge of the performance-based approach for the design of reinforced concrete structures in chloride bearing environment

The performance-based approach, published by the International Federation for Structural Concrete (fib), was applied for the design of a RC element in a marine environment, with corrosion resistant reinforcement, to analyse the potentiality of the model as well as the possible reasons which limit its use. Results showed that the fib model allows to compare different solutions and to consider the benefits connected with the use of preventative measures. However the definition of reliable values for some input parameters, as the critical chloride threshold for corrosion resistant reinforcement, is demanded to the designer and this aspect clearly limits a widespread use

Development of TaqMan Real-Time PCR protocols for simultaneous detection and quantification of the bacterial pathogen ralstonia solanacearum and their specific lytic bacteriophages

Ralstonia solanacearum is the causal agent of bacterial wilt, one of the most destructive diseases of solanaceous plants, affecting staple crops worldwide. The bacterium survives in water, soil, and other reservoirs, and is difficult to control. In this sense, the use of three specific lytic R. solanacearum bacteriophages was recently patented for bacterial wilt biocontrol in environmental water and in plants. To optimize their applications, the phages and the bacterium need to be accurately monitored and quantified, which is laborious and time-consuming with biological methods. In this work, primers and TaqMan probes were designed, and duplex and multiplex real-time quantitative PCR (qPCR) protocols were developed and optimized for the simultaneous quantification of R. solanacearum and their phages. The quantification range was established from 108 to 10 PFU/mL for the phages and from 108 to 102 CFU/mL for R. solanacearum. Additionally, the multiplex qPCR protocol was validated for the detection and quantification of the phages with a limit ranging from 102 targets/mL in water and plant extracts to 103 targets/g in soil, and the target bacterium with a limit ranging from 103 targets/mL in water and plant extracts to 104 targets/g in soil, using direct methods of sample preparation

Corrosion of Steel in Concrete and Its Prevention in Aggressive Chloride-Bearing Environments

This keynote paper deals with the durability of reinforced concrete (RC) structures exposed to aggressive environments characterized by high concentration of chloride ions, namely, marine environments or the use of de-icing salts. The mechanism of chloride-induced corrosion of steel in concrete is introduced, and its influence on the service life of RC structures is analyzed. Factors affecting the time to corrosion initiation are described with regard to both concrete properties and environmental exposure conditions. Design approaches available for achieving durability targets associated with the design service life are analyzed, focusing on studies carried out by the authors in recent years at the mCD Concrete Durability lab of Politecnico di Milano, which were aimed at improving the protection provided to the steel bars by the concrete cover, investigating the advantages of using corrosion-resistant stainless steel bars and developing the electrochemical technique of cathodic prevention

Effects of nanosilica on compressive strength and durability properties of concrete with different water to binder ratios

The effects of the addition of different nanosilica dosages (0.5%, 1%, and 1.5% with respect to cement) on compressive strength and durability properties of concrete with water/binder ratios 0.65, 0.55, and 0.5 were investigated. Water sorptivity, apparent chloride diffusion coefficient, electrical resistivity, and carbonation coefficient of concrete were measured. The results showed that compressive strength significantly improved in case of water/binder = 0.65, while for water/binder = 0.5 no change was found. Increasing nanosilica content, the water sorptivity decreased only for water/binder = 0.55. The addition of 0.5% nanosilica decreased the apparent chloride diffusion coefficient for water/binder = 0.65 and 0.55; however, higher nanosilica dosages did not decrease it with respect to reference value. The resistivity was elevated by 0.5% nanosilica for all water/binder ratios and by 1.5% nanosilica only for water/binder = 0.5. The carbonation coefficient was not notably affected by increasing nanosilica dosages and even adverse effect was observed for water/binder = 0.65. Further information of microstructure was also provided through characterization techniques such as X-ray diffraction, thermal gravimetric analysis, mercury intrusion porosimetry, and scanning electron microscopy. The effectiveness of a certain nanosilica dosage addition into lower strength mixes was more noticeable, while, for the higher strength mix, the effectiveness was less

Modification of properties of reinforced concrete through nanoalumina electrokinetic treatment

An attempt was made to drift nanoalumina particles into concrete pores through electrokinetic treatment. An external electric current (current density = 3 A/m2) was applied for 3 and 15 days in reinforced concrete blocks toward steel reinforcement and microstructural characterizations (i.e. morphology observation and porosity analysis) were performed on concrete fragments of different depth from exposure surface. The morphology observation evidenced transport of nanoalumina from the exposure surface even reaching the rebar-concrete interface (up to 25–30 mm, in 15 days treatment). The porosity analysis of treated samples revealed that reduction of porosity of rebar interface was more pronounced as compared to the exposure surface and the treatment for 15 days was more beneficial for porosity refinement than treatment for 3 days. Effects of the electrokinetic NA treatment on strength of rebar-concrete interface were evaluated through pull-out test. The results showed that by increasing current density, bond strength of rebar-concrete interface increased

EDITORIAL

Presentación del Dossier a cargo de las coordinadoras y el coordinador que integran el Grupo de Investigación Consolidado: Formación Docente y Educación Infantil (FODEI) de la Secretaría General de Ciencia y Técnica de la UNNE

Stacking non-BPS D-Branes

We present a candidate supergravity solution for a stacked configuration of stable non-BPS D-branes in Type II string theory compactified on T^4/Z_2. This gives a supergravity description of nonabelian tachyon condensation on the brane worldvolume.Comment: 10 pages, revte

EDITORIAL: INFANCIAS, SUBJETIVIDADES Y PRÁCTICAS DE EDUCACIÓN Y CUIDADO: APORTES PARA NUEVOS COMIENZOS

Presentación del Dossier a cargo de las coordinadoras y el coordinador que integran el Grupo de Investigación Consolidado: Formación Docente y Educación Infantil (FODEI) de la Secretaría General de Ciencia y Técnica de la UNNE