预防碳化提高钢筋混凝土的耐久性

碳化是影响混凝土耐久性的一个重要因素,将加速混凝土中钢筋的腐蚀,从而缩短工程寿命。目前碳化破坏引起更加特别的重视,因全球关心的气候变化过程实质上就是空气中CO2浓度增加的过程。其次是因为使用高掺量混合材加剧了水泥的碳化破坏。加以高速发展新的基础设施,其中地下工程、地铁、火车站、城市中心等地的微气候中CO2的浓度显著增加。在施工过程中若养护不够,表面层空隙率增加,将更加加重CO2的腐蚀程度。本文将详细叙述这些因素,并提出解决碳化问题应采取的措施。     

Natural carbonation of self-compacting concretes

This experimental work involves a study of the carbonation depth in self-compacting concretes at different ages and also analyses their porous microstructure, since these aspects are directly related to each other. Eight different concretes were used, four self-compacting (SCC) and four normally-vibrated (NVC). The carbonation rate was found to be lower in SCC than NVC, due to the fact that limestone fines produce less porosity and a finer microstructure. The difference between both types of concrete tends to disappear as their fines content becomes similar. It was also observed that, under the test conditions (RH 60.8%), for pore sizes under 0.065 μm CO₂ diffusion in the interior of the concrete is not significant. An expression is proposed to estimate carbonation rate in concrete from the volume of pores over 0.065 μm and the threshold diameter.     

Measurement Of Indoor Relative Humidity Using A Passive Sampler For Water Vapour

As part of the energy and indoor climate survey recently carried out in Sweden (the ELIB study) a simple, inexpensive but reliable passive sampler for estimating monthly averages of relative humidity has been developed. The diffusion sampler consists of a 5 ml plastic tube prepared with lithium chloride monohydrate (LiCl. H₂O) as trapping medium. After necessary calibration of this particular design of sampler the relative humidity can be calculated from the weight change of the sampler, the time of sampling and the average temperature during this period. The estimated accuracy of the method is better than k 2% RH up to65% RH.     

Associations of indoor carbon dioxide concentrations,air temperature,and humidity with perceived air quality and sick building syndrome symptoms in Chinese homes

The indoor environment influences occupants’ health. From March 1, 2018, to February 28, 2019, we continuously monitored indoor temperature (T), relative humidity (RH), and CO₂ concentration in bedrooms via an online system in 165 residences that covered all five climate zones of China. Meanwhile, we asked one specific occupant in each home to complete questionnaires about perceived air quality and sick building syndrome (SBS) symptoms at the end of each month. Higher CO₂ concentration was significantly associated with a higher percentage of perceived stuffy odor and skin SBS symptoms. Higher relative humidity was associated with higher percentage of perceived moldy odor and humid air, while lower RH was associated with a higher percentage of perceived dry air. Occupants who lived in residences with high RH were less likely to have mucosal and skin SBS symptoms (adjusted odds ratio (AOR): 0.73–0.78). However, the benefit of high humidity for perceived dry air and skin dryness symptoms is weaker if there is a high CO₂ concentration level.     

Climate change impact and risks of concrete infrastructure deterioration

Atmospheric CO₂ is a major cause of reinforcement corrosion in bridges, buildings, wharves, and other concrete infrastructure in Australia, United States, United Kingdom and most other countries. The increase in CO₂ levels associated with global warming will increase the likelihood of carbonation-induced corrosion. Moreover, temperature rises will increase corrosion rates. Clearly, the impact of climate change on existing and new infrastructure is considerable, as corrosion damage is disruptive to society and costly to repair. The paper describes a probabilistic and reliability-based approach that predicts the probability of corrosion initiation and damage (severe cracking) for concrete infrastructure subjected to carbonation and chloride-induced corrosion resulting from elevated CO₂ levels and temperatures. The atmospheric CO₂ concentration and local temperature and relative humidity changes with time over the next 100 years in the Australian cities of Sydney and Darwin are projected based on nine General Circulation Models (GCMs) under (i) high CO₂ emission scenario, (ii) medium CO₂ emission scenario, and (iii) CO₂ emission reduction scenario based on policy intervention. The probabilistic analysis included the uncertainty of CO₂ concentration, deterioration processes, material properties, dimensions, and predictive models. It was found that carbonation-induced damage risks can increase by over 400% over a time period to 2100 for some regions in Australia. Damage risks for chloride-induced corrosion increase by no more than 15% over the same time period due to temperature increase, but without consideration of ocean acidity change in marine exposure. Corrosion loss of reinforcement is not significant. The results were most sensitive to increases in atmospheric CO₂.     

