Impacts of climate change on optimal mixture design of blended concrete considering carbonation and chloride ingress

Many studies on the mixture design of fly ash and slag ternary blended concrete have been conducted.However,these previous studies did not consider the effects of climate change,such as acceleration in the deterioration of durability,on mixture design.This study presents a procedure for the optimal mixture design of termary blended concrete considering climate change and durability.First,the costs of CO2 emissions and material are calculated based on the concrete mixture and unit prices.Total cost is equal to the sum of material cost and CO2 emissions cost,and is set as the objective function of the optimization.Second,strength,slump,carbonation,and chloride ingress models are used to evaluate concrete properties.The effect of different climate change scenarios on carbonation and chloride ingress is considered.A genetic algorithm is used to find the optimal mixture considering various constraints.Third,ilustrative examples are shown for mixture design of ternary blended concrete.The analysis results show that for termary blended concrete exposed to an atmospheric environment,a rich mix is necessary to meet the challenge of climate change,and for termary blended concrete exposed to a marine environment,the impact of climate change on mixture design is marginal.

Chloride ingress and macro-cell corrosion of steel in concrete made with recycled brick aggregate

An investigation on chloride ingress and macro-cell corrosion of steel bars in concrete made with recycled brick aggregate(RBA)was carried out.As control cases,virgin brick aggregate(BA)and stone aggregate(SA)were also investigated.Both cylindrical and cracked prism specimens were studied for 16 different cases.The prism specimens were made with a segmented steel bar providing electrical connection from outside of the specimens to measure macro-cell corrosion current continuously under seawater splash exposure for a period of 30 d using a data logger.Cylindrical specimens were submerged in 3%NaCl solution at a temperature of 40℃ to investigate chloride ingress in concrete made with RBA,BA,and SA after 120 and 180 d.Half-cell potential,corrosion area,and depths of corrosion were also investigated.The chloride ingress as well as corrosion of steel bars in concrete made with the different types of aggregate is ordered as RBA>BA>SA.

Detection of Chlorides in Pore Water of Cement Based Materials by Potentiometric Sensors

Free chloride ions dissolved in pore water of cement based materials initiate the corrosion of steel rebars used for reinforcement of concrete and thus pose serious constant danger to the safety of reinforced concrete structures.The in-situ monitoring of the content of chlorides in pore water with second order Ag/AgCl electrodes is an elegant approach which, however,suffers from poor stability of Ag/AgCl electrodes in highly alkaline environment like pore water.In this work the electrochemical stability of plain and stabilized electrodeposited Ag/AgCl sensors in extreme alkaline conditions are presented. The electrochemical processes were elucidated which govern the potentiometric response of Ag/AgCl sensors and determine their performance.The results have shown suitable stability and reproducibility of stabilized Ag/AgCl sensors over five months, which was sufficient for laboratory testing of chloride ingress into cement based materials.The embedded calibrated sensor elements were used to monitor the chloride uptake into cylindrical mortar specimen dipped into simulated pore water containing about 3 wt% of chloride ions.The velocity of diffusing front and the diffusion coefficient of chloride ions in pore water of cylindrical mortar sample were estimated from concentration-time profiles determined by chloride sensors positioned at defined positions in the sample wall.

Probabilistic Lifetime Assessment of Marine Reinforced Concrete with Steel Corrosion and Cover Cracking

In order to study the durability behavior of marine reinforced concrete structure suffering from chloride attack, the structural service life is assumed to be divided into three critical stages, which can be characterized by steel corrosion and cover cracking. For each stage, a calculated model used to predict the lifetime is developed. Based on the definition of durability limit state, a probabilistic lifetime model and its time-dependent reliability analytical method are proposed considering the random natures of influencing factors. Then, the probabilistic lifetime prediction models are applied to a bridge pier located in the Hangzhou Bay with Monte Carlo simulation. It is found that the time to corrosion initiation to follows a lognormal distribution, while that the time from corrosion initiation to cover cracking t~ and the time for crack to develop from hairline crack to a limit crack width t2 can be described by Weibull distributions. With the permitted failure probability of 5.0%, it is also observed that the structural durability lifetime mainly depends on the durability life to and that the percentage of participation of the life to to the total service life grows from 61.5% to 83.6% when the cover thickness increases from 40 mm to 80 mm. Therefore, for any part of the marine RC bridge, the lifetime predictions and maintenance efforts should also be directed toward controlling the stage of corrosion initiation induced by chloride ion.

Corrosion Resistance and Physical-Mechanical Properties of Reinforced Mortars with and without Carbon Nanotubes

Following the evolution of currently enforced Performance Based Design standards of reinforced concrete (RC) structures for durability, the designer, rather than complying with given prescriptive limits, may instead specify a cementitious mix design that is proven to exhibit a code prescribed resistance level (class) to a given exposure environment. Such compliance will lead to the protection of the steel reinforcement from corrosion and the cementitious mortar from degradation, during the design lifespan of the structure, under aggressive environmental exposure conditions such as, marine or deicing salts and carbonation. In this context, the enhancement of the physical and durability properties of common cement-based mortars under chloride exposure are experimentally investigated herein. In particular, the experimental program reported herein aims to evaluate the influence of incorporating multi-walled carbon nanotubes on the physical and mechanical properties of reinforced mortars against chloride ions. Furthermore, the anticorrosion protection of cementitious composites prepared with nanomaterials at 0.2% w/w is further investigated, by comparing all test results against reference specimens prepared without any additive. Electrochemical (Half-cell potential, corrosion current) and mass loss of reinforcement steel measurements were performed, while the porosity, capillary absorption and flexural strength were measured to evaluate the mechanical and durability characteristics of the mortars, following a period of exposure of eleven months;SEM images coupled with EDX analysis were further recorded and used for microstructure observation. The test results indicate that the inclusion of the nanomaterials in the mix improved the durability of the mortar specimens, while the nano-modified composites exhibited higher chloride penetration resistance and flexural strength than the corresponding values of the reference mortars. The test results and the comparison between nanomodified and reference mortars showed that the use of CNTs as addition led to protection of steel reinforcing bars against pitting corrosion and a significant improvement in flexural strength and porosity of the mortars.

Chloride binding and time-dependent surface chloride content models for fly ash concrete

Corrosion of embedded rebars is a classical deterioration mechanism of reinforced concrete structures exposed to chloride environments. Such environments can be attributed to the presence of seawater, deicing or sea-salts, which have high concentrations of chloride ion. Chloride ingress into concrete, essential for inducing rebar corrosion, is a complex interaction between many physical and chemical processes. The current study proposes two chloride ingress parameter models for fly ash concrete, namely： 1） surface chloride content under tidal exposure condition; and 2） chloride binding. First, inconsistencies in surface chloride content and chloride binding models reported in literature, due to them not being in line with past research studies, are pointed out. Secondly, to avoid such inconsistencies, surface chloride content and chloride binding models for fly ash concrete are proposed based upon the experimental work done by other researchers. It is observed that, proposed models are simple, consistent and in line with past research studies reported in literature.