CO2-optimization of reinforced concrete frames by simulated annealing

This paper describes a methodology to design reinforced concrete (RC) building frames based on minimum embedded CO₂ emissions. The design involves optimization by a simulated annealing (SA) algorithm applied to two objective functions, namely the embedded CO₂ emissions and the economic cost of RC framed structures. The evaluation of solutions follows the Spanish Code for structural concrete. The methodology was applied to six typical building frames with 2, 3 and 4 bays and up to 8 floors. The largest example has 153 design variables and a combinatorial solution space of 10²³². Results from the SA algorithm application indicate that embedded emissions and cost are closely related and that more environmentally-friendly solutions than the lowest cost solution are available at a cost increment which is quite acceptable in practice. Further, the best solutions for the environment are only at the most 2.77% more expensive than the best cost solutions. Alternatively, the best cost solutions increase CO₂ emissions by 3.8%. Finally, the methodology described will enable structural engineers to mitigate CO₂ emissions in their RC structural designs.     

Evaluation of calcium hydroxide contents in pozzolanic cement pastes by a chemical extraction method

The aim of the study is to investigate the calcium hydroxide (Ca(OH)₂) contents in pozzolanic cement pastes analyzed by the chemical extraction method and thermal analysis (DTA/TG). The second part of the study involves the carbonation of pozzolanic cement pastes and its influence on Ca(OH)₂ reduction. The Ca(OH)₂ contents in cement pastes after being subjected to accelerated carbonation were investigated by a chemical extraction method and compared to those values from DTA/TG analysis. The experimental results show that the chemical method reveals overall results comparable to those from DTA/TG analysis. However, in case of pastes containing high amounts of mineral admixtures or pastes subjected to carbonation which contain very low Ca(OH)₂, only the chemical method yields precise reliable results by showing the pseudo-negative contents.     

Planning for Place and Plexus: Metropolitan Land Use and Transport

Problem: Human and natural systems will probably have to adapt to climate change impacts, but this cannot be planned for using the traditional approach based on predictions because of the subject's great complexity, its planning horizon more than 50 years away, and uncertainty about the future climate and how effectively CO₂ emissions will be reduced.

Purpose: This article proposes a more appropriate basis for planning climate change adaptation. Anticipatory governance is a flexible decision framework that uses a wide range of possible futures to prepare for change and to guide current decisions toward maximizing future alternatives or minimizing future threats. Rather than trying to tame or ignore uncertainty, this approach explores uncertainty and its implications for current and future decision making.

Methods: I review and summarize the literature on anticipatory governance and provide three case studies to demonstrate its application to climate change planning.

Results and conclusions: Denver Water, New York City, and the City of Phoenix are all using scenarios to anticipate the range of global climate changes that may impact their communities and to develop adaptation strategies to address these impacts. Each is developing a decision framework for implementing adaptation strategies incrementally based on climate monitoring. An incremental approach minimizes the resources that must be allocated to address these risks and has allowed these cities to plan in spite of the high uncertainty associated with climate change science and social change.

Takeaway for practice: The complexity, uncertainty, and distant planning horizon associated with climate change cannot be managed sufficiently for the traditional predict-and-plan approach to yield good decisions about the significant social and capital investments likely to be required for adaptation. To be successful, social institutions must embrace new methods that explore uncertainty and that provide strategic guidance for current and future decisions.

Research support: None.     

Carbonation of concrete in the tropical environment of Singapore

The experimental investigation was performed to study the influence of unit cement content and w/c ratio on the rate of carbonation under natural exposure in the tropical environment of Singapore together with laboratory accelerated carbonation test. To represent concrete of lean to rich mixes, the cement content in a total of 21 mixtures was varied from 250 to 450 kg/m³, while the w/c ratios ranged between 0.40 and 0.70 with an increment of 0.05. After 14 years of natural exposure, the 28-day compressive strength (f ₂₈) and w/c ratio (w/c) were identified as reliable parameters for estimating the depth of carbonation. However, within the range of values studied, there is no significant effect of the cement content on the carbonation behaviour. The relationship between the 14-year natural exposure and the samples from the 7% CO₂ concentration accelerated test were also correlated. Tentative recommendations on the use of the findings in durability design against carbonation for required service life in a tropical environment are proposed.     

冻融前后混凝土碳化性能试验研究

通过对四种不同配合比的混凝土先后进行冻融循环试验(0、50、150次)和加速碳化试验(0、1、2周),研究冻融前后混凝土碳化性能变化情况.结果表明:冻融循环增大了混凝土孔隙率,加速了碳化进程,循环次数越多,加速作用越大;在冻融和高浓度CO2环境下,粉煤灰的掺入增加了混凝土碳化量,并造成较大强度损失,对混凝土结构不利;碳化使混凝土抗压强度略有提高,但不足以弥补冻融造成的强度损失,水灰比越大,强度损失越大.对混凝土结构进行耐久性设计和使用寿命预测时,必须考虑冻融、碳化及其它因素的复合作用.     

Mesoscopic simulation method of concrete carbonation process

Concrete structures in the atmosphere may suffer from the problem of steel corrosion induced by the carbonation process. Most of previous research on concrete carbonation problems has been performed on the macro-scale. These studies did not consider the influence of the aggregates, which should be involved on meso-scale. In order to extend the present studies on the problems of concrete carbonation from macro-scale to meso-scale, an improved method to simulate the random aggregate structure (RAS) in concrete has been proposed. This method can be divided into two steps: the process of RAS in a three-dimensional model, and the process of numerical analysis in a cross-section. In this method, the effect of the randomness on the results of RAS is analysed. Based on the mass conservation law and kinetic model of chemical reactions, a simplified numerical model of concrete carbonation is developed. The effect of the key parameters in this model such as effective diffusion coefficient of CO₂ and carbonation reaction constant is discussed. The comparison of the simulations of concrete carbonation process on macro-scale and meso-scale is discussed as well. Finally, an application of the method to determine the proper composition of concrete in the practical engineering is presented.     

The environmental impact of stock market capitalization and energy transition: Natural resource dynamics and international trade

This paper examines the effect of stock market capitalization and energy transition on the environment between 1994 and 2020, considering the roles of economic growth, natural resources, and international trade in Asian countries. The results indicate that stock market capitalization, energy transition, and natural resources reduce CO₂ emissions. International trade and economic growth are positively associated with CO₂ emissions. There is a suggestion that the government should encourage companies to determine the material business risks they face. Aside from traditional risks related to operations, technology, and the market, climate change-related risks should likewise be considered significant.     

Effects of stabilizers on CO2 fixation capacity in neutralization of alkali construction sludge

Construction sludge, generated from tunneling and piling, is typically in a liquid state. It can be improved via physical treatments, such as dehydration, and/or chemical treatments, using stabilizers, in order to to be recycled as construction material. To adjust the strength of sludge, chemical treatments are often preferred. However, chemical treatments frequently result in alkali leaching. Methods to reduce alkalinity by curing the alkaline sludge under CO₂ gas at a certain concentration have been proposed in Japan. In recent years, technologies that utilize CO₂ to improve the quality of cementitious material have received considerable attention in terms of carbon capture. Therefore, the effects of stabilizers on the CO₂ fixation capacity of alkaline sludge during pH neutralization were investigated in this study. Accelerated carbonation and carbonate content measurement tests were conducted to detect the CO₂ content fixed in alkaline sludge specimens treated with various stabilizers. The test results showed that the fixed maximum CO₂ content per gram of dry mass of sludge, (m_CO2)^max, increased with the calcium oxide (CaO) content of the stabilizer(s) per gram of dry sludge, C_CaO. However, the rate of increase in (m_CO2)^max with C_CaO was significantly affected by the type of stabilizer used. In the case of quicklime (QL), the ratio of (m_CO2)^max to C_CaO was approximately 0.5, whereas, in the cases of fly ash (FA) and steel slag (SS), the ratio was approximately 0.25. The ratios for biomass ash and paper sludge ash were between that for QL and that for FA and SS. Detailed analyses of the test results suggest that the CaO content per gram of stabilizer(s) in the sludge, C*_CaO, can provide an estimate of the fixed maximum amount of CO₂ per gram of stabilizer(s) in the sludge, (m*_CO2)^max. However, other factors, including the amount of water-soluble Ca, should be considered for a precise evaluation. Additionally, the experimental results showed that the decrease in pH owing to neutralization increases with the increasing C_CaO. However, the type of stabilizer did not significantly affect the relationship between the degree of CO₂ fixation and the degree of neutralization.     

Characteristics of ultra-high performance ‘ductile’ concrete and its impact on sustainable construction

This article presents an overview of the material characteristics of a Malaysia blend of ultra-high performance ‘ductile’ concrete (UHPdC). Examples of the environmental impact calculations of UHPdC structures compared to that of conventional reinforced concrete (RC) design are presented. The comparison studies show that many structures constructed from UHPdC are generally more environmentally sustainable than built of the conventional RC with respect to the reduction of CO₂ emissions, embodied energy and global warming potential. The enhanced durability of UHPdC also provides for significant improvements in the design life, further supporting the concept of sustainable development.     

Evaluation of CO2 captured in alkaline construction sludge associated with pH neutralization

Recently, the capture of carbon dioxide (CO₂) using alkaline waste and byproducts has garnered considerable interest. Construction sludge may be categorized as alkaline waste, as it often exhibits high alkalinity during its generation or intermediate treatment. Hence, researchers have attempted to accelerate pH neutralization and recycle alkaline construction sludge by curing it under a high CO₂ concentration. By exposing concentrated CO₂ gas to an alkaline sludge, cement hydrates such as calcium hydroxide and calcium–silicate–hydrate gels form calcium carbonate (CaCO₃). Subsequently, the generation of CaCO₃ is expected to reduce the pH of the sludge. However, the amount of CO₂ captured in sludge has not been investigated extensively, unlike those of other alkaline wastes. Therefore, the amount of CO₂ captured in alkaline sludge that is associated with pH neutralization is evaluated in this study. It is demonstrated that accelerated carbonation tests using a CO₂ incubator and carbonate content evaluation tests based on the gas pressure method successfully reveal the amount of CO₂ captured in the alkaline sludge that is associated with pH neutralization. Additionally, the test results show that the amount of m_CO2 (i.e., the amount of CO₂ captured per 1 g of dry mass of alkaline sludge) increases with ΔpH (ΔpH is the difference between the initial pH and the pH after the alkaline sludge is neutralized). A maximum of 0.021 g of CO₂ is captured per 1 g of dry mass of alkaline sludge when the addition ratio of quicklime A_QL = 3% and 0.040 g when A_QL = 6%. The CO₂ capture ratio m_CO2/m_CO2^max, which represents the ratio of CO₂ captured in the sludge to the maximum capturing capacity, increases with ΔpH. CO₂ capture ratios of up to 90.0% and 84.9% are recorded when A_QL = 3% and A_QL = 6%, respectively. It is discovered that a higher A_QL results in a higher m_CO2. Moreover, the test results indicate that a higher A_QL causes a more significant change in the CO₂ capture ratio, even when the pH decreases slightly.     

低CO_2浓度下混凝土的加速碳化试验研究

选用CO2体积浓度为3%的低浓度碳化试验研究了水胶比及粉煤灰与矿粉单双掺对混凝土碳化性能的影响。通过对比试验,分析了低CO2浓度下碳化深度与抗压强度之间的相关性,并采用XRD研究了混凝土的碳化产物,显示了物相分析结果与碳化试验结果的一致性。结果表明,低浓度碳化试验结果能很好反映理论规律,并且碳化周期长时与抗压强度之间具有较好的对应关系。进行低浓度加速碳化试验时,建议采用混凝土的碳化龄期为90d。XRD分析表明,混凝土的碳化速率与其内部可碳化物质CH的含量密切相关。     

Effects of sample crumbling and particle size on accelerated carbonation of alkaline construction sludge treated with paper-sludge ash-based stabilizers

Construction sludge generated from underground and pile construction works frequently appears in a liquid state. For its utilization as a construction geomaterial, it can be improved by mixing in cement or lime. The target strength can be achieved relatively easily by adjusting the amount of cement or lime added to the sludge. However, this type of chemical treatment is associated with high alkalinity, which causes environmental concerns associated with alkali leaching. In this study, therefore, the accelerated carbonation of alkaline construction sludge through treatment with a paper-sludge ash-based stabilizer (PSAS) was experimentally attempted to neutralize the pH. The alkaline sludge treated with the PSAS was exposed to pure CO₂ gas to accelerate the carbonation. The effects of granulation via crumbling prior to the accelerated carbonation on the pH neutralization period were analyzed from the test results. It was observed that crumbling the sludge prior to the CO₂ gas exposure accelerated the pH neutralization. These results suggest that a short pH neutralization period can be achieved by reducing the particle size. Therefore, the effect of the particle size of the PSAS-treated sludge on the pH neutralization period was also investigated. The results showed that the pH neutralization period decreased with an increasing particle size. However, it was also observed that, when the addition ratio of the PSAS was increased, the difference in the pH neutralization period induced by the change in particle size became less significant.     

Accelerated carbonation of alkaline construction sludge by paper sludge ash-based stabilizer and carbon dioxide

Construction sludge frequently has high alkalinity after its generation or during the intermediate treatment process. The aim of this study is to experimentally investigate the potential of combining accelerated carbonation and a paper sludge ash-based stabilizer (PSAS) to neutralize the alkalinity of construction sludge in a short period and to improve its strength for use as a recycled material. The experimental results indicate that the addition of a PSAS significantly granulated the alkaline sludge, and once granulated, the PSAS successfully accelerated the pH neutralization of the alkaline sludge. It was also found that the decrease in dry density ρ_d and the degree of saturation S_r of the PSAS-treated sludge was able to reduce the period required for the pH neutralization, t_N. The decrease in ρ_d is thought to allow fresh CO₂ gas to penetrate the specimen more easily. However, if S_r is below a certain limit, it does not strongly facilitate the reduction of t_N. This implies that pH neutralization cannot be accelerated when the amount of water in the sludge is below a certain level. Moreover, it was found that mean particle diameter D₅₀ also affected t_N. The strength development of the PSAS-treated sludge was evaluated using a series of cone index tests. It was found that the strength of the alkaline sludge without the PSAS was significantly decreased by accelerated carbonation, but was significantly increased even after accelerated carbonation when the PSAS was present. Due to the porosities of the remaining PS ash particles, most of the contribution of the water absorption and retention performance of the PSAS to the strength development of the PSAS-treated sludge was secured after accelerated carbonation. In addition, the granulated particles of the PSAS-treated sludge retained their granular shape to some extent. Therefore, it is presumed that the friction and interlocking of the particles did not decrease significantly. It was also found that, after carbonation, the q_c of the PSAS-treated sludge increased more rapidly than that of the alkaline sludge without the PSAS. A further detailed examination of the test results showed that under air-curing conditions, the q_c of the treated sludge with accelerated carbonation increased relatively gradually compared to that of the treated sludge without accelerated carbonation.     

粉煤灰混凝土二维和三维碳化研究

本文首先建立了2D和3D碳化测试方法,在此基础上系统研究了粉煤灰混凝土在加速碳化试验中2D和3D碳化深度的依时变化规律,探讨了粉煤灰掺量(0%、10%、20%、40%、60%)、粉煤灰种类(Ⅰ级、Ⅱ级)、水胶质量比(0.3,0.35,0.4)、胶凝材料用量(458,380kg/m3)、养护龄期(28,90d)5个重要因素对粉煤灰混凝土2D和3D碳化深度的影响规律。另外,还将2D和3D碳化试验结果与同条件下1D碳化进行了定量比较,发现2D和3D碳化明显存在交互作用,为量化该交互作用提出了2D和3D碳化交互系数概念,并给出了2D和3D碳化交互系数随时间变化的数学表达式。以期为准确预测实际混凝土结构在真实状态下碳化寿命提供科学依据。     

Evaluation of carbonated incineration bottom ash using exhaust gas and CO2 discharged from waste incineration facilities as ground material

This study focused on a carbonation treatment that immobilizes trace elements such as lead (Pb) in incineration bottom ash (IBA) using the exhaust gas and the refined carbon dioxide generated (CO₂) recovered at the Japan's first incineration facility, which recovers CO₂ in the gaseous form. This study describes (1) the bearing capacity of IBA from the municipal solid waste (MSW) obtained through the California bearing ratio (CBR) test to investigate the possibility of using IBA as a base course material, (2) mechanical characteristics of IBA using the cone index test, to investigate the possibility of using as an embankment material, and (3) leaching properties of IBA from MSW obtained through a single batch leaching test and the long-term outdoor exposure of the leaching test to evaluate the effect of carbonation treatments. Results reveal that there was no effect of trace components not included in the separated and recovered CO₂, and that the effect of carbonation could be obtained even if the exhaust gas was used directly. The carbonated IBA can be applied as a sub-base course material based on its mechanical properties, regardless of the carbonation conditions. In addition, carbonated IBA can be used as an embankment material owing to its sufficient cone index value. Regarding the heavy metal and metalloid leaching behavior, it was revealed that all the IBA samples satisfied the soil environmental standards set by the Ministry of the Environment, Japan, except for Cr (VI). The long-term leaching characteristics of carbonated IBA showed that Pb could be immobilized over a long period.